Wall accumulation and spatial localization in particle-laden wall flows

2012 ◽  
Vol 699 ◽  
pp. 50-78 ◽  
Author(s):  
G. Sardina ◽  
P. Schlatter ◽  
L. Brandt ◽  
F. Picano ◽  
C. M. Casciola
Keyword(s):  
Turbulent Flows ◽  
Pair Correlation ◽  
Turbulent Channel Flow ◽  
Wall Turbulence ◽  
Small Scale ◽  
Wall Region ◽  
Inertial Particles ◽  
Wall Flows ◽  
Domain Truncation ◽  
Near Wall

AbstractWe study the two main phenomenologies associated with the transport of inertial particles in turbulent flows, turbophoresis and small-scale clustering. Turbophoresis describes the turbulence-induced wall accumulation of particles dispersed in wall turbulence, while small-scale clustering is a form of local segregation that affects the particle distribution in the presence of fine-scale turbulence. Despite the fact that the two aspects are usually addressed separately, this paper shows that they occur simultaneously in wall-bounded flows, where they represent different aspects of the same process. We study these phenomena by post-processing data from a direct numerical simulation of turbulent channel flow with different populations of inertial particles. It is shown that artificial domain truncation can easily alter the mean particle concentration profile, unless the domain is large enough to exclude possible correlation of the turbulence and the near-wall particle aggregates. The data show a strong link between accumulation level and clustering intensity in the near-wall region. At statistical steady state, most accumulating particles aggregate in strongly directional and almost filamentary structures, as found by considering suitable two-point observables able to extract clustering intensity and anisotropy. The analysis provides quantitative indications of the wall-segregation process as a function of the particle inertia. It is shown that, although the most wall-accumulating particles are too heavy to segregate in homogeneous turbulence, they exhibit the most intense local small-scale clustering near the wall as measured by the singularity exponent of the particle pair correlation function.

scholarly journals Velocity and spatial distribution of inertial particles in a turbulent channel flow

2019 ◽  
Vol 872 ◽  
pp. 367-406 ◽  
Author(s):  
Kee Onn Fong ◽  
Omid Amili ◽  
Filippo Coletti
Keyword(s):  
Spatial Distribution ◽  
Particle Velocity ◽  
Turbulent Channel Flow ◽  
Wall Turbulence ◽  
Buffer Layers ◽  
Working Fluid ◽  
Wall Region ◽  
Inertial Particles ◽  
Velocity Fluctuations ◽  
Near Wall

We present experimental observations of the velocity and spatial distribution of inertial particles dispersed in turbulent downward flow through a vertical channel at friction Reynolds numbers $\mathit{Re}_{\unicode[STIX]{x1D70F}}=235$ and 335. The working fluid is air laden with size-selected glass microspheres, having Stokes numbers $St=\mathit{O}(10)$ and $\mathit{O}(100)$ when based on the Kolmogorov and viscous time scales, respectively. Cases at solid volume fractions $\unicode[STIX]{x1D719}_{v}=3\times 10^{-6}$ and $5\times 10^{-5}$ are considered. In the more dilute regime, the particle concentration profile shows near-wall and centreline maxima compatible with a turbophoretic drift down the gradient of turbulence intensity; the particles travel at speed similar to that of the unladen flow except in the near-wall region; and their velocity fluctuations generally follow the unladen flow level over the channel core, exceeding it in the near-wall region. The denser regime presents substantial differences in all measured statistics: the near-wall concentration peak is much more pronounced, while the centreline maximum is absent; the mean particle velocity decreases over the logarithmic and buffer layers; and particle velocity fluctuations and deposition velocities are enhanced. An analysis of the spatial distributions of particle positions and velocities reveals different behaviours in the core and near-wall regions. In the channel core, dense clusters form which are somewhat elongated, tend to be preferentially aligned with the vertical/streamwise direction and travel faster than the less concentrated particles. In the near-wall region, the particles arrange in highly elongated streaks associated with negative streamwise velocity fluctuations, several channel heights in length and spaced by $\mathit{O}(100)$ wall units, supporting the view that these are coupled to fluid low-speed streaks typical of wall turbulence. The particle velocity fields contain a significant component of random uncorrelated motion, more prominent for higher $St$ and in the near-wall region.

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

2011 ◽  
Vol 687 ◽  
pp. 141-170 ◽  
Author(s):  
C. Manes ◽  
D. Poggi ◽  
L. Ridolfi
Keyword(s):  
Turbulent Flows ◽  
Laser Doppler Anemometry ◽  
Wall Turbulence ◽  
Shear Instability ◽  
Wall Structure ◽  
Wall Region ◽  
Mean Velocity ◽  
Wide Range ◽  
Permeable Walls ◽  
Near Wall

AbstractThis 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 Kármán 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.

