Does turbulence speed up or slow down settling heavy particles?
In plain words
Turbulence can make heavy particles fall faster by steering them into downward-moving fluid, or slower through drag effects and lingering in upward flow. Which effect wins under given conditions is disputed.
Precise statement
For heavy particles with $\mathrm{St}\sim 1$ and settling parameter $\mathrm{Sv}=v_s/u_{\eta}\sim 1$ (still-fluid settling speed over Kolmogorov velocity) in homogeneous turbulence, preferential sweeping (Wang and Maxey 1993) raises the mean settling velocity while nonlinear drag and loitering lower it. Determine the sign and magnitude of the change in mean settling velocity as a function of St, Sv, particle Reynolds number and $\mathrm{Re}_{\lambda}$, including two-way coupling at mass loading of order 1.
What would settle it
Laboratory measurements and DNS mapping the settling-velocity change over $(\mathrm{St}, \mathrm{Sv}, \mathrm{Re}_{\lambda})$ with consistent results in overlapping ranges.