Showing papers on "Open-channel flow published in 2011"

Interface-resolved DNS of vertical particulate channel flow in the turbulent regime

Abstract: We have conducted a direct numerical simulation (DNS) study of dilute turbulent particulate flow in a vertical plane channel, considering thousands of finite-size rigid particles with resolved phase interfaces. The particle diameter corresponds to approximately 11 wall units and their terminal Reynolds number is set to 136. The fluid flow with bulk Reynolds number 2700 is directed upward, which maintains the particles suspended upon average. Two density ratios were simulated, differing by a factor of 4.5. The corresponding Stokes numbers of the two flow cases were O(10) in the near-wall region and O(1) in the outer flow. We have observed the formation of large-scale elongated streak-like structures with streamwise dimensions of the order of 8 channel half-widths and cross-stream dimensions of the order of one half-width. At the same time, we have found no evidence of significant formation of particle clusters, which suggests that the large structures are due to an intrinsic instability of the flow, triggered by the presence of the particles. It was found that the mean fluid velocity profile tends towards a concave shape, and the turbulence intensity as well as the normal stress anisotropy are strongly increased. The effect of varying the Stokes number while maintaining the buoyancy, particle size and volume fraction constant was relatively weak.

Fox and McDonald's Introduction to Fluid Mechanics

Abstract: CHAPTER 1 INTRODUCTION. 1.1 Note to Students. 1.2 Scope of Fluid Mechanics. 1.3 Definition of a Fluid. 1.4 Basic Equations. 1.5 Methods of Analysis. 1.6 Dimensions and Units. 1.7 Analysis of Experimental Error. 1.8 Summary. Problems. CHAPTER 2 FUNDAMENTAL CONCEPTS. 2.1 Fluid as a Continuum. 2.2 Velocity Field. 2.3 Stress Field. 2.4 Viscosity. 2.5 Surface Tension. 2.6 Description and Classification of Fluid Motions. 2.7 Summary and Useful Equations. References. Problems. CHAPTER 3 FLUID STATICS. 3.1 The Basic Equation of Fluid Statics. 3.2 The Standard Atmosphere. 3.3 Pressure Variation in a Static Fluid. 3.4 Hydraulic Systems. 3.5 Hydrostatic Force on Submerged Surfaces. 3.6 Buoyancy and Stability. 3.7 Fluids in Rigid-Body Motion (on the Web). 3.8 Summary and Useful Equations. References. Problems. CHAPTER 4 BASIC EQUATIONS IN INTEGRAL FORM FOR A CONTROL VOLUME. 4.1 Basic Laws for a System. 4.2 Relation of System Derivatives to the Control Volume Formulation. 4.3 Conservation of Mass. 4.4 Momentum Equation for Inertial Control Volume. 4.5 Momentum Equation for Control Volume with Rectilinear Acceleration. 4.6 Momentum Equation for Control Volume with Arbitrary Acceleration (on the Web). 4.7 The Angular-Momentum Principle. 4.8 The First Law of Thermodynamics. 4.9 The Second Law of Thermodynamics. 4.10 Summary and Useful Equations. Problems. CHAPTER 5 INTRODUCTION TO DIFFERENTIAL ANALYSIS OF FLUID MOTION. 5.1 Conservation of Mass. 5.2 Stream Function for Two-Dimensional Incompressible Flow. 5.3 Motion of a Fluid Particle (Kinematics). 5.4 Momentum Equation. 5.5 Introduction to Computational Fluid Dynamics. 5.6 Summary and Useful Equations. References. Problems. CHAPTER 6 INCOMPRESSIBLE INVISCID FLOW. 6.1 Momentum Equation for Frictionless Flow: Euler's Equation. 6.2 Euler's Equations in Streamline Coordinates. 6.3 Bernoulli Equation-Integration of Euler's Equation Along a Streamline for Steady Flow. 6.4 The Bernoulli Equation Interpreted as an Energy Equation. 