NP6.2 | Exploring Gravity Currents, Waves, and Eddies: from Physical Modeling to Geophysical Applications
EDI
Exploring Gravity Currents, Waves, and Eddies: from Physical Modeling to Geophysical Applications
Co-organized by OS4
Convener: Kevin Lamb | Co-conveners: Maria Eletta Negretti, Chris Johnson, Cyril GadalECSECS, Yvan Dossmann, Marek Stastna, Kateryna Terletska
Orals
| Fri, 19 Apr, 08:30–12:30 (CEST)
 
Room 0.94/95
Posters on site
| Attendance Tue, 16 Apr, 16:15–18:00 (CEST) | Display Tue, 16 Apr, 14:00–18:00
 
Hall X4
Posters virtual
| Attendance Tue, 16 Apr, 14:00–15:45 (CEST) | Display Tue, 16 Apr, 08:30–18:00
 
vHall X4
Orals |
Fri, 08:30
Tue, 16:15
Tue, 14:00
The nonlinear nature of fluid flow gives rise to a wealth of interesting and beautiful phenomena. Many of these are of fundamental importance in the understanding of lakes, oceans and the atmosphere because of their role in such things as transport, the energy cascade and, ultimately, in mixing.

This session is intended to bring together researchers interested in the fundamental nature of nonlinear processes. Particular attention will be paid to intrinsically nonlinear flows that are driven by a gravitational forces acting on density variations, e.g. those due to temperature (e.g. katabatic winds) and/or salinity (e.g. density currents) differences, and/or the presence of particles (e.g. snow avalanches, debris-flows turbidites). While occurring in various planetary environments, and involving different fluids and particles, they share numerous features due to the common and similar physical processes that govern their dynamics. Yet, a universal description of their dynamics remains elusive, as specifically the feedback on the flow of various processes is difficult to predict.

We therefore welcome contributions including (but not limited to) diverse occurrences of geophysical gravity currents, nonlinear and solitary waves, wave-mean flow and wave-wave interactions, flow instabilities and their nonlinear evolution, frontogenesis, double diffusion and the nonlinear equation of state, convection, and river plumes.

This session then aims to present complementary physical-based approaches, by gathering researchers from different communities, all focusing on these flows by studying field data, using analogue laboratory experiments or numerical simulations or focusing on analytical modelling. We particularly encourage the participation of early-career researchers and students.

Orals: Fri, 19 Apr | Room 0.94/95

Chairpersons: Kateryna Terletska, Marek Stastna
08:30–08:35
08:35–08:45
|
EGU24-3505
|
NP6.2
|
ECS
|
On-site presentation
Saulo Mendes, Ina Teutsch, and Jérôme Kasparian

Theoretical studies on the modulation of unidimensional regular waves over a flat bottom due to a current typically assign an asymmetry between the effects of opposing/following streams on the evolution of major sea variables, such as significant wave height. The significant wave height is expected to monotonically increase with opposing streams and to decrease with following streams. To some extent, observations on data sets containing a few thousand of waves or over a continuous series of about a day confirm this prediction. Here we show that in very broad-banded seas with high directional spread, the asymptotic behavior of sea variables over large data sets is highly non-trivial and does not follow the theoretical predictions, especially at high values of the ratio between tidal stream and group velocity. Furthermore, we analyze the anomalous statistics originating from both forward and opposing non-stationary currents. Despite the sea states being dominantly broad-banded and featuring a large directional spread, we found that anomalous statistics are of the same order of magnitude of those observed in unidirectional laboratory experiments and symmetrical in regard to the orientation of the tidal current.

How to cite: Mendes, S., Teutsch, I., and Kasparian, J.: Direction Symmetry of Wave Field Modulation by Tidal Current and its Consequences for Extreme Nonlinear Waves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3505, https://doi.org/10.5194/egusphere-egu24-3505, 2024.

08:45–08:55
|
EGU24-4294
|
NP6.2
|
ECS
|
On-site presentation
Xiaolin Bai

Internal waves have a wide range of scales but are typically unresolved in climate or global models. With an unprecedented capability of observing and simulating these processes, they are becoming increasingly important to quantify the upscale effect of these processes. As the largest marginal sea in the western Pacific, the South China Sea has the most energetic and frequent internal waves around the world. These waves are also affected by multiscale processes, climate changes, and anthropogenic impacts. There have been considerable advances in exploring the generation and propagation of internal waves in recent years. However, the understanding of the formation and fate of internal solitary-like waves on the continental shelf is still very limited. It is widely accepted that these internal waves generally originate from the Luzon Strait. They usually have regular occurrence and are phase-locked to tidal forcing in the Luzon Strait. However, we present field measurements showing an irregular occurrence of nonlinear internal waves on the northern shelf of the South China Sea. This irregular occurrence is in striking contrast to the prominent predictability of internal waves originating from the Luzon Strait. We reveal that the intermittent nature of the occurrence is due to the local generation of nonlinear internal waves on the continental shelf, in addition to the fission of shoaling internal waves. The results reported here are expected to apply to other shelf regions of the world's oceans.

How to cite: Bai, X.: Intermittent Generation of Nonlinear Internal Waves on Continental Shelf, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4294, https://doi.org/10.5194/egusphere-egu24-4294, 2024.

08:55–09:05
|
EGU24-8014
|
NP6.2
|
ECS
|
On-site presentation
Sam Hartharn-Evans, Marek Stastna, and Magda Carr

Well established energy-based methods of quantifying diapycnal mixing in process-study numerical models are often used to provide information about when mixing occurs, and how much mixing has occurred. However, describing how and where this mixing has taken place remains a challenge. Moreover, methods based on sorting the density field struggle with under resolution and uncertainty as to the definition of the reference density when bathymetry is present. Here, an alternative method of understanding mixing is proposed. Paired histograms of user selected variables (which we abbreviate USP) are employed to identify mixing fluid, and are then used to display regions of fluid in physical space that are undergoing mixing. Here, two case studies are presented to showcase this method: shoaling internal solitary waves and a shear instability in cold water influenced by the nonlinearity of the equation of state. For the first case, the USP method identifies differences in the mixing processes associated with different internal solitary wave breaking types, including differences in the horizontal extent and advection of mixed fluid. For the second case, the method is used to identify how density, and passive tracers are mixed within the core of the asymmetric cold-water Kelvin-Helmholtz instability.

How to cite: Hartharn-Evans, S., Stastna, M., and Carr, M.: A new approach to understanding fluid mixing in process-study models of stratified fluids, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8014, https://doi.org/10.5194/egusphere-egu24-8014, 2024.

09:05–09:15
|
EGU24-15282
|
NP6.2
|
On-site presentation
|
Kiori Obuse, Yusuke Hagimori, and Michio Yamada

Two-dimensional Navier-Stokes turbulence on a rotating sphere is one of the most fundamental mathematical models for describing the dynamics of planetary atmospheres and oceans. Despite its great simplicity, this system is known to have a solution with an anisotropic non-uniform large-scale structure similar to the zonal flows similar to those on Jupiter and other giant gas planets. Although westward circumpolar zonal flows are formed in the free-decay problem [1, 2] and multiple zonal band structure is formed in the forced problem [3, 4], the formation mechanism of the large-scale zonal flows has not yet been fully clarified. From the mean flow non-acceleration theorem based on the weakly non-linear theory [5, 6, 7], the effect of viscosity is sometimes considered to be its essential factor. However, there is no guarantee that the suggestion from the weakly nonlinear theory hold for fully nonlinear systems. In fact, Obuse and Yamada [8] reported the formation of large-scale westward circumpolar zonal flows in an unforced two-dimensional turbulence on a rotating sphere even when considering inviscid flows, which strongly suggests that the main factor in the formation mechanism of large-scale zonal flows is the nonlinearity of the Navier-Stokes or Euler equations, not the dissipation by viscosity.

