Flow characterization using a laser Doppler vibrometer

This paper shows how a scanning laser Doppler vibrometer (LDV), an instrument designed to measure vibrations of structures or objects, can be used in a non-traditional fashion to measure the flow around a cylinder. In particular the von Karmann vortex street which appears in the cylinder wake will be visualized. This is achieved by measuring the changes in the optical path induced by local fluctuation of air refraction index to which the laser vibrometer is sensitive. The measurements obtained with the LDV will be compared visually to measurements done with particle image velocimetry and also with CFD computations for one test case. The specific frequency of the Von Karmann street predicted by the vibrometer, will be compared numerically to the other techniques for two different sized cylinders at three different velocities.