Dynamics of an impinging jet. Part 2. The noise generation

Abstract

The aerodynamic noise generated by a subsonic jet impinging on a flat plate is studied from measurements of near-field and surface-pressure fluctuations. The far-field noise measured at 90° to the jet axis is found to be generated by two different physical mechanisms. One mechanism is the impinging of the large coherent structures on the plate, and the other is associated with the initial instability of the shear layer. These two sources of noise radiate to the far field via different acoustical paths.

Flow Features near Plate Impinged by Ideally Expanded and Underexpanded Round Jets

“…The tones are generated by an aeroacoustic feedback mechanism occurring between the nozzle and the plate. A good agreement has been found between the tone frequencies of the present simulated jet, those found in the experimental studies of Henderson et al [7] and Krothapalli et al [6], and the frequencies predicted by the classical feedback model [3,4]. The corresponding comparisons are available in previous papers [16,17].…”

“…For subsonic impinging round jets, the tone frequencies are well predicted by the model proposed by Ho and Nosseir [3] and Nosseir and Ho [4]. Round supersonic jets impinging on a flat plate normally have been investigated experimentally by Henderson and Powell [5], Krothapalli et al [6], and Henderson et al [7], among others.…”

“…For each of these tone frequencies, the corresponding axisymmetric or helical jet oscillation and the associated mode number in the classical model proposed by Ho and Nosseir [3] and Nosseir and Ho [4] have been identified [16,17]. The results are given in Table 3, and they will be used in the next sections.…”

Self-excited oscillations of a high-speed impinging planar jet

“…The Strouhal number is a dimensionless frequency comprised of f, the frequency in Hertz of the dominant acoustic tone, a length scale, L, and a velocity scale, U. Previous authors, such as Ho and Nosseir (1981), Nosseir and Ho (1982), Tam and Ahuja (1990) and Panickar and Raman (2007) who investigated the tone generation process in the impinging axisymmetric jet flow, have used the diameter of the nozzle outlet (D) as the relevant length scale, and the flow velocity of the jet at the nozzle outlet (U o ) as the velocity scale. The same authors have attributed the generation of acoustic tones in impinging jet flows to a feedback cycle, consisting of a coupling between instabilities in the free shear layer and acoustic perturbations provided by the impingement of coherent structures on the plate.…”

“…Various aspects of the feedback excitation mechanism for the subsonic axisymmetric case have been investigated by Ho and Nosseir (1981), Nosseir and Ho (1982), Tam and Ahuja (1990) and Panickar and Raman (2007), among many others. Extensive work on the axisymmetric case involving supersonic flows has been performed by Henderson and Powell (1993), Krothapalli et al (1999) and Henderson (2001).…”