Koji Kikuyama

Development of three-dimensional turbulent boundary layer in an axially rotating pipe

The time-mean velocities and turbulent fluctuations inside the turbulent boundary layers which developed in an axially rotating pipe were measured in the case where an undeveloped flow with a rectangular axial velocity distribution was introduced in the pipe. The pipe rotation gives two counter effects on the flow: one is a destabilizing effect due to a large shear caused by the rotating pipe wall and the other is a stabilizing effect due to the centrifugal force of the swirling velocity component of the flow. The destabilizing effect prevails in the inlet region, but the stabilizing effect becomes dominant in the downstream sections. The intensity of turbulence in the rotating pipe decreases ultimately below that in a stationary state of the pipe. Using the experimental results, the relationship between the mixing length and Richardson number proposed by Bradshaw was examined for the turbulent boundary layer that develops in the rotating pipe.

Numerical simulation for the propulsive performance of a submerged wiggling micromachine

The effect of the Reynolds number on the propulsive performance of a micromachine wiggling in water is analysed numerically. The micromachine, whose geometry is similar to an NACA65-010 hydrofoil, is set in a straight conduit. The wiggling motion is expressed by a progressive wave with reference to the swimming motions of fish. The flow around the micromachine is analysed by using a two-dimensional finite element method. The Reynolds number, Re, ranges from 1 to 100. The analysis reveals that the decrement in Re deteriorates the propulsive performance, and that marked deterioration occurs at Re < 10. The decrement in the thrust force is caused by the increment in the skin friction, and the increment in the power is attributable to the increase of the work against the pressure around the micromachine. The flow induced by the wiggle motion reduces with decreasing Re.

Aerodynamic Loads on Horizontal Axis Wind Turbine Rotors Exerted by Turbulent Inflow

†† Wind turbines, which are installed under the natural wind conditions, suffer complicated loads on the rotor blades due to the turbulent wind. The present paper describes a numerical method for calculating the effects of turbulence characteristics on the aerodynamic loads, and shows the numerical results. The turbulent wind field is simulated with the Veers ’ model. This model is based on a Fourier synthesis method and capable of considering spatial crosscorrelations between any two points in the rotor plane, where the turbulent wind is generated. For the calculation of aerodynamic loads on the rotor blades, an inviscid aerodynamic model based on the asymptotic acceleration potential method is adopted.

Numerical study on the propulsive performance of a submerged wiggling micromachine in straight conduit

Abstract The propulsive performance of a submerged micromachine wiggling in a straight conduit is numerically analysed. The micromachine, whose geometry is similar to a hydrofoil, is mounted in a straight conduit of width W, and the wiggling motion is expressed by a progressive wave. The flow around the micromachine is simulated by a two-dimensional finite element method. The Reynolds number Re, based on the flow velocity upstream of the micromachine and the micromachine length L, is varied from 1 to 100, while the blockage ratio L/W is independently changed in the range of 0.36 ≤ L/W ≤ 1.25. The flow around the micromachine varies according to the wiggling motion, and it presents complicated behaviour owing to the large-scale eddies. The increment of L/W reduces the minimum pressure on the micromachine surface, while it increases the velocity near the micromachine. The propulsive performance is more affected by L/W at Re = 10 than at Re = 100. When L/W is increased in the case of Re = 10, the thrust force decreases at 0.36 ≤ L/W ≤ 0.81 and increases at L/W ≥ 0.81. The decrement is due to the increase of the skin friction, and the increment is attributable to the change in the pressure distribution. The lift force and the power become larger with L/W. The efficiency takes its minimum value at L/W = 0.81.

Turbulent flows inside a rotating curved rectangular channel for different aspect ratios

Abstract Effects of channel rotation have been investigated both experimentally and theoretically by using channels taht rotate orthogonally in order to clarify the flow nature inside passages of turbomachinery. In this article we present velocity and turbulence measurements inside a rotating curved rectangular channel for three aspect ratios of the cross section, 1:1, 2:1, and 4:1. In a channel having a high aspect ratio, direct effects of the channel rotation and wall curvature on turbulence are dominant. With the decrease in aspect ratio, secondary flows generated by pressure imbalance deform the velocity distribution, and the prediction of turbulence behavior tends to be intactable because of the occurrence of the secondary motion and because of the highly three-dimensional natue of the flow.

