[haihek]

Hello Everyone,

I am trying to write a UDF for unsteady flow, in which the velocity at the intlet of 2d cylinder is increased by time (or by time step). I am using a VOF model, and it is pressure based. If it is easy, can anyone write me the code, if not, then point me to the right place. I am currently reading the UDF manual, so i know how to compile the c file and load it.

Thanks
Alex.

[ComputerGuy]

Alex,

Please look here: http://www.cfd-online.com/Forums/flu...tml#post287097

[haihek]

Computerguy,

Thank you for your reply, that was helpful to read..

but that code is made for velocity change at a defined time,

how can have that code to change the velocity at everytime step between 0 m/s till 1m/s, within lets say 30000 time steps. And make it only for the inlet boundary.

Many thanks,

Alex

[ComputerGuy]

Alex,

Are you asking how to perform a ramp of the velocity between 0 and 1 m/s over some defined period of time which takes 30000 time steps? If so, it's simply linear interpolation.

I haven't checked the following code, but it should be clear enough to fix if it's broken:

#include "udf.h"
DEFINE_PROFILE(pressure_magnitude, t, i)
{
	real default_velocity_mag,velocity_mag;
	real start_velocity,end_velocity;
	real ramp_start,ramp_end;
	real line_slope, line_intercept;
	
	face_t f;
	
	default_velocity_mag=0.0;
	start_velocity=0.0;
	end_velocity=1.0;
	ramp_start=0.0;
	ramp_end=3.0;
	
	
	velocity_mag=default_velocity_mag;
	the_current_time = CURRENT_TIME;

	line_slope=(end_velocity-start_velocity)/(ramp_end-ramp_start);
	line_intercept=start_velocity-line_slope*ramp_start;
	
	if ((the_current_time>=ramp_start) && (the_current_time<=ramp_end))
	{
		velocity_mag=line_slope*the_current_time+line_intercept;
	}

	begin_f_loop(f,t)
		{
			F_PROFILE(f,t,i) = velocity_mag;
		}
	end_f_loop(f,t)
}

• default velocity is the value that the code will use outside the ramp time
• start velocity is the value of the velocity at the beginning of the ramp (in your case, 0 m/s)
• end velocity is the value of the velocity at the beginning of the ramp (in your case, 1 m/s)
• ramp start and ramp end are the times, in seconds, you want to start and end the velocity ramp, respectively. This is easier to do (and slightly more robust) than selecting the number of time steps you'd like to subdivide the ramp into, as it gives you the option to take variable time steps.

Let me know if this is clear.

ComputerGuy

[haihek]

Dear Computerguy,

Thank you so much. The code is very clear. However i have few questions, they may sound stupid but still i thought i should ask.

for the DEFINE_PROFILE(pressure_magnitude, t, i) .. shouldn't be DEFINE_PROFILE(velocity_magnitude, t, i)? or it really doesn't make a difference?

My other question is: after lets say 3 seconds of flow time, where the velocity becomes 1 m/s, will the code maintain that velocity at a later time (i.e t>3seconds)?

Thanks again CG .. i will do the compiling using FLUENT today.. and i will then run it in the simulation.. i will let you know how it goes.

Regards,
Alex

[ComputerGuy]

haihek,

Good questions.

1. It doesn't matter what name you use on a UDF, I just forgot to change the name
2. The code will not use the last value from the previous timestep. It will always default to "default_velocity_mag"

The updated code you're looking for is below:

#include "udf.h"
DEFINE_PROFILE(velocity_magnitude, t, i)
{
	real default_early_velocity_mag,default_late_velocity_mag,velocity_mag;

	real start_velocity,end_velocity;
	real ramp_start,ramp_end;
	real line_slope, line_intercept;
	
	face_t f;
	
	default_early_velocity_mag=0.0;
	default_late_velocity_mag=3.0;
	start_velocity=0.0;
	end_velocity=1.0;
	ramp_start=0.0;
	ramp_end=3.0;
	
	
	velocity_mag=default_early_velocity_mag;
	the_current_time = CURRENT_TIME;

	line_slope=(end_velocity-start_velocity)/(ramp_end-ramp_start);
	line_intercept=start_velocity-line_slope*ramp_start;
	
	if ((the_current_time>=ramp_start) && (the_current_time<=ramp_end))
	{
		velocity_mag=line_slope*the_current_time+line_intercept;
	}

	if ((the_current_time>ramp_end))
	{
		velocity_mag=default_late_velocity_mag;
	}

	begin_f_loop(f,t)
		{
			F_PROFILE(f,t,i) = velocity_mag;
		}
	end_f_loop(f,t)
}

ComputerGuy

[AstralD]

Dear ComputerGuy,
I have a problem with the code. I have defined a negative velocity outlet (0.5 m/s) on a curved pipe and defined the flow inlet (let say 7 L/m) for the inlet. in my problem velocity will change slightly and decreases in 2 seconds, starts from 0s and begin to peak to the highest velocity then drops to 0m/s in the 2s. imagine it is a vacuum and works for 2 sec and stops.
Should I use the exact same code for this problem if not can you please advise the proper code? I really appreciate that!

