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In openFOAM, there are multiple options for choose in LES, such as WALE model, Smagorinsky model and so on. Which one is suitable for dynamic mesh and why ?
I am a bigginner of LES simulation, as far as I know , the only difference between them is WALE model includes the rotation rate in the calculation of νsgs, and Smagorinsky not. What else should I take into account in simulation?
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Different simulation tasks may require different turbulence models. I suggest, you look for literature on your application (or similar applications) as a reference case. And either rely on previous work or do test runs with different models to see which one gets you closest to the reference case.
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  • I am doing LES
  • Subcritical and supercritical flow
  • Either circle cylinder or square cylinder would work
  • Preferably an experimental case for validation
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You need not only to compare your result with the experiment, but also to have a comparison with DNS. If you are applying a wall modeled LES, I would strongly suggest to also try or compare the results with a wall-resolved LES.
Here are ERCOFTAC classic database which might be suitable for your validation:
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Having the required time-series data and in order to analyze food demand system in a country (the aggregate level), How to implement regression procedures in one of following models: LES, DDS, AIDS or LA/AIDS?
I am open for collaboration
Thanks
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Thank you very much Mr. Boubaker Dhehibi
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Hello, I have a project coming up with Delayed detached eddy simulations.
I am trying to understand how LES works right now. can someone suggest a lecture series or video series on how to understand Delayed detached eddy simulation? How does LES evolve into Delayed detached eddy simulation?
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First of all, what about the textbooks you are already considering to understand LES?
In general you have to consider first the idea of the evolution of LES into DES and then into DDES:
P.R. Spalart, S. Deck, M.L. Shur, K.D. Squires, M.K. Strelets, and A. Travin. A New Version of Detached-eddy Simulation, Resistant to Ambiguous Grid Densities. Theoretical and Computational Fluid Dynamics, 20(3):181–195, 2006.
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Hello,
I am doing a parameter study for different grids doing wall modeled large eddy simulation for a channel flow. Could someone recommend me some papers which discuss the recommendations for such grids? I read papers which give clear recommendations but I dont find a lot why specific grids perform well. Its clear that at some point the grid is just too coarse, so I relate to fine enough grids with different aspect ration for example. I thought maybe it has to do with typical sizes of eddies in the turbulent channel flow but could not find many information about this. I know that it has much to do with the numerics used but are there also some physical reasons eg. elongated eddies in streamwise direction so that x+/y+ > 1 is reasonable? . Also I wonder about some results, eg. a 160*160*160 (x+*y+*z+) grid performing better than a way finer 40*40*40 grid in means of the mean velocity results. The simulations are carried out at Re_tau = 2000 on a finite volume code using WALE and Werner Wengle wall model.
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As stated by Filippo, in my experience, comparing the velocity profile is never enough in LES, especially for implicitly filtered LES in the channel flow. For example, with certain codes and discretizations, a mass error due to the pressure discretization can directly affect the mean velocity profile and make it look good while all the other quantities would signal a completely wrong solution.
For what concerns the wall and SGS model, in my opinion, a single combination doesn't allow to really take effects apart. Also, there isn't much around on the Werner & Wengle model, which makes it more difficult to understand.
Are you using a specific code or is this an in-house solver?
More generally, you want to build confidence on your tool for this case, but you are exploring a very little portion of the parameter space that is known to strongly affect results, especially at high Re.
The main rule of thumb that I can suggest is that, because the main dynamics at the wall is due to the streamwise streaks, you don't want to represent it uncorrectly. Now, dx+ = 40 is kind of good also for a wall resolved LES so, one effect you might be observing is that your choice of completely equal grid sizes in the 3 coordinate directions is kind of altering (trough the numerics etc.) a local dynamics that is obviously anisotropic and demands a different aspect ratio for the cells. But, again, this is just a guess. There are no two LES codes that perform the same given identical conditions.
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Hi everyone. I am using DES model to simulation turbulent flow with RE 6500. I am only getting one vortex and not getting any vortex shedding. I tried to use URans model, but it shows no vortices and only wake region behind cylinder. Please inform me whats effect of model on vortex shedding.? Will LES provide vortex shedding or there is some other reason of not getting vortex shedding. Thanks
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I do suppose that your DES model is looking for unsteady (time dependent) solutions?
regards
mico
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There are different versions of OpenFOAM. I want to know which version is more suitable for LES.
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@Kazi Md. Jahid Hasan, I agree that we can do transient vortex-resolving simulation of flows with OpenFoam. I have used it for such simulations several times. But since it is 2nd order, truncation error can be larger or comparable to modelled properties. See for example Smagorinsky model, where nut is proportional to dx^2. Therefore, I don't think OpenFoam implements LES technology strictly, but most likely it works as something similar.
As i know, there is one third order implementation for OpenFoam by Wildkatze and Arjun Yadav, but I'm not sure whether it is open to community.
The second question is diffusion introduced in implementation of Rhie-Chow interpolation. This artificial diffusion term presents in all transient OpenFoam viscous solvers, regardless of version. Value of this term was unsatisfactory large for some of my problems, that's why I've changed original algorithm. If you want to do LES-like simulations with OpenFoam, then probably you should tune original solver.
Finally, you can take any latest version from openfoam.org (v.9) or openfoam.com (v. 2106) and it should give you similar results if you use incompressible viscous flow model.
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hi.
I have a problem in my simulation work. I use flow-3d for simulating a surface vortex with an air core in a pipe-tank system. I have been successful simulating the vortex with 1cm mesh size, but I haven't been able to see the air core. Due to this problem I had to had to use finer mesh with size of 0.6cm.
When using finer mesh ,my streamlines of the near surface particles change dramatically in comparison with the streamlines I had when using coarser mesh with 1cm mesh cell size.
In coarser mesh my streamlines are strongly sucked and driven into my pipe but when i reduce my mesh cell to 0.6cm, with exactly the same setup my streamline suddenly get shorter and incomplete. It seems that after changing the mesh size the flow of water in to the pipe is not enough strong to create the enough suction for near surface particles of water and due to this there i see no air entrainment afterward.
I will upload the photos of my streamlines in both conditions, I'll be so delightful if anyone can tell what is the cause of this problem I have and how I can solve it?
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Giuseppe Altieri thanks very much for your answer
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Dear all,
I am trying to run a Stress-Blended Eddy Simulation with a modified turbulent viscosity for the RANS part, only.
However, when I use DEFINE_TURBULENT_VISCOSITY to modify the RANS turbulent viscosity, it also modifies the LES subgrid model (what I want to avoid)...
