Problem with implementing tau in udf
 

HI!
I HAVE TO WRITE A UDF FOR FLAPPING MOTION OF A FLATE PLATE IN WHICH ALPHA IS ALSO CHANGING.
IM UNABLE TO UNDERSTAND HOW TO INCORPORATE TAU OF THE FOLLOWING EQUATIONS.

a" + = 0.5a'+{1 - cos[2pi(t-tr)/Dtr]}; tr<t<tr+Dtr

a"=alpha dot , a'=alpha dot subscript knot, t=tau

IF SOMEONE CAN PLEASE HELP, I'LL BE REALLY VERY GRATEFUL.
REGARDS

what does it mean "UNABLE TO UNDERSTAND HOW TO INCORPORATE TAU OF THE FOLLOWING EQUATIONS."

and what is tau?
show the cod you already have

Thank you so much for your reply Sir.
Sir my problem consists of flapping in three parts:
1. Pitching down rotation and translation acceleration
2.translation at constant angle of attack and constant speed
3. pitching up rotation and translational deceleration at end of stroke.
I have all the equations and the intervals of tau (non dimensional )time as well.
I am unable to write it in UDF.
If you can please help