Plotting backflow on a mass flow graph (two way FSI)

Cirion0000 · September 21, 2009, 12:34

Hi, I'm trying to figure out how to show the backflow on a graph, considered as "negative" flow. When I plot a graph using the expression Rho * Average Velocity * Cross-sectional area, I get a plot that is pretty reasonable and expected (a sinusoidal wave [my input pressure is sinusoidal]) but it never crosses below the X axis. I know this is wrong because you can see from the animations that flow comes back into the device as well. Is there a way to get CFX to properly plot this? I would try using the MassFlow expression but I get errors using this (so I had to improvise with the Rho*Avg. Velocity*Cross-sectional area)

Also, is there a way to plot the overlay/summation of two locations at once? (There are essentially two openings for outlet/inlet)

ghorrocks · September 21, 2009, 19:30

The MassFlow function works fine for backflow. The problem is more likely to be that you are using an inlet as your boundary condition and that does not allow backflow. Try using an opening instead.

Cirion0000 · September 28, 2009, 13:16

I did use the BC "opening" in CFX-pre. And I can visually inspect, and there is definitely backflow in the vector plot.

Also, the massflow command does work, but only for the final timestep (so it is working... sort of), which does not make sense. Otherwise, it gives me this error:

"WARNING
Unable to calculate mass flow the surface.
Some or all of the required variables are not present:
Density, Velocity, and, for multi-phase cases, Volume Fraction."

And just to clarify again the solution makes sense. The vector profile and deflection behave as expected. I'm just not entering the right expression, or something.

ckleanth · September 28, 2009, 14:41

if you haven't asked to save the requested variable at a particular timestep during your simulation how you want cfx to plot it?

Cirion0000 · October 7, 2009, 11:50

Allright, I solved that issue now I think. Now however I have a new concern, this one should be simple to solve but I can't locate how to do so. Basically I need to increase the max number of warning messages from the /NERR command but I do not know where to input it since I used ANSYS workbench to create the input file and I cannot locate the menu for it there. I can see it in the ANSYS multiphysics application but not workbench. Since I have 2000 timesteps I reach the 10,000 warning limit very quickly. (Not fatal warnings just minor warnings basically telling me that the loads are small and such.)

lingdeer · November 29, 2011, 13:30

Though it is a post 2 years ago, I hope somebody can give me some advice:

Here is the /NERR command:
http://www.kxcad.net/ansys/ANSYS/ans...lp_C_NERR.html

I have the same problem with no. of warning message. I m using ANSYS 13

I actually added
/uis,msgpop
/nopr
/NERR,,,-2

So that ANSYS won't prompt me for warning message
Finally I can successfully start a simulation.
But after a lot of timesteps, the no. of warning messages exceed 10000 and stopped my simulations.

I tried to change to
/NERR,,99999,-2

But seems ANSYS cannot recognize this command and aborted my run.

September 21, 2009, 12:34	Plotting backflow on a mass flow graph (two way FSI)	#1
Cirion0000 New Member Join Date: Jul 2009 Posts: 8 Rep Power: 17	Hi, I'm trying to figure out how to show the backflow on a graph, considered as "negative" flow. When I plot a graph using the expression Rho * Average Velocity * Cross-sectional area, I get a plot that is pretty reasonable and expected (a sinusoidal wave [my input pressure is sinusoidal]) but it never crosses below the X axis. I know this is wrong because you can see from the animations that flow comes back into the device as well. Is there a way to get CFX to properly plot this? I would try using the MassFlow expression but I get errors using this (so I had to improvise with the RhoAvg. VelocityCross-sectional area) Also, is there a way to plot the overlay/summation of two locations at once? (There are essentially two openings for outlet/inlet)

September 28, 2009, 14:41		#4
ckleanth Senior Member George Join Date: Mar 2009 Location: Birmingham, UK Posts: 257 Rep Power: 18	if you haven't asked to save the requested variable at a particular timestep during your simulation how you want cfx to plot it? __________________ Top 4 tips 1. Knowledge is everything and Ignorance is dangerous. 2. Understand your limitations and try to eliminate them. 3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window. 4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
mass flow in is not equal to mass flow out	saii	CFX	12	March 19, 2018 06:21
mass flow inlet boundary condition	vivian	FLUENT	5	April 21, 2006 07:23
mass flow	Wenbin Song	FLUENT	0	September 27, 2005 14:00
mass flow rate error	Masood	FLUENT	0	May 22, 2005 01:32
Mass flow rate	Neser	CFX	4	February 14, 2004 01:27

September 21, 2009, 19:30		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	The MassFlow function works fine for backflow. The problem is more likely to be that you are using an inlet as your boundary condition and that does not allow backflow. Try using an opening instead.

September 28, 2009, 13:16		#3
Cirion0000 New Member Join Date: Jul 2009 Posts: 8 Rep Power: 17	I did use the BC "opening" in CFX-pre. And I can visually inspect, and there is definitely backflow in the vector plot. Also, the massflow command does work, but only for the final timestep (so it is working... sort of), which does not make sense. Otherwise, it gives me this error: "WARNING Unable to calculate mass flow the surface. Some or all of the required variables are not present: Density, Velocity, and, for multi-phase cases, Volume Fraction." And just to clarify again the solution makes sense. The vector profile and deflection behave as expected. I'm just not entering the right expression, or something.

October 7, 2009, 11:50		#5
Cirion0000 New Member Join Date: Jul 2009 Posts: 8 Rep Power: 17	Allright, I solved that issue now I think. Now however I have a new concern, this one should be simple to solve but I can't locate how to do so. Basically I need to increase the max number of warning messages from the /NERR command but I do not know where to input it since I used ANSYS workbench to create the input file and I cannot locate the menu for it there. I can see it in the ANSYS multiphysics application but not workbench. Since I have 2000 timesteps I reach the 10,000 warning limit very quickly. (Not fatal warnings just minor warnings basically telling me that the loads are small and such.)

November 29, 2011, 13:30		#6
lingdeer Member Lingdeer Join Date: Jul 2011 Posts: 49 Rep Power: 15	Though it is a post 2 years ago, I hope somebody can give me some advice: Here is the /NERR command: http://www.kxcad.net/ansys/ANSYS/ans...lp_C_NERR.html I have the same problem with no. of warning message. I m using ANSYS 13 I actually added /uis,msgpop /nopr /NERR,,,-2 So that ANSYS won't prompt me for warning message Finally I can successfully start a simulation. But after a lot of timesteps, the no. of warning messages exceed 10000 and stopped my simulations. I tried to change to /NERR,,99999,-2 But seems ANSYS cannot recognize this command and aborted my run.