Atomization of a Powder by a Swirling Flow with a Recirculation Zone

The problem of the motion of a swirling flow in an axisymmetric channel with permeable walls is investigated numerically. Various flow regimes including those with the formation of recirculation zones are obtained. The problem of atomization of a powder by a swirling flow for the purpose of obtaining a finely dispersed mixture is considered. Particle concentration distributions in the flow are calculated, the formation of characteristic deposition zones is demonstrated, and the unsteady process of particle transfer is investigated with allowance for deposition on the lateral surface of the channel.     

Experimental and numerical investigation of a viscoplastic Carbopol gel injected into a prototype 3D mold cavity

A numerical model for free-surface flow of a viscoplastic liquid into a cavity is presented. This flow is regarded as a basic model of injection molding, which is a widely used processing technology. Model experiments of the injection process are performed with a water-based gel with shear-thinning behavior. The filling process is visualized by tracing the free surface of the gel within the cavity. Filling times of the cavity are deduced from the experimental observations. The filling process is also analyzed by means of numerical simulation.The flow equations are integrated according to the finite-volume method. The volume-of-fluid method is employed in order to describe the flow of two incompressible, immiscible phases, the phase interface is resolved by the method of geometric reconstruction or alternatively by the method of surface compression. The Herschel–Bulkley model is used in order to describe the shear-thinning behavior of the gel and the effects of a yielding point. The governing equations of the flow are solved by means of the commercial code Fluent as well as the Open Source code OpenFOAM.The results of the numerical simulations are analyzed in detail. They are compared with the experimental findings. Cavity filling times in the experiments and the simulations are in good agreement. Different patterns of the filling flow depending on the injection parameters are evident in the experiments and the simulations. They are characterized and arranged with respect to the similarity parameters of the flow configuration. Again, the results of the simulation are found to agree well with the experimental observations.     

Transient removal of a contaminated fluid from a cavity

A numerical and experimental study of the time-dependent hydrodynamic removal of a contaminated fluid from a cavity on the floor of a duct is presented. The duct flow has a parabolic inlet velocity profile and laminar flows are considered in a Reynolds number range between 50 and 1600 based on the duct height. The properties of the contaminated cavity fluid are assumed to be the same as for the fluid flowing in the duct. Attention is focussed on the convective transport of contaminated fluid out from the cavity and the effect of duct flow acceleration on the cleaning process. Passive markers which are convected with the flow are used in the numerical simulation for the purpose of identifying the contaminated cavity fluid. It is shown that the cleansing of the cavity is more pronounced during the unsteady start-up of the duct flow and the rate of cleaning decreases as the flow reaches a steady state. The cleaning process is enhanced as the cavity aspect ratio is increased and as the duct Reynolds number increases. A ‘volumetric’ approach based on the spread of markers is shown to be useful in determining the fraction of the cavity that remains contaminated after steady conditions have been reached. The distribution of the contaminant in a cavity during the unsteady stage and after steady conditions are reached are identified using passive markers.     

Numerical modeling of the self-oscillation onset near a three-dimensional backward-facing step in a transonic flow

The results of a numerical investigation of the nature of self-oscillation processes occurring in transonic flow past a backward-facing step and a cavity with a flow of the open type are presented. The turbulent flow past the above-mentioned bodies is modeled using the NOISEtte software package intended for solving problems of aerodynamics and aeroacoustics on unstructured grids. The modeling is performed using the eddy-resolving IDDES method that belongs to the class of hybrid RANS-LES approaches. The adequacy of the calculations is confirmed by means of comparing the results obtained with the available experimental data. The structure and the salient features of the self-oscillatory, hydrodynamic-in-nature process, which arises in flow past a cavity and a backward-facing step, are established.     

