Rotation–entrainment control of Lagrangian dispersion in a turbulent horizontal jet: core–edge contrasts and transient attracting barriers

Rotation and turbulence jointly shape transport and mixing in jet-like flows, from boundary currents to atmospheric plumes. Even under weak rotation (Rossby number O(1)), particle spreading can become strongly inhomogeneous because material barriers reorganize pathways and constrain exchange across the turbulent–non-turbulent interface. Here we use a laboratory horizontal jet to quantify how rotation regulates Lagrangian dispersion in distinct jet sub-regions (core versus edges) and to link the observed trends in dispersion and diffusivity to the geometry of transient attracting barriers.

We analyse three experiments (datasets previously presented in De Serio et al., 2021): a non-rotating reference case (EXP14) and two rotating cases with increasing rotation rate (EXP15 and EXP16). Experiments were conducted using a turbulent, non-buoyant jet released horizontally into ambient water (initial diameter d=0.08m, exit velocity u=1.14m/s). Planar PIV velocity fields are integrated to compute Lagrangian trajectories of numerical neutrally buoyant particles. We evaluate single-particle absolute dispersion A(t) and direction-dependent absolute diffusivities K(t). 

We also diagnose barrier-structured transport without time-integration, using Transient Attracting Profiles (TRAPs), an instantaneous diagnostic of the most attracting regions of the flow derived from local minima of the strain-rate tensor (Serra et al., 2020; Kunz et al., 2024). TRAPs mark hyperbolic skeletons of maximal compression on the measurement plane, predicting where tracers accumulate and where strong stretching develops. In our jet, TRAPs provide a compact geometric context for interpreting when and where lateral spreading is inhibited (reduced A or analogously K) or promoted (enhanced stretching and growth of A and K).

Across all cases, we note that A(t) exhibits an initial ballistic regime consistent with inertial short-time behaviour. Rotation then introduces a clear, region-dependent ordering. In the jet core, focusing on intermediate dispersion values (i.e. structures of order 10–100 cm), these levels are reached first in EXP14 (no rotation), then in EXP15, and last in EXP16, demonstrating that core dispersion decreases as rotation increases. Consistently, the growth of K(t) is progressively suppressed under stronger rotation, indicating stabilization and more coherent pathways. At the jet edges, the rotating cases show the same ordering, so that stronger rotation implies lower dispersion. In contrast, without rotation (EXP14) edge-region dispersion is minimal. Interpreted through TRAP geometry, stronger rotation favours tighter attracting pathways and enhanced accumulation along compressive skeletons, reducing cross-interface wandering and lowering edge-region diffusivities, while non-rotating edges remain weakly dispersive because velocities are small and entrainment is limited.

Overall, rotation reduces dispersion in both core and edge regions, but through distinct mechanisms: stabilization driven by the Rossby number in the core and entrainment-mediated limitation at the edges, with TRAPs offering an immediate geometric interpretation of the observed A and K trends.

References

De Serio et al. 2021: https://doi.org/10.1007/s00348-021-03297-2.

Serra et al. 2020: https://doi.org/10.1038/s41467-020-16281-x.

Kunz et al. 2024: https://doi.org/10.5194/os-20-1611-2024.