Towards shear flow measurements using femtosecond laser flow tagging

The Femtosecond Laser Electronic Excitation Tagging (FLEET) method is employed to measure wall shear stress in a supersonic flow. This study relies on the relationship between shear stress and velocity. We demonstrate the capability of FLEET to resolve velocity measurements in the viscous sublayer for shear stress calculations. In two of the studies, a 100µm thick, 2:5cm long fluorescent line is written into the flow and the displacement of the line is recorded after a time delay to give velocity measurements in the boundary layer of the flow, forming a velocity gradient that can then be translated into shear stress. In a third study, a single fluorescent point is written into the flow to reduce the projection ambiguity of a displaced line. Measurements are obtained from three different experimental configurations and a comparison of their respective advantages and disadvantages is given. It is shown that FLEET provides the spatial and temporal scales necessary to interrogate the boundary layers in these experiments. This study also sheds light on some diffculties of using a molecular tagging method to visualize supersonic flows in a confined space.