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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.

An experimental method is proposed to study dispersed two-phase flows at an airwater interface, a family of flows of practical significance in environmental and industrial seĴings. The applicability of this technique is demonstrated through the study of a lightly-deformed turbulent free-surface laden with floating particles (`floaters'). A low-mean turbulent flow is generated in a turbulence box actuated by a 10×10 synthetic jet array. Using LEDs and a single camera, free-surface flow measurements are carried out by Particle Image Velocimetry (PIV) simultaneously with Lagrangian tracking of the floaters, allowing the potential to characterise the coupling between the floater dynamics and the (sub)surface flow. Discrimination of the dispersed and continuous phases is carried out based on size. Individual floaters and clusters of floaters are successfully tracked throughout the field of view while they navigate through elongated and circular regions of high and low vorticity, characteristic features typically observed when a subsurface turbulent flow interacts with a free surface. Preliminary results of the floater-fluid interactions are presented to highlight the potential of this technique to beĴer our understanding of floaterladen turbulent free surfaces.