Flow field measurements in a screeching rectangular jet

The detailed flow characteristics of a screeching rectangular jet with a Mach number of 1.69 was examined in order to investigate the role of coherent vortical structures on the characteristics of the screech tones. For this purpose the evolution of the three-dimensional flow field and in particular, the vorticity field is captured in time and space for the first time.; The experiments were carried out for a converging-diverging (C-D) nozzle designed for a properly expanded Mach number of 1.44 with a rectangular exit area of aspect ratio 4. A stereoscopic PIV technique developed with the contribution of the author was successfully applied in order to obtain the periodic evolution of the jet flow by means of phase locked measurements. A near field microphone signal was used to generate the phase signal synchronized with the vortex generation process in the shear layer of the jet. The phase locked ensemble averaged velocity and the vorticity field as the curl of the corresponding velocity field are obtained with good resolution.; The results obtained from the detailed velocity field measurements were first verified to be consistent and in good agreement with the global characteristics of previous observations, and then these measurements were further examined to determine the role of the large scale coherent vortical structures on the screech tone frequency and intensity variation with fully expanded Mach number. The unsteady jet characteristics were analyzed using the phase averaged velocity field. The data was decomposed into its mean, periodic and random components by using conventional statistical methods. The organized coherent structures were identified through the calculation of the space-time correlation function of the velocity field. The area defined by the extent of this function was used to determine the circulation and the convective speed of the coherent structures along the shear layer. An observed frequency jump in the screech tone for underexpanded and overexpanded operating regimes was explained with the measured difference in the convective speed of coherent vortical structures for these two regimes.