Showing papers on "Streamlines, streaklines, and pathlines published in 2006"

Steady solutions of the Navier-Stokes equations by selective frequency damping

Abstract: A new method, enabling the computation of steady solutions of the Navier-Stokes equations in globally unstable configurations, is presented. We show that it is possible to reach a steady state by damping the unstable (temporal) frequencies. This is achieved by adding a dissipative relaxation term proportional to the high-frequency content of the velocity fluctuations. Results are presented for cavity-driven boundary-layer separation and a separation bubble induced by an external pressure gradient.

Enhanced mixing and reaction through flow focusing in heterogeneous porous media

Abstract: [1] Transverse dispersion across adjacent streamlines can control the amount of mixing and reaction between one or more contaminants and a limiting substrate along the fringes of groundwater plumes. Streamlines in groundwater converge and diverge in heterogeneous porous media, depending on the permeability distribution. When flow is focused in a high-permeability zone, the distance required for a solute to cross a given number of streamlines decreases, and the time allowed for mixing and reaction is reduced. Because the first effect outweighs the latter, the overall result is an enhancement of transverse mixing and reaction. Here we develop a conceptual model of heterogeneous two-dimensional structures facilitating flow focusing. We use the conceptual model to develop simple analytical expressions quantifying the extent to which mixing and reaction are enhanced when flow focusing occurs and compare these to results of numerical simulations. Significant enhancement of transverse mixing and reaction by flow focusing is observed; for the cases considered, flow focusing enhances the amount of reaction by a factor ranging from 1.8 to 11.9. The relatively simple analytical expressions demonstrate that the fraction of the domain height made up by high-permeability inclusions, the fraction of flow that passes through the inclusions, and the fringe bypassing of inclusions determine the amount of mixing and reaction enhancement for the permeability distributions considered. These results partially explain why field-scale dispersivities are larger than laboratory derived dispersivities, where homogeneous and isotropic sediments are typically used. Further work is needed to verify the theoretical results presented here with laboratory and field experiments and to expand the relatively simple analytical expressions to consider more heterogeneous three-dimensional permeability fields.

Numerical simulation of unconfined flow past a triangular cylinder

Abstract: Numerical simulations of two-dimensional laminar flow past a triangular cylinder placed in free-stream at low Reynolds number (10⩽Re⩽250) are performed. A finite volume method, second-order accurate in space and time, employing non-staggered arrangement of the variables with momentum interpolation for the pressure–velocity coupling is developed. Global mode analysis predicts Recr=39.9 which confirms the results of earlier studies. Vortex shedding phenomena is found to be similar to the square cylinder with no second bifurcation in the range of Re studied. A discussion on the time-averaged drag coefficient, rms of lift coefficient and Strouhal number is presented. Particle tracking and the instantaneous streaklines provide an excellent means of visualizing the von Karman vortex street. Copyright © 2006 John Wiley & Sons, Ltd.

Streamline-based simulation of carbon dioxide storage in a North Sea aquifer

Abstract: [1] We model carbon dioxide (CO2) storage into a deep North Sea aquifer using streamline-based simulation. We assume incompressible flow of liquid-like CO2 and aqueous phases. We simulate dissolution of CO2 and a rate-limited precipitation reaction. Advective transport and reactions are solved along streamlines, while dispersion and flow due to gravity segregation of the phases are solved on the underlying grid. Geological storage is modeled on a one million cell model. The distribution of CO2 after injection is dominated by advective transport due to multiphase flow, and CO2 moves preferentially through high-permeability channels. Without reaction the regional groundwater flow causes the CO2 to continue to migrate until it reaches residual saturation, where it continues, slowly, to dissolve. Precipitation leads to a decrease in porosity and permeability, while CO2 is stored in the solid phase. The storage efficiency is low, around 2%, because of aquifer heterogeneity.

