Influence of particle's buoyancy on turbidity currents from continuous influx

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 U_f 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 U_f over (Δρ₀/ρ₀ . 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 <h_c>), 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).