Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session G06: Acoustics: Aeroacoustics I |
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Chair: Adrian Sescu, Mississippi State Room: 205 |
Sunday, November 24, 2019 3:48PM - 4:01PM |
G06.00001: Modelling peak supersonic heated jet noise at fixed jet or fixed acoustic Mach number using an acoustic analogy-based non-parallel flow asymptotic approach Mohammed Afsar, Adrian Sescu, Edmondo Minisci This presentation provides a summary of our recent results on supersonic jet noise prediction using the generalized acoustic analogy approach. The analogy poses a formula for the far-field sound as a convolution product of propagator tensor and the fluctuating Reynolds stress auto-covariance tensor. We found that approximating the propagator tensor using a non-parallel flow asymptotic approximation (Afsar et al., 2019. Phil. Trans. A, in press) of the vector Green's function that satisfies the adjoint linearized Euler equations allows accurate prediction of the peak jet noise. That is, in the small observation angle region, the predictions remain within 1-2 dB of experimental data up to a Strouhal number (based on jet diameter) of at least 0.5. We based the analogy on a model of the Reynolds stress auto-covariance that agrees with Large-Eddy Simulations in isothermal conditions. In general, our predictions recover both the spectral quietening observed in heated jets at fixed acoustic Mach number (Afsar et al. AIAA J., vol.49, p.2522, 2011) and the noise enhancement at fixed jet Mach number. We observed that in either of these cases, using the simpler non-spreading locally parallel flow based Green’s function results in a significant under prediction of sound. [Preview Abstract] |
Sunday, November 24, 2019 4:01PM - 4:14PM |
G06.00002: Large eddy simulations of supersonic twin rectangular jets including screech Jinah Jeun, Gao Jun Wu, Sanjiva K Lele In this work the aeroacoustics of supersonic jets including screech tones, issuing from military-style twin rectangular nozzles with aspect ratio of 2:1 and design Mach number of 1.5, is studied numerically. Large eddy simulations (LES) are performed using the unstructured compressible flow solver “CharLES” developed by Cascade Technologies, to fully replicate the nozzle geometry including the upstream duct with flow divider and sharply turning converging-diverging nozzle. Equilibrium wall-model is used to account for the internal boundary layers. Three different nozzle operating conditions, which correspond to two over-expanded conditions and one ideally-expanded condition, are considered. At each operating condition, the far-field acoustic spectra are obtained from the near-field LES data using the Ffowcs Williams-Hawkings method. Both near-field flow statistics and the far-field acoustics are compared with experiments conducted at the University of Cincinnati, to examine predictive capabilities of the LES solver. Interactions between closely placed two jet plumes as they spared are captured, and the effects of nozzle operating conditions on them are discussed. [Preview Abstract] |
Sunday, November 24, 2019 4:14PM - 4:27PM |
G06.00003: Sound Sources in Subsonic Twin Turbulent Jets Arnab Samanta, Nishanth Muthichur, Santosh Hemchandra We analyse the sound from merging identical twin $M_j = 0.9$ turbulent cold jets to explore how the radiation mechanisms differ from corresponding single jets via focusing on the role of jet shear layers in breaking down the organized structures of the individual jets. Such twin jets are known to reduce noise via shielding, at least at their merging planes, while these may dynamically couple and increase sound at the other cross planes, once placed close to each other. Here, we study this via varying the point of first interaction between the pair of jets which include a case where the jets first interact approximately at their potential core breakdown location, while in the other cases they merge some distance upstream and downstream of this point. We follow a hybrid approach where the near-field is computed using an LES, while a Lighthill's-based analogy yields the radiated sound. On comparing with the well-validated single jet results, the twin jet evolution and their sound spectra appear very different, especially for the jets with the closest spacing which also show higher-frequency discrete tones. We seek to explain the observed differences via careful analysis of the respective sound sources using a range of techniques including constructing the respective SPOD modes. [Preview Abstract] |
Sunday, November 24, 2019 4:27PM - 4:40PM |
G06.00004: Wavenumber-frequency spectra of wall-pressure fluctuations in compressible turbulent channel flow Yi Liu, Kan Wang, Meng Wang Knowledge of wall pressure fluctuations in wall-bounded flows is important for predictions of structural vibration and noise. In this study, direct numerical simulations with a high-order, non-dissipative finite difference scheme are employed to accurately capture the spatiotemporal characteristics of wall pressure fluctuations in compressible turbulent channel flows at Mach 0.4 and friction Reynolds number of 180. Acoustic peaks are barely visible in the one-dimensional streamwise wavenumber-frequency spectra. However, in the two-dimensional wavenumber-frequency spectra at the zeroth spanwise wavenumber, acoustic peaks associated with both longitudinal and oblique propagating waves are clearly identifiable, although they are several orders of magnitude weaker than the convective peak. The number of oblique-wave peaks at each frequency matches the theoretical prediction for duct acoustic modes. The effect of a small two-dimensional hump on the channel surface is also investigated. The results suggest that acoustic scattering by the hump vastly increases the acoustic energy for both longitudinal and oblique modes. [Preview Abstract] |
