Simultaneous measurements of velocity fields in thermal turbulence by USV and PIV

To test the validity of the ultrasonic velocimetry (USV) in thermal turbulence, we performed an experiment in water thermal turbulence. In this experiment, the velocity field was simultaneously measured by USV and particle image velocimetry (PIV), and then the measured velocity profiles, $w_{\mathrm{USV}}(t,z)$ and $w_{\mathrm{PIV}}(t,z)$, were compared. Due to equipment limitations, the measured velocity range was narrow $(\left|w\right|?8\mathrm{mm}/s)$ compared to the measurable range of USV (±91.3 mm/s), which made the test a severe one. Nevertheless, it was found that the correlation coefficient $C_{\mathrm{corr}}$ between $w_{\mathrm{USV}}(t,z)$ and $w_{\mathrm{PIV}}(t,z)$ exceeds 0.9 and that $C_{\mathrm{corr}}$ increases as $w$ increases. The results of this test show that USV method outputs data agreeing with PIV data except when the fluid velocity is so small that the digitization unit of USV ($\Delta u=0.72\mathrm{mm}/s$ in this test) is elicited explicitly. This encourages us to believe that in thermal turbulence the USV is as reliable a technique as the PIV method.