6.5 Energy Grade Line and Hydraulic Grade Line. 6.6 Unsteady Bernoulli Equation: Integration of Euler's Equation Along a Streamline (on the Web). 6.7 Irrotational Flow. 6.8 Summary and Useful Equations. References. Problems. CHAPTER 7 DIMENSIONAL ANALYSIS AND SIMILITUDE. 7.1 Nondimensionalizing the Basic Differential Equations. 7.2 Nature of Dimensional Analysis. 7.3 Buckingham Pi Theorem . 7.4 Determining the PI Groups. 7.5 Significant Dimensionless Groups in Fluid Mechanics. 7.6 Flow Similarity and Model Studies. 7.7 Summary and Useful Equations. References. Problems. CHAPTER 8 INTERNAL INCOMPRESSIBLE VISCOUS FLOW. 8.1 Introduction. PART A. FULLY DEVELOPED LAMINAR FLOW. 8.2 Fully Developed Laminar Flow between Infinite Parallel Plates. 8.3 Fully Developed Laminar Flow in a Pipe. PART B. FLOW IN PIPES AND DUCTS. 8.4 Shear Stress Distribution in Fully Developed Pipe Flow. 8.5 Turbulent Velocity Profiles in Fully Developed Pipe Flow. 8.6 Energy Considerations in Pipe Flow. 8.7 Calculation of Head Loss. 8.8 Solution of Pipe Flow Problems. PART C. FLOW MEASUREMENT. 8.9 Direct Methods. 8.10 Restriction Flow Meters for Internal Flows. 8.11 Linear Flow Meters. 8.12 Traversing Methods. 8.13 Summary and Useful Equations. References. Problems. CHAPTER 9 EXTERNAL INCOMPRESSIBLE VISCOUS FLOW. PART A. BOUNDARY LAYERS. 9.1 The Boundary-Layer Concept. 9.2 Boundary-Layer Thicknesses. 9.3 Laminar Flat-Plate Boundary Layer: Exact Solution (on the Web). 9.4 Momentum Integral Equation. 9.5 Use of the Momentum Integral Equation for Flow with Zero Pressure Gradient. 9.6 Pressure Gradients in Boundary-Layer Flow. PART B. FLUID FLOW ABOUT IMMERSED BODIES. 9.7 Drag. 9.8 Lift. 9.9 Summary and Useful Equations. References. Problems. CHAPTER 10 FLUID MACHINERY. 10.1 Introduction and Classification of Fluid Machines. 10.2 Turbomachinery Analysis. 10.3 Pumps, Fans, and Blowers. 10.4 Positive Displacement Pumps. 10.5 Hydraulic Turbines. 10.6 Propellers and Wind-Power Machines. 10.7 Compressible Flow Turbomachines. 10.8 Summary and Useful Equations. References. Problems. CHAPTER 11 FLOW IN OPEN CHANNELS. 11.1 Basic Concepts and Definitions. 11.2 Energy Equation for Open-Channel Flows. 11.3 Localized Effect of Area Change (Frictionless Flow). 11.4 The Hydraulic Jump. 11.5 Steady Uniform Flow. 11.6 Flow with Gradually Varying Depth. 11.7 Discharge Measurement Using Weirs. 11.8 Summary and Useful Equations. References. Problems. CHAPTER 12 INTRODUCTION TO COMPRESSIBLE FLOW. 12.1 Review of Thermodynamics. 12.2 Propagation of Sound Waves. 12.3 Reference State: Local Isentropic Stagnation Properties. 12.4 Critical Conditions. 12.5 Summary and Useful Equations. References. Problems. CHAPTER 13 COMPRESSIBLE FLOW. 13.1 Basic Equations for One-Dimensional Compressible Flow. 13.2 Isentropic Flow of an Ideal Gas: Area Variation. 13.3 Normal Shocks. 13.4 Supersonic Channel Flow with Shocks. 13.5 Flow in a Constant-Area Duct with Friction. 13.6 Frictionless Flow in a Constant-Area Duct with Heat Exchange. 13.7 Oblique Shocks and Expansion Waves. 13.8 Summary and Useful Equations. References. Problems. APPENDIX A FLUID PROPERTY DATA. APPENDIX B EQUATIONS OF MOTION IN CYLINDRICAL COORDINATES. APPENDIX C VIDEOS FOR FLUID MECHANICS. APPENDIX D SELECTED PERFORMANCE CURVES FOR PUMPS AND FANS. APPENDIX E FLOW FUNCTIONS FOR COMPUTATION OF COMPRESSIBLE FLOW. APPENDIX F ANALYSIS OF EXPERIMENTAL UNCERTAINTY. APPENDIX G SI UNITS, PREFIXES, AND CONVERSION FACTORS. APPENDIX H A BRIEF REVIEW OF MICROSOFT EXCEL (ON THE WEB). Answers to Selected Problems. Index.