In this study, we consider two-dimensional Navier-Stokes equations on a rotating sphere, and focus on three-wave nonlinear interactions of Rossby waves, which are linear solutions of this system, to investigate the factors directly involved in the mechanism of large-scale zonal flow formation. The three-wave non-resonant nonlinear interactions of Rossby waves are investigated in detail, by calculating the time derivative of energy of zonal Rossby modes. The obtained results suggest that the formation of the westward circumpolar large-scale zonal flows is directly caused by non-local energy transfer due to near-resonant interactions.

[1] S. Yoden and M. Yamada, “A numerical experiment on two-dimensional decaying turbulence on a rotating sphere,"  J. Atomos. Sci., 50, 631-643 (1993)

[2] S. Takehiro, M. Yamada, Y.-Y. Hayashi, "Energy accumulation in easterly circumpolar jets generated by two-dimensional barotropic decaying turbulence on a rapidly rotating sphere", J. Atmos. Sci., 64, 4084-4097 (2006)

[3] T. Nozawa and S. Yoden, "Formation of zonal band structure in forced two-dimensional turbulence on a rotating sphere," Phys. Fluids, 9, 2081-2093 (1997)

[4] K. Obuse, S. Takehiro, M. Yamada, "Long-time asymptotic states of forced two-dimensional barotropic incompressible flows on a rotating sphere", Phys. Fluids., 22, 156601 (2010)

[5] G. Charney and P.G. Drazin,"Propagation of planetary-scale disturbances from the lower into the upper atmosphere",  J. Geophys. Res., 66, 83-110 (1961)

[6] A. Eliassen and E. Palm, "On the transfer of energy in stationary mountain waves",  Geofys Publ., 22(3), 1-23 (1961)

[7] D.G. Andrews and M.E. McIntyre, "Planetary waves in horizontal and vertical shear: The generalized Eliassen-Palm relation and the mean zonal acceleration", J. Atoms. Sci., 30(11), 2031-2048 (1976)

[8] K. Obuse and M. Yamada, in preparation

 

 

How to cite: Obuse, K., Hagimori, Y., and Yamada, M.: Rossby wave nonlinear interactions and large-scale zonal flow formation in two-dimensional turbulence on a rotating sphere, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15282, https://doi.org/10.5194/egusphere-egu24-15282, 2024.

09:15–09:25
|
EGU24-3654
|
NP6.2
|
On-site presentation
Francisco Beron-Vera

We investigate the properties of, and can carry out a stability analysis of a baroclinic current in, a stratified thermal rotating shallow-water model for the subinertial dynamics of the upper ocean, a key player in the global climate system where most of the ocean variability is concentrated affecting the lateral transport of floating material such as plastic garbage, oil, and Sargassum seaweed.  Unlike the standard thermal model, the model considered here includes linear buoyancy variation in the vertical, still maintaining its two-dimensional structure and that of the adabatic (constant density) model.  Like the standard thermal model, the stratified thermal model produces submesoscale circulations resembling those observed in satellite imagery, yet taking longer to manifest.  Our study is motivated by this numerical observation. The model possesses a Lie--Poisson Hamiltonian structure.  A particular aspect of the model is that it supports motion integrals which neither form the kernel of the corresponding bracket nor are related to any explicit symmetries via Noether's theorem.  Among other things, we investigate the role of these conservation laws in constraining the growth of finite-amplitude perturbations to a zonal flow with quadratic vertical shear. Joint work with Maria J. Olascoaga.

How to cite: Beron-Vera, F.: Properties and baroclinic instability of stratified thermal ocean flow, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3654, https://doi.org/10.5194/egusphere-egu24-3654, 2024.

09:25–09:35
|
EGU24-10281
|
NP6.2
|
On-site presentation
Jean-Baptiste Roustan, Lucie Bordois, Franck Dumas, Francis Auclair, and Xavier Carton

Internal solitary waves with large amplitude have long been observed in the Strait of Gibraltar (SoG). Beautiful satellite images depict the ISW observed at the entrance of the Alboran Sea, generated by the interaction between tides and the topography at Camarinal Sill, the most important topographic obstacle in the Strait. As the tidal current heads west, it becomes supercritical over the sill, leading to the development of an internal hydraulic jump. When the current weakens, the flow reverts to subcritical,  the internal hydraulic jump is released, leading to the eastward propagation of an internal bore. This bore progressively transforms into an ISW train due to non-hydrostatic dispersion and non-linear effects. While the main mechanism of generation is now well understood, there are still open questions about the intricate dynamics of these nonlinear internal waves and their evolution along the Strait. Previous studies show an important variability in the shape, intensity and arrival time of this internal solitary train. 

 

Recent field experiments have revealed a complex network of local hydraulic jumps forming near Camarinal Sill. From the same dataset, the potential generation of not just one but two ISW trains has been addressed.  Then, we have investigated the implication on the evolution of these trains during their journey in the Strait of Gibraltar. 

 

Our mooring data reveal the presence of two trains of ISWs with slightly different north-south fronts east of Camarinal Sill. We propose hypotheses to explain the tilting of the fronts based on differential mixing and meridional tidal variability . Moreover, these findings prompt us to reconsider our understanding of the physics responsible for the observation of non-rank ordered ISW trains in the eastern part of the strait. To deeply investigate the consequences of nonlinear wave-wave interaction in the disorganization of the train, we implemented a simplified 3D non-hydrostatic configuration. 

How to cite: Roustan, J.-B., Bordois, L., Dumas, F., Auclair, F., and Carton, X.: Analyzing the Dynamics of Multiple ISW Emissions in the Strait of Gibraltar, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10281, https://doi.org/10.5194/egusphere-egu24-10281, 2024.

09:35–09:45
|
EGU24-16445
|
NP6.2
|
On-site presentation
Uwe Harlander and Ana M. Mancho

Isaac M. Held writes in his introduction of the book by Schneider and Sobel (2007) "A theory for the general circulation of the atmosphere has at its core a theory for the quasi-horizontal eddy fluxes of energy, angular momentum, and water vapor by the macro-turbulence of the troposphere." An analog of this atmospheric macro-turbulence can be studied by using data from the differentially heated rotating annulus laboratory experiment (e.g. Fowlis and Hide, 1965). From simultaneous measurements of surface temperature and horizontal flow it is possible to study elementary structures of Lagrangian tracer fluxes (Agaoglou, 2024) as well as eddy fluxes of heat and momentum. In a fluid layer, the three fluxes for PV, momentum M, and heat  T are connected via the Margules equation PV=∂M/∂y + f/H T. Here y is the north-south direction, f is the Coriolis parameter and H the layer depth. From our data we are able to compute all three fluxes and it is instructive to compare the results with fluxes from simplified models. E.g., in the Eady model T does not depend on z and we can thus obtain the total heat flux from this model using the surface data. Moreover, since M is zero for the Eady model, the PV flux is proportional to the heat flux. Using an even simpler model, the surface geostrophic approximation, we can deduce the flow from the temperature field alone. However, this model does not have the correct phase difference between the velocity and the temperature field and gives a wrong mean heat flux. Applying a phase difference such that the heat flux becomes comparable to the one from the Eady model allows to estimate the vertical flow structure from the temperature field alone. The result might be helpful for the construction of flow fields from satellite sea surface temperature data (LaCasce and Mahadevan, 2006).  

M. Agaoglou and V. J. García-Garrido and U. Harlander and A. M. Mancho (2024) Building transport models from baroclinic wave experimental data, Physics of Fluids, in press.

W. W. Fowlis and R. Hide (1965) Thermal convection in a rotating annulus of liquid: effect of viscosity on the transition between axisymmetric and non-axisymmetric flow regimes, J. Atmos. Sci., 22, 541-558.
 