cheers,
Alex

#include "udf.h"
DEFINE_PROFILE(pressure_magnitude, t, i)
{
real default_velocity_mag,velocity_mag;
real start_velocity,end_velocity;
real ramp_start,ramp_end;
real line_slope, line_intercept;

face_t f;

default_velocity_mag=0.55;
start_velocity=0.0;
end_velocity=0.0;
ramp_start=0.0;
ramp_end=3.0;


velocity_mag=default_velocity_mag;
the_current_time = CURRENT_TIME;

line_slope=(end_velocity-start_velocity)/(ramp_end-ramp_start);
line_intercept=start_velocity-line_slope*ramp_start;

if ((the_current_time>=ramp_start) && (the_current_time<=ramp_end))
{
velocity_mag=line_slope*the_current_time+line_inte rcept;
}

begin_f_loop(f,t)
{
F_PROFILE(f,t,i) = velocity_mag;
}
end_f_loop(f,t)
}

[Dr.TDAJones]

Thanks for the code ComputerGuy,

I have used it to setup my own ramp code. It is quite complicated and I thought that for anyone else wanting to use multiple ramps i have posted it here and it may assist.

If anyone could suggest a method of simplifying the code further that would be helpful.

It involves:
1st, a ramp from v=0 to v=-0.1 m/s, for 0.005 s.
2nd, to maintain at -0.1 m/s, for 0.041 s
3rd, to ramp from v=-0.1 m/s to 0 m/s, in 0.005 s,
4th, to ramp from v=0 m/s to 0.06 m/s, in 0.002 s.
5th, to maintain at 0.06 m/s for 0.001 s,
6th, to ramp from v=0.06 m/s to 0 m/s, in 0.002 s.
7th, the code repeats itself looping back to the first stage.

#include "udf.h"
DEFINE_PROFILE(inlet_y_velocity, t, position)
{
real velocity_mag;


real start_velocity1,end_velocity1;
real start_velocity2, end_velocity2;
real start_velocity3, end_velocity3;
real start_velocity4, end_velocity4;

real ramp_start1,ramp_end1;
real ramp_start2, ramp_end2;
real ramp_start3, ramp_end3;
real ramp_start4, ramp_end4;


real line_slope1, line_intercept1;
real line_slope2, line_intercept2;
real line_slope3, line_intercept3;
real line_slope4, line_intercept4;

real cycle_period = 0.056;
real flow_time = RP_Get_Real("flow-time");
real cycle_number = flow_time / cycle_period;
real time_in_cycle = cycle_period * (cycle_number - floor(cycle_number));

face_t f;

start_velocity1=0.0;
end_velocity1=-0.1;
start_velocity2 = -0.1;
end_velocity2 = 0;
start_velocity3 = 0;
end_velocity3 =0.06;
start_velocity4 =0.06;
end_velocity4 =0.0;

ramp_start1=0.0;
ramp_end1=0.005;
ramp_start2=0.046;
ramp_end2=0.051;
ramp_start3=0.051;
ramp_end3=0.053;
ramp_start4=0.054;
ramp_end4=0.056;

line_slope1 = (end_velocity1 - start_velocity1) / (ramp_end1 - ramp_start1);
line_slope2 = (end_velocity2 - start_velocity2) / (ramp_end2 - ramp_start2);
line_slope3 = (end_velocity3 - start_velocity3) / (ramp_end3 - ramp_start3);
line_slope4 = (end_velocity4 - start_velocity4) / (ramp_end4 - ramp_start4);

line_intercept1 = start_velocity1 - line_slope1 * ramp_start1;
line_intercept2 = start_velocity2 - line_slope2 * ramp_start2;
line_intercept3 = start_velocity3 - line_slope3 * ramp_start3;
line_intercept4 = start_velocity4 - line_slope4 * ramp_start4;

if ((time_in_cycle >=ramp_start1) && (time_in_cycle <=ramp_end1))
{
velocity_mag=line_slope1* time_in_cycle +line_intercept1;
}
else if ((time_in_cycle >ramp_end1) && (time_in_cycle <=ramp_start2))
{
velocity_mag=-0.1;
}

else if ((time_in_cycle >= ramp_start2) && (time_in_cycle <= ramp_end2))
{
velocity_mag=line_slope2* time_in_cycle + line_intercept2;
}

else if ((time_in_cycle >= ramp_start3) && (time_in_cycle <= ramp_end3))
{
velocity_mag = line_slope3 * time_in_cycle + line_intercept3;
}
else if ((time_in_cycle >ramp_end3) && (time_in_cycle < ramp_start4))
{
velocity_mag = 0.06;
}

else if ((time_in_cycle >= ramp_start4) && (time_in_cycle <= ramp_end4))
{
velocity_mag = line_slope4 * time_in_cycle + line_intercept4;
}

begin_f_loop (f,t)
{
F_PROFILE(f,t,position) = velocity_mag;
}
end_f_loop(f,t)
}