scholarly journals Exact regularised point particle (ERPP) method for particle-laden wall-bounded flows in the two-way coupling regime

2019 ◽  
Vol 878 ◽  
pp. 420-444 ◽  
Author(s):  
F. Battista ◽  
J.-P. Mollicone ◽  
P. Gualtieri ◽  
R. Messina ◽  
C. M. Casciola
Keyword(s):  
Turbulent Flows ◽  
Parameter Space ◽  
Density Ratio ◽  
Stokes Number ◽  
Point Particle ◽  
Turbulent Pipe Flow ◽  
Solid Boundary ◽  
Mass Loading ◽  
Fluid Density ◽  
Extra Stress

The exact regularised point particle (ERPP) method is extended to treat the inter-phase momentum coupling between particles and fluid in the presence of walls by accounting for vorticity generation due to particles close to solid boundaries. The ERPP method overcomes the limitations of other methods by allowing the simulation of an extensive parameter space (Stokes number, mass loading, particle-to-fluid density ratio and Reynolds number) and of particle spatial distributions that are uneven (few particles per computational cell). The enhanced ERPP method is explained in detail and validated by considering the global impulse balance. In conditions when particles are located close to the wall, a common scenario in wall-bounded turbulent flows, the main contribution to the total impulse arises from the particle-induced vorticity at the solid boundary. The method is applied to direct numerical simulations of particle-laden turbulent pipe flow in the two-way coupling regime to address turbulence modulation. The effects of the mass loading, the Stokes number and the particle-to-fluid density ratio are investigated. The drag is either unaltered or increased by the particles with respect to the uncoupled case. No drag reduction is found in the parameter space considered. The momentum stress budget, which includes an extra stress contribution by the particles, provides the rationale behind the drag behaviour. The extra stress produces a momentum flux towards the wall that strongly modifies the viscous stress, the culprit of drag at solid boundaries.

scholarly journals Model for the dynamics of micro-bubbles in high-Reynolds-number flows

2019 ◽  
Vol 879 ◽  
pp. 554-578 ◽  
Author(s):  
Zhentong Zhang ◽  
Dominique Legendre ◽  
Rémi Zamansky
Keyword(s):  
Reynolds Number ◽  
Stochastic Model ◽  
Turbulent Flows ◽  
Dissipation Rate ◽  
Density Ratio ◽  
Stokes Number ◽  
Small Scale ◽  
Fluid Inertia ◽  
Joint Orientation ◽  
Inertia Forces

We propose a model for the acceleration of micro-bubbles (smaller than the dissipative scale of the flow) subjected to the drag and fluid inertia forces in a homogeneous and isotropic turbulent flow. This model, that depends on the Stokes number, Reynolds number and the density ratio, reproduces the evolution of the acceleration variance as well as the relative importance and alignment of the two forces as observed from direct numerical simulations (DNS). We also report that the bubble acceleration statistics conditioned on the local kinetic energy dissipation rate are invariant with the Stokes number and the dissipation rate. Based on this observation, we propose a stochastic model for the instantaneous bubble acceleration vector accounting for the small-scale intermittency of the turbulent flows. The norm of the bubble acceleration is obtained by modelling the dissipation rate along the bubble trajectory from a log-normal stochastic process, whereas its orientation is given by two coupled random walks on a unit sphere in order to model the evolution of the joint orientation of the drag and inertia forces acting on the bubble. Furthermore, the proposed stochastic model for the bubble acceleration is used in the context of large eddy simulations (LES) of turbulent flows laden with small bubbles. To account for the turbulent motion at scales smaller than the mesh resolution, we decompose the instantaneous bubble acceleration in its resolved and residual parts. The first part is given by the drag and fluid inertia forces computed from the resolved velocity field, and the second term refers to the random contribution of small unresolved turbulent scales and is estimated with the stochastic model proposed in the paper. Comparisons with DNS and standard LES, show that the proposed model improves significantly the statistics of the bubbly phase.

scholarly journals Oscillatory switching centrifugation: dynamics of a particle in a pulsating vortex

2018 ◽  
Vol 857 ◽  
Author(s):  
Francesco Romanò
Keyword(s):  
Dynamical Systems ◽  
Systems Theory ◽  
Limit Cycles ◽  
Vortex Core ◽  
Density Ratio ◽  
Lab On A Chip ◽  
Stokes Number ◽  
Pulsation Frequency ◽  
Fluid Density ◽  
Asymptotic Dynamics

