scholarly journals Optimal fluxes and Reynolds stresses

2016 ◽  
Vol 809 ◽  
pp. 585-600 ◽  
Author(s):  
Javier Jiménez
Keyword(s):  
Numerical Procedure ◽  
Momentum Equation ◽  
Turbulent Shear ◽  
Spectral Energy ◽  
Reynolds Stresses ◽  
Large Eddy ◽  
Solenoidal Field ◽  
Long Time ◽  
Time Averages ◽  
Scalar Conservation

It is remarked that fluxes in conservation laws, such as the Reynolds stresses in the momentum equation of turbulent shear flows, or the spectral energy flux in anisotropic turbulence, are only defined up to an arbitrary solenoidal field. While this is not usually significant for long-time averages, it becomes important when fluxes are modelled locally in large-eddy simulations, or in the analysis of intermittency and cascades. As an example, a numerical procedure is introduced to compute fluxes in scalar conservation equations in such a way that their total integrated magnitude is minimised. The result is an irrotational vector field that derives from a potential, thus minimising sterile flux ‘circuits’. The algorithm is generalised to tensor fluxes and applied to the transfer of momentum in a turbulent channel. The resulting instantaneous Reynolds stresses are compared with their traditional expressions, and found to be substantially different. This suggests that some of the alleged shortcomings of simple subgrid models may be representational artefacts, and that the same may be true of the intermittency properties of the turbulent stresses.

scholarly journals Flow around a Complex Building: Experimental and Large-Eddy Simulation Comparisons

2005 ◽  
Vol 44 (5) ◽  
pp. 571-590 ◽  
Author(s):  
Ronald Calhoun ◽  
Frank Gouveia ◽  
Joseph Shinn ◽  
Stevens Chan ◽  
Dave Stevens ◽  
...  
Keyword(s):  
Large Eddy Simulation ◽  
Initial Conditions ◽  
Sonic Anemometer ◽  
Momentum Equation ◽  
Full Potential ◽  
Reynolds Stresses ◽  
Additional Insight ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Complex Building

Abstract A field program to study atmospheric releases around a complex building was performed in the summers of 1999 and 2000. The focus of this paper is to compare field data with a large-eddy simulation (LES) code to assess the ability of the LES approach to yield additional insight into atmospheric release scenarios. In particular, transient aspects of the velocity and concentration signals are studied. The simulation utilized the finite-element method with a high-fidelity representation of the complex building. Trees were represented with a canopy term in the momentum equation. Inflow and outflow conditions were used. The upwind velocity was constructed from a logarithmic law fitted to velocities obtained on two levels from a tower equipped with a 2D sonic anemometer. A number of different kinds of comparisons of the transient velocity and concentration signals are presented—direct signal versus time, spectral, Reynolds stresses, turbulent kinetic energy signals, and autocorrelations. It is concluded that the LES approach does provide additional insight, but the authors argue that the proper use of LES should include consideration of cost and may require an increased connection to field sensors; that is, higher-resolution boundary and initial conditions need to be provided to realize the full potential of LES.

Calculation of turbulence-driven secondary motion in non-circular ducts

1984 ◽  
Vol 140 ◽  
pp. 189-222 ◽  
Author(s):  
A. O. Demuren ◽  
W. Rodi
Keyword(s):  
Eddy Viscosity ◽  
Three Dimensional ◽  
Numerical Procedure ◽  
Momentum Equation ◽  
Shear Stresses ◽  
Reynolds Stresses ◽  
Calculation Methods ◽  
Mean Flow ◽  
Square Duct ◽  
Secondary Motion

Experiments on and calculation methods for flow in straight non-circular ducts involving turbulence-driven secondary motion are reviewed. The origin of the secondary motion and the shortcomings of existing calculation methods are discussed. A more refined model is introduced, in which algebraic expressions are derived for the Reynolds stresses in the momentum equations for the secondary motion by simplifying the modelled Reynolds-stress equations of Launder, Reece & Rodi (1975), while a simple eddy-viscosity model is used for the shear stresses in the axial momentum equation. The kinetic energy k and the dissipation rate ε of the turbulent motion which appear in the algebraic and the eddy-viscosity expressions are determined from transport equations. The resulting set of equations is solved with a forward-marching numerical procedure for three-dimensional shear layers. The model, as well as a version proposed by Naot & Rodi (1982), is tested by application to developing flow in a square duct and to developed flow in a partially roughened rectangular duct investigated experimentally by Hinze (1973). In both cases, the main features of the mean-flow and the turbulence quantities are simulated realistically by both models, but the present model underpredicts the secondary velocity while the Naot-Rodi model tends to overpredict it.

