Tomonori Nakano

Visualization of Aerodynamic Noise Source around a Rotating Fan Blade

The purpose of this study is to understand the aerodynamic noise source distribution around a rotating fan blade by measuring the noise signal and velocity field around the blade. The local noise-level distribution over the fan blade is measured by microphone arrays, and the flow field is visualized by smoke and phase-averaged PIV measurement. The noise source distribution is examined by cross-correlation analysis between noise signal and velocity fluctuation. It is found that the noise source is located near the rotating fan blade, especially around leading and trailing edges. The separation and reattachment of flow are observed near the leading edge, and the tip vortices and vortex shedding are found near the trailing edge. The cross-correlation distribution of the noise signal and the radial velocity fluctuation shows large magnitude in the correlated regions, which indicates the noise generation by the formation of vortex structure around the blade.

Measurement of shear-stress distribution over NACA0018 airfoil by liquid-crystal coating

The shear-stress measurement by liquid-crystal visualization was extended to the flow along curved surfaces for application to the airfoil study in wind tunnel. The measurement system consists of stereoscopic observation system with two color CCD cameras and stroboscope, which are rotational about the axis of the airfoil. It was found that the magnitude of the shear stress changes along the airfoil surface, which is consistent with the visual observation of separation and reattachment point.

Study of Unsteady Fluid Forces on a Rotationally Oscillating Circular Cylinder

The characteristics of fluid forces on a rotationally oscillating circular cylinder in a uniform flow are studied experimentally at various forcing frequencies and rotational amplitudes of cylinder oscillation. The measurements are made on the pressure fields around the circular cylinder by using the velocity data measured by PIV in conjunction with the pressure Poisson equation. The present result indicates that the fluid forces are enhanced by the low-frequency oscillation and are reduced at high-frequency oscillation, which are due to the modification of the wake structure by the influence of rotational oscillation. This trend is further magnified by an increase in rotational amplitude.

Reduction mechanism of discrete frequency noise from a symmetrical aerofoil by cylinder wake

The discrete frequency noise is generated from an airfoil in a uniform flow at small angle of attack to the free stream at moderate Reynolds number. The present experiment is conducted to find the reduction condition of discrete frequency noise by the influence of cylinder wake using a surface-flow visualization technique by shear-stress sensitive liquid crystal coating. Utilizing the color image analysis, the point of flow separation and reattachment is evaluated from the visualized images. It is found that the region of flow separation over the airfoil was fairly reduced by the influence of cylinder wake, where the suppression of discrete frequency noise was observed. The laminar boundary-layers over the airfoil seems to be shifted to a turbulent state by the influence of cylinder wake.