The dynamics of a small rigid spherical particle in an unbounded pulsating vortex is considered, keeping constant the particle Stokes number $St$ and varying the particle-to-fluid density ratio $\unicode[STIX]{x1D71A}$ and the pulsation frequency of the vortex $\unicode[STIX]{x1D714}$ . We show that the asymptotic dynamics of a particle of given $St$ and $\unicode[STIX]{x1D71A}$ can be controlled by varying $\unicode[STIX]{x1D714}$ , turning the vortex core either into an attractor or a repellor. The creation of non-trivial particle limit cycles characterizes the boundaries between centrifugal and centripetal regions in parameter space. The discovered phenomenon is termed oscillatory switching centrifugation and its implications for particle demixing processes, biological protocols, lab-on-a-chip devices and dynamical systems theory are discussed at the end.

Modulation of turbulence by dispersed solid particles in a spatially developing flat-plate boundary layer

2016 ◽  
Vol 802 ◽  
pp. 359-394 ◽  
Author(s):  
Dong Li ◽  
Kun Luo ◽  
Jianren Fan
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Fluid Velocity ◽  
Plate Boundary ◽  
Stokes Number ◽  
Point Particle ◽  
Solid Particles ◽  
Inertial Particles ◽  
Mass Loading ◽  
Turbulence Modulation

Direct numerical simulations of particle-laden spatially developing turbulent boundary layers over a flat plate have been performed to investigate the effect of inertial particles on turbulence modulation, using the Eulerian–Lagrangian point-particle approach with two-way coupling. The particles are smaller than the Kolmogorov length scale of the dilute flow, and inter-particle collisions are not considered. The simulation results show that the addition of small solid particles increases the mean streamwise fluid velocity, which in turn leads to a reduction in the boundary layer integral parameters and an increase in the skin-friction drag. These effects become more pronounced as the particle Stokes number and mass loading increase. The streamwise turbulence intensity is slightly enhanced in the close vicinity of the wall but damped in the outer layer. In contrast, the Reynolds stress and the turbulence intensities in the wall-normal and spanwise directions are substantially attenuated across the entire boundary layer, and the levels of attenuation increase monotonically with both particle Stokes number and mass loading. The exchange of kinetic energy between particles and fluid indicates that particle–fluid interactions cause extra energy dissipation, which plays a crucial role in turbulence modulation.

Turbulent Mass Transfer Simulations of Binary Electrolyte in Parallel-Plate Electrode Channel

2012 ◽  
Vol 550-553 ◽  
pp. 2014-2018
Author(s):  
Xiao Lan Zhou ◽  
Cai Xi Liu ◽  
Yu Hong Dong
Keyword(s):  
Mass Transfer ◽  
Turbulent Flows ◽  
Primary Objective ◽  
Wall Turbulence ◽  
Limiting Current ◽  
Solid Boundary ◽  
Wall Region ◽  
Schmidt Numbers ◽  
Near Wall ◽  
Turbulent Mass Transfer

Electrochemical mass transfer in turbulent flows and binary electrolytes is investigated. The primary objective is to provide information about mass transfer in the near-wall region between a solid boundary and a turbulent fluid flow at different Schmidt numbers. Based on the computational fluid dynamics and electrochemistry theories, a model for turbulent electrodes channel flow is established. The turbulent mass transfer in electrolytic processes has been predicted by the direct numerical simulation method under limiting current and galvanostatic conditions, we investigate mean concentration and the structure of the concentration fluctuating filed for different Schmidt numbers from 0.1 to 100 .The effect of different concentration boundary conditions at the electrodes on the near-wall turbulence statistics is also discussed.

scholarly journals Concentration and velocity statistics of inertial particles in upward and downward pipe flow

2017 ◽  
Vol 822 ◽  
pp. 640-663 ◽  
Author(s):  
J. L. G. Oliveira ◽  
C. W. M. van der Geld ◽  
J. G. M. Kuerten
Keyword(s):  
Liquid Velocity ◽  
Fluid Velocity ◽  
Three Dimensional ◽  
Stokes Number ◽  
Terminal Velocity ◽  
Point Particle ◽  
Inertial Particles ◽  
Carrier Fluid ◽  
Velocity Statistics ◽  
The Difference

