Global Skin Friction Sensor for Cryogenic Wind Tunnels

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 into cryogenic wind tunnels has proven difficult. A system that can provide global measurements of mean and unsteady skin friction is of significant interest. ISSI and WMU propose exploiting existing measurement systems for temperature and utilizing a new variational mathematical approach that can extract global skin friction from high spatial resolution distributions of surface temperature acquired using Temperature-Sensitive Paint (TSP) and IR thermography. The process is based on the well-established relationship between the energy equation and skin friction. This approach has recently been demonstrated by the proposing team as part of a Phase II Air Force SBIR program, thus demonstrating the technical maturity of the technique. Temperature-Sensitive Paint and IR thermography are high TRL tools that are currently used in wind tunnels at both room temperature and cryogenic conditions. These systems, combined with surface heating devices such as carbon nanotubes, provide the high spatial resolution measurements of temperature gradients that are required for extraction of skin friction using this new approach. Combining the proven measurement capabilities of TSP and IR thermography with this new variational approach to extraction of skin friction results in a low risk approach to providing global skin friction measurements in cryogenic wind tunnels.