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

Periodic channel flow with time dependent mass flow rate

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   May 9, 2013, 20:59
Default Periodic channel flow with time dependent mass flow rate
  #1
New Member
 
Saâd Debbahi
Join Date: Oct 2010
Posts: 26
Rep Power: 15
QBeast is on a distinguished road
Send a message via Yahoo to QBeast
Dear fluent users,

I am using a translational periodic channel configuration to explore a specific Reynolds number range, here I am constrained with the periodicity to reduce cells number and keep the computation affordable but the issue is about accessing the mass flow rate targeted value which must be achieved by the pressure gradient, looking at the GUI I see no UDF hook available to set a time dependent mass flow rate ? the problem doesn't occur with velocity inlet BC but for the periodic channel of interest I have searched the forum and found this thread (guys are debating the issue and no definitive solution found):

http://www.cfd-online.com/Forums/flu...flow-rate.html

I would like to know if it is possible to do it using journal file reading?

I've found this document gathering some fluent tips, in pages 48, 49 the author shows a kind of work around for variables lacking udf hooks but this needs the very exact variables name:

http://willem.engen.nl/uni/fluent/do...ips-Tricks.pdf


I am not sure about this, but I am tempted to use a DEFINE_ADJUST to patch periodic faces cells and force their mass flow rate modification (this is surely too simple to get the job done) but the solver would keep looking to satisfy the target mass flow rate value....

time dependent mass flow rate across periodic channel flow?


thanks in advance, need your help guys

best regards.
QBeast is offline   Reply With Quote

Old   May 10, 2013, 03:36
Default
  #2
Senior Member
 
sbaffini's Avatar
 
Paolo Lampitella
Join Date: Mar 2009
Location: Italy
Posts: 2,150
Blog Entries: 29
Rep Power: 39
sbaffini will become famous soon enoughsbaffini will become famous soon enough
Send a message via Skype™ to sbaffini
I have this UDF that i never even tried to test because i don't actually need it (yet).

It basically is (intended to be) a workaround for the fact that a fixed mass flow rate can't be used in the Fractional step method (NITA).

So what i try to do is implementing a PID controller which time step by time step tries to fix the mass flow rate at the desired value by adjusting the forcing in the domain.

What you can try to do is to modify in time the desired value for the mass flow rate (which instead i assumed constant here) and try to figure out the best setting for the PID controller.

The UDF is intended for the parallel Fluent but, it should work for serial too if you modify the following lines:

#if RP_NODE
m_new=PRF_GRSUM1(massflow);
e_new=(m_des-m_new)/(m_des);
F_new=F_old+e_new*k_new+e_old*k_old;
#endif

with:

e_new=(m_des-m_new)/(m_des);
F_new=F_old+e_new*k_new+e_old*k_old;

However, i hope it is VERY CLEAR that i really never tried this UDF and some additional work could be required.
Attached Files
File Type: c massflow.c (3.5 KB, 159 views)
sbaffini is offline   Reply With Quote

Old   May 10, 2013, 11:07
Default
  #3
New Member
 
Saâd Debbahi
Join Date: Oct 2010
Posts: 26
Rep Power: 15
QBeast is on a distinguished road
Send a message via Yahoo to QBeast
Dear Paolo,

Thanks for the quick reply, I am examining the file you sent me line by line, I have only one concern about the solver being constrained with the mass flow value condition across the periodic boundaries, I'm trying to make periodic boundary conditions using velocity_inlet / pressure outlet inthis sequential order:

in a DEFINE_ADJUST macro:


1. perform first time iteration

2. loop over outlet boundary nodes
2.1 get nodes values of P,U,V,W from outlet boundary (I think using a plane )
2.2 patch the inlet with P,U,V,W of outlet boundary kept from the previous time step

3. perform next iteration

4. set new x-velocity U=Uprevious+deltaU to shift it up at the inlet and accordingly update the mass flow target value the solver have to meet for pressure outlet boundary

I'm having an issue figuring out how to patch the inlet value of P,U,V,W with those of the previous state of the outlet boundary, in other words how to completly clone (if possible) the outlet boundary cells faces or cells on the inlet plane?

these are previous calculated mean field variables from the outlet I wanna patch the inlet with

C R M1(c,t)
C P M1(c,t)
C U M1(c,t)
C V M1(c,t)
C W M1(c,t)



thanks helping

regards
QBeast is offline   Reply With Quote

Old   May 10, 2013, 14:14
Default
  #4
Senior Member
 
sbaffini's Avatar
 
Paolo Lampitella
Join Date: Mar 2009
Location: Italy
Posts: 2,150
Blog Entries: 29
Rep Power: 39
sbaffini will become famous soon enoughsbaffini will become famous soon enough
Send a message via Skype™ to sbaffini
Dear Saad,

i want to stress some points:

1) The concept behind my UDF is that of a forcing, varying in time trough a control process (a PID), and trying to achieve the desired mass flow rate. This is exactly the same thing done by Fluent for fixed mass flow rates with periodic boundary conditions. However, the iterative method used by Fluent is intended to work within outer iterations, which are not present with the NITA approach. Also, i think that my approach is more representative of an actual device trying to control the mass flow.

2) My UDF is intended to work with periodic boundary conditions already set-up in Fluent, but with the pressure gradient equal to zero. Instead, the forcing is provided by you trough the DEFINE_SOURCE part of the UDF which has to be activated for the cell_zone(s) between the two periodic faces. I want to highlight the fact that it is still Fluent handling the periodicity, the UDF only handles the forcing driving the flow.

3) I understand some influence from what you have read in the post you cited above. Forget it. Periodicity is periodicity. If you use velocity-inlet/pressure outlet or any combination of pressure-inlet/pressure-outlet that's not periodicity and the UDF above will not apply (actually, in that case, you can just provide a proper time dependent profile trough an UDF).

4) I don't think i understand why you want to perform the mapping of velocity variables from outlet to inlet (which is actually what periodicity already do, just it does it in both direction). However, if you want to pursue this path, you have to consider that the full set of flow variables is not generally available on interior face threads or even domain boundaries, so the most practical option is to use the variables in the c0 cells (which are always available) of the given faces to be mapped.

5) I already wrote the kind of mapping UDF you are talking about (but without mass flow control, i have to merge the two after the test of the latter) and i use it to map, for each time step, the velocity field from an auxiliary domain to the inlet of my principal domain. This is useful in LES to specify a proper inflow condition. However, the mapping from a face which is in the same domain of your inlet can be a true pain in the ass unless the two faces are really far away. As a matter of fact, despite some fancy paper published with OpenFOAM on the subject, it has no clear advantage at all.
sbaffini is offline   Reply With Quote

Reply

Tags
channel, mass flow rate, periodic, transient, udf

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
mass flow in is not equal to mass flow out saii CFX 12 March 19, 2018 06:21
Low Mixing time Problem Mavier CFX 5 April 29, 2013 01:00
Mass flow rate boundary condition for continuity equation for oscillating flow p07ip705 Main CFD Forum 0 February 28, 2013 02:33
Mass flow rate sepidecent CFX 0 August 9, 2011 01:15
Orifice Plate with a fully developed flow - Problems with convergence jonmec OpenFOAM Running, Solving & CFD 3 July 28, 2011 06:24


All times are GMT -4. The time now is 00:34.