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why downstream height is defined?

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Old   August 28, 2008, 06:15
Default why downstream height is defined?
  #1
NITIN DEWANGAN
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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?
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Old   August 28, 2008, 18:25
Default Re: why downstream height is defined?
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MAB
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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.
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Old   August 28, 2008, 23:37
Default Re: why downstream height is defined?
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NITIN DEWANGAN
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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.
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Old   August 28, 2008, 23:37
Default Re: why downstream height is defined?
  #4
NITIN DEWANGAN
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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.
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Old   August 29, 2008, 02:41
Default Re: why downstream height is defined?
  #5
NITIN DEWANGAN
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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??
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Old   August 29, 2008, 07:28
Default Re: why downstream height is defined?
  #6
MAB
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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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