Broad Band Shock Associated Noise Modelling for High-Area-Ratio Under-Expanded Jets

Broadband Shock Associated Noise (BBSAN) is an important component of supersonic jet noise for jets at off-design conditions when the pressure at the nozzle exit is different from the ambient. Two high area ratio under-expanded supersonic jets at Nozzle Pressure Ratios (NPRs) 3.4 and 4.2 are considered. The jets correspond to conditions of the experiment in the Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC) in the Supersonic Jet Facility of Monash University. Flow solutions are obtained by the Large Eddy Simulation (LES) and Reynolds Averaged Navier-Stokes (RANS) methods. The solutions are validated against the Particle Image Velocimetry (PIV) data. For noise spectra predictions, the LES solution is combined with the time-domain Ffowcs Wiliams -Hawkings method. To probe accuracy of the reduced-order method based on acoustic analogy, the RANS solutions are substituted in the Morris and Miller BBSAN method, where different options for modelling of the acoustic correlation scales are investigated. The noise spectra predictions are compared with the experimental data from the non-anechoic LTRAC facility and the NASA empirical sJet model. Apart from the low-frequencies influenced by the jet mixing noise, the RANS-based acoustic predictions align with those from LES for most frequencies in the range of Strouhal numbers (St) 0.4<St<2 within 1-2 dB

Analysis of the non-parallel flow-based Green's function in the acoustic analogy for complex axisymmetric jets

This paper considers how a complex axisymmetric jet modifies the structure of the propa- gator tensor in Goldstein’s generalized acoustic analogy. The jet flow we consider is in general a dual stream flow that operates either as a single jet or a complex co-axial jet flow. The latter of which is of interest to turbofan engine manufacturers. The form of the acoustic analogy that we use here is based on our recent work on jet noise modeling (Afsar et al. 2019, PhilTrans. A., vol. 377) that highlighted the importance of non-parallel flow effects in the correct calcu- lation of the propagator. The propagator calculation takes advantage of the fact that mean flow non-parallelism enters the lowest order asymptotic expansion of the former at sufficiently low frequencies of the same order as the jet spread rate. Whilst this might seem restrictive, our previously reported calculations at high subsonic and mildly supersonic jets indicate that the subsequent jet noise predictions remain accurate up to the peak frequency (typically at a Strouhal number based on jet velocity and diameter of ≈ 0.5 − 0.6) for the small angle acoustic radiation. One of critical assumptions of this approach is that the mean flow speed of sound squared is given by either the Crocco relation (in unheated jets) or the Crocco-Busemann relation for heated flows. Our analysis for the dual stream complex axisymmetric jet however shows that the latter assumption (in the form of Crocco-Busemann formula) is no longer an accurate representation of the speed of sound variation. We therefore present a more general form of the asymptotic analysis than that used in Afsar et al. (2019a & b). For the complex jet mean flow field, the mean flow speed of sound is otherwise arbitrary but must remain a single-valued function of the streamwise mean flow. The predictions based on this approach are shown to remain accurate up to the peak frequency. We discuss how to extend the range of validity by utilizing a suitable composite asymptotic solution for the Green’s function problem

Effect of large-scale mixing on the axisymmetric structure of turbulence correlations in complex dual stream jets

Dual-stream flows are a ubiquitous feature of turbofan engines used in civil aviation. In this paper we analyze the spatial structure of turbulence correlations in a high speed round coaxial jet operating at heated conditions. In particular we consider the effect of axisymmetry of a second rank correlation tensor and the usual fourth order Reynolds stress auto-covariance tensor that enters the Goldstein’s generalized acoustic analogy formulation. The invariants of these tensors can be reduced to a simpler form depending on whether isotropy or axisymmetry was assumed. We show that an axisymmetric turbulence approximation remains accurate in the core region but tends to break down in the bypass stream and especially in the interfacial region between both streams where high level of mixing of turbulence takes place. In the paper we present some of our latest results and provide a road map for the future calculations that we have planned

Statistical analysis of high-speed jet flows

The spatiotemporal dynamics of pressure fluctuations of a turbulent jet flow is examined from the viewpoints of symbolic permutations theory and Kolmogorov-Smirnov statistics. The methods are applied to unveil hidden structures in the near-field of the two jets corresponding to the NASA SHJAR SP3 and SP7 experiments. Large Eddy Simulations (LES) are performed using the high-resolution Compact Accurately Boundary-Adjusting high-REsolution Technique (CABARET) accelerated on Graphics Processing Units (GPUs). It is demonstrated that the decomposition of the LES pressure solutions into symbolic patterns of simpler temporal structure reveals the existence of some orderly structures in the jet flows. To separate the non-linear dynamics of the revealed structures from the linear part, the results based on the pressure signals obtained from LES are compared with the surrogate dataset constructed from the original data

Effect of tensor representations for high-order turbulence correlations in complex axi-symmetric flow fields

Dual-stream flows are common feature of turbofan engines in civil aviation. For jet flows with a significant (i.e. proportional of total mass flow) bypass stream, transport of momentum through mixing between streams may cause an apparent departure of axi-symmetric turbu- lence conditions. One of the pertinent features of such departure is the difference in amplitude of transverse diagonal components of velocity and Reynolds stress based correlation function– that is correlation tensors of rank 2 and 4 respectively. Since these functions play a direct role in, among other things, the sound generation process, their accurate representation into an irreducible tensor form is vital in Engineering analysis. This paper builds on our previ- ous work (AIAA 2020-2573, [1]) in which we assessed the kinematic structure of generalized auto-covariance tensor using turbulence data obtained by highly resolved Large-Eddy Simu- lations of complex dual stream jets. Now, however, we extend this work further by showing how a generalized form of the axisymmetric representation theory of the appropriate tensor correlation functions can be determined by exploiting the experimental observation that such correlation metrics are localized in small azimuthal separations for axisymmetric jets. The paper summarizes the new theory and shows initial comparisons of an appropriate tensor form using LES data reported in our previous work

Correlation Analysis of High-Resolution Particle Image Velocimetry Data of Screeching Jets

The authors would like to acknowledge the support of the Australian Research Council, the Research at Cloud Monash National eResearch Collaboration Tools and Resources project funded by the Australia Commonwealth Government, and the School of Engineering and Materials Sciences at Queen Mary University of London

Determination of atomic hydrogen in hydrocarbons by means of the reflected electron energy loss spectroscopy and the X-ray photoelectron spectroscopy

Elastic peaks electron spectroscopy (EPES) is a perspective tool for measuring the hydrogen atomic density in hydrocarbons. It is known that hydrogen elastic peaks overlap inelastic energy loss spectra. This fact complicates the quantitative interpretation of EPES spectra. In this paper, a novel technique based on the joint use of EPES and X-ray photoelectron spectroscopy (PES) is proposed. A key part of the method is the inelastic scattering background subtraction which is performed in two steps. At the first step, differential inelastic scattering cross-sections are retrieved from PES spectra, while at the second step, the retrieved cross-sections are used to remove the inelastic scattering signal from EPES spectra. Both REELS and PES spectra are described on the base of the invariant imbedding method forming a consistent framework for the surface state analysis. A good agreement is obtained between calculated spectra and experimental data