Wake-Airfoil Interaction as Broadband Noise Source: A Large-Eddy Simulation Study

Abstract

A large-eddy simulation is carried out on a rod-airfoil configuration and compared to an accompanying experiment as well as to a RANS computation. A NACA0012 airfoil (chord c = 0.1 m) is located one chord downstream of a circular rod (diameter d = c/10, Red = 48 000). The computed interaction of the resulting sub-critical vortex street with the airfoil is assessed using averaged quantities, aerodynamic spectra and proper orthogonal decomposition (POD) of the instantaneous flow fields. Snapshots of the flow field are compared to particle image velocimetry (PIV) data. The acoustic far field is predicted using the Ffowcs Williams & Hawkings acoustic analogy, and compared to the experimental far field spectra. The large-eddy simulation is shown to accurately represent the deterministic pattern of the vortex shedding that is described by POD modes 1 & 2 and the resulting tonal noise also compares favourably to measurements. Furthermore higher order POD modes that are found in the PIV data are well predicted by the computation. The broadband content of the aerodynamic and the acoustic fields is consequently well predicted over a large range of frequencies ([0 kHz; 10 kHz]).

Characterisation of sheet cavity noise of a hydrofoil using the Ffowcs Williams–Hawkings acoustic analogy

“…Acoustic analogies have been widely used for aeroacoustic noise characterisation [31,32,33]. Among the research related to marine propeller noise is the work by Ianniello et al [7] who investigated the noise induced by a full scale ship in non-cavitating conditions using a porous FW-H analogy coupled with a RANS solver.…”

“…This could be considered sufficient for an aeroacoustic analysis, where it is often practical to simulate only a fraction of the span [49,32,33,6]. Doing so limits the size of the largest turbulent length scale that may be captured [32].…”

“…This could be considered sufficient for an aeroacoustic analysis, where it is often practical to simulate only a fraction of the span [49,32,33,6]. Doing so limits the size of the largest turbulent length scale that may be captured [32]. For a compressible simulation, the maximum permitted wavelength propagating in the span-wise direction is similarly limited, effectively only allowing for certain frequencies to be resolved in a three-dimensional manner for a given domain width [6].…”

“…A typical approach is outlined by Boudet et al [32], where a coherence function is used to correct the radiated sound pressure levels given the simulation and experimental domain sizes.…”

Characterization of the Noise Sources in a Rod-Airfoil Configuration by Means of Time-Resolved Tomographic PIV

“…Casalino 2 developed an aeroacoustic code based on a porous Ffowcs-Williams and Hawkings in combination with URANS simulations for the acoustic prediction. Improvements in the evaluation of the broadband spectral component were achieved by Boudet et al 3,4 using LES simulations which allowed the detection of the sub-Kármán vortical structures in the cylinder wake embedded in the larger Kármán rollers which are responsible for emission at higher frequency. In both the cases the characterization of the noise sources relied on numerical data obtained by CFD computations.…”