It is known experimentally that an imperfectly expanded rectangular jet from a thin-lip convergent nozzle emits only a single dominant screech... Show moreIt is known experimentally that an imperfectly expanded rectangular jet from a thin-lip convergent nozzle emits only a single dominant screech tone. The frequency of the screech tone decreases continuously with an increase in jet Mach number. However, for a supersonic jet issued from a bevelled nozzle or a convergent-divergent nozzle with straight side walls, the shock cell structure and the screech frequency pattern are fairly complicated and have not been predicted before. It is shown that the shock cell structures of these jets can be decomposed into waveguide modes of the jet flow. The screech frequencies are related to the higher-order waveguide modes following the weakest link screech tone theory. The measured screech frequencies are found to compare web with the predicted screech frequency curves. Show less
Jets with complex shock-cell structures exist in numerous technological applications. This paper describes a fundamental study of shock... Show moreJets with complex shock-cell structures exist in numerous technological applications. This paper describes a fundamental study of shock-induced flow resonance (commonly referred to as "jet screech") in supersonic jets with spanwise nonuniform shock-cell structures. Experiments that involve flow visualization and detailed mapping of the near field reveal unsteady aspects of shock-induced flow resonances, mode transitions, and directivity of the radiated noise. The following important results about the role of spanwise nonuniform shock-cells emerged: (1) It is possible to have two coexisting, independent feedback loops at nonharmonically related frequencies and different spanwise modes. (2) The same type of spanwise asymmetric mode was produced by two entirely different source configurations. (3) Nozzle geometry significantly altered the intensity and directivity of screech and broadband shock noise. The results presented here provide considerable insight into the fluid dynamics and acoustics of jets with spanwise oblique shock-cell structures and provide grounds for believing that shock-induced noise can be controlled by tailoring nozzle geometry. (C) 1999 American Institute of Physics. [S1070-6631(99)00103-8]. Show less
