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October 18, 2017, 21:37 
need help with natural convection problem

#1 
New Member
heesang Yoo
Join Date: Jul 2017
Posts: 13
Rep Power: 4 
hello everyone
I am having trouble with solving simple natural convection problem. The problem is that momentum residual is so high and do not go down... I attached my design image... the red one is heat source so maybe there has to be movement of inner gas, circulation around heat source design.jpg When I set laminar model and set initial velocity as 0m/s momentum residual was as below momentum residual.png and velocity profile was very dirty... for example region that has to be have downstream had upstream and so on... so i ask for colleague how to reduce momentum residual he said that I have to change turbulence model or set initial velocity as twice of my goal velocity so I set ke model (because it's residual was lower than sst) and set initial velocity as 0.1m/s. then, momentum and turbulence residual were as below momentum residual2.jpg turbulence residual2.jpg velocity profile was clearer than before but, still momentum residual has to be lower and velocity profile was still incorrect. is there somebody know well about natural convection? and is there other factors to lower momentum residual and make clear velocity profile?? thank u! 

October 18, 2017, 22:54 

#2  
Senior Member
Lane Carasik
Join Date: Aug 2014
Posts: 693
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Quote:
How are you accounting for density changes? Is gravity set in the correct direction? Are you properly resolving the highest gradients in your flow? 

October 20, 2017, 02:14 

#3  
New Member
heesang Yoo
Join Date: Jul 2017
Posts: 13
Rep Power: 4 
Quote:
thanks for reply. It is natural convection so, I couldn't know whether the flow is turbulent or laminar. So, I assumed the flow as laminar flow but, its reynold's number was far larger than 2300 so, I set the flow as turbulent flow. i made fluid properties (conductivity,viscosity, density, heat capacity) as function of temperature. also, I set gravity as 9.8m/s^2 and i can't understand ur last sentence... resolve highest gradient of flow?? what's that mean? now I'm trying various trials to make residual lower. such as increasing heat transfer coefficient, make pressure loss 0 in porous media...etc... but, I can't lower momentum residual more below than 1e2... 

October 20, 2017, 02:16 

#4  
Senior Member
Lane Carasik
Join Date: Aug 2014
Posts: 693
Rep Power: 10 
Quote:


October 20, 2017, 03:01 

#5 
New Member
heesang Yoo
Join Date: Jul 2017
Posts: 13
Rep Power: 4 

Tags 
ansys, cfd, cfx, momentum residual, natural convection 
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