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April 6, 2015, 23:19 
dynamic mesh using define grid motion udf

#1 
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fredrick
Join Date: Apr 2015
Location: China
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hello ladies and gentlemen. I am trying to simulate deflecting a beam that is acting like a fan for a heat sink. i am using define grid motion udf but every time i try "smoothing" and "remeshing", i get a problem of negative volume error. Kindly help me to workaround this problem. this is my code based on the simple deflection equation of the fluent udf tutorial.
the equation for my deflection is s(z)=amplitude*sin(2*pi*f*time)*(z/0.075)^2 hence the velocity becomes velocity = amp * w * cos(w * time)*(z/0.075)^2; z ranges from zero to 75mm but i want deflection to start at 20mm this is my code #include "udf.h" DEFINE_GRID_MOTION(fan1,main, dt, time, dtime) { Thread *tf = DT_THREAD (dt); face_t f; Node *v; real NV_VEC (velocity), NV_VEC (axis); real NV_VEC (origin), NV_VEC (rvec); real velc; real freq; real amp; real w; int n; /* set deforming flag on adjacent cell zone */ SET_DEFORMING_THREAD_FLAG (THREAD_T0 (tf)); freq=50; amp=0.003; w=2*3.141592654*freq; velc = amp * w * cos(w * time); NV_S (velocity, =, 0.0); NV_D (axis, =, 0.0,0.0, 1.0); NV_D (origin, =, 0.00307, 0.023, 0.075);/*center of gravity for my beam*/ begin_f_loop (f, tf) { f_node_loop (f, tf, n) { v = F_NODE (f, tf, n); /* update node if z position is greater than 0.02 and that the current node has not been previously visited when looping through previous faces */ if (NODE_Z (v) > 0.020 && NODE_POS_NEED_UPDATE (v)) { /* indicate that node position has been update so that it's not updated more than once */ NODE_POS_UPDATED (v); velocity[0] = velc * pow (NODE_Z (v)/0.075, 2); NV_V_VS (rvec, =, NODE_COORD (v), +, velocity, *, dtime); NV_V (NODE_COORD (v), =, rvec); } } } end_f_loop (f, tf); } kindly guys, assist me with any suggestion. 

May 29, 2015, 03:43 
Dear Fredrick

#2 
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MaSoud Rahimi
Join Date: Jan 2015
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I am also working on beam and dynamic mesh,i think it is really sensitive to time step and cell size. a minimum time step you could have is :1/8(1/freq).


July 14, 2015, 02:56 
thanks masoud6

#3 
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fredrick
Join Date: Apr 2015
Location: China
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hello. thanks for your reply. do you think the code i have written for the dynamic mesh is okay?


July 14, 2015, 06:19 
another strategy for grid motion

#4 
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MaSoud Rahimi
Join Date: Jan 2015
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it's true logically but it depends on your geometry. if it possible put the geometry. however you can carry out another strategy for code implement as following bellow (use your own equation and beam characteristic):
#include"udf.h" #include"unsteady.h" #include"dynamesh_tools.h" #include "udf.h" DEFINE_GRID_MOTION(beam,domain,dt,time,dtime) { Thread *tf; face_t f; int n; Node *v; /* get the thread pointer for which this motion is defined */ tf=DT_THREAD(dt); begin_f_loop(f,tf) { f_node_loop(f,tf,n) { v = F_NODE(f,tf,n); NODE_Y(v)=NODE_Y(v)+fabs(NODE_X(v))*dtime; } } end_f_loop(f,tf); } 

August 28, 2015, 03:32 
definition

#5 
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MaSoud Rahimi
Join Date: Jan 2015
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Dear Fredrick
could you please tell me what does these three lines mean, specially line 3 ? NV_S (velocity, =, 0.0); NV_D (axis, =, 0.0,0.0, 1.0); NV_D (origin, =, 0.00307, 0.023, 0.075);/*center of gravity for my beam*/ 

August 28, 2015, 05:16 

#6  
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Amin
Join Date: Oct 2013
Location: Germany
Posts: 377
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Quote:
I'm working on same projects, the first question, Fredrick, what's the direction of the beam in your code? Is it in X direction? I've also prepared a same code based on fluent tutorial but it's not working, actually the domain is completely stationary! My domain is 2D and I've checked the beam in X and Y direction, bot it's still not working 

August 28, 2015, 05:17 

#7  
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Amin
Join Date: Oct 2013
Location: Germany
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Quote:
Based on your code, is the beam bending? or only rotate as a rigid beam? 

August 31, 2015, 00:14 
bending grid motion

#8 
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MaSoud Rahimi
Join Date: Jan 2015
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Dear Amin
the geometry has been attached. as can be seen from fig. this cod is expedient for bending of a beam. best 

October 11, 2015, 22:25 
masoud6

#9  
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fredrick
Join Date: Apr 2015
Location: China
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Quote:
my thoughts are, since I'm working with velocity equation, the first line is to reset vel as 0; the second line means my beam lies in the z axis. the third line i suspect I'm wrong.. maybe i could have ignored x, y and just indicate z (0.0, 0.0, 0.075) since the starting point for my beam is 75mm in the Z axis... I'm restarting the project and I am still stuck. I guess your code looks nice but i don't understand this line "NODE_Y(v)=NODE_Y(v)+fabs(NODE_X(v))*dtime; " what do u mean by fabs? couldyou also kindly send me your entire code so to my email if you don't mind "irungufred@yahoo.com". thanks 

October 11, 2015, 22:35 

#10  
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fredrick
Join Date: Apr 2015
Location: China
Posts: 5
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Quote:
sorry that I could not reply on time. my domain is 3D. mine works and I can see the bean vibrating but the problem of negative volume is the headache. My beam lies in the Z axis. It vibrates in X direction. which equation of vibration did you use? 

October 14, 2016, 02:37 
define grid motion

#11 
New Member
Amit soni
Join Date: Jul 2015
Posts: 10
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In my case it is showing
Warning: incorrect cg motion UDF beam::libudf on zone 14 (assuming no motion), What does it mean 

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