CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

Problem with massflow BC at inlet

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   November 3, 2016, 08:41
Default Problem with massflow BC at inlet
  #1
New Member
 
Alex
Join Date: Jun 2015
Posts: 22
Rep Power: 0
MrNavierStokes is on a distinguished road
Hello CFD community,

I am currently working on a simulation of a radial turbine. At the inlet I have a massflow boundary condition for which the massflow is calculated through a "p-regulator" loop I implemented in CFX using expressions. I set the required pressure ratio between two planes in the inlet and outlet section respectively and the p-regulator calculates the appropriate massflow which is then set at the inlet. BUT the "iteratively" calculated massflow mdotIter is only being set after 50 iterations. For the first 50 iterations a constant massflow of e.g. 0.04 kg/s is used.

The loop works fine for one case. After I changed the geometry of the inlet pipe however it stops working. Eventhough the loop should start after 50 iterations, no massflow passes through the turbine (monitor point for massflow equals 1e-016 kg/s) from the beginning. After 50 iterations, when the regulator loop starts, the simulation crashes. My expression for the inlet massflow is the following:

BC mdot = if(citern<50,0.04 [kg s^-1],mdotIter)

I believe the terminology is correct, because I don't recieve any errors from CFX and it works perfectly fine for the first case.
I also suppose there is nothing wrong with the new inlet geometry, because when I change the inlet BC to:

BC mdot = 0.04 [kg s-^1]

it works again.
I hope someone is able to help me, because this problem really confuses me.

Thank you guys very much!

Regards, Alex.
MrNavierStokes is offline   Reply With Quote

Old   November 3, 2016, 13:36
Default
  #2
Senior Member
 
Join Date: Jun 2009
Posts: 1,608
Rep Power: 29
Opaque will become famous soon enough
Your simulation crashes/fail, how does it fail ?

The error message might help understand the source of the problem.
Opaque is offline   Reply With Quote

Old   November 5, 2016, 04:00
Default
  #3
Senior Member
 
omid
Join Date: Apr 2011
Posts: 102
Rep Power: 13
omidiut is on a distinguished road
Dear Alex
Hi

could you explain more about your problem?
what is 'mdotIter' in your expression?
omidiut is offline   Reply With Quote

Old   November 7, 2016, 03:55
Default
  #4
New Member
 
Alex
Join Date: Jun 2015
Posts: 22
Rep Power: 0
MrNavierStokes is on a distinguished road
Hello guys,

first of all thanks for your help!

@Opaque: Of course you're right, sorry. Should've said something about the error message I receive from the solver. It only says:

Fatal Overflow in linear Solver

after 50 iterations, that ist when my inlet BC switches from a constant massflow of 0.04 kg s^-1 to the regulator-loop massflow mdotIter.

@omidiut: I work on a turbine stage simulation and would like to do a comparison between the CFD and measurement results. In order to do that I have to figure out the correct position of the VTG blades (Variable Turbine Geometry, stationary guide vanes to regulate the turbine massflow) that has been used during the measurements. In order to find the right position and calculate the correct turbine map (they differ dependent on the VTG position), I set a certain pressure ratio (measurement data) between two section planes of my domain and the loop iterates until the massflow at the inlet corresponds to this pressure ratio. Of course I could just set the massflow at the inlet since I have the data, but this wouldn't work for the whole map, because a massflow inlet BC would cause instabilities at high pressure ratios. Now the inlet BC mdotIter is part of a p-regulator loop. It is set as a BC after 50 iterations. Before that a constant massflow is used. Know the strange thing is, that it worked perfectly fine for one case, but after I changed the inlet pipe (it has a slightly different shape) the loop does not work anymore. For a constant massflow the calculation runs normally, but as soon as i write:

BC mdot = if(citern<50,0.04 [kg s^-1],mdotIter)

instead of

BC mdot = 0.04 [kg s-^1]

it crashes with the error message above.

Thank you guys for your time and help.

Regards, Alex.
MrNavierStokes is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Inlet problem for turbulent flow in cavity Sebastian A FLUENT 0 January 21, 2016 06:54
Problem with mass flow inlet vainilreb OpenFOAM Running, Solving & CFD 4 April 8, 2013 07:41
Flux Problem at Inlet piyushsharma0487 Fluent UDF and Scheme Programming 9 April 19, 2012 03:57
Problem with gas inlet in an external flow vitor.monjardim ANSYS 0 January 30, 2012 07:35
Transient simulation (Problem with massflow and Pressure) ChristianF CFX 4 August 22, 2011 22:42


All times are GMT -4. The time now is 23:06.