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Estimation of convection speed in underexpanded jets from schlieren pictures

机译：从schlieren图片估计膨胀不足射流中的对流速度

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In this paper, the quality of the estimation of the convection velocity in jet shear layers using schlieren pictures is investigated. The aim is to discuss whether the convection velocity is likely to be biased if determined from schlieren images obtained at a high frame rate, as in previous experiments using the phase shift method. For this, a numerical procedure is developed in order to generate schlieren-like images on the basis of simulation data, and applied to the results provided by the large-eddy simulation (LES) of an under-expanded round jet at an ideally expanded Mach number of 1.56. The results obtained from the schlieren pictures are compared with those obtained directly from the LES density fields. It is notably found that the location of the maximum of gray level fluctuations in the schlieren pictures corresponds well to that of the maximum of density fluctuations, and that the convection velocity estimated for low frequencies using schlieren pictures is underestimated for small separation distances between the two points used for the phase shift calculation.
机译：本文研究了使用schlieren图像估算射流剪切层中对流速度的质量。目的是讨论如从先前以相移方法进行的实验中那样，如果从以高帧频获得的纹影图像中确定对流速度是否可能偏向于对流速度。为此，开发了一种数值程序，以便在模拟数据的基础上生成类似schlieren的图像，并将其应用于在理想扩展的马赫数下扩管不足的圆形射流的大涡模拟（LES）所提供的结果数为1.56。从schlieren图片获得的结果与直接从LES密度场获得的结果进行比较。值得注意的是，在schlieren图片中，最大灰度波动的位置与密度波动最大值的位置非常吻合，并且对于两者之间的较小分隔距离，低估了使用schlieren图片为低频估计的对流速度用于相移计算的点。

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