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-   -   Experimental results do not match with simulation results (http://www.cfd-online.com/Forums/openfoam-verification-validation/96429-experimental-results-do-not-match-simulation-results.html)

robingilbert January 23, 2012 04:00

Experimental results do not match with simulation results
 
Hello everyone,

I work on CFD simulations of data centers using OpenFoam. I am trying to validate the simulations with experimental data. I use transientSimpleFoam solver for my simulations. I place a sensor at the inlet of the cooling unit to see how the temperature varies over time. I perform the simulations using the same conditions but my results are way off. Although the steady state temperatures match, the path followed by simulations are not accurate. I am attaching the results below. The blue line is the experimental result while the red one is simulation result. Please give you suggestions to improve my simulation.
http://img14.imageshack.us/img14/4197/steadystate.jpg

gschaider January 23, 2012 12:24

Quote:

Originally Posted by robingilbert (Post 340579)
Hello everyone,

I work on CFD simulations of data centers using OpenFoam. I am trying to validate the simulations with experimental data. I use transientSimpleFoam solver for my simulations. I place a sensor at the inlet of the cooling unit to see how the temperature varies over time. I perform the simulations using the same conditions but my results are way off. Although the steady state temperatures match, the path followed by simulations are not accurate. I am attaching the results below. The blue line is the experimental result while the red one is simulation result. Please give you suggestions to improve my simulation.
http://img14.imageshack.us/img14/4197/steadystate.jpg

You don't give us very much information to work with. For instance a description of the case would be nice to see whether the solver is appropriate.

About the solver: which OF-version are you using? Because the transientSimpleFoam that I found is quite old and doesn't solve the temperature equation (it's incompressible)

robingilbert January 23, 2012 13:09

Hello Bernhard Gschaider,

Sorry about not giving much details... so here goes...

What I am trying to solve for is a data center room with so many servers in it and one cooling unit. The air flow through the servers and the cooling unit are constant and I modified the solver based on channelFoam to fix the flow rate (it works because I tested it with swak4Foam application- thanks to you!!). Now the heat added by the servers is modeled as a source term in the temperature equation. Here is the temperature equation that I solve. I added this to the transientSimpleFoam solver. I am using OF version 1.6.x.

Code:

fvScalarMatrix TEqn
        (
        fvm::ddt(T)
        + fvm::div(phi, T)
        - fvm::laplacian(DT, T)
    == Q/(rho0*Cp0)-
        );
    TEqn.solve();

Q is the source term which is active only where the servers are present. For cooling unit, the temperature follows a linear curve with respect to inlet temperature to the unit. so I fix the temperature of the air within the cooling unit for every iteration as follows:

Code:

T[cellI]= (0.21739*T[labelOfClosestCell]+225.739);
T[labelOfClosestCell] is the temperature at the inlet of cooling unit. Maybe this is where I am wrong? I am not sure about the above step that I took. Anyway, as shown in the plot above, the steady state temperatures look reasonable but the path it takes does not look good. Am I missing something? or is my set up wrong? I would like to fix this issue so please let me know if you need any more details.

Thank you so much :)

gschaider January 23, 2012 18:15

Quote:

Originally Posted by robingilbert (Post 340670)
Hello Bernhard Gschaider,

Sorry about not giving much details... so here goes...

What I am trying to solve for is a data center room with so many servers in it and one cooling unit. The air flow through the servers and the cooling unit are constant and I modified the solver based on channelFoam to fix the flow rate (it works because I tested it with swak4Foam application- thanks to you!!). Now the heat added by the servers is modeled as a source term in the temperature equation. Here is the temperature equation that I solve. I added this to the transientSimpleFoam solver. I am using OF version 1.6.x.

Code:

fvScalarMatrix TEqn
        (
        fvm::ddt(T)
        + fvm::div(phi, T)
        - fvm::laplacian(DT, T)
    == Q/(rho0*Cp0)-
        );
    TEqn.solve();

Q is the source term which is active only where the servers are present. For cooling unit, the temperature follows a linear curve with respect to inlet temperature to the unit. so I fix the temperature of the air within the cooling unit for every iteration as follows:

Code:

T[cellI]= (0.21739*T[labelOfClosestCell]+225.739);
T[labelOfClosestCell] is the temperature at the inlet of cooling unit. Maybe this is where I am wrong? I am not sure about the above step that I took. Anyway, as shown in the plot above, the steady state temperatures look reasonable but the path it takes does not look good. Am I missing something? or is my set up wrong? I would like to fix this issue so please let me know if you need any more details.

Thank you so much :)

Don't know. Fixing the temperature that way IS kinda ugly and probably messes up conservation.

Anyway. One more thing that comes to mind: have you checked the VALUE of DT (because you probably wil have "dived" rho*cp out of that too).

robingilbert January 23, 2012 18:39

Quote:

Originally Posted by gschaider (Post 340740)
Anyway. One more thing that comes to mind: have you checked the VALUE of DT (because you probably wil have "dived" rho*cp out of that too).

Yes the thermal diffusivity that I set was 0.00002 m^2/s for air.


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