Infrared Based Wall Shear Stress Measurement Techniques

The paper presented here introduces two novel, infrared based wall shear stress measurement techniques. The first provides wall shear stress visualizations with a high spatial and temporal resolution as well as spatial quantitative information. The other technique enables sensor-based measurements of the wall shear stress magnitude and direction. Both techniques are based on the close link between momentum and heat transport in the boundary layer and correlate the surface temperature distribution of a heated surface (first technique) or a heated spot (second technique), which is measured using infrared thermography, with the wall shear stress. For the spatial qualification and quantification, the temporal surface temperature evolution of a heated structure subjected to a flow is linked to the wall shear stress distribution. The second, sensor-based technique heats a small spot on an otherwise unheated surface. The temperature distribution, or thermal tuft, around this heated spot is closely related to the local wall shear stress magnitude and direction. Results are presented for both techniques and compared to reference measurements and visualizations, respectively. Reference measurements of the wall shear stress were obtained using a skin friction balance; oilflow visualizations were used as a reference visualization technique.