Showing papers on "Open-channel flow published in 2018"

Hall effects on unsteady MHD oscillatory free convective flow of second grade fluid through porous medium between two vertical plates

Abstract: The effects of radiation and Hall current on an unsteady magnetohydrodynamic free convective flow in a vertical channel filled with a porous medium have been studied. We consider an incompressible viscous and electrically conducting incompressible viscous second grade fluid bounded by a loosely packed porous medium. The fluid is driven by an oscillating pressure gradient parallel to the channel plates, and the entire flow field is subjected to a uniform inclined magnetic field of strength Ho inclined at an angle of inclination α with the normal to the boundaries in the transverse xy-plane. The temperature of one of the plates varies periodically, and the temperature difference of the plates is high enough to induce the radiative heat transfer. The effects of various parameters on the velocity profiles, the skin friction, temperature field, rate of heat transfer in terms of their amplitude, and phase angles are shown graphically.

Estimating large-scale structures in wall turbulence using linear models

Abstract: A dynamical systems approach is used to devise a linear estimation tool for channel flow at a friction Reynolds number of . The estimator uses time-resolved velocity measurements at a single wall-normal location to estimate the velocity field at other wall-normal locations (the data coming from direct numerical simulations). The estimation tool builds on the work of McKeon & Sharma (J. Fluid Mech., vol. 658, 2010, pp. 336–382) by using a Navier–Stokes-based linear model and treating any nonlinear terms as unknown forcings to an otherwise linear system. In this way nonlinearities are not ignored, but instead treated as an unknown model input. It is shown that, while the linear estimator qualitatively reproduces large-scale flow features, it tends to overpredict the amplitude of velocity fluctuations – particularly for structures that are long in the streamwise direction and thin in the spanwise direction. An alternative linear model is therefore formed in which a simple eddy viscosity is used to model the influence of the small-scale turbulent fluctuations on the large scales of interest. This modification improves the estimator performance significantly. Importantly, as well as improving the performance of the estimator, the linear model with eddy viscosity is also able to predict with reasonable accuracy the range of wavenumber pairs and the range of wall-normal heights over which the estimator will perform well.

Numerical evidence of logarithmic regions in channel flow at R e τ = 8000

Abstract: Direct numerical simulations of turbulent channel flows up to $R\phantom{\rule{0}{0ex}}{e}_{\ensuremath{\tau}}=8000$ find logarithmic variations both in the mean velocity and streamwise turbulent variance, although these logarithmic regions do not agree with each other.

Determination of epsilon for Omega vortex identification method

Abstract: In the present paper, epsilon (e) in the Omega vortex identification criterion (Ω method) is defined as an explicit function in order to apply the Ω method to different cases and even different time steps for the unsteady cases. In our method, e is defined as a function relating with the flow without any subjective adjustment on its coefficient. The newly proposed criteria for the determination of e is tested in several typical flow cases and is proved to be effective in the current work. The test cases given in the present paper include boundary layer transition, shock wave and boundary layer interaction, and channel flow with different Reynolds numbers.

Cross-stream migration of active particles

Abstract: For natural microswimmers, the interplay of swimming activity and external flow can promote robust directed motion, for example, propulsion against (upstream rheotaxis) or perpendicular to the direction of flow. These effects are generally attributed to their complex body shapes and flagellar beat patterns. Using catalytic Janus particles as a model experimental system, we report on a strong directional response that occurs for spherical active particles in a channel flow. The particles align their propulsion axes to be nearly perpendicular to both the direction of flow and the normal vector of a nearby bounding surface. We develop a deterministic theoretical model of spherical microswimmers near a planar wall that captures the experimental observations. We show how the directional response emerges from the interplay of shear flow and near-surface swimming activity. Finally, adding the effect of thermal noise, we obtain probability distributions for the swimmer orientation that semiquantitatively agree with the experimental distributions.

Viscoelastic Pipe Flow is Linearly Unstable.