J. H. LaCasce and A. Mahadevan (2006) Estimating subsurface horizontal and vertical velocities from sea-surface temperature, Journal of Marine Research 64, 695–721.
    
T. Schneider and A. H. Sobel (Eds.) (2007) The Global Circulation of the Atmosphere, Princeton University Press.

How to cite: Harlander, U. and Mancho, A. M.: Transport and fluxes of atmospheric models deduced from laboratory data , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16445, https://doi.org/10.5194/egusphere-egu24-16445, 2024.

09:45–09:55
|
EGU24-17638
|
NP6.2
|
ECS
|
On-site presentation
Shi-Wei Jian and Peter Haynes

A complex system of alternating zonal jets has been observed in the low latitude Pacific ocean, both on the equator and extending away into the subtropics. The dynamics of the subtropical jets, which also seem to be associated with strong cross-jet gradients in chemical tracers, may be similar to those of much-studied beta-plane zonal jets, but the effect of longitudinal boundaries, which are present in oceanic cases, on such jets is still puzzling. The forcing mechanism behind this set of oceanic zonal jets and what determines its horizontal and vertical structure are also poorly understood. We consider a beta-plane two-dimensional model with rigid boundary conditions and apply stochastic forcing, which without rigid boundaries would generate zonal jets. The instantaneous flow field is strongly time-dependent, with a large component of stochastically forced basin modes. This seems to disrupt the formation of alternating zonal jets, which, in contrast to the case without rigid boundaries, are observable only in the time-mean field and much weaker than the instantaneous flow. There is some evidence that these apparent time-mean jets are primarily the signature of stochastically forced basin modes rather than genuinely persistent jet-like flows. Adding tracers to the model allows the investigation of the relation between jet structure and the transport and mixing of tracers, and act as an important diagnostic to verify the presence or absence of jets. The instantaneous tracer field in the case with rigid boundaries is highly unsteady and, unlike the doubly periodic case, the jet structure is not manifested in the tracer field. Two-dimensional simulations with simple rigid boundary geometry may overestimate the generation of basin modes relative to the real ocean. Non-uniform damping is applied as a model device to break the interaction between the flow and boundaries and test whether it can inhibit basin modes and hence allow generation of steady zonal jets. With this non-uniform damping, jets present in the zonal mean field become more persistent as our redefined zonostrophy parameter increases, but there exist caveats we need to examine further, such as small ratio of energy in the zonal flow to the total energy as compared to much-studied cases without boundaries.

How to cite: Jian, S.-W. and Haynes, P.: The dynamics of low-latitude sub-surface oceanic jets, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17638, https://doi.org/10.5194/egusphere-egu24-17638, 2024.

09:55–10:05
|
EGU24-13002
|
NP6.2
|
On-site presentation
Jordi Isern-Fontanet, Antonio Turiel, Cristina González-Haro, and Viktor G. Gea

The upper ocean is crowded with fronts of different intensities and extensions,  which are known to play a major role in the dynamics of the oceanic upper layers and contribute to set some properties such as the spectral slopes of Sea Surface Temperatures (SST), among others. Here, we show that the upper layers of the ocean can be modeled by the multifractal theory of turbulence and, then, we use this theory to predict the link between the most intense fronts and the scaling properties of the structure functions. This prediction is, finally, verified with a wide range of observations and numerical simulations.

In particular, we show that the behavior of thermal gradients at small enough scales can be characterized by singularity exponents. Then, we use the singularity exponents of thermal gradients as a measure of the intensity of thermal fronts; and the fractal dimension of the set of points with the same singularity exponent, known as the singularity spectrum, as a measure of their extension. This allows us to connect fronts with the structure functions of temperature using the multifractal formalism. Assuming that the turbulent cascade can be modeled with the log-Poisson model, we analytically shown that the anomalous scaling of the structure functions is a function of the intensity of the strongest front, i.e. the smallest singularity exponent. This prediction is verified using the SST provided by numerical simulations of an upwelling system; simulations of the global ocean; and satellite observations. Moreover, we show that the predicted relationship is also valid for other variables such as velocities.

Our results not only provide insight on the functioning of the upper ocean, but also provide a guide to develop and adjust numerical models. Indeed, our results imply that numerical models have to correctly model, or parametrize, those processes generating the most intense fronts, in order to properly reproduce some of the statistics of ocean temperatures.

 

How to cite: Isern-Fontanet, J., Turiel, A., González-Haro, C., and Gea, V. G.: On the contribution of ocean fronts to the anomalous scaling of the structure functions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13002, https://doi.org/10.5194/egusphere-egu24-13002, 2024.

10:05–10:15
|
EGU24-14146
|
NP6.2
|
On-site presentation
Yulong Zhao, Yanwei Zhang, Pengfei Ma, Xiaodong Zhang, and Zhifei Liu

Subaqueous sand waves are widely observed in the world′s oceans, but few in-situ observations have been performed to understand the dynamic processes of how they form and migrate. Large-amplitude subaqueous sand waves, 1.5 to 20 m in height and 55 to 510 m in length, are found on upper continental slope of the northern South China Sea, at water depths of ~ 150 to 800 m. Herein, high-resolution tripod observations were conducted in this sandwave field to understand the dynamic mechanism how sandy sediments are remobilized by oceanic dynamic processes, in particular internal solitary waves and internal tides. Our results indicate that near-critical reflection of diurnal internal tides at the continental slope can cause high suspended sediment concentration within the bottom water at a narrow belt further upslope to the observation site downslope locations. These sediments are transported downslope by the ebb tides to the observation site, forming the daily-recurring high suspended sediment concentration. The passage of episodic extreme internal solitary waves can result in much denser high sediment clouds with a thickness of up to ~ 40–50 m above the seafloor. These high suspended sediment concentration events are caused by in-situ resuspension of sediments from the seabed and upward transport of these sediments out of the boundary layer in response to passing of internal solitary waves. The two sub-processes of sediment resuspension are regulated by distinct dynamic mechanisms: incipient sediment resuspension from seafloor is controlled by current-induced strong bed shear stress, while the upward transport of sediments out of the bottom boundary layer is driven by the upwelling convergent currents at the rear of the internal solitary waves. Such results provide new insights into understanding the dynamic mechanism of the so-called ‘resuspension’ process in marine sedimentology. Our results also highlight the importance of internal solitary waves and internal tides in modulating sediment remobilization over subaqueous sandwave fields.

How to cite: Zhao, Y., Zhang, Y., Ma, P., Zhang, X., and Liu, Z.: Sediment remobilization over subaqueous sand waves: Insights from in-situ observation in the northern South China Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14146, https://doi.org/10.5194/egusphere-egu24-14146, 2024.

Coffee break
Chairpersons: Yvan Dossmann, Chris Johnson
10:45–10:50
Particle Laden Gravity Currents
10:50–11:00
|
EGU24-1427
|
NP6.2
|
ECS
|
On-site presentation
Mostafa Shehata, Marie Rastello, Florence Naaim, and Herve Bellot

Numerous previous studies have been done to understand the physics of gravity currents. Some considered the propagation over smooth or rough horizontal surfaces (Tokyay et al. (2014), Zhou et al. (2017)), and others studied inclined surfaces without any roughness. What about the more complex situation of inclined rough surfaces? We have studied, experimentally, how a finite volume of heavy fluid (salted water with a clay suspension) rushing down a slope (20° & 30°) is affected by the multiple obstacles it counters on its way. Our setup is a classic volume release configuration in a 2D flume immersed in a 20 m3 water tank at INRAE Grenoble. The study tests (352 experiments per tilt) different initial conditions of the released flow (volume and density) and various surface conditions from smooth to rough:

  • For the initial conditions: the experiments show that the front velocity is monotonically related to its initial volume and density. Although the initial mass of the flow is the product of its density times volume, the mass effect on the flow front velocity cannot come in replacement of both volume and density effects, as it was found that the front velocity is non-monotonically related to its initial mass.
  • For the surface conditions: besides testing the smooth case, we have covered a wide range of roughness configurations using obstacles with different shapes, heights, and spacings. Walls or barriers blocking the whole width of the flume have been used (see Fig.1-a). Testing various heights and spacings shows that higher barriers decrease the flow front velocity, while non-monotonic relations were found when the spacing between successive barriers in the flow direction is changed. Flow propagation over and through an array of obstacles has also been studied with various obstacles arrangement (in-line and staggered) and different obstacles' cross-sections (rectangular and circular). For circular obstacles, the (x𝑓-t) curve is no longer smooth but takes the shape of stairsteps, and they are found to be more efficient in decelerating the flow (see Fig.1-b).

