Tonal Noise of an Isolated Axial Fan Rotor due to Inhomogeneous Coherent Flow Structures at the Intake

C1a - Tonal Noise Reduction

The overall acoustic performance of a fan is sometimes dominated by the tonal components, especially at blade passing frequency (BPF) and higher harmonics. In the case of an isolated fan rotor with a perfectly symmetric inlet nozzle, running at low characteristic circumferential Mach number, no BPF noise should exist, while measurements show just the opposite. Many authors have shown that the tonal noise at BPF is closely linked to ingested flow inhomogeneities. However, there is no consensus about what causes those incoming inhomogeneous flow structures and the subsequent BPF noise in the case of an isolated rotor.

An isolated fan is investigated on a standardized measurement test rig in an anechoic room. Supporting struts are positioned one rotor diameter downstream, the inlet nozzle is perfectly round with a large 1/4 rotor diameter radius and no obstructions are present. Hence, up- and downstream disturbances are thought to be minimal. The blades are instrumented with several flush mounted miniature pressure transducers on the suction and pressure side as well as at the tip. Hot-wire measurements are made in a plane 1/2 rotor diameters upstream of the rotor plane to characterize the incoming flow. The sound pressure is measured by microphones at different positions around the inlet and an in-duct microphone. A state-of-the-art hemispherical inflow control device (ICD), as used for aircraft engine tests, is employed to homogenize the incoming flow. The tones at BPF and higher harmonics are clearly reduced with ICD but they do not disappear completely.

A correlation technique is applied to investigate the flow characteristics as measured by the flush mounted pressure transducers. Nearly stationary coherent structures are detected. A flow visualization at the intake reveals stretched helical vortex structures exist in the intake which may act as a source of the tones.