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June 13, 2012, 11:25 |
LES for wind over buildings
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#1 |
Senior Member
Julien
Join Date: Jun 2012
Location: France
Posts: 152
Rep Power: 13 |
hi dear Foamers,
I am trying to compute some cases about buildings in a turbulent atmospheric boundary layer. I want to use a LES formulation , but I have absolutely no idea about a correct model and correct parameters to take? Does anybody has some advices for me? Another challenge will be to use a controlled inlet. I can generate some correct wind fields (with Jin, Lutes and Sarkani method for example), but how to tell openFoam to take this as inlet? |
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June 14, 2012, 06:19 |
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#2 |
Senior Member
Mahdi Hosseinali
Join Date: Apr 2009
Location: NB, Canada
Posts: 273
Rep Power: 18 |
about the first question read some papers in your context and see what they have done?
about the second question, how did you produced that? I'm sure in LES you can't have an static inlet profile and it should changes with time, there are some threads here about boundary conditions that change with time |
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June 15, 2012, 04:38 |
Part of solution
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#3 |
Senior Member
Julien
Join Date: Jun 2012
Location: France
Posts: 152
Rep Power: 13 |
Hi !
I found some answers to my questions, and I would like to help other foamers so I post these advices: This is an excellent source of knowledge on the WEB: http://www.tfd.chalmers.se/~lada/pos...-modelling.pdf Concerning LES, they say that it is necessary to choose: - a 3D model (but I succeded with a 2D one, with an OpenFoam warning) - unsteady analysis: I used pisoFoam - central diff. schemes for spatial derivatives: I used Gauss Linear (and Gauss linear corrected for laplacian) - A second order time scheme: I used CrankNicholson .5 All this is to avoid numerical viscosity; it is quite important because natural air viscosity is very small. The turbulence model choice seems to be of second order. 'hope this wil help some bodyelse! |
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June 15, 2012, 05:03 |
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#4 |
Senior Member
Mahdi Hosseinali
Join Date: Apr 2009
Location: NB, Canada
Posts: 273
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And what happened to your inlet condition?
btw when you are solving LES you have to do it 3D, there is 2D turbulence but it's not widely accepted because the main turbulence production mechanism which is vortex stretching does not occur in 2D. in your LES case you are having sub-grid scale but your grid-scales don't do what it has to do |
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June 15, 2012, 06:41 |
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#5 |
Senior Member
Julien
Join Date: Jun 2012
Location: France
Posts: 152
Rep Power: 13 |
Thanks Mahdi; you are right, my turbulence (little scales) is for sure approximate. but I have the main unsteady behavior with large vortices.
By the way, I still have bigger problems with my BC.
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June 15, 2012, 08:31 |
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#6 |
Senior Member
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Hi,
Guidebook for Practical Applications of CFD to Pedestrian Wind Environment around Buildings & http://www.opencae.jp/wiki/OpenFOAM-VandV-SIG |
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June 15, 2012, 08:47 |
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#7 |
Senior Member
Julien
Join Date: Jun 2012
Location: France
Posts: 152
Rep Power: 13 |
Thanks Elvis. It will help me to check my modelization. Nevertheless, results are mean values and BC are not detailed. It will help me to validate my calculations, but nothing to make them...
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June 15, 2012, 09:35 |
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#8 |
Senior Member
Dr. Fabian Schlegel
Join Date: Apr 2009
Location: Dresden, Germany
Posts: 222
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I have some remarks to your problem. With the outlet you will always have trouble if you use a Dirichlet or Neuman BC or a combination of both. But there are ways to overcome this trouble. Look for sponge layer (http://www.cfd-online.com/Forums/ope...utflow-bc.html) or convective Outlet BCs.
The inflow condition is also a bit tricky I guess. If you use a time variing BCs you have to ensure to apply natural turbulence (power spectra) and not some syntetic turbulence. You could ensure this with a longer domain to allow the flow to adjust and develop a natural turbulance before it reaches your buildings. The easiest way to overcome both problems inlet and outlet is to use periodic BCs and drive your flow e.g. with a mesocale pressure gradient (see channelFoam). I think the choice of your SGS model depends on your problem. I would recomend to check literature. May be the Deardorff model based on the TKE equation (one-equation eddy in OF) is a good choice. Check your discretization for your convective term carefully. I experience some problems with Gauss linear and switched to Gauss limited linear and apply the TVD flux limiter. This behaves much better in my cases. Kind regards, Fabian |
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June 25, 2012, 05:06 |
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#9 |
Senior Member
Julien
Join Date: Jun 2012
Location: France
Posts: 152
Rep Power: 13 |
Thanks Fabian ('excuse me for the delayed answer).
