In order to enhance the aggressive intensity of a cavitating jet for practical applications, such as cavitation peening, a nozzle equipped with an additional nozzle upstream from the main nozzle, i.e., a cavitator, and a guide pipe downstream from the main nozzle was proposed and optimized. The aggressive intensity of the jet was evaluated by the residual plastic deformation, i.e., the radius of curvature, in duralumin plate specimens subjected to the jet perpendicullary. The radius of curvature can be considered to be directly related to the aggressive intensity of the jet, as the plastic deformation such as introduction of compressive residual stress and/or work hardening in metallic materials is important parameters on cavitation peening. The deformation occurs because the pressure due to cavitation impacts is beyond the yield stress of the specimens. It was demonstrated that a nozzle equipped with an optimized cavitator and optimized guide pipe increased the aggressive intensity of the jet by a factor of 4.2 without an increase of the jet power, compared to the jet obtained with a conventional nozzle with neither cavitator nor guide pipe, as the cavitator fed cavitation nuclei for the jet and the guide pipe enlarged the cavitation clouds of the jet.
A broad investigation into the hydrodynamics of liquid mercury has been motivated of late by its use in MW-scale spallation neutron sources. One area of particular concern relates to the erosion suffered by vessel walls from the cumulative effects of liquid mercury droplet impacts arising from the collapse of cavitation bubbles. The low speed (< 5m/s) range of such events forms the focus of this paper and to this end a series of experiments is conducted on spherical droplets of diameter 2:5mm impacting upon a dry quartz surface. A reasonable simulation of such impacts is made possible by using the VOF (volume of fluid) solver interFoam (a part of the open source package OpenFOAM) in combination with an empirical expression for the dynamic contact angle of the air-mercury-quartz system. This latter represents a ‘best fit’ to data obtained from high resolution imaging of the droplet profile for a range of contact line velocities. Experiment and simulation are subsequently compared throughout the stages of initial deposition, spread, recession with break-up and, finally, bounce.
The laminar-turbulent transition of a boundary layer induced by a jet injection in the inlet region of a circular pipe was experimentally investigated. The jet was periodically injected radially from a small hole in the inlet region into the pipe flow. Axial velocity was measured by a hot-wire anemometer. The turbulence induced by the jet within the boundary layer developed into turbulent patches which grew in the axial, circumferential and radial directions downstream. Turbulent fluctuations within the patch were maximum a little inside the leading edge in the axial direction and slightly inside the circumferential interface in the circumferential direction. Non-turbulent fluid was entrained into the patch through the leading and trailing edges. This was the main reason for the axial growth of the patch downstream. If the entrainment is suppressed, the axial growth is inhibited. The axial distance between the leading edge and maximum fluctuation position varied little downstream. On the other hand, the distance between the maximum fluctuation position and the trailing edge increased downstream due to the increase in the entrainment of non-turbulent fluid across the trailing edge.
To improve the prediction accuracy of large eddy simulation, an anisotropy-resolving subgrid-scale (SGS) model is a promising strategy. Although an anisotropic term in this type of SGS model is known to effectively enhance unsteady motions of vortex structures particularly in the near-wall region, it has not been made clear how well this term reproduces the real SGS-stress components. Therefore, we performed a detailed investigation of the model performance by an a priori test using the direct numerical simulation (DNS) data of a plane channel flow. The anisotropic SGS model is constructed by combining an isotropic linear eddy-viscosity model with an extra anisotropic term that does not produce undesirable energy transfer between the grid-scale and SGS components. This modeling concept contributes to a reasonable prediction, while maintaining computational stability. Comparison of the SGS stresses evaluated by the model expressions with those obtained directly from the DNS data provided several insights useful for further development of this type of SGS models. From the present investigation, this anisotropic SGS model was found generally to produce a reasonable trend for the SGS-stress anisotropy.
The effects of external and internal disturbances on the development of boundary layer with heat transfer are investigated by means of direct numerical simulation (DNS) based on the finite difference scheme. The fractional step method is used to solve the governing equations. The external disturbance is generated by a regular turbulence-generating grid, while the internal disturbance is generated by a tripping object mounted on the wall. In order to clarify the momentum and heat transfer mechanism in a boundary layer under these effects, the instantaneous and statistical characteristics of velocity and temperature fields are presented and discussed along with their interactions. The results show that the boundary layer in the case with grid turbulence becomes turbulent even though the Reynolds number based on the momentum thickness is low. On the other hand, in the case with the tripping object, only low-amplitude fluctuations are generated in the vicinity of the tripping object and the boundary layer does not fully developed. The grid increases the skin friction and enhances heat transfer more significantly than the tripping object. It is also found that strong strain in the viscous sublayer, which is induced by the vortical motion in the buffer layer, contributes to the enhancement of heat transfer.
