what the purpose of "auto forced convection" in 2013?

[pei-ying.hsieh]

Dear Simulation CFD experts,

I am working on a conjugate heat transfer project using CFD 2013.  It involves natural convection, but, not forced convection (ie, no active flow such as fan or inlet velocity).  When should be the "auto forced convection" be turned on?  What does it do? 

Also, there is a "quick natural convection" switch in 2013, but, it looks like it was removed in 2014.  Under what conditions, should this be turned on?

I have tried to use wikihelp, but, it is Soooooo slow, and often it timed out.

Pei-Ying Hsieh

[nmuoio · Accepted solution]

"Auto forced convection" is, as its name implies, for forced convection.  Auto forced convection breaks up the solution into two parts.  The first part is a flow-only solution.  The solver completes a flow solution with heat transfer turned off.  When the flow solution is complete the solver turns flow off and turns on heat transfer.  The solver then completes a heat transfer solution, based on the flow solution.  This can be quicker because it is not calculating temperatures at each iteration.  It is valid when the flow field is not significantly influenced by the expected temperature variation.

In most of my forced convection simulations I use the same strategy, but I do it manually.  In many of my simulations the flow solution may take a few hundred iterations to converge, but the heat transfer solution usually converges after only a few dozen iterations.  I typically run a flow solution to convergence and then clone the scenario.  The cloned scenarios are then switched to heat transfer, and I can quickly run through various heat transfer scenarios using the same flow field.

I am not familiar with "quick natural convection" but, as you stated, it has been removed in 2014.  It would appear that Autodesk feels that there is no longer an advantage for it to ever be turned on.

For natural convection I would recommend you start by:

1) make sure the environment is set to "variable" in the fluid material properties.  This allows density to change with temperature.

2)  Set pressure=0 to all inlet and outlet boundaries and specify an inlet temperature.

3)  Make sure both "flow" and "heat" are turned on, as they are dependent on each other.

4)  Make sure Gravity Method is set to "earth", and specify the gravity direction.

Any other tips would be dependent on your specific application.

The help file, that was placed on your local computer during the software installation, has a lot of useful information.  I have found it to be far more useful than wikihelp.  I would recommend you open the help file and browse the "Analysis Guidelines" and "Application Best Practices" sections.  You'll find strategies and tips that should help with your natural convection simulation.

[Jon.Wilde]

Good response.

I would also add that for natural convection you need to have air set to variable.

There is a nice guide for this here. Lots of infomation on domain sizing, boundary conditions and solver settings.

Also be aware that you can get to the relevant help page at any time by pressing the blue '?' symbol.

[pei-ying.hsieh]

Thanks for the detailed explanation!

Pei-Ying