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首页> 外文期刊>Experiments in Fluids: Experimental Methods and Their Applications to Fluid Flow >Particle relaxation and its influence on the particle image velocimetry cross-correlation function
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Particle relaxation and its influence on the particle image velocimetry cross-correlation function

机译：粒子弛豫及其对粒子图像测速互相关函数的影响

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摘要

A study of some aspects of tracer particle responses to step changes in fluid velocity is presented. The effect of size distribution within a seed material on measured relaxation time is examined, with polydisperse particles of the same median diameter shown to possess a significantly higher relaxation time than their monodisperse counterparts when measured via a particle image velocimetry algorithm. The influence of a shock wave-induced velocity gradient within a PIV interrogation window on the correlation function is also examined using the noiseless cross-correlation function of Soria (Turbulence and coherent structures in fluids, plasmas and nonlinear media. World Scientific, Singapore, 2006). The presence of a shock is shown to introduce an artificial fluctuation into the measurement of velocity. This fluctuation is a function of the shock position, shock strength, spatial ratio and particle distribution. When the shock is located at the middle of the window, the magnitude of the fluctuation increases monotonically with increasing spatial ratio, increases asymptotically with shock strength, and decreases for increasing particle polydispersity. When the shock is located at the left-hand edge of the window, the magnitude of the artificial fluctuation is highest for intermediate spatial ratios, going to zero at infinitely high and low values. In this instance, particle polydispersity acts to increase the magnitude of fluctuations in measured velocity. In both cases, particle polydispersity serves to broaden the PDF of measured velocity. For the cases presented herein, with a shock located within the interrogation window, the root mean square of the artificial velocity fluctuations reaches values in excess of 30% of the freestream velocity.
机译：提出了对示踪粒子对流体速度阶跃变化的某些响应的研究。检查了种子材料中尺寸分布对测得的弛豫时间的影响，当通过粒子图像测速算法进行测量时，具有相同中值直径的多分散颗粒显示出比其单分散对应物显着更高的弛豫时间。还使用Soria的无噪声互相关函数（流体，等离子体和非线性介质中的湍流和相干结构）研究了PIV询问窗口中冲击波引起的速度梯度对相关函数的影响（新加坡，世界科学，2006年） ）。结果表明，存在电击会在速度测量中引入人为的波动。该波动是冲击位置，冲击强度，空间比和颗粒分布的函数。当冲击位于窗口的中间时，波动的大小随空间比的增加而单调增加，随冲击强度而渐近地增加，而随着颗粒的多分散性增加而减小。当冲击位于窗口的左边缘时，对于中间空间比率，人为波动的幅度最高，在无限高和低值处变为零。在这种情况下，颗粒的多分散性可增加测得速度波动的幅度。在这两种情况下，颗粒的多分散性都可以扩大测量速度的PDF。对于本文提出的情况，在询问窗口内有电击的情况下，人工速度波动的均方根值达到自由流速度的30％以上。

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