Abstract: Newtonian pipe flow is known to be linearly stable at all Reynolds numbers. We report, for the first time, a linear instability of pressure-driven pipe flow of a viscoelastic fluid, obeying the Oldroyd-B constitutive equation commonly used to model dilute polymer solutions. The instability is shown to exist at Reynolds numbers significantly lower than those at which transition to turbulence is typically observed for Newtonian pipe flow. Our results qualitatively explain experimental observations of transition to turbulence in pipe flow of dilute polymer solutions at flow rates where Newtonian turbulence is absent. The instability discussed here should form the first stage in a hitherto unexplored dynamical pathway to turbulence in polymer solutions. An analogous instability exists for plane Poiseuille flow.

Application of Atangana–Baleanu fractional derivative to MHD channel flow of CMC-based-CNT's nanofluid through a porous medium

Abstract: The objective of this article is to apply the Atangana–Baleanu derivative fractional in Caputo sense to convective flow of Carboxy–Methyl–Cellulose (CMC) based Carbon nanotubes (CNT's) nanofluid in a vertical microchannel. The magnetohydrodynamic (MHD) flow through a porous medium together with heat transfer is considered. The Atangana–Baleanu fractional derivative without singular and the non-local kernel is used in the mathematical formulation to get the time fractional governing equations subject to physical initial and boundary conditions. The Laplace transform technique is used to obtain the exact analytical solutions for velocity and temperature distributions. Finally, the influence of parameters of interest is studied through plots and discussed physically.

The common mechanism of turbulent skin-friction drag reduction with superhydrophobic longitudinal microgrooves and riblets

Abstract: Turbulent skin-friction drag reduction with superhydrophobic (SH) longitudinal microgrooves and riblets is investigated by direct numerical simulation (DNS), using lattice Boltzmann methods, in channel flow. The liquid/gas interfaces in the SH longitudinal microgrooves were modelled as stationary, curved, shear-free boundaries, with the meniscus shape determined from the solution of the Young–Laplace equation. Interface protrusion angles of were investigated. For comparison, the same geometries as those formed by the SH interfaces were also studied as riblets. Drag reductions of up to 61 % and up to 5 % were realized in DNS with SH longitudinal microgrooves and riblets, respectively, in turbulent channel flows at bulk Reynolds numbers of ( ) and ( ), with arrays of SH longitudinal microgrooves or riblets of size and on both walls, where and denote the widths and spacings of the microgrooves in base flow wall units, respectively. An exact analytical expression is derived which allows the net drag reduction in laminar or turbulent channel flow with any SH or no-slip wall micro-texture to be decomposed into contributions from: (i) the effective slip velocity at the wall, (ii) modifications to the normalized structure of turbulent Reynolds shear stresses due to the presence of this effective slip velocity at the wall, (iii) other modifications to the normalized structure of turbulent Reynolds shear stresses due to the presence of the wall micro-texture, (iv) modifications to the normalized structure of mean flow shear stresses due to the presence of the wall micro-texture and (v) the fraction of the flow rate through the wall micro-texture. Comparison to DNS results shows that SH longitudinal microgrooves and riblets share a common mechanism of drag reduction in which of the drag reduction arises from effects (i) and (ii). The contributions from (iii)–(v) were always drag enhancing, and followed a common scaling with SH longitudinal microgrooves and riblets when expressed as a function of the square root of the microgroove cross-sectional area in wall units. Extrapolation of drag reduction data from DNS to high Reynolds number flows of practical interest is discussed. It is shown that, for a given geometry and size of the surface micro-texture in wall units, the drag reduction performance of micro-textured surfaces degrades with increasing bulk Reynolds number of the flow. Curved SH interfaces at low protrusion angle ( ) were found to enhance the drag reduction by up to 3.6 % compared to flat interfaces, while reducing the instantaneous pressure fluctuations on the SH interfaces by up to a factor of two. This suggests that the longevity of SH interfaces in turbulent flow may be improved by embedding the SH surface within the microgrooves of shallow, scalloped riblets.

