[amin.z]

Hi friends!

I'm working on a problem about projectiles moving!
I need a UDF to set mass, inertia and initial velocity of the projectile!
the problem is that I don't find a macro to set the velocity at start time!
Does it need a macro? or it should be set in fluent?

[ghost82]

Hi Amin,
to set mass and inertia you can use DEFINE_SDOF_PROPERTIES.
To set initial velocity you can use DEFINE_CG_MOTION to be applied to rigid bodies.

[amin.z]

Hi Daniele!
tnx! I haven't enough experience in dynamic mesh field!
I was thinking CG-MOTION is for a constant speed and uniform motion!
How can I use this macro for initial velocity? only a value for "vel" argument? what about values of "time" and "dtime"?

[ghost82]

Better explain with an example:

This udf, for example:

# include "udf.h"
# include "dynamesh_tools.h" 

DEFINE_CG_MOTION(mymotion, dt, vel, omega, time, dtime)
{
Thread *t;
face_t f;
vel[0]= 0.1;
vel[1] = 0.0;    
vel[2] = 0.0;
}

sets a constant translational velocity of 0.1 m/s in +x direction.
So, if you apply it to your rigid body at the first time step it will translate with 0.1 m/s velocity.

omega refers to rotational velocity (omega[0], omega[1], omega[2]).

If you want to change velocities with time, then you can use directly 'time' without use the macro CURRENT_TIME.

So if you have a linear increasing velocity you can replace for example in the udf:

vel[0]= 0.1*time;

dtime is the timestep; as 'time' you can use dtime directly into the udf if you need it.

[amin.z]

ok!
I need just a constant speed at first time step and want to fluent calculates the speed at final step! so did I just need "vel[0]= 0.1"?
and what about X , Y compnents!?

[ghost82]

Explain better your problem, as I cannot understand why you want a positive velocity only for the first timestep. And after the first one?

[amin.z]

yeah! I have a projectile! it's a component of a bigger moving part and released from it! so it has a initial velocity and I want to know what happen for it after a few seconds! it hasn't any internal force! only gravity and drag reduce its speed!
Is it clear dear?

[ghost82]

Can't this be simulated by patching with high pressure and temperature a small space adjacent to the projectile? Then it will be the pressure that will move the projectile, which will move with 6dof (or restricted 6 dof)

[amin.z]

I think it maybe causes significant error in simulation! and also will effects on next time steps!
there is a tab as "center of gravity velocity" in dynamic mesh tab! do you know what is this? I've attached a pic from this tab...

[ghost82]

It is the velocity of the center of gravity.
At time 0, your projectile has a defined position and a defined velocity.
For your case before igniting the high pressure and temperature zone, your projectile is at rest, so you can set velocity to 0 m/s.
Moreover you have to set the initial position of the center of gravity.
These values will be automatically updated during the simulation, as your projectile moves.

[amin.z]

Originally Posted by ghost82

It is the velocity of the center of gravity.
At time 0, your projectile has a defined position and a defined velocity.
For your case before igniting the high pressure and temperature zone, your projectile is at rest, so you can set velocity to 0 m/s.
Moreover you have to set the initial position of the center of gravity.
These values will be automatically updated during the simulation, as your projectile moves.

Ok! so I could use this tab for applying an initial velocity to the projectile! this velocity only use for time 0 and changes during the solution based on the domain conditions! is this correct!?

[ghost82]

Yes, I think so.

[amin.z]

Originally Posted by ghost82

Yes, I think so.

hooom! good! tnx friend

[AUGUZUN]

Hello.
Amin, did it helped ? Did you manage to solve your problem?
I have very simular one.