Measurements of Wall-Shear-Stress Distribution in Hypersonic Flows

Accurate prediction of the state of the boundary layers and regions of separated flows is of fundamental importance in the design of hypersonic vehicles. The ability to predict boundary layer characteristics and how these affect surface shear stress and its influence on the surface skin friction is directly related to flight performance. Most measurement techniques rely upon point measurement of the local shear stress value and do not provide a 2-dimensional mapping of the aerodynamic surface. However, one methodology that has the ability to provide 2-dimensional surface shear stress data and is non-intrusive is the use of Shear Stress Sensitive Liquid Crystals. Such crystals will change in colour with changes in the local surface shear stress, going from Red at the lower shear stress levels through Green to Blue at the higher stress levels and have been used relatively successfully in low speed flows and some specifically targeted high speed flows, namely Mach 1, 2 and 3. Their use in hypersonic flows of Mach 5 and above has been neglected due to the hostile flow regime where there is extremely high shear stress levels combined with elevated temperatures, and where it is expected that current liquid crystals would rapidly erode. However, given that Liquid Crystals can react close to the kHz range and are able to measure skin friction from 100 to 1500 Pa (depending on mixture and flow properties) and is truly 2 dimensional thus providing very small measurement area (depending on camera capability) of less than 3 mm x 3 mm, they would be suitable for this application. Therefore this Phase II of the STTR is concerned with developing a system for measuring surface shear stress in Hypersonic flow facilities for the US Navy and previous experience by the University of Cincinnati and Engineering & Scientific Innovations have shown that the measurement parameters needed for use in Hypersonic flow may be fulfilled using Liquid Crystals.