scholarly journals Shear-flow dispersion in turbulent jets

2015 ◽  
Vol 781 ◽  
pp. 28-51 ◽  
Author(s):  
John Craske ◽  
Antoine L. R. Debugne ◽  
Maarten van Reeuwijk
Keyword(s):  
Shear Flow ◽  
Concentration Field ◽  
Turbulent Jets ◽  
Passive Scalar ◽  
Similar Process ◽  
Scalar Flux ◽  
Front Position ◽  
Longitudinal Mixing ◽  
Using Data ◽  
Integral Transport

We investigate the transport of a passive scalar in a fully developed turbulent axisymmetric jet at a Reynolds number of $\mathit{Re}=4815$ using data from direct numerical simulation. In particular, we simulate the response of the concentration field to an instantaneous variation of the scalar flux at the source. To analyse the time evolution of this statistically unsteady process we take an ensemble average over 16 independent simulations. We find that the evolution of $C_{m}(z,t)$, the radial integral of the ensemble-averaged concentration, is a self-similar process, with the front position and spread both scaling as $\sqrt{t}$. The longitudinal mixing of $C_{m}$ is shown to be primarily caused by shear-flow dispersion. Using the approach developed by Craske & van Reeuwijk (J. Fluid Mech., vol. 763, 2014, pp. 538–566), the classical theory for shear-flow dispersion is applied to turbulent jets to obtain a closure that couples the integral scalar flux to the integral concentration $C_{m}$. Model predictions using the dispersion closure are in good agreement with the simulation data. Application of the dispersion closure to a two-dimensional jet results in an integral transport equation that is fully consistent with that of Landel et al. (J. Fluid Mech., vol. 711, 2012, pp. 212–258).

Investigation of Modeling Approaches for Scalar Transport and Mixing in LES

2006 ◽  
Author(s):  
Olivia S. Sun ◽  
Lester K. Su ◽  
Tristan M. Burton
Keyword(s):  
Mixed Model ◽  
Concentration Field ◽  
Model Performance ◽  
Eddy Diffusivity ◽  
Passive Scalar ◽  
Scalar Transport ◽  
Implicit Time ◽  
Scalar Flux ◽  
Spherical Coordinate ◽  
Eddy Simulation

LES models for subgrid scalar transport are assessed through large eddy simulation of passive scalar mixing in a turbulent round jet. The simulation is performed on a spherical coordinate grid using semi-implicit time advancement. The models used to close the subgrid scalar flux term are varied while the same modeling approach is used for the momentum model in order to isolate the effects of the scalar modeling. The subgrid scalar flux models tested include an eddy diffusivity model and a one-parameter mixed model. Model performance is analyzed by examining mean resolved-scale quantities and PDFs of the scalar concentration field.

scholarly journals Spectral modelling for passive scalar dynamics in homogeneous anisotropic turbulence

2016 ◽  
Vol 799 ◽  
pp. 159-199 ◽  
Author(s):  
A. Briard ◽  
T. Gomez ◽  
C. Cambon
Keyword(s):  
Isotropic Turbulence ◽  
Reynolds Numbers ◽  
Passive Scalar ◽  
Homogeneous Isotropic Turbulence ◽  
Scalar Flux ◽  
Anisotropic Turbulence ◽  
Theoretical Predictions ◽  
High Reynolds ◽  
Shear Driven Flows ◽  
Very High

The present work aims at developing a spectral model for a passive scalar field and its associated scalar flux in homogeneous anisotropic turbulence. This is achieved using the paradigm of eddy-damped quasi-normal Markovian (EDQNM) closure extended to anisotropic flows. In order to assess the validity of this approach, the model is compared to several detailed direct numerical simulations (DNS) and experiments of shear-driven flows and isotropic turbulence with a mean scalar gradient at moderate Reynolds numbers. This anisotropic modelling is then used to investigate the passive scalar dynamics at very high Reynolds numbers. In the framework of homogeneous isotropic turbulence submitted to a mean scalar gradient, decay and growth exponents for the cospectrum and scalar energies are obtained analytically and assessed numerically thanks to EDQNM closure. With the additional presence of a mean shear, the scaling of the scalar flux and passive scalar spectra in the inertial range are investigated and confirm recent theoretical predictions. Finally, it is found that, in shear-driven flows, the small scales of the scalar second-order moments progressively return to isotropy when the Reynolds number increases.

scholarly journals An explicit algebraic model for the subgrid-scale passive scalar flux

2013 ◽  
Vol 721 ◽  
pp. 541-577 ◽  
Author(s):  
Amin Rasam ◽  
Geert Brethouwer ◽  
Arne V. Johansson
Keyword(s):  
Channel Flow ◽  
Rotation Rate ◽  
Model Performance ◽  
Turbulent Channel Flow ◽  
Passive Scalar ◽  
Subgrid Scale ◽  
Scalar Flux ◽  
Algebraic Form ◽  
New Model ◽  
Flux Model