Regeneration mechanisms of near-wall turbulence structures

1995 ◽  
Vol 287 ◽  
pp. 317-348 ◽  
Author(s):  
James M. Hamilton ◽  
John Kim ◽  
Fabian Waleffe
Keyword(s):  
Turbulent Flows ◽  
Wall Turbulence ◽  
Direct Result ◽  
Wall Region ◽  
Computational Domain ◽  
Streamwise Vortices ◽  
Turbulence Structures ◽  
Wall Structures ◽  
Fully Developed Turbulent Flow ◽  
Near Wall

Direct numerical simulations of a highly constrained plane Couette flow are employed to study the dynamics of the structures found in the near-wall region of turbulent flows. Starting from a fully developed turbulent flow, the dimensions of the computational domain are reduced to near the minimum values which will sustain turbulence. A remarkably well-defined, quasi-cyclic and spatially organized process of regeneration of near-wall structures is observed. This process is composed of three distinct phases: formation of streaks by streamwise vortices, breakdown of the streaks, and regeneration of the streamwise vortices. Each phase sets the stage for the next, and these processes are analysed in detail. The most novel results concern vortex regeneration, which is found to be a direct result of the breakdown of streaks that were originally formed by the vortices, and particular emphasis is placed on this process. The spanwise width of the computational domain corresponds closely to the typically observed spanwise spacing of near-wall streaks. When the width of the domain is further reduced, turbulence is no longer sustained. It is suggested that the observed spacing arises because the time scales of streak formation, breakdown and vortex regeneration become mismatched when the streak spacing is too small, and the regeneration cycle at that scale is broken.

Some insights for the prediction of near-wall turbulence

2013 ◽  
Vol 723 ◽  
pp. 126-139 ◽  
Author(s):  
Farid Karimpour ◽  
Subhas K. Venayagamoorthy
Keyword(s):  
Turbulent Flows ◽  
Time Scales ◽  
Time Scale ◽  
Eddy Viscosity ◽  
Wall Turbulence ◽  
Wall Region ◽  
Velocity Scale ◽  
The Mean ◽  
Near Wall Turbulence ◽  
Near Wall

AbstractIn this paper, we revisit the eddy viscosity formulation to highlight a number of important issues that have direct implications for the prediction of near-wall turbulence. For steady wall-bounded turbulent flows, we make the equilibrium assumption between rates of production ($P$) and dissipation ($\epsilon $) of turbulent kinetic energy ($k$) in the near-wall region to propose that the eddy viscosity should be given by ${\nu }_{t} \approx \epsilon / {S}^{2} $, where $S$ is the mean shear rate. We then argue that the appropriate velocity scale is given by $\mathop{(S{T}_{L} )}\nolimits ^{- 1/ 2} {k}^{1/ 2} $ where ${T}_{L} = k/ \epsilon $ is the turbulence (decay) time scale. The difference between this velocity scale and the commonly assumed velocity scale of ${k}^{1/ 2} $ is subtle but the consequences are significant for near-wall effects. We then extend our discussion to show that the fundamental length and time scales that capture the near-wall behaviour in wall-bounded shear flows are the shear mixing length scale ${L}_{S} = \mathop{(\epsilon / {S}^{3} )}\nolimits ^{1/ 2} $ and the mean shear time scale $1/ S$, respectively. With these appropriate length and time scales (or equivalently velocity and time scales), the eddy viscosity can be rewritten in the familiar form of the $k$–$\epsilon $ model as ${\nu }_{t} = \mathop{(1/ S{T}_{L} )}\nolimits ^{2} {k}^{2} / \epsilon $. We use the direct numerical simulation (DNS) data of turbulent channel flow of Hoyas & Jiménez (Phys. Fluids, vol. 18, 2006, 011702) and the turbulent boundary layer flow of Jiménez et al. (J. Fluid Mech. vol. 657, 2010, pp. 335–360) to perform ‘a priori’ tests to check the validity of the revised eddy viscosity formulation. The comparisons with the exact computations from the DNS data are remarkable and highlight how well the equilibrium assumption holds in the near-wall region. These findings could prove to be useful in near-wall modelling of turbulent flows.