I tried a simple if/else statement based on the value of the SBES blending function:
DEFINE_TURBULENT_VISCOSITY(user_mu_t,c,t)
{
if(SBES_BlendingFunction == 1)
{
mu_t = compute_my_RANS_mu_t(...);
}
else
{
mu_t = C_MU_T(c,t);
}
return mu_t;
}
But It does not give the expected results... I also tried to loop over cells and compute it locally but had issues with the implementation...
Any help or advices would be highly appreciated !
Thanks in advance!
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Leo Cotteleer use DEFINE_TURBULENT_VISCOSITY macro and after writing the UDF, you may hook it in define/model/viscous panel.To be honest, DEFINE_TURBULENT_VISCOSITY macro replaces the new definition of viscosity in turbulent model. If you would like use the default one and add some extra expression, you can do it by this macro, too. You can combine the default turbulent viscosity and others. Please find the attached manual herewith.
best regards
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I want to use LES to simulate the flow past a sphere at high Re by non-iterative PISO method, which is much faster than iterative method. However, the result of non-iterative PISO is far from the result in literatures, especially in Strouhal number. Can this method stable and accurate enough to apply to LES in turbulent flow? How can I improve the accuracy?
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Try the NITA based on fractional time step and compare to see if you have better solutions.
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1.I'm using Ansys 2021 R2 fluent and the base of the meshed cuboid becomes finer when multizone method is used, to have a structured mesh. How do I overcome this issue?
2. How can I mesh a structured hexahedral mesh with the sizing I want ?
Note : The mesh is used to analyze the wind loads on high rise buildings using LES.
I have used body of influence to create multiple scales in the fluid domain.
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Cells clustering is entirely decided by the mesh generator based on the definitions you made before. If the high building you defined through the boundary condition is placed where the mesh is finer, you have the answer.
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I used to use RANS and I am now reading in LES. I just need a book about how LES is "applied" in equations. Most books, Pope's, Sagaut, ..., approach the problem mathematically. I just need, maybe, a flow chart or something.
I used, as an example, Finite-Volume in solving N-S equations and when it came to Turbulence I was using RANS. Here, I believe it is the same but to the limit of Sub-grid scales and then I need to model those.
I appreciate any help other than Pope, Sagaut ..
Thank you.
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Difficult to address any textbook that explain the details about how LES is practically performed. You have to think that there are different codes based on different formulations and different SGS models. You can have big differences from a FD or FV or FEM or SM based LES codes. Often, a LES code is just a derivation of a NS code where an SGS model is added.
If you want to focus mainly on FV-based LES, you can read the historical paper of Schumann. The CTR has a lot of material but you should read also the fundamenta papers published on journals. I suggest also to see the general forum of cfd-online where many details are discussed.
You can find also some files in my RG home page with some lectures about LES
But, I strongly suggest to ask for specific issues about LES to get useful answers.
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Which model is best to capture separation when simulating either a 2D or 3D of an oscillating airfoil?
For 3D simulation which model of hybrid LES/RANS is most suitable?
How do I improve on the model length scale?
Thanks
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If you have the resources (time and equipment ) go for LES.
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Both PANS (Girimaji, 2006) and TFLES (Pruett et al, 2003) provide a self-consistent formulation that allows one to recover RANS versus DNS at opposite limits. Are the two approaches fundamentally different (e.g., in the sense that RANS vs LES employ different ansatz and thus yield different types of closures to be modeled: the Reynolds stress vs the subfilter stress)? Or are they conceptually the same approach, independently developed and thus mainly differing in preferred modeling choices/perspectives?
(note: I am unfamiliar with PANS and only marginally familiar with TFLES; apologies for any misinterpretations)
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in RANS-like formulation the numerical discretization has much less implication as the turbulence model acts on all the scales.
I don’t know the specific issues in PANS but only for the case of a scale separation the numerical discretization assumes relevance
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For the RANS/LES hybrid turbulence model, is it possible to theoretically determine the distribution of grid nodes, including the RANS part near the wall and the respective grids of the LES part in the turbulent core area?
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Documents that analyze LES grids like this: "Grid-point requirements for large eddy simulation: Chapman’s estimates revisited" or "Grid Construction Strategies for Wall-Resolving Large Eddy Simulation and Estimates of the Resulting Number of Grid Points".
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I am trying to have a c-grid with airfoil unit cell extension through Z for LES, when I assemble special domain along with airfoil upper lower and c curve the cells are not orthogonal near the boundary.
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I think Muhammad provide you a good references
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UPD (29/08/2018): Is the RANS/LES way of turbulence modeling a converging process, i.e. will it hypothetically converge to a hypothetical big but single PDE system? Or will it inevitably disassemble into several non-overlapping PDE systems/approaches each of which is valid only for a certain turbulent regime? 
After more than 5 decades of active search, it seems that the hierarchy of moment equations for the Reynolds stress and higher moments derived from the Navier-Stokes equations does not have a universal closure that could be applied to more or less wide range of flows. Numerous closures have been proposed since then. All of them contain heuristic arguments which are usually changed from case to case. So, does it mean that our understanding of turbulence will never be complete? Or do people still believe that a “magic closure” exists which can explain everything?
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Completely agree with you in the computational context, not for the real-world scenarios. What I mean is somehow deal with the butterfly effect which implies even the farthest smallest decimals may have a role to play in the holistic simulation of turbulence like in nature.
Regards,
Hamed
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I am currently working on a project involving shock / boundary layer interactions using experiments and CFD. I am looking for some books / papers where I can get the theoretical understanding of such interactions.
I plan to model the flow using RANS if possible, but I'm not counting LES or DNS out just yet.
Any additional info on this topic will also be highly appreciated.
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Several researchers in this field: Zheltovodov, Bushnell, Borovoy.
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What the differences between the SST k-omega and LES turbulence models for Pumps ?
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The real difference is that LES is NOT a turbulence model but a different formulation of the governing equations based on local filtering of the variable.
That strongly differs from the formulation based in the statistical averaging (RANS/URANS).
The variables that are computed are totally different.
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My classmate is trying to run a CFD simulation over a farward facing step with conjugate heat transfer over the step and the working fluid is air and at Re= 5000, 7000,1000. But the size of the physical domain is about 1m length with 0.01 m width and a height of about 0.250m . If the meshing is done with a count of 1 milloin cells the simulation is taking months of time to run with a huge divegrence in the timestep. Is it possible to perform a LES simulation over such a big domain. For performing LES what would be the size of the physical domain and the respective mesh size.
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As I mentioned above, in principle such a LES simulation with CHT is possible, but only on the expense of massive Linux Cluster computation time. If I say "massive ressources" then I mean something like several hundreds of CPU cores.
