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 pressure and utilizing a new variational mathematical approach that can extract global skin friction from high spatial resolution distributions of surface temperature acquired using Temperature- and Pressure-Sensitive Paint (TSP/PSP) or IR thermography. PSP and TSP are high TRL tools that are currently used in wind tunnels at both room temperature and cryogenic conditions. During the Phase I program, NASA provided the ISSI/WMU team with a set of cryogenic boundary layer transition data. This data was acquired using TSP data on a CRM model in the NTF. This data was reprocessed to produce global distributions of skin friction on the wing. The data clearly shows variations in skin friction over the wing and high-resolution features such as separation and attachment lines in the turbulent wedges at the leading edge of the wing. Combining the proven measurement capabilities of TSP and PSP with this new variational approach to extraction of skin friction from surface temperature and pressure distributions results in a low-risk approach to providing global skin friction measurements in cryogenic wind tunnels.
The clear NASA targets for this measurement capability are high Reynolds number facilities such as the NTF and 1/3-meter cryogenic wind tunnels. Quantitative measurements of skin friction should have immediate impact in NASA efforts to develop and design newer aerospace vehicles. Efforts to deploy laminar flow wings or coatings on legacy aircraft would also benefit from this measurement. Even without quantitative capability, the measurement offers something similar to oil flow visualization, which is not possible in these facilities.
The final product from this program should be a system capable of acquiring high spatial resolution distributions of skin friction on a model using Temperature-and Pressure-Sensitive Paint data. This is a technological capability that is of interest to ISSI’s current commercial, research, and military wind tunnel customers. ISSI expects to aggressively market this capability to these customers.