scholarly journals Passive scalar anisotropy in a heated turbulent wake: new observations and implications for large-eddy simulations

2001 ◽  
Vol 442 ◽  
pp. 161-170 ◽  
Author(s):  
HYUNG SUK KANG ◽  
CHARLES MENEVEAU
Keyword(s):  
Kinetic Energy ◽  
Eddy Diffusivity ◽  
Passive Scalar ◽  
Separation Distance ◽  
Large Eddy Simulations ◽  
Wake Flow ◽  
Scalar Dissipation ◽  
Moderate Reynolds Number ◽  
Large Eddy ◽  
Scalar Variance

The effects of passive scalar anisotropy on subgrid-scale (SGS) physics and modelling for large-eddy simulations are studied experimentally. Measurements are performed across a moderate Reynolds number wake flow generated by a heated cylinder, using an array of four X-wire and four cold-wire probes. By varying the separation distance among probes in the array, we obtain filtered and subgrid quantities at three different filter sizes. We compute several terms that comprise the subgrid dissipation tensor of kinetic energy and scalar variance and test for isotropic behaviour, as a function of filter scale. We find that whereas the kinetic energy dissipation tensor tends towards isotropy at small scales, the SGS scalar-variance dissipation remains anisotropic independent of filter scale. The eddy-diffusion model predicts isotropic behaviour, whereas the nonlinear (or tensor eddy diffusivity) model reproduces the correct trends, but overestimates the level of scalar dissipation anisotropy. These results provide some support for so-called mixed models but raise new questions about the causes of the observed anisotropy.

A scalar subgrid model with flow structure for large-eddy simulations of scalar variances

2000 ◽  
Vol 407 ◽  
pp. 315-349 ◽  
Author(s):  
P. FLOHR ◽  
J. C. VASSILICOS
Keyword(s):  
Flow Structure ◽  
Kinematic Model ◽  
Scalar Fields ◽  
Passive Scalar ◽  
Inertial Range ◽  
Large Eddy Simulations ◽  
Tracer Particles ◽  
Particle Pairs ◽  
Large Eddy ◽  
Scalar Variance

A new model to simulate passive scalar fields in large-eddy simulations of turbulence is presented. The scalar field is described by clouds of tracer particles and the subgrid contribution of the tracer displacement is modelled by a kinematic model which obeys Kolmogorov's inertial-range scaling, is incompressible and incorporates turbulent-like flow structure of the turbulent small scales. This makes it possible to study the scalar variance field with inertial-range effects explicitly resolved by the kinematic subgrid field while the LES determines the value of the Lagrangian integral time scale TL. In this way, the modelling approach does not rely on unknown Lagrangian input parameters which determine the absolute value of the scalar variance.The mean separation of particle pairs displays a well-defined Richardson scaling in the inertial range, and we find that the Richardson constant GΔ ≈ 0.07 which is small compared to the value obtained from stochastic models with the same TL. The probability density function of the separation of particle pairs is found to be highly non-Gaussian in the inertial range of times and for long times becomes Gaussian. We compute the scalar variance field for an instantaneous line source and find good agreement with experimental data.

Assessment of two-equation models of turbulent passive-scalar diffusion in channel flow

1992 ◽  
Vol 238 ◽  
pp. 405-433 ◽  
Author(s):  
Kiyosi Horiuti
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Eddy Diffusivity ◽  
Passive Scalar ◽  
Scalar Flux ◽  
Scalar Fluxes ◽  
Large Eddy ◽  
Scalar Variance ◽  
Flux Model ◽  
Kim Model

