Natural Convection ! Boussinesq Vs Ideal Gas density

 User Name Remember Me Password
 Register Blogs Members List Search Today's Posts Mark Forums Read

 LinkBack Thread Tools Search this Thread Display Modes
 December 16, 2018, 04:36 Natural Convection ! Boussinesq Vs Ideal Gas density #1 New Member   Join Date: Oct 2018 Posts: 24 Rep Power: 7 Hi Everyone! I am simulating cooling of a component by natural convection. The component is placed in rectangular box at middle location with rectangular big box inlet at bottom and outlet at top. What should be density selections in create/edit material tab (ideal gas or Boussinesq)? Why operating density should be zero in case of ideal gas? I run simulation for above two types of air density keeping gravity y=-9.81m/s (inlet is at y=0, outlet is at y>0) For density as ideal gas , flow of air is downward (towards inlet) For density as Boussinesq, flow of air is upward (towards outlet) Avg velocity at inlet or outlet for ideal gas density is 1.5m/s and that of Boussinesq density its 0.3m/s (which seems okay for natural convection) Can anyone please explain why there is difference in direction of velocity flow by just varying density of air in material tab? Thank you in advance. Sai Krishna likes this.

 December 16, 2018, 23:02 #2 New Member   Join Date: Oct 2018 Posts: 24 Rep Power: 7 Hope someone take few minutes to reply As of now I understood that density of fluid should NOT be selected as "Ideal Gas" In 'create/material' tab of Fluent for an OPEN system. For a CLOSED system with no inlet or outlet it will work.

 December 18, 2018, 09:18 #3 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,703 Rep Power: 66 Ideal gas can work for an open system... You are just doing something wrong. With Boussinesq you do not need to specify any operating density. With ideal gas, you need to specified an operating density. Pressures in Fluent are a little funny and some bookkeeping is needed. You specify not the actual static pressure but with the hydrostatic part removed. Some people like to set the operating density to 0 so that they can set the static pressure on the boundary (just like setting 0 Pa operating pressure lets you specify absolute pressure instead of a gauge pressure). So my question is, what are your boundary conditions? Don't think they are the same when "all I did was change density."

December 18, 2018, 10:14
#4
New Member

Join Date: Oct 2018
Posts: 24
Rep Power: 7
Quote:
 Originally Posted by LuckyTran Ideal gas can work for an open system... You are just doing something wrong. With Boussinesq you do not need to specify any operating density. With ideal gas, you need to specified an operating density. Pressures in Fluent are a little funny and some bookkeeping is needed. You specify not the actual static pressure but with the hydrostatic part removed. Some people like to set the operating density to 0 so that they can set the static pressure on the boundary (just like setting 0 Pa operating pressure lets you specify absolute pressure instead of a gauge pressure). So my question is, what are your boundary conditions? Don't think they are the same when "all I did was change density."

Thank you so much for reply.

With Boussinesq i did not specified operating density. For ideal gas, I tried specifying zero and non-zero density as well.

My boundary conditions are 'pressure inlet=0 pa' 'pressure outlet= 0' remaining 4 side of big box are adiabatic and some heat source term to the component cell zone. operating pressure= default (101325 pa)
All these BC are same for both cases.

For ideal gas as density case,
Changing "type" under boundary conditions from 'pressure inlet' and 'pressure outlet' to 'Wall' gives me flow upward.

September 15, 2020, 08:46
#5
Member

Sai Krishna
Join Date: May 2018
Posts: 37
Rep Power: 8
Quote:
 Originally Posted by cfd_user_pune Thank you so much for reply. With Boussinesq i did not specified operating density. For ideal gas, I tried specifying zero and non-zero density as well. My boundary conditions are 'pressure inlet=0 pa' 'pressure outlet= 0' remaining 4 side of big box are adiabatic and some heat source term to the component cell zone. operating pressure= default (101325 pa) All these BC are same for both cases. For ideal gas as density case, Changing "type" under boundary conditions from 'pressure inlet' and 'pressure outlet' to 'Wall' gives me flow upward.
Hi,
Have you found any solution, because iam also facing the same issue. Iam also trying to model natural convection around a pcb exposed to ambient conditions. when i used ideal gas flow vectors are drooping downwards. I expected they should move upwards and leave from outlet.
my model consists of inlet on LHS and outlet on RHS with rest as walls.

I got flow as per physical intuition with boussinesq model. but in that i got temp change >75degrees. so i cant conclude them as final results.

Any help would be greatly appreciated.

Thanks in advance.

 Thread Tools Search this Thread Search this Thread: Advanced Search Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post kevinmccartin CFX 12 October 13, 2022 21:43 Badi CFX 8 April 10, 2018 05:52 rahul62 OpenFOAM 0 February 21, 2018 05:48 Jervds CFX 6 October 2, 2016 02:53 Dan Moskal Main CFD Forum 0 October 24, 2002 22:02

All times are GMT -4. The time now is 23:45.

 Contact Us - CFD Online - Privacy Statement - Top