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May 2, 2005, 08:20 
Fluid Structure Interaction. Tut 20 like Problem

#1 
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I have been trying to simulate geometry with Moving Wall boundary condition, the equations are similar like the FSI Tutorial 20 in CFX. the Problem which i am facing is that i cannot model these equation using Expression editor because you cant define variable dependent on Time, i was wondering if some one has developed a Fortran Routine for case like tutorial 20. For reference i can post these equation too. F1=known; Fopp=0 (First Time Step); V1=Known; FNEW=F1; V2=(FNEWFopp)*DeltaT/mball; FNEW=F1Fopp; Unfortunetly i dont have any idea with Fortran in CFX other wise these equation wouldnt be a problem to program it........ What i feel Thanks Aziz 

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May 2, 2005, 13:36 
Re: Fluid Structure Interaction. Tut 20 like Probl

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Hi Aziz,
The FSI tutorial demonstrates how simple FSI cases involving solid body dynamics w/o deformation can be solved in CFX. If you want to simulate structural deformations, you'll need to process loads on a perelement basis, rather than the perbody or perboudary condition basis that is used in the tutorial. This will require extensive UserCEL or User FORTRAN... or you could order the licenses that are required to take advantage of the new twoway 'MFX' coupling between CFX and ANSYS in their upcoming release, 10.0. What sort of application do you have in mind? Regards, SteveR 

May 2, 2005, 18:54 
Re: Fluid Structure Interaction. Tut 20 like Probl

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Hi Aziz,
Can you clarify what you mean by "the Problem which i am facing is that i cannot model these equation using Expression editor because you cant define variable dependent on Time"? Do you mean the motion is a function of the flow and other parameters and therefore the simulation determines the motion; or do you do know the motion of the object but you cannot get CFX to use an expression as a function of time? I say this as you can define mesh deformation motions as functions of time, and if it does not work for you then you have done something wrong. Just checking. Regards, Glenn 

May 3, 2005, 04:08 
Re: Fluid Structure Interaction. Tut 20 like Probl

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Hi SteveR,
You are right, the best way to model Fluid Structure together would be to use 2 way coupling with CFX and ANSYS together, but to know the basic phenonmenon i need to do it as simple as possible. My requirement is to simple model Impulse Equation and for that i know the initial velocity with that a ball is moved inside let say Water and produces a local increase in pressure which tends to deccelerate the ball and now ball moves with new velocity untill it is stoped and moves backward. Thats what i only need to model. With equation editor it is not easy as you cant store the velocity from the last time step or we can i dont know exactly...... This is equation which i need to model V1=VoDeltaF*DeltaT/mass So here i need Vo from last time step and after calculating V1 i need to store it again as Vo. Do you know i have model it on Excel sheet by my own but with CFX it seems that i must use Fortran which i dont really want to go in. Thanks Aziz 

May 3, 2005, 04:29 
Re: Fluid Structure Interaction. Tut 20 like Probl

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Hi Glenn,
Actually if you need the values from last time to be used for calculating new values you cant store them using Expression editor. Have a look on the equation perhaps you will understand better V1=VoDeltaF*DeltaT/mass So here i need Vo from last time step and after calculating V1 i need to store it again as Vo. Do you know i have model it on Excel sheet by my own but with CFX it seems that i must use Fortran which i dont really want to go in. If it would have possible using Expression editor that you could define variables as a function of time then things were very easy for me. thanks Aziz 

May 3, 2005, 08:42 
Re: Fluid Structure Interaction. Tut 20 like Probl

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Hello Aziz,
In the documented tutorial, there is a CEL expression something like velBallOld=areaAve(Mesh Velocity v)@Ball. I suspect that this is what you are looking for... Mesh updates/motion occurs as a preprocessing step at the start of a time step, and the areaAve call back is last evaluated at the end of the previous time step  This, and other callbacks, are *not* updated during mesh motion; They are only updated during the coefficient iterations within the timestep. Thus, during the mesh update at time n+1, the value of velBallOld is constant, and corresponds to the mesh velocity at time n. In your equation, V0 would be evaluated as velBallOld above, and V1 would be evaluated as V1=(disp1disp0)/dt where disp1 is the displacement you apply to the mesh at time n+1 and disp 0 is evaluated using another areaAve callback. Hope it helps... SteveR 

May 4, 2005, 09:34 
Re: Fluid Structure Interaction. Tut 20 like Probl

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Thanks SteveR for your reply, i have tried your way but still not succeed, infact i know intial velocity Vo and V1 can be calculate using Vo but the problem is now to store new V1 into Vo as Vo is already been defined as initial values.
Regards, Aziz 

May 4, 2005, 19:24 
Re: Fluid Structure Interaction. Tut 20 like Probl

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Hi,
There is no way of storing a variable for use in the next timestep in CEL. Steve's described method works fine and I have used it many times. Glenn Horrocks 

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