1,720,958 research outputs found
Turbulence modulation by dense suspensions in channel flows
No full textDense suspensions are usually investigated in the laminar limit where inertial effects are insignificant. In this regime, the main effect of the suspended phase is to alter the rheological behavior of the flow which always displays higher effective viscosity with respect to the carrier fluid. When the flow rate is high enough, i.e. at high Reynolds number, the flow may become turbulent and the interaction between solid and liquid phase modifies the turbulent dynamics that we know in single-phase fluids. In the present work, we study turbulent channel flows laden with finite-size particles at high volume fraction (F = 0:2) by means of Direct Numerical Simulations. A direct-forcing Immersed Boundary Method has been adopted to couple liquid and solid phases. The two-phase simulations have been performed fixing the bulk Reynolds number at Reb = Ub 2h=n = 12000 (Ub bulk velocity, h channel half-width and n the fluid kinematic viscosity). The particle size is relatively large with respect to the viscous length, i.e. 10 and 20 times, but smaller than large scales. We will present a detailed comparison of the statistical behavior of the particle-laden flow and the corresponding single-phase flow. The presence of the solid phase strongly alters the wall turbulence dynamics and its effect cannot be accounted only considering the higher rheological effective viscosity.Fluid MechanicsMulti Phase System
Structure and dynamics of turbulent flows over highly permeable walls
No full textHighly porous materials are found in various industrial applications and environmental flows. In previous studies it was found that a turbulent flow along a highly porous wall experiences a higher skin friction as compared to a solid wall with similar surface roughness when the so-called permeability Reynolds number (Re_K) is larger than O(1). The main objective of the present study was to gain understanding of the characteristic structures and auto-generation mechanisms of turbulence for Re_K >> 1. To this purpose the Volume-Averaged Navier-Stokes (VANS) equations were solved in a Direct Numerical Simulation (DNS) of a turbulent flow through a plane channel with an upper solid wall and a lower porous wall at Re_K = 5.91. The DNS results are in good agreement with available Particle Image Velocimetry (PIV) data for the same flow geometry. A linear stochastic estimation technique was used to capture the structure associated with the characteristic ejection event that contributes most to the Reynolds shear stress near the porous wall. This structure is similar to a horseshoe vortex. Contrary to the conventional hairpin vortex found near solid walls, this horseshoe vortex has a significantly higher inclination angle with the wall and its legs are much shorter. The latter is consistent with the observed absence of low and high-speed streaks near highly permeable walls. Next, the auto-generation mechanisms of the horseshoe vortex were studied in another DNS in which the horseshoe vortex was released in the Reynolds-averaged flow field obtained from the former DNS. Two distinct auto-generation mechanisms were observed: (1) the generation of new structures at the upstream end of the horseshoe vortex, which evolve rapidly into a turbulent spot with an arrowhead shape, and (2) the interaction of the horseshoe vortex with spanwise oriented Kelvin-Helmholtz vortex rollers originating from the inflexion point in the mean velocity profile near the porous wall
Direct numerical simulations of drag reduction in turbulent channel flow over bio-inspired herringbone riblet-texture
No full textThe use of drag reducing surface textures is a promising passive method to reduce fuel consumption. Probably most wellknown is the utilisation of shark-skin inspired ridges or riblets parallel to the mean flow. They can reduce drag up to 10%. Recently another bio-inspired texture based on bird flight feather riblets has been proposed. It differs from the standard riblets in two ways. First, the riblets are arranged in a converging/diverging or herringbone pattern. Second, the riblet height or groove depth changes gradually. Drag reductions as high as 20% have been claimed [2]. The objective of the present work is to study the drag reducing properties and mechanisms of this texture. To that purpose Direct Numerical Simulations (DNSs) of turbulent plane channel flow have been performed. Structured roughness has been applied to both walls and several geometric parameters have been varied. Marginal drag reductions on the order of 2.5% and significant drag increases well beyond 100% were found. The latter is attributed to a strong secondary flow that mixes momentum through the whole channel. In future optimization studies we might look for conditions at which secondary motions affect the near-wall cycle of turbulence only
Mechanics of dense suspensions in turbulent channel flows
No full textDense suspensions are usually investigated in the laminar limit where inertial effects are insignificant. When the flow rate is high enough, i.e. at high Reynolds number, the flow may become turbulent and the interaction between solid and liquid phases modifies the turbulence we know in single-phase fluids. In the present work, we study turbulent channel flows laden with finite-size particles at high volume fraction by means of Direct Numerical Simulations. A direct-forcing Immersed Boundary Method has been adopted to couple liquid and solid phases. We will show that the turbulence is attenuated in dense cases, even though the overall drag is increased because of the particle contribution to the total stress
Flow regimes of inertial suspensions of finite size particles
