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# Large Eddy Simulation - Science topic

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Dear experts
I am conducting research about the vortex shedding frequency of a cylindrical pole. The Reynolds numbers fall in the subcritical regime (Re = Uref D/ν ≈ 10^3 − 2 × 10^5 ).
I need to simulate in 2D and 3D in Ansys Fluent and I am wondering what model to use to get the best results to calculate the shedding frequency and Strouhal numbers of the vortices behind the pole using FFT.
I think k-omega SST does not provide the correct results comparing it to my calculations. Based on my research I consider using:
• transition k-kl-omega
• Detached Eddy Simulation (DES)
• Large Eddy Simulation (LES)
Which model will give the best results with a not too long calculation time?
And also what wall-treatment would best suit my situation?
When I used K-omega SST (RANS) model, it gave me the correct static results (mean forces) and the Strouhal Number. However, this model did not give the correct results for the fluctuating forces both the across-wind and along-wind direction.
FYI, I compared the results with experimental wind tunnel tests done for the same circular model for Re=13,000 to 89,000. The diameter of the cylinder was 0.1m and the height was 0.4m.
To be honest, RANS model is expected to give correct average results, since the formulation is also taken to be the average of the original NS-equation with the Reynolds decomposition. So, I would recommend LES models which is a far superior model to give the accurate fluctuating results along with the mean.
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Hello everyone,
My goal is to know how accurate is my simulation using the Wall Modeled Large Eddy Simulation (WMLES) S-omega and to calculate the total turbulent kinetic energy.
Therefore I would like to know if I'm resolving 80% of the turbulent kinetic energy with my grid and to do that I need to compute the SGS turbulent kinetic energy and compute the ratio between the resolved and total turbulent kinetic energy (k_resolved / (k_resolved + k_sgs) )
Assuming the resolved turbulent kinetic energy can be calculated by:
• k_resolved = 0.5 * (U_RMSE ^2 + V_RMSE ^2 + W_RMSE ^2 )
Is there a way to compute k_sgs using the various parameters given by Fluent ?
The turbulent parameters that Fluent has available are:
• Subgrid Turbulent Viscosity
• Subgrid Effective Viscosity
• Subgrid Turbulent Viscosity Ratio
• Subgrid Filter Length
• Effective Thermal Conductivity
• Effective Prandtl Number
• Wall Yplus
The article referenced in the Fluent user theory guide is (PDF) A hybrid RANS-LES approach with delayed-DES and wall-modelled LES capabilities (researchgate.net)
According to the guide, the subgrid filter length is given by the equation (4) and the subgrid turbulent viscosity is given by equation (19).
Greetings.
Depending on the type of simulation that is performed, many features appear. In the “run calculation” tab, the variables can be extrapolated, and more parameters can be added. These are located by pressing the “other/add variables” button. In case what you need does not appear, it can be accessed through the console and even perform the operations you mention. However, there are many hidden options, perhaps you can find some more information in the user guide. Remember that it is commercial software, which means that it is closed source and many of the calculations are black box, so what you are looking to calculate would be very complicated.
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I am using Detached eddy simulation model (DES) in fluent. I want to make sure that the simulation is being switched to LES model away from the wall. How do we know that this swtich happen. Is there any quantity/indicator that can prove that this switch is happen.
Thank you
Yes there is, however it depends on the DES model being used. From what I can check in the manuals:
If you use DES, DDES, IDDES you have a turbulence quantity named DES TKE Dissipation Multiplier which represent a blending function multiplying in the definition of the length scale.
If you use the SBES or SDES if you have a turbulence quantity named Shielding Function for SBES or SDES which goes from 0 to 1 depending if you are in DES or RANS, which is easier to check.
For the DES with BSL/SST or Transition SST in the Fluent Users guide 2021 R1 p1824 it is said that the function represents FDES or FIDDES and when FDES >1 you're in a LES region.
I recommend also you look to 13.2.2.2 in the Users guide which helps to choose the adequate DES model.
More details about these parameters are in the Fluent Theory Guide in chapters 4.12, 4.13 and 4.14 and in Fluent Users Guide chapter 13.20.
I also would like to know if there is a more clear way to see if we're in RANS or LES region with the DES TKE Dissipation Multiplier.
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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?
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.
