Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session A4: Acoustics I: Aeroacoustics (General) |
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Chair: Philip McCarthy, University of Toronto Room: B112 |
Sunday, November 20, 2016 8:00AM - 8:13AM |
A4.00001: Acoustic mode coupling of two facing, shallow cylindrical cavities Philip McCarthy, Alis Ekmekci Cavity mode excitation by grazing flows is a well-documented source for noise generation. Similarly to their rectangular equivalents, single cylindrical cavities have been shown to exhibit velocity dependent self-sustaining feedback mechanisms that produce significant tonal noise. The present work investigates the effect of cavity mode coupling on the tonal noise generation for two facing, shallow cylindrical cavities. This geometric arrangement may occur for constrained flows, such as those within ducts, silencers or between aircraft landing gear wheels. For the latter configuration, the present study has observed that the tonal frequency dependence upon the freestream Mach number, associated with the single cavity feedback mechanism, no longer holds true. Instead, two simultaneously present and distinct large amplitude tones that are independent (in frequency) of speed, propagate to the far field. These two, fixed frequency tones are attributable to the first order transverse mode, and the first order transverse and azimuthal modes for the two combined cavities and the volume between them. Altering either the cavity aspect ratio or the inter-cavity spacing thus changes the acoustic resonant volume and translates the centre frequencies of the observed tones correspondingly. [Preview Abstract] |
Sunday, November 20, 2016 8:13AM - 8:26AM |
A4.00002: ABSTRACT WITHDRAWN |
Sunday, November 20, 2016 8:26AM - 8:39AM |
A4.00003: Direct numerical simulation of broadband trailing edge noise from a NACA 0012 airfoil Mohammad Mehrabadi, Daniel Bodony Commercial jet-powered aircraft produce unwanted noise at takeoff and landing when they are close to near-airport communities. Modern high-bypass-ratio turbofan engines have reduced jet exhaust noise sufficiently such that noise from the main fan is now significant. In preparation for a large-eddy simulation of the NASA/GE Source Diagnostic Test Fan, we study the broadband noise due to the turbulent flow on a NACA 0012 airfoil at zero degree angle-of-attack, a chord-based Reynolds number of 408,000 and a Mach number of 0.115 using direct numerical simulation (DNS) and wall-modeled large-eddy simulation (WMLES). The flow conditions correspond to existing experimental data. We investigate the roughness-induced transition-to-turbulence and sound generation from a DNS perspective as well as examine how these two features are captured by a wall model. Comparisons between the DNS- and WMLES-predicted noise are made and provide guidance on the use of WMLES for broadband fan noise prediction. [Preview Abstract] |
Sunday, November 20, 2016 8:39AM - 8:52AM |
A4.00004: Large Eddy Simulation of Surface Pressure Fluctuations on a Stalled Airfoil Sanjiva Lele, Joseph Kocheemoolayil The surface pressure fluctuations beneath the separated flow over a turbine blade are believed to be responsible for a phenomenon known as Other Amplitude Modulation (OAM) of wind turbine noise. Developing the capability to predict stall noise from first-principles is a pacing item within the context of critically evaluating this conjecture. We summarize the progress made towards using large eddy simulations to predict stall noise. Successful prediction of pressure fluctuations on the airfoil surface beneath the suction side boundary layer is demonstrated in the near-stall and post-stall regimes. Previously unavailable two-point statistics necessary for characterizing the surface pressure fluctuations more completely are documented. The simulation results indicate that the space-time characteristics of pressure fluctuations on the airfoil surface change drastically in the near-stall and post-stall regimes. The changes are not simple enough to be accounted for by straight-forward scaling laws. The eddies responsible for surface pressure fluctuations and hence far-field noise are significantly more coherent across the span of the airfoil in the post-stall regime relative to the more canonical attached configurations. [Preview Abstract] |
Sunday, November 20, 2016 8:52AM - 9:05AM |
