Biased sampling reduces particle settling velocities in turbidity currents

arXiv:2606.08241v1 Announce Type: new Abstract: We investigate the mechanisms governing particle settling in turbidity currents using two-way coupled Eulerian-Lagrangian direct numerical simulations. The effective particle settling velocity is decomposed into a fluid velocity sampled at particle positions and a particle-fluid slip velocity. Their Eulerian mean profiles are obtained using a concentration-weighted average of the coarse-grained fields.The mean sampled fluid velocity is shown to be approximately equal to the ratio of the vertical turbulent flux of particles to their mean concentration. This velocity remains predominantly positive in both inertial-particle and passive-tracer cases, despite the zero Eulerian mean vertical fluid velocity. We therefore infer that this upward bias is inertia-independent and outweighs downward-directed biases associated with particle inertia. The passive-tracer cases further indicate that the upward bias arises from turbulent transport acting on an inhomogeneous concentration field, rather than from the so-called loitering effect (Nielsen, J. Sedim. Petrol., vol. 63, 1993, pp. 835-838).The mean slip velocity closely follows the terminal settling velocity predicted for a quiescent fluid with a correction for finite particle Reynolds number. This is consistent with a leading-order balance between buoyancy and drag in the slope-normal direction. Combining the two velocity components yields a simple model for the effective settling velocity across the entire flow depth, in good agreement with the simulation data.