Subgrid combustion modelling for large-eddy simulations

2000 ◽  
Vol 1 (2) ◽  
pp. 209-227 ◽  
Author(s):  
S Menon
Keyword(s):  
Turbulent Flows ◽  
Internal Combustion Engines ◽  
Large Scale ◽  
Large Eddy Simulations ◽  
Accurate Prediction ◽  
Pollutant Emissions ◽  
Combustion Model ◽  
Small Scale ◽  
Large Eddy ◽  
Combustion Processes

Next-generation gas turbine and internal combustion engines are required to reduce pollutant emissions significantly and also to be fuel efficient. Accurate prediction of pollutant formation requires proper resolution of the spatio-temporal evolution of the unsteady mixing and combustion processes. Since conventional steady state methods are not able to deal with these features, methodology based on large-eddy simulations (LESs) is becoming a viable choice to study unsteady reacting flows. This paper describes a new LES methodology developed recently that has demonstrated a capability to simulate reacting turbulent flows accurately. A key feature of this new approach is the manner in which small-scale turbulent mixing and combustion processes are simulated. This feature allows proper characterization of the effects of both large-scale convection and small-scale mixing on the scalar processes, thereby providing a more accurate prediction of chemical reaction effects. LESs of high Reynolds number premixed flames in the flamelet regime and in the distributed reaction regime are used to describe the ability of the new subgrid combustion model.

Buoyancy- to inertial-range transition in forced stratified turbulence

2001 ◽  
Vol 427 ◽  
pp. 205-239 ◽  
Author(s):  
GEORGE F. CARNEVALE ◽  
M. BRISCOLINI ◽  
P. ORLANDI
Keyword(s):  
Wave Breaking ◽  
Large Scale ◽  
High Strain ◽  
Inertial Range ◽  
Large Eddy Simulations ◽  
Small Scale ◽  
Stratified Turbulence ◽  
Large Eddy ◽  
Scale Turbulence ◽  
Range Density

The buoyancy range, which represents a transition from large-scale wave-dominated motions to small-scale turbulence in the oceans and the atmosphere, is investigated through large-eddy simulations. The model presented here uses a continual forcing based on large-scale standing internal waves and has a spectral truncation in the isotropic inertial range. Evidence is presented for a break in the energy spectra from the anisotropic k−3 buoyancy range to the small-scale k−5/3 isotropic inertial range. Density structures that form during wave breaking and periods of high strain rate are analysed. Elongated vertical structures produced during periods of strong straining motion are found to collapse in the subsequent vertically compressional phase of the strain resulting in a zone or patch of mixed fluid.

scholarly journals Statistical Analysis of Dynamic Subgrid Modeling Approaches in Large Eddy Simulation

2021 ◽  
Author(s):  
Mohammad Khalid Hossen ◽  
Asokan Mulayath Variyath ◽  
Jahrul M Alam
Keyword(s):  
Kinetic Energy ◽  
Large Eddy Simulation ◽  
Turbulent Flows ◽  
Large Scale ◽  
Physical Space ◽  
Statistical Characteristics ◽  
Small Scale ◽  
Eddy Simulation ◽  
Wide Range ◽  
Large Eddy

In large eddy simulation (LES) of turbulent flows, the most critical dynamical processes to be considered by dynamic subgrid models to account for an average cascade of kinetic energy from the largest to the smallest scales of the flow is not fully clear. Furthermore, evidence of vortex stretching being the primary mechanism of the cascade is not out of the question. In this article, we study some essential statistical characteristics of vortex stretching and its role in dynamic approaches of modeling subgrid-scale turbulence. We have compared the interaction of subgrid stresses with the filtered quantities among four models using invariants of the velocity gradient tensor. This technique is a single unified approach to studying a wide range of length scales in the turbulent flow. In addition, it also provides a rational basis for the statistical characteristics a subgrid model must serve in physical space to ensure an appropriate cascade of kinetic energy. Results indicate that the stretching mechanism extracts energy from the large-scale straining motion and passes it onto small-scale stretched vortices.

