Free convection from a torsionally oscillating vertical plane

Free convective flow of a viscous incompressible fluid from a uniformly heated vertical plane lamina, undergoing small-amplitude sinosoidal torsional oscillations, is investigated. The non-axisymmetric fluid motion consists of the primary Rosenblat flow and the secondary buoyancyinduced cross-flow. Numerical-analytical and asymptotic solutions of the energy equation, covering the whole range of the values of the Prandtl number of the fluid, are derived for their subsequent use in the analysis of the unsteady cross-flow. The mean cross-flow is found to dominate the steady components of the primary flow.     

A numerical analysis of free-surface flow in curved open channel with velocity–pressure-free-surface correction

This paper presents a numerical study onfree-surface flow in curved open channel. An improved SIMPLEC algorithm with velocity-pressure-free-surface coupled correction is developed and validated. Such algorithm differs from the traditional SIMPLEC algorithm and includes three correction equations which are named as the velocity correction equation, the free-surface correction equation derived from the continuity equation with the kinematic boundary conditions on the free-surface and the bottom bed, and the pressure correction equation taking the same formulation as the traditional SIMPLEC algorithm does. In this study, the improved method is used to solve the incompressible, three-dimensional, Reynolds-averaged Navier–Stokes equation set combined with the standard k– model and/or the low Reynolds number k– model for free-surface viscous flow in curved open channels. The power law scheme (POW) is used to discretize the convection terms in these equations with a finite-volume method. The practical cases studied are free-surface flow through the 180° curved open channel with different hydraulic discharge rates. The comparisons between computations and experiments reveal that the model is capable of predicting the detailed velocity field, including changes in secondary motion, the distribution of bed shear, and the variations of flow depth in both the transverse and the longitudinal directions. In summary, the improved SIMPLEC algorithm is feasible and effective for numerical study of free-surface viscous flows in curved open channels.Authors acknowledge the financial sponsor of the Strategic Research Grants #7001371 (BC), and 7001463 (BC), City University of Hong Kong, and Research Fund of the State Key Hydraulics Laboratory, Wuhan University, PRC.     

Unsteady three-dimensional MHD-boundary-layer flow due to the impulsive motion of a stretching surface

Summary The development of velocity and temperature fields of an incompressible viscous electrically conducting fluid, caused by an impulsive stretching of the surface in two lateral directions and by suddenly increasing the surface temperature from that of the surrounding fluid, is studied. The partial differential equations governing the unsteady laminar boundary-layer flow are solved numerically using an implicit finite difference scheme. For some particular cases, closed form solutions are obtained, and for large values of the independent variable asymptotic solutions are found. The surface shear stresses inx-andy-directions and the surface heat transfer increase with the magnetic field and the stretching ratio, and there is a smooth transition from the short-time solution to the long-time solution.     

Exact solutions of non-Newtonian fluid flows with prescribed vorticity

Summary The equations of motion of a non-Newtonian second-grade fluid flow are highly nonlinear partial differential equations. For this reason, there exists only a limited number of exact solutions. Due to the complexity of the equations, inverse methods described by Nemenyi [1] have become attractive in the study of non-Newtonian fluids. In these methods, certain physical or geometrical properties of the flow field are assumed a priori.Lin and Tobak [2] studied steady plane viscous incompressible flows for a chosen vorticity function by decomposing the nonlinear fourth-order partial differential equation in the streamfunction. This excellent approach yielded two second-order linear partial differential equations in the streamfunction. Hui [3] used this approach to study unsteady plane viscous incompressible flows.During the past decade, there has been substantial interest in flows of viscoelastic liquids due to the occurrence of these flows in industrial processes. In this paper, we study the steady and unsteady incompressible viscous non-Newtonian second-grade fluid flows in which the vorticity is proportional to the streamfunction perturbed by a uniform stream. The solutions obtained are exact solutions and represent various non-parallel flows of second-grade fluids.The plan of this paper is as follows: In Sect. 2, the equations of motion of an unsteady plane incompressible second-grade fluid are given, and the vorticity function is assumed to be ² =A(–Ux–BUy ²). In Sect. 3, solutions for the steady flow are obtained. In Sect. 4, solutions for unsteady flow are obtained.     

