Estimation of convection speed in underexpanded jets from schlieren pictures

Abstract

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 framerate, 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.