# Problem with UDF for CG Circular Motion

 Register Blogs Members List Search Today's Posts Mark Forums Read

 September 15, 2016, 15:21 Problem with UDF for CG Circular Motion #1 New Member   Luthfilaudri Nadhira Join Date: Sep 2016 Posts: 4 Rep Power: 3 Hi Everyone, I am quite new in C++ programming, especially in UDF for Fluent. I have tried writing a UDF for circular motion for a solid body with radius "R" (0.05 meter) and constant rotational speed "rpm" (3000 rpm) on x-z plane in clockwise direction. Below is the UDF I wrote: =============================================== #include #include #include "udf.h" DEFINE_CG_MOTION(circl, dt, vel, omega, time, dtime) { real rpm, pi, R, theta; pi = 3.14159265; rpm = 2*pi*3000/60; R = 0.05; NV_S(vel, = , 0.0); NV_S(omega, = , 0.0); vel[0] = R*rpm*sin(rpm*time); vel[2] = -R*rpm*cos(rpm*time); theta = rpm*time*180/pi; Message("time:%f vel0:%f vel2:%f theta:%f \n", time, vel[0], vel[2], theta); } =============================================== The problem that I have is the preview motion of the UDF is different for different time step that I used. I believe that different time step should not change the trajectory of the motion, right? It should only affect the smoothness of the motion. I don't what is wrong with my UDF. Does anyone know what is the problem with my UDF? Thanks in advance!

 September 16, 2016, 02:08 #2 Member   Davoud Malekian Join Date: Jan 2016 Posts: 53 Rep Power: 3 Hello, i give you an example here so maybe we can understand your motion better: assume that you want to calculate vel[0] for these time steps: 0.1s & 0.25s until you reach 1 second. the results are listed below: for time_step=0.1s *******************for time_step=0.25s 0.1s vel[0]=-129.9******************** 0.25s vel[0]=75 0.2s vel[0]=-129.9 0.3s vel[0]=0 0.4s vel[0]=129.9 0.5s vel[0]=129.9******************** 0.5s vel[0]=129.9 0.6s vel[0]=0 0.7s vel[0]=-129.9 0.8s vel[0]=-129.9********************0.75s vel[0]=150 0.9s vel[0]=0 1s vel[0]=129.9******************** 1s vel[0]=129.9 as you can see in the results, for the time_step=0.25 we didnt get any negative(minus) velocity nor zero velocity, but when the time_step is 0.1s we have zero and negative velocities, the only thing that can be said about your project is that by reducing the time_step your motion can be simulated more accurately but by increasing time_step your motion will be just an approximation of your real motion and it won't be that accurate(so many fluctuations and ... will be dismissed) and one more thing, i think(not sure) your code is ok. regards,

 Thread Tools Display Modes Linear 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 On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Rizwan Fluent UDF and Scheme Programming 39 June 29, 2017 19:04 hadii FLUENT 9 July 20, 2016 04:46 leoarmas Fluent UDF and Scheme Programming 8 May 14, 2016 03:04 sohail_27 Fluent UDF and Scheme Programming 4 October 4, 2010 13:36 anant FLUENT 2 January 17, 2007 01:15

All times are GMT -4. The time now is 13:05.