Showing papers on "Streamlines, streaklines, and pathlines published in 2002"
TL;DR: Experimental observations of the fluid flow profile obtained by superimposing images of particle movement in a plane normal to the electrode surface are presented and good agreement is found between the numerical and experimental streamlines.
Abstract: The application of a nonuniform ac electric field to an electrolyte using coplanar microelectrodes results in steady fluid flow. The flow has its origin in the interaction of the tangential component of the nonuniform field with the induced charge in the electrical double layer on the electrode surfaces. Termed ac electro-osmosis, the flow has been studied experimentally and theoretically using linear analysis. This paper presents experimental observations of the fluid flow profile obtained by superimposing images of particle movement in a plane normal to the electrode surface. These experimental streamlines demonstrate that the fluid flow is driven at the surface of the electrodes. Experimental measurements of the impedance of the electrical double layer on the electrodes are also presented. The potential drop across the double layer at the surface of the electrodes is calculated numerically using a linear double layer model, and also using the impedance of the double layer obtained from experimental data. The ac electro-osmotic flow at the surface of the electrodes is then calculated using the Helmholtz-Smoluchowski formula. The bulk fluid flow driven by this surface velocity is numerically calculated as a function of frequency and good agreement is found between the numerical and experimental streamlines.
404 citations
TL;DR: In this article, a two-dimensional, rectangular enclosure with sinusoidal temperature profile on the upper wall and adiabatic conditions on the bottom and sidewalls is numerically investigated.
Abstract: Natural convection in a two-dimensional, rectangular enclosure with sinusoidal temperature profile on the upper wall and adiabatic conditions on the bottom and sidewalls is numerically investigated. The applied sinusoidal temperature is symmetric with respect to the midplane of the enclosure. Numerical calculations are produced for Rayleigh numbers in the range 10 2 to 10 8 , and results are presented in the form of streamlines, isotherm contours, and distributions of local Nusselt number. The circulation patterns are shown to increase in intensity, and their centers to move toward the upper wall corners with increasing Rayleigh number. As a result, the thermal boundary layer is confined near the upper wall regions. The values of the maximum and the minimum local Nusselt number at the upper wall are shown to increase with increasing Rayleigh number. Finally, an increase in the enclosure aspect ratio produces an analogous increase of the fluid circulation intensity.
154 citations
TL;DR: In this paper, anisotropic seismic structure due to flow-induced mineral alignment is investigated for a series of models designed to simulate deformation in the upper mantle within a few hundred kilometers of a plate boundary.
Abstract: [1] The anisotropic seismic structure due to flow-induced mineral alignment is investigated for a series of models designed to simulate deformation in the upper mantle within a few hundred kilometers of a plate boundary. The orientation distributions of olivine:enstatite aggregates evolve along streamlines of each flow model, based on each grains plastic response to the local stress/strain field. The effective elastic tensor for these textured aggregates provides predictions of P wave anisotropy and shear wave splitting throughout the model space. P and S travel time delay patterns and fast shear wave polarization angles are found to vary significantly with incidence angle for a given model. Comparison of predicted fast P direction for our method versus a finite-strain based estimate shows that agreement is acceptable for much of the model space, but notable differences occur in regions up to several tens of kilometers in size. Two-dimensional models of spreading center flow are presented for slow and fast rates and for several cases in which the ridge migrates over the deeper mantle. The effect of flow in the third dimension is addressed in a few calculations. For one comparison of flow in the mantle wedge at a subduction zone, the introduction of trench parallel flow causes significant changes in the predicted patterns of P wave anisotropy (magnitude, more than orientation) and SKS splitting.
124 citations
TL;DR: In this paper, the authors describe a numerical prediction of heat transfer and fluid flow characteristics inside an enclosure bounded by two isothermal wavy walls and two adiabatic straight walls.
119 citations
TL;DR: In this paper, a two-dimensional solution for unsteady natural convection in an enclosure with a square body was obtained using an accurate and efficient Chevyshev spectral collocation method.