Three-dimensional particle tracking velocimetry is applied to particle-laden turbulent pipe flows at a Reynolds number of 10 300, based on the bulk velocity and the pipe diameter, for developed fluid flow and not fully developed flow of inertial particles, which favours assessment of the radial migration of the inertial particles. Inertial particles with Stokes number ranging from 0.35 to 1.11, based on the particle relaxation time and the radial-dependent Kolmogorov time scale, and a ratio of the root-mean-square fluid velocity to the terminal velocity of order 1 have been used. Core peaking of the concentration of inertial particles in up-flow and wall peaking in down-flow have been found. The difference in mean particle and Eulerian mean liquid velocity is found to decrease to approximately zero near the wall in both flow directions. Although the carrier fluid has all of the characteristics of the corresponding turbulent single-phase flow, the Reynolds stress of the inertial particles is different near the wall in up-flow. These findings are explained from the preferential location of the inertial particles with the aid of direct numerical simulations with the point-particle approach.

A Modeling Study on Particle Dispersion in Wall-Bounded Turbulent Flows

2014 ◽  
Vol 6 (06) ◽  
pp. 764-782 ◽  
Author(s):  
Jian-Hung Lin ◽  
Keh-Chin Chang
Keyword(s):  
Turbulent Flows ◽  
Particle Dispersion ◽  
Hard Sphere Model ◽  
Wall Roughness ◽  
Mass Loading ◽  
Particle Collisions ◽  
Physical Mechanisms ◽  
Parametric Studies ◽  
Low Mass ◽  
Bounded Particle

AbstractThree physical mechanisms which may affect dispersion of particle’s motion in wall-bounded turbulent flows, including the effects of turbulence, wall roughness in particle-wall collisions, and inter-particle collisions, are numerically investigated in this study. Parametric studies with different wall roughness extents and with different mass loading ratios of particles are performed in fully developed channel flows with the Eulerian-Lagrangian approach. A low-Reynolds-numberk–εturbulence model is applied for the solution of the carrier-flow field, while the deterministic Lagrangian method together with binary-collision hard-sphere model is applied for the solution of particle motion. It is shown that the mechanism of inter-particle collisions should be taken into account in the modeling except for the flows laden with sufficiently low mass loading ratios of particles. Influences of wall roughness on particle dispersion due to particle-wall collisions are found to be considerable in the bounded particle–laden flow. Since the investigated particles are associated with large Stokes numbers, i.e., larger thanO(1), in the test problem, the effects of turbulence on particle dispersion are much less considerable, as expected, in comparison with another two physical mechanisms investigated in the study.

scholarly journals Enhancing CFD predictions in shape design problems by model and parameter space reduction

2020 ◽  
Vol 7 (1) ◽  
Author(s):  
Marco Tezzele ◽  
Nicola Demo ◽  
Giovanni Stabile ◽  
Andrea Mola ◽  
Gianluigi Rozza
Keyword(s):  
Turbulent Flows ◽  
Parameter Space ◽  
Dimensional Space ◽  
Lift Coefficient ◽  
Target Function ◽  
Dynamic Mode Decomposition ◽  
Dynamic Mode ◽  
Space Reduction ◽  
Mode Decomposition ◽  
Computational Resources

Abstract In this work we present an advanced computational pipeline for the approximation and prediction of the lift coefficient of a parametrized airfoil profile. The non-intrusive reduced order method is based on dynamic mode decomposition (DMD) and it is coupled with dynamic active subspaces (DyAS) to enhance the future state prediction of the target function and reduce the parameter space dimensionality. The pipeline is based on high-fidelity simulations carried out by the application of finite volume method for turbulent flows, and automatic mesh morphing through radial basis functions interpolation technique. The proposed pipeline is able to save 1/3 of the overall computational resources thanks to the application of DMD. Moreover exploiting DyAS and performing the regression on a lower dimensional space results in the reduction of the relative error in the approximation of the time-varying lift coefficient by a factor 2 with respect to using only the DMD.

