Surface pressure visualization by 3D PTV

Some recent trends in the development of PIV techniques are leading to the realization of velocity measurement at large scale. Moreover, methods for evaluating the fluid flow pressure from PIV data have been successfully demonstrated at small scale and for simple geometries. Methods for quantitative visualization of surface flow properties such as skin-friction and pressure are comparatively less developed, despite the great interest for the aerodynamic insight they provide for the study of the flow around 3D complex objects.
The accuracy of the pressure evaluation at the surface is inquired in this work by dedicated measurements over a sphere at Red = 8 × 104. The measurements made with robotic PIV are compared to direct surface pressure measurements. The solution of the Reynolds averaged momentum equations is performed with the Poisson problem formulation and the results are compared with potential flow theory and some simplified models. The accuracy of stagnation pressure depends upon the measurement resolution. Instead, the suction peak region is generally underestimated. The pressure in the separated region is retrieved with sufficient accuracy.
The surface flow properties visualization of a time-trialing full-scale cyclist replica at 14 m/s (Re = 5.5×105) is performed as a demonstration of the above method to a complex 3D problem at large scale. The data obtained with the robotic PIV technique is analyzed in close proximity of the solid surface and the skin friction lines of the time averaged velocity field are inspected. The topological analysis of the skin friction lines yields clearly the evolution of the flow around the body segments returning the details of the separated flow regions and local recirculation zones. The analysis of the surface pressure distribution yields results in accordance with the skin-friction lines, the outer flow velocity field as well as the vortex topology.