alphaEff
 

Hello foamers,
I would like to find the difinition of alphaEff() wich is used in hEqn.H in following form :

fvm::laplacian(turb.alphaEff(), h)


Does anyone know where "alphaEff()" is defined?:confused:

Hi,

I guess, turb in your message is compressible turbulent model, so for RAS alphaEff is defined as

for LES it is

and finally thermo.alphaEff() is http://foam.sourceforge.net/docs/cpp...b7cebc3adf4b1d

It took me a very long time to figure this one out. The reason why it took me (and probably you) so long is because of the complicated structure of openFoam's thermophysical classes.

As alexeym indicates, no matter which type of turbulence model you choose (RAS or LES), the turbulence model will have a call to a thermophysical model. Searching through the various files, i figured out that in

/src/thermophysicalModels/basic/heThermo/heThermo.C

you will find a line that says something along the lines that

alphaEff = CpByCpv* (alpha + alphat)

I haven't found any other thermophysical model that also says this (if anyone discovers another one like this, please post a reply!!)

Note: alpha is defined in the

/src/thermophysicalModels/basic/basicThermo/basicThermo.H

as having the units kg/m/s. This probably looks strange to you knowing that thermal conductivity (kappa) usually has units of kg*m/s^3/k, where k is the degrees in kelvin.

In effect, alpha is known as the thermal diffusivity for the enthalpy of a mixture, where as kappa is the thermal diffusivity for temperature. The relation between the two is that

kappa = alpha * Cp,

where Cp is the specific heat capacity at constant pressure.

You may also notice that the definition of alphaEff above is also scaled by a factor of CpByCpv. CpByCpv = Cp/Cpv, where Cpv is the specific heat capacity at constant pressure/volume.

Depending on which energy model you use, e.g. sensibleEnthalpy, then

cpbycpv = 1

since it is assumed that cp=cpv.

If you use sensibleInternalEnergy, then

cpbycpv = Cp/Cv,

where cv is the specific heat capacity at constant volume.

The different types of energy models can be found in the directory

src/thermophysicalModels/specie/thermo

However, keep in mind that some solvers limit which of these energy models that you can use.

Did you mean, alphaEff is known as the thermal diffusivity for the enthalpy of a mixture, where as kappaEff is the thermal diffusivity for temperature?

You mean that kappa is the thermal conductivity, right?

Dear Paritosh,

kappa is thermal diffusivity in OpenFOAM, isn't it correct? And I think it is Alpha / Cp as OpenFOAM SIMPLE solvers neglect the rho.

Dear alexy,
The link has been corrupted. Please include a new one, as your description is insightful.