Studying both 20° and 30°-flume tilts enables us to look through the slope effect. The analysis shows that, in general, increasing the slope results in higher front velocity values. Nevertheless, the degree of influence is dependent on diverse factors (volume, density, bed surface conditions). In addition, we have studied the effect of the initial flow parameters on the flow height just after the lock release (at an accurate predetermined distance from the lock chosen based on 252 experiments). This height depends only on the initial volume and density effect is negligible. Determination of this height is essential for our non-dimensional analysis: to study the temporal evolution of the non-dimensional front position (𝑥𝑓−𝑥o)/𝑥o versus the non-dimensional time (t𝑓/to). Indeed, it will enable us to avoid using the initial flow depth at the lock that is highly dependent on the inclination angle, or an estimated virtual height after the lock that would be less representative (see Fig.1-c).

Fig. 1: 

 

 

How to cite: Shehata, M., Rastello, M., Naaim, F., and Bellot, H.: Experimental study of gravity current propagation over rough tilted surfaces, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1427, https://doi.org/10.5194/egusphere-egu24-1427, 2024.

11:00–11:10
|
EGU24-16782
|
NP6.2
|
ECS
|
On-site presentation
|
Manohar Kumar Sharma, Cyrille Bonamy, Marie Rastello, Cyril Gadal, Laurent Lacaze, and Julien Chauchat

Particle-laden gravity currents are extremely important in geophysical flow applications. They are the major pathway of sediments in subaqueous environments such as deep lakes and oceans and, to some extent, in the shallower seas of the continental shelves [1]. Various approaches have been developed to model these currents with different complexities ranging from box models, shallow water models, single-phase and two-phase flow models. The starting point of the present analysis is based on Gadal et al. [2] who investigated the role of non-dimensional parameters such as the bed slope (α), the Reynolds number (Re), the Stokes number (St), and the volume fraction (φ) on the dynamics of the front velocity at the early stage of the current propagation using both experimental and numerical approaches. The front velocity at short time scales as the square root of the reduced gravity times the initial lock-height. Overall, it is an increasing function of the bed slope and a decreasing function of the initial volume fraction (for φ>0.45). It is also shown that the duration of the initial constant velocity regime decreases with the particle settling velocity or Stokes number at small bed slope angles. The 2D two-fluid simulations performed with sedFOAM [3] have been shown to reproduce almost quantitatively these trends however a comprehensive description of the detailed underlying physical mechanisms is still missing. In this contribution, we propose to use the two-fluid model to address this question. To achieve this goal, 3D two-fluid simulations have been performed and the numerical results have been depth-averaged over the current shape. The mass balance is used to quantify the entrainment at the current interface and the various terms entering in the momentum balance are extracted from the simulation results. These analysis are used to understand the origin of the current dynamics attenuation such as fluid viscous and turbulent stresses, particle-particle interactions, and fluid-particle interactions.

References:

[1] Meiburg, E. and Kneller, B. (2010). Turbidity currents and their deposits. Annual Review of Fluid Mechanics, 42(1):135–156.

[2] Gadal C., Mercier M. J., Rastello M., and Lacaze L. (2023). “Slumping regime in lock-release turbidity currents”,. J. Fluid Mech., 974:A4,

[3] Chauchat, J., Cheng, Z., Nagel, T., Bonamy, C., and Hsu, T.-J. (2017). Sedfoam- 2.0: a 3-d two-phase flow numerical model for sediment transport.    Geoscientific Model Development, 10(12):4367–4392.

How to cite: Sharma, M. K., Bonamy, C., Rastello, M., Gadal, C., Lacaze, L., and Chauchat, J.: Investigating the dynamics of particle-laden gravity currents using two-fluid simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16782, https://doi.org/10.5194/egusphere-egu24-16782, 2024.

11:10–11:20
|
EGU24-16920
|
NP6.2
|
On-site presentation
|
Matthieu Mercier, Jean Schneider, Cyril Gadal, and Laurent Lacaze

We investigate experimentally the dynamics of particle-laden gravity currents originating from a continuous influx in a nearly steady regime. We seek to characterize the influence of suspended particles on the inner structure of the current, depending on their settling/rising properties.

The compositional fluid for the current is obtained by adding salt (NaCl) to the ambient fluid of the density ρ0, the density difference of the current with the ambient fluid is noted Δρ0. In the case of particle-laden gravity currents, we add plastic particles (diameter 500 µm and density 1050 kg/m3) in a semi-dilute concentration (Φ ~ 1%). We can also change the settling/rising properties of the particles by choosing the density of the fluid in the current, the density ratio varying between R=0.98 (rising particles) and R=1.02 (settling particles). Once prepared, the compositional fluid, stored and continuously stirred in a large reservoir, is injected in a nearly two-dimensional long tank by a pump at fixed flow rate Q, long durations of injection are possible by using either a very long tank, or an open channel configuration. Two setups have been used, associated to turbulent regimes, with typical Reynolds number of the order of 103 to 104 respectively. Within the body of the current, the density/concentration profiles are estimated by light attenuation technique [1].

Our results show that the front velocity Uf is well controlled by a characteristic velocity based on the buoyancy of the current and the flow rate per unit width, as shown in Figure 1(a). Depending on the height of the injection inlet h0, the current can exhibit different hydraulic features near the front, depending on the value of the Froude number at the inlet F0, defined as the ratio of Uf over (Δρ00 . g . h0)1/2, with g the gravity constant, being supercritical (F0>1) or subcritical (F0<1). For the body of the current, as shown in Figure 1(b) with extractions made along a vertical profile 1.8m after the inlet, the mean concentration profiles are very different for slightly floating (R=0.98) or settling (R=1.02) particles from the case of neutrally buoyant particles (R=1.0). They all differ from the density profile of saline gravity currents (no particles case). Implications for the transport of particles and mixing processes within and at the interface of the current will be discussed.

Figure 1: (a) Front velocity vs. injection properties. (b) Mean concentration profiles with depth (rescaled by the mean current depth <hc>), extracted at 1.8m from the inlet for rising (red)-neutral (magenta)-settling (blue) particles. Density profile for a density current is indicated with a dashed (black) line. Shaded areas indicate fluctuations around the mean profiles.
 

References
[1] Schneider et al. Investigation of particle laden gravity currents using the light attenuation technique. Exp. Fluids 64, 23 (2023).

How to cite: Mercier, M., Schneider, J., Gadal, C., and Lacaze, L.: Influence of particle's buoyancy on turbidity currents from continuous influx, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16920, https://doi.org/10.5194/egusphere-egu24-16920, 2024.

