Boundary layer measurements over a body of revolution using long-distance particle image velocimetry

This study reports the development and application of long-distance high magnification Particle Image Velocimetry (PIV) to measure the mean and fluctuating velocity components in a turbulent boundary layer over a body of revolution. A 400 mm lens coupled with an extension tube and a novel aperture control mechanism has enabled high resolution images at a working distance of 1.5 m. The body of revolution used in the investigation is representative of a conventional submarine hull without appendages, as per (Joubert, 2006). The flow over the model was measured at a nominal Reynolds number, Re_L=4.0 × 10⁶, based on the length of the body. A comparison of single-point statistics from the PIV experiments were found to compare well with pitot probe and hot-wire measurements. Further, the boundary layer thickness and mean streamwise velocity were found to conform closely with a traditional flat plate turbulent boundary layer in the parallel mid-body section of the geometry. However, a comparison of the streamwise and wall-normal Reynolds stresses with Direct Numerical Simulations (DNS) of a turbulent boundary layer on a flat plate with zero pressure gradient indicated lower turbulence levels. This was partly attributed to the spatial averaging evident both in the hot-wire and PIV measurements. We also report that in the tail section of the model, the boundary layer rapidly grows and departs from the flat plate solution due to the conical section and adverse pressure gradient imposed on the flow.