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May 6, 2013, 00:51 
2D Pitching Airfoil

#1 
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samrat himvanth nanduri
Join Date: May 2012
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Dear all,
I'm using a UDF for pitching airfoil . The function i'm using is omega[2] = 0.5 * pitchampl * (1cos(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 

May 6, 2013, 00:56 

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

May 6, 2013, 01:13 

#3  
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samrat himvanth nanduri
Join Date: May 2012
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Quote:
Thanks for your reply. I really didnt get wat u were saying. I want the airfoil to pitch using omega[2] = 0.5 * pitchampl * (1cos(w*time));. what changes do i make.. thanks in advance, regards, sam. 

May 6, 2013, 01:52 

#4 
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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 * (1cos(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. 

May 31, 2013, 01:37 

#5 
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samrat himvanth nanduri
Join Date: May 2012
Posts: 25
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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... 

May 31, 2013, 02:56 

#6 
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Thanks, I am OK.
The sliding mesh is sufficient for your purpose. Use Code:
DEFINE_TRANSIENT_PROFILE 

May 31, 2013, 03:00 

#7 
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samrat himvanth nanduri
Join Date: May 2012
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Thanks for your reply.. but wen i use sliding mesh the grid does not rotate at all..i cant see rotation in animation also...


February 3, 2014, 15:25 

#8 
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Rakesh
Join Date: Jan 2014
Location: Delft
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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 Zaxis. 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. Last edited by rsarma; February 5, 2014 at 12:23. 

February 22, 2014, 05:54 

#9  
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Musa
Join Date: Mar 2012
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Quote:
I am having the same problem, have you manage to solve it ? please share the solution with me.. Thanks 

February 22, 2014, 09:26 

#10 
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Rakesh
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Dear Fkanyako,
Yeah it is solved now. I replaced sin by cos and it actually worked. 

February 25, 2014, 06:35 

#11 
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Musa
Join Date: Mar 2012
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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 

February 25, 2014, 07:08 

#12 
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Rakesh
Join Date: Jan 2014
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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! 

January 10, 2015, 14:14 

#13 
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Hello everybody
I'm working on incompressible unsteady force measurement of twodimensional naca0012 under pitchup, hold and pitchdown 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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