Wall normal focused laser differential interferometry for measurements of flow disturbances on flat-plate models

Abstract

A new approach for measuring boundary layer disturbances with focused laser differential interferometry at planar models is presented. By integrating a glass window into a flat plate, the optical axis was aligned normal to the model surface and the focal plane was set inside the boundary layer. By determining the extent of the sensitive volume along the optical axis and calculating the analytical transfer function of the setup, the implications of the FLDI properties on the measured data are analyzed. Measurements performed on a flat plate at Mach 6 are used to demonstrate the effects of the laminar-turbulent transition on the spectral distribution of the power density, and to explicitly verify the detectability of the expected second-mode instabilities. Advantages and disadvantages of the proposed setup compared to the conventional one are discussed.