Integral formulation to simulate the viscous sintering of a two-dimensional lattice of periodic unit cells

In this paper a mathematical formulation is presented which is used to calculate the flow field of a two-dimensional Stokes fluid that is represented by a lattice of unit cells with pores inside. The formulation is described in terms of an integral equation based on Lorentz's formulation, whereby the fundamental solution is used that represents the flow due to a periodic lattice of point forces. The derived integral equation is applied to model the viscous sintering phenomenon, viz. the process that occurs (for example) during the densification of a porous glass heated to such a high temperature that it becomes a viscous fluid. The numerical simulation is carried out by solving the governing Stokes flow equations for a fixed domain through a Boundary Element Method (BEM). The resulting velocity field then determines an approximate geometry at a next time point which is obtained by an implicit integration method. From this formulation quite a few theoretical insights can be obtained of the viscous sintering process with respect to both pore size and pore distribution of the porous glass. In particular, this model is able to examine the consequences of microstructure on the evolution of pore-size distribution, as will be demonstrated for several example problems.     

Boundary integral equation solution of viscous flows with free surfaces

Summary solutions of the biharmonic equation governing steady two-dimensional viscous flow of an incompressible Newtonian fluid are obtained by employing a direct biharmonic boundary integral equation (BBIE) method in which Green's theorem is used to reformulate the differential equation as a pair of coupled integral equations which are applied only on the boundary of the solution domain.An iterative modification of the classical BBIE is presented which is able to solve a large class of (nonlinear) viscous free surface flows for a wide range of surface tensions. The method requires a knowledge of the asymptotic behaviour of the free surface profile in the limiting case of infinite surface tension but this can usually be obtained from a perturbation analysis. Unlike space discretisation techniques such as finite difference or finite element, the BBIE evaluates only boundary information on each iteration. Once the solution is evaluated on the boundary the solution at interior points can easily be obtained.     

Interfacial capillary–gravity waves due to a fundamental singularity in a system of two semi-infinite fluids

The interfacial capillary–gravity waves due to a transient fundamental singularity immersed in a system of two semi-infinite immiscible fluids of different densities are investigated analytically for two- and three- dimensional cases. The two-fluid system, which consists of an inviscid fluid overlying a viscous fluid, is assumed to be incompressible and initially quiescent. The two fluids are each homogeneous, and separated by a sharp and stable interface. The Laplace equation is taken as the governing equation for the inviscid flow, while the linearized unsteady Navier–Stokes equations are used for the viscous flow. With surface tension taken into consideration, the kinematic and dynamic conditions on the interface are linearized for small-amplitude waves. The singularity is modeled as a simple mass source when immersed in the inviscid fluid above the interface, or as a vertical point force when immersed in the viscous fluid beneath the interface. Based on the integral solutions for the interfacial waves, the asymptotic wave profiles are derived for large times with a fixed distance-to-time ratio by means of the generalized method of stationary phase. It is found that there exists a minimum group velocity, and the wave system observed will depend on the moving speed of the observer. Two schemes of expansion of the phase function are proposed for the two cases when the moving speed of an observer is larger than, or close to the minimum group velocity. Explicit analytical solutions are presented for the long gravity-dominant and the short capillary-dominant wave systems, incorporating the effects of density ratio, surface tension, viscosity and immersion depth of the singularity.     

Plane-strain propagation of a fluid-driven fracture during injection and shut-in: Asymptotics of large toughness

This paper considers the problem of plane-strain fluid-driven fracture propagating in an impermeable elastic medium under condition of large toughness or, equivalently, of low fracturing fluid viscosity. We construct an explicit solution for a fracture propagating in the toughness-dominated regime when the energy dissipated in the viscous fluid flow inside the fracture is negligibly small compared to the energy expended in fracturing the solid medium. The next order corrections in viscosity to this limiting solution are then derived, allowing the range of problem parameters corresponding to the toughness-dominated regime to be established. The first-order small viscosity (large toughness) solution is shown to provide an excellent approximation of the solution for the crack length in the wide range of the viscosity parameter. Furthermore, this solution, when combined with the first-order small-toughness solution of Garagash and Detournay [Journal of Applied Mechanics, 2005], provides a simple analytical approximation of the crack length solution in practically the entire range of viscosity (toughness). It is also shown that the established method of asymptotic expansion in small parameter is equally applicable to study other small effects (e.g., fluid inertia) on the otherwise toughness-dominated solution. A solution for the fracture evolution during shut-in (i.e., after fluid injection rate is suddenly stopped) is also obtained. This solution, which corresponds to a slowing fracture evolving towards the toughness-dominated steady state, draws attention to the possibility of substantial fracture growth after fluid injection is ceased especially under conditions when the fracture propagation during injection phase is dominated by viscous dissipation.     