Numerical analysis of the heat and fluid flow in a weld pool with a dynamic keyhole

In keyhole plasma arc welding, the interaction between the keyhole and the weld pool occurs when the keyhole appears inside the weld pool. The change of the keyhole shape and dimensions has direct effect on the heat and fluid flow in the weld pool, and the latter will also influence the keyhole geometry. In this study, the coupled behaviors of weld pool and keyhole are treated to develop a three-dimensional model for analyzing the heat and fluid flow inside a weld pool with a dynamic keyhole. In view of the characteristics of PAW process, a combined volumetric heat source model (double-ellipsoid plus conic body source) is established, and one of its distribution parameters is adjusted dynamically with the variation of the depth of keyhole. The physical phenomena, such as the weld pool development, the keyhole formation, the evolution of fluid flow and thermal field, the full-penetration of the test plates, and the transformation from a blind keyhole to an open keyhole, are quantitatively analyzed. The numerical results reveal the regularity of fluid flow in weld pool with a keyhole. The calculated keyhole shape and the fusion zone of plasma arc welds are compared with the experimental measurements. Both agree with each other generally. It lays foundation for optimizing the welding process parameters and improving the stability of plasma arc welding process.     

Analysis of the Time Behaviour of a Diffusive Transport in a Stratified Medium

The paper presents a study of the diffusive transport of passive solute plumes in a two-dimensional non-homogeneous depth stratified flow domain. All the properties of the process are expressed by depth dependent deterministic functions. The method of moments, combined with the method of Green functions are chosen to determine the relevant characteristics of the flow (mass, center of mass, variance, etc.) used to describe the behaviour of the transient motion. General relationships for the n-order concentration moments are proved. Further, it is derived that the transient motion defined by time-dependent parameters tends asymptotically at large time to a stable regime whose characteristics are determined. Consequently, under certain hypotheses, an equivalence between the mean original process and a Fickian diffusive transport at large time may be established. The time required by the process to reach its asymptotic behaviour is also calculated.     

On passing a non-Newtonian circulating tumor cell (CTC) through a deformation-based microfluidic chip

Circulating tumor cells (CTCs) are potential indicators of cancer. Detection of CTCs is important for diagnosing cancer at an early stage and predicting the effectiveness of cancer treatment. Among various devices, deformation-based CTC microchips have shown a strong promise for CTC detection. This type of devices involves a process where CTCs are trapped while allowing more deformable blood cells to squeeze through the filtration geometry at the specified operating pressure. For designing a reliable CTC microchip, in-depth understanding of the cell passing process through the deformation-based microfluidic device is of high value to the device performance enhancement. In this paper, the CTC squeezing process through a microfluidic filtering channel is studied with a non-Newtonian CTC model employed to account for shear-thinning properties of the cell. Detailed microscopic multiphase flow characteristics regarding the filtering process are discussed including the pressure signatures, flow details, cell deformation, and viscosity variation. Critical pressure for the non-Newtonian CTC at different flow rates is analyzed as it plays a crucial role in the device operation in ensuring a successful passing event. Our study provides insights into the non-Newtonian cell squeezing process, which can guide in the design and optimization of next-generation deformation-based CTC microfilters.     

Free Fluid Flow Through a Visco-Deformable Porous Medium with a Moving Boundary

One-dimensional Darcy-law flow through a porous matrix representing a high-viscosity liquid is investigated. The flow develops in a region which depends on time due to sedimentation. The problem considered simulates the geological process of sedimentation in a basin. In accordance with geological data, the permeability and viscosity coefficients of the matrix are assumed to depend nonlinearly on the porosity. The asymptotic properties of the flow are described for large times. The agreement between the results of asymptotic and numerical solutions is satisfactory at intermediate times and good at large times under the realistic sedimentary basin conditions. The simplicity of the asymptotic solution obtained makes it possible to vary the problem parameters and determine the porosity, pressure, and velocities for particular geological conditions by means of simple calculations.     

A concise method for determining a valve flow coefficient of a valve under compressible gas flow

This work presents a novel method of determining a valve flow coefficient for a valve and transient mass flow rate of compressible gas discharged from a reservoir. The proposed method consists of a set of equations to express the physical phenomena and measurement equipment to measure the indispensable data used in the above equations. Regardless of the kind of valve, the valve flow coefficient can be obtained efficiently and feasibly. The results of this study indicate not only that the valve flow characteristics of the diaphragm valve significantly differ from those of the ball valve, but also that the C_v flow equation conventionally used is no longer valid for the diaphragm valve. The valve flow coefficient of the ball valve determined by the proposed method is about 48 and the representative one proposed by the ANSI/ISA is about 56. In addition, the mass flow rate of gas flow through a valve under transient process can be estimated without using a flow meter. Moreover, the cumulated masses discharged predicted by the method proposed herein are consistent well with those of the experimental results. The deviations are smaller than 6%.     

