# Having difficulties in writing udf define_CG_motion for a piston

 Register Blogs Members List Search Today's Posts Mark Forums Read May 20, 2009, 17:35 Having difficulties in writing udf define_CG_motion for a piston #1 New Member   antonio santos Join Date: May 2009 Posts: 2 Rep Power: 0 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   May 5, 2012, 21:32 #2 New Member   Jessica Simberg Join Date: Mar 2012 Posts: 18 Rep Power: 13 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; vel = Y; vel = 0; } this udf is for the speed not the position..... chaitanyaarige and alireza.deimekar like this.   February 20, 2015, 06:02 #3
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 Originally Posted by Jessica.S 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; vel = Y; vel = 0; } this udf is for the speed not the position.....

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?   February 20, 2015, 07:30 #4
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 Originally Posted by tsram90 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?
If you have the equation of displacement, it should be easy to derive the equation for velocity, don't you think?

So if I were you, I would go for the easy solution, and use the equation of velocity.   April 7, 2015, 06:14 #5 New Member   umair Join Date: Mar 2015 Posts: 1 Rep Power: 0 hi friends, I have a piston sinusoidal motion code ... #include "udf.h" #include 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; vel = a * w * sin(w*time); vel = 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 chaitanyaarige and Chakra like this.   April 13, 2015, 05:03 #6 Senior Member   Join Date: Nov 2013 Posts: 1,965 Rep Power: 25 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.  Tags dynamic mesh, piston, udf Thread Tools Search this Thread Show Printable Version Email this Page Search this Thread: Advanced Search Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are On Refbacks are On Forum Rules Similar Threads Thread Thread Starter Forum Replies Last Post lakeat OpenFOAM Community Contributions 58 December 23, 2021 02:36 [snappyHexMesh] crash sHM H25E OpenFOAM Meshing & Mesh Conversion 11 November 10, 2014 11:27 Harish FLUENT 2 July 23, 2004 06:13 KKLAU FLUENT 1 April 6, 2004 23:58 umesh FLUENT 1 June 13, 2003 04:20

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