AbstractIn Marstorpet al. (J. Fluid Mech., vol. 639, 2009, pp. 403–432), an explicit algebraic subgrid stress model (EASSM) for large-eddy simulation (LES) was proposed, which was shown to considerably improve LES predictions of rotating and non-rotating turbulent channel flow. In this paper, we extend that work and present a new explicit algebraic subgrid scalar flux model (EASSFM) for LES, based on the modelled transport equation of the subgrid-scale (SGS) scalar flux. The new model is derived using the same kind of methodology that leads to the explicit algebraic scalar flux model of Wikströmet al. (Phys. Fluids, vol. 12, 2000, pp. 688–702). The algebraic form is based on a weak equilibrium assumption and leads to a model that depends on the resolved strain-rate and rotation-rate tensors, the resolved scalar-gradient vector and, importantly, the SGS stress tensor. An accurate prediction of the SGS scalar flux is consequently strongly dependent on an accurate description of the SGS stresses. The new EASSFM is therefore primarily used in connection with the EASSM, since this model can accurately predict SGS stresses. The resulting SGS scalar flux is not necessarily aligned with the resolved scalar gradient, and the inherent dependence on the resolved rotation-rate tensor makes the model suitable for LES of rotating flow applications. The new EASSFM (together with the EASSM) is validated for the case of passive scalar transport in a fully developed turbulent channel flow with and without system rotation. LES results with the new model show good agreement with direct numerical simulation data for both cases. The new model predictions are also compared to those of the dynamic eddy diffusivity model (DEDM) and improvements are observed in the prediction of the resolved and SGS scalar quantities. In the non-rotating case, the model performance is studied at all relevant resolutions, showing that its predictions of the Nusselt number are much less dependent on the grid resolution and are more accurate. In channel flow with wall-normal rotation, where all the SGS stresses and fluxes are non-zero, the new model shows significant improvements over the DEDM predictions of the resolved and SGS quantities.

Experimental study on longitudinal mixing in open channel flow with various vegetation patterns

2020 ◽  
Author(s):  
Hyoungchul Park ◽  
Jinhwan Hwang
Keyword(s):  
Shear Flow ◽  
Dispersion Coefficient ◽  
Longitudinal Dispersion ◽  
Velocity Shear ◽  
Vegetation Patterns ◽  
Longitudinal Dispersion Coefficient ◽  
Mean Velocity ◽  
Natural Streams ◽  
Longitudinal Mixing ◽  
The Impact

<p>In natural streams, vegetation considerably has an influence on the flow characteristics in a variety of ways. For example, vegetation distorts flow structure in both lateral and vertical directions and changes the magnitude of turbulence and shear flow. Due to these effects, diluted contaminants in river transport and disperse differently. Accordingly, many previous researchers have investigated the impact of vegetation on the mixing process. Most of them have estimated the dispersion coefficient since this is the crucial parameter to quantify the degree of dispersion of contaminants numerically. They mainly studied in diverse characteristics of vegetation, such as density or submergence, etc., and identified the change in hydraulic parameters involving the dispersion coefficient.</p><p>In this work, considering the vegetation distributed in various forms in the natural river, we studied the effect of vegetation patterns on the longitudinal mixing coefficient. Six types of spatial patterns considered in this study are represented numerically by introducing the standardized Morisita index. Laboratory experiments with artificial emergent vegetation were performed in multiple vegetation patterns, and the longitudinal dispersion coefficient was estimated from the measured concentration curves by applying the routing technique. And we analyzed the cause of change in dispersion coefficient by calculating not only the dispersion coefficient but also the magnitude of mean velocity, shear flow, turbulence, etc.</p><p>According to the experimental results, the mean velocity in the vegetated channel is almost the same regardless of the type of pattern but is always lower than that in the non-vegetated channel. The longitudinal dispersion coefficient gets larger as the arrangement changes from uniform to 2D clumped pattern. The cause of change in coefficient is closely related to the spatial velocity gradients in both lateral and vertical directions since the spatial heterogeneity of velocity increases the magnitude of shear flow.</p>

Second-order modeling of a passive scalar in a turbulent shear flow

AIAA Journal ◽  
1990 ◽  
Vol 28 (4) ◽  
pp. 610-617 ◽  
Author(s):  
T.-H. Shih ◽  
J. L. Lumley ◽  
J.-Y. Chen
Keyword(s):  
Shear Flow ◽  
Passive Scalar ◽  
Second Order ◽  
Turbulent Shear ◽  
Turbulent Shear Flow

Transport of passive scalar in turbulent shear flow under a clean or surfactant-contaminated free surface

2011 ◽  
Vol 670 ◽  
pp. 527-557 ◽  
Author(s):  
HAMID R. KHAKPOUR ◽  
LIAN SHEN ◽  
DICK K. P. YUE
Keyword(s):  
Free Surface ◽  
Shear Flow ◽  
Solid Wall ◽  
Surface Flux ◽  
Scalar Fields ◽  
Transport Processes ◽  
Turbulent Shear ◽  
Turbulent Shear Flow ◽  
Scalar Flux ◽  
Conditional Averaging

