Unsteady Skin Friction and Pressure for Wind Tunnel Measurements

While skin friction is a key parameter for characterizing fluid flows, it has proven to be a difficult quantity to measure. Currently, skin friction is measured at discrete locations using different sensors; however, determining the proper measurement locations a priori is a significant challenge. Measurement techniques that provide global distributions of skin friction, such as oil film interferometry, shear sensitive liquid crystals, and surface stress sensitive films, have demonstrated steady state skin friction distributions in specific settings. Unfortunately, deployment of these measurement techniques for use in large scale production wind tunnels has proven difficult. The issue of optimum skin friction sensor placement is even more complicated in regions where the flow is highly unsteady, and therefore, a system that can provide distributed measurements of unsteady skin friction is of significant interest. ISSI and WMU propose utilizing a new variational mathematical approach known as boundary enstrophy flux (BEF) that can be used to extract qualitative skin friction from distributions from Pressure-Sensitive Paint (PSP) data. This approach will be combined with an unsteady PSP (uPSP) system, which has been demonstrated to operate effectively in production wind tunnels. The Phase I program will seek to validate the application of the BEF methodology to uPSP data using surface stress sensitive films data on a stalled airfoil. Efficient data processing approaches for analyzing the large volume of data acquired by uPSP systems using the BEF approach will be evaluated. By applying the BEF analysis to the uPSP data, it is possible to produce high spatial resolution distributions of both quantitative unsteady pressure and qualitative unsteady skin friction using a single system. This data can then be used to evaluate numerical models and select optimum positions for point sensors.