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-   -   boundary layer (http://www.cfd-online.com/Forums/fluent/125329-boundary-layer.html)

Arvin October 23, 2013 09:10

boundary layer
 
Hey guys, I am really stuck here. I am new to fluent and I am modelling a steady state 2d tank for natural convection problem. Size of the tank being 100 by 100 mm. I have attached a boundary layer as well as face mesh. Total no. of nodes is 1000000. Do I need to check internal continuity?. Despite many attempts, I am not able to get converged solution, not even the plumes from bottom surface. My boundary conditions are 1.Heat flux is supplied at bottom plate. 2.Other three sides are kept at ambient temperature. Working fluid is water. Please do help me out. I want to know the settings that needs to be applied in fluent. Boussinesq or Polynomial for water? Standard or PRESTO? Any other important things to be set?
Thanks. I do hope I get help as soon as possible as I am running out of time.

delaneyluke October 24, 2013 09:38

Could you be more specific about what's not converging.
What have you tried so far to improve convergence?
Natural convection is an unsteady phenomenon, have you tried a transient analysis?

Regards
Luke

Arvin October 24, 2013 09:55

No, I have not yet tried transient analysis since I am not getting convergence on steady state itself. I have a strong doubt on using Boussinesq approximation. Can you guide me on this?
Thanks.

delaneyluke October 28, 2013 15:11

Well the reason you dont get convergence with a steady state solver is because the phenomenon is transient. A transient phenomenon will not converge with a steady state solver.
Whats your question about the Boussinesq approx

Regards
Luke

Arvin October 29, 2013 05:42

Hi, thanks Luke. My question is obviously we need to specify density as a function of temperature. And we can do that either by specifying density as Boussinesq approx or by using polynomial and other piecewise functions. But for natural convection problem, generally Boussinesq approx is used. Why is that? It can be used in phenomena involving small temperature changes, so how do we know that? So, how to select the appropriate function of density?
Secondly what is the difference between pressure based solver and density based solver? For natural convection problems why can't we use density based solver?
Thanks.


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