Recombination of two vortex filaments and jet noise

The recombination of two vortex filaments in a viscous incompressible fluid is analysed by the use of the vorticity equation. The analysis is confined to a local flow field, where the recombination process occurs, and is based on several assumptions, such as the conservation of the fluid impulse, spatial symmetry of the flow field etc. The flow field is expanded as polynomials of coordinates, and variations of their coefficients are obtained by the use of the vorticity equation. It is proved that the process is completed within a short time ofO (σ ²/Γ) and the viscous effect is essential;σ and Γ are the size and the circulation of the vortex filaments, respectively. This result is applied to predict the far-field noise of a circular jet by assuming that the main noise source is the recombination process in deformed vortex rings in the jet near field. The predicted noise intensity shows theU dependence and has an additional new factor (d/σ)⁶;U is the jet velocity andd is the average spacing between vortex rings.     

Fluid reaction on a vibrating disc in a viscous medium

This article studies the fluid reaction on a vibrating disc immersed in a viscous fluid. The fluid is considered incompressible and Newtonian. The disc which is of negligible thickness vibrates harmonically in the direction perpendicular to its surface with an amplitude much smaller than the radius of the disc, in such a way that the non-linear terms can be neglected. The flow is axisymmetric and the velocity tends to zero away from the disc. Different approaches to this problem are presented. The first method consists in solving numerically an integral equation obtained from the Navier–Stokes equation. The second method calculates in an analytic fashion the asymptotic series for the pressure differential across the plate for large values of the dimensionless parameter β, equal to the frequency times the radius squared divided by the kinematic viscosity. The limit when β tends to zero is also studied. The analytical expressions give more reliable results when approaching the limits β large and β small than the numerical solution.     

Thermal radiation effects on magnetohydrodynamic mixed convection flow of a micropolar fluid past a continuously moving semi-infinite plate for high temperature differences

Summary An analysis is presented to study the effect of radiation on magnetohydrodynamic mixed convective steady laminar boundary layer flow of an optically thick electrically conducting viscous micropolar fluid past a moving semi-infinite vertical plate for high temperature differences. A uniform magnetic field is applied perpendicular to the moving plate. The density of the micropolar fluid is assumed to reduce exponentially with temperature. The usual Boussinesq approximation is neglected because of the high temperature differences between the plate and the ambient fluid. The Rosseland approximation is used to describe the radiative heat flux in the energy equation. The resulting governing equations are transformed using a similarity transformation and then solved numerically by applying an efficient technique. The effects of radiation parameter R, magnetic parameter M, couple parameter Δ and density/temperature parameter n on the velocity, angular velocity and temperature profiles as well as the local skin friction coefficient, wall couple stress and the local Nusselt number are presented graphically and in tabular form.     

Nonlinear streaming due to the oscillatory stretching of a sheet in a viscous fluid

Summary An elastic sheet is stretched back and forth in a viscous fluid. The problem is governed by a nondimensional parameterS which represents the relative magnitude of frequency to stretching rate. The Navier-Stokes equations are solved by matched asymptotic expansions for largeS. Due to nonlinearity there exists boundary layers ofO(S ^–1/2). The unsteady oscillatory flow contains both basic and higher harmonic oscillations. The induced steady streamlines show a saddle like flow which is different from that of acoustic streaming.With 5 Figures     

Hall effects on the unsteady hydromagnetic flows of an Oldroyd-B fluid

The influence of Hall currents and rotation on the oscillatory flows of an infinite plate is investigated. Exact solutions for the two problems are obtained.The fluid considered is a homogeneous Oldroyd-B. During the mathematical analysis it is found that governing differential equation for steady flow in an Oldroyd-B fluid is identical to that of viscous fluid. Further, it is observed that in absence of the strength of transverse magnetic field (B₀) the solution in resonance case does not satisfy the boundary condition at infinity. Physical significance of mathematical results is also discussed.     

两黏性颗粒间切向阻力的渐近解析解

从分析两黏性颗粒的相对切向运动着手,化二阶变系数非齐次液桥流体压力微分方程为欧拉方程,解得具有相对运动的不等径颗粒间液桥流体压力和切向黏性阻力的渐近解析解,并与Goldman意义上的近似解和其他文献中的数值解进行对比。结果表明:利用这些解析解可直接定义黏性颗粒力学模型,也可分析不同参数条件下液桥流体压力与颗粒间切向阻力的变化规律。     

On extra boundary condition in the stagnation point flow of a second grade fluid

The two dimensional stagnation point flow of a second grade fluid is considered. The flow is governed by a boundary value problem in which the order of differential equations is one more than the number of available boundary conditions. It is shown that without augmenting the boundary conditions at infinity it is possible to obtain a numerical solution of the problem for all values of K, where K is the dimensionless viscoelastic fluid parameter. The numerical results using the algorithm foreshadow an asymptotic behavior for large K. The asymptotic solution is derived up to terms of O(K⁻¹). Perturbation solutions are also obtained up to the terms of O(K²). Finally an approximate solution is developed, based on stretching of the independent variable and minimizing the residual of the differential equation in the least square sense. All these solutions are compared with the exact numerical solution and the appropriate conclusions are drawn.     

