Takahiro Kiwata

Conjugate natural convection in porous media

Abstract This paper reviews conduction-convection conjugated natural convection from plates or bodies in a fluid-saturated porous medium. Heating condition at constant temperature is specified on the surface of a conductive solid body positioned away from the solid-porous interface. The solid-porous boundary temperature is obtained as a part of the solution of temperature field over the domain of interest. Several different configurations including slender bodies, rectangular slabs, horizontal cylinders and spheres are discussed. Associated convective flows and heat transfer characteristics are illustrated. A compact expression for the average Nusselt number is derived from a simple one-dimensional analysis. Analytical solutions for time dependent processes and steady temperature distributions in conductive solids are also included

Conjugate natural convection from a vertical heated slab

Abstract Conjugate natural convection heat transfer from a vertical plate has been investigated analytically and experimentally. Assuming the existence of vertically averaged interfacial temperature between plate and fluid, it is shown that there is a unique nondimensional parameter to characterize the problem. The interfacial temperature is obtained as a root of 5th order polynomial for laminar natural convection, and it is presented as a function of the conjugate parameter. For turbulent convection it is also shown that the polynomial becomes 4th order. A simple expression for the average heat transfer is given as a function of conjugate parameter and the apparent Rayleigh number defined by an overall temperature difference. An experiment, using a water vessel with a heating plate on a side wall, was carried out in order to test a proposed theory for the heat transfer. Three different materials, copper, stainless steel and ceramics, were used as a conductive slab. The measured average heat transfer rates are in good agreement with the theory.

The blockage effects for an oscillating rectangular cylinder at moderate Reynolds number

The results of an experimental study concerning blockage effects on the drags and lifts of a rectangular cylinder which is stationary or forced to oscillate transversely in a uniform flow at moderate Reynolds number Re = 4 x 10 3 are presented in this paper. The experiments were conducted in a towing water tank at different blockage ratios up to 40%, with rectangular cylinders of side ratios of 1, 2, 3 and 4 being stationary or forced to oscillate at frequencies of St c = 0.1-0.5 and at amplitudes of a/H = 3.5-28%. Based on the measurements, it is found that the mean values of drag forces of an oscillating rectangular cylinder and the Strouhal numbers of a stationary cylinder depend greatly on blockage ratios, while rms values of lift forces and phase differences between the lift and the oscillatory displacement are much less dependent of them. Furthermore, it is found that, for B/H = 1 and 2 rectangular cylinders and with the increase of blockage ratio, the drag forces do not show the monotonic increase tendency that one would expect from the knowledge of blockage effects on circular cylinder and thin plates. Instead, the drag forces decrease to a minimum values at first and then increase gradually. The reason for this first-dropping phenomenon is supposed to be that, for short rectangular cylinder, the base pressure depends mainly on reattachment or non-reattachment of shear layers separated from leading edges onto the afterbody of the cylinder. For B/H = 3 and 4 rectangular cylinders, such first-dropping phenomenon disappeared because the flow gets reattached even at free-upstream conditions. This dependence of drag forces on blockage is also a function of oscillatory frequency and amplitude.

Flow Visualization of Vortex Structure of an Excited Coaxial Jet

This paper describes the results of experimental and numerical studies concerning the near-field vortical structure and dynamics of a coaxial jet. The effect of excitation of annular and circular jets on the vortical structure in the inner and outer shear-layers was studied. The flow visualization and the measurements of mean and fluctuating velocities, by a hot-film probe and the particle image velocimetry (PIV) technique, were carried out in an open water tank. The results of flow visualization and numerical simulation are in good agreement. With the excitation of a half of the initial vortex frequency, it is found that the vortex-pairing event is promoted by the forced excitation and it results in the rapid expansion of the jet width and the increase of velocity fluctuation and the entrainment rate. For the velocity ratio near unity, the dominant peak frequency coincides with the initial vortex frequency, i.e., the lip wake vortex frequency, independent of the forced excitation frequency.

Flow-Induced Transverse Vibration of a Cantilevered Prism for Energy Harvesting

Various types of hydroelectric power generators have been developed, and small-scale hydroelectric power generation is expected to provide an independent, dispersed power supply for harnessing hydro-energy of small-sized rivers and agricultural waterways. In order to investigate the effects of the geometric shape of a bluff body’s cross section on the flow-induced transverse-galloping vibration, we performed free-vibration tests of a cantilevered prism with three kinds of cross sections, i.e., rectangular, D-section, and triangular prisms. The effects of the side ratio D/H (where D: depth of a bluff body in the flow direction, H: height of a bluff body normal to the flow direction) and the turbulence intensity of the free stream on the flow-induced vibration were also investigated. It was found that the vibrations of the thin rectangular, D-section, and triangular prisms occur at lower reduced velocity than that of the thick prisms. We also carried out a power-generation experiment using iron-gallium alloy that is a kind of magnetostrictive material to investigate a possibility of harvesting energy from the transverse-galloping vibration in a water flow. The energy harvester using magnetostrictive material and a rectangular prism with D/H = 0.2 generated a maximum electric power of 0.35 W at a reduced velocity of Vr = 4.4.Copyright