What are the reasons:
1) LES by itself is already a rather expensive simulation method. Usually a good quality LES takes a couple of days on - let's say - 100-200 CPU cores. Mesh resolution depends on the simulated Re number and resolved flow/geometry features, but it is absolutely clear, that you cannot do a good LES on 1 million mesh elements. LES is in any case full 3d and even for a backward facing step you need a certain minimum resolution in the 3rd coordinate direction. And it is forbidden (physically invalid !!) to apply symmetry boundary conditions in a scale-resolving simulation like LES. Even periodicity is a wrong assumption, sine real turbulence is chaotic but not periodic or symmetric.
2) Second, combining LES with conjugate heat transfer is extremely expensive. The reason is, that the time-scales of the LES and the thermal conduction processes in solids are orders of magnitude different in terms of applicable and required time-scales. In the LES you need very small time-scales in order to resolve the motion of large and medium size turbulent eddies and their turbulent fluctuations for pressure, velocity, temperature. In the solid you need to run from your thermal initialization (which is usually not a thermal equilibrium state) to the fully developed temperature distribution in both the solid and fluid domain. Since it is a transient simulation, you cannot apply different timescales in the different domains, like this is usually done in steady-state simulations in order to speed-up this process of finding the new thermal equilibrium, after temperature distributions have fully developed. So you need to run the LES for a substantially linger time for combined LES/CHT in comparison with doing "only" a full LES simulation without CHT. This usually makes LES/CHT so expensive, that with "normal" computational resources it is not possible to do it. But at the peak of science with supercomputers it can be done.
3) In order to quicker arrive at the seeked thermal equilibrium state in fully developed LES conditions one can apply a "trick". In order to let the temperature distribution develop itself more quickly - in particular in the solid domain - one can artificially increase the thermal conductivity value of the solid material by 1-2 orders of magnitude over a certain simulation time period. By that the solid material can quicker heat up to the desired thermal state from the applied boundary conditions and one can save simulation time. But afterwards, if the thermal conductivity value has been set back to normal, one need to run still a larger number of LES through-flow times for the geometry in order to get rid of the influence of this artificially increased physical parameter in the obtained solution.
Finally - I have tried with colleages in an industrial project with VOITH Hydro a similar CFD analyses. And we finished this project with a bit mixed success. The computational resources (~350 CPU cores at that time several years ago in 2014) were not yet really sufficient to obtai a 100% good result of SAS_SST/CHT simulation. But maybe you like to study our publication:
Best regards,
Dr. Th. Frank.
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Dear All,
Which type of simulations is good to investigate the rotating stall in axial compressor? RANS, URANS, LES, DES, and DNS. Secondly, a single passage or multi passage is recommended for the investigation of rotating stall.
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Filippo Maria Denaro Thank you for your suggestion. I have enough computational power and i have checked papers too. some people used RANS, some LES and DES. I was thinking that which technique will give me the good results
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Anyone know what should be the first layer thickness for the large eddy simulation. Also the quality of the mesh should be how much for LES.
Regards,
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Agree with all above. 10 - 20 local dn+ ( local wall unit). 50 if you have a good wall modeling. Use 1-2 dn+ if you want DNS-like solution, say 5 -10% error relative to DNS.
The practical question is how to find dn+.
In practice, there is no way to know that for arbitrary shapes body. Just do a simulation on reasonable grid. Compute overall total shear stress. This value should be in the same order of magnitude of the actual value. Use that to determine your next grid.
You can use some smart means/analytical cases to guess your first grid.
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In turbulent boundary layer, the transition and turbulent regions could be clearly known by seen the Cf plot and averaged velocity profiles via transformation. However, how can I know where the flow enters streamwise fully-turbulent region in supersonic mixing layer by numerical simulation ? As I learn from open literature , combined with my own work, I get three ideas
1. Some empirical equation to ensure a sufficient length of the computational domain. For example, the criterion for xeff/dm1 > 500 proposed by Papamoschou and Roshko (1988).
2. Velocity power spectrum. Calculate the spectrum of a point and get the -5/3 and -7 slope.
3. The Reynolds stress profiles, which should be self-similar. However, the Urms profiles are hard to overlap, so what is the criterion to define 'self-similar'?
Actually, which one or more are rational? Do you have more answers?
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Well, you could perform several analyses. For example you can compute the TKE over several planes normal to the streamwise direction and associate the evaluation of the energy spectra.
A qualitative approach could be the identification of the vortical structures.
I don't think it exists a unique approach.
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I am trying to use WRF_LES to simulate a dust deposition case. My horizontal spatial resolution is 10m, and vertical spatial resolution is stretched from 0.96 m to 24 meter. As the vertical eddy viscosity is important for me and it has a big difference between using 'isotropic (lx=lz=(dx*dy*dz)^1/3)' and 'anisotropic (lx=(dx*dy)^1/2, lz=(dz))'.
So that would be great if somebody can give me some information about when isotropic/anisotropic is used and why?
Any feedback is welcomed, Thanks!
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The concept of LES, while an attractive alternative to the prohibitively expensive direct numerical simulation, poses some significant problems on its own. The main difficulty is that there is no common agreement on the form of a subgrid scale model. In the best known case of the Smagorinsky-Lilly formula, we have a simple algebraic relationship involving the magnitude of a strain rate tensor and the length scale determined by the grid size.
In the more general case, we can propose various expressions involving the characteristic velocity and length scales. Given the different physical mechanisms and geometric constraints affecting vertical and horizontal turbulence, it is advisable to have different values for mixing lengths mentioned in the question.
We should remember, however, that evaluation of diffusivities based on the concept of a mixing length is only an approximation. The precise formula for the mixing coefficients in the LES model can be obtained from the realistic sub-grid-scale model.
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For performing LES initial soundings have been taken from ERA data. But large scale forcings and surface fluxes also needs to be computed, to provide these as input. Kindly suggest some suitable methods .
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how to use steady state solution as initial solution for les ?
the problem is that Rans model produce average velocity field while les need instentenous velocity field,so it there a way to solve that problem?
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as you see from the literature, there are many proposals for generating inflow conditions for LES (and DNS). I can not say that one specific formulation is the best.
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I want to calculate the skin friction for channel flow.I use openfoam with shearStress utility for calculating the shear stress on the wall (vector form).Is it possible from the shear stress vector to calculate the friction factor using paraview?
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It is known that Cf=wall shear stress (tau wall)/0.5*rho*v^2.....from openFoam you can use the postProcess command to calculate wallShearStress.
So, it is either you can calculate Cf from wallShearStress and then divide it manually by 0.5*rho*v^2 (using an excel sheet for example) .......or you can create your own post processing function in openFoam by modifying the wallShearStress function.