J-A 2 Characterization of Crack-Tip Fields: Extent of J-A 2 Dominance and Size Requirements

The requirements for J-dominance, limits of the single-parameter criterion to characterize the fracture of engineering structures, and two-parameter fracture analyses are first reviewed. Through comparison, it is argued that the two-parameter fracture methodology based on the J-A ₂ theory is a reasonable extension of the single parameter (J-integral) fracture methodology. Consequently the extent of J-A ₂ dominance in various specimens under either tension or bending is investigated in detail in this paper. Using the J ₂ flow theory of plasticity and within the small-strain framework, full field finite element solutions are obtained for both deep and shallow crack geometries of single edge notch bar under pure bending [SEN(B)] and central cracked panel in uniform tension [CC(T)]. These crack-tip stresses are compared with those in the HRR singularity fields and the J-A ₂ asymptotic fields at the same level of applied J. The comparison indicates that the size R of the region dominated by the J-A ₂ field is much larger than that of the HRR field around the crack tip. Except for deeply-cracked SEN(B) in low hardening material (n=10) under fully plastic conditions, the numerical results near the crack tip in both SEN(B) and CC(T) match very well with the J-A ₂ asymptotic solutions in the area of interest 1<r/(J/σ₀)<5 from well-contained to large scale plasticity. The implications of these results on the minimal specimen size requirements essential to a two-parameter fracture criterion based on the J-A ₂ asymptotic solution are then discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Unsteady flow and heat transfer of a viscous fluid in the stagnation region of a three-dimensional body with a magnetic field

The unsteady incompressible flow and heat transfer of a viscous electrically conducting fluid in the vicinity of a stagnation point of a general three-dimensional body have been studied when the velocity in the potential flow varies arbitrary with time. The magnetic field is applied normal to the surface. The effects of viscous dissipation and Ohmic heating are included in the analysis. Both nodal-point region (0?c?1, where c=b/a is the ratio of the velocity gradients in y and x directions in the potential flow) and saddle-point region (−1?c<0) are considered. The semi-similar solution of the Navier-Stokes equations and the energy equation are obtained numerically using an implicit finite difference scheme. Also a self-similar solution is found when the velocity in the potential flow, the magnetic field and the wall temperature vary with time in a particular manner. The asymptotic behaviour of the self-similar equations for large η is obtained which enables us to find the upper limit of the unsteady parameter λ. One interesting result is that the magnetic field tends to delay or prevent flow reversal in y-component of the velocity. The surface shear stresses in x and y directions and the surface heat transfer increase with the magnetic field as well as with the accelerating free stream velocity.     

Modeling coating flow and surfactant dynamics inside the alveolar compartment

The unsteady viscous flow induced by streamwise-travelling waves of spanwise wall velocity in an incompressible laminar channel flow is investigated. Wall waves belonging to this category have found important practical applications, such as microfluidic flow manipulation via electro-osmosis and surface acoustic forcing and reduction of wall friction in turbulent wall-bounded flows. An analytical solution composed of the classical streamwise Poiseuille flow and a spanwise velocity profile described by the parabolic cylinder function is found. The solution depends on the bulk Reynolds number R, the scaled streamwise wavelength \(\lambda \), and the scaled wave phase speed U. Numerical solutions are discussed for various combinations of these parameters. The flow is studied by the boundary-layer theory, thereby revealing the dominant physical balances and quantifying the thickness of the near-wall spanwise flow. The Wentzel–Kramers–Brillouin–Jeffreys (WKBJ) theory is also employed to obtain an analytical solution, which is valid across the whole channel. For positive wave speeds which are smaller than or equal to the maximum streamwise velocity, a turning-point behaviour emerges through the WKBJ analysis. Between the wall and the turning point, the wall-normal viscous effects are balanced solely by the convection driven by the wall forcing, while between the turning point and the centreline, the Poiseuille convection balances the wall-normal diffusion. At the turning point, the Poiseuille convection and the convection from the wall forcing cancel each other out, which leads to a constant viscous stress and to the break down of the WKBJ solution. This flow regime is analysed through a WKBJ composite expansion and the Langer method. The Langer solution is simpler and more accurate than the WKBJ composite solution, while the latter quantifies the thickness of the turning-point region. We also discuss how these waves can be generated via surface acoustic forcing and electro-osmosis and propose their use as microfluidic flow mixing devices. For the electro-osmosis case, the Helmholtz–Smoluchowski velocity at the edge of the Debye–Hückel layer, which drives the bulk electrically neutral flow, is obtained by matched asymptotic expansion.