Particulates and Continuum : Multiphase Fluid Dynamics

Abstract: INTRODUCTION Scope and Applications Elementary Particle-Fluid Interactions Heat and Mass Transfer from a Sphere Deformable Particles Momentum Transfer in a Nonuniform Fluid Motion of a Particle Suspended in a Fluid Particle Diffusivity Electrostatic Charging Multiphase Flow Systems Exercise Problems BASIC EQUATIONS Intraphase Equations and Balances at the Interfaces Significance of Phase Configurations Averages and Averaging Theorems Volume Averaged Equations of Conservation and Interface Balance Equations Closure Relations Simplified Formulations Exercise Problems TRANSPORT PROPERTIES AND PROCESSES Drag, Heat, and Mass Transfer of a Particle Cloud Interaction of Particles with Surfaces and Momentum Transfer Transport Processes among Particle Clouds-Single Scattering Viscosity and Thermal Conductivity of Particle Clouds Flow Regimens of a Cloud of Particles in a Turbulent Fluid Brownian Motion, Coagulation, and Agglomeration Exercise Problems EFFECTS OF WAVES AND ELECTRICITY AND SURFACE BOUNDARY CONDITIONS Interaction with Radiation Interaction with Sound Waves Interaction with an Ionized Gas Mobility and Electrical Conductivity Charge Distribution Boundary Conditions of Suspensions Exercise Problems ONE-DIMENSION MOTIONS Adiabatic Flow One-Dimentional Steady Motion Adiabatic Flow through a Nozzle Gas-Liquid Systems Unsteady Flow Shock Waves in Dusty Gas Settling in an External Field Settling of Charged Dusts Exercise Problems PIPE FLOW OF A SUSPENSION Experimental Studies on Dilute Suspensions Basic Relations of Pipe Flow Fully Developed Pipe Flow Sedimentary Flow Flow of Suspension of Fibers Heat Transfer in Pipe Flow Cyclone Separation Exercise Problems GENERAL MOTION OF DILUTE SUSPENSIONS Vortex Motion Eletrohydrodynamic Flow Laminar Boundary Layer Motion over a Flat Plate Particulate Suspension in a Turbulent Fluid Jets and Sprays Diffusion and Fall-Out from Point Sources Flow Over a Cylinder Exercise Problems DENSE SYSTEMS Some Fundamental Nature of Dense Suspensions Pipe Flow Porous Media and Moving Beds Observations on Fluidized Beds Bubbles in Fluid Beds Circulatory Motion in Fluidized Beds Transport Prodesses in Fluidized Beds Exercise Problems

Comparison of solutions of Saint-Venant equations by characteristics and finite difference methods for unsteady flow analysis in open channel

Abstract: The unsteady flow can be analysed by Saint-Venant equations These equations can be solved by characteristics and finite difference methods The Saint-Venant equations are changed into four complete differential equations in characteristics method and these equations are solved by drawing two characteristics lines The Saint-Venant equations are changed into set nonlinear equations and are solved using Preissman scheme in finite difference method This set of equation is changed into linear equation using Newton-Rafson method and can be solved using Sparce method In this research, the results of the two method were compared and this was shown that: 1) these two methods can draw the surface profiles and flow hydrograph as well; 2) the finite difference method is more accurate than that one; 3) the mesh size in finite difference method can be larger than that one; 4) the difference between two methods are increased by increasing the time and distance

Formation of liquid–liquid slug flow in a microfluidic T-junction: Effects of fluid properties and leakage flow

Abstract: Characteristics of liquid–liquid slug flow are investigated in a microchannel with focus on the leakage flow that bypasses droplets through channel gutters. The results show that the leakage flow rate varies in a range of 10.7–53.5% and 8.3–30.9% of the feed flow rate, during the droplet formation (i.e., at T-junction) and downstream flow (i.e., in the main channel), respectively, which highly depends on Ca number and wetting condition. Empirical correlations are proposed to predict them for perfectly and partially wetting conditions. Leakage flow contribution is further used to improve the Garstecki model for size scaling in order to extend its suitability for both squeezing and shearing regimes. The instantaneous flow rates of the immiscible phases are found to fluctuate periodically with the formation cycles, but in opposite behavior. The effect of the presence of leakage flow on such fluctuation are investigated and compared with gas–liquid systems. © 2017 American Institute of Chemical Engineers AIChE J, 2017

