VISUALIZATION OF SUPERSONIC SCREECHING JETS USING A PHASE CONDITIONEDFOCUSING SCHLIEREN SYSTEM

This study describes a technique that combines the benefits of focusin g schlieren and phase conditioning. Focusing schlieren blurs and drops contrast of non-critical features whereas phase conditioning emphasiz es periodic flow features, and their combination produces unique resul ts. The supersonic jets that we studied produced an intense tone refer red to as screech. The measured screech tone signal was used as input to the phase conditioning circuit that adjusted the strobing light sou rce to the vertical synchronization pulse of a CCD camera. The sharp v ideo images obtained by this technique could either be frozen or conti nuously swept through one period of screech to acquire a slow motion v ideo record of the jet unsteadiness. Two cases were visualized in this study: first, an underexpanded jet from a convergent rectangular nozz le at various fully expanded Mach numbers. Second, a supersonic jet em erging from a convergent-divergent rectangular nozzle at a design Mach number of 1.4, artificially excited by impingement tones. The results of this study illustrate the usefulness of this system in visualizing oscillatory flows.