We have discussed the feasibility of the lattice Boltzmann method as a simulation technique with multi-body hydrodynamic interactions for many particle dispersions. To do so, we consider aggregation phenomena in a two-dimensional suspension composed of magnetic particles. The validity of the present results has been discussed by comparing with those of Monte Carlo and Brownian dynamics simulations. The results obtained here are summarized as follows. The density ratio between magnetic particles and the ambient liquid does not significantly influence the cluster formation. If relatively coarse lattices are used, physically reasonable aggregate structures can be obtained, but fine lattice systems cause invalid aggregate formation of particles, which means that the particle Brownian motion cannot be activated properly in the latter case. The results concerning the pair correlation function and magnetization curves clearly show that the lattice Boltzmann results are in good agreement with those of Monte Carlo and Brownian dynamics methods. We may conclude from these results that if relatively coarse lattice systems are used, the translational and rotational Brownian motion of particles are reasonably activated. Hence, some modifications are necessary to the basic equations of the lattice Boltzmann method for using fine lattice systems in simulations of magnetic dispersions.
In this study, we discuss the development of a mixer that incorporates a moving interface formed by placing two gas–liquid free interfaces into a conventional straight-flow micromixer. We restricted the movement of the interfaces in a direction normal to the flow of the liquid, in order to first promote molecular diffusion by reducing the diffusion distance, and second enhance convective diffusion caused by unsteady flow. We investigated these mixing effects and characteristics in physical experiments along with computational fluid dynamics (CFD) simulations, and compare the results. We found that the imposition of the gas–liquid free interfaces in the channel affected mixing, and that the effect of unsteady flow was small near the moving interfaces. We concluded that the acceleration of convective transport caused by the change of flow direction enhanced mixing.
A new technique for the particle depth and size determination is implemented and tested by using synthetic as well as experimental hologram images. The particle depth and size are detected by geometric measures of the particle signal peak in the reconstructed images. Performance tests are carried out with different holograms patterns consisting of different sizes particles and overlapping interference fringes. The results obtained in both synthetic and in experimental holograms demonstrate the potential of the present method to the flow measurements.
The objective of this study was to develop a simulation method to analyze the body behavior and to clarify the most stable body position while freefalling during a skydive. Using the simulation method, we conducted an optimizing calculation to maximize an objective function with respect to the stability in the freefall. It was found that the most stable position became an arched one. In order to clarify the reason why the most stable position became arched, optimization with respect to a simple shaped object consisting of 20 cylinders was conducted. Then the angle to maximize the restoring moment for each cylinder element was analytically calculated and compared with the optimized angle. From the results, we conclude that the most stable position becomes arched mainly since the restoring moment for each individual part is maximized at that angle. We also conclude that the magnitude of the arch in the most stable position is determined by the ratio of the normal and tangential drag coefficients.
Positron emission particle tracking (PEPT) with positron annihilation spectroscopy was possible to visualize the high speed behavior of particulates within a fluid bed in a mixer, which is an important basic operation in materials processing. This study provides new findings on a number of different regions with particle flow as well as on the performance resulting from different impeller blade geometries within a high shear mixer. There are marked differences in the flow pattern between the two blade designs. PEPT also revealed different characteristic impact velocities in each region (depending on the location within the granulator) as well as significant differences in powder flow when using flat or beveled impeller blades. These new results should prove useful for the design as well as for the operation and scale-up of high shear mixers.
In this study, we experimentally deal with the tumbling, which is a rotating motion of a plate with its axis perpendicular to the falling direction. As the plate, we consider a rectangular-cross-section prism with a depth-to-width ratio λ = 0.3, an aspect ratio AR = 10, and inertia moment ratios I* = 0.75 - 43, together with a wide range of a non-dimensional control parameter C = 7.2×101 - 1.6×103. As a result, we specify fundamental aerodynamic characteristics in terminal condition, like the reduced rotating rate Ω*, the lift coefficient CL, the drag coefficient CD and the lift-to-drag ratio CL/CD as functions of both C and I*. Moreover, the relation between the actual Reynolds number Re and the Reynolds number Re(Vd) based on Vd can be approximately determined, being independent of C and I*. Eventually, we successfully propose a series of empirical formulae to predict Ω*, CL, CD and CL/CD as functions of Re(Vd) alone, which is available for a wide parameter range except for large C and small I*.
Hot-wire measurements were carried out in a fully developed turbulent pipe flow disturbed by rough wall sections (containing d- and k-type roughness) with emphasis being placed on the dependence of the statistical turbulence properties on the type of roughness. A sufficient length of pipe was provided downstream of the disturbance to ensure recovery to the equilibrium state. Noticeable differences between d- and k-type rough walls were found during the initial stage of recovery. In the case of k-type roughness, a considerable increase in the maximum turbulence intensity and velocity gradient was observed and momentum exchange violations were found to occur near the wall. For the d-type roughness, only a small variation in the velocity gradient appeared in the core region. No qualitative differences between d- and k-type flows were found for the intermediate and final recovery stages. The mean flow over the rough wall enhances the production of turbulence that depends on the type of roughness present. The rough wall produces an internal boundary layer which is found to eventually overwhelming turbulent diffusion in the recovery process.