Direct numerical simulation of turbulent channel flow over a surrogate for Nikuradse-type roughness

Abstract: A tiled approach to rough surface simulation is used to explore the full range of roughness Reynolds numbers, from the limiting case of hydrodynamic smoothness up to fully rough conditions. The surface is based on a scan of a standard grit-blasted comparator, subsequently low-pass filtered and made spatially periodic. High roughness Reynolds numbers are obtained by increasing the friction Reynolds number of the direct numerical simulations, whereas low roughness Reynolds numbers are obtained by scaling the surface down and tiling to maintain a constant domain size. In both cases, computational requirements on box size, resolution in wall units and resolution per minimum wavelength of the rough surface are maintained. The resulting roughness function behaviour replicates to good accuracy the experiments of Nikuradse (1933 VDI-Forschungsheft, vol. 361), suggesting that the processed grit-blasted surface can serve as a surrogate for his sand-grain roughness, the precise structure of which is undocumented. The present simulations also document a monotonic departure from hydrodynamic smooth-wall results, which is fitted with a geometric relation, the exponent of which is found to be inconsistent with both the Colebrook formula and an earlier theoretical argument based on low-Reynolds-number drag relations.

Experimental study of tsunami-like waves generated with a vertical release technique on dry and wet beds

Abstract: Tsunamis, impulse waves, and dam failures are disasters that challenge humanity, often leading to massive casualties and extreme economic losses. The highly unsteady flow conditions generated by such events are often in the form of turbulent bores. The purpose of this study was to investigate and validate a new generation system for bores propagating over dry and wet bed conditions. There are multiple techniques to generate such waves experimentally, and the study focused on the generation of tsunami-like inundation conditions through the vertical release of a water volume. A detailed methodology to characterize the generated waves hydraulically, in terms of their wave heights and flow velocities, is presented, and good agreement with the classical dam-break case for both dry bed surges and wet bed bores was demonstrated. Because of the importance of estimating the impact forces induced by such waves, particular attention was given to the wavefront celerity and the velocity profiles measured behind the wavefront; these were found in agreement with Prandtl's power law for open channel flows, and in-depth measurements allowed for the definition of an expression to estimate flow deceleration behind the wavefront. Along with considerations of the Froude number and momentum, this paper provides relevant information to assist engineers in designing safer infrastructures in areas prone to such extreme loading.

Inertial focusing of finite-size particles in microchannels

Abstract: At finite Reynolds numbers, , particles migrate across laminar flow streamlines to their equilibrium positions in microchannels. This migration is attributed to a lift force, and the balance between this lift and gravity determines the location of particles in channels. Here we demonstrate that velocity of finite-size particles located near a channel wall differs significantly from that of an undisturbed flow, and that their equilibrium position depends on this, referred to as slip velocity, difference. We then present theoretical arguments, which allow us to generalize expressions for a lift force, originally suggested for some limiting cases and , to finite-size particles in a channel flow at . Our theoretical model, validated by lattice Boltzmann simulations, provides considerable insight into inertial migration of finite-size particles in a microchannel and suggests some novel microfluidic approaches to separate them by size or density at a moderate .

