Having difficulties in writing udf define_CG_motion for a piston
Please can someone help me write a udf to define CG_motion for a piston inside a cilinder.
the equation for the position is Y=27*cos(A)+sqrt(100^2  (27^2)*sin^2(A)) Thank you so much everybody 
hope it's not too late....
DEFINE_CG_MOTION(piston,dt,vel,omega,time,dtime) { real Y = ; real A = ; /* reset velocities */ NV_S(vel, =, 0.0); NV_S(omega, =, 0.0); if (!Data_Valid_P()) /* Do nothing if gradient isn’t allocated yet. */ return; Y = 27*cos(A)+sqrt(100^2  (27^2)*sin^2(A)); /* sets the components of velocity */ vel[0] = 0; vel[1] = Y; vel[2] = 0; } this udf is for the speed not the position..... 
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There are many UDFs available for velocity. But I don't know the equation for velocity. I have equation of displacement. How do I implement that? 
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So if I were you, I would go for the easy solution, and use the equation of velocity. 
hi friends,
I have a piston sinusoidal motion code ... #include "udf.h" #include <stdio.h> DEFINE_CG_MOTION(piston_mov, dt, vel, omega, time, dtime) { real a, w, pi; pi = 3.1415; /* define motion variables */ a = 0.007; /* 0.012m movement amplitude */ w = 2 * pi * 30; /* 30Hz (1800rpm) frequency */ /* define object movement law */ vel[0] = 0; vel[1] = a * w * sin(w*time); vel[2] = 0; } its working very well. But i want to set the velocity and displacement as according below mentioned equations Let suppose equation of displacement A=0.0036; theta=0:360; Xp=A*sin(theta)+(cos(theta))^0.5 And velocity Vp=A*w(cos(theta)sin(theta))^0.5 Please tell me, how i can modify the above udf according these equations Thanks 
You should write your equations as functions of time. Currently it is a function of theta (which is probably an angle), but you should specify how the angle depends on time.

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