Abstract: A two-dimensional solution for unsteady natural convection in an enclosure with a square body is obtained using an accurate and efficient Chevyshev spectral collocation method. A spectral multidomain methodology is used to handle a square body located at the center of the computational domain. The physical model considered here is that a square body is located at the center between the bottom hot and top cold walls. To see the effects of the presence of a body on natural convection between the hot and cold walls, we considered the cases that the body maintains the adiabatic and isothermal thermal boundary conditions for different Rayleigh numbers varying in the range of 103 to 106. When the Rayleigh number is small, the flow and temperature distribution between the hot and cold walls shows a symmetrical and steady pattern. At the intermediate Rayleigh number, the fluid flow and temperature fields maintain the steady state but change their shape to the nonsymmetrical pattern. When the Rayleigh number is hi...
111 citations
TL;DR: In this paper, a joint formulation is employed for the optimal control of flow around a rotating cylinder, governed by the unsteady Navier-Stokes equations, with the main objective of suppressing Karman vortex shedding in the wake of the cylinder by controlling the angular velocity of the rotating body.
Abstract: Adjoint formulation is employed for the optimal control of flow around a rotating cylinder, governed by the unsteady Navier-Stokes equations. The main objective consists of suppressing Karman vortex shedding in the wake of the cylinder by controlling the angular velocity of the rotating body, which can be constant in time or time-dependent. Since the numerical control problem is ill-posed, regularization is employed. An empirical logarithmic law relating the regularization coefficient to the Reynolds number was derived for 60 ≤ Re ≤ 140. Optimal values of the angular velocity of the cylinder are obtained for Reynolds numbers ranging from Re = 60 to Re = 1000
108 citations
TL;DR: In this article, a deterministic hairpin vortex signature is determined by conditional averaging techniques, consistent with the hairpin signature defined by Adrian and co-workers: circular streamlines with a strong ejection of low-speed fluid away from the wall (a Q 2 event) just upstream of the vortex head.
Abstract: Time-resolved particle-image velocimetry measurements are made in the streamwise-wall-normal plane of turbulent channel flow at Re τ = 550 and 1747. Temporal and convective derivatives of velocity are computed from this data in order to evaluate the small-scale behaviour of these quantities as well as of the velocity itself. Instantaneous velocity fields indicate that the flow is dominated by small-scale vortex cores believed to be associated with hairpin/hairpin-like vortices. These vortices have been observed in realizations of the random velocity in other wall turbulence studies. In this work, a deterministic ‘vortex signature’ is determined by conditional averaging techniques. This average signature is consistent with the hairpin vortex signature defined by Adrian and co-workers: circular streamlines with a strong ejection of low-speed fluid away from the wall (a Q 2 event) just upstream of the vortex head. In addition, the spatial extent of these small-scale vortices appears to remain relatively cons...
94 citations
TL;DR: The generalized Taylor dispersion theory for suspensions of Brownian particles was developed to study the dispersion of gyrotactic swimming micro-organisms in a linear shear flow.
Abstract: The theory of generalized Taylor dispersion for suspensions of Brownian particles is developed to study the dispersion of gyrotactic swimming micro-organisms in a linear shear flow. Such creatures are bottom-heavy and experience a gravitational torque which acts to right them when they are tipped away from the vertical. They also suffer a net viscous torque in the presence of a local vorticity field. The orientation of the cells is intrinsically random but the balance of the two torques results in a bias toward a preferred swimming direction. The micro-organisms are sufficiently large that Brownian motion is negligible but their random swimming across streamlines results in a mean velocity together with diffusion. As an example, we consider the case of vertical shear flow and calculate the diffusion coefficients for a suspension of the alga Chlamydomonas nivalis. This rational derivation is compared with earlier approximations for the diffusivity.
86 citations
TL;DR: In this article, the authors presented a systematic numerical study on a two-dimensional, steady state and laminar natural convection in a rectangular enclosure with discrete heat sources on walls.