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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When planning and designing buildings, and urban environments, especially cities with many high-rises, simulating airflow is an important tool for studying how the city’s layout might affect expected temperatures, among other things. Other areas of use include modeling and predicting wind load on buildings to study Fluid-Structure Interaction(FSI). The local climate around a building in an urban environment can differ significantly from the more general weather data often used in the computation. Consequently, local wind and temperature conditions affect heating and cooling as energy requirements for buildings. However, there is still a stumbling block. Existing simulations use extremely simplified geometric descriptions of the urban environment, which results in poor computational precision. (The photo has been taken from Chalmers university of technology.)
The answer depends on a scale of your problem. In some application there is a lot of emphasis on representation of the extremely fine details. Please see for example
In the end, it all depends on the capacity of your computer and the ability to digitize complex structures.
For a problem shown in your figure, you can represent the effect of a missing structure by appropriate parameterization of the surface roughness.
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In general, there are several automated eddy detection algorithms. However, each identification method poses a multinuclear eddy identification problem, e.g., multiple SLA extremes. This problem can occur when multiple eddies are physically close together. We tried to solve this problem with the following paper "A new mononuclear eddy identification method with simple splitting strategies" and the paper "Technical Note: Watershed strategy for oceanic mesoscale eddy splitting" in Ocean Science, which can be downloaded from http://www.ocean-sci-discuss.net/11/1719/2014/osd-11-1719-2014.html. We hope these strategies be helpful for the investigations in ocean dynamics.
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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)
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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Dear Enthusiasts,
I hope you are enjoying your research. I have a new topic (well, to me at least) and I want your valuable ideas on it. I want to compute Buffet Forcing Function on an aerodynamic body, like VEga or SLS. Can anyone tell me the simplest way to do it (a) on WT model and (b) on Full scale model.
I have fair idea of computing unsteady pressure values and then computing fluctuations but what after that...? I have a grey area ahead. Please, if someone knows, do help me.
Regards,
Shamoon
Thanks Janet
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We have been numerically investigating the thermal-fluid characteristic of sCO2 laminar flowing in a 0.5 mm diameter circular microtube in upward direction. Depending on sharp density change along the tube, this case can be admitted as a compressible flow. When applied pressure-based solver option, the pressure firstly decreases until a certain point along the tube, then increases towards the outlet. So, this situation is impossible. Can this situation result from pressure-based solver? Can it be fixed by using density-based solver? I would like to be grateful if you could share your recommendations. (Boundary conditions: mass flow inlet, pressure outlet (75 bar), constant wall heat flux)
Dear Sir, thank you for your reply. We have already defined the properties of the sCO2 by using Fluent Console (NIST tables for CO2 is current in the Fluent database). Although the entire graph is very reasonable, the pressure change along the flow direction is interesting as seen in the attached file. When calculated theoretically (considered friction depending on both wall shear and momentum change) by using Ansys data (wall shear stress, density, axial velocity, etc.), the pressure drop is found as expected (full decreasing along flow direction).
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Hello,
I am studying land surface processes using a Large Eddy Simulation model. I need data related to the land surface parameters, especially soil moisture at different depths, soil temperatures, root fraction etc. Where can I get these data from?
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I can't understand from a physical standpoint how fluctuating velocity components in a turbulent flow lead to formation of spinning structures of different sizes known as eddies. Can anyone describe the physical underlying mechanism in simple tangible terms?
The turbulent flow is indeed characterized by spinning structures in the form of entangled vorticity tubes. When observing the evolution of the fluid, we can easily notice that these tubes meander, oscillate and change their connectivity in result of the process known as the vortex reconnection.
The mechanical model explaining the behaviour of the vorticity filaments was investigated by Da Rios in close collaboration with Levi-Civita. One of the first publication on the subject is available in Italian: L.S. Da Rios, Sul Moto d’un liquido indefinito con un filetto vorticoso di forma qualunque, Rend. Circ. Mat. Palermo 22 (1906) 117–135. where Da Rios derived the equations of the vortex filaments in the turbulent flow. The mechanical processes expressed in these equations answer the original question.
Levi Civitta continued work on the geometric model of turbulence, but unfortunately most of the results remained obscure for more than half a century and were only rediscovered in the 1960s. The theory is elaborated in the review article by Renzo L. Ricca (1996)
“The contributions of Da Rios and Levi-Civita to Asymptotic Potential Theory and Vortex Filament Dynamics, (Fluid Dynamics Research, vol. 18, pp 245-268).
