Simultaneous measurements of large- and small-scale flow properties in turbulent convection by cross-correlation based optical flow velocimetry

How to accurately measure both large-scale flow structure and small-scale turbulent statistics simultaneously is crucial for understanding the energy transfer mechanism in turbulent flows. Although Particle image velocimetry (PIV) has become the most prominent technique for velocity field measurement, the requirement of its basic principle and the limited spatial resolution of commercial cameras have made current PIV impractical to acquire large- and small-scale flow properties simultaneously. In recent years, optical flow velocimetry has received growing attention because of its capacity in measuring large-area flow field with single-pixel resolution. However, its accuracy is limited by the detailed application circumstances. In this work, we apply a cross-correlation based optical flow velocimetry to analyze the particle images of Rayleigh-Bénard convection, based on which we obtain the global velocity field and small-scale turbulent statistics simultaneously. It is found that this method is capable to obtain the mean flow field that is in good agreement with the PIV result, and meanwhile can resolve the velocity structure function in the dissipation range. Moreover, the energy dissipation rate obtained from the velocity structure function and that directly calculated from the definition agree with each other quantitatively, and they are consistent with previous direct numerical simulation and theoretical prediction as well. These results suggest that the cross-correlation based optical flow velocimetry is a promising tool for simultaneous measurements of large- and small-scale properties in turbulent flows.