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

Impulse input -Residence time distribution

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

Reply
 
LinkBack Thread Tools Display Modes
Old   February 25, 2011, 10:20
Post Impulse input -Residence time distribution
  #1
New Member
 
abhik banerjee
Join Date: Jul 2010
Location: Roorkee
Posts: 9
Rep Power: 6
abhik.banerjee is on a distinguished road
I would like to obtain the residence time distribution in the glass tank of a float glass furnace.I have already solved the steady state problem involving flow and energy equations.I am now carrying out a unsteady state analysis using a UDS equation which includes only the transient and convective term.I am going to track the weighted average of the uds variable at the otlet .However i am not able to give the impulse input necessary as an input boundary condition.

How can a give the impulse input ...please help?
abhik.banerjee is offline   Reply With Quote

Old   February 25, 2011, 10:30
Default
  #2
Senior Member
 
Amir's Avatar
 
Amir
Join Date: May 2009
Location: Shiraz, Iran
Posts: 739
Blog Entries: 1
Rep Power: 13
Amir is on a distinguished road
Hi,
you can do that with a simple UDF by DEFINE_PROFILE macro.
Amir is offline   Reply With Quote

Old   February 25, 2011, 10:33
Default
  #3
New Member
 
abhik banerjee
Join Date: Jul 2010
Location: Roorkee
Posts: 9
Rep Power: 6
abhik.banerjee is on a distinguished road
@amir ...well i can use a define profile macro to define the input but how do i account for the time..i have to give the input for a short time duration
abhik.banerjee is offline   Reply With Quote

Old   February 25, 2011, 12:43
Default
  #4
Senior Member
 
Amir's Avatar
 
Amir
Join Date: May 2009
Location: Shiraz, Iran
Posts: 739
Blog Entries: 1
Rep Power: 13
Amir is on a distinguished road
your case is unsteady and DEFINE_PROFILE macro is evaluated at every time step. you can retrieve the time by CURRENT_TIME command so you can implement such algorithm:
if (CURRENT_TIME<...)
F_PROFILE(...)=...;
else
F_PPROFILE(...)=0.0;
Amir is offline   Reply With Quote

Old   April 1, 2011, 07:11
Default
  #5
New Member
 
abhik banerjee
Join Date: Jul 2010
Location: Roorkee
Posts: 9
Rep Power: 6
abhik.banerjee is on a distinguished road
@Amir

I tried to use an impulse input ..The code worked ok,but the value of the uds scalar variable (area weighted average value) which I am tracking at the outlet did not decrease even after a very long time as should happen in case of an impulse input .

So I am using a step input instead and the code is as below...

DEFINE_PROFILE(boundary_profile,t,i)
{
real x[ND_ND];
real y;
face_t f;
real current_time;
current_time = RP_Get_Real("flow-time");
begin_f_loop(f,t)
{
F_CENTROID(x,f,t);
y=x[0];
if(current_time>=0)
{
F_PROFILE(f,t,i) = 1.0;
}
else
{
F_PROFILE(f,t,i) = 0.0;
}
}
end_f_loop(f,t)
}
The code worked ok...I am using the ratio of the area weighted average value of the uds scalar variable at the outlet to inlet to get the F curve..
The shape is ok....but the problem is that when I calculate the mean residence time using the above data the value comes more than the volumetric residence time(Volume of tank/volumetric flow rate)...which I think is not correct...

I think I am making some mistake soemwhere in UDS application...
Should the UDS equation have any source term..or is my method of using the area weighted average value faulty...
abhik.banerjee is offline   Reply With Quote

Old   April 1, 2011, 08:51
Default
  #6
Senior Member
 
Amir's Avatar
 
Amir
Join Date: May 2009
Location: Shiraz, Iran
Posts: 739
Blog Entries: 1
Rep Power: 13
Amir is on a distinguished road
I proposed you to make some changes in UDF for impulse input as follow:
Code:
#include "udf.h"
DEFINE_PROFILE(boundary_profile,t,i)
{
real x[ND_ND]; 
real y;
face_t f;
real time=CURRENT_TIME;
begin_f_loop(f,t)
{
F_CENTROID(x,f,t);
y=x[1];
if(time<=CURRENT_TIMESTEP)
{
F_PROFILE(f,t,i) = 1.0;
}
else
{
F_PROFILE(f,t,i) = 0.0; 
}
}
end_f_loop(f,t)
}
Amir is offline   Reply With Quote

Old   April 1, 2011, 09:17
Default
  #7
New Member
 
abhik banerjee
Join Date: Jul 2010
Location: Roorkee
Posts: 9
Rep Power: 6
abhik.banerjee is on a distinguished road
@ Amir

Thank u very much...I willl try the code and let u know the results....One more thing ..I am tracking the area weighted average value of the UDS scalar variable at the outlet surface with time ...is this approach right....
abhik.banerjee is offline   Reply With Quote

Old   April 1, 2011, 11:59
Default
  #8
Senior Member
 
Amir's Avatar
 
Amir
Join Date: May 2009
Location: Shiraz, Iran
Posts: 739
Blog Entries: 1
Rep Power: 13
Amir is on a distinguished road
first of all in this approach, you have to disable inlet diffusion in setting UDSs.
for evaluating residence time, I have another suggestion:
in transient flow iteration, you can monitor (and/or write) volume average of UDS in whole domain and check when it drops lower than the criteria.
Amir is offline   Reply With Quote

Reply

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Particle residence time Eric1 FLUENT 0 May 2, 2010 22:13
calculation diverge after continue to run zhajingjing OpenFOAM 0 April 28, 2010 04:35
Differences between serial and parallel runs carsten OpenFOAM Bugs 11 September 12, 2008 11:16
Residence time contours? Carlos FLUENT 0 February 8, 2008 18:42
AMG versus ICCG msrinath80 OpenFOAM Running, Solving & CFD 2 November 7, 2006 16:15


All times are GMT -4. The time now is 21:28.