A4.00005: On the intermittent route to resonance for flow through an orifice in a duct Manu Kamin, Joseph Mathew In experiments done by Vineeth et. al. (2015) on flow in a duct-orifice arrangement, flow enters through the duct inlet, and leaves into the atmosphere through the orifice exit, “whistling” was observed at a Reynolds number of 4200, where large amplitude pressure oscillations were observed. At slightly lower Reynolds numbers, bursts of smaller amplitudes of pressure oscillations were observed to appear intermittently. For a similar configuration, LES were carried out. Both whistling and intermittency were observed in the simulations. As air flows from the duct into the orifice, it turns sharply around the corner at the duct-orifice interface, and hence, flow separation occurs, and a shear layer is formed at the mouth of the orifice. The mechanism of whistling was found to be the shear layer within the orifice flapping about and hitting the trailing edge of the orifice periodically, thus causing the shear layer to break and roll up into a vortex. Hurst exponent was measured in the time series data obtained. It was found to gradually drop to zero as the flow approached the state of whistling, since the growth rates of all the long term and short term trends in the time series vanish. A loss of multifractality in the time series was also observed as flow approached whistling. [Preview Abstract] |
Sunday, November 20, 2016 9:05AM - 9:18AM |
A4.00006: Computation and analysis of rotor-noise generation in grid turbulence Junye Wang, Kan Wang, Meng Wang The noise of a ten-bladed rotor interacting with a grid-generated turbulent flow at low Mach number is computed using large-eddy simulation and the Ffowcs Williams-Hawkings extension to Lighthill's theory. The grid turbulence is approximated as convected homogeneous and isotropic turbulence generated by a separate simulation and provided as inflow boundary conditions. The sound pressure spectrum predicted by the simulation exhibits overall agreement with previous experimental measurements in terms of the spectral shape and level. The turbulence ingestion noise is broadband with small peaks at the blade passing frequency and its harmonics. It is significantly stronger than the rotor self-noise generated by blade trailing-edge vortex shedding. Consistent with experimental observations, decreasing the rotor advance ratio at fixed mean inflow velocity leads to an increase in the sound pressure level. Different levels of acoustic compactness assumptions are examined, and the results indicate that the blade chord is acoustically compact over the frequency range of interest. Blade to blade correlations of the acoustic dipole sources are shown to be small. [Preview Abstract] |
Sunday, November 20, 2016 9:18AM - 9:31AM |
A4.00007: Large-eddy simulation of propeller noise Jacob Keller, Krishnan Mahesh We will discuss our ongoing work towards developing the capability to predict far field sound from the large-eddy simulation of propellers. A porous surface Ffowcs-Williams and Hawkings (FW-H) acoustic analogy, with a dynamic endcapping method (Nitzkorski and Mahesh,2014) is developed for unstructured grids in a rotating frame of reference. The FW-H surface is generated automatically using Delaunay triangulation and is representative of the underlying volume mesh. The approach is validated for tonal trailing edge sound from a NACA 0012 airfoil. LES of flow around a propeller at design advance ratio is compared to experiment and good agreement is obtained. Results for the emitted far field sound will be discussed. \\ \textit{Nitzkorski and Mahesh, Phys. Fluids, 2014, 26(11):115101} [Preview Abstract] |
Sunday, November 20, 2016 9:31AM - 9:44AM |
A4.00008: Prediction of Flow-Induced Noise Over a Realistic Automotive Vehicle Jaeyong Jeong, Junshin Park, Donghyun You Turbulent flow interacting with the front parts of an automotive vehicle, such as the cowl-top, A-pillars, and side mirrors are known to be significant sources of acoustic noise. In the present study, sources and propagation of acoustic noise generated over the front parts of a realistic automotive vehicle, known as the DrivAer model are predicted using a novel hydrodynamics-acoustics splitting method. Large eddy simulations are conducted to predict the turbulent flow field which is employed to compute noise sources, while of which accuracy is validated against experimental data. Acoustic fields are predicted using immersed-boundary linearized perturbed compressible equations. Discussion on turbulent flow fields, acoustic sources, and acoustic wave propagation are presented. [Preview Abstract] |
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