Effect of Free Jet on Refraction and Noise

Abstract

This article investigates the role of a free jet on the sound radiated from a jet. In particular, the role of an infinite wind tunnel, which simulates the forward flight condition, is compared to that of a finite wind tunnel. The second configuration is usually used in experiments, where the microphones are located in a static ambient medium far outside the free jet. To study the effect of the free jet on noise, both propagation and source strength need to be addressed. In this work, the exact Green's function in a locally parallel flow is derived for a simulated flight case. Numerical examples are presented that show a reduction in the magnitude of the Green's function in the aft arc and an increase in the forward arc for the simulated flight condition. The effect of finite wind tunnel on refraction is sensitive to the source location and is most pronounced in the aft arc. A Reynolds-averaged Navier-Stokes solution (RANS) yields the required mean flow and turbulence scales that are used in the jet mixing noise spectrum calculations. In addition to the sound/flow interaction, the separate effect of source strength and elongation of the noise-generating region of the jet in a forward flight is studied. Comparisons are made with experiments for the static and finite tunnel cases. Finally, the standard free-jet shear corrections that convert the finite wind tunnel measurements to an ideal wind tunnel arrangement are evaluated. I. IntroductionForward flight is generally believed to reduce the jet noise emission due to the reduced shear. Experimentally, simulation of the flight and its effect on jet noise is carried out in either an open wind tunnel (OWT) or an ideal wind tunnel (IWT). In the first configuration, the primary jet is surrounded by a co-flow (free-jet) with Mach number M ∞ that simulates the flight Mach number, while the microphones are positioned outside the free stream in the stationary atmosphere. In the second arrangement, also referred to as an infinite wind tunnel, the microphones are positioned inside the co-flow to avoid the ambiguities associated with the refraction correction. In order for the microphones to be in the far field of the acoustic sources, the tunnel flow needs to extend far enough in the span-wise direction. Obviously, in either case, the measurements are carried out with fixed microphones.In practice, the OWT measurements are corrected for the secondary shear layer refraction, using semi-empirical corrections equations. The usual practice (ref. 1) is to replace the free jet shear layer with a vortex-sheet and to consider the source along the jet center line-and use a three-step calibration procedure for angle, distance, and sound amplitude. This procedure takes advantage of the high frequency geometric acoustics arguments, i.e., conservation of acoustic energy along a ray tube, and converts the noise measured outside the free jet to the levels that would be measured in an IWT. These simplifications, among other factors, ignore the difference between the on-axis and offaxis sourc...

Robustness of Reduced-Order Jet Noise Models

“…, which was previously reported in the experiment [47]. In addition, the recommended set of parameters of Khavaran's model originally suggested in [23,24] (Table A1) leads to a large offset of the predicted noise spectra in comparison with the experiment. The GAA model captures the noise spectra for the cold SILOET jet in comparison with the RANS-based results (comp.…”

“…The second considered jet noise model corresponds to the model of Khavaran and co-workers [17,[23][24][25] The third jet noise model corresponds to the Goldstein Generalised Acoustic Analogy (GAA) [26,27].…”

Aeroacoustic catastrophes: upstream cusp beaming in Lilley's equation

“…Ring sources are frequently used in aeroacoustics as a model simplification (e.g. [5,37]). They reflect the turbulence physics by concentrating the source in the shear layer of the jet.…”

Hot Jets and Sources of Jet Noise

“…As recommended by Chidambaram et al [32], value of C λ = 0.14 was used. The local turbulent Prandtl number is then defined as the ratio of the eddy viscosity to the thermal eddy diffusivity where q, in general, is a fourth-rank tensor that correlates velocity-velocity or velocitytemperature fluctuations at two points A and B separated by space and time τ, and G is the relevant Green's function [33]. The product of the GF and its conjugate may be evaluated at the center of the correlation times a phase factor where is directed as and k = ω/c ∞…”

“…where f m (r s , k, θ) is solved numerically as a solution to second-order, self-adjoint, compressible Rayleigh operator (eqn C2), corresponding to mode number m, wave number k = ω /c ∞ (to avoid confusion, notation κ will be used to denotes turbulent kinetic energy TKE in this appendix), and observer angle θ as Using the convolution integral, the GF can be obtained for other types of singularities of interest in jet noise [33]. For example, the GF associated with a moving monopole with source frequency ω s and convection velocity î U c and including operator D is C3where M s = U(r s )/c ∞ and M c = U c /c ∞ are the local acoustic Mach number and convection Mach number respectively, and convective derivative D is defined in eqn (11a).…”