Effect of length and inclination of a thin fin on natural convection in a square enclosure

Abstract: A numerical investigation of steady, laminar, natural convective fluid flow in a square enclosure with an inclined heated thin fin of arbitrary length attached to the hot wall is considered. A transverse temperature gradient is applied on two opposing walls of the enclosure, while the other two walls are adiabatic. Attachment of highly conductive inclined thin fins with lengths equal to 20%, 35%, and 50% of the side, positioned in the middle of the hot left wall of the enclosure, is examined. The problem is formulated in terms of the vorticity–stream function procedure. A numerical solution based on the finite-volume method is obtained. Representative results illustrating the effects of the thin-fin inclination angle and length on the streamlines and temperature contours within the enclosure are reported. In addition, results for the local and average Nusselt numbers at the heated wall of the enclosure are presented and discussed for various parametric conditions. It is found that the Rayleigh number and ...

Measurements of Unsteady Wake Interference Between Tandem Cylinders

Abstract: A multi-phase, experimental study in the Basic Aerodynamics Research Tunnel at the NASA Langley Research Center has provided new insight into the unsteady flow interaction around cylinders in tandem arrangement Phase 1 of the study characterized the mean and unsteady near-field flow around two cylinders of equal diameter using 2-D Particle Image Velocimetry (PIV) and hot-wire anemometry These measurements were performed at a Reynolds number of 166 x 10(exp 5), based on cylinder diameter, and spacing-to-diameter ratios, L/D, of 1435 and 37 The current phase, Phase 2, augments this dataset by characterizing the surface flow on the same configurations using steady and unsteady pressure measurements and surface flow visualization Transition strips were applied to the front cylinder during both phases to produce a turbulent boundary layer upstream of the flow separation For these flow conditions and L/D ratios, surface pressures on both the front and rear cylinders show the effects of L/D on flow symmetry, pressure recovery, and the location of flow separation and attachment Mean streamlines and instantaneous vorticity obtained from the PIV data are used to explain the flow structure in the gap and near-wake regions and its relationship to the unsteady surface pressures The combination of off-body and surface measurements provides a comprehensive dataset to develop and validate computational techniques for predicting the unsteady flow field at higher Reynolds numbers

Identification and analysis of coherent structures in the near field of a turbulent unconfined annular swirling jet using large eddy simulation

Abstract: Large eddy simulations of incompressible turbulent flow in an unconfined annular swirling jet at Reynolds number 81500 are reported, based on the outer radius of the jet. The results are in excellent agreement with experimental data for mean flow, turbulent statistics, and power spectral densities of velocity fluctuations. Two dominant families of large-scale coherent structures are identified in the flow. Both are orthogonal to the mean three-dimensional streamlines, which suggests that they are formed as the result of a Kelvin-Helmholtz instability. Instantaneous vortex structures as well as different types of spectra and two-point correlations are presented to further elucidate the properties of the flow.

On electro-osmotic flows through microchannel junctions

Abstract: We study the electro-osmotic flow through a T-junction of microchannels whose dielectric walls are weakly polarizable The present global analysis thus extends earlier studies in the literature concerning the local flow of an unbounded electrolyte solution around nearly insulated wedges The velocity field is obtained via superposition of an irrotational part associated with the equilibrium zeta potential and the induced-charge electro-osmotic flow originating from the interaction of the externally applied electric field and the charge cloud it induces owing to field leakage through the polarizable dielectric channel walls Along the channel walls the latter component gives rise to fluid velocities converging toward the corner which dominate the flow in its immediate vicinity Recent experimental observations in the literature regarding the appearance and subsequent expansion of flow reversal and vortices downstream (initially) and upstream (subsequently) of the junction, are both rationalized in terms of the growing relative importance of this induced contribution to the global velocity field with increasing intensity of the externally applied electric field

Analytical solutions for uniform potential flow past multiple cylinders

Abstract: The problem of uniform potential flow past a circular cylinder is a basic one in fluid dynamics and the solution is well-known. In this paper, an analytical construction is presented to generalize this fundamental result to find solutions for steady irrotational uniform flow past a multi-cylinder configuration in a planar flow in the case when the circulations around the obstacles is taken to vanish. More generally, if a conformal mapping from a canonical multiply connected circular region to the unbounded fluid region exterior to a finite collection of non-cylindrical obstacles of more general shape is known, the formulation also provides solutions for the uniform flow past those obstacles.

