Pseudo turbulence in PIV breaking-wave measurements

The particle image velocimetry (PIV) technique was employed to measure the instantaneous velocity distribution under nonbreaking and breaking water wa ves. The corresponding turbulence intensity was calculated by the ensemble average of repeated measurements. The pseudo turbulence found was large eno ugh to affect the accuracy of the turbulence measurements. We follow Prasad et al.'s (1992) approach to demonstrate that the pseudo turbulence is rela ted to the bias error, which is the discrepancy between the true position o f the particle image and the position calculated from the pixel array data with inadequate pixel resolution. To reduce the bias error (or the pseudo t urbulence), we first calculate it from a turbulence-free flow with the same experimental set-up as that used for the targeted experiments (i.e., we us e the same size of field of view, seeding particles, seeding density, lens aperture, and laser wavelength in both experiments). Then we minimize the b ias error from the turbulence measurements in the actual experiments. To de monstrate the procedure, the evolution of a breaking wave is investigated.