The wake of two side-by-side square cylinders

Abstract: Aerodynamic interference between two cylinders involves most of the generic flow features associated with multiple structures, thus providing an excellent model for gaining physical insight into the wake of multiple cylindrical structures. This work aims to provide an experimental systematic study of the flow behind two side-by-side square cylinders. The square cylinder is a representative model for bluff bodies with sharp corners, characterized by a fixed flow separation point, which are distinct from those of continuous curvature with oscillating separation points, typically represented by the circular cylinder. Experiments were performed at a Reynolds number Re of 4.7 × 10 4 and a cylinder centre-to-centre spacing ratio T/ d (d is the cylinder height) of 1.02–6.00. The flow was measured using different techniques, including hot wires, load cell, particle imaging velocimetry and laser-induced fluorescence flow visualization. Four distinct flow regimes and their corresponding T/ d ranges are identified for the first time on the basis of the flow structure and the Strouhal number. Physical aspects in each regime, such as interference between shear layers, gap flow deflection and changeover, multiple flow modes, entrainment, recirculation bubble, vortex interactions and formation lengths, are investigated in detail and are connected to the characteristics of the time-averaged and fluctuating fluid forces. The flow displays a marked difference in many facets from that behind two side-by-side circular cylinders, which is linked to their distinct flow separation natures. A crucial role played by the gap flow and its passage geometry in contributing to the observed difference is also unveiled.

Hydraulic Radius for Evaluating Resistance Induced by Simulated Emergent Vegetation in Open-Channel Flows

Abstract: The resistance induced by simulated emergent vegetation in open-channel flows has been interpreted differently in the literature, largely attributable to inconsistent uses of velocity and length scales in the definition of friction factor or drag coefficient and Reynolds number. By drawing analogies between pipe flows and vegetated channel flows, this study proposes a new friction function with the Reynolds number that is redefined by using a vegetation-related hydraulic radius. The new relationship is useful for consolidating various experimental data across a wide range of vegetation density. The results clearly show a monotonic decrease of the drag coefficient with the new Reynolds number, which is qualitatively comparable to other drag coefficient relationships for nonvegetated flows. This study also proposes a procedure for correcting sidewall and bed effects in the evaluation of vegetation drag.

Turbulence modification by stable stratification in channel flow

Abstract: Direct numerical simulations of stably stratified, turbulent channel flow at low to moderate Reynolds number have been performed using large computational boxes and considering a wide range of stratification levels. For weak stratification or high Reynolds number, the turbulence is affected by buoyancy in the core of the channel, but the near-wall region differs little from the neutral case. With strong stratification, large laminar patches appear in the near-wall region and turbulent momentum and buoyancy fluxes vanish in the core of the channel. With increasing stratification, the near-wall streaks remain essentially unmodified, while large-scale global modes are damped. In the central region, internal gravity waves are dominant. In addition, there is an intermediate outer layer where the dynamics of the turbulent structures is governed by local fluxes. In this region, energy spectra collapse when using local Obukhov scaling.