This review also contains some references to modern publications but I think it is a good idea to familiarize yourself with the original contributions of Da Rios and Levi-Civita. It is perhaps important to mention that the renewed interest in the geometrical methods developed by these authors ultimately led to the topological fluid dynamics and to the description of turbulence based on the theory of knots (Moffat's articles are a excellent introduction to this area).
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Currently, I have developed Large-eddy model in Lattice Boltzmann Method. When i use 1024 * 512 mesh, it works fine and reasonable match with literature is found. However, by increasing the mesh quality, drag and lift values got disturbed and values move far away from the literature. I do not know why fine mesh is causing problem. I think i need to add kind of filter in my coding to filter the distribution function or something. Can anybody help me how to solve this problem. (Using low quality mesh 512 * 256), result matches with literature.
I need fine mesh around the body for complex bodies, like, square with chamfer ratios.
Hi Muhammad, First of all you should know how to perform a mesh sensitivity analysis. One of the most recommended method is to decrease the cell size by increasing the number of cells, this would lead you to desirable results. Which means the cell size from which you cannot get any high difference in magnitude of various physical values. It should be noted that an over sensitive mesh could lead you to high time consumption and error.
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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,
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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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
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 want to evaluate a multiphysics fluid pump problem. This is a conceptual fuel pump for automotive application. The current model has magnetic elements (in pump itself), reciprocating components and an integrated BLDC motor. Architecture of this pump is considerably different from existing reciprocating and rotary pumps. I hope to find some vital information like delivery pressure and flow rate fluctuation, net heat generation, flow pattern inside the pump, dynamics of the moving components etc.
I am a Product Development Engineer and have very little knowledge of CFD or FEM. I am looking for suggestions on,
1. How to move forward with this project ?
2. Do I need a team and different softwares to solve different problems or it is possible to evaluate them by one person?
3. Which kind of softwares are being used to address similar problems.
ANSYS has a good Multiphysics capabilities. I did a structural-thermal-electrical analysis in conjunction with CDF analysis. However I don't know much about electro magnetic simulation. A few Multiphysics elements are available in WB that you can have degrees of freedom in different physics. In addition to that, there is methode called "Load Transfer" that you can you use. Basically you Perform a simulation in a physic and then transfer the result to another physic. You need to identify which method is the best in your application based on a lot of parameters such as complexity, computational resources, time and etc. Hope this information helps.
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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.
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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For as long (or rather, short) as I've been in the fields of turbulence research and computational fluid dynamics, I've been told that the Reynolds-averaged Navier-Stokes (RANS) approach of modeling and simulating turbulent flows is based on ensemble or time averaging of the Navier-Stokes equations. Here, the Reynolds stresses have to be modeled to close the resulting equations before they can be solved.
On the other hand, large-eddy simulations (LES) are (formally) based on (spatially) filtering the Navier-Stokes equations. In this case, the subgrid-scale or subfilter-scale stresses have to be modeled. In a commonly used approach to large-eddy simulations, the theoretical approach/description of filtering is just that: a theoretical formality that is not used in practice. A practical large-eddy simulation then consists in solving the Navier-Stokes equations on a grid that is too coarse to resolve all scales up to the Kolmogorov scale, but fine enough to still resolve the large scales. An extra forcing term (the subgrid-scale model) is added to model all the missing physics (for example, the dissipation of kinetic energy).
I wonder now how actual RANS simulations are performed, especially given the existence of hybrid RANS/LES methods? Does time or ensemble averaging ever play a role in an actual practical RANS simulation? I could imagine that time averaging implicitly plays a role if practical RANS simulations are like iterative methods that try to obtain a steady-state solution of the Navier-Stokes equations on a coarse grid. But then I don't understand how switching between RANS and LES modes, like in hybrid RANS/LES is possible. Or is the time/ensemble averaging of RANS just part of the theoretical description that is not used in practice, as is the case with filtering in the practical approach to LES I describe above? Is a practical RANS simulation then 'just' a very-coarse-grid simulation of the Navier-Stokes equations? (In which, for example, eddy viscosity or Reynolds stress models just serve as extra forcing terms to capture any missing physics?)
Hi Maurits,
I think it is safe to say that time/ensemble averaging is never performed in an actual RANS simulation, just like explicit filtering is not (or rarely) performed in practical large-eddy simulations. It is mostly the turbulence model that makes all the difference, and provides the variables in each formulation with the proper meaning.