Interaction Between a Slow Magnetohydrodynamic Shock Wave and a Tangential Discontinuity

The self-similar problem of the oblique interaction between a slow MHD shock wave and a tangential discontinuity is solved within the framework of the ideal magnetohydrodynamic model. The constraints on the initial parameters necessary for the existence of a regular solution are found. Various feasible wave flow patterns are found in the steady-state coordinate system moving with the line of intersection of the discontinuities. As distinct from the problems of interaction between fast shock waves and other discontinuities, when the incident shock wave is slow the state ahead of it cannot be given and must to be determined in the process of solving the problem. As an example, a flow in which the slow shock wave incident on the tangential discontinuity is generated by an ideally conducting wedge located in the flow is considered. The basic features of the developing flows are determined.     

Investigation of the Flow Structure on a Miniature Gas-Dynamic Setup: Identification of the Secondary Flow in a Clustered Supersonic Jet Escaping Into a Rarefied Space

A possibility of using small-scale vacuum setups for experimental investigations of supersonic jets escaping from supersonic nozzles into vacuum or rarefied space is considered. Results of studying the structure of the secondary supersonic flow formed in supersonic jets with developed condensation, which is detected for the first time, are reported. The present investigations are carried out with the use of photometry and spectrometry of jets with the use of radiation excited by an electron beam; flow visualization is also performed. The results obtained in the study are analyzed; capabilities and specific features of various methods of flow registration are considered. An empirical model, which establishes the dependence between the detected secondary flow and the process of formation of large clusters in the flow, is developed and justified.     

Selection of convective rolls in a thin evaporating-fluid layer in an air flow

The process of selection of longitudinal convective rolls in a thin layer of evaporating fluid immersed in an air turbulent boundary layer flow is studied numerically. The dependence of the two-dimensional flow patterns on the Rayleigh number and boundary conditions is analyzed. Calculations with account for the thermocapillary effect are carried out. The numerical results are compared with experimental data.     

Numerical simulation of time‐dependent hydrodynamic removal of a contaminated fluid from a cavity

The time‐dependent hydrodynamic removal of a contaminated fluid from a rectangular cavity on the floor of a duct is analysed numerically. Laminar duct flows are considered for Reynolds numbers of 50 and 1600 where the characteristic length is the duct height. Two cases are considered where: (1) the fluid density in the cavity is the same as that for the duct fluid and (2) the cavity fluid has a higher density than the duct fluid but the two fluids are miscible. The flow is solved by a numerical solution of the time‐dependent Navier–Stokes equations. Attention is focused on the convective transport of contaminated fluid out from the cavity and the effect of duct flow velocity profile on the cleaning process. Passive markers are used in the numerical simulation for the purpose of identifying the contaminated cavity fluid. The results show that the flow patterns in the cavity are influenced by the type of duct flow. From a cleaning perspective, the results suggest that it is easier for the duct flow to penetrate a cavity and to remove contaminated cavity fluid when the duct flow is of the Poiseuille type and the aspect ratio is large. Copyright © 2003 John Wiley & Sons, Ltd.     

Numerical investigation of toroidal shock wave focusing in a cylindrical chamber

In this paper, focusing of a toroidal shock wave propagating from an annular shock tube into a cylindrical chamber was investigated numerically with the dispersion controlled dissipation (DCD) scheme. The first case for an incident Mach number of 1.5 was conducted and compared with experiments for validation. Then, several cases were calculated for higher incident Mach numbers varying from 2.0 to 5.0, and complicated flow structures were observed. The numerical study was mainly focused on two aspects: focusing process and flow structures. The process, including diffraction, focusing, and reflection, is displayed to reveal the focusing mechanism, and the flow structures at different incident. Mach numbers are used to demonstrate shock reflection styles and focusing characteristics. PACS 47.40.Ki; 47.40.Nm; 52.35.Tc     