Turbulent boundary layers over permeable walls: scaling and near-wall structure

Abstract: This paper presents an experimental study devoted to investigating the effects of permeability on wall turbulence. Velocity measurements were performed by means of laser Doppler anemometry in open channel flows over walls characterized by a wide range of permeability. Previous studies proposed that the von Karman coefficient associated with mean velocity profiles over permeable walls is significantly lower than the standard values reported for flows over smooth and rough walls. Furthermore, it was observed that turbulent flows over permeable walls do not fully respect the widely accepted paradigm of outer-layer similarity. Our data suggest that both anomalies can be explained as an effect of poor inner–outer scale separation if the depth of shear penetration within the permeable wall is considered as the representative length scale of the inner layer. We observed that with increasing permeability, the near-wall structure progressively evolves towards a more organized state until it reaches the condition of a perturbed mixing layer where the shear instability of the inflectional mean velocity profile dictates the scale of the dominant eddies. In our experiments such shear instability eddies were detected only over the wall with the highest permeability. In contrast attached eddies were present over all the other wall conditions. On the basis of these findings, we argue that the near-wall structure of turbulent flows over permeable walls is regulated by a competing mechanism between attached and shear instability eddies. We also argue that the ratio between the shear penetration depth and the boundary layer thickness quantifies the ratio between such eddy scales and, therefore, can be used as a diagnostic parameter to assess which eddy structure dominates the near-wall region for different wall permeability and flow conditions

On the fluctuating wall-shear stress in zero pressure-gradient turbulent boundary layer flows

Abstract: Recent direct numerical simulation (DNS) results relating to the behavior of the fluctuating wall-shear stress τw,rms+ in turbulent boundary layer flows are discussed. This new compilation is motivated by a recent article [Wu and Moin, “Transitional and turbulent boundary layer with heat transfer,” Phys. Fluids 22, 085105 (2010)], which indicates a need for clarification of the value of τw,rms+. It is, however, shown here, based on other recent DNS data, that most results, both in boundary layer and channel geometry, yield τw,rms+≈0.4 plus a small increase with Reynolds number coming from the growing influence of the outer spectral peak. The observed discrepancy in experimental data is mainly attributed to spatial resolution effects, as originally described by Alfredsson et al. [“The fluctuating wall-shear stress and the velocity field in the viscous sublayer,” Phys. Fluids 31, 1026 (1988)].

Spatial distribution of deposition within a patch of vegetation

Abstract: [1] This laboratory study describes the spatial pattern of deposition observed in a patch of vegetation located at the wall of a channel. There are two sources of sediment flux to the patch: the advection of particles across the upstream leading edge and the lateral dispersion of particles from the adjacent open channel. The relative contribution of these two supplies determines the spatial pattern of net deposition in the patch. We define the advection length scale within the patch as the longitudinal distance over which advection contributes a significant sediment source. At distances from the leading edge that are within the advection length scale, net deposition in the patch is laterally uniform, reflecting the laterally uniform mean flow delivering the particles. At distances farther than the advection length scale from the leading edge, the net deposition is highest near the flow-parallel edge and decreases into the patch, which is the signature of dispersive transport from the patch edge. Two processes contribute to the lateral dispersion, both of which are associated with the shear-layer vortices formed at the flow-parallel interface between the patch and the channel. The vortices generate turbulence and enhance the turbulent diffusion of sediment across the interface. In addition, the vortices induce a wave oscillation in the flow field within the patch that appears to enhance the lateral transport inside the patch.

Near-wall turbulence statistics and flow structures over three-dimensional roughness in a turbulent channel flow

Abstract: Utilizing an optically index-matched facility and high-resolution particle image velocimetry measurements, this paper examines flow structure and turbulence in a rough-wall channel flow for Reτ in the 3520–5360 range. The scales of pyramidal roughness elements satisfy the ‘well-characterized’ flow conditions, with h/k ≈ 50 and k+ = 60 ~ 100, where h is half height of the channel and k is the roughness height. The near-wall turbulence measurements are sensitive to spatial resolution, and vary with Reynolds number. Spatial variations in the mean flow, Reynolds stresses, as well as the turbulent kinetic energy (TKE) production and dissipation rates are confined to y < 2k. All the Reynolds stress components have local maxima at slightly higher elevations, but the streamwise-normal component increases rapidly at y < k, peaking at the top of the pyramids. The TKE production and dissipation rates along with turbulence transport also peak near the wall. The spatial energy and shear spectra show an increasing contribution of large-scale motions and a diminishing role of small motions with increasing distance from the wall. As the spectra steepen at low wavenumbers, they flatten and develop bumps in wavenumbers corresponding to k − 3k, which fall in the dissipation range. Instantaneous realizations show that roughness-scale eddies are generated near the wall, and lifted up rapidly by large-scale structures that populate the outer layer. A linear stochastic estimation-based analysis shows that the latter share common features with hairpin packets. This process floods the outer layer with roughness-scale eddies, in addition to those generated by the energy-cascading process. Consequently, although the imprints of roughness diminish in the outer-layer Reynolds stresses, consistent with the wall similarity hypothesis, the small-scale turbulence contains a clear roughness signature across the entire channel.

