CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

"Momentum imbalance"

Register Blogs Community New Posts Updated Threads Search

Like Tree1Likes
  • 1 Post By Gert-Jan

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   February 3, 2009, 02:28
Default "Momentum imbalance"
  #1
Nitin
Guest
 
Posts: n/a
i am doing simulation of hydraulic jump, which is similar to the tutorial 7 of Free surface flow over a bump. it is a free surface multiphase model for air and water. steady state, k-e model. i have defined ccl expressions

the purpose of hydraulic jump study is to calculate energy dissipation. i am getting momentum imbalance of some .005% for water. of course there is some mass imbalance also. i am confused about physics of fluid mechanics, how is there any momentum balance. i accept always there be an energy dissipation. but how it follows law of conservation of momentum?

thanks..
  Reply With Quote

Old   February 3, 2009, 17:22
Default Re: "Momentum imbalance"
  #2
Glenn Horrocks
Guest
 
Posts: n/a
Hi,

CFD is a numerical approximation of the Navier Stokes equations. It converges toward the exact solution (hopefully!) with increasing convergence, mesh resolution, time step resolution etc - but it never has an error of zero, there is always a small error/imbalance/residual. You just need to make sure the error/imbalance/residual is small enough such that the result it gives is accurate enough for what you are trying to do.

Glenn Horrocks
  Reply With Quote

Old   February 4, 2009, 00:40
Default Re: "Momentum imbalance"
  #3
Nitin
Guest
 
Posts: n/a
thanks Glenn, there is always imbalance of mass, momentum in flow situation. my question is that.. is any hydraulic jump situation satisfy conservation of momentum?

what factors effect the flow, other than mass and velocity? if mass at inlet equals to outlet, than velocity should be equal at both ends. isn't it? if so than how momentum conservation for dam flow situation???

thanks,,
  Reply With Quote

Old   February 4, 2009, 17:31
Default Re: "Momentum imbalance"
  #4
CycLone
Guest
 
Posts: n/a
Hi Nitin,

Your momentum imbalance is 0.005% (i.e. 1/20,000th difference between what came in and what came out), which is very close to round-off and not at all significant, so for all intents and purposes momentum has been conserved.

It is impossible to fully conserve a quantity when computing with a finite number of digits. See http://en.wikipedia.org/wiki/Round-off_error.

-CycLone
  Reply With Quote

Old   May 11, 2021, 06:51
Default
  #5
New Member
 
Bondhan
Join Date: Oct 2020
Posts: 1
Rep Power: 0
bondhanCFD is on a distinguished road
Quote:
Originally Posted by Nitin
;91768
i am doing simulation of hydraulic jump, which is similar to the tutorial 7 of Free surface flow over a bump. it is a free surface multiphase model for air and water. steady state, k-e model. i have defined ccl expressions

the purpose of hydraulic jump study is to calculate energy dissipation. i am getting momentum imbalance of some .005% for water. of course there is some mass imbalance also. i am confused about physics of fluid mechanics, how is there any momentum balance. i accept always there be an energy dissipation. but how it follows law of conservation of momentum?

thanks..
Hello, How do you check the momentum balance?
bondhanCFD is offline   Reply With Quote

Old   May 11, 2021, 07:34
Default
  #6
Senior Member
 
Gert-Jan
Join Date: Oct 2012
Location: Europe
Posts: 1,928
Rep Power: 28
Gert-Jan will become famous soon enough
- Run your calculation in the Solver Manager.
- Create a New monitor
- Give it any name you like
- Click on "Imbalance" with your Right Mouse Button
- Go for "Select All IMBALANCE Variables"
- Press OK

Alternatively.......... in the bottom of your output file, all imbalances are listed. But that is only the end result. The monitor tells you how it developed during the calculation.
aero_head likes this.
Gert-Jan is offline   Reply With Quote

Reply


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 Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Domain Imbalance HMR CFX 5 October 10, 2016 06:57
Simulation of Axial Fan Flow using A Momentum Source Subdomain Liam CFX 28 July 16, 2013 09:24
Derivation of Momentum Equation in Integral Form Demonwolf Main CFD Forum 2 October 29, 2009 20:53
momentum imbalance calculation sudhakar CFX 0 August 6, 2007 08:38
Nonstaggered Grids and Momentum Interpolations S. Wang Main CFD Forum 3 March 2, 2001 14:44


All times are GMT -4. The time now is 15:23.