Flow Visualization of Excited Coaxial Water Jet With Phase Difference

Flow visualization and measurements of mean and fluctuating velocities were performed on a coaxial jet at a Reynolds number of 3000 in an open water tank using hot-film anemometry, particle image velocimetry (PIV) and laser-induced fluorescence (LIF). The coaxial jet, with a velocity ratio of 0.6, was excited at the forcing frequencies fe = fn , fn /2, where fn is the initial vortex frequency of the unexcited jet. The annular and circular jets were subjected to differently phased excitation of the vortical structure in the inner and outer shear-layers, and the effects were studied. For a coaxial jet excited at half of the initial vortex frequency, the vortex pattern and interaction dynamics in the near-field of the jet depend on the phase of the excitations. Vortex-pairing is promoted by forced excitation with specific phase differences, which produces rapid diffusion of the jet width. For the forcing frequency fe = fn , forced excitation with various phase differences has little effect on the vortex pattern in the shear-layers, but the center-line velocity increases more than that of a coaxial jet subjected to a forced excitation of fe = fn /2.Copyright

Analysis of Computational Fluid Dynamics and Particle Image Velocimetry Models of Distal-End Side-to-Side and End-to-Side Anastomoses for Coronary Artery Bypass Grafting in a Pulsatile Flow

BACKGROUND Intimal hyperplasia (IH) is a major cause of graft failure. Hemodynamic factors such as stagnation and disturbed blood flow are involved in IH formation. The aim of this study is to perform a comparative analysis of distal-end side-to-side (deSTS) and end-to-side (ETS) anastomoses using computational fluid dynamics (CFD) after validating the results via particle image velocimetry (PIV).Methods and Results:We investigated the characteristics of our target flow fields using CFD under steady and pulsatile flows. CFD via PIV under steady flow in a 10-times-actual-size model was validated. The CFD analysis revealed a recirculation zone in the heel region in the deSTS and ETS anastomoses and at the distal end of the graft, and just distal to the toe of the host artery in the deSTS anastomoses. The recirculation zone sizes changed with the phase shift. We found regions of low wall shear stress and high oscillating shear index in the same areas. The PIV and CFD results were similar. CONCLUSIONS It was demonstrated that the hemodynamic characteristics of CFD and PIV is the difference between the deSTS and ETS anastomoses; that is, the deSTS flow peripheral to the distal end of the graft, at the distal end and just distal to the toe of the host artery is involved in the IH formation.

Flow Characteristics of Multiple Round Jets Issuing from In-line Nozzle Arrangement

This research reports on an experimental investigation of the flow characteristics of multiple round jets issuing from 6 × 6 in-line nozzle arrangement at low-Reynolds number, Re ≈ 4 × 103, with five spacing ratios of l/d = 1.5, 2, 3, 4 and 5 (where d is a diameter of nozzle, and l is a spacing between the center of nozzles). The mean and fluctuating velocities of the jet flow were measured by constant-temperature type hot-wire anemometer. We found that the bending of the outside jet decreases with increasing the spacing ratio l/d. In the further downstream, the multiple jets merge into a single jet flow and the merging of multiple jets occur more downstream with increasing the spacing ratio l/d. In the case of the small spacing ratio, the maximum velocity of merged jet increases and the merged jet spreads from the near field of jet exit.

Two-Phase Flow Visualization in the throttle of expansion valve

Recently, there has been an increase in demand for low-noise air conditioner. The reduction of noise generating from the expansion valve in an air-conditioner is particularly important. In order to design a low-noise operating valve, it is necessary to understand the flow characteristics of the refrigerant passing through the throttle of the valve. Therefore, we manufactured an expansion valve with a new throttle design. We compared the performances of the original and new throttle design by monitoring the refrigerant flow conditions and the acoustic noise generation. As results, it was confirmed that refrigerants flow of the upstream of throttle influences in predominance. The reduction in the noise power spectra in the high frequency domain has been made, which is the one that poses particular problems for the human ear.

Flow-Induced Streamwise Oscillation of Two Square Cylinders in Tandem Arrangement

Flow-induced streamwise oscillation of two tandem square cylinders has been studied by means of free-oscillation testing in a wind tunnel. One cylinder was elastically supported so as to allow it to move in the streamwise direction; the other was fixed to the tunnel sidewalls. Small values of the reduced mass-damping parameter (Cn ≤ 1.63) have been considered. When the upstream cylinder is free to oscillate, there are two excitation regions: the first for reduced velocity, Vr, in the range 2.5 ≤ Vr ≤ 5 and cylinder gap distance to reference-length ratio, s, between 0.3 and 2, is due to movement-induced excitation accompanied by symmetrical vortex shedding, while the second, for 0.75 ≤ s ≤ 1.5 and 4.5 ≤ Vr ≤ 6.5, is due to vortex excitation by alternate Karman vortex shedding, accompanied with unstable limit-cycle oscillation. For wide gap distances over 2.5, an excitation region of the upstream cylinder occurs for 3.5 ≤ Vr ≤ 4.7, which is due to alternate Karman vortex shedding, and resembles the streamwise oscillation of a single cylinder. On the other hand, when the downstream cylinder is free to oscillate for narrow gap distances of 0.3 ≤ s ≤ 0.75, the response characteristics have an excitation region due to alternate Karman vortex shedding from the two cylinders, connected by dead water region between them, for 3.2 ≤ Vr ≤ 5.4. When s is greater than 1, the downstream cylinder experiences buffeting by wake fluctuation of the upstream cylinder.Copyright