Turbulence statistics in an open-channel flow over a rough bed

Abstract: This paper presents the results of a large-eddy simulation (LES) of turbulent flow over a channel bed artificially roughened by hemispheres. The Reynolds number of the flow based on the channel depth is 13,680 at a relatively low submergence of 3.42. First- and second-order statistics are compared with corresponding laboratory experiments to validate the LES. The effect of roughness heterogeneity on higher-order statistics is quantified and discussed. The contribution of the dominating turbulent events (i.e., sweeps, ejections) to the Reynolds stress and the anisotropy of turbulence are quantified. Visualizations of the complex three-dimensional turbulence structures reveal the occurrence of a number of different vortex types in the flow. The contribution of turbulence structures to the turbulent kinetic energy and their scaling is assessed through proper orthogonal decomposition.

The structure of turbulent flow in an open channel bend of strong curvature with deformed bed: Insight provided by detached eddy simulation

Abstract: [1] Results of a detached eddy simulation (DES) are used to better understand the effects of the mean flow three-dimensionality and secondary currents on turbulence and boundary shear stresses and the mechanisms through which the momentum and Reynolds stresses are redistributed in a strongly curved 193° bend with fixed deformed bed corresponding to the later stages of the erosion and sedimentation process. The ratio between the radius of curvature of the curved reach and the channel width is close to 1.3. The large channel curvature and the point bar induce flow separation near the inner bank and the formation of several strong separated shear layers (SSLs), where production by mean shear dominates. DES shows that in addition to the main cell of cross-stream circulation developing in the deeper part of the bend, several streamwise-oriented vortices (SOV) form at the inner bank. DES satisfactorily captures the distribution of the streamwise velocity and streamwise vorticity in relevant cross sections compared to experiment. Comparison with a Reynolds-averaged Navier-Stokes (RANS) simulation shows that DES predicts more accurately the velocity redistribution and cross-stream motions in the channel. This is because RANS significantly underpredicts the circulation and turbulence amplification inside the cores of the SOV vortices. DES is then used to clarify the influence of the SOV vortices and SSLs on the boundary shear stress. DES reveals the presence of several regions of large amplification of the pressure RMS fluctuations near the inner and outer banks, which can locally increase the bed erosion and affect the bank stability in the case of a bend with erodible banks. The mean flow bed shear stress distribution predicted by DES is significantly different than that predicted by RANS, while DES predictions of the mean flow are more accurate. This means that use of eddy-resolving techniques like DES in mobile bed simulations of flow in curved alluvial channels should result in more accurate predictions of bathymetry.

Three dimensional flow around a circular cylinder confined in a plane channel

Abstract: This paper presents two- and three-dimensional direct numerical simulations of the flow around a circular cylinder placed symmetrically in a plane channel. Results are presented in the Reynolds number range (based on the cylinder diameter and centerline velocity) of 10 to 390 for a blockage ratio (ratio of the cylinder diameter to the channel height) of 0.2. The aim of this work was to investigate in detail the confinement effect due to the channel’s stationary walls on the force coefficients and the associated Strouhal numbers, as well as on the generated flow regimes. Present results suggest a transition from a 2-D to a 3-D shedding flow regime between Re = 180 and Re = 210. This transition was found to be dominated by mode A and mode B three dimensional instabilities, similar to those observed in the case of an unconfined circular cylinder. This is the first time that the existence of the two modes, and of naturally occurring vortex dislocations, has been confirmed via full 3-D simulations for the case...

Inertial migration of deformable capsules in channel flow

Abstract: Using three-dimensional computer simulations, we study the cross-stream inertial migration of neutrally buoyant deformable particles in a pressure-driven channel flow. The particles are modeled as elastic shells filled with a viscous fluid. We show that the particles equilibrate in a channel flow at off-center positions that depend on particle size, shell compliance, and the viscosity of encapsulated fluid. These equilibrium positions, however, are practically independent of the magnitude of channel Reynolds number in the range between 1 and 100. The results of our studies can be useful for sorting, focusing, and separation of micrometer-sized synthetic particles and biological cells.