Direct numerical simulation is performed to study the turbulent transport of passive scalars near clean and surfactant-contaminated free surfaces. As a canonical problem, a turbulent shear flow interacting with a flat free surface is considered, with a focus on the effect of splats and anti-splats on the scalar transport processes. Using conditional averaging of strong surface flux events, it is shown that these are associated with coherent hairpin vortex structures emerging from the shear flow. The upwelling at the splat side of the oblique hairpin vortices greatly enhances the scalar surface flux. In the presence of surfactants, the splats at the surface are suppressed by the surface tension gradients caused by spatial variation of surfactant concentration; as a result, scalar flux is reduced. Conditional averaging of weak surface flux events shows that these are caused by anti-splats with which surface-connected vortices are often associated. When surfactants are present, the downdraught transport at the surface-connected vortices is weakened. Turbulence statistics of the velocity and scalar fields are performed in terms of mean and fluctuation profiles, scalar flux, turbulent diffusivity and scalar variance budget. Using surface layer quantification based on an analytical similarity solution of the mean shear flow, it is shown that the depth of the scalar statistics variation is scaled on the basis of the Schmidt number. In the presence of surfactants, the scalar statistics have the characteristics of those near a solid wall in contrast to those near a clean surface, which leads to thickened scalar boundary layer and reduced surface flux.

On the geometry of turbulent mixing

1999 ◽  
Vol 393 ◽  
pp. 123-147 ◽  
Author(s):  
EMMANUEL VILLERMAUX ◽  
CLAUDIA INNOCENTI
Keyword(s):  
Fractal Dimension ◽  
Scalar Field ◽  
Turbulent Mixing ◽  
Concentration Field ◽  
Volume Ratio ◽  
Passive Scalar ◽  
Global Features ◽  
Integral Scale ◽  
Scalar Dispersion ◽  
Concentration Threshold

We investigate the temporal evolution of the geometrical distribution of a passive scalar injected continuously into the far field of a turbulent water jet at a scale d smaller than the local integral scale of the turbulence. The concentration field is studied quantitatively by a laser-induced- fluorescence technique on a plane cut containing the jet axis. Global features such as the scalar dispersion from the source, as well as the fine structure of the scalar field, are analysed. In particular, we define the volume occupied by the regions whose concentration is larger than a given concentration threshold (support of the scalar field) and the surface in which this volume is enclosed (boundary of the support). The volume and surface extents, and their respective fractal dimensions are measured as a function of time t, and the concentration threshold is normalized by the initial concentration Cs/C0 for different injection sizes d. All of these quantities display a clear dependence on t, d and Cs, and their evolutions rescale with the variable ξ = (ut/d)(Cs/C0), the fractal dimension being, in addition, scale dependent. The surface-to-volume ratio and the fractal dimension of both the volume and the surface tend towards unity at large ξ, reflecting the sheet-like structure of the scalar at small scales. These findings suggest an original picture of the kinetics of turbulent mixing.

Passive-scalar wake behind a line source in grid turbulence

2000 ◽  
Vol 416 ◽  
pp. 117-149 ◽  
Author(s):  
D. LIVESCU ◽  
F. A. JABERI ◽  
C. K. MADNIA
Keyword(s):  
Line Source ◽  
Scalar Fields ◽  
Passive Scalar ◽  
Transport Equations ◽  
Source Size ◽  
Scalar Dissipation ◽  
Grid Turbulence ◽  
Scalar Flux ◽  
Convective Regime ◽  
Scalar Variance

The structure and development of the scalar wake produced by a single line source are studied in decaying isotropic turbulence. The incompressible Navier–Stokes and the passive-scalar transport equations are solved via direct numerical simulations (DNS). The velocity and the scalar fields are generated by simulating Warhaft's (1984) experiment. The results for mean and r.m.s. scalar statistics are in good agreement with those obtained from the experiment. The structure of the scalar wake is examined first. At initial times, most of the contribution to the scalar variance is due to the flapping of the wake around the centreline. Near the end of the turbulent convective regime, the wake develops internal structure and the contribution of the flapping component to the scalar variance becomes negligible. The influence of the source size on the development of the scalar wake has been examined for source sizes ranging from the Kolmogorov microscale to the integral scale. After an initial development time, the half-widths of mean and scalar r.m.s. wakes grow at rates independent of the source size. The mixing in the scalar wake is studied by analysing the evolution of the terms in the transport equations for mean, scalar flux, variance, and scalar dissipation. The DNS results are used to test two types of closures for the mean and the scalar variance equations. For the time range simulated, the gradient diffusion model for the scalar flux and the commonly used scalar dissipation model are not supported by the DNS data. On the other hand, the model based on the unconditional probability density function (PDF) method predicts the scalar flux reasonably well near the end of the turbulent convective regime for the highest Reynolds number examined. The scalar source size does not significantly influence the models' predictions, although it appears that the time-scale ratio of mechanical dissipation to scalar dissipation approaches an asymptotic value earlier for larger source sizes.

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