[abhik.banerjee]

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?

[Amir]

Hi,
you can do that with a simple UDF by DEFINE_PROFILE macro.

[abhik.banerjee]

@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

[Amir]

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;

[abhik.banerjee]

@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...

[Amir]

I proposed you to make some changes in UDF for impulse input as follow:

#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)
}

[abhik.banerjee]

@ 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....

[Amir]

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.