Coeval thrusting and extension during lower crustal ductile flow - implications for exhumation of high-grade metamorphic rocks

Abstract: Numerical models of tectonic processes in large hot orogens offer insight into the styles, controls and consequences of ductile flow of middle and lower orogenic crust. We present results from crustal-scale thermal-mechanical models illustrating contrasting styles of syn- and post-convergent ductile flow. Crustal viscosity in the models is controlled by temperature-dependent flow laws, scaled to represent lithological strength variations. An additional viscosity decrease at high temperature can be imposed to represent weakening during incipient partial melting. Two end-member tectonic styles, in which lower crustal flow is driven either by gravity or tectonics, develop in response to the initial properties and evolving strength of the model crust. Homogeneous channel flow, driven by the gravitational potential energy difference between a plateau and its foreland, is characteristic of Himalayan–Tibetan (HT-series) models with relatively weak, laterally homogeneous crust and moderately high heat production. In contrast, Grenville Orogen (GO-series) models with relatively strong, laterally heterogeneous crust do not reach the low effective viscosity (ηeff≤1019 Pa.s) required for gravity-driven channel flow. Instead, tectonically driven lateral flow and explusion of lower crust is triggered by underthrusting of a strong indentor. The dominant structures are ductile fold nappes that may be transported hundreds of kilometres towards the foreland. When tectonic convergence is turned off, the model orogens undergo gravitational spreading, in which ductile middle and lower crust flow laterally from the plateau towards the foreland, driving thrusting on the orogenic flanks and ductile thinning in the orogenic core. The resulting geometry can resemble that produced by syn-convergent ductile flow, except for the development of regional-scale normal-sense shear zones in the post-convergent stage. Model feasibility is tested against data from the western Grenville Orogen in Ontario. The comparison suggests that high-grade metamorphic rocks in the orogenic core record the effects of protracted syn- and post-convergent ductile flow, whereas those on the flanks record mainly post-convergent thrusting and extension. The models suggest a number of ways to distinguish among different styles of syn- and post-convergent ductile flow; metamorphic P–T–t paths alone are not diagnostic of tectonic process without additional information on crustal-scale structure and timing. In combination with erosion, both types of flow may contribute to exhumation of regionally extensive high-grade gneiss terranes.

A wall-layer model for large-eddy simulations of turbulent flows with/out pressure gradient

Abstract: In this work, modeling of the near-wall region in turbulent flows is addressed A new wall-layer model is proposed with the goal to perform high-Reynolds number large-eddy simulations of wall bounded flows in the presence of a streamwise pressure gradient The model applies both in the viscous sublayer and in the inertial region, without any parameter to switch from one region to the other An analytical expression for the velocity field as a function of the distance from the wall is derived from the simplified thin-boundary equations and by using a turbulent eddy coefficient with a damping function This damping function relies on a modified van Driest formula to define the mixing-length taking into account the presence of a streamwise pressure gradient The model is first validated by a priori comparisons with direct numerical simulation data of various flows with and without streamwise pressure gradient and with eventual flow separation Large-eddy simulations are then performed using the present wall model as wall boundary condition A plane channel flow and the flow over a periodic arrangement of hills are successively considered The present model predictions are compared with those obtained using the wall models previously proposed by Spalding, Trans ASME, J Appl Mech 28, 243 (2008) and Manhart et al, Theor Comput Fluid Dyn 22, 243 (2008) It is shown that the new wall model allows for a good prediction of the mean velocity profile both with and without streamwise pressure gradient It is shown than, conversely to the previous models, the present model is able to predict flow separation even when a very coarse grid is used

