Realizable versus non-realizable dynamic subgrid-scale stress models

The concept of dynamic large eddy simulation (LES) is highly attractive: such methods can dynamically adjust to changing flow conditions, which is known to be highly beneficial. For example, this avoids the use of empirical, case dependent approximations (like damping functions). Ideally, dynamic LES should be local in physical space (without involving artificial clipping parameters), and it should be stable for a wide range of simulation time steps, Reynolds numbers, and numerical schemes. These properties are not trivial, but dynamic LES suffers from such problems over decades. We address these questions by performing dynamic LES of periodic hill flow including separation at a high Reynolds number Re = 37 000. For the case considered, the main result of our studies is that it is possible to design LES that has the desired properties. It requires physical consistency: a PDF-realizable and stress-realizable LES model, which requires the inclusion of the turbulent kinetic energy in the LES calculation. LES models that do not honor such physical consistency can become unstable. We do not find support for the previous assumption that long-term correlations of negative dynamic model parameters are responsible for instability. Instead, we concluded that instability is caused by the stable spatial organization of significant unphysical states, which are represented by wall-type gradient streaks of the standard deviation of the dynamic model parameter. The applicability of our realizability stabilization to other dynamic models (including the dynamic Smagorinsky model) is discussed.

In this article, we describe the use of a new dynamic cubic nonlinear model, a new nonlinear subgrid-scale model, for simulating the cavitating flow around an NACA66 series hydrofoil. For comparison, the dynamic Smagorinsky model is also used. It is found that the dynamic cubic nonlinear model can capture the turbulence spectrum, while the dynamic Smagorinsky model fails. Both models reproduce the cavity growth/destabilization cycle, but the results of the dynamic cubic nonlinear model are much smoother. The re-entrant jet is clearly captured by the models, and it is shown that the re-entrant jet cuts the cavity into two parts. In general, the dynamic cubic nonlinear model provides improvement over the dynamic Smagorinsky model for the calculation of cavitating flow.

Nonlinear dynamic modeling and model reduction strategy are studied in this paper for a rotating thin cylindrical shell. The nonlinear dynamic model is first established in terms of ordinary differential equations, in which the effects of Coriolis and centrifugal forces are considered, as well as the initial hoop tension due to rotation. This model describes both the in‐plane vibrations and the flexural vibration and reflects the coupling effects of those deformations. Based on this original model, a novel model reduction strategy is proposed to reduce the degrees of freedom by neglecting vibration modes predominated by in‐plane vibrations. Meanwhile, for the reduced‐order model, the in‐plane vibrations’ contributions to the rotating shell’s response are still preserved. To validate the dynamic model and the model reduction strategy, comparisons and simulations are carried out. Subsequently, nonlinear dynamic behaviors are investigated preliminarily by analyzing the rotating cylindrical shell’s amplitude‐frequency responses under different excitation levels.

A Nonlinear Sub-grid Scale Model for Compressible Turbulent Flow

The present study investigates the effect of the subgrid-scale (SGS) stress model on large eddy simulation of flow over a finite-length square cylinder mounted vertically on a ground plane. The study specifically considers a cylinder of aspect ratio AR = 5 immersed in a thin boundary layer for a Reynolds number of Re = 500 (based on the free stream velocity and cylinder width.) Three different subgrid-scale models are considered: a Smagorinsky model, a dynamic Smagorinsky model and a dynamic nonlinear model. The mean flow structure is similar for all SGS models on both grids, although noticeable differences are observed in the size of the recirculation zone, level of resolved-scale turbulence and vortical structures for different SGS models. The dynamic Smagorinsky model appears to be especially sensitive to the grid resolution.

In this paper, the effect of subgrid-scale (SGS) modelling, grid resolution and anisotropy of the subgrid-scales on large eddy simulation (LES) is investigated. LES of turbulent channel flow is performed at Re τ=934, based on friction velocity and channel half width, for a wide range of resolutions. The dynamic Smagorinsky model (DS), the high-pass filtered dynamic Smagorinsky model (HPF) based on the variational multiscale method and the recent explicit algebraic model (EA), which accounts for the anisotropy of the SGS stresses are considered. The first part of the paper is focused on the resolution effects on LES, where the performances of the three SGS models at different resolutions are compared to direct numerical simulation (DNS) results. The results show that LES using eddy viscosity SGS models is very sensitive to resolution. At coarse resolutions, LES with the DS and the HPF models deviate considerably from DNS, whereas LES with the EA model still gives reasonable results. Further analysis shows that the two former models do not accurately predict the SGS dissipation near the wall, while the latter does, even at coarse resolutions. In the second part, the effect of SGS modelling on LES predictions of near-wall and outer-layer turbulent structures is discussed. It is found that different models predict near-wall turbulent structures of different sizes. Analysis of the spectra shows that although near-wall scales are not resolved at coarse resolutions, large-scale motions can be reasonably captured in LES using all the tested models.