The effect of wall-normal gravity on particle-laden near-wall turbulence

2019 ◽  
Vol 873 ◽  
pp. 475-507 ◽  
Author(s):  
Junghoon Lee ◽  
Changhoon Lee
Keyword(s):  
Turbulent Channel Flow ◽  
Viscous Sublayer ◽  
Stokes Number ◽  
Wall Turbulence ◽  
Small Scale ◽  
Gravitational Acceleration ◽  
Lower Wall ◽  
Low Speed ◽  
Near Wall Turbulence ◽  
Near Wall

We performed two-way coupled direct numerical simulations of turbulent channel flow with Lagrangian tracking of small, heavy spheres at a dimensionless gravitational acceleration of 0.077 in wall units, which is based on the flow condition in the experiment by Gerashchenko et al. (J. Fluid Mech., vol. 617, 2008, pp. 255–281). We removed deposited particles after several collisions with the lower wall and then released new particles near the upper wall to observe direct interactions between particles and coherent structures of near-wall turbulence during gravitational settling through the mean shear. The results indicate that when the Stokes number is approximately 1 on the basis of the Kolmogorov time scale of the flow ($St_{K}\approx 1$), the so-called preferential sweeping occurs in association with coherent streamwise vortices, while the effect of crossing trajectories becomes significant for $St_{K}>1$. Consequently, in either case, the settling particles deposit on the wall without strong accumulation in low-speed streaks in the viscous sublayer. When particles settle through near-wall turbulence from the upper wall, more small-scale vortical structures are generated in the outer layer as low-speed fluid is pulled farther in the direction of gravity, while the opposite is true near the lower wall.

Characteristics of large- and small-scale structures in the turbulent boundary layer over a drag-reducing riblet surface

2019 ◽  
Vol 234 (3) ◽  
pp. 796-807
Author(s):  
Zi-Liang Zhang ◽  
Ming-Ming Zhang ◽  
Chang Cai ◽  
Yu Cheng
Keyword(s):  
Boundary Layer ◽  
Turbulent Flows ◽  
Small Scale ◽  
Wall Region ◽  
Hot Wire ◽  
Logarithmic Region ◽  
Scale Structures ◽  
Riblet Surface ◽  
Small Scale Structures ◽  
Near Wall

Riblet is one of the most promising passive drag reduction techniques in turbulent flows. In this paper, hot-wire measurements on a turbulent boundary layer perturbed by a drag-reducing riblet surface are carried out to further understand the riblet effects on the turbulent flows and the drag reduction mechanism. Compared with the smooth case, different energy variations in the near-wall region and the logarithmic region are observed over riblets. Then, by using a spectral filter of a given wavelength, the time series of the hot-wire data are decomposed into large- and small-scale components. It is indicated that large-scale structures in the logarithmic region impose a footprint (amplitude modulating effect) on the near-wall small-scale structures. By quantifying this footprint, it is found that the interactions between large- and small-scale structures over riblets are weakened in the near-wall region. Furthermore, the bursting process of large and small scales is studied. For both large- and small-scale structures, a shorter bursting duration and a higher bursting frequency are observed over the riblet surface, which indicates that riblets impede the formation of large- and small-scale bursting events. The flow physics behind these phenomena are also discussed in detail.

On the role of gravity and shear on inertial particle accelerations in near-wall turbulence

2010 ◽  
Vol 658 ◽  
pp. 229-246 ◽  
Author(s):  
V. LAVEZZO ◽  
A. SOLDATI ◽  
S. GERASHCHENKO ◽  
Z. WARHAFT ◽  
L. R. COLLINS
Keyword(s):  
Isotropic Turbulence ◽  
Response Times ◽  
Quantitative Agreement ◽  
Wall Turbulence ◽  
Wall Region ◽  
Inertial Particles ◽  
Inertial Particle ◽  
Homogeneous And Isotropic Turbulence ◽  
Near Wall

Recent experiments in a turbulent boundary layer by Gerashchenko et al. (J. Fluid Mech., vol. 617, 2008, pp. 255–281) showed that the variance of inertial particle accelerations in the near-wall region increased with increasing particle inertia, contrary to the trend found in homogeneous and isotropic turbulence. This behaviour was attributed to the non-trivial interaction of the inertial particles with both the mean shear and gravity. To investigate this issue, we perform direct numerical simulations of channel flow with suspended inertial particles that are tracked in the Lagrangian frame of reference. Three simulations have been carried out considering (i) fluid particles, (ii) inertial particles with gravity and (iii) inertial particles without gravity. For each set of simulations, three particle response times were examined, corresponding to particle Stokes numbers (in wall units) of 0.9, 1.8 and 11.8. Mean and r.m.s. profiles of particle acceleration computed in the simulation are in qualitative (and in several cases quantitative) agreement with the experimental results, supporting the assumptions made in the simulations. Furthermore, by comparing results from simulations with and without gravity, we are able to isolate and quantify the significant effect of gravitational settling on the phenomenon.

