Numerical Analysis of the Acoustic Field of an Axial Fan at Disturbed and Undisturbed Inflow Conditions

2.5 Noise Prediction by Analytical or Numerical Models

The flow in the tip region of axial fans plays a major role in the aerodynamic losses, thermal loads and acoustic emissions. In this paper the flow and the acoustic field of an axial fan will be analyzed by a hybrid CFD/CAA method.
In a first step, the tip clearance flow in a ducted axial fan is predicted by LES. The simulations are based on a multi-block structured mesh with 140 million grid points for a single blade which comprises a 72 degree segment of a rotating axial fan with periodic boundary conditions in the circumferential direction. The turbulent flow is simulated at a Reynolds number of 936000 operating at a design and off-design condition for various tip gap sizes in order to determine the acoustic sources. Computations are performed with undisturbed inflow as well as disturbed inflow conditions using a synthetic turbulence generation method (Roidl, B. et al. Int. J. Heat Fluid Flow 2013). Comparisons of the simulation results with the experimental data (Tao, Z. and Carolus T.H. ASME 2013) including spectra of the surface pressure fluctuations show good agreement.
In off-design operating condition the tip clearance vortex interacts with the downstream blade generating a cyclic transition with distinct interaction frequency on the suction side of the blade near the tip. The turbulent interaction at the inlet dissipates the tip leakage vortex and triggers transition on a larger surface area of the blade. The acoustic field on the near field and the far-field is determined by solving the Acoustic Perturbation Equations (APE) on a mesh for a single blade consisting of approx. 1.1 billion grid points. The maximum resolvable frequency achieved by the computational mesh is about 10 kHz. The acoustic source terms are determined from the flow field results. The acoustic results in terms of the noise spectra are in a good agreement with experimental data and show an increasing tip gap size to lead a higher broadband noise level. More detailed results with turbulent inflow conditions and acoustic source localization will be presented in the conference paper.