Shear flow past two-dimensional droplets pinned or moving on an adhering channel wall at moderate Reynolds numbers: a numerical study

Abstract: Numerical simulations are presented of shear flow past two-dimensional droplets adhering to a wall, at moderate Reynolds numbers. The results were obtained using a level-set method to track the interface, with measures to eliminate any errors in the conservation of mass of droplets. First, the case of droplets whose contact lines are pinned is considered. Data are presented for the critical value of the dimensionless shear rate (Weber number, of the shear flow. It is shown that a Cox–Voinov-type expression can be used to describe the motion of the downstream contact line. A qualitatively different relation is tested for the motion of the upstream contact line. In a third part of this paper, results are presented for droplets moving on a wall with position-dependent sliplength or contact-angle hysteresis window, in an effort to stabilize or destabilize the drop.

Numerical study of the steady-state uniform flow past a rotating cylinder

Abstract: Results from the numerical simulation of the two-dimensional incompressible unsteady Navier–Stokes equations for streaming flow past a rotating circular cylinder are presented in this study. The numerical solution of the equations of motion is conducted with a commercial computational fluid dynamics package which discretizes the equations applying the control volume method. The numerical set-up is validated by comparing results for a Reynolds number based on the free stream of as a unique effective thickness, the modification of Glauert's boundary-layer analysis and the VCVPF approach as proposed by Wang & Joseph (2006a) produce results which are in better general agreement with the values from numerical simulation than those from Glauert's solution.

Inertial effects on the transfer of heat or mass from neutrally buoyant spheres in a steady linear velocity field

Abstract: Microscale inertia is found to break the degenerate closed-streamline configuration that occurs in a shearing flow past a neutrally buoyant torque-free spherical particle in the inertialess limit. The broken symmetry at small but finite Re allows heat or mass to be convected away in an efficient manner in sharp contrast to the inertialess diffusion-limited scenario. Inertial forces scale with the particle Reynolds number, defined as Re=γa2∕ν, where a is the radius of the particle, γ is the characteristic magnitude of the velocity gradient, and ν is the kinematic viscosity of the suspending fluid. The dimensionless heat or mass transfer rate is then given by Nu=C(RePe)1∕3+O(1) when Re≪1 and RePe≫1, the constant C being a function of the flow in the vicinity of the particle. Here, Nu is the Nusselt number defined as Q∕(4πkaΔF), where Q is the dimensional heat/mass flux, k the appropriate transport coefficient, and ΔF the driving force viz. the temperature or concentration difference between the particle a...

Large-Eddy Simulation of Flow over a Wall-Mounted Hump with Separation Control

Abstract: Large-eddy simulation with a dynamic subgrid-scale model and nondissipative numerics is employed to predict the turbulent flow separation over a wall-mounted hump and its control Large-eddy simulation results for the baseline (no control), steady suction, and oscillatory-Jet control cases are compared with the results of experimental measurements and previous computational predictions using large-eddy simulation with a constant coefficient Smagorinsky model and dissipative numerics, implicit large-eddy simulation, detached eddy simulation, and unsteady Reynolds-averaged Navier-Stokes simulation The present large-eddy simulation is shown to be consistently more accurate than the previous numerical approaches in predicting the experimentally measured flow quantities such as the pressure coefficient, reattachment length, mean velocity, and turbulence statistics It is shown that steady suction and synthetic jet oscillations cause a reduction of the reattachment length by about 128 and 73 %, respectively, compared with the uncontrolled case

Fluid flow and mixing in rough-walled fracture intersections

Abstract: [1] Fracture intersections play a basic role in contaminant transport through fracture networks because they allow different fluids to mix and disperse along the flow paths. We use experimental and numerical methods to understand and improve predictions of these phenomena. Laboratory experiments of mixing between two miscible fluids were performed within an artificial, rough-walled fracture intersection made of textured glass. We also develop a numerical model of mixing based on local application of streamline routing within the irregular aperture distribution of the intersection. This model shows good agreement with the laboratory experiments, both in the amount of average mixing and in the spatial distribution of dye streamlines. The numerical model is used to generalize our results based on aperture statistics, and shows that mixing is significantly affected by how well apertures correlate across the intersection, especially as fractures are closed. We conclude that flow channelization through rough-walled intersecting fractures significantly enhances physical mixing compared to intersecting parallel plates. Relative to transport through parallel plate aperture networks, surface roughness may reduce solute dilution and increase solute dispersion.

