Dispersed two-phase flows at air–water interfaces are ubiquitous in environmentally relevant flows such as in the dispersion of floating microplastics or transport processes across the air–sea interface. In the current study, we propose a method to study such flows through the study of a relatively flat turbulent free surface laden with spherical floating particles (“floaters”). The free surface is perturbed by a relatively low-mean nearly homogeneous subsurface turbulent flow that is produced in a turbulence box actuated by a 10×10 synthetic jet array. The free surface flow field is characterized using planar particle image velocimetry (PIV) simultaneously with Lagrangian tracking of floaters allowing insight into the floater dynamics and the surface flow coupling. This is enabled by a relatively simple setup of LED panels and a single camera. Distinction between the continuous (flow tracers) and the dispersed (floaters) phase is carried out by exploiting their size disparity and number density. The proposed method is employed to characterize the single-phase flow field and the clustering statistics of floaters for different turbulence levels, the latter achieved by varying the distance of the free surface from the jet array. Specifically, we study the effect of different turbulence levels on the floater clustering behavior. We observe that the time required for floaters to reach a clustered quasi-steady state decreases with increasing vorticity and surface divergence amplitude. In addition, the growth rate of the mean cluster size is observed to increase with increasing vorticity and surface divergence amplitude, with its temporal evolution exhibiting two distinct phases: an agglomeration phase and an equilibrium phase. In contrast, in the absence of a subsurface flow, floaters are observed to cluster at a relatively slower rate characterized by a prolonged agglomeration phase. Finally, to highlight the potential of this technique in studying floater-laden turbulent free surfaces, preliminary results of flow–floater interactions are discussed.

Cambridge University Press apologise for an error with the supplementary material of the above article. Additional materials from an unrelated article were erroneously published alongside the intended supplementary material. This has been corrected.

The turbulent boundary layer (TBL) development over an air cavity is experimentally studied using planar particle image velocimetry. The present flow, representative of those typically encountered in ship air lubrication, resembles the geometrical characteristics of flows over solid bumps studied in the literature. However, unlike solid bumps, the cavity has a variable geometry inherent to its dynamic nature. An identification technique based on thresholding of correlation values from particle image correlations is employed to detect the cavity. The TBL does not separate at the leeward side of the cavity owing to a high boundary layer thickness to maximum cavity thickness ratio (δ/t_max = 12). As a consequence of the cavity geometry, the TBL is subjected to alternating streamwise pressure gradients: from an adverse pressure gradient (APG) to a favourable pressure gradient and back to an APG. The mean streamwise velocity and turbulence stresses over the cavity show that the streamwise pressure gradients and air injection are the dominant perturbations to the flow, with streamline curvature concluded to be marginal. Two-point correlations of the wall-normal velocity reveal an increased coherent extent over the cavity and a local anisotropy in regions under an APG, distinct from traditional APG TBLs, suggesting possible history effects.