# why downstream height is defined?

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 August 28, 2008, 06:15 why downstream height is defined? #1 NITIN DEWANGAN Guest   Posts: n/a hi, can anybody tell why we are using downstream height downH in the tutorial problem no 7 of flow over free surface bump? i am confused, what will happen if we remove that expression. is there any effect on the outflow side?

 August 28, 2008, 18:25 Re: why downstream height is defined? #2 MAB Guest   Posts: n/a For an outlet boundary condition , in CFX , you have to specify volume fractions for each fluid . You can avoid this specification by changing it to an opening with a sepcified pressure. or in outlet change volume fractions to zero gradient.

 August 28, 2008, 23:37 Re: why downstream height is defined? #3 NITIN DEWANGAN Guest   Posts: n/a thank MAB, this is correct, ifwe take some pressure, the result would be same , but in my case, there is a twist in the problem: i am continuing this problem by looking hydraulic jump after and over the bump. i simply take static pressure value at outlet, the results differ from the previous cases when i used downstream height. i just want to know, what is the fluid mechanics fundamental involve with this outflow boundary condition? thanks.

 August 28, 2008, 23:37 Re: why downstream height is defined? #4 NITIN DEWANGAN Guest   Posts: n/a thank MAB, this is correct, if we take some pressure, the result would be same , but in my case, there is a twist in the problem: i am continuing this problem by looking hydraulic jump after and over the bump. i simply take static pressure value at outlet, the results differ from the previous cases when i used downstream height. i just want to know, what is the fluid mechanics fundamental involve with this outflow boundary condition? thanks.

 August 29, 2008, 02:41 Re: why downstream height is defined? #5 NITIN DEWANGAN Guest   Posts: n/a with this post, i want to ask one more thing is that. if downstream height is defined, what should be its height? how it is considered??

 August 29, 2008, 07:28 Re: why downstream height is defined? #6 MAB Guest   Posts: n/a I think they used a lower height at downstream as compared to the one at inflow boudary. The reason is that water level would decrease gradually as the water goes out of the tank . so the downH is the rough estimate of this lower level. You can even keep the level same as at inflow boundary.

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