Extending the Frequency Limits of "Postage-Stamp PIV" to MHz Rates

The effective frequency limits of postage-stamp PIV, in which a pulse-burst laser and very small fields of view combine to achieve high repetition rates, have been extended by increasing the PIV acquisition rate to very nearly MHz rates (990 kHz) by using a faster camera. Charge leaked through the camera shift register at these framing rates but this was shown not to bias the measurements. The increased framing rate provided oversampled data and enabled use of multi-frame correlation algorithms for a lower noise floor, increasing the effective frequency response to 240 kHz where the interrogation window size begins to spatially filter the data. The velocity spectra suggest turbulence power-law scaling in the inertial subrange steeper than the theoretical -5/3 scaling, attributed to an absence of isotropy.

Single Pixel Evaluation of Microchannel Flows

Recently a new μPIV interrogation algorithm has been proposed in which the interrogation window size is reduced to a single pixel. Such small interrogation window sizes are possible using correlation averaging to increase the effective particle concentration to levels required for correlation analysis to succeed. The random error exhibits the expected behavior of decreasing roughly in proportion to N−1/2 while the bias error exhibits unexpected peak-locking behavior with zero bias error at integer and half integer pixel displacements and maximal errors at one-quarter and three-quarter pixel displacements. Accompanying experiments show the potential of this technique but have not yet been sufficiently refined to confirm this unexpected bias error behavior.