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CL history for pitching airfoil

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Old   February 14, 2014, 05:16
Default CL history for pitching airfoil
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Harry
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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
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Old   February 14, 2014, 05:35
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Quote:
Originally Posted by star View Post
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!
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Old   February 14, 2014, 11:45
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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.
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Old   February 22, 2014, 05:44
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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
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Old   February 22, 2014, 07:10
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Originally Posted by fkanyako View Post
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);
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Old   February 22, 2014, 07:50
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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
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Old   February 22, 2014, 09:00
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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]
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