81 citations
01 Jan 2002
TL;DR: In this article, the authors extended the range of applications of DES towards a fully unsteady three-dimensional case with strong streamline curvature, which is known to be a major problem for Reynolds-averaged Navier-Stokes equation (RANS) methods.
Abstract: Latest developments in the simulation of turbulence by detached eddy simulation (DES) have suggested that this technique might be able to replace large eddy simulation (LES) within the next decade. The results of the flow past a square cylinder show that this approach is quite inexpensive compared to LES while capturing the most important features of the flow. This study extends the range of applications of DES towards a fully unsteady three-dimensional case with strong streamline curvature, which is known to be a major problem for Reynolds-averaged Navier-Stokes equation (RANS) methods. The case considered is the turbulent flow over wall-mounted cubes at a Reynolds number of Re = 1.3 x 10 4 . The results demonstrate that DES is able to capture the most dominant flow patterns like LES, while RANS only gives a only a poor representation of the unsteady flow phenomena
72 citations
TL;DR: In this article, a finite element scheme based on the Galerkin method of weighted residuals is used to derive the governing equations for mixed convection heat transfer in open-ended enclosures.
TL;DR: In this paper, a spatial dependence in the velocity field was introduced by periodic modulation of the shape of the wall driving the flow; this perturbs the steady-state streamlines in the direction perpendicular to the main flow.
Abstract: By experiments and supporting computations we investigate two methods of transport enhancement in two-dimensional open cellular flows with inertia. First, we introduce a spatial dependence in the velocity field by periodic modulation of the shape of the wall driving the flow; this perturbs the steady-state streamlines in the direction perpendicular to the main flow. Second, we introduce a time dependence through transient acceleration–deceleration of a flat wall driving the flow; surprisingly, even though the streamline portrait changes very little during the transient, there is still significant transport enhancement. The range of Reynolds and Reynolds–Strouhal numbers studied is 7.7[les ]Re[les ]46.5 and 0.52[les ]ReSr[les ]12.55 in the spatially dependent mode and 12[les ]Re[les ]93 and 0.26[les ]ReSr[les ]5.02 in the time-dependent mode. The transport is described theoretically via lobe dynamics. For both modifications, a curve with one maximum characterizes the various transport enhancement measures when plotted as a function of the forcing frequency. A qualitative analysis suggests that the exchange first increases linearly with the forcing frequency and then decreases as 1/Sr for large frequencies.
TL;DR: In this paper, a direct numerical simulation is used to investigate stratified turbulent flow over a series of prescribed moving water waves at a bulk Reynolds number Re=8000 and waveslope ak=0.1.
Abstract: Direct numerical simulation is used to investigate stratified turbulent flow over a series of prescribed moving water waves at a bulk Reynolds number Re=8000 and waveslope ak=0.1. Unstable, neutral, and stable stratifications are considered for a range of wave phase speeds c. Stratification is shown to significantly alter the mean vertical profiles of velocity and temperature, turbulence variances, wave-induced flow fields, and surface form stress. For the range of conditions considered, the surface form stress (drag) and flow patterns (critical-layer height and streamlines) are well correlated with the friction velocity u*, which therefore contains the essential information about stratification influences. Nonseparated sheltering [Belcher and Hunt, Annu. Rev. Fluid. Mech. 30, 507 (1998)], which determines the drag in neutral flow over stationary topography, is modified by stratification and the movement of the underlying waves. The variation of the form stress with phase speed is correlated with the movement of the critical layer above the surface. Compared to neutral flow at a given phase speed, the flow patterns with unstable stratification are similar to the flow patterns over slower moving waves while stable stratification results in flow patterns typical of faster moving waves. This behavior is qualitatively captured by the wave age parameter c/u*. The wave-induced temperature field responds to the wave-induced velocity fields by forming positive and negative patches over the wave crests and troughs, respectively, with the resulting wave-induced heat flux as much as 15% of the total surface heat flux. Estimates of wave growth from the DNS are in reasonable agreement with field observations and laboratory experiments, and they are larger than predictions from high Reynolds-number, second-order closure models for c/u* 10, the present calculations predict less negative form stress (or less damping) of the waves compared to second-order closure models.
