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Old   March 7, 2011, 05:01
Default Student need help; transient CFD.
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Dennis Hamborg
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Hi. I am a student at the faculty of engineering at the university of southern Denmark.

I have been briefly introduced to Ansys workbench 12.1. I know how to make “steady state” CFD analysis but I need to make a transient analysis. I have a tube with some “tunnels” leading to different arias with “outlets”, “openings” and a "walls" as domains also I have an "inlet" domain. I need to make the inlet transient. So that a pressure of 6 bars is let in, in a time periode of 0,2 sek. Also I would like to see how the air moves after the valve has closed. I am estimating a total run time of about 2 sek.

What I have figured out so far: I know how to change from “steady state to “transient” when I do that two error messages turn up, The first message I get is:

“In Analysis 'Flow Analysis 1' - Domain 'Default Domain': Transient analyses require that initial conditions are specified unless an Initial Values file is specified at run-time.”

The second message is that ansys needs an output file. I know how to set up the output by using the “output control”

So my questions is:

1 - How do I get rid of the first error message?

2 – How do I allow pressure to enter for 0,2 seconds and let the analysis run for 2 seconds?

Hope someone in here is able to help me out on this.

Thanks in advance.
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Old   March 7, 2011, 06:53
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In the case of a steady simulation it does not matter whether you initialize your solution or not it only affects the time it takes for convergence.

However in case of a transient case the solution progresses on the basis of the initial conditions that you provide, so it "neccessary" to provide the intial conditions, otherwise ANSYS is lost since there is no place for it to start frm.
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Old   March 7, 2011, 07:22
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1. in the domain initialisation you should define the initial parameters with exact values for every domain, except you continue your solution by interpolating from a steady state result

2. define an expression for pressure which is using a 1D interpolating user function. In user function you can define the pressure as function of time, look in the help. Maybe there are easier solution but it works.

On the other hand, by setting the inlet boundary as a wall by setting pressure to zero there can cause convergence difficulties. I've made simulation like this, however in my case the massflow became 0 at the outlet. It converged well, but I don't now how precise it numerically.
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Old   March 9, 2011, 05:15
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Thank you for your replys it is greatly apreaciated.

I have got a better understanding of the transient analysis now and I have got it to work with a constant pressure.

However I have tried to make a function of pressure according to time. I use the function in the expression on time like this:

“expression 1(t)”.

I use the expression as the pressure on the inlet. This works fine for steady state but when I try to use it on transient analysis it stops with an error.

When I try to change the pressure on the inlet to a number instead of the expression the transient analysis works fine. So my guess is that something is wrong with the expression?

Problem solved. Once again thanks for your help guys..

Last edited by Deham08; March 9, 2011 at 06:16.
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Old   September 10, 2011, 01:42
Default rigid body motion problem (^ DOF)
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Hi all,
In continuation of earlier discussion, I am also facing the same problem of
"In Analysis 'Flow Analysis 1' - Domain : Transient analyses require that initial conditions are specified unless an Initial Values file is specified at run-time."
The situation is like, I am simulating a rigid body 6DOF turbine problem. its an transient case. turbine runner has to rotate on there own axis as the water strikes to the runner blades.
Look in to the image for better understanding.
I have three domains, two stationary and one rotating. these two domain is not conected to each other.
one stationary domain is the domain, which will carry the water and will be the portion inside the runner blade.
I am getting the above error on that domain.
I have not run the problem with steady state.
What should I do to run the problem.
I have set all rigid body options to runner.

Please guide.

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Old   September 12, 2011, 20:07
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Are you looking for the steady state performance of this device, or are you interested in the transient response (ie speed up and slow down)?
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cfd, initial conditions, transient

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