Radial-vaned air separators produce strong stall improvement effect in axial flow fans. Changes in the fan internal flow patterns achieved by the device are confirmed by Pitot traverse measurements. For the air separator condition, with a decrease in the fan flow rate the meridional streamline inclinations through the rotor blades have become increasingly steep and the circumferential velocities increase toward the tip region, in contrast to relatively little changes in the behaviors of the streamlines and flow patterns in the solid wall condition. It is suggested that the stall suppression is achieved by a combination of the following effects of the air separator; (1) suction of casing boundary layers and elimination of embryos of stall, (2) separation of reversed flow from the main flow, (3) induction of the flow toward the casing wall and strengthening of meridional streamline inclination, thus keeping the Euler head increasing, and (4) reinforcement of axi-symmetric flow structure. Appearance of these effects in the descending order is considered to achieve the great stall suppression effect that would be difficult by other types of stall prevention devices.
Rarefied gas flows in the slip regime are investigated by the CIP method. The two-dimensional Couette/Poiseuille flows and two/three-dimensional rid-driven cavity flows are calculated using the Navier-Stokes (N-S) equation with the Maxwell's velocity slip boundary condition for various Knudsen numbers and accommodation coefficients. Numerical solutions show that the N-S equation with the slip boundary can give comparable solutions to analytical solutions and kinetic-type approaches. The continuum model is sufficiently applicable to the slip flow regime on the range of Knudsen numbers of some typical MEMS as 10-3 < Kn < 10-1 .
Through computational fluid dynamics (CFD) and experiments, we studied the spiral flow and mixing process inside a novel cylindrical micromixer. Because mixing efficiency is generally poor in microflows, we expected spiral flow to enhance mixing. Parametric studies evaluated mixing efficiency in terms of flow rate and outer cylinder diameter (i.e., channel width). The patterns of streamlines and the concentration distribution obtained by CFD simulations agreed well with those obtained experimentally by flow observation and concentration measurements. Moreover, we found that the mixing efficiency was strongly influenced by the number of revolutions of the spiral flow. To predict the number of revolutions, we propose a simple equation derived by considering friction forces and angular momentum balance. Consequently, we found that when the distance between the inner and outer cylinders and/or the flow rate was increased, the number of revolutions in the spiral flow increased, which resulted in an enhanced mixing efficiency.
The characteristics of functional activated air microbubble jet with UV irradiation are experimentally clarified for high performance of liquid decomposition. The decolorization of methylene blue solution improves by UV irradiation onto activated air microbubble jet for any solution pH. The effect of UV irradiation on decolorization is most intensive for neutral solution due to the enhanced radical generation through photochemical reaction.
The formation and breakup of an axisymmetric immiscible, viscous, laminar compound jet flowing vertically downward into and breaking up in another immiscible liquid is studied numerically. We use a front-tracking/finite difference method to track the unsteady motion and the breakup of the compound jet interfaces, which are governed by the incompressible Navier-Stokes equations for Newtonian fluids. We consider the formation and breakup of a three-fluid compound jet in which the inner fluid density is greater than the shell's fluid density, and compare with the case when the inner fluid density is less than the shell's fluid density. The effects of interfacial tensions in terms of Weber number and interfacial tension ratio are investigated. An increase in Weber number leads to an increase in the breakup length of the compound jet and a decrease in the size of compound drops.
The objective of this study was to investigate the stability during a skydiving freefall, in which the diver has to take various body positions in order to control precisely their distance, velocity, and direction relative to the other divers for group performance. For this objective, the state equation for a simple elliptic cylinder model was initially derived, considering its equations of motion and fluid force characteristics. Next, using the form of the state equation derived for the elliptic cylinder and input/output data obtained from the developed simulation method for the body behavior of the skydiver, the state equations of the skydiver were identified for various body positions. Finally, roots of the state equations were obtained to investigate the stability. As a result, the causes of instability such as the spin and spiral phenomena were clarified as an unstable natural modes, and the stable limit of the body position was obtained as a value of the parameter which is related to the arch magnitude of the diver's body.
In our previous paper (Kanagawa et al., J. Fluid Sci. Tech., 5, 2010), we have proposed a systematic method for derivation of various types of nonlinear wave equations for plane waves in bubbly liquids. The method makes use of an asymptotic expansion with multiple scales in terms of a small wave amplitude as an expansion parameter and a set of scaling relations of physical parameters, based on basic equations of two-fluid model of bubbly flows. In this paper, we extend the method so as to handle a weakly diffracted ultrasound beam in a quiescent liquid containing a number of spherical gas bubbles distributed with a weak nonuniformity. Because of the high expandability of the original method, the extension can be accomplished by adding a scaling relation of the diameter of the beam to the original set of scaling relations. As a result, we derive a generalized Khokhlov—Zabolotskaya—Kuznetsov (KZK) equation [or a generalized Kadomtsev—Petviashvili (KP) equation] for a long wave and low frequency case.