[star]

Hi,
In pitching airfoil case the angle of attack changes with time. In Solve/Monitors, we can plot CL history but that is for some fixed angle of attack (as there is no udf option there). Can anybody tell me how to get CL history in this case?
Thanks in advance

[Anna Tian]

Originally Posted by star

Hi,
In pitching airfoil case the angle of attack changes with time. In Solve/Monitors, we can plot CL history but that is for some fixed angle of attack (as there is no udf option there). Can anybody tell me how to get CL history in this case?
Thanks in advance

Monitor -> edit 'Cl' -> click on 'write' and fill in the file name

Then after running the simulation, you can find the cl-history file in the case folder. That is the same for the changing AoA.

Good luck!

[star]

Thanks Meimei for your reply. I am not sure whether i should write the X and Y components in CL because it depends on angle of attack (in my case it's changing) but i will try it.

[fkanyako]

Hi Star,
I am also running simulation pitching airfoil
using the equation a(t) = a0 +a1 * sin(wt).
where a0 = 15, a1 = 10, (ao is the mean angle of attack). I took the time derivative of this equation to get the angular velocity and I wrote the udf like this omega(2) = w *a1* cos(wt). As for the a0 i set it in fluent that the velocity is coming at 15 degrees which is the mean angle of attack.
x = -5sin(15)
y = 5cos(15) let say free stream velocity is 5.
I also set the force vectors for lift and drag under solve/monitor. at this same angle 15 deg.

But the lift coefficient history when plotted against the angle of attack its not comparable to experiment. Please let me know if am writing the udf correctly or do you mind just to show me your udf... thanks

[star]

Originally Posted by fkanyako

Hi Star,
I am also running simulation pitching airfoil
using the equation a(t) = a0 +a1 * sin(wt).
where a0 = 15, a1 = 10, (ao is the mean angle of attack). I took the time derivative of this equation to get the angular velocity and I wrote the udf like this omega(2) = w *a1* cos(wt). As for the a0 i set it in fluent that the velocity is coming at 15 degrees which is the mean angle of attack.
x = -5sin(15)
y = 5cos(15) let say free stream velocity is 5.
I also set the force vectors for lift and drag under solve/monitor. at this same angle 15 deg.

But the lift coefficient history when plotted against the angle of attack its not comparable to experiment. Please let me know if am writing the udf correctly or do you mind just to show me your udf... thanks

Before running, you must check pitch motion through Display Zone Motion. If it works as you want then udf is ok. I am not expert in this. you can find more udf's in this forum. Here i have attached mine. It works fine.

#include <stdio.h>
#include <math.h>
#include "udf.h"

#define pi 3.141592653589793 //

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

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

/* motion */
pitchampl = 4 * pi / 180; /* angular pitch amplitude */
omg = 2 * pi * 40; /* angular frequency */
pitchdispl = pitchampl * sin(omg * time); /* resultant displacement */

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

[fkanyako]

Thanks a lot Star.
Looking your UDF, we basically have the same. When I preview mesh motion its working fine. But the result is not so good. The problem am having maybe its with the setting especially the boundary condition. are you ptiching your airfoil at any mean angle of attack ?.

Also I get the lift coefficient between 0.4 and -0.1 vs time, sinusoidal. But the experimental is between 3.0 and -0.5 vs time. If you manage to validate your result please explain the settings and also how u computed the lift coefficient vs angle of attack. Thanks

[star]

It depends on no. of factors. This question is not specific. Quality of your mesh, Reynolds number, time step size and other boundary conditions. You should search previous posts related to your problem.
[Be sure you are using correct pitching amplitude and frequency]

[lfla]

I am currently doing something similar and i have set inlet velocity in x and y components. x=30cos8 and y:30sin8.
I applied the udf to the fluid using mesh motion and selecting the udf

#include <stdio.h>
#include <math.h>
#include "udf.h"

DEFINE_CG_MOTION(airfoil_pitch, dt, vel, omega, time, dtime)
{
real pitchampl, pitchdispl, omg, pi, count;

pi = 3.14159265;


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

/* motion */
pitchampl = 5.5 * pi / 180; /* angular pitch amplitude */
omg = 2 * pi * 1.2; /* angular frequency */
pitchdispl = pitchampl * sin(omg * time); /* resultant displacement */

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

Do you have any advice for, thank you i would reallu appreciate it

[lfla]

Originally Posted by fkanyako

Hi Star,
As for the a0 i set it in fluent that the velocity is coming at 15 degrees which is the mean angle of attack.
x = -5sin(15)
y = 5cos(15) let say free stream velocity is 5.
I also set the force vectors for lift and drag under solve/monitor. at this same angle 15 deg.

Hi fkanyako,
Could i ask you why did you not use x=5cos15 and y 5sin15 Thank you