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-   -   How do I specify a decrease in pressure? (http://www.cfd-online.com/Forums/cfx/104461-how-do-i-specify-decrease-pressure.html)

Somebody July 10, 2012 07:45

How do I specify a decrease in pressure?
 
I am trying to simulate a turbine in a pipe simply with a decrease in pressure - but I am unsure how to do this. Some help would be greatly appreciated.
Thank you.

ghorrocks July 10, 2012 19:22

You mean you want to replace a turbine with a pressure drop versus flow rate? This can be done with an interface or a source term. You can choose the best approach for your needs.

Somebody July 11, 2012 00:49

Thanks. Could you give me a few more details on how I can do that? I am quite a beginner with ANSYS software.

ghorrocks July 11, 2012 01:01

The easiest way is using an interface. Have a look at the tutorials for examples with interfaces.

You can add interface models which modify the flow through the interface. You could use a pressure change model and make it a function of mass flow rate to model the fan curve.

Alternately you can use source terms. This is a more advanced technique so best leave it until you have a bit more experience I suspect.

Somebody July 12, 2012 04:25

Thank you.
I've spent some time reading up on interfaces and working through a tutorial. I'm still a bit unsure about how they work, however. To create an interface, I first have to have separate domains, yes? Is there something I need to do in DesignModeler to be even able to create separate domains in CFX-Pre?

ghorrocks July 12, 2012 08:09

No, an interface can connect a single domain. In fact a single domain can be two separate meshed regions connected by an interface. You do not need two domains.

In designmodeller you need to make it different parts on both sides of the interface.

Somebody July 13, 2012 04:19

Thanks for the help.

I have attempted to create this interface, with Pressure Change of -0.5 [bar], but when I try to update the solution I get the following error messages:

CFX Solver Manager: "An error has occurred in cfx5solve: The ANSYS CFX solver has terminated without writing a results file. Command on host wts4 exited with return code 0."
ANSYS Workbench: "The CFX Solver for system (file name) did not produce a results file. Detailed information can be found in the output fil for the run, which can be viewed by selecting Display Monitors from the Solution Component. Cell A5"

Any ideas for other key things I could be missing or misunderstanding?

ghorrocks July 13, 2012 07:53

This is the workbench error message. It does not tell you much. You need to open the CFX output file and see the error message there, that will tell you why it bombed out.

Somebody July 13, 2012 09:20

That's what I figured - the actual error message seemed awfully long to post, though.

So first, in CFX-Pre, I had the following warning: "In Analysis 'Flow Analysis 1' - Solver Control: When defining Mass and Momentum for a domain interface, it is recommended that a convergence criteria is specified for the Domain Interface Target."
I imagine that this might be my issue (or at least, one of my issues), but I could not figure out what to do about it. Any advice here?

(And again, thank you so much.)

ghorrocks July 14, 2012 05:35

Until you post the actual error message we cannot help you.

Post the output file as an attachment to a post.

Somebody July 23, 2012 04:20

1 Attachment(s)
Thanks again. Here is the Out File from CFX Solver Manager.

ghorrocks July 23, 2012 08:02

The error message is quite clear. You have A GGI interface which does not have any intersection. If this was not intended you better fix your mesh to make sure things line up. If this is correct and the interface does not join at times then use the expert parameter as it says.

Somebody July 23, 2012 10:15

I cannot figure out where the error with the mesh is. It seems to me that all parts are connecting? How do I figure out where the problem is?

ghorrocks July 23, 2012 18:41

Load your mesh in CFD-Post. There should be a variable (can't remember its name) which also shows you the connected and unconnected parts of the GGI. You should see there the unconnected GGI.

Somebody July 24, 2012 03:23

But if the CFX Solver "did not produce a results file," I can't load the problematic mesh in CFD Post, can I? (I am assuming that what I see when I click on Results is from before I tried messing with pressure...)

ghorrocks July 24, 2012 03:32

You can load def and cfx (CFX-Pre setup files) in CFD-Post.

Also you should make a results file at time step zero which will output the initial condition.

Somebody July 24, 2012 04:06

Hmm. I still can't figure it out, and considering the time limit I now have (until the end of the week), I don't think I will be able to.

But thanks again for all your help. I otherwise would not have known how to even start. Do you have a favorite charity? I'd like to send a few dollars their way.

ghorrocks July 24, 2012 19:29

Very generous of you. I don't know of any charities like CFD addicts anonymous or anything like that :). As I suspect you are not in Australia none of the charities I support will be relevant to you so you will have to choose one yourself.


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