Numerical Investigation of the Refraction Effects by Jet Flows in Anechoic Wind Tunnels, with Application to NASA/LaRC Quiet Flow Facility

Abstract

In regard to the problem of aircraft noise mitigation, the present study focuses on the refraction effects to be possibly induced by anechoic facility jet flows on the measured acoustic signatures, during typical airframe noise experiments. To this end, Computational AeroAcoustics (CAA) calculations based on the solving of the Perturbed Euler Equations (PEE) are conducted, enabling the estimation of the refraction effects characterizing a typical open-jet, anechoic wind tunnel, namely the NASA Langley's Quiet Flow Facility (QFF). Coming along with their validation against analytical results obtained through a Ray Tracing (RT) technique, the analysis of these CAA/PEE calculations highlights the refraction effects by the facility jet flow, delivering preliminary insights about what are the key parameters (jet height, jet curvature, jet spreading angle, shear layer thickness, etc.) to play a major role in these refraction phenomena.

Landing Gear Noise Identification Using Phased Array with Experimental and Computational Data

“…However, an indirect objective of the present study was to assess the ability of noise localization methods to account for the refraction effects of realistic background flows when equipped properly (e.g., the round spreading-out jet of the C19 anechoic wind tunnel). Therefore, in the present case, the propagation kernel was allotted a geometrical acoustics (GA) approach based on works by Candel [39,40]; implying solely a highfrequency approximation (i.e., assuming an acoustic wavelength that is small enough compared with the length scale of flow heterogeneities), this approach accurately accounts for the refraction effects of any type of mean flow, as was recently confirmed through a dedicated numerical study [41] by the present authors. Please note that, in the present case, the GA-based propagation kernel was each time adjusted to the configuration considered; whenever the latter configuration involved a homogeneous medium (e.g., the F2 experiment, first CFD-CAA computation), the latter kernel was reduced to the convected Helmholtz equation (Green function).…”

An advanced hybrid method for the acoustic prediction

“…43 Relying on the so-called 'porous surface' FWH integration technique. 44 Especially for this particular tonal component, which acoustic energy is more likely to be confined within the jet region because of multiple backscattering of the radiating waves by the shear layer interfaces [53].…”

“…Table 2 provides the set of weighting coefficients associated with either the 11-point (6th order) or 15-point (8 th order) IOFD scheme. As was said, these two IOFD schemes were validated on the basis of various academic test cases [29] (among which the one of Section 5.4), being then applied to various problems [31,52,53] (among which the aerodynamic problems provided in Sections 6.1 and 6.2, respectively).…”

“…As a corollary, regarding the present CAA-based hybrid approach and weakly-coupled CFD-CAA calculation matters, such a relaxation of the constraints weighting on the CAA stage can be directly turned into a beneficial increase of the CFD storage density -which, at the present date, constitutes the best way for minimizing the signal degradation to be possibly induced by the manipulation of CFD data [43]. These IOFD schemes were thoroughly validated on the basis of various test cases [29][30][31] (among which the one of Section 5.3), before they were applied to various problems [31,52,53] (among which the aerodynamic problems provided in Section 6).…”

“…Table 4 lists the weighting coefficients f j associated with IOFD filtering schemes of either 11-point (6 th order) or 21-point (12 th order). Please, note that the two latter schemes were thoroughly validated on the basis of various test cases [30], before they were applied to various problems [31,52,53] for which they were used along with IOFD derivation schemes, offering thus to maintain the accuracy level reached by the latter.…”