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-   -   Problem with negative velocity values (http://www.cfd-online.com/Forums/cfx/106960-problem-negative-velocity-values.html)

Andris September 13, 2012 08:58

Problem with negative velocity values
 
Hi all,
I am trying to simulate a transient analysis on blood flow in an artery. I have imported into CFX a velocity profile that includes a point where there is backflow of fluid. However, when I run it in solver and set up a monitor point to evaluate the inlet and outlet velocity, it does not plot the negative velocity values.
Can someone tell me how to deal with this problem?
Also I have used an "opening" for the outlet rather than an "outlet", how will this impact my results?
Many Thanks

Benfa September 13, 2012 15:28

Hi,

if you monitor the velocity it will show you the absolute value of it. If you want to see a change in the sign in front of the velocity you need to monitor the u,v and w velocity components

Ben

Andris September 14, 2012 04:47

Thanks Ben,
I was wondering also, if it is possible to use the outlet option for my outlets if I have backflow? I am using an opening at the moment instead and I wondering if this will adversely affect my results.
Andris

Lance September 14, 2012 04:56

Quote:

Originally Posted by Andris (Post 381754)
Thanks Ben,
I am using an opening at the moment instead and I wondering if this will adversely affect my results.

If the outlet is far away from the region of interest it might not affect your results, but it is impossible for anyone else but yourself to know that. Why dont you try it and look at the results?

Benfa September 14, 2012 12:34

Hi,

if you get backlflow at your outlet you should use the opening bc. That is the flow that will develop. If you use outlet bc you are disturbing your solution in that region. Why is it a problem to use opening?

Andris September 17, 2012 04:47

Hi,
Thanks for your help, its not a problem, I was just wondering what repercussions there would be if I used an opening rather than an outlet seeing as there are important distinctions between the two.
Regards,
Andris

Andris September 17, 2012 07:14

Also, while setting up monitor points, I get the problem that after a certain number of timesteps, the values are no longer plotted and I cannot access them. how can I remedy this problem? Is there a limit on the number of timesteps CFX solver can hold?
Andris

ghorrocks September 17, 2012 18:51

No limit to my knowledge, I have had tens of thousands of points and it works fine. Are you sure you have the x axis set to all time steps, not just the latest few?

Lance September 18, 2012 04:04

Glenn, just out of curiosity, what is the computational overhead when using tens of thousands of monitor points?

Andris September 18, 2012 06:12

Yep, I go to monitor properties, set 'range settings' to 'display all', put 'plot data' by 'simulation time', the run is for 5 seconds at a time step of 0.01. Yet for every simulation, it stops plotting the monitor points even though the run keeps on going. I exported the results to excel to see if perhaps I could access them there but still no luck.

Andris September 18, 2012 07:18

One of the errors i sometimes read is "Size mismatch, expected 76 values but found 77", it is at this point that the monitor points stop plotting. Can anyone help please?

Lance September 18, 2012 08:36

I would guess that the solver manager expects to find 76 monitors but finds 77 and decides to not plot any of them. Have you tried to "clear generated data" (or whatever it is called in workbench) to reset the monitor displays in the solver manager?

Andris September 18, 2012 08:52

Hi,
No not yet, I have changed the path file name of my file to something simpler to see if it makes a difference based on a previous post. If that doesn't work, I will try what you recommended. I don't really use Workbench atm, just CFX, is there an equivalent option in CFX and if so where can it be found?
Thank you,
Andris

Andris September 18, 2012 12:15

Hi,
I still get the same problem, the error message comes up randomly at different times, sometimes it goes to 1.2 seconds, sometimes to 0.4 seconds with the same mesh and the same settings. I'm really confused as to why this is happening? Could it be maybe to do with the license settings that my institution has?
Andris

ghorrocks September 18, 2012 19:36

Quote:

Glenn, just out of curiosity, what is the computational overhead when using tens of thousands of monitor points?
Sorry, my comment was unclear - From a small number of monitor points I have tens of thousands of readings and that works fine.

I do recall a few years back trying to generate a few hundred monitor points and the CEL parser failed. So I think the CEL parser is the limiting factor, not the solver. When I cut it back to something the parser could handle I cannot recall any significant slowing of the solver. But this was a long time ago and my memory might not be right.

Andris September 19, 2012 05:41

Ok, so the CEL parser can hold only a certain number of monitor points? So far it plots a monitor point for every timestep that I go through. How can I change this?
Regards,
Andris

ghorrocks September 19, 2012 06:05

I am not sure precisely, but I suspect it is the overall count of "bits" in the CCL which is the limit, rather than monitor points specifically.

BTW: I meant to say CCL parser, not CEL parser.

Andris September 19, 2012 06:10

Ok, how do I remedy this problem?

Lance September 19, 2012 06:12

Quote:

Originally Posted by ghorrocks (Post 382443)
Sorry, my comment was unclear - From a small number of monitor points I have tens of thousands of readings and that works fine.

Ok, I thought so :)
Didnt really see the purpose of having that many monitor points. I've done a couple of hundred thousands of iterations with monitors and that works fine, but the solver manager can be a bit slow to open.

So, yes, the limit is in the CCL rather than the number of iterations (or monitor points).

Andris September 19, 2012 06:29

Hi,
This is very confusing. What can I do ensure that the monitor points keep on plotting throughout my simulation? Your help would be greatly appreciated.


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