Noise reduction properties of a multiple-source schlieren system

Abstract

Schlieren images often contain noise that is caused by density gradients in the air that surrounds the wind-tunnel test section. It has been shown that multiple-source, or sharp-focusing, schlieren systems may reduce the region of space in which the schlieren system is most sensitive to density gradients. In this paper, the potential for noise reduction by minor modifications to traditional schlieren systems is examined. The modifications result in a multiple-source (sharp-focusing) schlieren system using the existing mirrors. Analysis shows that, for a system with an incoherent narrowband source, the imaging system, the slit spacing, and the transverse length scale of the noise govern the depth of focus. Also shown is that the maximum reduction of the noise scales linearly with the number of source slits.

A modified schlieren technique for micro flow visualization

“…However, for micro-scale flow fields the background noise in the schlieren image increases and contrast decreases such that density gradients are difficult to visualize. To increase the sensitivity of the schlieren technique, lens and knife-edge quality must be improved and a more complicated design must be used, adding to experimental effort and expense [3,4]. In the micronozzle flow, schlieren imaging has been used to detect shock wave patterns at pressure conditions above ambient, but the shock wave was not observed at vacuum conditions [5].…”