these approches numeriques pour la simulation du bruit a large ...

Abstract
This study investigates the capability of unsteady CFD approaches to predict
broadband noise sources. A special configuration is set-up, in which a rod (d=chord/10)
sheds a subcritical vortex street (Red ∼ 48 000) that impinges onto a NACA0012
airfoil. This is a relevant test-case for noise generation in turbomachines, combining
tonal and broadband sources around an airfoil.
The numerical approach undertaken relies on two steps:
• First, unsteady aerodynamic computations are carried out and compared to results
from an accompanying experiment. Comparisons include velocity and pressure spec-
tra, as well as instantaneous flow field snapshots processed using recent techniques
(Normalized Angular Momentum, Proper Orthogonal Decomposition).
• Second, the acoustic far field is computed in the time domain using a version of
the Ffowcs Williams & Hawkings analogy for arbitrary control surfaces, with an
advanced time formulation. Results are compared to far field measurements both in
terms of spectra and Sound Pressure Levels.
Numerical simulation of broadband sources is very challenging because it involves
a wide range of turbulent scales, and sound levels are far below aerodynamic ones.
Two alternative ways of representing the broadband content are investigated (both
implemented according to the two-step approach described above):
• First, Unsteady Reynolds Averaged Navier Stokes (U-RANS) computations are
tested with various k−ω turbulence closures. They all predict satisfactorily the mean
flow quantities as well as the deterministic fluctuations, but the shedding frequency
is overestimated (+25%). By construction, U-RANS does not simulate the turbulent
fluctuations, so a stochastic model of the spanwise fluctuations is implemented in
the acoustic analogy to take into account the broadband dynamics. However, such
a model might not be straightforward to develop for a turbomachine.
• Conversely, Large-Eddy Simulation (LES) directly represents the main turbulent
fluctuations. An auto-adaptive subgrid scale model is designed to be possibly applied
to complex geometries, and is validated on classical benchmarks. On the rod-airfoil,
LES accurately simulates the mean quantities, and the turbulent flow dynamics. The
snapshot comparisons show that the LES simulates the various scales inside the rod
wake, and their interaction with the airfoil, in agreement with the PIV (U-RANS
only predicts the two major POD modes). The broadband spectrum of the far field
is very close to the experimental one for a wide range of frequencies.
LES, despite its high computational cost, is a promising tool for the study of
isolated turbomachinery broadband sources. It can also help to design affordable
broadband models to apply to U-RANS data.