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Dynamics >>24
Aeroacoustics
gets DES treatment
B. Greschner & F. Thiele, Hermann-Foettinger-Institute
of Fluid Mechanics Berlin University of Technology, Berlin, Germany
The modeling of turbulence
is a vital component of the prediction of broadband noise generated
in complex industrial fluid applications. The non-zonal hybrid DES
approach is applied to efficiently compute the compressible flow
around a rod-airfoil configuration, whereas the far-field sound
propagation is described by an aeroacoustic analogy. Special attention
is paid to the correct prediction of sound field spectra and directivity,
which both show a strong dependency on the turbulence model applied.
DES an optimized approach
In industrially relevant noise prediction scenarios, the correct modeling of the energy dissipation in separated flows is the key to the proper determination of aeroacoustic broadband noise. The application of traditional turbulence modeling techniques, such as (U)RANS, consistently predicts only the large structures and fail to resolve the broadening spectra. Simulating such phenomena, however, does not require Large Eddy Simulation (LES) or even full Direct Numeric Simulation (DNS). Detached Eddy Simulation (DES) is a less expensive hybrid method which operates as a traditional RANS turbulence model in the near-wall region, and in an LES-mode in the outer, separated flow region. One key advantage of this approach is the efficiency due t minimal time and spatial resolution requirements in the boundary layer taken from RANS, not LES. The aeroacoustic analogy (FW-H) is attached by the unsteady flow data on a penetrable surface.
DES and broadening of acoustic spectra
An airfoil located in the wake of a rod results in both periodic (large-scale) and broadband fluctuations. The instationary flow is computed numerically using STAR-CD. The highly threedimensional flow with larger amount of small-scale content represents an advantage of the k-? model. This results in a more accurate prediction of the shedding frequency and the statistical mean values. However, the spectral content of the flow is well reproduced by both SA-DES (Spalart Allmaras) and k-?-DES simulations. Due to the calculation of the rod wake in LES mode, the DES is able to correctly predict the dominating flow physics and turbulent scales. The snapshots of vorticity iso-surfaces plotted in Fig. 2 are comparable for both turbulence models. The far-field noise, calculated by an aero-acoustic analogy [1] is in good agreement with the experimental result for both directivity and magnitude [2].
Conclusions
The basic advantage of the hybrid DES approach for aeroacoustics is clearly demonstrated in terms of computational costs and accuracy of the flow prediction over bluff bodies. The improvement in predictive accuracy is attributed to the performance of LES-mode calculation in the separated turbulent region, which also supports the good acoustic prediction.
[1]
Jacob, M., Boudet, J., Casalino, D. and Michard, M., “A rodairfoil
experiment as benchmark for broadband noise modeling”, Proceedings
of 3rd SWING Aeroacoustic Workshop, Stuttgart, Sep. 2002 [2] Greschner,
B., Thiele, F., Casalino, D. and Jacob, M.C., “ Influence of turbulence
modeling on the broadband noise simulation for complex flows”,
AIAA Paper, No. 2004-2943, Manchester, 2004
Figures
01:Instantaneous
pressure iso-surfaces of k-?-DES
02:Instantaneous vorticity iso-surfaces.
a: SA-DES, b: k-?-DES
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