In RANS, the eddy viscosity is such that the velocity field obtained from the numerical solution represents (a reasonable approximation of) the time/ensemble-averaged flow field of the problem under study. Since the expected velocity field is typically smooth and often two-dimensional, considerable simplifications are allowed, including 2D domains, coarse grids, and low-order/dissipative numerics.
Francesco
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Dear all,
I'm looking for the difference between WALE and Smagorinsky sub-grid scale models in filtering the scalar transport equation.
Does anyone know something about it?
Thanks
Dear Shahin,
The original and dynamic Smagorinsky-Lilly models are essentially algebraic models in which subgrid-scale stresses are parameterized using the resolved velocity scales. The underlying assumption is the local equilibrium between the transferred energy through the grid-filter scale and the dissipation of kinetic energy at small subgrid scales. The subgrid-scale turbulence can be better modeled by accounting for the transport of the subgrid-scale turbulence kinetic energy.
Wall-Adapting Local Eddy-viscosity (WALE) model is designed to return the correct wall asymptotic behavior for wall bounded flows.
Here is a good paper where 3 (SGS) models are compared:
Assessment of SubGrid-Scale Modeling for Large-Eddy Eimulation of a Spatially-Evolving Compressible Turbulent Boundary Layer”.
Regards
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I am currently working on performing an LES WALE simulation to extract pressure data, to be later used for acoustic simulation. I have some information about the grid density requirements in terms of acoustic simulation. However, I am in dearth of information concerning the mesh density requirements for the CFD mesh using a LES WALE turbulence model. Are there specific mesh element size restrictions that need to be accounted for the fluid domain rhather than the smallest turbulence scales?
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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?
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 Dr. Launder,
What are the second moment turbulence models? Do they belong to the Reynolds Averaged Navier Stokes methodology or to the Large Eddy Simulation?
Thank you and kind regards,
George
Definitely you will get better results in comparison to EVM's. Through RSM you can get complete turbulence structure, since we are solving six additional transport equations for each independent components of Reynold stress. but computational cost will be high.
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Hi,
I am using ANSYS Fluent to develop LES of turbulent open channel flow with surface heating flux. The shear stress number is 400. There are 5 subgrid-scale models (i.e., Smagorinsky-Lilly Model, WALE model, WMLE model, Dynamic Kinetic Energy Subgrid-Scale Model) under the Large Eddy Simulation option. Could anyone tell me the difference between these models and which one is suitable for my case? Thanks a lot.
Cheers
I suggest to have a look to the user guide (the references) but also having a look to some specific textbook about LES (for example Sagaut's book)
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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,
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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Dear community;
Unlike the bulk of paper in LES where the streamwise and spanwise direction are homogeneous (periodic boundary condition) my case is in-homogeneous. Specifically, I have an inflow and outflow. Therefore, filtering close the boundaries is not straight forward.
I have look into the web but the papers that I have found with something similar do not use dynamic procedure. I wanted to know if some of you have read something similar in a journal that can recommend me or if you have experience in something similar.
I was advised to use a different stencil close to the inlet and outlet, but I would like to have a paper to reproduce their case. I am planning a top hat filter.
Thank professors; very interesting your comments.  Indeed, I am using  a stencil of 3x3x3. Far from boundaries, the discrete filtering is the same as other researchers have done. I used the Simpson rule to obtain the discrete representation of the convolution integral.
I did not use compact scheme, since the code was written by my advisor. Nonetheless, I have several ideas that I want to test in a more controlled problem (i.e: 1D Burger equation) with compact schemes and many other factors.
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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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I am trying to simulate transitional flow in a smooth circular pipe using Ansys FLUENT. Is LES is suitable for transition flow modeling? if yes, which SGS model  can capture the transition behaviour?
yes, you can use LES formulation using the dynamic SGS model (an option in ANSYS/FLUENT). However, use second order discretization (time and space) and refine the grid near the wall to ensure to have at least 3-4 nodes within y+=1
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Hello,
I have done LE simulation for a flow over flat plate to study about transition and saved the results for every 100 time steps. I ran simulation for the time period of 2.5s and I have sampled the data for the last 1.5s. Now I would like to generate a plot for time averaged skin friction or shear stress distribution along the inclined plate using tecplot or CFD post. I am having hard time generating the result. If someone could spare some time and suggest me something it would be of great help.
Thank you
Kind Regards,
Kumar
Have you though about using paraview?. I Think it is easier to do what you want. In fact I have already done that kind of post processing before and I can help you a lot.