TL;DR: In this article, a four-poles magnet was installed to apply the cusp-shaped magnetic field to air in a cubic enclosure, and a simple model equation was derived for magnetizing force and numerically computed for the system.
Abstract: Magnetizing force, which acts in a magnetic field of steep gradient, was applied to air in a cube heated from above and cooled from below, and with the four vertical walls thermally insulated. A four-poles magnet was installed to apply the cusp-shaped magnetic field to air in the cubic enclosure. A simple model equation was derived for magnetizing force and numerically computed for the system. Without a magnetic field, the conduction was stable, but under the magnetizing force a strong downward flow occurred from the center of the top heated plate and the average Nusselt number attained Nu = 1.17 at Ra = 10 5 and γ=0.5, which is equivalent to a temperature difference of 4 [°C] between the top and bottom walls under a maximum magnetic induction of 0.9 [T] inside a cube of (0.064) 3 [m 3 ] heated from above. The flow visualization experiment with hot incense smoke proved the downward flow from the top hot plate
TL;DR: In this article, the problem of hydromagnetic double-diffusive convective flow of a binary gas mixture in a rectangular enclosure with the upper and lower walls being insulated is solved numerically by the finite-difference methodology.
TL;DR: In this paper, the stability of injection molding flow is investigated numerically using a transient finite element method, and the authors conclude that surface defects are caused by a flow instability near the free surface during filling of the mold.
Abstract: In order to elucidate the mechanism of flow mark surface defects, the stability of injection molding flow is investigated numerically using a transient finite element method. Experiments performed by Schepens and Bulters [Bulters, M., and A. Schepens, “The origin of the surface defect ‘slip-stick’ on injection moulded products,” Paper IL-3-2, in Proceedings of the 16th Annual Meeting of the Polymer Processing Society, Shenghai, China, 2000a, pp. 144–145] using a novel two color injection molding technique are summarized and they indicate that surface defects are caused by a flow instability near the free surface during filling of the mold. Steady finite element calculations of a model injection molding flow using a single mode, exponential Phan-Thien–Tanner constitutive equation supply information about the base state streamlines and polymer stresses. By varying the parameters of the model, the degree of strain hardening in the extensional viscosity can be controlled. Then a linear stability analysis is u...
TL;DR: In this paper, the flow and heat transfer characteristics of impinging laminar multiple square jets have been investigated numerically through the solution of the three-dimensional Navier-Stokes and energy equations in steady state.
TL;DR: In this paper, the sequence of transitions from steady to unsteady chaotic flow in a close-packed face-centred cubic array of spheres is examined using lattice-Boltzmann simulations.
Abstract: The sequence of transitions in going from steady to unsteady chaotic flow in a close-packed face-centred cubic array of spheres is examined using lattice-Boltzmann simulations. The transition to unsteady flow occurs via a supercritical Hopf bifurcation in which only the streamwise component of the spatially averaged velocity fluctuates and certain reflectional symmetries are broken. At larger Reynolds numbers, the cross-stream components of the spatially averaged velocity fluctuate with frequencies that are incommensurate with those of the streamwise component. This transition is accompanied by the breaking of rotational symmetries that persisted through the Hopf bifurcation. The resulting trajectories in the spatially averaged velocity phase space are quasi-periodic. At larger Reynolds numbers, the fluctuations are chaotic, having continuous frequency spectra with no easily identified fundamental frequencies. Visualizations of the unsteady flows in various dynamic states show that vortices are produced in which the velocity and vorticity are closely aligned. With increasing Reynolds number, the geometrical structure of the flow changes from one that is dominated by extension and shear to one in which the streamlines are helical. A mechanism for the dynamics is proposed in which energy is transferred to smaller scales by the dynamic interaction of vortices sustained by the underlying time-averaged flow.