11:20–11:30
|
EGU24-17856
|
NP6.2
|
ECS
|
On-site presentation
Stan Thorez, Ulrich Lemmin, D. Andrew Barry, and Koen Blanckaert

Hyperpycnal river inflows discharging into lakes or reservoirs will plunge and trigger gravity-driven underflows near the bed. Such underflows are called turbidity currents if the density excess is mainly caused by a high sediment concentration. These underflows can reach the bottom of the lake or, alternatively, detach from the bed and intrude horizontally into the lake waters to form an interflow if a layer of equal density is encountered. As underflows carry a number of constituents fed to them by the river or eroded from the bed, such as sediment, contaminants, nutrients and oxygen, their pathway and final destination have an impact on lake or reservoir water quality. The mixing processes in the plunging region are known to be of dominant importance for the dilution of the inflowing river water by entrainment of ambient water, thereby exerting a primary control on the final intrusion depth of underflows. Understanding and quantifying these processes is therefore key. Until now, the majority of estimations of the mixing extent in the plunging region were made within laterally confined laboratory experiments or via passive tracer methodologies. This study focuses on quantifying plunging mixing from flow velocity measurements in a laterally unconfined river inflow in the field and investigating its dependency on inflow conditions.

Field measurements of the plunging Rhône River entering Lake Geneva were conducted using a boat-towed ADCP along a grid of transects for six inflow conditions characterized by the inflow densimetric Froude number Frd. The plunging mixing coefficient Ep, which compares the underflow discharge immediately post-plunging to the initial river inflow discharge, was used to quantify the plunging mixing.

Results indicate that for larger Frd values (Frd > 4) Ep estimates align with laterally confined lab experiments (Ep = O(0.1)). Conversely, for smaller Frd values Ep estimates correlate with field tracer measurements of laterally unconfined inflows (Ep = O(1)). Ep decreases with increasing Frd, challenging existing numerical simulations predicting the opposite relationship.

This study offers key insights into turbulent mixing rates associated with hyperpycnal river inflows and highlights the need to incorporate realistic field conditions for accurate modeling of plunging river inflows and intrusion depth.

How to cite: Thorez, S., Lemmin, U., Barry, D. A., and Blanckaert, K.: Quantifying Turbulent Mixing in Plunging River Inflows: Insights from Field Measurements in Lake Geneva, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17856, https://doi.org/10.5194/egusphere-egu24-17856, 2024.

Gravity currents: Effects of topography, stratification and rotation
11:30–11:40
|
EGU24-22329
|
NP6.2
|
On-site presentation
Maarten van Reeuwijk, Lianzheng Cui, and Graham Hughes

We explore the dynamics of inclined temporal gravity currents using direct numerical simulation, and find that the current creates an environment in which the flux Richardson number, gradient Richardson number and turbulent flux coefficient are constant across a large portion of the depth of the outer layer. Changing the slope angle modifies these mixing parameters, and the flow approaches a maximum Richardson number of approx. 0.15 as the angle tends to zero, for which the entrainment coefficient E->0.

The turbulent Prandtl number remains O(1) for all slope angles, demonstrating that E->0 is not caused by a switch-off of the turbulent buoyancy flux. Instead, E->0 occurs as the result of the turbulence intensity going to zero as the angle tends to zero, due to the flow requiring larger and larger shear to maintain the same level of turbulence. We develop a conceptual model which is in excellent agreement with the DNS data.

How to cite: van Reeuwijk, M., Cui, L., and Hughes, G.: Mixing and entrainment in inclined gravity currents, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22329, https://doi.org/10.5194/egusphere-egu24-22329, 2024.

11:40–11:50
|
EGU24-10029
|
NP6.2
|
ECS
|
On-site presentation
Mohamad Harrouk, Rabah Mehaddi, Boris Arcen, and Yvan Dossmann

The present work investigates the dynamical characteristics of a gravity current produced by a continuous and steady release of a dense fluid, with a source density ρ0, into a lighter ambient fluid, with density ρa < ρ0. A planar geometry is considered in which the ambient fluid is initially at rest inside a rectangular tank and the dense current is continuously released from a source located at the bottom-left corner of the tank.  The released current propagates along the horizontal bottom boundary of the domain displacing the ambient fluid. This configuration has been considered experimentally by Sher & Woods (2017). A numerical study has been carried out using the Direct Numerical Simulations (DNS) of the governing equations via the NEK5000 solver. Instantaneous three-dimensional velocity, pressure and density fields were extracted and two-dimensionalized by width-averaging. The state of the release is characterized by the source Froude number Fr0 = u0 / √(g'0 h0 ) with u0 being the velocity of the release, h0 being the height at the inlet, and g'0 = g (ρ0 - ρa)/ρa being the source buoyancy. Throughout the series of simulations, we control the state of the current at the source by only varying the source density ρ0, resulting in a range of source Froude number between 0.6 < Fr0 < 2.7, and we seek to record the effects of this variation on the dynamics. The source discharge Q0 = u0 h0 and buoyancy flux B0 = Qg'0 are kept constant over time. The front speed, uf, was shown to remain steady; a well-known feature of continuous gravity currents. A dimensionless parameter, λ = uf/B01/3, that characterizes the front speed was computed as a function of Fr0 and the result shows a good agreement with the range recorded by Sher & Woods (2017). The entrainment of ambient fluid into the current is parametrized with two methods. First, we estimate the rate of change of the volume of the current, dV/dt, and we recorded the range 1.8Q0 < dV/dt < 2.1Q0 for the selected Fr0  range. Secondly, the theory of inclined plumes introduced by Ellison & Turner (1959) was considered to estimate a local entrainment parameter, E, as a function of the local stratification represented by the local Richardson number Ri. The well-known relation, E proportional to Ri-1, was held when Ri < 0.8; otherwise, the entrainment parameter tends to near-zero values.

How to cite: Harrouk, M., Mehaddi, R., Arcen, B., and Dossmann, Y.: The characterization of the dynamics of a continuous gravity current, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10029, https://doi.org/10.5194/egusphere-egu24-10029, 2024.

11:50–12:00
|
EGU24-6404
|
NP6.2
|
ECS
|
Highlight
|
On-site presentation
Adrien Lefauve, Miles Couchman, and Paul Linden

We present a relatively recent laboratory experiment, the Stratified Inclined Duct (SID), that sustains a buoyancy-driven exchange flow and allows to accurately control and measure stratified sheared turbulence. The submesoscale turbulent mixing of momentum, heat, and salinity in the ocean have a leading-order but poorly constrained large-scale impact. We argue that SID may serve as a fruitful testbed for studying these processes, especially near boundaries, such as in estuaries.

First, we introduce SID, which consists of two large (400 litres) reservoirs containing salt solutions connected by a long rectangular duct. The long-lasting  exchange within the duct has a Reynolds number of order Re ~ 1,000-10,000. The apparatus can be tilted at a small angle θ with respect to the horizontal, which energises the flow and increases turbulence levels, due to the emergence of 'hydraulic control'.

Second, we present high-resolution experimental measurements of the three-dimensional velocity and density field within the duct which allow to delve into the energetics of stratified turbulence. SID uniquely allows the experimenter to control the level of turbulent kinetic energy dissipation, and to sweep through increasingly turbulent regimes by varying the key product Re*θ. The levels of turbulent intensity are comparable to those found in moderately turbulent patches in the ocean.

Third, we demonstrate that a data-driven analysis combining automated image analysis, data reduction and unsupervised clustering discovered previously unsuspected patterns in a large SID turbulence dataset. Multiple types of energetic turbulence were found, as well as intermittent turbulence that cycles between these types through distinct transition pathways. We argue that this data-driven identification of turbulence is a stepping stone towards better physics-based parameterizations.

How to cite: Lefauve, A., Couchman, M., and Linden, P.: The Stratified Inclined Duct: a new canonical laboratory experiment to study ocean turbulence and mixing, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6404, https://doi.org/10.5194/egusphere-egu24-6404, 2024.

