児玉 好雄

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.

Effects of Tip Clearance on the Noise Generated from Low Pressure Axial Flow fans : Difference Caused by the Width and Location of a Ring Set in the Tip Clearance and the Blade Tip Profile

The effect of the width and the location of a ring set in the clearance between a duct inner wall and a blade tip and the profile of a blade tip on fan noise are experimentally investigated by using three rings of different widths and four impellers with different tip geometry. The results are summarized as follows : A considerable amount of noise reduction can be expected by setting a ring within a tip clearance, when its initial gap is rather large. Both the noise and the aerodynamic characteristics at the maximum efficiency point are better in the case with a larger gap on the leading edge side than on the trailing edge side. In the lower flow rate range, however, the fan with a smaller tip clearance on the leading edge side is superior.

低圧軸流送風機の共鳴騒音に関する研究 : 丸棒の位置、回転数および翼先端形状の影響

Resonance tone is minimized by reducing the tip clearance, say less than about 1 mm. Even in these conditions it occurs with a sufficiently high level if a rod is attached near the leading edge on the suction surface of the blade, as well as if a part of the blade tip is cut off. The fundamental resonance frequency, f, is about 60 70% of the blade passing frequency and is correlated by a Strauhal number as f = St. WD, where St = 0.2 and W is the relative velocity. D is the characteristic length, which is without exception equal to the integer multiple of 0.0078 t, where t is the blade pitch.