A Doppler sensor for high spatial resolution measurements of unsteady surface pressure

The Doppler frequency shift is used as the basic sensing mechanism for a new technique for measurement of unsteady surface pressure. The frequency shift is experienced by a focused laser beam reflected off the aluminized top of a flexible polymer diaphragm subjected to the unsteady pressure. Prototype sensors based on this concept, with different sizes and diaphragm materials and thicknesses, are constructed as well as evaluated. The results provide an understanding of the limits of the sensor's sensitivity, bandwidth, resolution and noise level. Moreover, analysis of typical wall-pressure spectra beneath boundary layers of high and low Reynolds number in the light of these limits underlines the potential advantage of the new sensor in resolving the signature of small-scale turbulent structures at high Reynolds numbers.