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Old   January 12, 2008, 21:35
Default Question on Unsteady Flow
  #1
Ben
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Hi all, I have question on unsteady flow. I have the velocity of the flow at the inlet around 500m/s. The simulation is for total time of t=5s. During the simulation the flow conditions change, the flow at the inlet will have an acceleration, The qtion is how to set an acceleration condition to the flow in CFX.

Any help will be very much appreciated. Ben

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Old   January 13, 2008, 18:35
Default Re: Question on Unsteady Flow
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Glenn Horrocks
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Hi,

Set the inlet velocity as a CEL expression and define a CEL expression as a function of time. Then you can ramp the flow up against time.

Regards, Glenn Horrocks
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Old   January 15, 2008, 14:28
Default Re: Question on Unsteady Flow
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Ben
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Hi Glenn,

Thank you so much for your help, I did tried to use CEL, but I couldn't make it work, cuz i do not have experience using CEL. Could you please give help, if possible an exemple. and how to set an acceleration = 2500 m/s^2 over t = 0,23ms.

thank you Glenn.
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Old   January 15, 2008, 17:53
Default Re: Question on Unsteady Flow
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Glenn Horrocks
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Hi,

In CFX the inlet flow can be defined as a velocity. Therefore V=initial velocity+acceleration*time, and use this V as the inlet velocity.

For how to get CFX to use CEL expressions for boundary conditions look in the tutorial examples.

Glenn Horrocks
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Old   January 16, 2008, 19:50
Default Re: Question on Unsteady Flow
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Ben
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Hi Glenn,

But this does not solve the problem, cuz I need to have at t= 0 s V_inlet= 1000 m/s for exemple, and at, t+ dt the flow start having an acceleration, over total time 0.23ms so I think using simply the formulat u mentioned it doesnt solve this problem ( v= v0 + a*t = (1000+270000*0.23x10^-3)= 1057. so instead to have 1000m/s at the inlet , it will be the new value 1057 m/s which is wrong. It should be there is a way where I can use time step instead to use the totale time.

and I'm note sure how to do that in CFX.

Thank you Gleen
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Old   January 17, 2008, 18:04
Default Re: Question on Unsteady Flow
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Glenn Horrocks
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Hi,

Timestep size is a variable available in CFX. Look in the CFX documentation reference manual for CEL. It explains the available variables.

Glenn Horrocks
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