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-   -   "Momentum imbalance" (https://www.cfd-online.com/Forums/cfx/26974-momentum-imbalance.html)

 Nitin February 3, 2009 02:28

"Momentum imbalance"

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..

 Glenn Horrocks February 3, 2009 17:22

Re: "Momentum imbalance"

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

 Nitin February 4, 2009 00:40

Re: "Momentum imbalance"

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,,

 CycLone February 4, 2009 17:31

Re: "Momentum imbalance"

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

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