High-Cycle Thermal Fatigue in Mixing Tees: Large-Eddy Simulations Compared to a New Validation Experiment

2008 ◽  
Author(s):  
Johan Westin ◽  
Pascal Veber ◽  
Lars Andersson ◽  
Carsten ’t Mannetje ◽  
Urban Andersson ◽  
...  
Keyword(s):  
Experimental Data ◽  
Large Scale ◽  
Turbulent Viscosity ◽  
Large Eddy Simulations ◽  
Small Scale ◽  
Wall Velocity ◽  
Fine Mesh ◽  
Validation Experiment ◽  
Large Eddy ◽  
Near Wall

The present paper describes new experimental data of thermal mixing in a T-junction compared with results from Large-Eddy Simulations (LES) and Detached Eddy Simulations (DES). The experimental setup was designed in order to provide data suitable for validation of CFD-calculations. The data is obtained from temperature measurements with thermocouples located near the pipe wall, velocity measurements with Laser Doppler Velocimetry (LDV) as well as single-point concentration measurements with Laser Induced Fluorescence (LIF). The LES showed good agreement with the experimental data also when fairly coarse computational meshes were used. However, grid refinement studies revealed a fairly strong sensitivity to the grid resolution, and a simulation using a fine mesh with nearly 10 million cells significantly improved the results in the entire flow domain. The sensitivity to different unsteady inlet boundary conditions was however small, which shows that the strong large-scale instabilities that are present in the mixing region are triggered independent of the applied inlet perturbations. A shortcoming in the performed simulations is insufficient near-wall resolution, which resulted in poor predictions of the near-wall mean velocity profiles and the wall-shear stress. Simulations using DES improved the near-wall velocity predictions, but failed to predict the temperature fluctuations due to high levels of modeled turbulent viscosity that restrained the formation of small scale turbulence.

Turbulence: the filtering approach

1992 ◽  
Vol 238 ◽  
pp. 325-336 ◽  
Author(s):  
M. Germano
Keyword(s):  
Large Eddy Simulation ◽  
Turbulent Flows ◽  
Large Scale ◽  
Stokes Equations ◽  
Small Scale ◽  
Mean Values ◽  
Eddy Simulation ◽  
Turbulent Field ◽  
Large Eddy ◽  
Different Levels

Explicit or implicit filtered representations of chaotic fields like spectral cut-offs or numerical discretizations are commonly used in the study of turbulence and particularly in the so-called large-eddy simulations. Peculiar to these representations is that they are produced by different filtering operators at different levels of resolution, and they can be hierarchically organized in terms of a characteristic parameter like a grid length or a spectral truncation mode. Unfortunately, in the case of a general implicit or explicit filtering operator the Reynolds rules of the mean are no longer valid, and the classical analysis of the turbulence in terms of mean values and fluctuations is not so simple.In this paper a new operatorial approach to the study of turbulence based on the general algebraic properties of the filtered representations of a turbulence field at different levels is presented. The main results of this analysis are the averaging invariance of the filtered Navier—Stokes equations in terms of the generalized central moments, and an algebraic identity that relates the turbulent stresses at different levels. The statistical approach uses the idea of a decomposition in mean values and fluctuations, and the original turbulent field is seen as the sum of different contributions. On the other hand this operatorial approach is based on the comparison of different representations of the turbulent field at different levels, and, in the opinion of the author, it is particularly fitted to study the similarity between the turbulence at different filtering levels. The best field of application of this approach is the numerical large-eddy simulation of turbulent flows where the large scale of the turbulent field is captured and the residual small scale is modelled. It is natural to define and to extract from the resolved field the resolved turbulence and to use the information that it contains to adapt the subgrid model to the real turbulent field. Following these ideas the application of this approach to the large-eddy simulation of the turbulent flow has been produced (Germano et al. 1991). It consists in a dynamic subgrid-scale eddy viscosity model that samples the resolved scale and uses this information to adjust locally the Smagorinsky constant to the local turbulence.