Your advices are very interesting, but so dense! You gave me some months of work! I have to:
Code:
bounding box: 4m x 4m x 14m Uo = 4 m/s dT = 2ms 220 000 cells non orthogonality: max 45, min 5.8 Tolerance: 1e-2 for all (but P : 1e-3 ) nCorrectors 2 nNonOrthogonalCorrectors 1 relaxation Factors: 0.7 for all (but p: 0.3) What tdo you think about these parameters? (I am still not sure it worked... I have to check for the Von Karman vortex shedding and its Strouhal frequency) Last edited by Djub; June 25, 2012 at 07:03. Reason: I forgot to mention Uo |
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July 2, 2012, 04:50 |
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#10 |
Senior Member
Dr. Fabian Schlegel
Join Date: Apr 2009
Location: Dresden, Germany
Posts: 222
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Sry for my late answer, but I was on holiday :-D
I never done LES for buildings. I am dealing with canopy flows. My indention was to give you some hints, to avoid some problems I have experienced. You should carefully think about your boundary conditions. If you are able to use periodic BCs your fine, the only "problem" to take care of is a the driving pressure gradient for the flow. If you are not able to use periodic BCs you should spent some work on that. The available outflow BCs (if you not know a value for the pressure at outlet, e.g. if you simulate a windtunnel this would be the case) normally have problems if strong vortices reaches the outflow domain. The easies BC to overcome this problem would be a sponge layer. The inflow is the next problem. You have to ensure natural turbulence and this is for my opinion difficult to ensure with synthetic velocity profiles for the inlet. A way to overcome this problem would be using a "beach" before the flow reaches your buildings. Another way probably be a Sommerfeld radiation BC, called convective BC. And for the remaining spanwise boundaries use periodic BC. I checked literature for BC with in and outflow but found nothing usable. For the SGS model I would recommend to check literature. The choice of an SGS model is very problem depended. But I think you find quickly an appropriate SGS model. For the problem with the discretization scheme, i just wrote my experience. I have read in the forum that the linear scheme should also work, but for my test with an infinite cylinder I experienced a problem with the SGS model. It generated much turbulence upstream in front of the cylinder, which is physical senseless. Using the limited linear scheme solved this problem for me, but limits the CFL number to less than 1. This is just my experience, so feel free to use other schemes, but check your results. Your setup seems to be OK. I use 3 orthogonal correctors and 2 nonOrthogonal with less mesh orthogonality, but I never tested it carefully. Its just a feeling :-D The tolerance seems to be a little low. I use 1e-06 for p and 1e-05 for U but this depends on what you expect from your results. kind regards, Fabian |
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July 12, 2012, 12:27 |
Current flow
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#11 |
Senior Member
Julien
Join Date: Jun 2012
Location: France
Posts: 152
Rep Power: 13 |
Hi,
I go slowly, but still progressing. So I am quite happy ! Now, I have a big (1M2 cells) 3D problem. I work with a LES model (smagorinsky), with PISO algorithm. But I have a comprehension problem: PISO is an Implicit method; thus, it should be not depending on a low current flow. Nevertheless, my simulation doesn't work with a Cfl > 1 . Did I miss something? I have also misunderstanding about the size of the LES filter. How to control this size? With the delta (I use cubeRootVol) and deltaCoeff (I use 1) ? Does it mean that my filter is a square gate with size equal to 1xcell size ? Last, I also have no idea about how to choose the size of the cells of my meshing... For the moment, my "choice" was to have a maximum of cells, but a number that paraFoam accepts! (I had some core dumps with high refinement meshes...) Thanks for help, Djub PS: I followed your advice and raised my tolerances to 10-5 and 10-6 for P ... |
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July 16, 2012, 21:22 |
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#12 |
Senior Member
Dr. Fabian Schlegel
Join Date: Apr 2009
Location: Dresden, Germany
Posts: 222
Rep Power: 18 |
Hi,
for me it tooks almost 1 year to get the first simulation running. So be patient and happy about every small step forward. Which method to you choose for the convective term? I use limitedLinerar and this limits the CFL number to less than one as far as I understood. To figure out how your filter lengthscale delta is calculated step into the source code. Just check $FOAM_SRC/turbulenceModels/LESdeltas/ and go through the .C and .H files and you will find your answer. The size of your mesh depends on what you need. If you doing a LES for the wake of a cylinder without a wall model you have to ensure y+ (wall coordinate) to be below one to resolve the boundary layer (important for viscous drag). For atmospheric flow this is not possible. You normally not be able to resolve the real boundary layer. You have to use a wall function instead (log law, monin-obukhov) depending on you specific problem. As far as I know the mesh resoltion depends also on your problem forumulation, e.g. neutral stability (no temperature) or instable boundary layer. As far as I know you have to have a finer grid resolution if you would like to get the temperature gradiends correctly, but this is just guessing To be honest I cannot give you an answer. You have to figure out the mesh resolution by yourself with a study of different meshes and checking the results if they change or not. This is research :-D But check literature and see what other people use for similar problems. kind regards, Fabian |
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January 29, 2013, 15:29 |
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#13 |
Member
Jubayer
Join Date: Oct 2009
Location: The University of Western Ontario, London, Ontario
Posts: 42
Blog Entries: 1
Rep Power: 16 |
Hi,
Did any one of you figure the burning question regarding LES in OpenFoam, which is, whenever you use a fluctuating inlet, pressure field becomes crazy. You can reproduce this just by running the pitzdaily tutorial and checking the pressure contours. Jubayer |
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January 30, 2013, 04:58 |
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#14 |
Senior Member
Julien
Join Date: Jun 2012
Location: France
Posts: 152
Rep Power: 13 |
Hi!
