FREQUENCY RESOLUTION OF HIGH-SPEED PIV

Kurzfassung

High speed particle image velocimetry (PIV) measurements with a sample frequency of 20 kHz are conducted in order to characterize the temporal response of such a system. The limits and drawbacks associated with the PIV measurement technique regarding the frequency analysis of the obtained velocity time series are investigated. Hot wire measurements are performed for comparison. In contrast to time resolved analog measurement techniques (e.g. hot wire anemometry) it is not possible to apply analog filters to the PIV signal in order to avoid aliasing effects. But the PIV processing itself acts like a filter in space and time. Here a special attention is turned on the influence of the interrogation window size for the PIV analysis on the frequency spectrum. Two types of flows are chosen for the here presented study. The first is grid turbulence. It provides nearly homogeneous and isotropic turbulence at a certain distance downstream of the grid. The second is the wake behind a cylinder, where the flow field has a more inhomogeneous character. It can be shown that the spatial averaging in the interrogation window results in a low pass filter for the frequency response and the transfer function is a modified sinc function.