An internal stereo-PIV system for measuring wall shear stress and an external system for wake characterization around a submerged body

Understanding three-dimensional boundary layers and flow separation on inclined bodies of revolution are challenging due to the large variation in pressure gradients and wall shear stresses. Two stereo-PIV (SPIV) systems are developed to characterize the flow and wall stresses around a 3m long, 6:1 prolate spheroid, at varying incidence angles and Reynolds numbers (0.15-1.2x10⁷). An internal high-resolution system with FOV of 10×8 mm² is placed inside the model to measure mean wall shear stress from the velocity gradients in the viscous sublayer. This system features local seeding, a laser beam transmitted via. an optical fiber, and cameras viewing the sample area through a curved window whose outer surface is made of a polymer with refractive index matched with that of water. Data at varying azimuthal angles are obtained by rotating the model around its axis. Data analysis involving ensemble correlations achieves a resolution of ~5 mm, as confirmed by calibration tests. The external SPIV system is mounted behind the model and has a FOV of 40×30 cm² focusing on the flow in the lee side of the spheroid. For towing tank tests, the remotely controlled, adjustable laser sheet optics are located in submersible enclosures mounted on a separate rigid strut. For both systems, the images are recorded by 12MP CMOS cameras, with the data multiplexed optically and transmitted to external acquisition systems. Sample results from a towing tank experiment will be introduced.