Considerations for the design of inlet flow conditioners for static fan noise testing

Reductions in distortion tone level of up to 10 dB have already been achieved with a hemispherical honeycomb flow conditioner. Ways of achieving further reductions are investigated in acoustic and aerodynamic tests on flow conditioner elements. Turbulence reduction measurements indicate that adding a gauze downstream of the honeycomb is much more effective than increasing the cell depth. Some tests concerning the importance of the duct boundary layer region which shed further light on earlier work are discussed. Aspects relevant to outdoor full-scale engine tests are emphasized and some preliminary results from such tests are given.