12:00–12:10
|
EGU24-5897
|
NP6.2
|
ECS
|
On-site presentation
Tassigny Axel, Negretti Maria Eletta, and Wirth Achim

Gravity currents play a crucial role in the formation of deep waters in the ocean, contributing to the vorticity and energy transfers towards the ocean interior. We present results from an experimental study on downslope intruding and rotating gravity currents into an initially two-layer stably stratified ambient at high buoyancy Reynolds numbers. A new turbulent process of downslope transport, intermittent and localized, is identified, taking the form of cascades. The lifetime of cascades presents a power law relationship, and the related transport does not exhibit any characteristic length scale, suggesting self-organized criticality. Cascades reveal to be the main contributor to the vorticity and turbulence in the ocean interior, with a dependence on the Coriolis parameter and the density anomaly to the surrounding ambient. Vorticity is produced both by the spreading of the cascade into the interior, and by the meandering and the break up of the deep boundary current (formed from downward Ekman transport). When the intrusion spreads at the pycnocline only, anticyclonic eddies are formed in the intrusion and top layers, whereas for intrusions spreading through the full bottom layer, vortices of both signs are generated due to bottom friction. The turbulence in the receiving ambient reveals to be horizontally isotropic, non-stationary and non-homogeneous. In the intrusion area close to the slope, the turbulence is forced by energy injection at the penetration length scale through the cascades. The central area far from the boundaries is characterized, instead, by freely evolving two-dimensional turbulence, forced at large scales.

 

How to cite: Axel, T., Maria Eletta, N., and Achim, W.: On the turbulent structures generated by intruding downslope rotating gravity currents, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5897, https://doi.org/10.5194/egusphere-egu24-5897, 2024.

12:10–12:20
|
EGU24-20862
|
NP6.2
|
Highlight
|
On-site presentation
Kiran Bhaganaagar

 Buoyancy-generated turbulence processes with substantial density variations within the flow are critical in oceanic systems. The talk highlights a universal framework to characterize density currents and turbulent plumes released from a source. Variable density effects in density currents and plumes alters the velocity field due to variation in density. Consequently the pathways for turbulence generation, mixing and transport are complicated than a passive scalar mixing. A large-eddy-simulation is used to simulate density currents and plumes released from a source.  In this talk, Favre-averaged turbulence transport equations for generalized compressible flows are presented within the context of density currents and plumes. A simplified conceptual turbulent model is presented.The study is focused on addressing a pivotal question: whether the kinetic energy spectrum follows the traditional inertial -$5/3$ spectrum, or displays a -$3$ buoyancy regime, or demonstrates a combination of both phenomena. Significant deviations from the inertial spectra are observed. New insights on the spectral transfer of energy, and turbulence mechanims for buoyancy-driven flows are presented

How to cite: Bhaganaagar, K.: Density-driven flows: Plumes and Density Currents, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20862, https://doi.org/10.5194/egusphere-egu24-20862, 2024.

12:20–12:30
|
EGU24-17941
|
NP6.2
|
ECS
|
On-site presentation
Yueh Shing Shen and Hervé Capart

Sediment deposition has significantly impacted the storage capacity of reservoirs in Taiwan. Delta progradation may be influenced by factors such as creep landslide, current-driven bedload, or suspended load. To better understand hyperpycnal mechanisms in reservoirs, small-scale experimental simulations were conducted in the laboratory. In order to observe and model more on-site phenomena, a series of experiments were performed, involving the fabrication of a 2-D channel using acrylic material. A specific type of sand was used to ensure that it neither settles too quickly nor remains continuously suspended in the water. Fortunately, the initial goal of generating a traveling wave in the laboratory using a moving source was successfully achieved. Velocity flow field and concentration distribution analyses were conducted using a high-speed camera with dense laser and the Spectrophotometric Method, respectively. Experimental results were compared with computational simulations based on the Shallow Water Theory. The conventional theory of two-layer flow proved insufficient to adequately describe the experimental results. To address this, an additional layer theory called the sub-layer was introduced into our numerical modeling. Ultimately, by incorporating shallow flow equations with settling and mixing, three long interfaces were simulated and compared favorably with the experimental results.

How to cite: Shen, Y. S. and Capart, H.: Hyperpycnal Delta Progradation in a Lake of Constant Depth: Mathematical Modeling and Laboratory Measurements of Morphology, Suspended Sediment Concentration, and Velocity Field, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17941, https://doi.org/10.5194/egusphere-egu24-17941, 2024.

Posters on site: Tue, 16 Apr, 16:15–18:00 | Hall X4

Display time: Tue, 16 Apr 14:00–Tue, 16 Apr 18:00
Chairpersons: Kevin Lamb, Yvan Dossmann
X4.98
|
EGU24-1871
|
NP6.2
Kateryna Terletska, Vladimir Maderich, and Elena Tobisch

Propagation of internal solitary waves (ISW) under the edge of the ice cover may lead to their
destabilization through overturning and breaking events. Factors such as ice cover depth, ridging
intensity, and internal wave amplitudes play crucial roles in the evolution and disintegration of ISW
beneath the ice cover. In the study, a numerical investigation of the transformation of ISW
propagating from open water in the stratified sea under ridged ice cover is carried out. A
nonhydrostatic numerical model, that is based on the Reynolds averaged Navier-Stokes equations in
the Boussinesq approximation for a continuously stratified fluid, was used in the investigation. The
study focused on an idealized scenario with a vertical distribution of potential density anomalies
designed to replicate the summer profile of potential density observed over the Yermak Plateau in
the Arctic Ocean. In the numerical experiments, number of ice keels were placed beneath a
uniform-thickness ice layer. The ice keel shape was approximated by the Versoria function. It is
carried out calculations with a different ridging intensity, that is, the ratio of the maximum height of
the keel to the distance between the keels. In present calculations, it varies from 1/1000 for
moderately ridged ice to 1/20 for heavily ridged ice, which is broadly consistent with the ocean
values. The transformation of ISW of depression is additionally governed by the blocking
parameter β for a single keel, which is the ratio of the height of the minimum thickness of the upper
layer under the ice keel to the incident wave amplitude. An important characteristic of the ISW-
ridged ice interaction is the loss of kinetic and available potential energy during the ISW
transformation. Energy transformation due to mixing leads to the transition of energy to background
potential energy and energy dissipation. To characterize the dependence of energy loss on keel
height and distance between keels, we introduced the parameter, which is the ratio of the sum of
submerged ice thickness and maximal keel penetration to the distance between keels. An energy
loss was estimated based on a budget of depth-integrated pseudoenergy before and after the wave
transformation. The results revealed that the energy loss increases with a decrease in distance
between keels or an increase in keel height. The level of energy loss is highest for β values near
zero. For values β greater than 0.8, interaction is moderate or weak, and distance between the keels
no longer affects energy loss.

How to cite: Terletska, K., Maderich, V., and Tobisch, E.: Internal solitary wave energy transformations under ridged ice cover, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1871, https://doi.org/10.5194/egusphere-egu24-1871, 2024.

X4.99
|
EGU24-2296
|
NP6.2
Keisuke Nakayama and Kevin Lamb

In a three-layer system with equal upper and lower thin layers with the same density jump across each interface, fully nonlinear governing equations have revealed that breathers exist under the Boussinesq approximation. Also, it has been demonstrated that breathers may occur in the Baltic Sea. Additionally, in previous studies, it has been shown that the larger the upper- and lower-layer thicknesses, the more the breathers behave like solitons and that phase shifts occur after two breathers interact, with a forward/backward shift of the faster/slower breather, while the properties of the breathers are preserved. Still, the oblique interaction of breathers has yet to be explored. Thus, we aimed to investigate oblique breather interactions in a three-layer system by using fully nonlinear numerical simulations to clarify the characteristics of breathers. The ratio of the thin layer thicknesses to the total depth was 0.25 in this study. Breathers have two significant parameters, p and q, corresponding to the wavelength of a breather and the envelope amplitude. So, we had several configurations to clarify the influence of incident angles and amplitudes on the breather interactions by changing the parameters p and q. Stably progressing breathers, where p and q are 0.025 and 0.006, were examined by changing the incident angles from 10 to 40 degrees to estimate a critical angle. Also, the oblique breather interactions with a larger envelope amplitude were simulated to analyse the amplitude dependence of the critical angle. A Mach stem was found to occur in oblique breather interactions. Also, the critical angle was revealed to decrease as the envelope amplitude decreases. The behaviour of obliquely-interacting breathers provides further evidence that breathers in a three-layer fluid have soliton-like characteristics.