No full textInertial regimes in a channel flow of suspension of finite-size neutrally buoyant particles are studied for a wide range of Reynolds numbers: , and particle volume fractions: . The flow is classified in three different regimes according to the phase-averaged stress budget across the channel \cite{Lashgari2014}. The laminar viscous regime at low and where the viscous stress is the dominating term in the budget, the turbulent regime at high and relatively low where the momentum is mainly transferred by the action of the Reynolds stress and the inertial shear-thickening regime where the particle stress contributes the most to the significant enhancement of the wall shear stress. Particle distribution and dispersion properties provide additional evidence for the existence of the three different regimes
The influence of wall permeability on laminar and turbulent flows: Theory and simulations
No full textThe study of flows over permeable walls is relevant to many applications. Examples are flows over and through porous river beds, vegetation, snow, heat exchangers of foam metal, and oil wells. The main objectives of this thesis are to gain insight in the influence of wall permeability on both laminar and turbulent flows, and to develop a formalism for Direct Numerical Simulations (DNS) of turbulent flows over permeable walls. To describe flow inside a permeable wall, we use the Volume--Averaged Navier--Stokes (VANS) equations for the volume--averaged flow. The latter is defined as a weighted volume average of the microscopic flow, and is continuous throughout the porous medium. To solve the VANS equations, closures are needed for the subfilter--scale stress and the drag force. The latter is investigated in more detail in chapter 4. In chapter 3, an analysis is given of the influence of wall permeability on the laminar boundary layer over a wedge. A generalized Falkner--Skan equation is derived. Results are shown for various wedge angles. In chapter 5, a formalism is developed for DNS of turbulent flow in a plane channel with a permeable bottom wall. The VANS equations are used to simulate the flow inside the permeable wall. Results are shown from four simulations, for which only the wall porosity was changed. The influence of wall permeability can be characterized by the permeability Reynolds number. Turbulence near a highly permeable wall is dominated by relatively large vortical structures, which originate possibly from a Kelvin--Helmholtz type of instability. These structures cause an exchange of momentum between the channel and the permeable wall and consequently the skin friction increases. In chapter 6, the formalism developed in chapter 5 is validated. A DNS has been performed of turbulent channel flow over a permeable wall consisting of a Cartesian grid of 30x20x9 cubes. An Immersed Boundary Method is used to enforce a zero velocity on the cubes. The results of the DNS compares very well with a second DNS in which the VANS equations are used for the flow inside the permeable wall.Mechanical, Maritime and Materials Engineerin
Self-Sustaining Mechanisms in Wall Bounded Turbulence: Merging & auto-generation of vortices
No full textFor channel flow, we explore how a hairpin eddy may reach a threshold strength required to produce additional hairpins by means of auto-generation. This is done by studying the evolution of two eddies with different initial strengths (but both below the threshold strength), initial sizes and initial stream-wise spacing between them. The numerical procedure followed is similar to Zhou et al [1999]. The two eddies were found to merge into a single stronger eddy in case of a larger upstream and a smaller downstream eddy placed within a certain initial stream-wise separation distance. Subsequently, the resulting stronger eddy was observed to auto-generate new eddies. Merging of eddies thus is a viable explanation for the creation of the threshold strength eddies.Solid and Fluid MechanicsProcess and EnergyMechanical, Maritime and Materials Engineerin
Influence of Concentration on Sedimentation of a Dense Suspension in a Viscous Fluid
No full textMacroscopic properties of sedimenting suspensions have been studied extensively and can be characterized using the Galileo number (Ga), solid-to-fluid density ratio (πp) and mean solid volume concentration (ϕ¯). However, the particle–particle and particle–fluid interactions that dictate these macroscopic trends have been challenging to study. We examine the effect of concentration on the structure and dynamics of sedimenting suspensions by performing direct numerical simulation based on an Immersed Boundary Method of monodisperse sedimenting suspensions of spherical particles at fixed Ga= 144 , πp= 1.5 , and concentrations ranging from ϕ¯ = 0.5 to ϕ¯ = 30 %. The corresponding particle terminal Reynolds number for a single settling particle is ReT= 186. Our simulations reproduce the macroscopic trends observed in experiments and are in good agreement with semi-empirical correlations in literature. From our studies, we observe, first, a change in trend in the mean settling velocities, the dispersive time scales and the structural arrangement of particles in the sedimenting suspension at different concentrations, indicating a gradual transition from a dilute regime (ϕ¯ ≲ 2 %) to a dense regime (ϕ¯ ≳ 10 %). Second, we observe the vertical propagation of kinematic waves as fluctuations in the local horizontally-averaged concentration of the sedimenting suspension in the dense regime.Multi Phase System
Inertial effects in sedimenting suspensions of solid spheres in a liquid
