Dynamics of an impinging jet. Part 1. The feedback phenomenon

Abstract

In a high-speed subsonic jet impinging on a flat plate, the surface pressure fluctuations have a broad spectrum due to the turbulent nature of the high-Reynolds-number jet. However, these pressure fluctuations dramatically change their pattern into almost periodic waves, if the plate is placed close to the nozzle (x0/d < 7·5). In the present study extensive measurements of the near-field pressure provide solid support for the hypothesis that a feedback mechanism is responsible for the sudden change observed in the pressure fluctuations at the onset of resonance. The feedback loop consists of two elements: the downstream-convected coherent structures and upstream-propagating pressure waves generated by the impingement of the coherent structures on the plate. The upstream-propagating waves and the coherent structures are phase-locked at the nozzle exit. The upstream-propagating waves excite the thin shear layer near the nozzle lip and produce periodic coherent structures. The period is determined by the convection speed of the coherent structures, the speed of the upstream-propagating waves as well as the distance between the nozzle and the plate. An instability process, herein referred to as the ‘collective interaction’, was found to be critical in closing the feedback loop near the nozzle lip.

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].…”

“…This structure is due to the generation of a hydrodynamic-acoustic standing wave by the aeroacoustic feedback mechanism. The number of cells in the standing wave is equal to the mode number of the feedback mechanism in the model of Ho and Nosseir [3], as shown by Gojon et al [33] using a model of an hydrodynamic-acoustic standing wave proposed by Panda et al [36]. The phase field at θ 0, in Fig.…”

“…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.…”

Numerical investigation of the compressible flow past an aerofoil

“…As shown in figure 7, the upstream-propagating pressure waves are generated by the impingement of largescale structures on the surface of the aerofoil, interact with the shock wave and impart energy to maintain its motion on the surface. Moreover, this kind of feedback (Ho & Nosseir 1981) is typical of flows with self-sustained motions, such as flows over cavities (e.g. Heller & Delfs 1996;Larchevêque et al 2003).…”