On transition in a pipe. Part 1. The origin of puffs and slugs and the flow in a turbulent slug

1973 ◽  
Vol 59 (2) ◽  
pp. 281-335 ◽  
Author(s):  
I. J. Wygnanski ◽  
F. H. Champagne
Keyword(s):  
Energy Balance ◽  
Turbulent Flows ◽  
Turbulent Energy ◽  
Turbulent Pipe Flow ◽  
Entire Cross Section ◽  
Turbulent Fluid ◽  
Dissipation Term ◽  
Onset Of Turbulence ◽  
Order Of Magnitude ◽  
Pressure Transport

Conditionally sampled hot-wire measurements were taken in a pipe at Reynolds numbers corresponding to the onset of turbulence. The pipe was smooth and carefully aligned so that turbulent slugs appeared naturally atRe> 5 × 104. Transition could be initiated at lowerReby introducing disturbances into the inlet. For smooth or only slightly disturbed inlets, transition occurs as a result of instabilities in the boundary layer long before the flow becomes fully developed in the pipe. This type of transition gives rise to turbulent slugs which occupy the entire cross-section of the pipe, and they grow in length as they proceed downstream. The leading and trailing ‘fronts’ of a turbulent slug are clearly defined. A unique relation seems to exist between the velocity of the interface and the velocity of the fluid by which relaminarization of turbulent fluid is prevented. The length of slugs is of the same order of magnitude as the length of the pipe, although the lengths of individual slugs differ at the same flow conditions. The structure of the flow in the interior of a slug is identical to that in a fully developed turbulent pipe flow. Near the interfaces, where the mean motion changes from a laminar to a turbulent state, the velocity profiles develop inflexions. The total turbulent intensity near the interfaces is very high and it may reach 15% of the velocity at the centre of the pipe. A turbulent energy balance was made for the flow near the interfaces. All of the terms contributing to the energy balance must vanish identically somewhere on the interface if that portion of the interface does not entrain non-turbulent fluid. It appears that diffusion which also includes pressure transport is the most likely mechanism by which turbulent energy can be transferred to non-turbulent fluid. The dissipation term at the interface is negligible and increases with increasing turbulent energy towards the interior of the slug.Mixed laminar and turbulent flows were observed far downstream for\[ 2000 < Re < 2700 \]when a large disturbance was introduced into the inlet. The flow in the vicinity of the inlet, however, was turbulent at much lowerRe. The turbulent regions which are convected downstream at a velocity which is slightly smaller than the average velocity in the pipe we shall henceforth call puffs. The leading front of a puff does not have a clearly defined interface and the trailing front is clearly defined only in the vicinity of the centre-line. The length and structure of the puff is independent of the character of the obstruction which created it, provided that the latter is big enough to produce turbulent flow at the inlet. The puff will be discussed in more detail later.

scholarly journals On the transition between turbulence regimes in particle-laden channel flows

2018 ◽  
Vol 845 ◽  
pp. 499-519 ◽  
Author(s):  
Jesse Capecelatro ◽  
Olivier Desjardins ◽  
Rodney O. Fox
Keyword(s):  
Reynolds Number ◽  
Fluid Phase ◽  
Stokes Number ◽  
Channel Flows ◽  
High Mass ◽  
Inertial Particles ◽  
Mass Loading ◽  
Two Phase ◽  
Phase Dynamics ◽  
Wide Range

Turbulent wall-bounded flows exhibit a wide range of regimes with significant interaction between scales. The fluid dynamics associated with single-phase channel flows is predominantly characterized by the Reynolds number. Meanwhile, vastly different behaviour exists in particle-laden channel flows, even at a fixed Reynolds number. Vertical turbulent channel flows seeded with a low concentration of inertial particles are known to exhibit segregation in the particle distribution without significant modification to the underlying turbulent kinetic energy (TKE). At moderate (but still low) concentrations, enhancement or attenuation of fluid-phase TKE results from increased dissipation and wakes past individual particles. Recent studies have shown that denser suspensions significantly alter the two-phase dynamics, where the majority of TKE is generated by interphase coupling (i.e.  drag) between the carrier gas and clusters of particles that fall near the channel wall. In the present study, a series of simulations of vertical particle-laden channel flows with increasing mass loading is conducted to analyse the transition from the dilute limit where classical mean-shear production is primarily responsible for generating fluid-phase TKE to high-mass-loading suspensions dominated by drag production. Eulerian–Lagrangian simulations are performed for a wide range of particle loadings at two values of the Stokes number, and the corresponding two-phase energy balances are reported to identify the mechanisms responsible for the observed transition.

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