Low-center-of-gravity wind turbines (LCGWTs) characterized by tapered blades whose chord length c increases nonlinearly from the top (where c = 0.11 m) to the bottom (where c = 0.17 m) of each blade. Further, turbines featuring these blades do not need any arms, or even a center pole in the rotor. Two experimental LCGWTs (diameter: 0.4 m; height: 0.25 m) with symmetrical blades (NACA 0018) and cambered blades were built. A dead band, which is a band of tip speed ratio (TSR) where the rotor has negative torque at TSR lower than that where the maximum power-coefficient condition is achieved, was observed when symmetrical blades were subjected to low wind speed. In contrast, no dead band was observed for the cambered blades. Under high wind speeds and over a wide range of TSR values, performance of the LCGWTs was better with cambered blades than with symmetrical blades. Computational fluid dynamics (CFD) analysis of 2-dimensional rotors whose blade sections corresponded to the blade sections at the equatorial planes of both types of LCGWTs showed the same tendency. Performance predictions by the blade element momentum (BEM) method using aerodynamic data on the NACA 0018 blades showed some agreement with the CFD analysis. For the cambered blade rotor, Wilson and Walker's empirical correction of the thrust coefficient, a correction that is typically used in simulations of horizontal axis wind turbines, brought the BEM prediction closer to the CFD prediction than Glauert's correction did. However, the agreement between the BEM prediction with Wilson and Walker's correction and the CFD prediction of the cambered blade rotor was thought to be just a coincidence due to large difference on the torque variations between BEM and CFD. At least, the Wilson and Walker's correction predicts larger torque than the Glauert's correction at high TSR region.
Jet flows have been applied in numerous fields to control flow separation. Over the last decade, several studies on the production of synthetic jets have been performed. However, little information is available about a number of aspects concerning synthetic jets, including details of the structure and the formation mechanism of such jets. The present study attempts to clarify some of the fundamental flow characteristics of free synthetic jets on the basis of experiments and numerical simulations. Experimental velocity measurements and flow visualizations are performed using the hot-wire anemometer and the smoke wire method, respectively. It is found that both the temporal change in the flow pattern and the time-averaged velocity distribution at the centerline depend on K = ReU/S2 (the ratio of the Reynolds number to the square of the Stokes number). The unsteady downstream flow characteristics are discussed in addition to the relation between the formative point of the synthetic jet and the value of K. Furthermore, the flow pattern and the unsteady flow characteristics of the synthetic jet are compared with those of a continuous jet.
In this study, numerical simulations of supercritical-water flows over an arbitrary geometry are presented. In the present method, the supercritical-fluids simulator (SFS) developed by the authors is coupled with the building-cube method (BCM). Further, an immersed boundary (IB) method is applied to the wall boundary treatment, and the mathematical models for water programmed in PROPATH are used for thermophysical property estimation. First, as a classical case, steady laminar flows over a circular cylinder are simulated for numerical validation because of the simplicity of the model. Next, as another classical case, unsteady laminar flows past two side by side circular cylinders are computed with more complex physics of flow interactions. Finally, an E-shaped fin geometry is employed as an arbitrary geometry model in practical applications, and natural and forced convection flows over the E-shaped fin are investigated using the present method. The obtained results indicate that the aerodynamic characteristics of supercritical water are identical to that of conventional fluid based on the same Reynolds number, whereas the heat transfer effects are significantly different based on the distinctive Prandtl numbers. In addition, it is revealed that in the practical applications of supercritical-water flows, the intensity of the natural convection flow tends to be much stronger than that in liquid and gas state water flows at the same scale of geometry. The present method is demonstrated to be a promising tool for two-dimensional practical supercritical-fluid flow simulations with arbitrary geometry and complex physics.
Progressive ultrasonic waves cause acoustic streaming in a liquid. Although theoretical and experimental studies on acoustic streaming for liquid phase have been carried out, acoustic streaming for a solid-liquid mixture does not seem to have been investigated. The purpose of this study is to clarify the velocity distribution of acoustic streaming in a solid-liquid mixture. An ultrasonic wave with a frequency of 485 kHz was horizontally irradiated on tap water with aluminum particles in a cylindrical tube with a diameter of 120 mm whose orientation was kept horizontal; the acoustic streaming velocities were measured with the irradiation time of the ultrasonic wave, initial particle concentration, and particle shape as the parameters. The following results were obtained: (a) The higher the initial particle concentration is, the faster the acoustic streaming velocity of a solid-liquid mixture becomes;(b) When ultrasonic waves are irradiated on a liquid with heavier solid particles, the acoustic streaming velocity of the solid-liquid mixture decreases with irradiation time to a certain extent.
The author has studied the wettability of micro-structured surfaces, some of which have exhibited drag reduction in the channel flow of water in previous experiments. All of the five different patterned surfaces tested here comprise microscale cylindrical pillars of constant length, thickness, and interval. The movement of the wetting front across the microstructured surface has been observed in order to assess the wettability. The surface with dense and long pillars exhibited the slowest movement of the wetting front; meaning that it has the best ability to maintain an air layer, which is the cause of drag reduction in channel flow. Those surfaces having the same number of pillars as the least wettable surface, but with two different pillar lengths, were next in terms of wettability. The experimental results agreed with analysis using a Washburn-type equation, which showed that the ratio between the actual surface area and the apparent area is an important parameter in estimating the wettability of patterned surfaces.
In many fluid-structure interaction problems, the virtual mass, namely, the added mass is one of important interests. In the present study, the authors investigate the validity of a numerical method previously proposed by them in order to specify the added mass coefficient of arbitrary two-dimensional solid bodies efficiently and conveniently. In this method, we consider a two-dimensional incompressible viscous fluid under the assumption of an infinitesimal oscillation amplitude of the body, and properly modify the Navier-Stokes equations into linear equations the Brinkman equations. The solving method is based on a discrete singularity method. In order to show the method's effectivity and validity, the authors compute some flows around the bodies with fundamental cross sections with/without sharp edges, which oscillate in infinite flow fields. In addition, the authors solve the full Navier-Stokes equations by a finite difference method, and compare with each other to specify the valid range for the method. Then, the authors confirm the nonlinear amplitude effect and specify the valid range for the method.
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