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-   -   Problem using Central differencing scheme with LES WALE (http://www.cfd-online.com/Forums/cfx/119105-problem-using-central-differencing-scheme-les-wale.html)

selvam2487 June 10, 2013 05:28

Problem using Central differencing scheme with LES WALE
 
Hello Friends,

I am trying to run a Large Eddy Simulation with WALE model for analysing thermal mixing of fluids in a T junction piping system. Currently I am performing simulations in fluid domain only.

(i) Initially I did a steady state simulation using SST model and gave its result file as input for transient simulations involving LES. I chose LES WALE model and gave Central differencing scheme for Advection scheme and Second order backward Euler scheme for transient scheme.

(ii) I chose IAPWS library for material properties (Water in this case). I selected Table Generation checkbox and gave Min. and Max. Temperature as 273.15 K and 1000 K and Min. and Max. Absolute Pressure as 1000 Pa and 1e07 Pa.

(iii) Inlet and boundary conditions of my simulation are:
Hot water: T = 393 K and mass flow rate = 600g/s
Cold water: T = 298 K and mass flow rate = 100g/s
Adiabatic wall boundary.
At outlet 'Relative Pressure' was set to zero.

No. of Nodes: 5 Million

When I run the simulation and check the trn files after few hundred iterations, the temperatures are very unphysical (218 K and 425 K in this case). Therefore, I stopped the simulation and chose Bounded Central Difference scheme this time and the same thing happens. I ran the simulations in two different meshes with aspect ratios 90 and 8 respectively and nothing changes. But when I performed the same simulatons with upwind scheme for just the energy equation, then the simulation is ok with temperature in the limits of 298 K to 393 K.

If anybody has experienced this problem before or if you know the reason why this happens and what's the mistake which I need to correct, please guide me in dealing with this problem.

Thanks in advance for your help.

ghorrocks June 10, 2013 22:19

You are certainly making it hard for yourself by running a LES model with IAPWS. When you run IAPWS you will have a much harder time converging. Do you really need IAPWS? If you can use a constant properties fluid your simulation will be much better behaved and run much faster.

selvam2487 June 11, 2013 03:03

Les wale, bcds
 
Can you please explain me why using IAPWS will not be good for the convergence of LES? My simulation involves mixing of fluids at two different temperatures (298 K and 393 K). I thought that using IAPWS library, one can know the variation of temperature dependent properties like density, viscosity etc. If using it makes the solution difficult to converge or produce unphysical results (in my case now), then can I incorporate expressions involving variation of water properties with temperature and run the simulation?

ghorrocks June 11, 2013 08:22

When material properties go non-linear then the equations get much harder to solve, even when the non-linearity is small. If you don't believe me then simply switch to a constant properties fluid and I bet it converges just fine.

You are absolutely correct in the IAPWS is the best way to make sure your water properties are correct for all conditions. But you pay a big price for that in difficulty of convergence, and to get around that you normally have to use much smaller timesteps = much longer simulation.

So the cost of the additional accuracy of IAPWS is high. So only use it when you really need it.

You can simplify it by defining simple functions for fluid properties in the conditions yo uare interested in. This is usually a lot more stable than IAPWS in my experience at least for small deviations from constant properties.

selvam2487 June 12, 2013 03:22

Dear Glenn,

I tried giving constant properties or expressions for water properties and it seems to be ok. The question which is bugging me is, "Inspite of the usage of IAPWS library being very difficult for convergence, why should it produce unphysical results when using Bounded Central Differencing Scheme for LES WALE model? But, when I switched to upwind scheme for only energy equations, then the temperature variation is within the defined limits of my simulation." Is it a problem related to my mesh or is it something else?

ghorrocks June 12, 2013 06:07

The unphysical results from IAPWS is due to it not converging, or having other numerical instabilities which cause it to result in unphysical results. To get IAPWS to work you are going to need to improve numerical stability -which means better mesh quality, smaller time steps, a better initial condition and/or double precision numerics.

selvam2487 June 20, 2013 07:28

Les wale
 
Hello Glenn,

Thank you for your reply. I am working on the numerical grid and will see how the solution behaves when I try to implement the changes you mentioned. Once again I wanted to thank you very much for your help. I will let you know how the solution behaves after my simulations are over. Take care :)


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