scholarly journals Statistical Analysis of Dynamic Subgrid Modeling Approaches in Large Eddy Simulation

Aerospace ◽  
2021 ◽  
Vol 8 (12) ◽  
pp. 375
Author(s):  
Mohammad Khalid Hossen ◽  
Asokan Mulayath Variyath ◽  
Jahrul M. Alam
Keyword(s):  
Large Eddy Simulation ◽  
Turbulent Flows ◽  
Large Scale ◽  
Joint Probability ◽  
Energy Dissipation Rate ◽  
Statistical Characteristics ◽  
Small Scale ◽  
Eddy Simulation ◽  
Large Eddy ◽  
The Individual

In large eddy simulation (LES) of turbulent flows, dynamic subgrid models would account for an average cascade of kinetic energy from the largest to the smallest scales of the flow. Yet, it is unclear which of the most critical dynamical processes can ensure the criterion mentioned above. Furthermore, evidence of vortex stretching being the primary mechanism of the cascade is not out of the question. In this article, we study essential statistical characteristics of vortex stretching. Our numerical results demonstrate that vortex stretching rate provides the energy dissipation rate necessary for modeling subgrid-scale turbulence. We have compared the interaction of subgrid stresses with the filtered quantities among four models using invariants of the velocity gradient tensor. The individual and the joint probability of vortex stretching and strain amplification show that vortex stretching rate is highly correlated with the energy cascade rate. Sheet-like flow structures are correlated with viscous dissipation, and vortex tubes are more stretched than compressed. The overall results indicate that the stretching mechanism extracts energy from the large-scale straining motion and passes it onto small-scale stretched vortices.

scholarly journals Entrainment and Mixing in Stratocumulus: Effects of a New Explicit Subgrid-Scale Scheme for Large-Eddy Simulations with Particle-Based Microphysics

2019 ◽  
Vol 76 (7) ◽  
pp. 1955-1973 ◽  
Author(s):  
Fabian Hoffmann ◽  
Graham Feingold
Keyword(s):  
Liquid Water ◽  
Large Scale ◽  
Large Eddy Simulations ◽  
Droplet Growth ◽  
Small Scale ◽  
Subgrid Scale ◽  
Mixing Processes ◽  
Large Eddy ◽  
Stratocumulus Clouds ◽  
Linear Eddy Model

Abstract The entrainment and mixing of free-tropospheric air is an essential component of the observed microphysical structure of stratocumulus clouds. Since the relevant scales involved in this process are usually smaller than the grid spacing of typical large-eddy simulations (LESs), their correct representation is difficult. To adequately accommodate these small-scale processes, we apply a recently developed approach that explicitly simulates LES subgrid-scale (SGS) turbulence fluctuation of supersaturation using the one-dimensional linear eddy model. As a result of reduced numerical diffusion and the ability to explicitly represent the SGS distribution of liquid water and supersaturation, entrainment rates tend to be lower in the new approach compared to simulations without it. Furthermore, cloud holes comprising free-tropospheric air with negligible liquid water are shown to persist longer in the stratocumulus deck. Their mixing with the cloud is shown to be more sensitive to the microphysical composition of the cloud as a result of the explicitly resolved inhomogeneous mixing, which is also confirmed analytically. Moreover, inhomogeneous mixing is shown to decrease the droplet concentration and to increase droplet growth significantly, in contrast to previous studies. All in all, the simulations presented can be seen as a first step to bridge the gap between ultra-high-resolution direct numerical simulation and LES, allowing an appropriate representation of small-scale mixing processes, together with the large-scale dynamics of a stratocumulus system.