The Influence of Small Particles on the Structure of a Turbulent Shear Flow

2004 ◽  
Vol 126 (4) ◽  
pp. 613-619 ◽  
Author(s):  
David I. Graham
Keyword(s):  
Shear Flow ◽  
General Trend ◽  
Turbulent Shear ◽  
Mass Loading ◽  
Turbulent Shear Flow ◽  
Algebraic Equations ◽  
Reynolds Stresses ◽  
Temporal Correlations ◽  
Fluid Velocities ◽  
Nonlinear Algebraic Equations

In this paper, an analytical solution is found for the Reynolds equations describing a simple turbulent shear flow carrying small, wake-less particles. An algebraic stress model is used as the basis of the model, the particles leading to source terms in the equations for the turbulent stresses in the flow. The sources are proportional to the mass loading of the particles and depend on the temporal correlations of the fluid velocities seen by particles, Rijτ. The resulting set of equations is a system of nonlinear algebraic equations for the Reynolds stresses and the dissipation. The system is solved exactly and the influence of the particles can be quantified. The predictions are compared with DNS results and are shown to predict trends quite well. Different scenarios are investigated, including the effects of isotropic, anisotropic and non-equilibrium time scales and negative loops in Rijτ. The general trend is to increase anisotropy and attenuate turbulence with higher mass loadings. The occurrence of turbulence enhancement is investigated and shown to be theoretically possible, but physically unlikely.

scholarly journals Sticky Thermals: Evidence for a Dominant Balance between Buoyancy and Drag in Cloud Updrafts

2015 ◽  
Vol 72 (8) ◽  
pp. 2890-2901 ◽  
Author(s):  
David M. Romps ◽  
Alexander B. Charn
Keyword(s):  
Large Eddy Simulation ◽  
Practical Interest ◽  
Momentum Equation ◽  
Severe Weather ◽  
Convective Clouds ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Great Practical Interest ◽  
Natural Boundaries ◽  
Drag Law

Abstract The vertical velocities of convective clouds are of great practical interest because of their influence on many phenomena, including severe weather and stratospheric moistening. However, the magnitudes of forces giving rise to these vertical velocities are poorly understood, and the dominant balance is in dispute. Here, an algorithm is used to extract thousands of cloud thermals from a large-eddy simulation of deep and tropical maritime convection. Using a streamfunction to define natural boundaries for these thermals, the dominant balance in the vertical momentum equation is revealed. Cloud thermals rise with a nearly constant speed determined by their buoyancy and the standard drag law with a drag coefficient of 0.6. Contrary to suggestions that cloud thermals might be slippery, with a dominant balance between buoyancy and acceleration, cloud thermals are found here to be sticky, with a dominant balance between buoyancy and drag.

Aerodynamic Investigation of a Passive Secondary Jet Exciting a Separation Bubble for a Blunt Flat Plate

2015 ◽  
Author(s):  
Christian Helcig ◽  
Stefan aus der Wiesche ◽  
Stephan Uhkoetter
Keyword(s):  
Boundary Layer ◽  
Laser Doppler Anemometry ◽  
Separation Bubble ◽  
Flow Characteristics ◽  
Numerical Procedure ◽  
Large Eddy Simulations ◽  
Reattachment Length ◽  
Large Eddy ◽  
Low Speed Wind Tunnel ◽  
Aerodynamic Investigation

The aim of this study is to examine the influence of passive jets interacting with the separation region of the flow around a blunt plate. Experimental and numerical analysis are used to measure the velocity within the separation and reattachment region of the blunt plate with different passive jet configurations. A blunt plate was placed in a low speed wind tunnel to conduct Laser-Doppler anemometry (LDA) measurements at Re = 2.06 × 104. For the numerical procedure a dynamical sub-grid model for Large Eddy Simulations (LES) was used. For all configurations the flow characteristics such as the reattachment length were determined to characterize the boundary layer. The passive jets showed a strong influence by interacting with the boundary layer of the blunt plate.

scholarly journals Physics-Based Simulations of Flow and Fire Development Downstream of a Canopy

Atmosphere ◽  
2020 ◽  
Vol 11 (7) ◽  
pp. 683
Author(s):  
Gilbert Accary ◽  
Duncan Sutherland ◽  
Nicolas Frangieh ◽  
Khalid Moinuddin ◽  
Ibrahim Shamseddine ◽  
...  
Keyword(s):  
Turbulent Flow ◽  
Forest Canopy ◽  
Prescribed Burning ◽  
Second Phase ◽  
Flow Conditions ◽  
Developed Turbulence ◽  
Large Eddy ◽  
Long Time ◽  
Two Phases ◽  
The Impact