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When the buoyancy effect is significant in supercritical fluids passing in the tube, some eddies can form in any location of tube, especially at the vicinity of supercritical temperature. So, what is the numerical method to solve like the compressible problem? As my experience and literature, there is no way to calculate this type of problem by using RANS models, so I have just started using LES models. Although it was used a very low time step size (about E-5) and tried all of the subgrid models, I couldn't solve this problem. I would like to be grateful if you could share your recommendations. Thanks
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Are you looking at density variation (compressible) and phase change at super critical temperature ?
Gravitational effects in horizontal ducts are observed at very low velocities such as free-forced convection situations .
i would advise dense phase model in cfd to assess such situations.
please elaborate the problem so that we can discuss in detail.
i would suggest to look into physics so that we can choose appropriate model for analysis
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I am trying to make sure I understand the limitations on the SST Transition model I'm using in FLUENT.
It is my understanding that RANS simulations have difficulty modeling large eddies (such as those which would form in the wheels of a race car) accurately, and it is my present understanding that the SST transition model would not completely capture that behavior, so even if one would expect such an eddy to form it would not be modeled accurately.
So if I wanted to model it with perfect accuracy, that would require an LES model.
Am I understanding that mostly right? I'm trying to make sure I have a thorough understanding because I might be questioned on it.
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If you wanted to model it with perfect accuracy, that would require a DNS model with a high order of accuracy in terms of your numerical grid resolution, a lot of computational power and time at your disposal.
To get an some understanding at least (not perfect accuracy) of the eddies and drag associated with something like a car wheel LES would describe the effect of the larger eddies, as the name suggests, whereas RANS is averaged, again as the name suggests, I recommend reading this thread on stack exchange for some more understanding of the differences "What are the differences between an LES-SGS model and a RANS based turbulence model?"
good luck
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I am interested in the numerical characterisation (3D-CFD) of aerodynamics and flow-induced noise (near-field) generated during the operation of a standalone horizontal axis wind turbine. Since I am attempting this activity for the first time, I was wondering if there are any publicly available CAD and performance data of any test wind turbine to validate the numerical methodology.
On a side note, if anyone could recommend me some pivotal pertinent literature, it'll be a great help.
Also, as my knowledge on the state-of-the-art is limited, I was wondering about the practices for modelling impeller rotation. I understand that specifying an angular velocity to the rotating domain should be relatively straightforward but are there any literature/methodologies available to model the motion of the blades based on air-velocity boundary conditions.
Thank you for reading and contributing.
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Olexiy Shynkarenko Thank you for the information Olexiy. Would it be possible to share some weblink or author information as i tried to google but couldn't get any relevant hit.
Thank you,
Sid
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Hello All
Would like to know whether any one has Best practices/guidelines for setting up Embedded LES in Ansys Fluent.
I am confused how to define RANS in the global region and define LES in specific regions in the mesh. ie, two schemes in a single mesh.
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Hi Joemon
As initial preperation step for Ansys meshing, I use spaceclaim to prepare the model. In spaceclaim, even if you have two volumes (LES inside RANS volume) you can define the topology option as 'merge'; then conformal mesh will be created later in Ansys meshing. However, ELES allows non-conformal mesh so it is not a big problem if the two volumes are not conformal, as long as the grid size difference is not beyond the allowing threshold. For the overlapping surfaces of the two volumes, you need to define them later as 'interior' in Fluent. Once they are defined as 'interior' you can define them as 'Les rans interface'. For non-conformal mesh, you need to use 'create/edit mesh interface dialogue box' to define the overlapping surfaces of LES and RANS volumes as interfaces.
How does the above sound to you?
Daniel
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I want to use UDF to modify the definition of subgrid length scale in LES model, which involves (1) judging the direction of local vortex axis (2) calculating the vector between grid vertex and grid center. I would like to ask you which macro command in UDF is needed for these two operations, or how to program them.
Thank you very much.
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It is difficult to use UDF to define a modified SGS model in Fluent. So we choose OpenFoam to do the job.
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I have done LES and I want to extract information from that data, so I can understand physical aspects of the problem better regarding turbulence structures and how they affect the shock structure.
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You can also try using modal techniques such as spectral POD, or Empirical Modal decomposition to extract coherent features from the flow.
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In LES non-premixed combustion of propane-air in Ansys fluent, there is no signs of combustion. The temp. is 300K everywhere. How to overcome it?
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Why shouldn't any species transport model (like Eddy Dissipation) not require temperature patching? Table-based models like equilibrium or flamelets do not, but if you use some form of species transport, you will have to provide an ignition source at some point to initiate combustion.... otherwise, you'd only simulate mixing of fuel and oxidizer. What can happen is that after temperature patching, the flame will not stabilize and the hot exhaust products are transported out of the computational domain.This would indicate that your flow field is not as stable as you'd want. Sometimes it also helps to not only patch temperature, but also some intermediate species like CO or H2 (I assume you talk about the combustion of natural gas).
Be careful with nomenclature here, by the way: in FLUENT, non-premixed combustion models are mixture-fraction-based models. This is not to be confused with non-premixed combustion in general which just means that fuel and oxidizer are separately injected into the combustion chamber. Thus, you can simulate non-premixed combustion with some kind of species transport model, which is in fact done quite often.
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Hello All!
I am doing LES for flow around the 2d circular cylinder in fluent. I want to verify the Navier-stokes from my CFD results. Can somebody guide me on how I can do that? Thanks!
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The filter width depends on the formulation: implicit or explicit filtering. In a software like Fluent the approach to the filtering is implicit and the filter width tends to zero according to the grid size tending to zero. This way the LES solution tends to be a solution of the unfiltered NS equations.
The only congruent way to work is performing the 3D LES and then averaging in time the results to get a statistically steady solution according to the 2D RANS.
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Hi
I am working on thermal stratification of dam reservoir. Could you please help me about choosing the right turbulence model?
K-epsilon
k-omega
LES
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K-epsilon model
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I need a periodic BC where mass flow rate is specified to model fully developed flow, and another periodic BC with no pressure drop. However, once I define the mass flow rate for one periodic BC, another periodic BC is defined with the same value. Can anyone help to solve this issue?
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That's probably the case, but you can't compute the mass flow rate in that way for a turbulent flow. That formula is valid for laminar cases.
Try using a pressure gradient in x direction only
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We are using a code where the divergence of velocity is neglected in the stress tensor for a low Mach approximation. We inject a pure helium plume in an air cavity. In PIV experiments the flow is more turbulent than in CFD corse DNS calculations. Kolmogorov scale is evaluated to be around 0.1mm. Mesh cell is around 0.3mm. We performed Smagorinsky LES and coarse DNS calculations. L2 norm convergence is estimated better than 7% of error. We wonder whether the Approx 20% of differences between experimental PIV measurements could be attributed to that neglected Div(u) term in the stress tensor, or not sufficiently converged DNS (or exp. problem). Has someone examples of the importance of this div U term in turbulence development ? Best regards?
