A rod-airfoil experiment as a benchmark for broadband noise modeling (original) (raw)

Influence of turbulence modeling on airfoil unsteady simulations of broadband noise sources

11 th AIAA/CEAS …, 2005

In the process of simulating the trailing edge noise of an airfoil, performing the unsteady simulations of these broadband noise sources cheaply and accurately remains a major challenge. In the present study, two different fine turbulent unsteady simulations are tested on a thin controlled diffusion airfoil at low speed and low angle of attack. The flow conditions correspond to a recent aeroacoustic experiment run in the large ECL anechoic open-jet windtunnel for which detailed wall pressure and wake velocity measurements are available at the same time as the far field acoustic sound. The first turbulence model used is a Large Eddy Simulation with the standard Smagorinsky sub-grid scale model, the second one is a k--based Detached Eddy Simulation with the same sub-grid scale model. In order to stick with our objective of a simulation on a single fast PC workstation, several grid refinements in all directions have also been tested. The present final topology has up to six different grid levels. The major flow features are captured by the Large Eddy Simulation and the mean flow quantities compare favorably with the measurements. Significant differences are observed with the Detached Eddy Simulation which fails to capture the experimental short laminar separation bubble at the leading edge and consequently misses the transition point. The Large Eddy Simulation overpredicts the size of this separation bubble and the use of the MARS scheme necessary for stability reason on all the grids tested, also prevented a good prediction of the wall pressure fluctuations close to the trailing edge and therefore consequent noise predictions. This study therefore stresses the high sensitivity of such simulations of low speed transitional flows over thin airfoils.

Aerodynamic Noise Prediction for a Rod-Airfoil Configuration using Large Eddy Simulations

20th AIAA/CEAS Aeroacoustics Conference, 2014

Aerodynamic noise produced by aerodynamic interaction between a cylinder (rod) and an airfoil in tandem arrangement is investigated using large eddy simulations. Wake from the rod convects with the flow, impinges of the airfoil to produce unsteady force which acts as a sound source. This rod-airfoil interaction problem is a model problem for noise generation due to inflow or upstream-generated turbulence interacting with a turbomachine bladerow or a wind turbine rotor. The OpenFoam and Charles (developed by Cascade Technologies) solvers are chosen to carry out the numerical simulations. The airfoil is set at zero angle of attack for the simulations. The flow conditions are specified by the Reynolds number (based on the rod diameter), Red = 48 K, and the flow Mach number, M = 0.2. Comparisons with measured data are made for (a) mean and root-mean-squared velocity profiles in the rod and airfoil wakes, (b) velocity spectra in the near field, and (c) far-field pressure spectra and directivity. Near-field flow data (on-and off-surface) is used with the Ffowcs Williams-Hawkings (FW-H) acoustic analogy as well as Amiet's theory to predict far-field sound. Disciplines

Prediction of Rod-Airfoil Interaction Noise Using the Ffowcs-Williams-Hawkings Analogy

AIAA Journal, 2003

Sound generated at low Mach number by an airfoil in the wake of a rod is investigated numerically. The Gaussian spanwise loss of coherence of the vortex shedding is shown to have a signi cant in uence on the broadband noise. Spanwise effects are successfully introduced into a time-domain formulation of the Ffowcs-Williams-Hawkings analogy, which is applied to aerodynamic data computed on various contours around the source region. It is shown that a careful choice of these contours is required. The ow eld is obtained from a two-dimensional Reynolds averaged Navier-Stokes calculation. Computed far-eld spectra compare very well to measurements obtained in an accompanying experiment.

Uncertainty quantification of the far-field noise from a rod-airfoil configuration

A Large-Eddy Simulation (LES) solver together with a Ffowcs-Williams and Hawking (FW-H) solver are coupled with a non-intrusive stochastic collocation method to propagate the geometric uncertainty observed in a rod-airfoil experiment. The LES are performed for several displacements of the rod with respect to its nominal position in the cross-wise direction. The positions are chosen to match the Clenshaw-Curtis (CC) points. The acoustic fields are obtained using an FW-H analogy on aerodynamic results sampled over the time on a porous surface embedded in the mesh. An uncertainty quantification is then performed on the mean and fluctuating root-mean-squared (rms) velocity at several transverse profiles, on the mean wall-pressure coefficient C p on the airfoil and on both the acoustic spectra and the directivity. The uncertainty of the rod position only partially recovers the discrepancies seen in the experimental data. An uncertainty in the position of the rod of ±0.004 m leads to an uncertainty of 0.5-1 dB in the Overall Sound Pressure Levels (OASPL).

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.

Three-dimensional predictions of the rod wake-airfoil interaction noise by hybrid methods

2007

Sound generated by an airfoil in the wake of a rod is predicted numerically by two different hybrid CFD/CAA approaches (Ma = 0.2). The configuration is a symmetric airfoil one chord downstream of a rod, whose wake contains both periodic and broadband vortical fluctuations. In particular, a significant broadening of the main Strouhal peak has been observed at subcritical vortex shedding conditions. This study addresses the overall ability of both CFD/CAA hybrid approaches to model broadband noise sources. The first approach computes the aerodynamic noise by solving the linearized perturbed compressible equations (LPCE) for the noise propagation, with the acoustic sources and hydrodynamic flow variables computed from the incompressible Large Eddy Simulation (iLES) using a computional grid of approximately 3 million grid cells and high-order compact finite difference schemes. The second approach uses the unsteady aerodynamic field of a compressible Detached Eddy Simulation (DES) and a ...

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.

Noise Radiated by a High-Reynolds-number 3-D Airfoil

11th AIAA/CEAS Aeroacoustics Conference, 2005

A large-eddy simulation (LES) of the flow around a NACA0012 airfoil at a chord-based Reynolds number of 5.0 × 10 6 and a Mach number of 0.22 is performed, and its results are compared with experimental data. The airfoil is placed without incidence to the turbulence-free incoming flow. The boundary layer in this configuration is expected to be initially laminar, and to transition to a turbulent state along the second half of the airfoil chord, and as such constitutes a challenging test-case for LES computations to reproduce. The LES calculation is performed with a parallel code resolving the full compressible Navier-Stokes equations on structured curvilinear grids with optimized explicit high-order finite-difference schemes and filters. A preliminary two-dimensional simulation and a full three-dimensional simulation are performed for the same airfoil configuration. The twodimensional simulation shows substantial discrepancies with available experimental data. On the other hand, flow results from the three-dimensional simulation compare favorably, and the LES is shown to capture boundary layer dynamics reasonably well. These results are a first step towards the development of the direct computation of noise emitted by a high-Reynolds-number airfoil.