Dynamic slip wall model for large-eddy simulation

Abstract: Wall modelling in large-eddy simulation (LES) is necessary to overcome the prohibitive near-wall resolution requirements in high-Reynolds-number turbulent flows. Most existing wall models rely on assumptions about the state of the boundary layer and require a priori prescription of tunable coefficients. They also impose the predicted wall stress by replacing the no-slip boundary condition at the wall with a Neumann boundary condition in the wall-parallel directions while maintaining the no-transpiration condition in the wall-normal direction. In the present study, we first motivate and analyse the Robin (slip) boundary condition with transpiration (nonzero wall-normal velocity) in the context of wall-modelled LES. The effect of the slip boundary condition on the one-point statistics of the flow is investigated in LES of turbulent channel flow and flat-plate turbulent boundary layer. It is shown that the slip condition provides a framework to compensate for the deficit or excess of mean momentum at the wall. Moreover, the resulting nonzero stress at the wall alleviates the well-known problem of the wall-stress under-estimation by current subgrid-scale (SGS) models. Secondly, we discuss the requirements for the slip condition to be used in conjunction with wall models and derive the equation that connects the slip boundary condition with the stress at the wall. Finally, a dynamic procedure for the slip coefficients is formulated, providing a dynamic slip wall model free of a priori specified coefficients. The performance of the proposed dynamic wall model is tested in a series of LES of turbulent channel flow, non-equilibrium three-dimensional channel flow, and flat-plate turbulent boundary layer. The results show that the dynamic wall model is able to accurately predict one-point turbulence statistics for various flow configurations, Reynolds numbers, and grid resolutions.

Wavelets solution of MHD 3-D fluid flow in the presence of slip and thermal radiation effects

Abstract: This article is devoted to analyze the magnetic field, slip, and thermal radiations effects on generalized three-dimensional flow, heat, and mass transfer in a channel of lower stretching wall. We supposed two various lateral direction rates for the lower stretching surface of the wall while the upper wall of the channel is subjected to constant injection. Moreover, influence of thermal slip on the temperature profile beside the viscous dissipation and Joule heating is also taken into account. The governing set of partial differential equations of the heat transfer and flow are transformed to nonlinear set of ordinary differential equations (ODEs) by using the compatible similarity transformations. The obtained nonlinear ODE set tackled by means of a new wavelet algorithm. The outcomes obtained via modified Chebyshev wavelet method are compared with Runge-Kutta (order-4). The worthy comparison, error, and convergence analysis shows an excellent agreement. Additionally, the graphical representation for var...

Discrete unified gas kinetic scheme for all Knudsen number flows. III. Binary gas mixtures of Maxwell molecules.

Abstract: Recently a discrete unified gas kinetic scheme (DUGKS) in a finite-volume formulation based on the Boltzmann model equation has been developed for gas flows in all flow regimes. The original DUGKS is designed for flows of single-species gases. In this work, we extend the DUGKS to flows of binary gas mixtures of Maxwell molecules based on the Andries-Aoki-Perthame kinetic model [P. Andries et al., J. Stat. Phys. 106, 993 (2002)JSTPBS0022-471510.1023/A:1014033703134. A particular feature of the method is that the flux at each cell interface is evaluated based on the characteristic solution of the kinetic equation itself; thus the numerical dissipation is low in comparison with that using direct reconstruction. Furthermore, the implicit treatment of the collision term enables the time step to be free from the restriction of the relaxation time. Unlike the DUGKS for single-species flows, a nonlinear system must be solved to determine the interaction parameters appearing in the equilibrium distribution function, which can be obtained analytically for Maxwell molecules. Several tests are performed to validate the scheme, including the shock structure problem under different Mach numbers and molar concentrations, the channel flow driven by a small gradient of pressure, temperature, or concentration, the plane Couette flow, and the shear driven cavity flow under different mass ratios and molar concentrations. The results are compared with those from other reliable numerical methods. The results show that the proposed scheme is an effective and reliable method for binary gas mixtures in all flow regimes.

MHD Slip Flow of Cu-Kerosene Nanofluid in a Channel with Stretching Walls Using 3-Stage Lobatto IIIA Formula

Abstract: In this paper, rheology of laminar incompressible Copper-Kerosene nanofluid in a channel with stretching walls under the influence of transverse magnetic field is investigated. The main structure of the partial differential equations was taken from the law of conservation of mass, momentum and energy equations. Governing boundary layer equations are transformed into nonlinear ordinary differential equations by using similarity variables and then solved with 3-stage Lobatto IIIA formula. Numerical results were compared with another numerical method (Runge-Kutta-Fehlberg) and found excellent agreement. The influence of physical parameters Reynolds number, magnetic number, solid volume fraction, momentum and thermal slip parameters on velocity and temperature profile considered. Numerical results revealed that solid volume fraction decreases the velocity of nanofluid particles near the lower wall of the channel and increase the thermal boundary layer thickness in the channel.

