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-   -   buoyantBoussinesqSimpleFoam (https://www.cfd-online.com/Forums/openfoam/95019-buoyantboussinesqsimplefoam.html)

 mikeP December 2, 2011 10:41

buoyantBoussinesqSimpleFoam

In the tutorials of buoyantBoussinesqSimpleFoam, Laminar Prandtl number is defined as 0.9 and turbulent Prandtl number is defined as 0.7.

Why is the difference? Shouldn't laminar Prandtl number for air be something around 0.7?

In Fluent, default settings for air are:
Cp: 1006.43
mu: 1.79e-5
k = 0.0242

which give Pr = 0.744

 Bernhard December 2, 2011 11:35

Does is really matter for a tutorial case?

 mikeP December 8, 2011 07:23

well not for the tutorial case, no.

I modified buoyantBoussinesqSimpleFOAM and added a heat source term into the equation.

my TEqn:

fvScalarMatrix TEqn
(
fvm::div(phi, T)
- fvm::Sp(fvc::div(phi), T)
- fvm::laplacian(kappaEff, T)
==
heatSource // heatSource is Q/(Cp*rho) - in units [K/s] where Q is [W/m3]
);

now I use a heatSource file in my 0/ directory and using setFields modify the internal values where I want my heat sources. However, the value of my heatSource input is Q/(Cp*rho). The value of heat capacity is defined in Prandtl number. That is why I asked.

Right now I assume rho = 1.225 and Cp = 1006.43 but my temperature results are not always the same as I get in Fluent.

If I define a boussinesq force - they are quite different
If I turn the boussinesq force off, then temperature results are similar
why would that be the case?

 mikeP December 9, 2011 12:56

The thing is I create a simple room with a size x,y,z= 1x1x1m and put a heatSource with a value of 1 at the cells where x=[0.2,0.4], y=[0.4,0.6], z=[0.4,0.6].

There is a flow from left to right with a velocity of (0.1,0,0) at the inlet.

so, the volumetric flow rate = 0.1 m3/s

The temperature increase at the outlet is less than I expect.

Using Q=m*c*deltaT, I expect a temperature increase of 0.08 K but I get 0.037 K (those are outlet average T - inlet average T)

There must be something wrong with my heatSource input, I guess?

Here is the domain and heatSource input: http://i.imgur.com/qDrBd.png

 Alex Lee July 27, 2013 09:50