Large Eddy Simulations of Jets in Crossflow: Large Scale Turbulence Effects

1999 ◽  
Author(s):  
Mayank Tyagi ◽  
Sumanta Acharya
Keyword(s):  
Large Scale ◽  
Large Eddy Simulations ◽  
Wave Number ◽  
Inertial Subrange ◽  
Small Scale ◽  
Freestream Turbulence ◽  
Large Eddy ◽  
Small Wave ◽  
Small Wave Number ◽  
Scale Turbulence

Abstract Large eddy simulations of jets in crossflow are performed to study the effect of energy containing scales present in the freestream on the penetration and spread of the coolant jet. Two specific freestream turbulence conditions are examined, one corresponding to 15% small scale Gaussian turbulence, and the other corresponding to a 15% freestream turbulence that satisfies the Von-Karman spectrum and has its peak energy specified in the small wave number range (large scales). The small-scale freestream turbulence can be viewed to be similar to grid generated turbulence. The large scale freestream turbulence spectrum has energy peak at a small wave number (corresponding to a specified length scale taken to be 4 hole diameters in this study) and has energy in the inertial subrange for large wave numbers. In the present study, the jets are issued through a row of square holes into the main crossflow. The jet to crossflow blowing ratio is 0.5 and the jet Reynolds number is approximately 4,700. Greater jet penetration and jet-mainstream mixing, in both the vertical and lateral directions, are observed for large-scale turbulence. The energy contained in large scales is mostly preserved although the energy carrying scales themselves undergo subsequent breakdown process due to the effect of the jet. In the nearfield of the jet, the large scales play a major role in enhancing the turbulent stresses, and the near wall transport. In the presence of the large scales, the horseshoe vortex is energized, and there is greater crossflow entrainment into the wake region. These large scale effects lead to significantly greater wall friction.

Investigation on hydrodynamic forces leading to coarse particle entrainment using Large-Eddy Simulations

2021 ◽  
Author(s):  
Gaston Latessa ◽  
Angela Busse ◽  
Manousos Valyrakis
Keyword(s):  
Experimental Data ◽  
Flow Field ◽  
Large Scale ◽  
Large Eddy Simulations ◽  
Flow Conditions ◽  
Mean Flow ◽  
Protection Measures ◽  
Practical Applications ◽  
Particle Entrainment ◽  
Large Eddy

<p>The prediction of particle motion in a fluid flow environment presents several challenges from the quantification of the forces exerted by the fluid onto the solids -normally with fluctuating behaviour due to turbulence- and the definition of the potential particle entrainment from these actions. An accurate description of these phenomena has many practical applications in local scour definition and to the design of protection measures.</p><p>In the present work, the actions of different flow conditions on sediment particles is investigated with the aim to translate these effects into particle entrainment identification through analytical solid dynamic equations.</p><p>Large Eddy Simulations (LES) are an increasingly practical tool that provide an accurate representation of both the mean flow field and the large-scale turbulent fluctuations. For the present case, the forces exerted by the flow are integrated over the surface of a stationary particle in the streamwise (drag) and vertical (lift) directions, together with the torques around the particle’s centre of mass. These forces are validated against experimental data under the same bed and flow conditions.</p><p>The forces are then compared against threshold values, obtained through theoretical equations of simple motions such as rolling without sliding. Thus, the frequency of entrainment is related to the different flow conditions in good agreement with results from experimental sediment entrainment research.</p><p>A thorough monitoring of the velocity flow field on several locations is carried out to determine the relationships between velocity time series at several locations around the particle and the forces acting on its surface. These results a relevant to determine ideal locations for flow investigation both in numerical and physical experiments.</p><p>Through numerical experiments, a large number of flow conditions were simulated obtaining a full set of actions over a fixed particle sitting on a smooth bed. These actions were translated into potential particle entrainment events and validated against experimental data. Future work will present the coupling of these LES models with Discrete Element Method (DEM) models to verify the entrainment phenomena entirely from a numerical perspective.</p>

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