Hi Guys,

I'm writing some code to use in a commercial software which models a wall in a manner similar to the vof method. Thus the wall can move through the domain - I won't go into the details...

Anyway, I'm most interested in the heat and mass transfer at the wall, however due to computational limiations I expect the near wall control volume to be at least several centimetres from the wall.

What I'd like to know is how best to model the heat and mass transfer to the wall.

It appears to me that the standard wall function approach still requires fine meshes near the wall for accurate results. Has anybody got any experience with using this approach but with large cell sizes near the wall?? What is the penalty in terms of accuracy etc?

Another method, used in chem eng a lot (but not cfd) is to use some sort of correlation, since the first cell could very well be in the bulk flow region. The problem with correlations is picking a good one, and that they only span a limited range of conditions.

So, I'm just wondering if anybody has come across this type of issue before. And what if any suggestions you have!

I know the ideal is to resolve the b/l at the wall etc., but this won't be possible in this particular model for various reasons, so I'm looking for another way. I don't need absolute accuracy but I would like to be able to capture the heat and mass transfers over a reasonable range of conditions....

Thanks Greg

It is difficult to give any advice without knowing what kind of flow you expect at the wall - is it an attached flow with a developed boundary layer? turbulent? impinging flow? separated flow? Or can you perhaps expect all kind of flows?

From what you say it sounds as if your best option is to use a correlation. Wall functions on a very coarse grid, perhaps even on a grid with varying grid-density, and with unknown and varying flow conditions does not sound very good to me.

Thanks Jonas,

I'm currently using correlations and having had a bit of a look at the wall functions stuff recently I agree with you.

Actually I expect a range of flow conditions but mainly some sort of natural convection at the wall.

To help me understand this I've decided to do some detailed runs using a model which resolves the b/l to help justify my correlation assumptions on the coarse model.

Thanks

Greg

For natural convection you are certainly much better off using a correlation - any good heat transfer book, like Incropera & Dewitt, should have a number of correlations for horisontal or vertical walls. A search for "natural convection correlation" on google gives several nice examples. Maya Heat Transfer Technologies also has a few online calculators for natural convection - Maya's tools are linked from CFD Online in the Resources/Online Calculators/Heat Transfer section.

Thanks alot Jonas

Greg