[vigii]

Hi Everyone,

I'm trying to simulate the pitching and oscillating motion of the airfoil.

I'm bit confused in defining the equations for oscillating motion and pitching in UDF.

How should I go about writing the UDF for the following equations.

h(t)=a*sin(wt)

where,
a=0.25; w=2*pi*0.2

Pitching happens at 1/3 length of the chord of equation

theta(t)=a*sin(wt+d)

Where,
a=pi/6; d=pi/2

Please help me in writing a UDF.

Regards,
Vignesh

[pakk]

Which aspect is giving you problems?
What did you try already?
The more specific your question is, the more specific your answer will be.

[vigii]

Hi,

Thanks for your reply.

My problem looks like this Video.

My case is a 2D Problem with airfoil. Here,I should oscillate the airfoil at 1/3 rd of the chord length. The equations which are defined above are responsible for its motion. I don't know how to proceed with the UDF. I know that I should use CG_Motion. But I'm not getting how to put my equations.

Please help me. Here is my UDF. Please tell me weather I'm correct.

#include "udf.h"

DEFINE_CG_MOTION(airfoil, dt, vel, omega, time, dtime)
{
real a, w, p, pi;

pi = 3.14159265;
/* define motion variables */

a = pi/6; 			/*	Amplitude 			*/
w = 2 * pi *0.2; 	/*	Angular Frequency	*/
p = pi / 2; 		/*	Phase				*/

/* define airfoil motion */

omega[2] = a*sin(w*time+p); /* z-component of angular velocity */

}

Regards,
Vignesh

[pakk]

Removed after udf was added

[pakk]

Now you have more chance of getting an answer.
I don't think I can help you much further (I don't even know what pitching means, and I have never used CG_MOTION), but somebody else might...

However, you could still easily give more information that would make it easier to answer your question.
-Does the udf you listed give any errors?
-Does the udf you listed have an effect on the results?
-If it has an effect, what is wrong with it?

I can give you only one advice, which will not help you to solve this problem, but make things easier: instead of

pi = 3.14159265;

you can use the constant M_PI, wich is defined as 3.14159... automatically.

[mooncfd]

I need your help please:
1- i have simulated an oscillating airfoil in fluent 13 used UDF (visual studio c 2008) under windows 7 32bit i have a result of Cp=0.72 (result correct and valid with literature ) .
2- the same case in 1 with the same parameters in fluent 13 used UDF (visual studio c 2008) under windows 7 64 bit i have a result of Cp=0.3 (result non valide)
.
why the results are different ?

[mooncfd]

I need your help please:
1- I've simulated an oscillating airfoil in fluent 13 used UDF (visual studio c 2008) under windows 7 32bit i have a result of Cp=0.72 (result correct and valid with literature ) .
2- the same case in 1 with the same parameters in fluent 13 used UDF (visual studio c 2008) under windows 7 64 bit i have a result of Cp=0.3 (result non valide)
.
I dond't understand Why the results are different ?

[fkanyako]

I'm attempting to analyze a 2D airfoil in a pitching motion. However I have seen so many UDF defining the pitching motion like these...

/* define motion variables */
pitchampl = 45 * pi / 180; /* 45-degree pitch amplitude */
w = 2 * pi * 25; /* 25 Hz frequency */

/* define airfoil motion */
(1) omega[2] = w * pitchampl * cos(w * time);

(2) omega[2] = w * pitchampl * sin(w * time);

I have also seen this equation

(3) omega[2] = 0.5 * pitchampl * sin(w*time);

Please I need help with the difference between these equations(1,2 and 3), which is the correct one for pitching motion of airfoil ? Is it sin or cos ?

Thanks

[pakk]

Go and read something about sinusoids and amplitudes, then it should become clear. They represent different situations, they can all be correct in the right situations. (1) and (2) seem strange to me, but I can imagine cases where they work. In this case, 'sin' and 'cos' are equally correct, they describe two possible motions.

[fkanyako]

Originally Posted by pakk

Go and read something about sinusoids and amplitudes, then it should become clear. They represent different situations, they can all be correct in the right situations. (1) and (2) seem strange to me, but I can imagine cases where they work. In this case, 'sin' and 'cos' are equally correct, they describe two possible motions.

Thanks Pakk, after some review I think you are right about equation 1 and 2. which is the angular velocity. I have seen many papers using either of them. But the third equation is basically the pitch angle. Using it as omega is wrong. You have to take the time derivative of equation(3) before using it as UDF. This is my understand so far I hope my simulation proves that and let me know if am on point.

[pradip.c1000]

Hi fkanyako,
I need your help in set up of meshing for airfoil Naca0012 for low Re in dynamic mesh. How to create a mesh for this problem? I am new to Dyn meshing using udf.

[Edgar Alejandro Martínez]

/************************************************** ***************************/
/* pitchingmotion.c */
/* UDF for specifying oscillatory motion */
/* */
/************************************************** ***************************/

#include "udf.h" // UDF header
#define OMEGA 18.67 // angular frequency (rad/sec)
#define AMP 15.00*M_PI/180 // amplitude (15 degrees)

DEFINE_CG_MOTION (pitching, dt, cg_vel, cg_omega, time, dtime)
{
real alpha_prime;

cg_vel[0] = 0.0;
cg_vel[1] = 0.0;
cg_vel[2] = 0.0;

cg_omega[0] = 0.0; // rotation around the x-axis
cg_omega[1] = 0.0; // rotation around the y-axis

// alpha = alpha_0 + AMP*sin(OMEGA*t)

/* The derivative of alpha is alpha prime. So, the
angular velocity we are looking for is:
alpha_prime = AMP*OMEGA*cos(OMEGA*t) */

alpha_prime = AMP*OMEGA*cos(OMEGA*time);

// alpha_prime is passed to cg_omega[2]:
cg_omega[2] = alpha_prime;
}