Junji Ochi

Effects of shear rate on the flow around a square cylinder in a uniform shear flow

Abstract This paper is concerned with the numerical simulation of the flow around a square cylinder in a uniform shear flow. In the simulation, the flow is assumed to be the superposition of a potential flow, which consists of the system of vortex filaments representing approximately the square cylinder and the shear layer originating from the separation at the corners of the cylinder, on the incident uniform shear flow. The effect of shear rate on the Strouhal number and the force acting on the cylinder is fairly small within the extent of our simulation, while the flow pattern is affected largely by the shear rate. At high shear rate, von Karmans vortex street is broken down and it seems that the flow pattern far down stream tends to be similar at any instant.

Effects of changes in frequency of ultrasonic vibrations on heat transfer

The heat transfer coefficients during natural convection of water on a downward-facing horizontal surface and that on a vertical surface were measured under the influence of ultrasonic vibrations. The multi-type oscillation used in this experiment was able to operate at three frequencies-28, 45, and 100 kHz-not only continuously, but also intermittently, When ultrasonic vibrations of 28 and 45 kHz were applied to water, cavitation bubbles were generated and the heat transfer coefficient increased remarkably. When an ultrasonic vibration of 100 kHz was applied, the increase in the heat transfer coefficient was small compared to those at 28 and 45 kHz. At 100 kHz, cavitation was not observed at this low ultrasonic power. The uniformity of heat transfer, which depends on the profile of the standing wave due to the ultrasonic vibration, was improved by the method of repeatedly applying multiple vibrations

Control of particles in a standing wave field using ultrasonic vibration

In order to control the motion of micromachines by acoustic power, the behavior of particles falling in several liquids with a standing wave field was studied experimentally. A PZT transducer with a frequency of 28 kHz was fixed to the bottom of a cylinder. Several small tubes were inserted in this cylinder, and glass particles with a diameter of about 1.5–3.5 mm were dropped in these tubes, respectively. Since cavitation bubbles are observed by applying ultrasonic vibration to a liquid, cavitation intensity as estimated from the erosion loss of an aluminum sheet was measured to investigate the effect of cavitation generation on the radiation pressure. Cavitation intensity in degassed water was weaker than that in ion‐exchanged water; however, the decrease in velocity of the glass particles by applying ultrasonic vibration exhibited the greatest deceleration in the degassed water. Ultrasonic power effectively acts on a particle as a radiation force when cavitation does not occur and there is almost no dam...

Near-Wall Bursting Phenomena and Quasi-Streamwise Vortex Models.

Near-wall bursting phenomena have been detected both by the VITA technique and by quadrant analysis. In addition to these detection schemes, visual burst detection has been performed in order to characterize the difference between VITA events and Q2 (second quadrant)events. It is shown that the yaw angle of visualized low-speed streaks for VITA events is greater than for Q2 events. Based on the experimental results and Euler equation, quasi-streamwise vortex models are proposed, and the results are compared with the experiments. Fluctuating velocity signals computed from the models are in good agreement with the experimental data for VITA events. Contour plots of the calculated streamwise velocity, spanwise vorticity and instantaneous Reynolds shear stress are presented. The generation processes of shear layer structures and Q2 and Q4 motions are shown.

Streaky Structures in a Turbulent Square-Duct Flow. Identification of Streaky Structures by Combined Flow Visualization-Velocity Measurement Techniques.

Streaky structures have been studied in a fully developed turbulent square-duct flow by using combined dye flow visualization-LDV velocity measurement techniques. Transverse distributions of low-speed streaks and mean streak spacing are shown in the near-wall region, i.e., yuτ/v < 25. At z/H < 0.16, the transverse distribution of the low-speed streaks is homogeneous and their spacing is about 90v/uτ, which suggests that the streaky structures near the wall bisector may be similar to those in turbulent boundary layers. In addition, the transverse distance between the visually identified low-speed streak and the velocity-measurement point was measured for each set of visual data. Conditioning the velocity fluctuations on this transverse distance, conditional probability density distributions of the axial, wall-normal and spanwise velocity fluctuations are shown. These distributions suggest the existence of quasi-streamwise vortical motions associated with the visualized streaky structures, their wall-normal dimension and the intermittency of their appearance.

Detection of Coherent Structures in LDV Data Using Combined Visualization in Square Duct Flow

Near-wall streaky structures have been studied in a fully-developed turbulent square-duct flow by using combined dye flow visualization — LDV velocity measurement techniques. The velocity fluctuations are conditioned on the topological properties, such as coalescing and branching, and the transverse locations of the visually-identified low-speed streaks. It is shown that a symmetric pair of counter-rotating quasi-streamwise vortices is associated with one low-speed streak in a statistical sense. However, upstream of coalescing point and downstream of branching point of the low-speed streaks, a single quasi-streamwise vortex is associated with one low-speed streak.

Intensity and Probability Density Distribution of Turbulence in a Fully Developed Square Duct Flow.

The turbulent velocity components and their probability density distributions were measured throughout the cross-sectional area in the fully developed turbulent flow through a square duct at Re≒7000 using LDV. The purpose of this work was to obtain data of value to engineers interested in the testing of a turbulent flow simulation model on a complicated flow pattern under a simple boundary condition. At this Reynolds number, the measured profiles of turbulent intensity near the wall bisector were in good agreement with Kim et al.s results of direct numerical simulation on the two-dimensional channel flow. For higher moments of turbulent velocity such as a skewness factor and a flatness factor, this agreement tends to be confined to the region adjecent to a wall. The flow field affected by the side wall was specified and the aspects of probability density of turbulence in this region were made clear through the measurements. Also, the velocity vector diagram of secondary flow was obtained. These results will contribute to a comprehension of turbulent flow in a square duct.

Single component two phase flow through a converging nozzle in critical state.

Single component two phase flow in a state of thermodynamic non-equilibrium has been investigated analytically. It is difficult in general to determine mass and heat transfer through the interface as vaporization and condensation. As the first step to reveal the characteristics of these complicated two phase flows occuring in variable area channels or in high speed flow, the analysis for the process of phase change between the liquid and the vapor is taken by the theoretical method, approximately, without any empirical conditions. The result of numerical calculation for steam-water critical flow in a state of thermodynamic non-equilibrium is shown to be in the range of 0.6 - 8.6 MPa saturated pressure at a nozzle inlet, and is compared with the equilibrium flow and experimental data on the critical flow in the past.

Transient Process of Non-equilibrium State Occurring in a Saturated Liquid-vapor System

A theoretical analysis is presented on a transient process through which non-equilibrium state occurring in saturated liquid-vapor system changes to stable equilibrium state. In this analysis, energy flux and mass flux between the liquid phase and the vapor are expressed by the phenomenological transport equations. The transfer functions between the change of the volume or mass in this system and the resulting change of the pressure or temperature of liquid and vapor are obtained, when these changes are fairly small. The natures of the two-phase transient phenomenon, especially the change of pressure and temperature, were discussed by using the results of numerical calculations in the case of carbon dioxide and water, whose material properties are different from each other.