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The stress tensor in the Newtonian model writes as
T = [-p+(lambda+2/3mu)div v]I+2mu D0
therefore if lambda+2/3mu=0 is assumed, also if the flow is not divergence-free this term has no relevance.
Taylor microscale can be estimated by means of the autocorrelation.
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After reading the proceeding: "Best Practices in Running Collaborative GPU Hackathons: Advancing Scientific Applications with a Sustained Impact" I came across the paper: "Porting the MPI Parallelized LES Model PALM to Multi-GPU Systems – An Experience Report". It is evident that GPU's are Single Instruction, multiple threads and therefore it is not a cache-optimized computer architecture. My experience is that unless you changed the whole kernel the Hybrid approach MPI+GPU affects the performance a lot. However, there is a boom in HPC with GPU's. In some instances, MPI alone performs better than the hybrid since we do not need to move information back and forth to the GPU and CPU, respectively. We have different good practices but there is no a standard or a reference that we can always take to the bank. The picture becomes even worse when high order schemes are used.
1) Which is your common approach for this issues in CFD?
2) Should we keep the structure of the source code as functional programming, imperative programming with a bunch of do .. end do in each subroutine that affects the performance a lot?
3) Should we use data region (where the data remains local in the GPU) where we packed all the computations even though we are hurting the readability of the source code?
4) Should we update the ghost cells in every time step?
Again the focus is only CFD
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While there is no universal answer to your question about hybrid computation, more and more people consider that the directive approach (OpenACC / OpenMP) to be the nominal way to go for porting large legacy codes, mostly because OpenAcc / OpenMP directives can be added incrementally in the code.
An interesting emerging alternative, which is probably more adapted to new codes aiming both at performance and portability across architectures (x86_64 multicore, nvidia GPU, AMD GPU, ARM, ...) is to use a "smart" library like Kokkos (developped at Sandia NL) or RAJA (devel at Livermore NL) or others to implement shared memory parallelism. These libraries provides "generic" concepts to express parallel loops (for, reduce, and so on). These libraries focus on node-level parallelism, and need to be used in combination with MPI for large scale distributed computation.
Another major advantage of these libraries, when compared to the directive approach, is that, and this is especially true for Kokkos, is that they also provide "architecture-aware" data containers. E.g. Kokkos provides arrays (called Kokkos::View) for CPU/GPU/... which allows you to access data like this data(i,j,k), very intuitively, and simplify the memory management when dealing with GPUs. Note that Kokkos library is a core building piece of the large parallel linear algebra package Trilinos.
I think that portability and more precisely "performance portability" is a keep concept today, that is being able to develop codes that can run efficiently on multiple platforms, not just one.
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I have a question on wind flow around the building using CFD. Why the most of the people are using steady RANS (RANS) models instead of Transient (URANS and LES). The flow will be stochastic turbulence in nature, but most of the reputed papers on wind flow around the building, ventilation, pedestrian comfort etc.., are used SRANS (STEADY RANS), models. 
Thanks advance
looking forward the answers.
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Rajasekarababu,
In most of one phase industrial applications, RANS is a pragmatic choice. This is explained by the robustness, ease of use and efficiency of the RANS.
However most of multiphase flows are highly transient and steady state approach does not work.
Some industrial applications expose limitations to the RANS modelling approach. For these applications, LES and DES provide a viable alternative at the expense of increased complexity and simulation time.
Regards
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Following this article
I sometimes had discussion with scientists about the "well-suited" using of URANS formulation for flow problems that are statistically steady (that is with no external time-varying driving force).
URANS solves the time-dependend Navier-Stokes equations on an unresolved grid, exactly as happens using LES. Without any explicit filtering or averaging, the only key difference between the two formulations is for the meaning of the turbulence model that adds some effect.
It is my opinion that for statistically steady flow, a real URANS formulation should drive, after a numerical transient, to a steady state solution to be congruent to its meaning of statistical solution (remembering that a statistical averaging is never really performed in the equations). Conversely, we see unsteady solutions that mimics the LES solution.
What really URANS is? Can be considered nothing else that a LES solution?
Hope to get your opinion,
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Thank you all above for this very interesting discussion. I hope you allow me to join. I have been giving this topic some thought as well over the past years, in the framework of the shallow water equations and the formation of dynamic eddies in URANS computations (or to be precise U-SWE computations) with steady boundary conditions. In other words, any dynamics that emerged in these computations came purely from the nonlinear excitation of discretization errors (which is I guess the topic of this discussion).
In these investigations, I experimented with the effect of the nonlinear advection scheme and the grid resolution on the emergence of eddies. Results indicate that most computations are performed with a 'numerical' Reynolds number which is not at all linked to the fluid viscosity (at least for practical grid resolutions), but to the artificial viscosity, effectively emerging from truncation errors from the discretization (and mostly from the nonlinear advection discretization). For numerical Reynolds numbers that are low enough (coarse grid, 'poor-quality' advection scheme), the results were steady and only resembed the time-averaged solution. For higher grid resolution and/or better advection schemes (not necessarily higher order, but with better conservation properties), the solution became time -dependent with vortex shedding (flow around a cylinder, flow around groynes in rivers).
Due to its link to the numerical viscosity, the aforementioned numerical Reynolds number is automatically space-, direction- and (in case of flow dynamics) time-dependent, resulting in an artificial/numerical solution.
For commonly-applied grid resolutions in practical environmental flow applications, the numerical Reynolds number is orders of magnitude lower than the theoretical one, based on the fluid viscosity. However, - and this is the main thing and also the reason why ILES methods were developed - this artificial solution resembles in some/many cases what happens in reality. In other words, despite the numerical/artificial nature of the (numerical) eddy viscosity, the flow dynamics and eddy formation, shedding frequency, etc. shows good agreement with reality, at least for certain advection schemes that exhibit certain conservation properties (momentum or energy).
To come back to the topic question:
In my opinion, URANS with/without a turbulence model and LES without explicit filtering differ really only in the turbulence model. The dynamic effects seen in URANS are artificial and in both models greatly depend on the grid resolution and the discretization of the nonlinear terms, whose discretization errors cause dispersion and spurious modes, which might look 'physically-correct'. The question for me then remains: do we except this and say that the results are valid despite being obtained for the wrong reasons in URANS (discretization errors), or do we strive for a correct solution that was obtained for the right reason (like in LES)? And perhaps linked to this: how do we know for sure whether a certain method always provides adequate results, for the wrong reasons?