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I have in my Geometry some cavities with almost zero flow. I was wondering if openFoam comes with some embedded LES model that uses RANS in bulk flows and employs LES near the wall. If yes what is its name?
so you produce also an adverse pressure gradient .... If your computational power is sufficient, I suggest using a wall-resolved LES and a dynamic SGS model. A mixed model is advisable
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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?
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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The simulation of turbulent flows by numerically solving the Navier–Stokes equations requires to resolve an ample range of time- and length-scales. Such a resolution can be achieved with Direct numerical simulation (DNS) but is computationally expensive and currently prohibitive for practical problems. The main idea behind LES is to reduce this computational cost by reducing the range of time- and length-scales that are being solved for via a low-pass filtering of the Navier–Stokes equations. Such a low-pass filtering, which can be viewed as a time- and spatial-averaging, effectively removes small-scale information from the numerical solution. This information is not irrelevant and needs further modeling, a task which is an active area of research for problems in which small-scales can play an important role, problems such as near-wall flows , reacting flows,and multiphase flows.
COMSOL is based on Finite elements to solve equations, and LES is focused for 3D grid mesh. In this context, you need a software based on Finite volumes as OpenFOAM, ANSYS Fluent, ANSYS CFX, etc. I think you can start with OpenFOAM for a better understanding of turbulence models.
kind regards
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Hello everyone,
I am wondering if a definite answer has now been provided to this question.
One issue is that hydrothermal vent fields have a lifetime of the order of 10,000 years (eg. TAG) whereas one needs to account for survival and evolution  of hydrothermal vent organisms over geological timescales exceeding 200 million years or so ?
One logical suggestion (eg. Lauren Mullineaux and coworkers)  is that larvae of vent organisms can be entrained and dispersed by hydrothermal venting and in particular by hydrothermal plumes (focused venting).
However several problems remain:
Are larvae capable of sniffing other active vent sites ?
It seems to me that colleagues have found that the answer to this question is yes.
Are larvae mobile enough to move down if necessary to reach such sites ?
Here too it seems that the answer may be yes also, from colleagues'past and ongoing research on this aspect.
Another question relates to the toxicity of hydrothermal vent fluids at the source of plume venting ? This is where entrainment is traditionally  expected to occur and yet this is also where plume fluid toxicity is greatest (and potentially lethal to most life forms ?).
So a related question would be: are larvae capable of surviving the toxicity of near-source fluids from hydrothermal vents that emit plumes ?
I have asked myself this question differently during the 1990s. I studied the dynamics of interaction between turbulent plumes and a crossflow in the lab (eg. Ernst et al 1994, Bull Volcanol; Ernst et al 1998, Bridge Newsletter; Palmer and Ernst 1998 Nature, Ernst et al 2000 EPSL...).
I observed that bent-over plumes in crossflow generate the water-equivalent of atmospheric tornadoes or what one may term wake tornadic vortices. I carried out lab experiments to try and understand what may control their generation and frequency of occurrence (unpublished research).
In a nutshell, incoming boundary layer vorticity generates a horseshoe vortex in the boundary layer around the plume source base. Vorticity in the trailing arms of the horseshoe vortex are periodically stretched in the intermittent entrainment field in the bent-over plume. This generates a train of wake "tornadic  vortices" downstream of the plume. These wake tornadoes can entrain what lies on the ground into the plume or just transport it a great distance.
Provided the plume remains bent-over, the more buoyant the plume is, the more intense vorticity stretching is. (Please note that megaplumes could also do a great larvae dispersal job as they are anticipated to have a "hurricane-like" dynamics; Palmer and Ernst 1998 Nature.)
If the plume is sourced from a conical mound, this strengthens the horseshoe vortex or a series of them in the boundary layer (still unpublished research I am afraid) and the wake tornadoes are even more intense.
In the latter case, the first "tornado" is generated on the mound and slightly upstream of the bent-over plume, fast accelerated around  the side edges of the plume, then sucked up into the plume in the near-wake.
In both cases, the tornadic vortices are advected in the wake region from the near-field into the far-field.
I have also verified that the above is consistent with common observations of tornadic whirlwinds, waterspouts and tornadoes generated by dynamically analogous bent-over plumes emitted by active volcanoes in a crossflow (also unpublished).
