Jet-Noise Reduction by Impinging Microjets: an Acoustic Investigation Testing Microjet Parameters

Abstract

A system of 36 impinging microjets was implemented on a round jet of Mach number 0.9, and the noise reduction was studied as a function of the microinjection mass flux, the number of microjets blowing, the layout of the blowing microjets, and the microjet diameter. Depending on the microinjection flux parameters, overall jet-noise reduction varied from 0 to 1.5 dB, showing some nonmonotonic behavior due to the change between subsonic to supersonic microjet regimes. The study of the microjet layout showed that the noise reduction decreased when the microjets were too close to each other and that certain configurations of microjet pairs could be favorable; this can be related to the flow structures induced by the microjets. Spectral analysis disclosed different control mechanisms, with highfrequency regeneration for high-injection flux, local control behavior at midfrequencies, and global behavior at low frequencies.

Simulation of Flow Control with Microjets for Subsonic Jet Noise Reduction

“…For both temperatures, the noise modifications caused by the microjets are similar to the literature results of Alkislar (Alkislar et al, 2007) and Castelain (Castelain et al, 2008) for isothermal jets, with essentially the most important spectral reduction observed close to the peak frequency and a reduction of the OASPL levels over the entire range of measurement angles caused by the medium frequency noise decrease. The integrated noise reduction of 1.1 dB observed at 90 degrees is besides very close to that of 1.2 dB measured by Castelain et al on a similar configuration and with the same mass flow rate ratio per microjet r m = 6.7 × 10 −4 .…”

“…In this last case, the larger medium frequency noise reduction observed numerically is partly compensated by the low frequency noise increase caused by the control, that finally leads to very close simulated and measured integrated levels. No experiments are available close to the jet axis but results from the literature (Alkislar et al, 2007;Castelain et al, 2008) indicate, for isothermal jets, that noise reduction is also expected at fore angles with for instance an attenuation of 0.8 dB at 30 degrees for a configuration similar to the isothermal jet considered in the present study. Such a reduction is not reproduced numerically even if noise levels are reduced above 7 kHz and 4 kHz, respectively for the isothermal and the heated jets.…”

Jet Noise Reduction by Microjets: A Parametric Study

“…Thus, if p  d were to reduce the jet Mach number by M, the noise reduction by I would be reconciled via Equation (10). If and why p  d might cause the jet Mach number to drop by M remains unclear.…”

Influence of the Nozzle-Exit Boundary-Layer Thickness on the Flow and Acoustic Fields of Initially Laminar Jets

“…For instance, if one aims at investigating the effects of devices such as chevrons, tabs or microjets, it should be required to ensure that the uncertainties due to the inflow conditions are lower than the variations of the sound pressure levels around −3 dB expected according to experiments. [40][41][42][43] For practical applications, vortex pairing noise appears in particular to be attenuated, which has motivated the developments of LES of initially turbulent jets. 12,44 especially to Jean-Michel Dupays for his technical assistance.…”