Towards Quantitative Wall Shear Stress Measurements: Calibration of Liquid Crystals

The wall shear stress distribution on an aerodynamically loaded component is of both practical and fundamental importance. Significant examples are the improvement of the performance of a vehicle, e.g. drag reduction, and more basic problems, such as the characterization of surface flows, e.g. with respect to flow control. The liquid crystal technique represents a promising diagnostics for measuring wall shear stress magnitude and orientation. In contrast to most other techniques, the direct measurement of two-dimensional wall shear stress distributions is straightforward. In order to establish quantitative measurements of wall shear stress using the liquid crystal technique, an in-depth understanding of the influencing parameters is required. For their investigation, a novel generic flat plate test section was designed. The experiments are performed in such a way that a turbulent boundary layer is triggered at a corresponding Reynolds number within the test section. Due to the generic test case, precise and well-known flow boundary conditions can be established, which in turn are validated by probe measurements. Velocity and temperature profiles are recorded with high spatial resolution using a miniaturized combined Pitot-thermocouple probe. Furthermore, the operational range of the new test rig is presented. Preliminary wall shear stress measurements confirm the well-defined flow conditions in the test section and the potential of the measurement technique.