  In this study, tonal noise produced by an isolated centrifugal fan is
investigated using unsteady Reynolds-averaged Navier-Stokes (URANS) equations.
This type of fans is used in ventilation systems. As the fan propagates tonal
noise in the system, it can severely affect the life quality of people that
reside in the buildings. Our simulation shows that turbulence kinetic energy
(TKE) is unevenly distributed around the rotation axis. Large TKE exists near
the shroud at the pressure sides of the blades. It is caused by the
recirculating flow. Moreover, the position of the largest TKE periodically
varies among the blades. The period corresponds to approximately 4 times the
fan rotation period, it was also found in acoustic measurements. The magnitude
of the tonal noise at the blade passing frequencies agrees well with
experimental data. By analyzing the wall-pressure fluctuations, it is found
that the recirculating flow regions with large TKE are dominant sources of the
tonal noise.