The behavior of a grassland fire propagating downstream of a forest canopy has been simulated numerically using the fully physics-based wildfire model FIRESTAR3D. This configuration reproduces quite accurately the situation encountered when a wildfire spreads from a forest to an open grassland, as can be the case in a fuel break or a clearing, or during a prescribed burning operation. One of the objectives of this study was to evaluate the impact of the presence of a canopy upstream of a grassfire, especially the modifications of the local wind conditions before and inside a clearing or a fuel break. The knowledge of this kind of information constitutes a major element in improving the safety conditions of forest managers and firefighters in charge of firefighting or prescribed burning operations in such configurations. Another objective was to study the behavior of the fire under realistic turbulent flow conditions, i.e., flow resulting from the interaction between an atmospheric boundary layer (ABL) with a surrounding canopy. Therefore, the study was divided into two phases. The first phase consisted of generating an ABL/canopy turbulent flow above a pine forest (10 m high, 200 m long) using periodic boundary conditions along the streamwise direction. Large Eddy Simulations (LES) were carried out for a sufficiently long time to achieve a quasi-fully developed turbulence. The second phase consisted of simulating the propagation of a surface fire through a grassland, bordered upstream by a forest section (having the same characteristics used for the first step), while imposing the turbulent flow obtained from the first step as a dynamic inlet condition to the domain. The simulations were carried out for a wind speed that ranged between 1 and 12 m/s; these values have allowed the simulations to cover the two regimes of propagation of surfaces fires, namely plume-dominated and wind-driven fires.

scholarly journals Three-Dimensional Turbulent Vortex Shedding From a Surface-Mounted Square Cylinder: Predictions With Large-Eddy Simulations and URANS

2014 ◽  
Vol 136 (6) ◽  
Author(s):  
B. A. Younis ◽  
A. Abrishamchi
Keyword(s):  
Experimental Data ◽  
Turbulence Model ◽  
Stokes Equations ◽  
Three Dimensional ◽  
Large Eddy Simulations ◽  
Navier Stokes ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Navier Stokes Equations ◽  
Large Eddy

The paper reports on the prediction of the turbulent flow field around a three-dimensional, surface mounted, square-sectioned cylinder at Reynolds numbers in the range 104–105. The effects of turbulence are accounted for in two different ways: by performing large-eddy simulations (LES) with a Smagorinsky model for the subgrid-scale motions and by solving the unsteady form of the Reynolds-averaged Navier–Stokes equations (URANS) together with a turbulence model to determine the resulting Reynolds stresses. The turbulence model used is a two-equation, eddy-viscosity closure that incorporates a term designed to account for the interactions between the organized mean-flow periodicity and the random turbulent motions. Comparisons with experimental data show that the two approaches yield results that are generally comparable and in good accord with the experimental data. The main conclusion of this work is that the URANS approach, which is considerably less demanding in terms of computer resources than LES, can reliably be used for the prediction of unsteady separated flows provided that the effects of organized mean-flow unsteadiness on the turbulence are properly accounted for in the turbulence model.

Evaluation of a Statistically Targeted Forcing Method for Synthetic Turbulence Generation in Large-Eddy Simulations and Hybrid RANS-LES Simulations

2020 ◽  
Author(s):  
Olalekan O. Shobayo ◽  
D. Keith Walters
Keyword(s):  
Turbulent Flow ◽  
Momentum Equation ◽  
New Method ◽  
Navier Stokes ◽  
Model Parameters ◽  
Mean Velocity ◽  
Spectra Analysis ◽  
Large Eddy ◽  
Mesh Resolution ◽  
Turbulence Generation

Abstract Computational fluid dynamics (CFD) results are presented for synthetic turbulence generation by a proposed statistically targeted forcing (STF) method. The new method seeks to introduce a fluctuating velocity field with a distribution of first and second moments that match a user-specified target mean velocity and Reynolds stress tensor, by incorporating deterministic time-dependent forcing terms into the momentum equation for the resolved flow. The STF method is formulated to extend the applicability of previously documented methods and provide flexibility in regions where synthetic turbulence needs to be generated or damped, for use in engineering level large-eddy and hybrid large-eddy/Reynolds-averaged Navier-Stokes CFD simulations. The objective of this study is to evaluate the performance of the proposed STF method in LES simulations of isotropic and anisotropic homogeneous turbulent flow test cases. Results are interrogated and compared to target statistical velocity and turbulent stress distributions and evaluated in terms of energy spectra. Analysis of the influence of STF model parameters, mesh resolution, and LES subgrid stress model on the results is investigated. Results show that the new method can successfully reproduce desired statistical distributions in a homogeneous turbulent flow.

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