Models for the transport of passive scalar in turbulent flow were investigated using databases derived from numerical solutions of the Navier—Stokes equations for fully developed plane channel flow, these databases being generated using large-eddy and direct numerical simulation techniques. Their reliability has been established by comparison with the experimental measurements of Hishida. Nagano & Tagawa (1986). The present paper compares these simulations and calculations using the Nagano & Kim (1988) ‘two-equation’ model for the scalar variance (kθ) and scalar variance dissipation (εθ). This model accounts for the dependence of flow quantities on the Prandtl number by expressing eddy diffusivity in terms of the ratio of the timescales of velocity and scalar fluctuations. However, the statistical analysis by Yoshizawa (1988) showed that there was an inconsistency in the definition of the isotropic eddy diffusivity in the Nagano—Kim model, the implications of which are clearly demonstrated by the results of this paper where large-eddy simulation and direct numerical simulation (LES/DNS) databases are used to compute the quantities contained in both models. An extension of the Nagano-Kim model is proposed which resolves these inconsistencies, and a further development of this model is given in which the anisotropic scalar fluxes are calculated. Near a rigid surface, a third-order ‘anisotropic representation’ of scalar fluxes may be used as an alternative model for reducing the eddy diffusivity, instead of the conventional ‘damping functions’. This model is similar but distinct from the algebraic scalar flux model of Rogers, Mansour & Reynolds (1989). A third aspect of this paper is the use of the LES/DNS databases to evaluate certain coefficients (those for modelling the pressure-scalar gradient terms) of another model of a similar type, namely the algebraic scalar flux model of Launder (1975).

Analogy between velocity and scalar fields in a turbulent channel flow

2009 ◽  
Vol 628 ◽  
pp. 241-268 ◽  
Author(s):  
ROBERT ANTHONY ANTONIA ◽  
HIROYUKI ABE ◽  
HIROSHI KAWAMURA
Keyword(s):  
Kinetic Energy ◽  
Prandtl Number ◽  
Channel Flow ◽  
Longitudinal Velocity ◽  
Turbulent Channel Flow ◽  
Scalar Fields ◽  
Passive Scalar ◽  
Scalar Dissipation ◽  
Turbulent Heat ◽  
Scalar Variance

The relationship between the fluctuating velocity vector and the temperature fluctuation has been examined using direct numerical simulation databases of a turbulent channel flow with passive scalar transport using a constant time-averaged heat flux at each wall for h+ = 180, 395, 640 and 1020 (where h is the channel half-width with the superscript denoting normalization by wall variables) at Prandtl number Pr=0.71. The analogy between spectra corresponding to the kinetic energy and scalar variance is reasonable in both inner and outer regions irrespective of whether the spectra are plotted in terms of kx or kz, the wavenumbers in the streamwise and spanwise directions respectively. Whereas all three velocity fluctuations contribute to the energy spectrum when kx is used, the longitudinal velocity fluctuation is the major contributor when kz is used. The quality of the analogy in the spectral domain is confirmed by visualizations in physical space and reflects differences between spatial organizations in the velocity and scalar fields. The similarity between the spectra corresponding to the enstrophy and scalar dissipation rate is not as good as that between the kinetic energy and scalar variance, emphasizing the prominence of the scalar sheets as the centre of the channel is approached. The ratio R between the characteristic time scales of the velocity and scalar fluctuations is approximately constant over a major part of the channel and independent of h+, when the latter is sufficiently large. This constancy, which is not observed in quantities such as the turbulent Prandtl number, follows from the spectral similarities discussed in this paper and has implications for turbulent heat transport models.

scholarly journals A-Priori Validation of Scalar Dissipation Rate Models for Turbulent Non-Premixed Flames

2020 ◽  
Author(s):  
M. P. Sitte ◽  
C. Turquand d’Auzay ◽  
A. Giusti ◽  
E. Mastorakos ◽  
N. Chakraborty
Keyword(s):  
Dissipation Rate ◽  
A Priori ◽  
Turbulent Flames ◽  
Large Eddy Simulations ◽  
Moment Closure ◽  
Scalar Dissipation ◽  
Combustion Model ◽  
Scalar Dissipation Rate ◽  
Jet Flame ◽  
Large Eddy

Abstract The modelling of scalar dissipation rate in conditional methods for large-eddy simulations is investigated based on a priori direct numerical simulation analysis using a dataset representing an igniting non-premixed planar jet flame. The main objective is to provide a comprehensive assessment of models typically used for large-eddy simulations of non-premixed turbulent flames with the Conditional Moment Closure combustion model. The linear relaxation model gives a good estimate of the Favre-filtered scalar dissipation rate throughout the ignition with a value of the related constant close to the one deduced from theoretical arguments. Such value of the constant is one order of magnitude higher than typical values used in Reynolds-averaged approaches. The amplitude mapping closure model provides a satisfactory estimate of the conditionally filtered scalar dissipation rate even in flows characterised by shear driven turbulence and strong density variation.