Implications from this are that wake tornadic vortices are expected to be generated intermittently around hydrothermal focused venting in crossflow. To me they offer a mechanism by which larvae are expected to be near-continuously sampled and entrained into proximal to medial regions of the bent-over plumes where they are considerably less toxic to life. As the wake tornadoes are advected into the far-field wake region of  a bent-over plume, there is also the possibility that some vent larvae are dispersed without ever experiencing toxic plume fluid.
I would very much  like to know if anyone may have considered this "tornadic" vortex entrainment / dispersal mechanism for hydrothermal vent larvae ?
And also if anyone may have documented direct or indirect evidence for "tornadic vortices" expected frequent occurrence in the wake region of hydrothermal vent plumes ?
Looking forward to hearing back,
With best wishes and kind regards to all,
Gerald
Dear Yuli,
Thank you for sharing your experience and insights.
I hear you are advocating for closer mutli-disciplinary research.
As you are coming from the fluid dynamics community, I'd like to share some of  the experience I have had.
Research focused on hydrothermal vents, all aspects of them has become truly multidisciplinary a long time ago now, so that you may find your happiness through linking with this most active and friendly community.
In the 1990s, I was working in the UK and was hired to focus on the fluid dynamics of hydrothermal plumes with funding for about 5 years (and work for about 7 years) from NERC. The programme was called the BRIDGE programme.
The BRIDGE Community has been a most active one and brought together scientists from all disciplines. This led to considerable advances in understanding of hydrothermal vents as systems and as systems of systems and became an important crucible of frontiers'science.
In the BRIDGE Community our group at Bristol was the main one that contributed to fluid dynamics work on hydrothermal plumes (with BRIDGE funding) in my recollections. There was also one other main  group (Cambridge) which focused on trying to understand the fluid dynamics of growth of vent constructs, and had considerable expertise on hydrothermal plume modelling. Besides you will no doubt be familiar with the DAMTP (Herbert Huppert, Andy Woods, et al), who revolutionized geophysical and environmental fluid dynamics together with relatively few other groups (eg. ANU at Canberra...).
Then there was already too little fundamental fluid dynamics in the mix of hydrothermal plume research which was generally truly exciting nonetheless.
Perhaps this was because hydrothermal vent research was still in the age of discovery of most vent sites on the planet (I believe this is still the case). This has been under the great impetus of colleagues such as Chris German and fairly numerous coworkers who discovered a huge number of sites in a rather short time.
Hence much more work, it is only natural I believe, focused on discovering and documenting all the new vent sites, from all viewpoints. This is also why there is now a considerable repository of datasets which still need to be fully mined by fluid dynamics modellers who may consider teaming up with these pioneers of the deep-sea. A most exciting venture that I recommend.
The BRIDGE Programme is only a part of a much larger programme that was called "Inter-Ridge" which preceded it and continues today. The Inter-Ridge Programme has been funded by NSF in the USA and has brought, just as for BRIDGE, many communities together and greatly contributed to advancing frontiers science and generally Earth Systems science.
If one puts all this together, there is a huge amount of data available today for fluid dynamics modellers. A few modellers have been dedicated to focusing on analyses of such data and I recommend, once again, that you consider contacting these colleagues and contribute your extensive expertise in fluid mechanics to joint ventures.
In the USA, fluid dynamics modelling authorities who have focused on hydrothermal plumes include Kevin Speer, Peter Rona and Bill Lavelle.
In the UK, many colleagues at Cambridge continue to make crucial contributions. You may want to contact Harry Elderfield, Mark Rudnicki and their colleagues. Formerly Steve Sparks has also contributed important modelling aspects with Chris German.
This could be a starting point for your exploration of potential collaborations, if you will.
Then, there are also in the Inter-Ridge community those experts who have focused on deep-sea velocity field measurements. I was hoping to hear from them. I have no doubt that evidence for "wake tornadoes" around hydrothermal plumes has already been collected long ago, but as I have not been following developments closely enough I cannot recall a particular name. These are also colleagues who may be keen to explore collaborating with you and with other fluid mechanics experts.
Traditionally in the plume in crossflow problem, the focus has been to try and understand how crossflows interact with and disperse plumes. Comparatively little attention has focused on how plumes affect cross-currents and change the whole velocity field. The wake tornadoes are an illustration of what one can find out when considering the latter angle of view.
In fluid mechanics, a classic problem is that of studying the interaction of hard obstacles in crossflows, such as cylinders or tapered cylinders. A turbulent jet or plume in a transverse crossflow is a special case where one can learn from the above provided the compliant nature of the plume is also taken into account.
