Influence of turbulence modeling on airfoil unsteady simulations of broadband noise sources (original) (raw)
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LES of the trailing-edge flow and noise of a controlled-diffusion airfoil at high angle of attack
Large-eddy simulations (LES) of flow over a low-speed airfoil at high angle of attack (15 • ) are performed using two different flow solvers, Fluent and CDP, on the same fine structured mesh, whose density was previously shown to provide a reasonably accurate trailing-edge flow for noise predictions at a lower incidence of 8 • . These simulations are compared with detailed pressure measurements made using flush-mounted remote microphone probes for validation and with the previous 8 • results to assess the effect of the fan-operating condition on sound radiation. Excellent agreement is found on the mean wall-pressure coefficient for the Fluent LES. The agreement on the wall-pressure spectra is satisfactory beyond 1 kHz. The observed discrepancies at low frequencies are similar at both incidences. They are shown to have a small impact on the acoustic predictions, which compare favorably with the anechoic wind tunnel measurements. On the contrary, the CDP run shows some extra large structure burst on the suction side and a flapping of the separated shear layer born at the leading edge. This triggers a too-large low-frequency content compared to experiment. † Von Karman Institute, FRIA fellowship, Brussels, Belgium
Direct Noise Computation of the Turbulent Flow Around a Zero-Incidence Airfoil
AIAA Journal, 2008
A large eddy simulation of the flow around a NACA 0012 airfoil at zero incidence is performed at a chord-based Reynolds number of 500,000 and a Mach number of 0.22. The aim is to show that high-order numerical schemes can successfully be used to perform direct acoustic computations of compressible transitional flow on curvilinear grids. At a Reynolds number of 500,000, the boundary layers around the airfoil transition from an initially laminar state to a turbulent state before reaching the trailing edge. Results obtained in the large eddy simulation show a well-placed transition zone and turbulence levels in the boundary layers that are in agreement with experimental data. Furthermore, the radiated acoustic field is determined directly by the large eddy simulation, without the use of an acoustic analogy. Third-octave acoustic spectra are compared favorably with experimental data.
Extensions and limitations of analytical airfoil broadband noise models
International Journal of Aeroacoustics, 2010
The present paper is a state-of-the-art of a special class of analytical models to predict the broadband noise generated by thin airfoils in a flow, either clean or disturbed. Three generating mechanisms are addressed, namely the noise from the impingement of upstream turbulence called turbulence-interaction noise, the noise due to the scattering of boundary-layer turbulence as sound at the trailing edge for an attached flow called trailing-edge noise, and the noise generated due to the formation of a coherent vortex shedding in the near wake of a thick trailing edge, called vortex-shedding noise. Different analytical models previously proposed for each mechanism are reviewed, as declinations of the same basic approach inherited from the pioneer work performed by Amiet in the seventies and based on an extensive use of Schwarzschild's technique. This choice is only an alternative to other models available in the literature and is made here for the sake of a unified approach. Issues dealing with the input data and related to the practical applications to fan noise predictions are rapidly outlined. The validity of the models is ckeched against dedicated experiments with thin airfoils and the limitations as the real configurations depart from the model assumptions are pointed out.
Wall-resolved large eddy simulation for aeroengine aeroacoustic investigation
Journal of the Royal Aeronautical Society, 2017
The work presented here forms part of a larger project on Large-Eddy Simulation (LES) of aeroengine aeroacoustic interactions. In this paper we concentrate on LES of near-field flow over an isolated NACA0012 airfoil at zero angle of attack and a chord based Reynolds number of = ×. A wall-resolved compressible numerical Large Eddy Simulation (NLES) approach is employed to resolve streak-like structures in the near-wall flow regions. The calculated unsteady pressure/velocity field will be imported into an analytically-based scheme for far-field trailing edge noise prediction later. The boundary layer mean and root-mean-square (rms) velocity profiles, the surface pressure fluctuation over the airfoil, and the wake flow development are compared with experimental data and previous computational simulations in our research group. It is found that the results from the wall-resolved compressible NLES are very encouraging as they correlate well with test data. The main features of the wallresolved compressible NLES, as well as the advantages of such compressible NLES over previous incompressible LES performed in our research group, are also discussed.
Large-Eddy Simulations (LES) of flow over a low-speed airfoil is performed using an unstructured-mesh flow solver with an efficient mesh-refinement strategy. Results from three different grids are compared against a reference structured-mesh solution, which was previously shown to provide a reasonably accurate trailing-edge flow for noise predictions. These simulations are compared with detailed pressure and velocity measurements made using flush-mounted remote microphone probes and hot-wire anemometry, respectively. The observed discrepancies are shown to have a small impact on the acoustic predictions, which compare favorably with the anechoic wind tunnel measurements. The effects of finite chord length and exact Green's function on the noise levels are also assessed.