My apologies for this long response.
Best wishes,
Frank Platzek
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Hello,
I am planning to run a research-led graduate course in turbulence modeling with a special emphasis on LES techniques. Please let me know which topics would you choose for such a course. Also, I'd like to hear your preferred teaching and assessment method. Any ideas are welcome.
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LES is so wide but also so niche that, in my opinion, a course is only useful if targeting basic stuff only (but with useful insights). Once you build that background correctly, the student can go deep in the preferred direction without the risk of being messed up by the so common copy&paste sources in the field.
Also, base material is nowadays largely available on most topics. To be really useful and new on a niche topic, that material should instead include also the minimal details, including implementations. Basically, the one who prepared it should have worked on that beforehand.
You can think of a two-part course. A first, basic one, but wide enough to cover all the LES arguments, giving the correct view. And then a second one, with each lecture dedicated to a single topic.
But you see how this can easily require a huge effort, not proportionate with respect to the scope (i.e., how many students are going to be interested in, say, implementation details of inlet specification methods, or commutation error treatment, or whatever?).
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The difference between the modeling system GAMS is that I can estimate the whole problem without thinking of the linear dependence problem with econometric system of equations estimations. However, I do not see how I could do the analysis without adding to external information like Frisch into the econometric model. All this I can get with GAMS in one model. For the interested persons, I attach the GAMS code (it is an amended version of a code I found in GAMS webpage).
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Thanks Kazi. However, it seems I will opt for SUREG procedure anyway. However, this leaves me to figure out n parameters of n equations, which I can solve in GAMS. I was hoping for to find a more elegant solution, but I cannot figure it out.
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hey guys
we are currently working on applying LES model on Lattice Boltzmann Method for simulating flow around cylinder and similar geometry.if there is any open source code which done this job its your kindness to address it to me
any relative comment is welcome
regard
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I do not find this publication in research gate. Can I introduce the corresponding full text and in case how can I proceed.
P. Freychet, J. Pelletier, G. Rosselin, Nicole Grenier, Christiane Rouel, G.Fouilleul
DOSAGE DE
L’HORMONE THYRÉOSTIMULANTE HYPOPHYSAIRE OVINE A L’AIDE D’UN SYSTÈME
RADIO-IMMUNOLOGIQUE BOVIN. EFFETS COMPARÉS DE LA THYROÏDECTOMIE ET DE
LA CASTRATION SUR LES TAUX DE TSH ET DE LH PLASMATIQUES CHEZ LES OVINS.
Annales de biologie animale, biochimie, biophysique, 1969, 9 (4), pp.483-496.
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I saw it has been done. Thank you very much.
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1) COMSOL program has only RANS model for the turbulent flow but I need LES model.
2) I want to work only on COMSOL program.
Thanks.
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LES is not a model but a totally different formulation...
You can try to set the code using a 3D model and a high order FV-based upwind reconstruction for the convective terms, if available. This is the ILES previously recommended. No turbulence model must be supplied, you should work as it were laminar conditions.
However, be careful that you need to solve the BL over the wall, that requires a weel refined grid close to all the walls.
You will have to wait that the solution develop a statistically energy equilibrium, then you have to continue the simulation for several time-unit and save the evolution of the fields to do a post-processing of the statistics. Depending on the Reynolds number and computational power, that could be quite long, also some weeks.
Good luck
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Among RANS and LES which one is the best for simulating transient flow in IC Engine.
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LES and RANS are two different worlds! The latter is based on statistical averaging, leading to steady equations (eventually also in 2d). A sort of time contribution is in the URANS brut it can be debated the le of the temporal mean.
LES is based in a local filtering, the equations are unsteady and 3d. As a consequence, the meaning of the turbulence model as well as of the final solution is totallui different.
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Hello All
I understand from literature that a completely resolved (Kolmogrov scale) LES is DNS itself. Any eddy resolving larger than kolmogrov scale are defined as LES. The effect of opted out eddies to the resolved eddies are modelled as subgrid stresses (SGS). Different modelling techniques for SGS include (1)Smagorinsky (2) WALES (3)WMLES etc. which are available in Ansys Fluent.
My doubts are :
1. Reference to Sagaut textbook, grid requirements for wall resolved and wall modeled LES is totally different, wall modelled being very coarse. (y+ min is ~30). In Fluent, various SGS models are available.
So Ansys Fluent provides a wall resolved LES with SGS models ? OR how can we distinguish wall resolved & wall modelled in Ansys Fluent ?
2. What is wall modelled LES (coarse meshes) and how can we implement it in Ansys Fluent. Is it just by making coarser meshes as per textbooks (y+~30) and applying required SGS models?
3. If we are making a very very fine mesh so as to capture Kolmogrov scale eddies, can fluent be used to solve DNS?
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1) The distinction appears from your choice of grid refinement close to the wall. Of course, then a proper SGS model have to be used ( Enhanced Wall Functions in https://www.sharcnet.ca/Software/Fluent6/html/ug/node514.htm).
2) see 1)
3) yes, any CFD code that solves the unsteady 3D form of the NS equations can perform a DNS.
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Hi all
I am trying to simulate turbulent pipe flow using LES method.
Literature:
Eggels (1994) used L/D = 5 for his studies.
In 1999, Blackburn used L/D = 2pi().
Hwang et al (2016) used L/D = 22 for their LES studies on pipe flow.
Q1. How do we arrive at required L/D of a pipe or length of a channel flow etc. when LES is used. As LES is computationally expensive, we cannot do a grid sensitivity. Does the two point correlations of required parameters (velocity, pressure etc.) help to obtain the minimum length required.
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Generally one sees at the spatial correlations, if they reaches zero, the lenght is sufficient.
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Hi,
I have the following parameters in my project (please see attachments).
I submitted a paper. Now, one of the Reviewer's comments is as follows:
Validation is not enough and convincing. The CFD results should compared with static and dynamic torque and efficiency performance with LES simulations available in the literature and or experimental measurements.
What is the exact meaning of "static and dynamic torque"?
I am grateful that give me useful links and papers in this case.
My used turbulence model in my project is K-W SST (RANS simulation). Also there are not any LES simulation available in this my field. My work is a 3D turbine.
Thanks.
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Torque is an important factor in much of the equipment on a factory floor. Measuring torque is often something that's misunderstood, which can lead to over- under-designing of measurement systems. This article addresses the many techniques and tradeoff of torque measurement techniques.
Torque can be divided into two major categories, either static or dynamic. The methods used to measure torque can be further divided into two more categories, either reaction or in-line. Understanding the type of torque to be measured, as well as the different types of torque sensors that are available, will have a profound impact on the accuracy of the resulting data, as well as the cost of the measurement.
