[Larrabee]

Hi, everyone. I am now working on a problem about a pitching 2D airfoil. I have written my UDF and it makes the airfoil oscillate. However, the airfoil is now oscillating around Z axis and I want to set the axis to be 1/4 of the chord length. What should I do to the UDF?


#include "udf.h"
#define Freq 2.0
#define w 2.0*M_PI*Freq
#define pitch 7.5*M_PI/180

DEFINE_CG_MOTION(shm, dt, cg_vel, omega, time, dtime)
{
real omega;

omega = pitch*w*cos(w*time);

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

cg_omega[0] = 0.0;
cg_omega[1] = 0.0;
cg_omega[2] = omega;
}

[AlexanderZ]

cg_omega[0] = 0.0;
cg_omega[1] = 0.0;
cg_omega[2] = omega;

cg_omega[0] means omega across x axis, cg_omega[1] - across y axis, cg_omega[2] - across z axis

if you want some "mix" axis you should find normalized vector your axis you want and put it here, for instance for vector {1,1,1}:

cg_omega[0] = 1*omega;
cg_omega[1] = 1*omega;
cg_omega[2] = 1*omega;

best regards

[Larrabee]

But, the rotating axis is still across the origin. I want to set the axis to be ,say line x=1, and rotate at omega_z=1.

[obscureed]

Hi Larrabee,

If you combine rotation about the origin with a translational velocity that keeps the axis-point stationary, then you get rotation about the axis-point.

You could also consider shifting the entire geometry until your axis *is* the origin.

Good luck!
Ed

[ronak]

Originally Posted by Larrabee

Hi, everyone. I am now working on a problem about a pitching 2D airfoil. I have written my UDF and it makes the airfoil oscillate. However, the airfoil is now oscillating around Z axis and I want to set the axis to be 1/4 of the chord length. What should I do to the UDF?


#include "udf.h"
#define Freq 2.0
#define w 2.0*M_PI*Freq
#define pitch 7.5*M_PI/180

DEFINE_CG_MOTION(shm, dt, cg_vel, omega, time, dtime)
{
real omega;

omega = pitch*w*cos(w*time);

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

cg_omega[0] = 0.0;
cg_omega[1] = 0.0;
cg_omega[2] = omega;
}

You are simulating a pitching motion in Fluent,yeah?
you could define the pitching axis in Fluent in this way :
Cell Zone Conditions , rotation-axis origin , x=0.25(if Leading Edge is at 0,0)

you may define this point (0.25*chord length) as the centre of gravity location,too.

[ronak]

I'm working on a similar case. It's been validated while angle of attack equals zero but the results doesn't match when there is A.O.A and I don't know how to input Alpha(angle of attack) in the UDF
my UDF is:


#include "udf.h"

#define pi 3.141592653589793 //

DEFINE_CG_MOTION(airfoil_pitch, dt, vel, omega, time, dtime)
{
real pitchampl,omg,frequency;

/* reset velocities */
NV_S (vel, =, 0.0);
NV_S (omega, =, 0.0);

/* motion */
frequency=0.06;
omg=2*pi*frequenc;
pitchampl = 15 * pi / 180; /* angular pitch amplitude */

omega[2] = pitchampl *omg* cos(omg * time);
}