Quantitative schlieren measurements of coherent structures in a cavity shear layer

Quantitative flow-field data were obtained in a planar shear layer spanning an open cavity with an extension of the schlieren method. The technique is based on the measurement of light-intensity fluctuations in a realtime sch lieren image. Data were collected using a fiberoptic sensor embedded in the imaging screen coupled to a photodetector. Time-resolved measurements of t he instantaneous density gradient at a point in the two dimensional flow cr oss section were thus obtained. Detailed surveys were carried out with both the optical instrument as well as a hot wire at a Mach number of 0.25 and with the optical instrument alone at a Mach number of 0.6. A comparison of the results shows that the non-intrusive technique can accurately measure t he growth rates of instability waves in the initial "linear" region of the shear layer. The density-gradient fluctuations measured at different locati ons (and times) were synchronized by using a microphone inside the cavity a s a reference and integrated to yield profiles of the density fluctuations associated with the dominant large-scale structures in the shear layer. Suc h quantitative visualization is expected to clarify the mechanism of sound generation by shear-layer impingement at the cavity trailing edge and eluci date the nature of this sound source.