基于互相关的光流测速法在湍流热对流中的应用

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

同时准确地测量大尺度流动结构和小尺度湍流统计特性对于理解湍流的能量传递规律有着重要意义。尽管粒子成像测速(PIV)技术得到了广泛的应用,但由于算法原理的要求以及商用相机分辨率的限制,PIV技术在同时获取大、小尺度流动特性方面并不实用。另一方面,近年来逐渐引起关注的光流测速法虽然可以实现单像素分辨率的大区域速度场测量,但其准确性通常受限于应用场景而难以保证。本文采用一种基于互相关的光流测速法来分析RayleighBénard对流的粒子图像,进而同时获得全局速度场和小尺度湍流特性。结果表明,这种方法不仅可以获得与PIV技术一致的全局场和流动强度,还能有效地解析出湍流耗散区的速度结构函数。进一步的,分别基于速度结构函数和定义直接计算而获得的能量耗散率不仅在数值大小上互相吻合,还与直接数值模拟的结果以及理论预测相一致。这些结果表明基于互相关的光流测速法在同时测量大尺度流动结构和小尺度统计特性上有很好的应用潜力。

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 singlepixel 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.