Noise Analysis of Axial Flow Fans

The total noise level of the fan is directly proportional to the sixth power of the blade speed. According to analysis, the noise source of the fan is basically dipole in nature. Furthermore, it can be inferred that the noise is caused by the pulsating force exerted by the blades on the air flowing through the fan. It can be considered that there are two sources of discrete frequency noise in wind turbines, one is the propeller type noise caused by the pressure field that moves with the rotor blades, and the other is the blade pulsating noise caused by aerodynamic interference. The distance between the moving and stationary blades of a wind turbine is an important factor in interference noise.
When this distance is small, changes in potential flow and wake can have an impact, and the blades may also act as sound barriers to enhance the sound radiation generated by lift pulsations on adjacent blade rows. This effect depends on the ratio of the wavelength of sound waves related to lift pulsation to the size of the blades acting as a barrier. The change in radiation intensity caused by this effect is most significant within the frequency range where the ratio is greater than 2. So, when the upstream and downstream of a radiating noise blade have the same number of blades, and each blade in these two columns meets a rotor blade at the same time, forming a sound barrier on both sides of the noise source, this effect will be stronger.
As the distance between the moving and stationary blades increases, the reduction of potential flow interference is much faster than the effect of wake velocity changes, and the role of the blades as sound barriers also decreases with increasing distance. From this, it can be seen that at least three parameters affect the magnitude of interference noise: the blade shape of the velocity field waveform (i.e. blade load), the distance between blade columns, and the blade radiation area as the sound source. A very small spacing may produce two acoustic effects. If the force pulsation established by the interference field of the stationary blade on the moving blade makes the moving blade a sound source, while the stationary blade is a sound barrier.