In order to validate the application of Large eddy simulation in the field of incompressible flow, the author used Large eddy simulation to compute 2-dimensional incompressible flows around an airfoil by software Fluent. In the beginning of computation, the paper prepared five types of mesh in order to check up the LES requiring for mesh. Then the author monitored lift coefficient and drag coefficient on the airfoil. The present results are compared with the experimental results. When the section angle of attack is low, the results of numerical simulation are very close to the experimental. However, at the attack angle of near stall condition there is some error. Large eddy simulation can simulate the origination and breaking off of the eddy very well. Strouhal number obtained is close to the theory. Validate the method of large eddy simulation by numerical simulation.

Large eddy simulation of round impinging jet with one-equation subgrid scale model

Large eddy simulation of a high-pressure homogenizer valve

In this paper, we investigate the transport of resolved turbulent stresses in a plane channel flow subjected to spanwise rotation using the method of large-eddy simulation (LES). We present both the general and simplified transport equations for the resolved turbulent stresses, which are essential for understanding the unique pattern of turbulent kinetic energy production in a rotating system. Numerical simulations are performed using a dynamic nonlinear model (DNM) for closure of the filtered momentum equation. The turbulent flow field studied in this research is characterized by a Reynolds number Reτ = 150 and various rotation numbers Roτ ranging from 0 to 7.5. In order to validate the LES approach, turbulent statistics obtained from the simulations are thoroughly compared with the available experimental and direct numerical simulation results.

The Discontinuous Galerkin (DG) method provides numerical solutions of the NavierStokes equations with high order of accuracy in complex geometries and allows for highly efficient parallelization algorithms. These attributes make the DG method highly attractive for large eddy simulation (LES). The main goal of this work is to investigate the feasibility of adopting an explicit filter to the numerical solution of the Navier-Stokes equations to increase the numerical stability of underresolved simulations such as LES and to use the explicit filter in dynamic subgrid scale (SGS) models for LES. The explicit filter takes advantage of DG’s framework where the solution is approximated using a polynomial basis. The higher modes of the solution correspond to a higher order polynomial basis, therefore by removing high order modes the filtered solution contains only lower frequency content. The explicit filter is successfully used here to remove the effects of aliasing in underresolved simulations of the Taylor-Green vortex case at a Reynolds number Re = 1600. The de-aliasing is achieved by evaluating a solution at a higher order polynomial (effectively increasing the number of quadrature points used for integration) and then projecting the solution down to a lower order polynomial. The SGS models investigated include the constant coefficient Smagorinsky Model (CCSM), Dynamic Smagorinsky Model (DSM), and Dynamic Heinz Model (DHM). The Taylor-Green Vortex case exhibits a laminar-turbulent transition and it is shown that the dynamic SGS models capture this transition more accurately than the CCSM when a sufficiently high polynomial order is used. The explicit test-filter operation for the dynamic models introduces a commutation error. A brief comparison of the effects of the commutative error that exists with this filter implementation is shown although further investigation is needed to determine the more appropriate order of operations.

Application of steady-state and dynamic modeling for the prediction of the BOD of an aerated lagoon at a pulp and paper millPart I. Linear approaches

Large eddy simulation (LES) was performed for a spatially developing round jet and its scalar transport at four steps of Reynolds number set between 1200 and 1,000,000. A simulated domain, which extends 30 times the nozzle diameter, includes initial, transitional, and established stage of jet. A modified version of convection outflow condition was proposed in order to diminish the effect of a downstream boundary. Tested were two kinds of subgrid scale (SOS) models: a Smagorinsky model (SM) and a dynamic Smagorinsky model (DSM). In the former model, parameters are kept at empirically deduced constants, while in the latter, they are calculated using different levels of space filtering. Data analysis based on the decay law of jet clearly presented the performance of SGS models. Simulated results by SM and DSM compared favorably with existing measurements of jet and its scalar transport. However, the quantitative accuracy of DSM was better than that of SM at a transitional stage of flow field. Computed parameters by DSM, coefficient for SGS stresses, CR and SGS eddy diffusivity ratio, ΓSGS, were not far from empirical constants of SM. Optimization of the model coefficient was suggested in DSM so that coefficient CR was nearly equal in the established stage of jet but it was reduced in low turbulence close to the jet nozzle. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(3): 175–188, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20001

A state-of-the-art spray modeling methodology, recently presented by Senecal et al. [1, 2], is applied to Large Eddy Simulations (LES) of vaporizing sprays. Simulations of non-combusting Spray A (n-dodecane fuel) from the Engine Combustion Network are performed. An Adaptive Mesh Refinement (AMR) cell size of 0.0625 mm is utilized based on the accuracy/runtime tradeoff demonstrated by Senecal et al. [2]. In that work it was shown that grid convergence of key parameters for non-evaporating and evaporating sprays was achieved for cell sizes between 0.0625 and 0.125 mm using the Dynamic Structure LES model. The current work presents an extended and more thorough investigation of Spray A using multi-dimensional spray modeling and the Dynamic Structure LES model. Twenty different realizations are simulated by changing the random number seed used in the spray sub-models. Multi-realization (ensemble) averaging is shown to be necessary when comparing to local spray measurements of quantities such as mixture fraction and gas-phase velocity. Through a detailed analysis, recommendations are made regarding the minimum number of LES realizations required for accurate prediction of Diesel sprays. Finally, the effect of a spray primary breakup model constant on the results is assessed.

Convergence of cognitive and adaptive decline in chronic schizophrenia