Quantitative schlieren visualization

Abstract

Using a closed circuit television camera (CCTV), a method was developed to directly obtain quantitative sound pressure measurements from the sound fields imaged in a Schlieren system. By using the exact timed format of the composite video signal and a series of time delay circuits, it was possible to take vertical intensity scans of a Schlieren image at any position on the television raster. The method used the camera's horizontal blanking pulse to trigger a broadband, high-speed sample-hold amplifier. This amplifier was triggered to sample each of the horizontal scan lines at a selected time in the horizontal scan sweep and to feed this value into a transient recorder. The signal was digitized and a continuous analog voltage signal corresponding to the black-white information from a Schlieren image was displayed on a scope. The same analog signal was also plotted on an X-Y recorder. The ease with which the Schlieren system can be operated is evident from the fact that only a single calibration curve at each frequency was needed. The curves plotted applied transducer voltage against the video output level. Thus, by comparing the video output levels for several vertical image intensity scans on a Schlieren image, relative sound pressure measurements were routinely obtained.