Sensitivity of Wavelet-Based Optical Flow (wOFV) to Sources of Error

The influence of several non-ideal experimental effects on the accuracy of a wavelet-based optical flow (wOFV) method is evaluated using data from a series of simulated flows. Out-of-plane particle displacements and image intensity non-uniformity are shown to decrease the accuracy of wOFV in a similar manner to correlation-based PIV, but with an increased sensitivity compared to correlation-based PIV. However, wOFV is still found to be more accurate than PIV when the magnitude of these effects remain within expected experimental bounds. It is noted that for some cases, the results are significantly improved by using image pre-processing filters. It is observed that flow divergence does not have an appreciable effect on the accuracy of wOFV velocity estimation, even though the fluid transport equation on which wOFV is based implicitly assumes that the motion is divergence-free. This is a significant finding for the broader applicability of planar velocimetry measurements using wOFV. Finally, it is noted that the accuracy of wOFV is not significantly reduced in regions of the image that do not contain particles, as long as the seeding density is sufficiently high. This explicitly demonstrates that wOFV (when applied to particle images) yields an accurate whole field measurement, and not just at the particle locations.