Particle Image Velocimetry (PIV) is the most widely used optical technique for measuring two-dimensional velocity fields in fluids. However, with the standard cross-correlation (CC) algorithm, improving the spatial resolution of instantaneous velocity fields and obtaining dense velocity fields in real time remains challenging. Optical Flow Velocimetry (OFV) offers a way to overcome these limitations. In this study, we demonstrate that dense velocity fields (one vector per pixel) with high spatial resolution can be obtained in real time at frequencies up to thousands of Hertz using an optical flow approach. We show that high resolution is achievable with optimized seeding, and that computational speed can be increased by choosing appropriate parameters and running on a single GPU. Using this method, 21 Mp velocity fields can be computed in real-time at 90 Hz, while 4 Mp velocity fields can be computed up to 460 Hz. These measurements enable the computation of various flow quantities in real time, during the experiment. It makes this technique perfectly suitable for many new type of experiments, from closed-loop flow control experiments based on OFV measurements, to very low frequency measurements or monitoring of the flow to find rare events. They also greatly accelerate post-processing leading to potential large time and energy gain for post-processing.
Pimienta et al. (Tue,) studied this question.