Spectral analysis of sound generation by supersonic impinging jets

Kurzfassung

The flow field in the stagnation point region of a supersonic jet impinging on a plate can be highly unsteady. This can lead to a lift loss in the case of hovering STOVL planes, and to increased noise levels in engineering processes in which supersonic jets are applied as, for example, in hypervelocity oxygen fuel spraying. A parameter governing the unsteady behavior is the distance between the exit of the nozzle from which the jet issues and the plate. At certain nozzle-to-plate distances the bow shock formed ahead of the plate oscillates strongly. The motion of the bow shock has been studied using a multi-exposure visualization technique (Klinkov et al., 2006). Synchronized pressure measurements performed at the stagnation point on the plate have shown that the pressure variations are strongly correlated with the oscillating shock. Under certain conditions these features are connected with the formation and destruction of a recirculation bubble in the impingement region of the supersonic jet (Klinkov Rein, 2004). Subject of the present contribution are the properties of the main frequencies of the unsteady flow field. Peak frequencies are determined by a spectral analysis of the pressure signal obtained at the stagnation point on the plate. Spectra obtained for different nozzle-to-plate distances are analyzed by forming multi-configuration spectra using the nozzle-to-plate distance as a parameter. This is repeated for different pressure ratios of the nozzle, i.e., for under-, ideally, and overexpanded jets. Multi-configuration spectra obtained as described above show a characteristic dependence of peak frequencies on the nozzle-to-plate distance. Frequencies related to primary peaks decrease with nozzle-to-plate distance then jump to a higher value and decrease again. The behavior of the primary peaks depends strongly on the characteristics of the supersonic jet (over/underexpansion). The frequency of primary peaks remains always limited to a small range near 30 kHz. The frequencies of further peaks, called secondary peaks hereafter, decrease about inversely with the nozzle-to-plate distance covering a much broader frequency range then that of primary peaks. The dependence of frequencies of secondary peaks on the characteristics of the jet is small. Considering multi-configuration spectra it will be shown that there exists a distinct difference in the dependence on the nozzle-to-plate distance of primary and secondary peaks. The latter ones are well-known to be caused by a feedback loop (Neuwerth, 1974). Disturbances in the shear layer of the jet are convected to the plate. On impingement sound is generated that propagates to the nozzle exit through the ambient gas. At the nozzle exit new disturbances are excited in the shear layer, and so on. In most publications on acoustic waves produced by impinging jets spectra of oscillations were traced to this mechanism. However, the approach of multi-configuration spectra shows that the behavior of primary peaks is influenced by an additional mechanism. In a model the behavior of primary peaks may be explained by adding the shock layer into the feedback loop or by considering entropy variations caused by an oscillating bow shock (Kuo Dowling, 1996). In addition to the features described so far low frequency bursts are present in the spectra for certain nozzle to plate distances. Then, the corresponding spectrum does no longer exhibit characteristic peaks. This is likely related to so-called zones of silence observed by other authors (Henderson, 2002).