Global Skin Friction Measurements for Hypersonic Wind Tunnels

Prediction of the state of boundary the layer and regions of separated flows is essential for the design of hypersonic vehicles. Flow separation, for example, influences the effectiveness of control surfaces, and therefore, control authority. In addition, skin friction measurements in hypersonic test facilities can be used to validate computational tools. Skin friction can currently be measured with some success at discrete locations, however, determining the proper measurement locations for a limited number of point sensors a priori is still 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 proven difficult to deploy in hypersonic wind tunnels. Measurement techniques that can provide global measurements of both mean and unsteady skin friction and operate in hypersonic test facilities would be of significant value. Over the last several years, Liu at Western Michigan University (WMU) has demonstrated the extraction of global maps of mean skin friction from a variety of surface visualization techniques such as luminescent oil flow, Temperature-Sensitive Paint (TSP), and Pressure-Sensitive Paint (PSP). Recently, this approach has been extended to operate with fast PSP as part of an Innovative Scientific Solutions, Inc. (ISSI)/WMU SBIR, resulting in unsteady global skin friction measurements in a supersonic shock wave boundary layer flow. Currently, Schmisseur at the University of Tennessee Space Institute (UTSI) is constructing a set of hypersonic wind tunnels for fundamental studies of hypersonic fluids. These tunnels provide an ideal test bed for demonstration of these new skin friction measurement techniques. Here, ISSI seeks to team with WMU and UTSI to demonstrate the use of these global skin friction tools into hypersonic wind tunnels. The experimental techniques that form the basis of these global skin friction measurements, TSP, PSP, and uPSP, are relatively high TRL systems that are commonly used in large production wind tunnels. Each of these measurements have also been used in hypersonic wind tunnels. The extraction of global skin friction from PSP and TSP data has been demonstrated in transonic and supersonic wind tunnels. This program presents a low risk high reward tool for research in hypersonic flows. The proposed system could provide not only experimental measurements of mean and unsteady skin friction with high spatial resolution, but also improve tunnel productivity by simultaneously acquiring pressure, unsteady pressure, temperature, or heat flux data with the skin friction measurements.