Questions related to Large Eddy Simulation
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?
Thanks for your answers! It means a lot to me.
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).
Thank you for reading,
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.
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?
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.
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.)
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.
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)
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.
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)
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?
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?
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.
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
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.
I will be more than happy to share any more information regarding the project to help you resolve my query.
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.
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?)
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?
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?
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?
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.
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,
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.
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?
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.
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 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?
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.
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,
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.
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.
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
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.
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 !
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 !!
Thank for your valuable comments.
Mesoscale cold core eddy observed in the North Eastern Arabian Sea is observed to be induced due to the baroclinic instability due to vertical shear in the flow. Can there be any link between the eddy and the internal waves? If yes what dynamics drive this?