Experimental investigation of the transient bleed valve noise

Abstract

This study presents the first step of an experimental study of the transient bleed valve noise. It was carried out on a simplified TBV geometry composed of a cylindrical inlet pipe leading to a diaphragm or a perforated disk for the purpose of generating pressure drops. Numerous diaphragms and grids have been tested in order to identify parameters that influence the acoustic radiation of the TBV and for NPR (Nozzle pressure ratio) from 1.2 to 3.6 to cover both subsonic and supersonic regimes. A large number of acoustic behaviors have been identified. For diaphragms far field acoustic spectra is dominated by mixing noise for all NPR and by shock-associated noise (screech and broadband shock associated noise (BBSAN)) when the critical value of the NPR delimiting the subsonic and supersonic behavior (NPRc = 1.89) is exceeded. For grids the mixing noise is still present but is composed of two humps. The parametric study allowed to associate the first hump to the noise of an equivalent jet having the smallest diameter encircling the grid perforations while the second is associated to the noise of the outer isolated jets. A first prediction model has thus been proposed based on this double source. Furthermore, the grids offer a significant noise reduction in the audible range with respect to a diaphragm of the same cross-section by shifting the radiation towards the high frequencies. The noise associated with supersonic phenomena (screech and BBSAN) are also strongly reduced and even suppressed in most of the tested cases.

Experimental investigation of the output duct role in the transient bleed valve noise

“…In the case of the diaphragms, similar characteristics as the mixing noise of jet issuing from more conventional nozzles has been observed. 1 The shape of the far-field acoustic spectra agrees with Tam et al similarity spectra 2 and the dual source of the mixing noise have been identified. In the case of the perforated plates, this mixing noise is strongly modified with the appearance of two distinct humps on the acoustic spectra.…”

“…The effect of the perforated plate/diaphragm geometry on these modal oscillations will be studied in Sec. V. Moreover, even if the phenomenon is less pronounced, the two humps characteristic of the perforated plate mixing noise shown in Laffay et al 1 are still present when the output duct is added. Indeed, for perforated plate with largely spaced perforations (Fig.…”

“…This can perhaps be explained by the stronger emergence of the output jet noise in this direction. 1,2 For Θ > 90 • , a strong attenuation of the resonances beyond 2000 Hz also appears. This attenuation is found in a majority of the studied configurations and may be explained by some acoustical reflections on the outlet surface of the duct.…”

“…A slight frequency shift of the different modes can also be observed especially for the higher frequencies and is probably explained by the absence of flow in the model. Thus, the combination of this kind of simple modeling with jet or grid noise models as given in Tam et al 2 and Laffay et al 1 will allow rapidly predicting the broadband radiation generated by this type of discharge systems in the case of a fairly simple duct geometry. Indeed, the curve ∆SP L in Fig.…”

“…In an attempt to reduce the acoustic radiation of such devices, an experimental study has been carried out to identify all the different acoustic sources responsible for this radiation and to propose solutions to attenuate or eliminate these sources. A first part of this study, 1 was focused on the acoustic radiation generated by the discharge of the high pressure flow through perforated plates and diaphragms. Indeed the passage of the flow through these perforations leads to very high speed jets responsible for an important acoustic radiation.…”