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Ben January 12, 2008 21:35

Question on Unsteady Flow
 
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


Glenn Horrocks January 13, 2008 18:35

Re: Question on Unsteady Flow
 
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

Ben January 15, 2008 14:28

Re: Question on Unsteady Flow
 
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.

Glenn Horrocks January 15, 2008 17:53

Re: Question on Unsteady Flow
 
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

Ben January 16, 2008 19:50

Re: Question on Unsteady Flow
 
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

Glenn Horrocks January 17, 2008 18:04

Re: Question on Unsteady Flow
 
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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