How to cite: Nakayama, K. and Lamb, K.: Obliquely interacting breathers in a three-layer fluid, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2296, https://doi.org/10.5194/egusphere-egu24-2296, 2024.

X4.100
|
EGU24-4662
|
NP6.2
Kevin Lamb

In certain stratifications internal wave breathers can exist. Such stratifications include symmetric double-pycnocline continuous stratifications provided the two pycnoclines are sufficiently far apart. Here the generation of breathers by steady flow over a small obstacle is investigated using a two-dimensional non-hydrostatic primitive equation numerical model under the Boussinesq approximation. The symmetric stratification consists of two thin hyperbolic tangent pycnoclines which separate the fluid into three nearly constant density layers. The two pycnoclines have the same thickness and the same density jump across them. The upper and lower constant density layers have the same thickness. Nonlinear internal waves in this type of stratification can be modelled with the modified KdV (mKdV) equation. If the pycnoclines are far enough apart the cubic coefficient in the mKdV equation is positive and weakly-nonlinear theory predicts the existence of breathers. In this numerical study a large number of simulations have been undertaken, varying the speed of the upstream flow, the amplitude and width of the obstacle and the thickness of the upper and lower layers. Multiple upstream propagating internal wave breathers are generated in a small region of parameter space. Their formation appears to arise from nonlinear-interactions in the lee wave field downstream of the obstacle. 

How to cite: Lamb, K.: Generation of internal wave breathers by steady flow over an obstacle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4662, https://doi.org/10.5194/egusphere-egu24-4662, 2024.

X4.101
|
EGU24-14658
|
NP6.2
|
ECS
Gauthier Rousseau, Nathan Pellerin, Benoît Fond, and Koen Blanckaert

Recent advancements in fluid flow imaging techniques have made it possible to visualize local temperature in flows by observing the response of photoluminescent dye or particles to light excitation. This has sparked increased interest in exploring laboratory-scale density currents induced by temperature differences. However, unlike the commonly investigated saltwater-freshwater or turbidity currents, heat transfer through boundaries can occur, potentially influencing the dynamics of the density current.

In this study, we utilize the dependence of the luminescence persistence time following pulse excitation on ambient fluid temperature of micrometric phosphor particles (YAG:Cr) to spatially and temporally resolve gravity currents produced by a lock-exchange flow. Notably, we introduce a novel inexpensive approach, which combine the use of LEDs and inexpensive high resolution CMOS sensors operated in a multi gate accumulation mode to extract temperature information with high spatial resolution. This simple method holds promise as it significantly enhances the accessibility of high resolution temperature imaging techniques for experimentalists. It can be applied to various thermal fluid experiments, to study for example thermal convection in fluid bodies.

How to cite: Rousseau, G., Pellerin, N., Fond, B., and Blanckaert, K.: Temperature imaging of density currents using phosphor micrometric particles, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14658, https://doi.org/10.5194/egusphere-egu24-14658, 2024.

X4.102
|
EGU24-9933
|
NP6.2
|
Highlight
Chris Johnson, Frank Millward, and Helen Webster

The plume of ash and gas released by large explosive volcanic eruptions rises to its neutral buoyancy level in the atmosphere, then spreads laterally to form an umbrella cloud. Density stratification of the atmosphere generates buoyancy forces in the cloud, which drive the outward spread as an intrusion. Although umbrella clouds are often modelled as circular axisymmetric structures, in practice they are usually influenced quite strongly by the meteorological wind, with spread in the upwind direction halted by the oncoming wind, and different rates of spreading in the downwind and crosswind directions. Here, we present a physically based shallow-layer intrusion model for wind-blown volcanic umbrella clouds, and derive a simple parametrization of non-axisymmetric umbrella cloud spreading based on this shallow-layer model. The simplified parametrization is quick to evaluate and so is suitable for use in operational Volcanic Ash Transport and Dispersion Models (VATDMs) that are used to predict ash hazard operationally. In contrast to previous parametrizations, in which there is assumed to be no interaction between a circular umbrella cloud and the meteorological wind, here the umbrella cloud is influenced by the wind and adopts a shape determined by the balance of buoyant spreading and downwind drag forces. We test our scheme within the UK Met Office 'NAME' dispersion model, and apply it to four diverse case studies of eruptions at Puyehue 2011, Pinatubo 1991, Ulawun 2019, and Calbuco 2015. We demonstrate that buoyant spreading is important even in plumes that are highly wind-blown, and obtain better descriptions of cloud spread and ash distribution than existing parametrizations based on an axisymmetric umbrella cloud dynamics.

How to cite: Johnson, C., Millward, F., and Webster, H.: An operational model for wind-blown volcanic umbrella clouds, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9933, https://doi.org/10.5194/egusphere-egu24-9933, 2024.

X4.103
|
EGU24-6447
|
NP6.2
Marek Stastna and Nico Castro-Folker

The simulation of instability and transport in the bottom boundary layer by internal solitary waves has been documented for over twenty years.  However, the challenge of shallow slopes and a disparity of scales between the large scale wave and the small scale boundary layer has proven challenging for simulations.  We present laboratory scale simulations that resolve the three-dimensionalisation in the boundary layer during the entire shoaling process.  We find that the late stage, in which the incoming wave fissions into boluses, provides the most consistent source of three-dimensionalisation.  In the early stage of shoaling, three-dimensionalisation occurs not so much due to separation bubble instability, but to the interaction of vortices shed from the separation bubble with the overlying pycnocline.

How to cite: Stastna, M. and Castro-Folker, N.: Simulations of the three-dimensional structure of instabilities beneath shoaling internal waves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6447, https://doi.org/10.5194/egusphere-egu24-6447, 2024.

X4.104
|
EGU24-15689
|
NP6.2
Laurent Lacaze, Cyril Gadal, Jean Schneider, Cyrille Bonamy, Julien Chauchat, Yvan Dossmann, Sébastien Kiesgen de Richter, Matthieu Mercier, Florence Naaim-Bouvet, and Marie Rastello

Particle-laden gravity currents (PLGCs) are driven by the mass difference between a heavy fluid-particle mixture and a lighter ambient liquid. They often occur in natural and industrial situations, among which a typical situation is the release of a finite volume. Here, we focus on such `dam-break' situations, which are studied using lock-release devices at the laboratory scale. The objective of the presententation is to provide a description at the macroscopic scale of the early moments of the flow, namely the slumping regime, with respect to the relevant dimensionless parameters. For this, we combine a total of 288 runs from three different lock-release devices and from two-fluids numerical simulations, which allow us to cover a large range of particle types (size and density), volume fractions, bottom slopes and geometries. By tracking the front propagation through time, we extract the dimensionless slumping velocity Fr and dimensionless characteristic slumping duration τ, on which we base our description. Our results show that the slumping velocity increases with the bottom slope, but decreases with the particle volume fraction when the latter exceeds a critical value. However, it remains independent of particle settling processes, which on the other hand affects the slumping duration. Hence, above a critical threshold, τ decreases as the ratio between the settling velocity and characteristic current velocity increases. For all these regimes, we derive scalings and energetic balances that reproduce the observed trends. The latter comparison confirms the role of initial energy transfer from the initial state towards the slumping phase on the resulting dynamics. This initial process and its characterisation remain crucial to prescribe relevant initial conditions for large-scale predictive modelling.

