Matt Silvia, Nootz Gero, Hellman Samuel, Hou Weilin
Ocean Sciences Division, US Naval Research Laboratory, Stennis Space Center, MS, 39529, USA.
Oceanography Division, NRC Research Associate at US Naval Research Laboratory, Stennis Space Center, MS, 39529, USA.
Sci Rep. 2020 Feb 7;10(1):2130. doi: 10.1038/s41598-020-58077-5.
Particle image velocimetry (PIV) is a well-established tool to collect high-resolution velocity and turbulence data in the laboratory, in both air and water. Laboratory experiments are often performed under conditions of constant temperature or salinity or in flows with only small gradients of these properties. At larger temperature or salinity variations, the changes in the index of refraction of water or air due to turbulent microstructure can lead to so-called optical turbulence. We observed a marked influence of optical turbulence on particle imaging in PIV. The effect of index of refraction variations on PIV has been described in air for high Mach number flows, but in such cases the distortion is directional. No such effect has previously been reported for conditions of isotropic optical turbulence in water. We investigated the effect of optical turbulence on PIV imaging in a large Rayleigh-Bénard tank for various path lengths and turbulence strengths. The results show that optical turbulence can significantly affect PIV measurements. Depending on the strength of the optical turbulence and path length, the impact can be mitigated in post-processing, which may reduce noise and recover the mean velocity signal, but leads to the loss of the high-frequency turbulence signal.
粒子图像测速技术（PIV）是一种成熟的工具，用于在实验室中收集空气和水中的高分辨率速度和湍流数据。实验室实验通常在恒温或恒盐条件下进行，或者在这些属性仅有小梯度的流动中进行。在较大的温度或盐度变化下，由于湍流微结构导致的水或空气折射率变化会产生所谓的光学湍流。我们观察到光学湍流对PIV中的粒子成像有显著影响。对于高马赫数流动，已在空气中描述了折射率变化对PIV的影响，但在这种情况下，畸变是有方向性的。此前尚未有关于水中各向同性光学湍流条件下此类影响的报道。我们研究了在大型瑞利-贝纳德槽中，不同光程长度和湍流强度下光学湍流对PIV成像的影响。结果表明，光学湍流会显著影响PIV测量。根据光学湍流的强度和光程长度，这种影响在后期处理中可能会减轻，这可能会降低噪声并恢复平均速度信号，但会导致高频湍流信号的丢失。
