|
[Sponsors] |
September 13, 2012, 08:58 |
Problem with negative velocity values
|
#1 |
Member
Join Date: Aug 2012
Posts: 35
Rep Power: 13 |
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 |
|
September 13, 2012, 15:28 |
|
#2 |
Member
Benny
Join Date: Apr 2012
Posts: 40
Rep Power: 14 |
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 |
|
September 14, 2012, 04:47 |
|
#3 |
Member
Join Date: Aug 2012
Posts: 35
Rep Power: 13 |
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 |
|
September 14, 2012, 04:56 |
|
#4 |
Senior Member
Lance
Join Date: Mar 2009
Posts: 669
Rep Power: 22 |
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?
|
|
September 14, 2012, 12:34 |
|
#5 |
Member
Benny
Join Date: Apr 2012
Posts: 40
Rep Power: 14 |
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? |
|
September 17, 2012, 04:47 |
|
#6 |
Member
Join Date: Aug 2012
Posts: 35
Rep Power: 13 |
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 |
|
September 17, 2012, 07:14 |
|
#7 |
Member
Join Date: Aug 2012
Posts: 35
Rep Power: 13 |
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 |
|
September 17, 2012, 18:51 |
|
#8 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,703
Rep Power: 143 |
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?
|
|
September 18, 2012, 04:04 |
|
#9 |
Senior Member
Lance
Join Date: Mar 2009
Posts: 669
Rep Power: 22 |
Glenn, just out of curiosity, what is the computational overhead when using tens of thousands of monitor points?
|
|
September 18, 2012, 06:12 |
|
#10 |
Member
Join Date: Aug 2012
Posts: 35
Rep Power: 13 |
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.
|
|
September 18, 2012, 07:18 |
|
#11 |
Member
Join Date: Aug 2012
Posts: 35
Rep Power: 13 |
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?
|
|
September 18, 2012, 08:36 |
|
#12 |
Senior Member
Lance
Join Date: Mar 2009
Posts: 669
Rep Power: 22 |
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?
|
|
September 18, 2012, 08:52 |
|
#13 |
Member
Join Date: Aug 2012
Posts: 35
Rep Power: 13 |
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 |
|
September 18, 2012, 12:15 |
|
#14 |
Member
Join Date: Aug 2012
Posts: 35
Rep Power: 13 |
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 |
|
September 18, 2012, 19:36 |
|
#15 | |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,703
Rep Power: 143 |
Quote:
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. |
||
September 19, 2012, 05:41 |
|
#16 |
Member
Join Date: Aug 2012
Posts: 35
Rep Power: 13 |
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 |
|
September 19, 2012, 06:05 |
|
#17 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,703
Rep Power: 143 |
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. |
|
September 19, 2012, 06:10 |
|
#18 |
Member
Join Date: Aug 2012
Posts: 35
Rep Power: 13 |
Ok, how do I remedy this problem?
|
|
September 19, 2012, 06:12 |
|
#19 | |
Senior Member
Lance
Join Date: Mar 2009
Posts: 669
Rep Power: 22 |
Quote:
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). |
||
September 19, 2012, 06:29 |
|
#20 |
Member
Join Date: Aug 2012
Posts: 35
Rep Power: 13 |
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. |
|
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
[ICEM] negative mesh volume problem (icem-cfd/cfx) | adam2008 | ANSYS Meshing & Geometry | 5 | April 16, 2010 12:21 |
Plot a graph of velocity & boundary condition problem | wanie | Fluent UDF and Scheme Programming | 0 | December 11, 2009 10:40 |
Problem Interface Solid Fluid with wall velocity Solver v12 | hills1 | CFX | 2 | October 12, 2009 05:36 |
Variables Definition in CFX Solver 5.6 | R P | CFX | 2 | October 26, 2004 02:13 |
Terrible Mistake In Fluid Dynamics History | Abhi | Main CFD Forum | 12 | July 8, 2002 09:11 |