Application of a Discontinuous Galerkin Based CAA Solver for Broadband Noise Prediction
2.5 Noise Prediction by Analytical or Numerical Models
Computational simulation plays a significant role in the development process of modern industrial fans. Computational fluid dynamics (CDF) is widely used for aerodynamic optimization and to gain a fan design with improved efficiency. In addition, the acoustic performance becomes more and more important as it constitutes another unique selling proposition. To obtain a low-noise fan design, efficient computational methods are necessary for the prediction of aerodynamically generated noise. The development of a first principle based broadband fan noise prediction capability is the aim of a cooperation between the industrial partner ebm-papst and the department of technical acoustics of the German Aerospace Center (DLR). A recently developed computational aeroacoustics (CAA) solver DISCO++ is used for broadband noise assessment utilizing stochastically generated sources.
The mechanism of noise generation by the flow field of an axial fan is a highly complex three dimensional process. A prominent example is the interaction between blade-tip vortices with the surrounding tip geometry and trailing edge noise from the fan blades. An efficient option for spatial discretization of such highly complex geometric features is provided by unstructured tetrahedral meshes. Such an approach is used in the CAA code DISCO++ to solve the acoustic perturbation equations (APE) with the discontinuous Galerkin (DG) method.
To provide a method with comparatively manageable numerical effort a two-step hybrid approach is chosen. In a first step, a Reynolds Averaged Navier Stokes (RANS) simulation is performed to obtain the stationary flow field and statistical turbulence parameters. Thereupon, time-resolved synthetic turbulence is generated by the Fast Random Particle Mesh (FRPM) method on a separate Cartesian grid. From the reconstructed stochastic turbulence an acoustic source term is computed and passed to DISCO++ which in turn computes the sound propagation in the CAA domain.
Within this work the results of the DISCO++ simulations of a five bladed ducted axial fan using an acoustic source term based on vorticity structures are presented. The results are compared with measurement and similar computational data. A good agreement over a wide range of frequencies is achieved. It is believed that a vorticity based acoustic source provides the capability to model anisotropic effects of vortex stretching in areas of highly accelerated flows occurring in the tip gap area.