Koichi Tsujimoto

Numerical simulation of channel flow with a rib-roughened wall

Turbulent channel flows where there are small two-dimensional rods of square cross-section regularly arranged along one wall, normal to the average flow, are simulated without using any model. The other wall is assumed to be flat because of the limitations on computer memory. Sparsely distributed ribs modulate the flow near the wall substantially, but when the ribs are densely distributed the property of flat-wall flow is retained to a large extent. The scales of time and length used to normalize near-wall turbulence are normally wall scales based on total wall drag and viscosity. However, it is thought that these scales may not be adequate, in view of the modification of the turbulence generation mechanism. On the other hand, the flow in the layer away from the wall has been confirmed to be little affected by roughness, and scaled by total wall drag and the distance normalized by the channel half-width. It has also been confirmed that the property of turbulence away from the wall in the case of a flat wa...

IDENTIFICATION OF NOISE SOURCE IN LOW MACH NUMBER FLOW BY DNS

Aeroacoustic field is simulated numerically by DNS (Direct Numerical Simulation) in order to understand the mechanism of generation of acoustic field. Particular interest is in the key element of sound source in turbulent flow near a wall. Acoustic field of coarse spatial resolution is calculated receiving data from the flow in the source field which necessitates fine spatial resolution. The source field considered in this simulation is of low Mach number flow, in which compressibility effect is neglected. The sound source is identified by second derivative of density fluctuation in the incompressible source field, converted from pressure fluctuation according to isentropic change. First, a 2-D cavity flow is simulated to demonstrate that global fluctuation of flow pattern such as due to vortex shedding which intuitively looks like a good measure of sound source, is not necessarily a key. This conclusion is confirmed by a simulation of 2-D acoustic field by a spinning vortex pair. Finally, DNS of turbulent channel flows are conducted to identify the key element of sound source in near-wall turbulence. It is revealed that coherent structures of streamwise vortices and larger scale hair-pin-like vortices are responsible for turbulent noise. This conclusion encourages the control of near-wall turbulence to reduce friction drag on the wall, the mechanism being the same in both noise and drag generation.

Flow and Heat Transfer of Petal Shaped Double Tube—Effects of Pipe Geometry

Heat exchangers are widely used to transfer heat from one fluid flow to another. Over time, various types of heat exchangers have been developed to decrease energy consumption and increase efficiency, including compact fin tube and double tube with special inner pipe geometries (ribbed, spiral, and petal shapes) [1, 2]. In this study, we experimentally investigated the heat transfer efficiency of petal-shaped double tubes with a large wetted perimeter or heat transfer area. Moreover, the influence of the number of petals was studied. Our results show that a larger heat transfer area improves heat transfer rate in heat exchangers but the heat transfer efficiency of the five petal-shaped double tube (5P-tube) is much higher than that of the 6P-tube because of a smaller flow resistance of the 5P-tube.

Interaction Between Water or Air-Water Bubble Flow and Tube Bundle—Effects of Arrangement of Tube Bundle and Void Fraction

Water or air-water bubble flow passing through tube bundle can be seen in many industrial equipments such as heat exchanger and chemical equipment. In addition, tube bundle can be used as a flow-straightener, -mixer, -resistor, and -damping device. In this study, the effects of tube arrangement of equally or unequally spaced in-line and staggered tube bundle and of void fraction on the flow characteristics such as flow pattern and flow resistance of a tube or tube bundle are examined experimentally.

Loss Reduction of the Flow in the Reducing Elbow Duct by Using Weir Shaped Small Obstacle

Reducing elbow is \( 90^{\circ }\) bend or elbow pipe/duct which has smaller outlet than inlet. This type of element is frequently used in the industrial facilities. After the corner of this element separated flow from the corner flows away from the inner wall. And then, effective cross-sectional area of downstream pipe/duct near the corner is reduced. Reduction of effective cross-sectional area causes extremely large flow resistance. In this study, loss reduction method with weir-shaped obstacle for reducing elbow duct is newly proposed. To clarify the effects of obstacle on the flow in the reducing elbow, pressure measurement and flow visualization were carried out. As a result, it is made clear that the loss of reducing elbow is reduced by maximum of about 63 % by using obstacle.

