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Turbulent Boundary condition, viscosity ratio and length scale |
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October 20, 2012, 14:13 |
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#41 |
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Francisco
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I've chosen standar values of density 1.225 kg/m3 and viscosity 1.7894e-5 kg/ms given by fluent.
I've chosen the timestep following the rule of CFL=1; delta_x/U=1. I even chose smaller ones. I can try hexa-mesh, but I don't know why in the references they get good results with worse meshes than mine. I prefer not to use a very fine mesh cause later I'll have to test the real scale case, and that's going to need even finner mesh. Also in the end I'll have to do the 3D case. |
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October 20, 2012, 14:32 |
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#42 |
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If one is getting the good results with bad mesh then he must the lucky one and atleast I cannot believe these results. In CFD one must first ensure the best practices instead of getting close to the experimental data.
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October 20, 2012, 14:35 |
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#43 |
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Francisco
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You are right, but I'm worried about computational time.
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October 20, 2012, 15:21 |
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#44 | |
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Francisco
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Quote:
Would you pass me the mesh you just made, please? Is there a tool to estimate computational time required? Thanks for your help |
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October 20, 2012, 15:27 |
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#45 | |
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Quote:
https://dl.dropbox.com/u/68746918/PS_2D_30Hx20H.zip 1. I would recommend SST-KW model 2. You should study the time step effects or do some literature survey and see how people have determined the time step. 3. Mesh is attached 4. You can estimate from the time step size. |
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October 21, 2012, 04:46 |
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#47 |
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Spot on. Thumbs up. You are 100 % right. We should stick to the basics first and apply proper physics to capture the desired phenomenon
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October 21, 2012, 04:48 |
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#48 | |
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October 21, 2012, 05:11 |
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#49 |
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Generally we do not apply the Mach number similarity when we are not interested in shock-wave interaction. So we must ensure the Reynolds number similarity.
One more thing, in making such analysis if one is interested in flow parameters change due to periodicity in flow then we one must be clever enough to set the density, dynamics viscosity, velocity and characteristics length in such a way to get the desired results with less man and machine hours. For this we should consult the work done by others in literature (Ansys customer portal is good place to find such data). In this way we can save our time and can use itin extending the previous work. To just give you example: If you want to simulate the flow around cylinder at the Re = 200. How would you set the parameters: density, viscosity, velocity and dia of cylinder? will you choose the default density = 1.225, default viscosity =1.78e-5 and then decide the velocity and diameter? Any idea even selecting the velocity and diameter? What will be the effect of selecting very high or very low velocity? PS: If one is interested I can share the Cylinder simulations at Re = 200 (Fluent and ICEM files) |
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October 21, 2012, 10:33 |
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#50 |
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Francisco
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@far I don't know how to use ICEM very well, is the mesh already generated in the files you sent me or do I have to specify number of division along the blocks sides?
I couldn't open fluent.cas either. Did you get any Cd or Cl results? Thanks a lot |
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October 21, 2012, 10:58 |
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#51 |
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Yes ICEM files are ready to be used. You are not able to open the case file becasue these are V 14 files and you are using older version. I am also attaching the .msh file which you should be able to open in any version of Fluent.
https://dl.dropbox.com/u/68746918/fluent.msh |
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October 21, 2012, 12:30 |
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#52 |
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SSL
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Dear friends,
I have not used ICEM till now. Is it really better than Gambit in meshing? Regards, Saeed Sadeghi |
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October 22, 2012, 12:51 |
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#54 | |
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October 22, 2012, 12:57 |
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#55 | |
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October 22, 2012, 14:35 |
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#56 |
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Philipp
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cfd seeker - I hope I didn't get you wrong: If you know the Re number, what is the problem? It absolutely doesn't matter how you "replicate" the Re number, just the number has to be the same. Results will also be the same (in normalized values).
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October 22, 2012, 14:39 |
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#57 |
Senior Member
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I would use density = 1.225 Kg/m3, V = 1 m/s , dia = 1 m and then viscosity = (1.225*1*1)/200. In this case stroul number is 0.22 approx and frequency will also be 0.22. Now I can decide the time step size.
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October 30, 2012, 10:26 |
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#58 |
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Francisco
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Hello everybody,
Sorry, I've been busy for some days. Returning to my problem I've tried to use the mesh Far made for me, but when I export it into Fluent v6 format it shows an error message (see picture). It's related to boundary conditions but I've specified all of them as usual: velocity inlet, symetryx2 and pressure outlet. Is anything left? By the way, Far, I'm interested in your cylinder Re=200 simulations if you still want to share! Thanks |
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October 30, 2012, 10:45 |
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#59 |
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I am always ready to share the knowledge. https://dl.dropbox.com/u/68746918/ICEM_CFDOnline.rar
By the way why file is giving error? are you using ICEM V14? You can directly read the .msh file in Fluent. PS : Drop box will take some time to upload the files. May be 10-20 hrs, meanwhile you will get error 404 or 403 |
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October 30, 2012, 10:57 |
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#60 | |
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Francisco
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Quote:
I've managed to export your mesh in 2D, but it gives error when importing it in parallel mode. An error message says: Build Grid: Aborted due to critical error. There are also several messages in the command window saying: 4:WARNING: cell id 28340 of thread 24 has NULL face pointer 3. Primitive Error at node 4: Build Grid: Aborted due to critical error. Which can be the reason? |
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Tags |
3d 2d, flat plate, turbulence models, viscosity limitation |
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