TL;DR: In this paper, the effect of anisotropic parameters on the existence of unsteady permanent oscillations and multiple steady-state solutions has been investigated using the Darcy model, where the principal directions of the permeability tensor are taken oblique to the gravity vector, while those of thermal and solutal diffusivity coincide with horizontal and vertical coordinate axes.
TL;DR: In this article, a thorough investigation of three-dimensional flow in a cubical cavity, subject to a constant velocity lid on its roof, is presented, where the weighting functions are properly chosen so that false diffusion errors can be largely suppressed by the equipped streamline operator.
TL;DR: In this article, a two-dimensional solution for unsteady natural convection in an enclosure with an adiabatic square body is obtained using an accurate and efficient Chevyshev spectral collocation method.
Abstract: A two-dimensional solution for unsteady natural convection in an enclosure with an adiabatic square body is obtained using an accurate and efficient Chevyshev spectral collocation method. A spectral multi-domain methodology is used to handle an adiabatic body located at the center of computational domain. The physical model considered here is that an adiabatic body is located at the center between the bottom hot and top cold walls. In order to see the effects of the presence of an adiabatic body on time-dependent natural convection between the hot and cold walls, we investigated the detail structure of fluid flow and heat transfer as a function of time for different Rayleigh numbers varying in the range of 103 to 106. When Ra=103, streamlines and isotherms reach the steady state without any oscillatory transients, and the flow and temperature distribution around the body in the enclosure shows a four-fold symmetrical pattern. At Ra=4×103, 104, and 105, streamlines and isotherms reach the steady state afte...
TL;DR: In this article, the authors examined the elastic instability of Boger fluids in the periodic channel (PC) and past a (linear) periodic array of closely spaced cylinders (PAC) in a channel.
Abstract: Purely elastic instabilities in the flows of polyisobutylene-based Boger fluids in the periodic channel (PC) and past a (linear) periodic array of closely spaced cylinders (PAC) in a channel have been experimentally examined. The test geometries have the same wavelength and amplitude associated with the periodic variation in the cross-sectional area. Pressure measurements show temporal fluctuations that appear when the Weissenberg number exceeds approximately 0.7 and 1.1 for the PC and PAC geometries, respectively. Flow visualization using digital particle imaging velocimetry (DPIV) shows that below the critical Weissenberg number, flows are two-dimensional and steady consisting of slow vortex motion in the wake region for the flow past the PAC flow and in the near wall region for the PC flow. Inception of flow instability results in the breakdown of the symmetric vortices in the flow past the PAC flow and in time-dependent vortex motion in the PC flow. The onset of flow instability is also accompanied by enhanced flow resistance. The power spectral composition of the pressure fluctuations is uniform along the flow direction for both geometries. This implies that in time-dependent simulations a unit cell with periodic boundary conditions can faithfully represent these geometries. The onset Weissenberg number values, when scaled with the maximum curvature of the streamlines, are very close to each other indicating that the instability arises from the coupling between streamline curvature and elastic normal stresses.
TL;DR: In this article, a modified Boussinesq approximation with temperature-dependent viscosity and thermal conductivity has been used to demonstrate the effects of temperaturedependent physical properties for natural convection in a concentric annulus.
Abstract: In this article fluid dynamic and thermal fields are presented for numerical simulations of laminar, steady, two-dimensional buoyancy-driven flows in an annulus between two vertically eccentric pipes using the penalty finite element method. The simulations were accomplished for Rayleigh numbers between 10 3 and 10 5 and radius ratio of 2.6, with various eccentricities. For most of the work, the usual Boussinesq approximation was made. However, in order to demonstrate the effects of temperature-dependent physical properties for natural convection in a concentric annulus, a modified Boussinesq approximation with temperature-dependent viscosity and thermal conductivity has been used. The formulation was based on primitive variables. Numerical results are presented in terms of isotherms, streamlines, and Nusselt numbers. The results are compared with recent publications and excellent agreement has been found. Stable solutions were obtained when the model was modified to incorporate temperature-dependent visco...