Numerical errors in the computation of subfilter scalar variance in large eddy simulations

2009 ◽  
Vol 21 (5) ◽  
pp. 055102 ◽  
Author(s):  
C. M. Kaul ◽  
V. Raman ◽  
G. Balarac ◽  
H. Pitsch
Keyword(s):  
Large Eddy Simulations ◽  
Numerical Errors ◽  
Large Eddy ◽  
Scalar Variance

Mixing Models for Large-Eddy Simulation of Nonpremixed Turbulent Combustion

2000 ◽  
Vol 123 (2) ◽  
pp. 341-346 ◽  
Author(s):  
S. M. deBruynKops ◽  
J. J. Riley
Keyword(s):  
Turbulent Combustion ◽  
Dissipation Rate ◽  
Mixture Fraction ◽  
Reaction Rates ◽  
Large Eddy Simulations ◽  
Scalar Dissipation ◽  
Mixing Models ◽  
Mixing Rate ◽  
Flamelet Model ◽  
Large Eddy

The application of mixture fraction based models to large-eddy simulations (LES) of nonpremixed turbulent combustion requires information about mixing at length scales not resolved on the LES grid. For instance, the large-eddy laminar flamelet model (LELFM) takes the subgrid-scale variance and the filtered dissipation rate of the mixture fraction as inputs. Since chemical reaction rates in nonpremixed turbulence are largely governed by the mixing rate, accurate mixing models are required if mixture fraction methods are to be successfully used to predict species concentrations in large-eddy simulations. In this paper, several models for the SGS scalar variance and the filtered scalar dissipation rate are systematically evaluated a priori using benchmark data from a DNS in homogeneous, isotropic, isothermal turbulence. The mixing models are also evaluated a posteriori by applying them to actual LES data of the same flow. Predictions from the models that depend on an assumed form for the scalar energy spectrum are very good for the flow considered, and are better than those from models that rely on other assumptions.

Large Eddy Simulations of Flow Past Two Pipelines in Tandem in Close Proximity to the Seabed

2017 ◽  
Author(s):  
Zhong Li ◽  
Mia Abrahamsen Prsic ◽  
Muk Chen Ong ◽  
Boo Cheong Khoo
Keyword(s):  
Drag Coefficient ◽  
Recirculation Zone ◽  
Separation Distance ◽  
Large Eddy Simulations ◽  
Plane Wall ◽  
Scale Model ◽  
Tandem Cylinders ◽  
Flow Past ◽  
Large Eddy ◽  
The Mean

Three-dimensional Large Eddy Simulations (LES) with Smagorinsky subgrid scale model have been performed for the flow past two free-spanning marine pipelines in tandem placed in the vicinity of a plane wall at a very small gap ratio, namely G/D = 0.1, 0.3 and 0.5. The ratio of cylinder center-to-center distance to cylinder diameter, or pitch ratio, L/D, considered in the simulations is taken as L/D = 2 and 5. This work serves as an extension of Abrahamsen Prsic et al. (2015) [1]. In essence, six sets of simulations have been performed in the subcritical Reynolds number regime at Re = 1.31 × 104. Our major findings can be summarized as follows. (1) At both pitch ratios, the wall proximity has a decreasing effect on the mean drag coefficient of the upstream cylinder. At L/D = 2, the mean drag coefficient of the downstream cylinder is negative since it is located within the drag inversion separation distance. (2) At L/D = 2, a squarish cavity-like flow exists between the tandem cylinders and flow circulates within the cavity. A long lee-wake recirculation zone is found behind the downstream cylinder at G/D = 0.1. However, a much smaller lee-wake recirculation zone is noticed at L/D = 5 with G/D = 0.1. (3) At L/D = 2, the reattachment is biased to the bottom shear layer due towards the deflection from the plane wall, which leads to the formation of the slanted squarish cavity-like flow where the flow circulates between the tandem cylinders.

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