LES of the trailing-edge flow and noise of a NACA6512-63 airfoil at zero angle of attack
Large-eddy simulations (LES) of flow over a low-speed highly cambered airfoil at small negative incidence is performed using two different flow solvers, CFX and Fluent, on a similar multi-block structured mesh. They mimic recent aeroacoustic experiments performed in the small anechoic wind tunnel at the University of Siegen. Focus has been put on the experiments where the airfoil was tripped to remove the extraneous Tollmien-Schlichting noise source. Two different tripping models have been simulated, where the actual serrated device is found to trigger the transition correctly as in the experiment. The LES and Reynolds-averaged Navier-Stokes (RANS) k − ω SST simulations are also compared with detailed velocity measurements made by a 3-D hot-wire in the wake. The LES predicts the wake thickness and deficit much better than the RANS and is the only one to yield the flow separation at the trailing edge. The acoustic predictions from two formulations of Lighthill's acoustic analogy compare favorably with the anechoic wind tunnel measurements at low and mid-frequencies. The formulation by Ffowcs shows better agreement with the tripped boundary layer experiments, whereas Curle's (1955) analogy predicts the untripped airfoil sound radiation better.
Direct Numerical Simulation of the Self-Noise Radiated by an Airfoil in a Narrow Stream
18th AIAA/CEAS Aeroacoustics Conference (33rd AIAA Aeroacoustics Conference), 2012
A direct numerical simulation (DNS) is conducted for the first time of an airfoil that is embedded in a wind-tunnel flow at a realistically high Reynolds number of Rec = 150, 000, based on the chord length. The nominal wind-tunnel speed corresponds to a Mach number of M = 0.25. The simulation domain comprises only the near field around the airfoil; the aerodynamic effect of the wind tunnel is included by using an appropriate set of inflow boundary profiles-a technique that has been used successfully in previous numerical airfoil studies by the authors of this paper. The boundary layer on the airfoil suction side is tripped using a purposely developed immersed-boundary method (IMBM). The results of this simulation are compared to an incompressible large-eddy simulation (LES) and experimental data. Both simulation approaches yield very good predictions of the steady and unsteady flow field and the acoustic far field. * Postdoctoral Associate, winklerj@bu.edu, AIAA member † Senior Lecturer, sandberg@soton.ac.uk, AIAA senior member ‡ Professor, stephane.moreau@usherbrooke.ca, AIAA member
Wall-Resolved Large Eddy Simulation over NACA0012 Airfoil
International Journal of Aerospace Sciences, 2013
The work presented here forms part of a project on Large-Eddy Simu lation (LES) of aeroengine aeroacoustic interactions. In this paper we concentrate on LES of near-field flo w over an isolated NA CA0012 airfo il at zero angle of attack with Re c =2e 5 . The pred icted unsteady pressure/velocity field is used in an analytically-based scheme for far-field trailing edge noise prediction. A wall resolved implicit LES or so-callednumerical Large Eddy Simu lation (NLES) approach is emp loyed to resolve streak-like structure in the near-wall flow regions. The mean and RMS velocity and pressure profile on airfoil surface and in wake are validated against experimental data and computational results fro m other researchers. The results of the wall-resolved NLES method are very encouraging. The effects of grid-refinement and h igher-order nu merical scheme on the wall-resolved NLES approach are also discussed. smth ctr conv J J J 1 ε + = (7) conv J , ctr J , smth J represents the interface flu x, its central difference term and s moothing term, respectively.
A rod-airfoil experiment as a benchmark for broadband noise modeling
Theoretical and Computational Fluid Dynamics, 2005
A low Mach number rod-airfoil experiment is shown to be a good benchmark for numerical and theoretical broadband noise modeling. The benchmarking approach is applied to a sound computation from a 2D unsteady-Reynolds-averaged Navier-Stokes (U-RANS) flow field, where 3D effects are partially compensated for by a spanwise statistical model and by a 3D large eddy simulation. The experiment was conducted in the large anechoic wind tunnel of the Ecole Centrale de Lyon. Measurements taken included particle image velocity (PIV) around the airfoil, single hot wire, wall pressure coherence, and far field pressure. These measurements highlight the strong 3D effects responsible for spectral broadening around the rod vortex shedding frequency in the subcritical regime, and the dominance of the noise generated around the airfoil leading edge. The benchmarking approach is illustrated by two examples: -the validation of a stochastical noise generation model applied to a 2D U-RANS computation; -the assessment of a 3D LES computation using a new subgrid scale (SGS) model coupled to an advanced-time Ffowcs-Williams and Hawkings sound computation. In both cases, the ability of computational fluid dynamics to model the source mechanisms and of the CAA approach to predict the far field are assessed separately.
Large-Eddy Simulation of the Flow Around a Controlled Diffusion Airfoil
In this study, a large eddy simulation of the flow around the Valeo-CD airfoil was carried out. The Valeo-CD airfoil is a thin, cambered, controlled-diffusion airfoil [1-3]. The simulation was intended to be a precursor for an aeroacoustic analysis of the broadband noise radiated from the airfoil. An embedded local mesh refinement technique was used to achieve very fine near-wall resolution while maintaining a coarse mesh away from the airfoil. The maximum achievable limits on the total grid size and the near-wall resolution while using a single PC were also determined in our study. The flow was simulated at a geometric angle of attack of 8° and a Reynolds number of 1.2×105. The spanwise width of the domain was equal to 5% of the airfoil chord length. Periodic boundary conditions were used in the spanwise direction. Experimental observations show that at this angle of attack, the flow exhibits laminar leading edge separation, transition to turbulence after reattachment and vortex sh...