Sunday, November 24, 2019 4:40PM - 4:53PM |
G06.00005: Analysis of Screech Generation in a Cold Supersonic Rectangular Jet with Large-Eddy Simulations Gao Jun Wu, Sanjiva Lele, Jinah Jeun Screech is an aeroacoustic resonance phenomenon found in non-ideally expanded supersonic jets. Using large-eddy simulations, this work studies screech generation for a cold under-expanded rectangular jet at several Nozzle Pressure Ratios (NPRs) around maximum screech. The aspect ratio of the converging-diverging rectangular nozzle is 4:1 and the design Mach number is 1.44. In the noise spectra, screech tones are detected at various observer locations. Spectral Proper Orthogonal Decomposition (SPOD) techniques are used to extract the temporal-spatial coherent structures from the LES data. At the screech frequency, a flapping motion is detected along the minor-axis plane, and a spatially modulating antisymmetric standing wave pattern is observed in the near field as a result of oppositely travelling hydrodynamic and acoustic wavepackets. In contrast to the conventional belief that screech feedback is closed by upstream-travelling acoustic waves outside the jet, recent research has indicated that the closure mechanism could arise from an internal acoustic mode supported by the jet plume. To further investigate this hypothesis, a linear instability analysis for the jet flow will be conducted, and the results will be compared with the SPOD data. [Preview Abstract] |
Sunday, November 24, 2019 4:53PM - 5:06PM |
G06.00006: Linear stability analysis of supersonic jet screech Michael Karp, Tim Flint, M. J. Philipp Hack Jet screech is an undesirable flow phenomenon which can pose severe limitations on the operation of jet engines. Screech is commonly understood as a feedback cycle involving interactions between instability waves, shear layers, shocks and acoustic waves. Our study seeks to advance the insight into the physics of jet screech by means of global linear stability theory. We consider steady laminar axisymmetric jets at supersonic conditions. Both under-expanded and perfectly expanded jets are investigated at various exit Mach numbers. The analysis connects the occurrence of screech to an absolute instability of the under-expanded jet. The features of the eigenmodes are discussed and their relevance to the elements of the screech cycle is explored. An adjoint analysis quantifies the receptivity of jet screech to internal and external perturbations. [Preview Abstract] |
Sunday, November 24, 2019 5:06PM - 5:19PM |
G06.00007: A New Instance of Crackle Noise Joseph Mathew, Sumit Patel Intense crackle noise has been identified and studied in high temperature, supersonic jets of engines operating at high specific thrust. We observed a new instance of crackle in LES of cold, plane, perfectly expanded supersonic jets (Mach 1.5, Reynolds number 100,000) impinging on plane wedges. LES is by explicit filtering, with a new adaptive filtering method for shock capturing. Crackle fronts appear when a detached normal shock stands about half a jet-width from the wedge tip, and the jet column is about 4 jet-widths (nozzle to wedge tip). The edges of this oscillating shock interact with the jet's bounding shear layer structures to emit acoustic wave fronts that steepen as they travel into the far field. Pressure signals exhibit sudden rises followed by gentle relaxations, intermittently. Consistently, pressure distributions along lines in the ambient show the existence and arrival of a sequence of sharp compression fronts. Skewness exceeds 0.4 in the far field. Crackle fronts do not appear in other configurations such as a short column (steady shock), short column and thin wedge (steady, attached, oblique shock), and a long column, thin wedge (large amplitude column oscillations). [Preview Abstract] |
Sunday, November 24, 2019 5:19PM - 5:32PM |
G06.00008: On the Origin of Secondary Tones in Trailing-Edge Noise at Moderate Reynolds Number Flows Tulio Ricciardi, William Wolf Direct numerical simulations are carried out to investigate the flow features responsible for secondary tones arising in trailing-edge noise at moderate Reynolds numbers. Simulations are performed for a NACA 0012 airfoil at freestream Mach number 0.3 at different angles of incidence. The Reynolds number based on the airfoil chord is fixed at $Re=10^5$. Flow configurations are investigated where scattering of boundary layer instabilities lead to tonal noise generation including equidistant secondary tones. Despite the geometric symmetry at zero deg. incidence, the flows become non-symmetric with a separation bubble only on one side of the airfoil. A separation bubble is also observed for the non-zero incidence flow. It is shown that low-frequency motion of the separation bubbles induces a frequency modulation of flow instabilities developed along the airfoil boundary layer. When the airfoil is at zero deg. angle of attack intense amplitude modulation and intermittency are also observed in the flow quantities, resulting in a complex vortex interaction mechanism at the trailing edge. Both amplitude and frequency modulations directly affect the velocity and pressure fluctuations that are scattered at the trailing edge, what leads to secondary tones in the acoustic radiation. [Preview Abstract] |
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