[Anissa A]

Hello,



I'm trying to modelize an oscillating liquid piston. I want to study the surface of the liquid according to the frequency and the amplitude of the piston. I need to use a dynamic mesh and create a sine motion for the piston. I want to use the following C code to program this motion:


#include "udf.h"
DEFINE_PROFILE(unsteady_velocity, thread, position)
{
face_t f;
real t = CURRENT_TIME;
begin_f_loop(f, thread)
{
F_PROFILE(f, thread, position) = 0.05*sin(3.14*t);
}
end_f_loop(f, thread)
}


However, I'm not sure of the units of the amplitude and the frequency I'm using. How can I know if 0.05 is in mm, cm, m ?
Also my simulation is not working well and I haven't seen if the piston is really moving up and down following a sine wave. Is the code correct ?



Thanks for your help

[`e`]

Fluent uses SI base units and therefore length scales are in metres and time is in seconds. The DEFINE_PROFILE macro is used for changing flow variables such as velocity to impose on a boundary condition (e.g. inlet). The dynamic mesh uses a different set of macros, see "Dynamic Mesh DEFINE Macros" in the UDF manual for reference.

[Anissa A]

Thank you for your reply,



I've checked all the possible macros and maybe you think that DEFINE_CG_MOTION is more suitable to define a piston motion ?

[Anissa A]

As I am working in 2D, if the piston motion is given by: y=A*sin(wt), the code will be:


#include "udf.h"

DEFINE_CG_MOTION(piston_motion, dt, vel, omega, time, dtime)
{
real A, w, pi;
pi = 3.14;

NV_S(vel, =, 0.0);
NV_S(omega, =, 0.0);


A = 0.005;
w = 2 * pi * 1; /* 1Hz frequency */


vel[0] = 0;
vel[1] = A * sin(w*time);
}


Is that correct ?

[`e`]

Is y a position or velocity? The DEFINE_CG_MOTION uses velocities. For an example of using this macro for a piston motion, see: CG Motion UDF Problem