No full textParticle-resolved Direct Numerical Simulations have been performed on the gravitational settling of mono-disperse solid spheres in a viscous fluid and triply periodic domain. In a comprehensive study, the bulk solid volume concentration was varied from ϕ=0.5 to 30%. To study the effect of inertia, three different Galileo numbers were considered in the inertial regime, Ga=144, 178 and 210, for which a single settling sphere exhibits distinctly different wake and path characteristics. The particle/fluid mass density ratio was fixed at 1.5. We find that for ϕ=2−30% the suspension microstructure and dynamics depend predominantly on the bulk concentration. In qualitative agreement with previous studies in literature, three different sedimentation regimes can be distinguished: (1) the dilute concentration regime for ϕ≲2% with preferential settling of particles in vertical trains, (2) the moderate concentration regime for 2%≲ϕ≲10% with preferential settling of particles in horizontal pairs with an interparticle distance of ∼ 1.5 particle diameters, and (3) the dense concentration regime for ϕ≳10% with a nearly random (“hard-sphere”) distribution of the particles in space. The clustering of particles is dictated by, respectively, trapping of particles in the wake of other particles, a drafting–kissing–tumbling (DKT) instability by which two vertically aligned particles quickly reorient themselves into a horizontally aligned particle pair, and short-range multiparticle interactions through viscous lubrication and to a lesser extent collisions between particles. In all cases, hindered settling at a reduced speed is observed as compared to a single settling sphere. The well-known Richardson–Zaki relation for the mean sedimentation velocity appears valid only for the dense concentration regime. We provide ample evidence that in the dense regime the characteristic velocity and time scales of particle motion are proportional to gDp and Dp/g, respectively, with g the gravitational acceleration and Dp the particle diameter. We also observe an ω−3 scaling of the particle velocity spectra for ωDp/g≳0.4 and we propose a model to explain this scaling behavior, based on the inertial response of the particles to small-scale flow perturbations. Kinematic waves, i.e., vertically propagating plane waves in the local concentration field, are observed in all cases, though unrelated particle motions are responsible for significant loss of the spatio-temporal coherence of the waves. The wave speed was determined from repeated space–time autocorrelations of the local concentration field and appears in reasonable agreement with Kynch sedimentation theory using the Richardson–Zaki relation. The passage of kinematic waves causes perturbations in the particle velocity at a frequency that matches well with peak frequencies in the particle velocity spectra for concentrations up to ϕ≈10%. The time-lagged cross-correlation of the vertical and horizontal particle velocity suggests that kinematic waves may trigger DKT instabilities, while conversely DKT instabilities may be responsible for the onset of kinematic waves. Finally, we suggest that obstruction and perturbation of the particle wake by neighboring particles could offer an explanation for the small influence of the Galileo number on the suspension behavior for ϕ=2−30%.</p
Turbulence in the wake of a roughness patch
No full textLittle research was done in the past concerning the propagation of three dimensional effect in shallow wake flow caused by a roughness patch. Today’s research on related subjects is dominated by emerging obstructions in shallow water where the flow can be assumed as (quasi) two dimensional. However the relevance of a submerged obstruction with increased roughness can be found in wake control, oyster reefs, river- and estuary bottoms and heterogeneous land occupancy. To get a better understanding of the consequences of the three dimensionality of the flow structures, experiments are performed in a wide shallow flume to examine these structures. The main objective is to examine whether the wake structure of a roughness patch can be treated as (quasi)-two-dimensional. The objective has been answered by a combination of a literature study and an experiment performed at the faculty’s laboratory. The results show four dominant mechanisms in the wake of a roughness patch: transverse mass flux, bottom friction, mixing layer and the secondary circulation. Based on a momentum balance the transverse mass flux and the bottom friction are the largest contributions to this balance. Although the contribution of the mixing layer and the secondary circulation to the recovery of the wake are of the order of 10%, their influence on the flow structure is more pronounced. The mixing layer is shifted towards the wake centerline due to the presence of a transverse mass flux forming a misalignment between the maximum spanwise Reynolds stress and the position of the wake half width. Since this shift is of limited influence on the position of the secondary circulation, a misalignment if formed between the maximum momentum transport by the secondary circulation and the mixing layer causing a lower streamwise velocity at the edge of wake with respect to the wake of an emerging obstruction. The secondary circulation is responsible for the transport of low momentum fluid towards the edge of the wake near the bottom, and high momentum fluid towards the wake centerline near the surface. This behavior is responsible for the cross gradient in the streamwise velocity profiles as shown by the data obtained. For modeling purposes of well mixed quantities, a (quasi)-two-dimensional approach only holds if the weaker streamwise velocity near the edge of the wake is taken into account. In the case a prediction of depth varying quantities is desired, the cross gradient caused by the secondary circulation needs to be implemented as well which results in the need of a three-dimensional modeling approach.Environmental Fluid MechanicsHydraulic EngineeringCivil Engineering and Geoscience
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