Well, I am not (yet ) an expert, so I don't know about pitzDaily. What I know, is that synthezised turbulence is very hard to create in order to be correct for CFD analysis. Usually, this kind of turbulence has nothing to see with Navier-Stokes: only with statistics within the velocity fields. For example, I don't know any method that involves fluctuation of Pressure. It sounds natural, for me, that a fluctuating veolicty inlet had a fluctuating pressure inlet. I don't remember this kind of pressure field... I am much more confident in the "recycling" method, with directMapfield. |
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September 3, 2018, 15:01 |
Boundary layer numerical trip
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#15 |
New Member
Pedro
Join Date: Feb 2016
Location: United States
Posts: 23
Rep Power: 10 |
Hi All,
I am trying to implement a numerical tripping by applying a Gaussian force in the wall-normal direction in OpenFOAM. The reason for this implementation is that I'd like to force the transition to turbulence in flat plate boundary layer flow. This idea can be done by adding a term to the momentum equation of the solver. The volume force has been defined in Schlatter and Orlu (2012) "Schlatter, P., & Örlü, R. (2012). Turbulent boundary layers at moderate Reynolds numbers: inflow length and tripping effects. Journal of Fluid Mechanics, 710, 5-34." I am seeking help regarding local force implementation into the momentum equation. Any help is greatly appreciated. Thanks, Pedram |
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September 3, 2018, 17:04 |
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#16 | |
Senior Member
Join Date: Jan 2014
Posts: 179
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Quote:
First: Turbulent Flat Plate Boundary Layer can be achieved by Lund Recycling Method or even better Synthetic Eddy Method. I implemented latter one (incompressible) recently for OpenFOAM 5.x during my PHD. My results agree well to Schlatter or Jiminez. Second: Laminar-Turbulent transition scenario is normally done by inserting TS-waves through a blowing-suction patch witch certain frequency and amplitudes. I assume that Schlatter used similar approach by adding some random behaviour to the signal in space and time in order destabilize the laminar boundary layer "faster". I could send you my TS-wave code. It is easy to add some random behaviour. So what do you really want to achieve? For a TBL with ZPG I would recommend the SEM Method. General concerns: Keep in mind that OpenFOAM is 2nd order in time and space. Latter one can be overcome by higher spatial resoltuion. In my simulations I used up to 120 points in the boundary layer. Simulations could be only run on HPC (I needed up to 10000 cores). |
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September 3, 2018, 17:39 |
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#17 |
New Member
Pedro
Join Date: Feb 2016
Location: United States
Posts: 23
Rep Power: 10 |
Thank you for your valuable feedback.
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September 3, 2018, 17:40 |
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#18 |
New Member
Pedro
Join Date: Feb 2016
Location: United States
Posts: 23
Rep Power: 10 |
Thank you for your valuable feedback.
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September 3, 2018, 18:10 |
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#19 | |
New Member
Pedro
Join Date: Feb 2016
Location: United States
Posts: 23
Rep Power: 10 |
Quote:
Actually, I'd like to capture the break up of structures in transition region. That is why I need to feed either superimposed TS waves at the inlet or a tripping mechanism near the inlet of the flow field. I would appreciate it if you can email me the TS-wave code (my email: pedram_tz@yahoo.com). Thanks, Pedram |
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September 8, 2018, 14:17 |
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#20 |
New Member
Pedro
Join Date: Feb 2016
Location: United States
Posts: 23
Rep Power: 10 |
I will get back to you all when I have my results.
Thanks |
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