Liquid/liquid dispersion in a chaotic advection flow

Mixing by chaotic advection in a twisted-pipe flow is used here to investigate the efficiency of this flow in the liquid/liquid dispersion process. This study focuses on water/oil dispersions produced by continuous water injection into a main oil flow, for small Dean numbers. The drop sizes obtained with the chaotic-advection twisted-pipe flow are compared with those in a straight pipe and a helically coiled flow for the same conditions. It is found that the resulting dispersions are finer and more mono-dispersed in the chaotic advection flow. These results are compared with the theoretical maximum diameter _dmax$d_{\text{max}}$ determined by the Grace theory in which the viscous stress controls the breakup phenomena. For this purpose, the kinematic field is computed from the theoretical formulae for Dean flow. The strain rate fields in the pipe cross-section are then analytically computed and used to predict the maximum drop diameter. The theoretical values are identical for the three configurations (straight, helically coiled, and twisted pipe) up to a critical Dean number, where the secondary flow becomes significant. Beyond this value, the shear stress is enhanced in the twisted-pipe flow compared with the straight-pipe flow, and the predicted drop diameters are smaller. An interpretation of the higher dispersive performance of the chaotic flow is provided by the Lagrangian trajectories of the particles.     

Evolution of a vortex street in the far wake of a cylinder

The evolution of a primary vortex street shed from a circular cylinder in the far wake is experimentally examined for 70 R 154 (R is the Reynolds number). According to the vorticity fields obtained using digital image processing for visualized flow fields, the primary vortex street breaks down into a nearly parallel shear flow of Gaussian profile at a certain downstream distance, before a secondary vortex street of larger scale appears further downstream. The process leading to the nearly parallel flow can be explained as the evolution of the vortex regions of an inviscid fluid if we invoke the observation that the distance between the two rows in the primary vortex street increases with the downstream distance, although the viscous effect probably contributes to this increase. Numerical computations with the discrete vortex method also support this explanation. The wavelengths and speeds of the primary and secondary vortex street are also measured.     

Onset of flow instability in a heated capillary tube

We study the stability of flow in a heated capillary tube with an evaporating meniscus. The behavior of the vapor/liquid system, which undergoes small perturbations, is analyzed by linear approximation, in the frame of a one-dimensional model of capillary flow, with a distinct interface. The effect of the physical properties of both phases, the wall heat flux and the capillary sizes, on the flow stability is studied. The velocity, pressure and temperature oscillations in a capillary tube with a constant wall heat flux or a constant wall temperature are determined. A scenario of a possible process at small and moderate Peclet numbers corresponding to the flow in capillaries is considered. The boundaries of stability, subdividing the domains of stable and unstable flows, are outlined, and the values of geometrical and operating parameters corresponding to the transition from stable to unstable flow are estimated. It is shown that the stable capillary flow occurs at relatively small wall heat fluxes, whereas at high ones, the flow is unstable, with continuously growing velocity, pressure and temperature oscillations.     

Redesign of a sharp heel draft tube by a validated CFD‐optimization

Numerical optimization techniques in flow design are often used to find optimal shape solutions, regarding, for instance, performance, flow behaviour, construction considerations and economical aspects. The present paper investigates the possibilities of using these techniques in the design process of a hydropower plant. This is realized by optimizing the shape of an existing sharp heel draft tube and validating the result with previously performed experiments. The actual shape optimization is carried out with the response surface methodology, by maximizing the average pressure recovery factor and minimizing the energy loss factor. The result from the optimization shows that it is possible to find an optimal solution on rather coarse grids. The location of the optimum is similar to the experiments, but the improvements are unexpectedly small. This surprising result indicates that the simulated flow field does not completely act as the real flow, which may be a result of the applied inlet boundary conditions, insufficient turbulence models and/or the steady flow assumption. Copyright © 2005 John Wiley & Sons, Ltd.     

Stokes flow inside and around a fluid drop distorted by the presence of a tube wall

Using a quadratic optimisation process to satisfy the boundary conditions, the drag coefficient and the flow patterns inside and outside a fluid drop translating axially in a tube have been determined accurately even when the drop is clearly elongated. Convincing comparisons with experimental results (flow visualizations, velocity and drag measurements) are presented. The effects of the viscosity and density ratios are examined.