Assessing flow resistance in gravel bed channels by dimensional analysis and self-similarity

Abstract: In this paper a new flow resistance equation for open channel flow, based on the integration of a power velocity profile, was tested for gravel bed channels. First this flow resistance equation, theoretically deduced by dimensional analysis and incomplete self-similarity condition, was reported. Then a relationship between the Γ function of the velocity profile, the channel slope and the Froude number was calibrated by the available laboratory measurements of flow velocity, water depth and bed slope carried out in 416 flume experimental runs with a gravel bed. Then the relationship for estimating Γ function and the theoretical resistance equation was tested by 83 independent flume measurements. The analysis also showed that the proposed flow resistance equation allows an estimate of the Darcy-Weisbach friction factor which is more reliable and accurate than that obtained by a semi-logarithmic flow resistance law or a variable-power resistance equation, calibrated with the same gravel bed measurements. For testing the applicability of the proposed Γ function (Eq. (17)), whose coefficients were estimated by flume measurements, available fields measurements were used. The analysis demonstrated that a scale factor (equal to 0.7611) is necessary to convert Γ values obtained by flume measurements into those corresponding to gravel bed rivers. The similitude between flow resistance in a gravel bed flume and in a gravel bed river is governed by the Γ function and a scale factor, equal to 1.6, is required to upscale the Darcy-Weisbach friction factor values obtained by flume measurements to the river case. In conclusion, the analysis showed that the Darcy-Weisbach friction factor for gravel bed channels can be accurately estimated by the proposed theoretical approach based on a power-velocity profile.

Influence of Hall current and wall conductivity on hydromagnetic mixed convective flow in a rotating Darcian channel

Abstract: In the present study, an analysis for steady hydromagnetic mixed convective generalised Couette flow between two infinite parallel plates of arbitrary electrical conductivities and finite thicknesses filled with a porous medium in the presence of a uniform transverse magnetic field in a rotating system with the Hall effect is presented. The heat transfer characteristics of the fluid flows are also investigated, taking viscous and Joule dissipations into account. Exact solutions of the resulting simultaneous ordinary differential equations governing the fluid flows are obtained in a closed form. The closed form analytical solutions for shear stress and mass flow rate are also obtained. To examine the physical consequences and flow characteristics, the numerical results for velocity, induced magnetic field, temperature field, shear stress, mass flow rate, and rate of heat transfer are computed for different values of various system parameters and are displayed in graphical and tabular forms. An interesting observation recorded that there arises flow reversal in the secondary flow direction when the permeability parameter is very small, i.e., when Darcian drag force is very large.

Diffusiophoresis in narrow channel flows

Abstract: Flows containing suspended colloidal particles and dissolved solutes are found in a multitude of natural and man-made systems including hydraulic fractures, water filtration systems and microfluidic devices, e.g. those designed for biological or medical applications. In these types of systems, unexpected particle dynamics such as rapid particle transport and focusing has been observed in the presence of local solute gradients due to the cooperating or competing effects of fluid advection and particle diffusiophoresis, the latter driven by local chemical gradients. We develop analytical expressions for the fluid, solute and particle dynamics in long, narrow channels due to the combined influence of pressure-driven channel flow with diffusiophoretic and diffusioosmotic effects. The results confirm a rapid particle focusing effect that can be controlled by manipulating the particle, solute and flow properties, as well as the channel’s geometry and surface chemistry. Thus, we propose a new approach for performing microfluidic zeta potentiometry, as well as techniques for sorting, concentrating and/or capturing particles based on their sizes or zeta potentials. Finally, we demonstrate that diffusioosmotic effects can be used to pump fluid against a pressure gradient.