林 秀千人

Turbulent Noise Generated from A Multiple-Disk Fan

We proposed a theory for estimating the spectral density distribution and the overall sound pressure level of the turbulent noise radiated from a multiple-disk fan. In the theory, experimental data of correlations of the longitudinal and the lateral correlation length as well as the pressure spectral densities were used. The effects of the casing on the radiated noise were clarified by experiments by comparing the spectral density distributions with and without casing. The validity of theoretically obtained formula was examined with respect to the effects of parameters such as the gap between two disks, the rotational speed, the number of disks, the thickness of disks, the radius of an impeller and the flow rate. The agreement between the theoretical and the experimental results was generally satisfactory.

Experimental Investigation into the Mechanism of Discrete Frequency Noise (DFN) Generation from a NACA 0012 Blade

An experimental investigation was made into the mechanism of discrete frequency noise ( DFN )generation from a NACA 0012 blade placed in a uniform oncoming flow. It was clarified that DFN was generated when the boundary layer on the blade surface was laminar and separated at the location upstream of the trailing edge. A dead air region was formed just after the trailing edge of the blade. Therefore a Karman vortex street occurred. Meanwhile we could not observe a close relationship between the existence of a separation bubble and the generation of DFN. That is, DFN was generated even when the attack angle of the flow was nearly zero and no separation bubble was observed. When a separation bubble existed, the boundary layer became turbulent and the flow did not separate up to the trailing edge, so a dead air region was not formed behind the blade, which resulted in the disappearance of DFN. The prediction of the sound pressure level of DFN generated from this stream lined blade by Fukanos theory for a flat plate model agreed well with the measured value.