# Turbine Flow - how many cells

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 April 3, 2001, 22:03 Turbine Flow - how many cells #1 Chetan Kadakia Guest   Posts: n/a Hi I am modeling turbine flow with various turbulence models. I have a Pentium 3 900MHz computer. How many cells should I be able to allow in my mesh? How long should it take to converge? How precise should I have my convergence criterion?

 April 4, 2001, 15:18 Re: Turbine Flow - how many cells #3 Chetan Kadakia Guest   Posts: n/a Lets say I have 12 hours and 256MB of RAM on a 900MHz processor with a good initial conditions. What do you think?

 April 4, 2001, 15:28 Re: Turbine Flow - how many cells #4 Scott W Guest   Posts: n/a For 3D flow I'd start with 200,000 cells (since that would take roughly 200MB of RAM, the remaining memory can be used by your operating system). You really need more memory for good 3D simulations. For 2D flows it may be fine to start with 20,000 cells. But please follow my test and try two different cell sizes, and then determine the number of cells you need. It will vary with each simulation.

 April 4, 2001, 15:51 Re: Turbine Flow - how many cells #5 Chetan Kadakia Guest   Posts: n/a I will try starting with 100,000 cells. Obtain an inviscid solution. Adapt the region of high gradients and boundary layer and obtain a laminar solution. Then I will refine the grid some more and obtain a turbulent solution with the standard k-epsilon solution. All of these runs will be steady such that I can provide an initial flow field for my unsteady models. After all the refinements, I should have 200,000 cells. I will then refine for a grid independent solution for the unsteady solutions. Until I can find grid independence. I may have to get into parallel processing. What do you think?

 April 4, 2001, 17:15 Re: Turbine Flow - how many cells #6 John C. Chien Guest   Posts: n/a (1). Even if you are using a custom made code for 3-D turbine passage flow (Navier-Stokes), to get a converged solution for compressible flow in 12 hours is somewhat tight for routine calculations, using a workstation. (2). To get a somewhat accurate result, you need to push the mesh size over 350,000 to one mega cells. (3). And even after that, we know that the solution will be incorrect in the loss prediction, unless you are an expert in turbulence modeling using your own code.

 April 4, 2001, 19:04 Re: Turbine Flow - how many cells #7 Chetan Kadakia Guest   Posts: n/a Perhaps I can parallel several machines together and get a million cells plus over several days. I can use LES, RSM or k-epsilon models. Do you think I may get a somewhat close solution then?

 April 5, 2001, 11:44 Re: Turbine Flow - how many cells #10 Scott W Guest   Posts: n/a I forgot to add: I fully agree with John on his points (6) and (7) on his 5:39 pm 4 Apr 2001 response. You need good experimental data if you want to have confidence in your CFD model.

 April 13, 2001, 16:37 Re: Turbine Flow - how many cells #11 Xiao Hu Guest   Posts: n/a The cell numbers depends on which turbulent model and wall function you use. If you use standard wall function or non-equilibrium wall function, check your y+ value at the first grid and make sure it is between 50 to 500. If you chose two-layer zone wall treatment, then make sure that y+ is approx. 1.

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