The Laminar Generalized Stokes Layer and Turbulent Drag Reduction

Abstract: This paper considers plane channel flow modified by waves of spanwise velocity applied at the wall and travelling along the streamwise direction. Both laminar and turbulent regimes for the streamwise flow are studied. When the streamwise flow is laminar, it is unaffected by the spanwise flow induced by the waves. This flow is a thin, unsteady and streamwise-modulated boundary layer that can be expressed in terms of the Airy function of the first kind. We name it the generalized Stokes layer because it reduces to the classical oscillating Stokes layer in the limit of infinite wave speed. When the streamwise flow is turbulent, the laminar generalized Stokes layer solution describes well the space-averaged turbulent spanwise flow, provided that the phase speed of the waves is sufficiently different from the turbulent convection velocity, and that the time scale of the forcing is smaller than the life time of the near-wall turbulent structures. Under these conditions, the drag reduction is found to scale with the Stokes layer thickness, which renders the laminar solution instrumental for the analysis of the turbulent flow. A classification of the turbulent flow regimes induced by the waves is presented by comparing parameters related to the forcing conditions with the space and time scales of the turbulent flow.

Lagrangian model of bed-load transport in turbulent junction flows

Abstract: Motivated by the need to gain fundamental insights into the mechanisms of bed-load sediment transport in turbulent junction flows, we carry out a computational study of Lagrangian dynamics of inertial particles initially placed on the bed upstream of a surface-mounted circular cylinder in a rectangular open channel (Dargahi, J. Hydraul. Engng, vol. 116, 1990, pp. 1197–1214). The flow field at Re = 39000 is simulated using the detached eddy simulation (DES) approach (Spalart et al., In Advances in DNS/LES, ed. C. Liu & Z. Liu, 1997, Greyden), which has already been shown to accurately resolve most of the turbulent stresses produced by the low-frequency, bimodal fluctuations of the turbulent horseshoe vortex (Paik et al., J. Hydraul. Engng, vol. 131, 1990, pp. 441–456; Escauriaza & Sotiropoulos, Flow Turbul. Combust., 2010, in press). The trajectory and momentum equations for the sediment particles are integrated numerically simultaneously with the flow governing equations assuming one-way coupling and neglecting particle-to-particle interactions (dilute flow) but taking into account bed–particle interactions and the effects of the instantaneous hydrodynamic forces induced by the resolved fluctuations of the coherent vortical structures. The computed results show that, in accordance with the simulated clear-water scour condition (i.e. the magnitude of the particle stresses is near the threshold of motion), the transport of sediment grains is highly intermittent and exhibits essentially all the characteristics of bed-load sediment transport observed in laboratory and field experiments. Groups of sediment grains are dislodged from the bed simultaneously in seemingly random bursting events and begin to move, saltating or sliding along the bed. Furthermore, particles that are not entrained into the bed-load layer are found to form streaks aligned with near-wall vortices around the cylinder. The global transport of particles is studied by performing a statistical analysis of the bed-load flux to reveal scale-invariance of the process and multifractality of particle transport as the overall effect of the coherent structures of the flow. A major finding of this work is that a relatively simple Lagrangian model coupled with a coherent-structure resolving simulation of the turbulent flow is able to reproduce the sediment dynamics observed in multiple experiments performed under similar conditions, and provide fundamental information on the initiation of motion and the multifractal nature of bed-load transport processes. The results also motivate the development of new Eulerian bed-load transport models that consider unsteady conditions and incorporate the intermittency of the unresolved scales of sediment motion.

Some exact solutions for debris and avalanche flows

Abstract: Exact analytical solutions to simplified cases of nonlinear debris avalanche model equations are necessary to calibrate numerical simulations of flow depth and velocity profiles on inclined surfaces. These problem-specific solutions provide important insight into the full behavior of the system. In this paper, we present some new analytical solutions for debris and avalanche flows and then compare these solutions with experimental data to measure their performance and determine their relevance. First, by combining the mass and momentum balance equations with a Bagnold rheology, a new and special kinematic wave equation is constructed in which the flux and the wave celerity are complex nonlinear functions of the pressure gradient and the flow depth itself. The new model can explain the mechanisms of wave advection and distortion, and the quasiasymptotic front bore observed in many natural and laboratory debris and granular flows. Exact time-dependent solutions for debris flow fronts and associated velocity...