An Experimental and Numerical Investigation of Crossflow Effects in Two-Phase Displacements

Abstract: Y. Cinar, SPE, and K. Jessen, SPE, Stanford U.; R. Berenblyum, SPE, Technical U. of Denmark; and R. Juanes, SPE,and F.M. Orr Jr., SPE, Stanford U.SummaryIn this paper, we present flow visualization experiments and nu-merical simulations that demonstrate the combined effects of vis-cous and capillary forces and gravity segregation on crossflow thatoccurs in two-phase displacements in layered porous media.We report results of a series of immiscible flooding experi-ments in 2D, two-layered glass bead models. Favorable mobility-ratio imbibition and unfavorable mobility-ratio drainage experi-ments were performed. We used pre-equilibrated immisciblephases from a ternary isooctane/isopropanol/water system, whichallowed control of the interfacial tension (IFT) by varying theisopropanol concentration. Experiments were performed for a widerange of capillary and gravity numbers. The experimental resultsillustrate the transitions from flow dominated by capillary pressureat high IFT to flow dominated by gravity and viscous forces at lowIFT. The experiments also illustrate the complex interplay of cap-illary, gravity, and viscous forces that controls crossflow. Theexperimental results confirm that the transition ranges of scalinggroups suggested by Zhou et al. (1994) are appropriate/valid.We report also results of simulations of the displacement ex-periments by two different numerical techniques: finite-differenceand streamline methods. The numerical simulation results agreewell with experimental observations when gravity and viscousforces were most important. For capillary-dominated flows, thesimulation results are in reasonable agreement with experimen-tal observations.IntroductionStreamline methods are very efficient numerical techniques forfield-scale reservoir simulation, but they are not without limita-tions. They treat flow along each streamline as independent ofadjacent streamlines and therefore do not typically represent cross-flow in the simulations. If users of streamline methods are tointerpret simulation results reliably, they will need to assess wheth-er any of the mechanisms not modeled in the simulations areimportant enough to limit the accuracy of the simulations appreciably.Transfer of fluid in the direction transverse to streamlines canresult from diffusion and dispersion, crossflow caused by viscousand capillary forces, and gravity segregation. The scaling of dif-fusion and dispersion has been investigated in a number of previ-ous studies. If the injected gas is miscible or partially miscible withthe oil, diffusion and dispersion mechanisms may play a signifi-cant role in the displacement (Mohanty and Johnson 1993; Fayersand Lee 1994; Tchelepi 1994; Jiang and Butler 1994; Burger andMohanty 1997). In particular, Burger and Mohanty (1997) showedthat diffusion through the oil phase can limit mass transfer from oilresiding in low-permeability regions. Similar arguments can alsoapply to other mechanisms of crossflow: viscous and capillarycrossflow as well as gravity segregation (Fayers and Lee 1994;Burger and Mohanty 1997; Zapata and Lake 1981; Zhou et al. 1994).Scaling analysis of crossflow mechanisms for displacements inheterogeneous media allows assessment of the relative contribu-tions of each driving force to flow (Tchelepi 1994; Zhou et al.1994; Shook et al. 1992). Starting from material balance equationsand defining them in dimensionless variables, scaling groups canbe obtained that determine the regime of flow during displacement.The relevant scaling groups for displacement of oil by water insystems that contain some simple heterogeneity such as layers arethe transverse gravity and capillary numbers, which are given by(Tchelepi 1994; Zhou et al. 1994):

Natural convection in inclined partitioned enclosures

Abstract: The problem of steady, laminar, natural convective flow of a viscous fluid in an inclined enclosure with partitions is considered. Transverse gradient of temperature is applied on the two opposing regular walls of the inclined enclosure while the other walls are maintained adiabatic. The problem is formulated in terms of the vorticity-stream function procedure. A numerical solution based on the finite volume method is obtained. Representative results illustrating the effects of the enclosure inclination angle and the degree of irregularity on the contour maps of the streamlines and temperature are reported and discussed. In addition, results for the average Nusselt number at the heated wall of the enclosure and the difference of extreme stream-function values are presented and discussed for various Rayleigh numbers, inclination angles and dimensionless partition heights.

