Accuray and Advantages of Hybrid Solutions Compared to Direct Noise Calculations for Low-Speed Fan Noise
E2 Noise Prediction by Analytical / Numerical Methods
In most industrial applications it is widely accepted that noise is a concern for fan manufacturers due to comfort trends and regulations. However, fan noise prediction is not a headache for engineers anymore thanks to advanced and mature simulation techniques and increasingly available computational resources. Based on their needs, engineers can still choose methods such as:
- a direct noise calculation from a compressible CFD solution as a reference solution
or hybrid solutions where;
- the compressible flow results are coupled with a time domain acoustic propagation technique
- the incompressible flow results are coupled with a time domain acoustic propagation technique
- the compressible flow results are coupled with a frequency domain acoustic propagation technique
- the incompressible flow results are coupled with a frequency domain acoustic propagation technique
and other non-listed methods.
This presentation focuses on the blade loading noise generated by a virtual low-speed five-blade axial fan which operates under a turbulent jet stream. All five alternative techniques listed above are applied to this fan using commercial finite volume software, which is coupled with a commercial finite element software for the last two bullet points above. It is worth to note that the frequency domain acoustic propagation solution uses a high order finite element model and reconstructs the source terms in frequency domain.
First, the competitiveness is challenged in a free-field radiation setup, to demonstrate that all five simulation approaches converge to the same acoustic levels in an anechoic-room like environment. Both tones and broadband responses are cross-validated using a finite-volume solution in one hand, and a finite element model on the other hand.
Secondly, the hybrid solution is applied to the free-field solution once more, in order to perform a contribution analysis between two zones representative of the leading-edge and trailing-edge of the blades. The leading-edge zone is found to be dominant in the simulations as expected since the fan operates downstream of a jet. This shows how a hybrid solution complements the workflow and how further acoustic insight can be obtained without rerunning the flow solution.
Finally, the acoustic response of an installed fan is computed using the same flow results around the axial fan. The reflection and scattering due to surrounding surfaces are then considered in the finite element acoustic solution. This shows how a frequency domain acoustic solution complements a time domain finite volume flow solution in presence of installation effects.