Free-Surface Viscous Flow Solution Methods for Ship Hydrodynamics

Abstract: The simulation of viscous free-surface water flow is a subject that has reached a certain maturity and is nowadays used in industrial applications, like the simulation of the flow around ships. While almost all methods used are based on the Navier-Stokes equations, the discretisation methods for the water surface differ widely. Many of these highly different methods are being used with success. We review three of these methods, by describing in detail their implementation in one particular code that is being used in industrial practice. The descriptions concern the principle of the method, numerical details, and the method’s strengths and limitations. For each code, examples are given of its use. Finally, the methods are compared to determine the best field of application for each. The following surface descretisation methods are reviewed. First, surface fitting/mesh deformation in PARNASSOS, developed by MARIN; the description focuses on the efficient steady-state solution method of this code. Then surface capturing with Volume-of-Fluid in ISIS-CFD, developed by CNRS/Ecole Centrale de Nantes; the main topic of this review are the compressive flux discretisation schemes for the volume fraction that are used in this code. And finally, the Level Set method in SURF, developed by NMRI; this description contains a modified formulation of the Level Set method that is optimised for ship flow computation.

Some aspects of aerodynamic flow control using synthetic-jet actuation.

Abstract: Aerodynamic flow control effected by interactions of surface-mounted synthetic (zero net mass flux) jet actuators with a local cross flow is reviewed. These jets are formed by the advection and interactions of trains of discrete vortical structures that are formed entirely from the fluid of the embedding flow system, and thus transfer momentum to the cross flow without net mass injection across the flow boundary. Traditional approaches to active flow control have focused, to a large extent, on control of separation on stalled aerofoils by means of quasi-steady actuation within two distinct regimes that are characterized by the actuation time scales. When the characteristic actuation period is commensurate with the time scale of the inherent instabilities of the base flow, the jets can effect significant quasi-steady global modifications on spatial scales that are one to two orders of magnitude larger than the scale of the jets. However, when the actuation frequency is sufficiently high to be decoupled from global instabilities of the base flow, changes in the aerodynamic forces are attained by leveraging the generation and regulation of 'trapped' vorticity concentrations near the surface to alter its aerodynamic shape. Some examples of the utility of this approach for aerodynamic flow control of separated flows on bluff bodies and fully attached flows on lifting surfaces are also discussed.

Force and torque acting on particles in a transitionally rough open channel flow

Abstract: Direct numerical simulation of open channel flow over a geometrically rough wall has been performed at a bulk Reynolds number of approximately 2900. The wall consisted of a layer of spheres in a square arrangement. Two cases have been considered. In the first case the spheres are small (with diameter equivalent to 10.7 wall units) and the limit of the hydraulically smooth flow regime is approached. In the second case the spheres are more than three times larger (49.3 wall units) and the flow is in the transitionally rough flow regime. Special emphasis is given on the characterisation of the force and torque acting on a particle due to the turbulent flow. It is found that in both cases the mean drag, lift and spanwise torque are to a large extent produced at the top region of the particle surface. The intensity of the particle force fluctuations is significantly larger in the large-sphere case, while the trend differs for the fluctuations of the individual components of the torque. A simplified model is used to show that the torque fluctuations might be explained by the spheres acting as a filter with respect to the size of the flow scales which can effectively generate torque fluctuations. Fluctuations of both force and torque are found to exhibit strongly non-Gaussian probability density functions with particularly long tails, an effect which is more pronounced in the small-sphere case. Some implications of the present results for sediment erosion are briefly discussed.

Representative roughness height of submerged vegetation

Abstract: [1] Roughness length scale is important in the evaluation of resistance caused by submerged vegetation in open channel flows. By transforming the concept of hydraulic radius, a representative roughness height is proposed in this study for quantifying effect of submerged vegetation on flow resistance in the surface layer. The proposed roughness height is characterized by its proportionality to both stem diameter and vegetation concentration and performs better than other length scales in collapsing resistance data collected under a wide range of vegetation conditions. An approach is then developed for estimate of the average flow velocity and thus resistance coefficients for both cases of rigid and flexible vegetation. Comparisons are also made between the present study and other four formulas available in the literature. This study also shows that all the formulas, if simplified for some simple conditions, can be unified in a general form.