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

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

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

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?

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?

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

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

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?

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

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.

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?

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?

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

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

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.

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

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.

Ok, how do I remedy this problem?

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).

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.

Sorry, a small tangent there :)

Are you displaying all the points? In solver manager, right click on the monitor point graph and select Monitor Properties, then Range Settings. Under Timestep Range Mode you can select whether it shows all points or only a subset.

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.

Something weird is happening with your simulation. I have used this feature many times and it works just fine for any length simulation.

Does the simulation continue otherwise OK? That is, the output file is still being written, the residuals are still coming up on the solver manager and when you stop the run the results file is written?

Yep, the run still keeps on going, a results file is written, all the residual values can still be seen. Everything works perfectly except that the monitor points just don't plot. They reach a certain time and stop, even though the simulation is running iterations for time steps way past those shown in the monitor graphs.