TL;DR: In this article, the authors studied flow past and within an isolated permeable spheroid directed along its axis of symmetry, and the flow velocity field was solved using the Stokes creeping flow equations governing the fluid motion outside the sphroid.
TL;DR: In this paper, a theoretical analysis of shell-side flow effects on the performance of hollow-fiber gas separation modules is presented, which uses Darcy's law to relate fiber packing, pressure fields, and velocity fields within the shell.
TL;DR: In this article, smoke patterns, velocity vectors, and streamlines are superimposed to clarify and compare the flow structures at different jet-to-crosswind momentum flux ratios, and four flow structures with apparent differences are identified.
TL;DR: In this article, the separation characteristics of fluid flow inside two parallel wavy plates for steady-laminar flow are investigated numerically in the presence of surface waviness and aspect ratio.
TL;DR: In this paper, a parallel-plate channel was used to model the behavior of an extremely dilute suspension of positively polarized spheres in an ac electric field under conditions such that the field lines were arranged in the channel cross section perpendicular to the streamlines of the main flow.
Abstract: Experiments were conducted in a parallel-plate channel in which an extremely dilute suspension of heavy, positively polarized spheres was exposed to an ac electric field under conditions such that the field lines were arranged in the channel cross section perpendicular to the streamlines of the main flow. To reduce the effects of the gravitational settling of the particles, the channel was slowly rotated around a horizontal axis. Following the application of a high-gradient strong ac field (∼ several kV/mm), the particles were found to move towards both the high-voltage and grounded electrodes and to form arrays of “bristles” along their edges. The process was also modeled theoretically by computing the trajectories of individual particles under the action of dielectrophoretic, viscous, and gravitational forces and under conditions of negligibly small particle Reynolds numbers. The model calculations required no fitting parameters because the particle polarizability was determined independently by measuri...
TL;DR: In this paper, the authors applied the differential quadrature (DQ) method to solve the vorticity-stream function formulation in the horizontal eccentric annulus and demonstrated that the DQ method is an efficient approach in computing the weak global circulation in the domain.
Abstract: In the present paper, natural convective heat transfer in horizontal eccentric annulus is numerically studied by applying the differential quadrature (DQ) method to solve the vorticity–stream function formulation. An explicit formulation for computing the stream function value on the inner cylinder wall is derived from the pressure single-value condition. It is demonstrated in this paper that the DQ method is an efficient approach in computing the weak global circulation in the domain. The present method was validated by comparing its numerical results with available experimental data. Very good agreement has been achieved. Then, a systematic study is conducted for the effect of eccentricity and angular position on the flow and thermal fields.
TL;DR: In this paper, the authors consider the slow motion of viscous fluid completely filling a rectangular container and show that the velocity field is governed by an inhomogeneous biharmonic equation, which depends on the dimensionless depth and lid speed alone.
Abstract: We consider the slow motion of viscous fluid completely filling a rectangular container. The motion is generated by the combined action of differential wall temperatures and the linear motion of the lid. If the relevant Reynolds and Peclet numbers and the lid speed are all small enough, the velocity field will be governed by an inhomogeneous biharmonic equation. In this approximation the temperature field, unaffected by the fluid motion, drives, at least in part, the fluid velocity field. Of interest here are the relative effects of buoyancy and lid motion. It is shown that the field, suitably scaled, depends on the dimensionless depth and lid speed alone. The mixed convection problem is solved for two pairs of wall heating protocols by sequentially solving, by an eigenfunction expansion method, up to four biharmonic problems. We present streamline patterns and quantitative data on the relative effects of lid motion on the buoyancy-driven fields in these containers.