In a discussion of static vs. dynamic torque, it is often easiest start with an understanding of the difference between a static and dynamic force. To put it simply, a dynamic force involves acceleration, were a static force does not.
The relationship between dynamic force and acceleration is described by Newton’s second law; F=ma (force equals mass times acceleration). The force required to stop your car with its substantial mass would a dynamic force, as the car must be decelerated. The force exerted by the brake caliper in order to stop that car would be a static force because there is no acceleration of the brake pads involved.
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Hi all
I understand that both are different flow problems.
When we try to simulate channel flow, top surface is defined wall for channel flows. For a flat plate of same dimensions, the top face shouldn't be a wall. Instead it is to be assumed that the flow is continous towards the top surface.
Can anyone help me with the boundary condition for a flat plate ?
Note - I am working with commercial code Ansys Fluent.
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I am trying to estimate turbulent wall pressure fluctuations using LES for zero pressure gradient flows. From the literatures(1,2), I arrive at a conclusion about the grid resolution in terms of viscous wall units. delta_x_+ = ~55, del_y+ ~ 1, del_z+ ~ 15.
1. How strictly we need to follow these numbers to get reasonably accurate LES results. I have seen some journal papers using LES without following these grid requirements. Are such results valid ?
2. Do we have to keep these resolutions throughout the domain or in the boundary layer region only? I am using ICEM CFD & Ansys Fluent s/w.
Ref 1: Large-Eddy Simulation : Current Capabilities , Recommended Practices , and Future Research. Georgiadis, Nicholas J et al.
Ref 2 : LES for Acoustics, Sagaut P.
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This estimation is generally used for channel flows where you have that x and z are directions of flow periodicity (homogeneous flow). These spatial directions are discretized by uniform grid steps in the whole domain. Conversely, along the wall normal direction (y) the resolution of the BL requires at least 3-4 nodes within y+=1, then the grid is via via enlarged approaching the center of the channel.
I suggest to read this paper
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Hi,
Please can I have  suggestions of articles and publications devoted for comparing LES results from those obtained by a DNS.
I have all the required statistical fields (all terms appearing in the RANS and FANS equations, in addition to the TKE and Reynolds Stress equations). I would like basically  to estimate, numerically,  a turbulent viscosity (scalar) from the tensors...
Best Regards
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Yes, I am aware ;-) I also worked and proposed some approach for LES using upwind.
But I think that in the LES community there are different schools that approach the LES equations in two ways, as either real filtered PDEs or numerical unresolved (by the discretization of equations and domain) equations. In my paper I tried to express my idea about those differences. I think that expecially the FV approach for LES was quite misunderstood.
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Hi,
In a simple form, the Boussinesq hypothesis reads
R = -2 nu_t * S,
where R and S are tensors (well known).
The previous hypothesis works if the two tensors are aligned; basically if the cosine
( | R : S | ) / ( ||R|| ||S|| ) > 0.86 (approximation).
I would like to have some suggestions for cases of  non-aligned tensors. Any method to estimate nu_t?
I thank you in advance and I look for your reply.
BR
Elie
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I never worked using hybrid formulation. I suspect that there are few strong theories on what solution one really get in the blended regions...
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i gave my first cell height is 0.0020 accordingly with roughness measurements (0.0019) Ks< first cell height. Can i run this model in LES. ( Model height is 0.7 , width is 0.15m and Domain height is 1.8 m all are accordingly with wind tunnel test section). 
Its is possible to run a terrain profiles simulation in ANSYS Fluent, accordingly with roughness parameters limitation (Ks< first cell height) in LES?
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Could you expand your question with the details of your problem? Why are you working with LES in such a problem?
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I want to combine my LES (3D time resolved field) with experimental data (2D planar distributions) in order to improve my results. The flow field is highly unsteady. Data assimilation can be applied but probably would not be perfect or even worse. Are there any similar studies?
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Dear Professor Ivan Kovalets,
Thanks for the answer. Data assimilation for modification of boundary conditions at the measured surface would be a great idea. I will try in this way.
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Dear authors,
How do you generate velocity time series for inlet boundary conditions for LES?
Thank you and kind regards,
George
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This way is quite simple but you need a very long inlet to let the flow correlate physically...but in some case where the separation is driven by the geometry (e.g. the backward facing step) that can work quite well
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Is there a later report on this evaluation - which tracks the outcomes after completing the program?
It would be good to talk about this if you have time.
And - thanks for uploading the Spingboard evaluation
Best regards Les
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no there isn't a follow up evaluation yet as far as I'm aware although we recommended one be conducted and the department people certainly hoped there would be. it might be worth contacting the department. regards
Eileen
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Dear colleague,
How to calculate the lateral stress exercise in the walls of the oedometer apparatus during a compression test in the field of non-linear behavior ?
Please let me offer methematical formul, or any experimental technique !
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Hi,
it is common to assume a K0 stress state, i.e. the lateral stress is the vertical stress multiplied by K0, where K0 is the earth pressure coefficient at rest. Jaky's formula can be used to estimate K0 for normally consolidated soils:
K0 = 1 - sin(phi), where phi is the friction angle.
See reference for other estimates.
Regards
Reference: R. F. Craig, Craig’s Soil Mechanics, 7th Edition, E & FN Spon, London, New York, 2007.
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I wish to know whether it is ok to have a comparably large positive Lyapunov exponents for a hyperchaotic system. I am working on a system which gives me LEs in the order like this : 19.4456, 1.45002, 0.0000, -28.4581, when simulated with the Wolf et al's algorithm. Is such a figure feasible for a 4D continuous-time hyperchaotic system? Thanks
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We can say the system is hyper-chaotic system because the LEs are +, +, 0, -.
There is no requirement related to the large Lyapunov exponent.
System is meant to be much more sensitive to initial conditions for higher Lyapunov exponent.
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Large eddy simulation grid  requirement mention about Delta X+ = 100, Delta Z+ = 30 and y+ = 1. Y+ is known but what do you mean by Delta x+ and Delta Z+, x is streamwise and z is spanwise directions,
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In the book "Turbulent Flows" by S.B. Pope (2001), the author explains non-dimensional wall units in physical terms instead of computational. The Reynolds stress at the wall must be zero, so the stress at the wall must be purely viscous (i.e. frictional). One can define a length scale for the purely viscous stress at the wall called the viscous length scale, defined as
d_v = nu*(rho/tau_w)^(1/2) = nu/u_tau, viscous length scale [m],
nu = kinematic viscosity (molecular), [m^2/s],
rho = fluid density, [kg/m^3],
tau_w = wall stress, [N/m^2],
u_tau = (tau_w/rho)^(1/2) friction velocity, [m/s],
A friction Reynolds number can be defined as
Re_tau = u_tau*d/nu, dimensionless,
d = a distance, [m],
such that if the distance is the viscous length scale d=d_v, Re_tau = 1. Non-dimensional "wall units" are x+ = x/d_v, y+=y/d_v, z+=z/d_v, so if you estimate the viscous lengthscale d_v then you can estimate normalized grid dimensions for your simulation.
