Tonal fan noise of an isolated axial fan rotor due to inhomogeneous coherent structures at the intake

In spite of low circumferential Mach number and no obvious disturbances up- and downstream, the sound of axial fans is often dominated by distinctive tones at blade passing frequency (BPF) and its higher harmonics. Flow visualization at the air intake and a correlation analysis of measured blade pressure fluctuations reveal that quasi-stationary coherent structures exist in the inflow which may act as a source of the tones. They can partly be suppressed by a hemispherical inflow control device which supports this finding.