CFD Online Logo CFD Online URL
Home > Forums > Software User Forums > OpenFOAM > OpenFOAM Running, Solving & CFD

buoyantPimpleFoam General Assumptions (thermophysical model, energy equation, etc)

Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Search this Thread Display Modes
Old   October 7, 2017, 03:37
Default buoyantPimpleFoam General Assumptions (thermophysical model, energy equation, etc)
New Member
Join Date: Oct 2017
Posts: 12
Rep Power: 7
AMK53 is on a distinguished road
I am trying to use buoyantPimpleFoam to simulate Rayleigh-Taylor instability of a gas (eventually a reacting simulation). I was looking through the source files and I had some general questions on the assumptions that the solver takes, and hope that someone can shed some light on it for me.

1. The energy equation is states as
fvScalarMatrix EEqn    (        fvm::ddt(rho, he) + fvm::div(phi, he)      + fvc::ddt(rho, K) + fvc::div(phi, K)      + (   == "e"          ? fvc::div            (                fvc::absolute(phi/fvc::interpolate(rho), U),                p,                "div(phiv,p)"            )          : -dpdt        )      - fvm::laplacian(turbulence->alphaEff(), he)     ==        rho*(U&g)      + radiation->Sh(thermo, he)      + fvOptions(rho, he)    );
All the terms look correct (though it is missing mechanical stress term), but the thermal diffusion term seems a bit odd to me. If we use Fourier's law of heat diffusion, we would have:

\nabla \cdot (-k\nabla T) = \nabla \cdot (-\frac{k}{C_p} [\nabla(C_pT) - T \nabla C_p] = \nabla \cdot (-\alpha [\nabla h - T \nabla C_p])

But in the solver energy equation, this is simply states as \nabla \cdot (-\alpha \nabla h). Obviously this does not matter for a thermally perfect gas as the specific heat is constant, but for gases with varying specific heats, there should be a gradient there. I have never seen the form used in buoyantPimpleFoam. Is this additional gradient term simply just ignored because it is negligible?

2. The divergence of gradients in OpenFoam (e.g. \nabla \cdot (\mu (\nabla U + \nabla U^T)) or \nabla \cdot (\alpha \nabla h)) are simply written as laplacians, i.e. laplacian(mu,D(U)). Again, this is only true if the transport properties do not vary. This is definitely not the case for reacting flows. Is this simply ignored in OpenFoam calculations or is this handled behind the scenes?

3. The thermophysical properties for gases seem to me to be overdefined. For example, a sample thermophysical model could be:

    type            heRhoThermo;  //Thermophysical Model Based on h/e and rho
    mixture         pureMixture; // Passive Gas Mixtures
    transport       const; // Constant transport properties (mu, Pr, etc.)
    thermo          hConst; // Constant Cp and heat of fusion
    equationOfState perfectGas; // Ideal Gas Law
    specie          specie;
    energy          sensibleEnthalpy; // Solve Energy Eqn. In Terms of Enthalpy
From my understanding, heRhoThermo calculates the enthalpy and density, but the density and the enthalpy are already defined by the perfect gas and 'hConst' assumptions. What additional things is the thermophysical type doing to calculate these? Is the type even necessary at this point?

4. Why is P_{rgh} \equiv P - \rho g \cdot x read from the initial conditions and then re-defined, i.e. in createFields.H

#include "readGravitationalAcceleration.H"
#include "readhRef.H"
#include "gh.H"

Info<< "Reading field p_rgh\n" << endl;
volScalarField p_rgh

// Force p_rgh to be consistent with p
p_rgh = p - rho*gh;
If you've correctly defined Prgh, then it should be consistent already. I'm not quite sure what they are doing here.

Thank you in advance.
AMK53 is offline   Reply With Quote

Old   October 23, 2018, 05:30
Senior Member
Jianrui Zeng
Join Date: May 2018
Location: China
Posts: 157
Rep Power: 7
calf.Z is on a distinguished road
Hello,I am also confused about these questions above.

Have you found the answer of it?

Any idea will be appreciated.
calf.Z is offline   Reply With Quote


assumptions, buoyantpimplefoam, consistency, laplacian operator

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
ATTENTION! Reliability problems in CFX 5.7 Joseph CFX 14 April 20, 2010 15:45
Viscosity and the Energy Equation Rich Main CFD Forum 0 December 16, 2009 14:01
Reynolds transport, turbulence model, etc Beginner Main CFD Forum 1 January 7, 2009 05:36
SIMPLE and energy equation convergence Fabio Main CFD Forum 0 June 1, 2007 06:06
Two-Phase Buoyant Flow Issue Miguel Baritto CFX 4 August 31, 2006 12:02

All times are GMT -4. The time now is 01:43.