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Changing the solver (from rhoSimpleFoam to rhoCentralFoam)

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Old   April 12, 2017, 09:47
Default Changing the solver (from rhoSimpleFoam to rhoCentralFoam)
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I want to simulate the pressure pulsation effect in Intake manifolds of IC engine. For that
I started my simulation with rhoSimpleFoam with massflow rate at outlet and total Pressure at inlet. I reached with steady state results from rhoSimpleFoam.
Now I want to change the boundary condition at outlet to zero (0) massflow rate and want to achieve the pressure pulsation effect. my new BC is like this:
        type            flowRateInletVelocity;
        rhoInlet        1.1792;
         massFlowRate    table
        (0   -3.2793e-4)// mass flow at the beginning
        (0.0005  0) // flow reduced to zero in 0.5 milliseconds
        (1 0)    

But I am facing a strange problem that my flowrate at a patch starts decreasing before the time 0.0005 s.
To me, the mass flow should remain constant to -3.2793e-4 kg/s before the time 0.5 milliseconds as the steady state has already been reached using rhoSimpleFoam.

I also made the same simulation using rhoPimpleFoam just to monitor massflow rate and I observed that it steady before 0.5 ms.

Kindly, suggest me something?

Last edited by Tobi; April 12, 2017 at 10:14. Reason: Code Tags Added
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Old   April 12, 2017, 10:10
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If I remember correctly, we make a linear interpolation inbetween the table:

massFlowRate    table
        (0   -3.2793e-4)// mass flow at the beginning
        (0.0005 -3.2793e-4) // mass constant till that time
        (0.001 0) // mass decrease in the next 0.5 ms
        (1 0)    
Try that one and give feedback.
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Old   April 13, 2017, 05:51
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Hello Tobi, thanks for the answer. I think it is okay, because i want a linear reduction in flow rate to zero in 0.5 milliseconds and zero flow after that. And i have tested it with rhopimplefoam.

My objective is use the steady state results from rhosimplefoam and then produce the unsteady effects due to reduction in mass flow with unsteady solver.
So, i want my unsteady solver to reach first the steady state (steady mass flow) using the rhosimplefoam results. I know that in beginning there will be some oscillations and the flow will stabilize quickly.
So , I specified the following:
massFlowRate table
(0 -3.2793e-4)
(1 -3.2793e-4)

And I observed that the mass flow rate gets stable after some time if I make the transition from rhoSimpleFoam to rhoPimplefoam (see the pic).But it is not the case with the rhocentralfoam, the solver i want to use.
I think, it is because of the fact that when I compare the the createfields.h for rhoSimpleFoam and rhoPimpleFoam they create the same fields.
But rhoCentraleFoam, has some extra fields like rhoE, rhoU, pos in its createfields.h. So, when I impose the steady-state results from rhosimplefoam to rhocentralfoam there are just like an initial guess to it. And that is why the flow does not stabilize quickly.
So, may be if I can modify rhoSimpleFoam to create the extra fields like rhoE, rhoU, pos. It can work. What is your opinion?

Last edited by DeepIndia; April 13, 2017 at 08:07.
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