When such an analysis is considered, one can account for why turbulent bent-over jets or plumes initially bifurcate (Ernst et al 1994, Bull Volcanol) and one can anticipate that bifurcation of hydrothermal plumes is common even though logistically this is difficult to document (eg. Ernst et al 2000, EPSL).
I am curious as to whether colleagues may now have documented more bifurcating hydrothermal plumes in this respect ?
When hydrothermal plumes behave as an obstacle to a crossflow, wake tornadoes are anticipated to occur and this can be regarded as an extension of vortex shedding by cylinders in crossflow to some form of vortex stretching and shedding by compliant tapered cylinders in crossflows.
I am still curious to find out if hydrothermal venting colleagues have considered this and their data accordingly ?
I shall leave you with these further réflexions.
I 'd still love to hear from Inter-Ridge colleagues.
With best regards and kind regards to all,
Gerald
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I would like to compute my diagnostics using this "full" EPflux not just using the ordinary quasi-geostrophic one. The results seem quite different so far.
Addtional to the paper above, you can have a feel about possible errors in estimatint the full EP flux and EP flux divergence based on reanalysis data sets from the followng paper:
Hua Lu, Thomas J. Bracegirdle, Tony Phillips, and John Turner, 2015: A Comparative Study of Wave Forcing Derived from the ERA-40 and ERA-Interim Reanalysis Datasets. J. Climate, 28, 2291–2311. doi: http://dx.doi.org/10.1175/JCLI-D-14-00356.1
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... I would like make an animation.
I want do simulate the cavity flow with large eddy simulation, but I have no idea for the post processing.
Hello,
I have a question similar to above question. I run LES for 1000 time steps and I save my fluent files every 100 time steps. I have 2 questions:
1. I would like to calculate turbulent dissipation rate. However, it is not available in fluent. Does anyone know how to calculate it in fluent?
2. I would like to calculate average value of a parameter lets say fluctuating velocites from those fluent files. is there a way that fluent opens all those files that I save in different time steps and take the average of those and give me the averaged value?
Thank you,
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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.
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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I want to simulate air core in hydrocyclone. How might I couple the multiphase model (VOF or mixture model) with Large eddy simulation to quantify the air core diameter
Here is an example of gas turbulent flow charged with solid particles.
Also is valid for gas with drops without evaporation.
It is difficult to apply LES to a flow with no disperse phase. You have to detect when the finite volume is gas or liquid. When you have gas the LES acts. When you have liquid the method attenuates by the density and viscosity.
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I am simulating flow in a francis turbine (containing runner and draft tube)
I have read about the index of Resolution Quality required for LES. And it was pointed out before that a good LES is that which tends to DNS as the grid Resolution tends to the smallest scale i.e Kolmogorov scales. Therefore there is no such things as grid-Independent LES in theory, because a grid-Independent LES is essentially DNS. And therefore, the philosophy of LES loses it's meaning if it is grid Independent.
(the Advantage of LES over DNS being that LES is much more economical while it only requires the Resolution of the most energetic Eddies that determine the essential flow properties)
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I am working on small jet engine. My field is to how to predict jet noise in the near field using ANSYS FLUENT. If some one help me for how to make this. I read that large eddy simulation(LES) can make this. I am working with ANSYS but i dont know how to use LES and if this is correct for my case or no or there is another method instead of LES. If some one have a tutorial or any another idea.Thank you.
Hi Tamer Raef
Here is a PDF about nozzle simulation by ANSYS FLUENT.
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Hope the focus be made on the lattice Boltzmann method if possible. for this method the use of large Eddy simulation (LES) is justifed by the direct calculation of the strain rate tensor Sij=(ui,j+uj,i)/2 at each node without finite differencing !
There is nothing different practically. Theoretically there is since turbulence is always 3D
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How does the eddy current affect the magnetic field? Specifically for a linear electromagnetic actuator, what is the influence of the eddy current (in the Armature, which is the only moving part in the actuator) to the magnetic field and current generated in the coil? By changing the shape of armature by means of embedding a different conductivity material in the form of a ring at different intervals will this make any change in the eddy current being generated? (Have a look at the figure- a cross section & a front view of cylindrical armature).
If it works good, what type of material should I use? Low or high conductivity? Or any other suggestion?
My aim is to generate a better current !!
Hi Vipin
Maybe you could consider an aluminium alloy, since aluminium itself is a good conductor and would yet contribute for the parasite currents.
Good luck.
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