scholarly journals Cascades and wall-normal fluxes in turbulent channel flows

2016 ◽  
Vol 796 ◽  
pp. 417-436 ◽  
Author(s):  
A. Cimarelli ◽  
E. De Angelis ◽  
J. Jiménez ◽  
C. M. Casciola
Keyword(s):  
Energy Source ◽  
Turbulent Flows ◽  
Wall Turbulence ◽  
Wall Region ◽  
Cascade Process ◽  
Driving Mechanisms ◽  
Energy Excess ◽  
Multidimensional Behaviour ◽  
Generation Mechanisms ◽  
Near Wall

The present work describes the multidimensional behaviour of scale-energy production, transfer and dissipation in wall-bounded turbulent flows. This approach allows us to understand the cascade mechanisms by which scale energy is transmitted scale-by-scale among different regions of the flow. Two driving mechanisms are identified. A strong scale-energy source in the buffer layer related to the near-wall cycle and an outer scale-energy source associated with an outer turbulent cycle in the overlap layer. These two sourcing mechanisms lead to a complex redistribution of scale energy where spatially evolving reverse and forward cascades coexist. From a hierarchy of spanwise scales in the near-wall region generated through a reverse cascade and local turbulent generation processes, scale energy is transferred towards the bulk, flowing through the attached scales of motion, while among the detached scales it converges towards small scales, still ascending towards the channel centre. The attached scales of wall-bounded turbulence are then recognized to sustain a spatial reverse cascade process towards the bulk flow. On the other hand, the detached scales are involved in a direct forward cascade process that links the scale-energy excess at large attached scales with dissipation at the smaller scales of motion located further away from the wall. The unexpected behaviour of the fluxes and of the turbulent generation mechanisms may have strong repercussions on both theoretical and modelling approaches to wall turbulence. Indeed, actual turbulent flows are shown here to have a much richer physics with respect to the classical notion of turbulent cascade, where anisotropic production and inhomogeneous fluxes lead to a complex redistribution of energy where a spatial reverse cascade plays a central role.

Evolution of the scalar dissipation rate downstream of a concentrated line source in turbulent channel flow

2014 ◽  
Vol 749 ◽  
pp. 227-274 ◽  
Author(s):  
E. Germaine ◽  
L. Mydlarski ◽  
L. Cortelezzi
Keyword(s):  
Scalar Field ◽  
Channel Flow ◽  
Line Source ◽  
Dissipation Rate ◽  
Turbulent Channel Flow ◽  
Scalar Fields ◽  
Small Scale ◽  
Wall Region ◽  
Small Scales ◽  
Near Wall

AbstractThe dissipation rate,$\varepsilon _{\theta }$, of a passive scalar (temperature in air) emitted from a concentrated source into a fully developed high-aspect-ratio turbulent channel flow is studied. The goal of the present work is to investigate the return to isotropy of the scalar field when the scalar is injected in a highly anisotropic manner into an inhomogeneous turbulent flow at small scales. Both experiments and direct numerical simulations (DNS) were used to study the downstream evolution of$\varepsilon _{\theta }$for scalar fields generated by line sources located at the channel centreline$(y_s/h = 1.0)$and near the wall$(y_s/h = 0.17)$. The temperature fluctuations and temperature derivatives were measured by means of a pair of parallel cold-wire thermometers in a flow at$Re_{\tau } = 520$. The DNS were performed at$Re_{\tau } = 190$using a spectral method to solve the continuity and Navier–Stokes equations, and a flux integral method (Germaine, Mydlarski & Cortelezzi,J. Comput. Phys., vol. 174, 2001, pp. 614–648) for the advection–diffusion equation. The statistics of the scalar field computed from both experimental and numerical data were found to be in good agreement, with certain discrepancies that were attributable to the difference in the Reynolds numbers of the two flows. A return to isotropy of the small scales was never perfectly observed in any region of the channel for the downstream distances studied herein. However, a continuous decay of the small-scale anisotropy was observed for the scalar field generated by the centreline line source in both the experiments and DNS. The scalar mixing was found to be more rapid in the near-wall region, where the experimental results exhibited low levels of small-scale anisotropy. However, the DNS, which were performed at lower$Re_{\tau }$, showed that persistent anisotropy can also exist near the wall, independently of the downstream location. The role of the mean velocity gradient in the production of$\varepsilon _{\theta }$(and therefore anisotropy) in the near-wall region was highlighted.

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