SIMULATION OF FLOW NOISE GENERATION IN A CIRCULAR CEILING DIFFUSER - A COMPARISON BETWEEN A SILENT AND A NOISY DESIGN BY USING CFD AND LIGHTHILL'S ACOUSTIC ANALOGY

In this paper, the internal flows inside two prototype ceiling diffusers are modelled by using standard CFD (Computational Fluid Dynamics) software. Based on the modelled velocity and density fields, the function generator of the CFD code is used to compute the source fields of the Lighthill's wave equation, telling the yield of flow noise at each point. Even though the two diffuser geometries closely resemble each other, the difference in their noise generation is very distinct. It is demonstrated that LES (Large Eddy Simulation) with very short time steps gives the source field accurately enough to resolve the small flow details responsible for the noise generation, and can reproduce the difference between the two designs. This implies that very short time step LES is a useful tool in locating the noise sources and comparing the noise productions of alternative geometries of flow handling components. A more advanced way to exploit a simulated velocity field would be to enter it into a separate acoustic software, calculating the source field in the Fourier domain and solving the exact acoustic field in the far field. Whichever approach is selected, the results suggest that prototype testing in HVAC industry could be cut down by applying computer simulation in flow noise analysis.