Direct Numerical Simulation of Dynamic-Rotational Controlled Jet

In order to develop a new mixing procedure, we conduct DNS (direct numerical simulation) of dynamic controlled free jets. As the dynamic control, it is assumed that the inflow velocity of jet is alternately rotated around streamwise direction. To realize the high accurate computation, the discretization in space is performed with hybrid scheme in which sine or cosine series and 6th order compact scheme are used. From view of instantaneous vortex structures, it is found that the flow pattern considerably changes according to the rotating frequency, i.e., according to the increasing frequency, helical and entangled mode appear in turn. From the ensemble averaged flow properties such as jet width and entrainment, it is confirmed that the jet widely spreads perpendicular to the rotating axis and that the mixing of upstream jet is markedly enhanced due to the dynamic control. Further in order to quantify the mixing efficiency of the dynamic control, as another mixing measure, a statistical entropy is examined. Compared to the uncontrolled jet, it is confirmed that the mixing efficiency is markedly improved, suggesting that the dynamic control can be expected to be useful for the improvement of mixing performance.

Flow and heat transfer of petal shaped double tube

In this study, the flow and heat transfer characteristics of petal-shaped double tube with 6 petals are examined experimentally for a compact heat exchanger. As results, the heat transfer rate, Q, of the 6 petal shaped double tube (6-p tube) is much larger than that, Qp, of conventional circular double tube in all Reynolds number Rein,h (where, the reference length is hydraulic diameter) ranges. For example, at Rein,h =(0.5～1.0)× 104 it is about 4 times of Qp. The heat transfer enhancement of 6-p tube is by the increase of heat transfer area, wetting perimeter, and a highly fluctuating flow, and Q of the 6-p tube can be expressed by Q [kW/m] = 0.54Rein,h + 2245.

Jet Flow from Resonance Nozzle and Flow Control by Notched Nozzle

The jet flow from an orifice nozzle shows a particular velocity profile with a large velocity gradient at the edge of the jet. Setting an orifice nozzle, second one, with a resonance room just after the first orifice nozzle the jet causes resonance phenomena and the resonance frequency depends on the jet velocity and the volume of resonance room. In this study, a notched nozzle was used as the second one to enhance the flow disturbance or fluctuation, and the effects of the notched nozzle on the mean and fluctuating velocities and the mixing or diffusion characteristics of the jet itself and with the surroundings were examined experimentally using a hot wire measurements. As a result, it was made clear that under a same operation power the flow rate near the nozzle exit of a notched resonance jet is larger than those of an orifice and resonance jets, for example, at x/d = 0.2 (x: distance from the nozzle exit to the downstream, d: nozzle diameter) it is about 2.5 and 1.3 times of theirs, respectively, and others.

Direct numerical simulation of vector-controlled free jets

We conduct DNS (direct numerical simulation) of vector controlled free jets. The inflow velocity of jet is periodically oscillated perpendicular to the jet axis. In order to realize the high accurate computation, a discretization in space is performed with hybrid scheme in which Fourier spectral and 6th order compact scheme are adopted. From visualized instantaneous vortex structures, it is found that the flow pattern considerably changes according to the oscillating frequency, i.e., according to the increasing the frequency, wave, bifurcating and flapping modes appear in turn. In order to quantify mixing efficiency under the vector control, as the mixing measure, statistical entropy is investigated. Compared to the uncontrolled jet, the mixing efficiency is improved in order of wavy, flapping and bifurcating modes. Thus the vector control can be expected for the improvement of mixing efficiency. Further to make clear the reason for the mixing enhancement, Snapshot POD and DMD method are applied. The primary flow structures under the vector control are demonstrated.

DNS of turbulent flow in a channel with an elastic cantilever

In various scientific fields, numerical solving of fluid-structure interaction (FSI) problems are of importance. Therefore the establishment of stable numerical scheme is needed to analyze the details of flow phenomena. In the present paper, a weak-coupling method for FSI problem is proposed: the rigorous equations of motion for a elastic body are discretized with finite volume method; the elastic body is reproduced via immersed boundary method in the flow computation. In order to demonstrate the performance of proposed scheme, the 3D structure analysis and 3D FSI problems of a elastic body are solved. It is found that the computation is stably conducted using the proposed method, in spite of the occurrence of fairly large deformation of the object. Also it turns out that the realistic flow including turbulence phenomena is well reproduce using the proposed FSI scheme.