The purpose of this study is to establish a method that more efficiently reduces drags for a three-dimensional object whose surfaces are shaped in some kind of wavy form. We use a truncated pyramid that is a relatively simple three-dimensional object. Experiments were carried out for truncated pyramids covered with flat surfaces or wavy surfaces in a turbulent open channel flow. The total drag was obtained by measuring the force acting on the pyramids. The local wall shear stress was estimated from the local mean velocity profile in the direction normal to the surfaces. The difference between the pressure on the uphill surface and that on the downhill surface of the pyramid was also measured with a pressure difference gauge. It was found that the total drag and the pressure difference for the truncated pyramid covered with the wavy surfaces were at their highest 7.9% and 13.7% lower respectively than the equivalent figures for the truncated pyramid covered with flat surfaces. The reductions of the pressure difference and total drag were caused by the recirculation flow intermittently occurring near the top face and the downhill face of the truncated pyramids.
Much attention has recently been given to high-efficiency cooling of pistons in internal combustion engines by cooling channels because of improved thermal efficiency. Cooling a piston efficiently requires a grasp of the gas-liquid multiphase flow state. However, because the magnitude and direction of the inertial force applied to the piston change depending on the crank angle, the flow field in the cooling channel that forms a complex gas-liquid multiphase flow remains a problem. Therefore, we developed a rig test apparatus simulating the reciprocating motion of a piston and visualized internal flows in a clear acrylic channel using a high-speed camera. This paper examines the effects of the Reynolds number of the oil jet and oscillation frequency of the reciprocating motion on flow characteristics in a right circular cylindrical channel. The Reynolds number and oscillation frequency were tested in the ranges 1000 to 2500 and 0 to 8.33 Hz, respectively. We found that the oil flow pattern in the channel forms the complex air-oil multiphase flow via air entrainment caused by the collision between the oil jet and the channel oil at the inlet. The gas phase area ratio in the channel increases with increasing Reynolds number, but its fluctuation is dominated by oscillation frequency. The fluctuation of the gas phase center of gravity becomes larger because of increases in inertial force with increases in oscillation frequency. The average bubble diameter in the channel decreases with increasing Reynolds number and oscillation frequency. We found that bubbles of small diameter are generated because the interfacial fluctuation of the oil jet increases as the jet goes downstream.
In pump’s sumps, the air entrainment sometimes occurs. Such flow accompanying the air entrainment becomes complicated owing to both of its unsteadiness with poor periodicity and its fully-three-dimensionality. In various industrial and environmental problems, the air entrainment often induces vibration, noise, low pumping efficiency or pump’s collapse at the worst, and is usually complicated owing to both their unsteadiness with poor periodicity and their fully-three-dimensionality. The present aim is to understand the air entrainment into a suction pipe, which appears inside a simple and basic suction sump in the vertical-wet-pit-pump configuration. In particular, we focus upon the influences of governing parameters upon the occurrence-time ratio γ of the air entrainment in over-critical-submergence condition, using a conductance-type electric sensor which can detect the existence of air bubbles through a suction pipe with no disturbances by the sensor probe and with a fine spatial resolution in order to achieve accurate measurements. As a result, we reveal the influences of such three kinetic parameters as the Froude number, the Reynolds number and the Weber number (or the Bond number) together with four geometric parameters upon the air entrainment and the free-surface pattern inside the suction sump.
In this study, we present a shape optimization analysis considering a rotational body in a flow field based on the adjoint variable and the finite element methods. The purpose of this study is to obtain an optimal shape so as to closely approach the target velocity in the target regions. It is necessary to apply the correct boundary conditions to the boundaries of the rotational structure to be able to obtain an appropriate shape that satisfies the conditions of the target velocity. It is also important to determine the position of the target domain given the target velocity. Based on these considerations, we have constructed a program code for shape optimization analysis considering rotational body in a flow field. Freefem++ was used to calculate the optimal shape.
Cavitation in the nozzle plays an important role in the atomization of the discharged liquid jet. Because of this, understanding in-nozzle cavitation is very important for the control of spray characteristics. However, in-nozzle cavitation is not fully understood at present, in part because it is affected by various factors, such as fluid properties, injector geometries, and ambient pressure. Although it is obvious that ambient pressure hinders cavitation development in the nozzle, the extent of the effect has not been quantitatively predicted yet. In the present study, visualization of cavitation in an enlarged two-dimensional (2D) transparent nozzle and the discharged liquid jet is carried out under various ambient pressure Pa (Pa = 0.1, 0.2, 0.3, 0.4, and 0.5 MPa). From the flow visualization result, an image analysis is carried out to obtain the experimental data on the effects of ambient pressure on cavitation length Lc, cavitation width Wc, and liquid jet angle θ. Finally, a set of correlations on Lc, modified cavitation number σc, jet angle θ, and the Weber number We at various ambient pressures are proposed, based on the image analysis results.
In this study, we compare the flow and temperature fields of a heated impinging jet interfering with a Couette flow and a Poiseuille flow through numerical simulations. We find that the instantaneous vortex structure in the Couette main flow has some periodicity and affects the heat transfer on the jet impinging surface. In time-averaged flow and temperature fields, the Nusselt number shows unique distributions. Further, a counter-rotating vortex pair is extracted, similar to the previously reported studies on a jet in a cross flow. It is found that this vortex pair involves the main flow fluid and causes a unique Nusselt number distribution. The overall heat transfer in the Couette main flow is lower than that in the Poiseuille main flow. From the mean velocity field, a back current occurs behind the jet inlet, which is similar to the separation in the wake flow of a cylinder crossing the main flow. In addition, the fluctuation velocity in the Couette main flow does not spread wider than that in the Poiseuille main flow, and high velocity fluctuations concentrate on the front edge of the jet flow. The distributions of both mean and fluctuation velocities of the Couette flow interfering with the jet are similar to those of the turbulent Couette flow.