Rotational Noise Analysis and Prediction for an Axial Fan With Unequal Blade Pitches

The rotational noise is one of the main sources of the noise emitted from a fan. This type of noise has discrete frequency components, which is so harsh that many methods have been developed to reduce the noise. Studies on the rotational noise, however, have hardly been made to clarify the generation mechanism and find concrete methods to reduce the noise.In order to suppress the discrete frequency components of the rotational noise, the spacing of the blades were changed and the comparison was made with a fan having equally spaced blades. The discrete frequency components were found to be suppressed in the unequally spaced fan and the noise tended to have a nature of whitenoise if the blade spacing and blade setting angles were properly chosen.The instantaneous pressure change on the shroud was measured in order that the pressure change was related to the discrete frequency noise was closely related to the pressure changes on the blade surfaces. By expressing the pressure change due to the blade passing in terms of the blade spacing and decomposing it into Fourier series, the noise characteristics was analyzed. The prediction result for the rotational noise emitted by an equally spaced fan was presented and found to agree well with the experiment.Copyright

Pressure Recovery of Rotating Diffuser With Distorted Inflows

The pressure recovery and velocity distributions in a two-dimensional rotating curved diffuser have been studied experimentally when even and uneven flows, respectively, were introduced to the diffuser. Two types of uneven flow were adopted; one has a linear velocity gradient on the surface of revolution and the other a linear velocity gradient in the meridian plane. The pressure recovery in the diffuser is improved by the unformalizing process of the uneven inlet velocities in the downstream sections if larger velocities are in the suction side region, but it is deteriorated if larger velocities are introduced in the pressure side region. When an uneven flow with a velocity gradient in the meridian plane is introduced to the diffuser, increased rotation speed and the gradient of the inlet velocity profile deteriorate the pressure recovery.

Unsteady Pressure Distributions on the Impeller Blades of a Centrifugal Pump-Impeller Operating Off-Design

Pressure distributions on the impeller blades of a centrifugal pump were examined experimentally. The periodic pressure change caused in the suction and delivery pipes by the interaction between the rotating impeller blades and the dividing ridge of the volute casing was small. However, a noticeable cyclic change in the pressure on the blade surface was measured and related to the nonuniform pressure distribution in the volute casing at off-design flow rates. In a lower than normal range of flow rates this pressure fluctuation was largely increased near the leading edge of the blade due to the turbulent fluctuations and the flow separation from the blade surfaces. These periodic pressure changes have a large effect upon cavitation inception along the impeller periphery.Copyright

Effects of Coriolis Force on the Turbulent Boundary Layer with Pressure Gradients

Distribution of velocities and turbulent intensities in turbulent boundary layers developed in a diffuser were studied experimentally when the diffuser was rotated about an axis perpendicular to it. With and increase in the opening angle of the diffuser, the turbulent intensities and the time mean velocity gradient were increased more near the pressure side wall, but decreased near the suction side wall due to the effects of the Coriolies force. On the pressure side wall, turbulent intensities were enhanced by the rotation, and the Monin-Oboukov formula was ascertained to be approximately established within the experiment range even in an adverse pressure gradient.

Effects of Coriolis force on the turbulent boundary layer with pressure gradients.

Distributions of velocity and turbulent intensities in turbulent boundary layers developed in a diffuser were studied experimentally when the diffuser was rotated about an axis perpendicular to it. with an increase in the opening angle of the diffuser, the turbulent intensities and the time mean velocity gradient increased more near the pressure side wall, but decreased further near the suction side wall due to the effects of the Coriolis force. On the pressure side wall, turbulent intensities was enhanced by rotation, and the Monin-Oboukhov formula was ascertained to be approximately established within the experimental range even in a adverse pressure gradient.