Low-speed rotating axial and radial fans are frequently used to manage engine temperature by ensuring adequate airflow through heat exchangers, especially at low vehicle speeds or idle. An undesirable side effect of these fans is generation of flow-induced noise, which is an annoyance to the operators and passengers, and a source of community noise, especially for commercial vehicles and heavy equipment. In most cases and particularly for high mass flow configurations, the cooling fan is a major contributor to the overall noise, and in some cases dominates relative to other sources such as engine, transmission, tire, mechanical, or exhaust contributions.
For heavy industry, operator and community noise regulations are imposed by governments, and a failure in the acoustic development of a product can lead to delays and expensive countermeasures to meet the requirements. In addition, cooling fan noise is a perceived quality issue that can affect brand image and customer satisfaction. Given the regulations and growing importance of acoustic comfort in these competitive markets, there is high value in addressing cooling fan flow-induced noise problems as early as possible during product development.
Experimental assessment of fan acoustic performance is limited by the difficulties of physical testing, and is typically performed in stand-alone test bench configurations. The results might not correlate well to the performance when integrated into a real system, as this tends to substantially alter the flow and acoustic environment of the fan. With physical testing, it is very difficult or impossible to identify the source of the noise. Using simulation has the potential to overcome these difficulties, but must meet the challenges of accurately capturing the key physical mechanisms related to flow-induced noise from cooling fans:
• Noise generation is, by definition, a transient phenomenon and traditional CFD codes typically have difficulty accurately predicting transient effects in reasonable timeframes.
• The complex interaction of rotating blades with nearby stationary geometry is a primary source of noise and the typical moving reference frame (MRF) technique fails to capture this effect.
• Tonal noise, especially associated with the blade passing frequency, can have significant dependence on the quality of the incoming flow, rotor casing interactions, existence of rotating stall conditions, and unsteadiness of the flow field.
• Broadband noise, which is usually related to vortex shedding, flow detachments, turbulent boundary layer noise, and tip vortex noise.
• Installation effects, which tend to influence the inlet flow conditions and the acoustic response of the system.
• Radiation of small amplitude pressure fluctuations (acoustics), outside the convective near field.
Exa provides a complete computation aeroacoustics solution with PowerFLOW and PowerACOUSTICS:
• PowerFLOW’s inherently transient solution accurately predicts the complex time dependent turbulent flow structures and the resulting noise sources induced by the cooling fan.
• PowerFLOW’s true rotating geometry capability provides time accurate simulation of rotation and captures all types of interactions more precisely than traditional MRF methods.
• PowerFLOW’s inherently compressible solution predicts the radiated noise simultaneously with the flow induced noise sources.
• PowerFLOW can handle fully detailed geometry and capture all the interactions between the fan and the surrounding components.
• For applications requiring prediction of noise propagation to the far field (community noise), PowerFLOW transient results can be coupled to the PowerACOUSTICS Far Field Noise module, which uses an acoustic analogy method based on the Ffowcs-Williams and Hawkings equation.
• Easy-to-use analysis and state-of-the-art 3D visualization capabilities with PowerACOUSTICS and PowerVIZ provide insight into sources of noise. Band-filtered pressure analyses can be used to isolate phenomena at specific frequency bands of interest (for example, to perform detailed investigation of the blade passing frequency noise).
F. Pérot, S. Moreau, M.S. Kim, M. Henner, D. Neal, Direct
Aeroacoustic Prediction of a Low-speed Axial Fan , AIAA paper 2010-3887, 14th AIAA/CEAS aeroacoustics conference, Stockhlom, June 2010.
F. Pérot, S. Moreau, M.S. Kim, D. Neal, Investigation of the Flow
Generated by an Axial 3-Blade Fan, 13th ISROMAC 2010-082, April 2010
F. Pérot, M.S Kim, V. Le Goff, X. Carniel, Y. Goth, C. Chassaignon, Numerical optimization of the noise reduction of a radial fan using flow obstructions, Fan2012 International Conference, April 2012, Senlys, France
F. Pérot, M.S. Kim, K. Wada, K. Norisada, M. Kitada, S. Hirayama, M. Sakai, S. Imahigasi, N. Sasaki, HVAC blower aeroacoustics predictions
based on the lattice boltzmann method, AJK Conference, AJK2011-23018, Japan
M.S. Kim, F. Pérot, M. Meskine, Aerodynamics and acoustics
predictions of the 2-blade NREL wind turbine using a Lattice Boltzmann Method, 14th ISROMAC, February 2012
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