Hump-Shaped Broadband Noise on a Fan at Off-design Conditions

D2 - Theoretical & Numerical Methods for Centrifugal Fans

Automotive fan systems are sometime used at off-design conditions, for instance at high flow rate when the car is moving. It has been observed that the resulting under-incidence of the flow on blades can trigger a particular acoustic phenomenon, with increased broadband noise in the frequency range of 3500-4500 Hz.
Some phenomenological studies were conducted by numerical means to understand this mechanism.
Velocities have been extracted from a 3D simulation of the fan, and these local conditions were applied in a compressible Large Eddy Simulation (LES) around the blade profile extruded in 3D. Results have shown that the under-Incidence creates a large boundary layer on the lower side of the profile, starting at roughly 20% of the chord.
At the same time, some vortical structures appear at the trailing edge and are convected in the wake with an almost constant periodicity. Even if this observed frequency corresponds to the hump observed on the fan acoustic spectrum, it does not explain the noise recorded during the experiment on the fan unless the flow fluctuations on the upper side of the profile are taken into account. In this area (where no turbulent boundary layer is developed), some instabilities are predicted by the compressible simulation: pressure and density vary at the frequencies of the structures in the wake, and act as an dipolar acoustic source on the blade. These fluctuations correlate with the frequency of the vortex shedding.
A second profile with a different camber distribution was additionally studied with a second LES. Results have shown that this phenomenon of pressure and density fluctuation can be strongly reduced with a less smooth profile.