Draping or overriding: The effect of horizontal stress gradients on internal layer architecture in ice sheets

Abstract: Internal isochronic layers in ice sheets sensed by radar show two characteristic relationships to the basal topography: Either they override it, with layers above the crests of rises lying essentially flat, or they drape over it, with the layers following rises and falls in basal topography. A mechanical theory is presented which shows that overriding is the expected behavior when topographic wavelengths are comparable with or less than the ice thickness, while draping occurs at longer wavelengths. This is shown with analytical perturbation solutions for Newtonian fluids, numerical perturbation solutions for nonlinear fluids, and finite element solutions for nonlinear fluids and large-amplitude variations. Bed variation from topography and changes in the basal boundary condition are considered, for fixed bed and sliding beds, as well as three-dimensional flows and thermomechanically coupled flows. In all cases, the dominant effect on draping/overriding is the wavelength of the topography or variation in basal boundary conditions. Results of these full mechanical system calculations are compared with those from the shallow ice approximation and the longitudinal stress approximation. Some calculations are carried out for zero accumulation, where the age of the ice and therefore isochrone geometry is not defined. It is shown that there is a close relationship between isochrones and streamlines, and that they behave similarly when bed wavelength divided by the ice thickness is small compared with the ratio of ice velocity and accumulation rate, which is a useful approximation. Numerical comparisons of isochrones and streamlines show them to be virtually coincident.

Analytical Study of Natural Convection in a Cavity With Volumetric Heat Generation

Abstract: An analytical study has been conducted for natural convection in a cavity of different aspect ratios with uniform volumetric heat generation. Two different boundary conditions are investigated for the cavity, viz., all walls are isothermal; two horizontal walls are adiabatic and two vertical walls are isothermal. A stream function vorticity formulation is used where the variables are expanded in terms of Rayleigh number, defined as Ra = gβh 5 q'''/αvk. The governing equations are reduced, to biharmonic equations, and these biharmonic equations are solved using one of the methods, available in the literature. It is observed that the horizontal component of velocity is smaller than the vertical component near the center and the vertical walls of the cavity. The results for velocity' profiles are compared with the simulations obtained from Fluent and they are found to be in good agreement.

Aerodynamic flow control using synthetic jet actuators

Abstract: The suppression of post-stall separation over an unconventional 2-D airfoil at moderate Reynolds numbers (up to 106) using synthetic (zero net mass flux) jet actuators is discussed. As shown by the authors in earlier investigations, the apparent modification of the surface shape by the interaction domain between the actuator jets and the cross flow results in a local displacement of the cross flow streamlines. The concomitant modification of the streamwise pressure gradient upstream of where the flow nominally separates in the baseline configuration can lead to complete suppression of separation over a significant range of angles of attack in the post-stall domain. While in the absence of flow control the airfoil is stalled at angles of attack exceeding 5°, actuation leads to either completely or partially attached flow within the entire range of angles tested (up to 25°) that is accompanied by a dramatic increase in lift and a corresponding decrease in pressure drag. Actuation is typically effected at frequencies that are an order of magnitude higher than the characteristic (shedding) frequency of the airfoil [i.e., F+ ∼ O(10) rather than F+ ∼ O(1)]. When the actuation frequency F+ is O(1), the reattachment is characterized by a Coanda-like tilting of the separated shear layer and the formation of large vortical structures at the driving frequency that persist beyond the trailing edge of the airfoil and lead to unsteady attachment and consequently to a time-periodic variation in vorticity flux and in circulation. In contrast, the suppression of separation at high actuation frequencies [i.e., F+=O(10)] is marked by the absence of organized vortical structures along the flow surface. The dynamics of the transient lift in controlled reattachment and separation are investigated using pulsed amplitude modulation of the actuation input and is exploited to improve the efficacy of the jet actuators by using pulse modulation of the excitation input.