How to cite: Lacaze, L., Gadal, C., Schneider, J., Bonamy, C., Chauchat, J., Dossmann, Y., Kiesgen de Richter, S., Mercier, M., Naaim-Bouvet, F., and Rastello, M.: Particle-laden gravity currents: the lock-release slumping regime at the laboratory scale, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15689, https://doi.org/10.5194/egusphere-egu24-15689, 2024.

X4.105
|
EGU24-5954
|
NP6.2
|
Highlight
Maria Eletta Negretti, Axel Tassigny, and Louis Gostiaux

Gravity currents are one of the key sub-mesoscale processes that drive energy transfer, impact the thermohaline structure and the vertical exchange of water masses in the ocean. At present, their representation remains difficult in numerical models. The targeted study area is the Strait of Gibraltar between the Mediterranean and the Atlantic Ocean. We present preliminary results of experiments obtained using the first realistic implementation of the Strait of Gibraltar with the adjacent Gulf of Cadiz and Alboran Sea, including all main forcings: the density difference, the barotropic tide, the Earth’s rotation and the realistic topography, scaled using available in-situ data. Detailed measurements of the velocity and density fields reveal that the large-scale circulation and the further faith of the Mediterranean waters flowing into the Atlantic Ocean are strongly influenced by the turbulent processes at small scale that take place in the main control areas, i.e. the Camarinal and Espartell sills. Two-dimensional velocity and density fields in these key regions and in several locations in the Gulf of Cadiz and Alboran Sea are obtained and turbulent fluxes and mixing are estimated. Finally, internal solitary waves are observed, possibly degenerating in a train of internal waves, generated by the tide in interaction with the topography at Camarinal sill and propagating toward the Alboran Sea. These results are analyzed to assess the impact of the parameter variation (barotropic and baroclinic forcings).

How to cite: Negretti, M. E., Tassigny, A., and Gostiaux, L.: A physical model of the Gibraltar Strait: the HERCULES experiment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5954, https://doi.org/10.5194/egusphere-egu24-5954, 2024.

X4.106
|
EGU24-6081
|
NP6.2
|
ECS
|
Nicolas Castro-Folker, Andrew P. Grace, and Marek Stastna

In a single-component system density depends only on temperature. If this dependence is linear, then we can expect a pair of floating and sinking gravity currents with i) the same absolute initial density difference between intruding and ambient fluids, and ii) initial conditions that (under a certain non-dimensionalisation) obey reflectional symmetry across mid-depth to maintain that symmetry throughout their evolution. However, water attains its maximum density at approximately 4°C, so the density of a cold (<10°C), freshwater system has an effectively quadratic, and therefore nonlinear, dependence on temperature. Work in two dimensions shows that the profile, speed, and shear instabilities of initially reflectional-symmetric currents evolve asymmetrically under the influence of a nonlinear equation of state. We extend this work to three-dimensional systems with no-slip boundary conditions. This allows us to also consider the lobe-cleft instability: an inherently three-dimensional instability that produces dynamic patterns of folds and protrusions along the front of gravity currents. In this talk we will discuss how the lobe-cleft instability is modulated by the nonlinear equation of state. We will also discuss how the lobe-cleft instability three-dimensionalises the billows produced by the shear instability along the top/bottom of sinking/floating currents, and how this, too, is affected by the nonlinear dependence on temperature.

How to cite: Castro-Folker, N., Grace, A. P., and Stastna, M.: The asymmetric evolution of three-dimensional gravity currents in cold, fresh water, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6081, https://doi.org/10.5194/egusphere-egu24-6081, 2024.

X4.107
|
EGU24-6066
|
NP6.2
Claudia Adduce, Maria Rita Maggi, and Giovanni Di Lollo

The hydrostatic imbalance between two adjacent fluids, driven by density variations associated with temperature, salinity, or sediment concentration gradients, often initiates the formation of gravity currents. These phenomena play a crucial role in various geophysical and engineering applications, influencing atmospheric, terrestrial, and subaqueous environments. In recent years, there has been growing research interest in understanding the interaction of gravity currents with obstacles on the seafloor. These obstacles can be artificial structures like pipelines and gas pipelines situated in the oceanic environment. Therefore, it is essential to investigate the dynamics of gravity currents over complex topography and analyze their characteristics and behavior as the initial conditions vary. This study experimentally examines the evolution of bottom-propagating gravity currents in the presence of an array of submerged cylindrical obstacles. The laboratory experiments were conducted within a Perspex tank with dimensions of 3 m in length, 0.3 m in height, and 0.2 m in width, using the lock-release technique by filling the left and right volumes of the tank to the same water depth. The density difference was reproduced through a salinity gradient. Submerged roughness was introduced by arranging a series of rigid plastic cylinders at a specified location, covering the entire width of the channel. Two different diameters, 2 cm and 2.5 cm, were analyzed, and the initial current depths were varied. A total of 24 full-depth lock-exchange experiments were performed. We employ an innovative image analysis technique based on light reflection to evaluate the instantaneous density fields. To apply the light attenuation technique and visualize the dense fluid, a controlled quantity of dye was introduced into the saline water. A calibration method was used to establish the correlation between light intensity and dye concentration for each pixel in the captured images. The conducted study clearly illustrates that an adequate height of obstacles results in a substantial portion of denser fluid being impeded by the foremost obstacle in an array. Additionally, transitioning from densified to less-densified array geometries induces distinct changes in flow morphologies. Upon concluding the analysis of this study, it is evident that all the experiments are affected by the presence of substantial bottom roughness.

How to cite: Adduce, C., Maggi, M. R., and Di Lollo, G.: Density-driven flow propagating over a bottom large-scale roughness, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6066, https://doi.org/10.5194/egusphere-egu24-6066, 2024.

Posters virtual: Tue, 16 Apr, 14:00–15:45 | vHall X4

Display time: Tue, 16 Apr 08:30–Tue, 16 Apr 18:00
Chairpersons: Kevin Lamb, Yvan Dossmann
vX4.15
|
EGU24-9523
|
NP6.2
|
ECS
Fei Yang, Qiang Wang, Yuangjian Wang, and Enhui Jiang

Turbidity currents venting is most efficient method for Xiaolangdi reservoir under normal water level when tackling sediment-laden floods to reduce sedimentation. The concept of continuous motion conditions of turbidity current was introduced as the requirements for the main part of turbidity current reach the dam site. Dynamics of turbidity currents in Xiaolangdi reservoir were investigated using the layer-averaged equations of motion. Water-sediment exchange with bed and above have been concealed for simplicity, by which the mechanism that drives continuous motion is subjected to the gravitational force. Two major control parameters, the relative longitudinal bed slope and the relative length of turbidity current, are proposed regarding topography and hydrodynamics. The ratio of the average riverbed longitudinal gradient J from the plunging point to the dam site to the critical gradient Jc of the turbidity current, J/Jc, represents the relative longitudinal bed slope. The ratio of the product of the equilibrium velocity u and duration T of the turbidity current to the radial distance L from the plunging point to dam site, uT/L, represents the relative length of turbidity current. Therefore, a discrimination diagram for continuous motion conditions of turbidity current was determined based on J/Jc and uT/L, successfully differentiated whether turbidity currents can reach the Xiaolangdi dam site.

How to cite: Yang, F., Wang, Q., Wang, Y., and Jiang, E.: Continuous motion conditions of turbidity current in Xiaolangdi reservoirs, China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9523, https://doi.org/10.5194/egusphere-egu24-9523, 2024.