The spanwise and streamwise resolution needed for turbulent boundary layer LES depends on your choice of wall model / your choice of SGS model. For incompressible LES, I recommend taking a look at the chapter on LES wall models in the book "LES for incompressible flows" by P. Saguat (2006). The author does a thorough review of the contemporary state of the art for LES wall models, and the author notes that part of the problem for LES of turbulent boundary layers is that turbulent kinetic energy is produced in the "buffer layer" just outside of the viscous sub layer in a turbulent boundary layer (Re_tau = order(1-10)). Therefore you either have to have a very fine resolution LES, like the y+=1 requirement that you stated, and/or use a wall model.
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wad
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You already have some tracks to follow with the first two answer, but I would like to present a different track for explaining this.
Fully resolving the NS equations (DNS) involves that you are able to resolve, with your mesh, all turbulent scales down to the smaller one: the Kolmogorov length scale. However, the latter greatly depends on the Reynolds number of your flow which explains why it is not practical (or even feasable) to solve high Re number flows using DNS.
To tackle this problem, the idea is to seperate the scales (large/small) by spacially/temporally filtering your field. That way you actually only need to have a mesh fine enough to solve the large scales, and then, model the small scales. Thus, the cost of the simulation can be greatly reduced. 
Also, I would like to support the reference suggestion from Leonardo Araujo as it is a great reference for LES. There is also a version covering compressible flows if you need.
"Large Eddy Simulation for Compressible Flows", Garnier, Eric, Adams, Nikolaus, Sagaut, P.
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les variables adéquates pour les utiliser dans TpsDig
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 I think pethyagoda sir is doing some modifications in barbus group classification s recently in Indian sp
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Hi all,
So after failing to get satisfying results to my simulation using the RANS model, I decided to switch to LES. As I am expecting some flow separation, after reading some papers, I decided to use the One Eddy Equation LES model. However, I am not sure about what BC should I set for the two fields; k (kinetic energy) and nuSgs (the subgrid scale viscosity), sepecially at the walls where I have the no Slip condition for the velocity.
Also should my mesh be fine enough at the walls?
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first, you did not provide details about your problem so that discussing BC.s in your setup is not possible...
In general:
1) Use dynamic SGS model, the turbulent viscosity will be determined by the flow conditions
2) Walls:Use natual BC.s for velocity if you have 3-4 grid nodes within y+<1
3) Inflow: many proposals exist, depending on your computational resource you can choose the best for you. Have a look at the Sagaut's book
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I want to perform LES simulation for airflow through Human Respiratory tract and I have approximately calculated the kolmogorov time scale of 10 microns getting confuse about the total number of time steps for ANSYS FLUENT. Please someone help me? 
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Just out of curiosity, you say your time step ~10^-06 s? How many cells does your mesh possess?
You definitely must have other informations regarding the time of your problem. For instance, you may want to simulate a period/cycle of breathing, or perhaps two. If they take, say, two seconds each you would need at least 2 million time steps. Perhaps you may also consider using fractional time step to assure numerical stability in such a complicated CFD problem.
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Hi,
I'm trying to validate LES with a turbulent channel case.
Assuming the SFS model works correctly (I actually tried several that gave very good agreement one to other), what can cause the results to be far off from DNS data?
Obviously using the same normalization as the DNS case and hopefully a sufficient mesh (was shown to be suffice in several papers).
Just to point out:
The shape of the results is generally OK, but the wall sheer stress / friction velocity are incorrect, therefore resulting in an incorrect mean velocity profile.
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a no-model LES is the only way to check for the effect of  the SGS model and isolate from the numerical errors..
Usually, I set the fixed pressure gradient, this way the non-dimensional equations are expressed directly in terms of the v+ velocity.
The effect of overestimation in the average velocity profile is quite common in some LES codes. But what about your energy spectra profiles?
I attach a paper of a project we developed few years ago.
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What do you think are the minimum benchmark cases that a LES solver must pass before being validated?
As implied in the question, this is strictly subjective so I would like to get as many opinions as possible. My interests lies in three-dimensional turbulent flows.
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Hi,
Can I devise a refinement criteria for AMR (Adaptive Mesh Refinement) in such a way it satisfies implicit LES requirements? What are the requirements for ILES exactly?
Or is it possible to think of a refinement criterion according to one of the explicit models of LES?
I'm mainly interested in flows over a bluff body.
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In so far as explicit LES is concerned, there are no practical obstacles to combine it with AMR. Say, set the filter length to the local size of the grid and the model will run stably. There are, however, theoretical (and thus accuracy) issues related to commutativity of filtering and differencing operations. For discussion see Marsden,  Vasilyev, and Moin, Construction of commutative filters for LES on unstructured meshes, J. Comput. Phys. 175 (2002) 584–603, and references therein.
Piotr
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When doing a turbulent simulations with models such as LES, how can I compute the ensemble average of the instantaneous velocities. I know that there is an option for calculating "time statistics" in Fluent, but I think that option just calculate the time average between the initial and final time points (1 value for the whole simulation). What I am looking for is unsteady ensemble average. Actually, I am not completely sure how it is supposed to be done, since velocity fluctuations do not have a single wavelength. So the more general question would be: If I have u(t) how can I calculate U_bar (t)  (assuming: u(t)=U_bar(t)+u'(t))?
Any help will be appreciated. 
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I think you will have to write UDF's for this kind of post processing. Just like a turbulent flow experimental researcher processing an instantaneous hot wire output.
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to simulating in-cylinder swirl, what kind of LES model is the best?
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Dynamic Samgorinsky model will be appropriate as sgs model for LES.
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I have been working with ILES in OpenFOAM for numerical simulation of partial sheet cavitation. I would like a backup of your opinions about the subgrid model and subgrid dissipation.
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Dear Dr. Thornber and Dr. Bensow,
Thank very much for your answers.
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I want to simulate the flow field with high resolution especially near the wall region in open channel flow with different Re. Can I use LBM, efficiently? Or VFM solution of Navier-Stokes equations using LES as turbulent model is more efficient from accuracy and computational cost (i.e. time) point of view.
I should be noted that my problem is a incompressible  multi-phase flow.
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Many thanks for your time and reply.
I want to know that which procedure has lower simulation time with same accuracy?