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-   -   2D Pitching Airfoil (http://www.cfd-online.com/Forums/fluent-udf/117270-2d-pitching-airfoil.html)

samcfd May 6, 2013 00:51

2D Pitching Airfoil
 
Dear all,

I'm using a UDF for pitching airfoil . The function i'm using is

omega[2] = 0.5 * pitchampl * (1-cos(w*time));

pitchampl = 25 * pi/180;

w = 2 * pi * 0.15;

when i use this function the airfoil is rotating 360 deg. i dont understand y..

but when i use omega[2] = 0.5 * pitchampl * sin(w*time); its pitching perfectly...

Could some1 pls tell me y this is happening.

thanks in advance,

regards,
sam

blackmask May 6, 2013 00:56

Note that omega is angular speed rather than the angle of attack. You angular speed is always non-negative so that your airfoil is always pitch in the same direction. You need something like
d 0.5 * pitchampl * (1-cos(w*time))
-----------------------------------
d time
which leads to
omega[2] = w*0.5 * pitchampl * sin(w*time);

samcfd May 6, 2013 01:13

Quote:

Originally Posted by blackmask (Post 425450)
Note that omega is angular speed rather than the angle of attack. You angular speed is always non-negative so that your airfoil is always pitch in the same direction. You need something like
d 0.5 * pitchampl * (1-cos(w*time))
-----------------------------------
d time
which leads to
omega[2] = w*0.5 * pitchampl * sin(w*time);

Dear Blackmask,

Thanks for your reply. I really didnt get wat u were saying. I want the airfoil to pitch using

omega[2] = 0.5 * pitchampl * (1-cos(w*time));.

what changes do i make..


thanks in advance,
regards,
sam.

blackmask May 6, 2013 01:52

I assume that you can tell the difference between pitch angle, say beta, and angular velocity, say omega. And you should know they are related by
omega[2] = d (beta) / dt (#)
While
beta(t) = 0.5 * pitchampl * (1-cos(w*time))
is the pitch angle in the above equation (#) (so that beta is in the range [0, pitchamp]), what you need to specify in the UDF is the angular velocity. That is what I tried to tell you in the last post.

samcfd May 31, 2013 01:37

Dear Blackmask,
How u keeping?? Hope all s gud.. I need a quick favor. Could u pls help me out on how to write a UDF for Rotating grid. Example: I have to circles one inside the otr n i need to rotate the inner circle at a particular RPM. mostly for turbine applications. could u help me..
thanks in advance...

blackmask May 31, 2013 02:56

Thanks, I am OK.

The sliding mesh is sufficient for your purpose. Use
Code:

DEFINE_TRANSIENT_PROFILE
for time varying rpm. Otherwise you do not need UDF at all.

samcfd May 31, 2013 03:00

Thanks for your reply.. but wen i use sliding mesh the grid does not rotate at all..i cant see rotation in animation also...

rsarma February 3, 2014 15:25

Dear Blackmask,

This is in continuation of the earlier message that I sent you. I tried adjusting sin by cos. But the problem still persists. Could you put forward any other suggestion. Here is my problem.
I am dealing with a pitching airfoil problem. I am using dynamic mesh with a udf.


My airfoil is pitching fine, but I see that there is some problem with the axis. It is pitching about the Z-axis. But it should have both positive and negative angles of attack. However my airfoil is either pitching above or below the zero angle of attack line. Could you kindly help me out?

Thanks.

fkanyako February 22, 2014 05:54

Quote:

Originally Posted by rsarma (Post 473186)
Dear Blackmask,

This is in continuation of the earlier message that I sent you. I tried adjusting sin by cos. But the problem still persists. Could you put forward any other suggestion. Here is my problem.
I am dealing with a pitching airfoil problem. I am using dynamic mesh with a udf.


My airfoil is pitching fine, but I see that there is some problem with the axis. It is pitching about the Z-axis. But it should have both positive and negative angles of attack. However my airfoil is either pitching above or below the zero angle of attack line. Could you kindly help me out?

Thanks.

Hi
I am having the same problem, have you manage to solve it ? please share the solution with me.. Thanks

rsarma February 22, 2014 09:26

Dear Fkanyako,

Yeah it is solved now. I replaced sin by cos and it actually worked.

fkanyako February 25, 2014 06:35

How did you determine your time step size ?

I normally cal. this way
omega = 2*pi *frequency

and t = 1/freq.

Dt = t/300 or 400 or 500 it depends on how many time steps per circle I want. If this approach is wrong then please tell how to do it.

Thanks

rsarma February 25, 2014 07:08

Well, your approach is correct. It all depends on the number of points you want in each cycle. I typically was interested in about 20 points in a cycle for my application. So you can accordingly adjust that.

Good Luck!

airbus January 10, 2015 14:14

Hello everybody
I'm working on incompressible unsteady force measurement of two-dimensional naca0012 under pitch-up, hold and pitch-down motion at a constant velocity about 50 m/s

Recently I examined the model in static mode at various AOA in Fluent , but i've a problem for write a correct""UDF"" for Dynamic mode test that airfoil at defined AOA by specified HOLD time (h : holding factor) set in during motion as pitch up, HOLD , and return for different cases by different amplitude angular pitching rates

I'll so grateful to help me for doing this part
Thank you


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