[john Lu]

Hi, everyone
I am doing a simplified simulation about one piston deforming axially(Z axis) while moving directly along X axis simultaneously.

Firstly, for piston deforming:
I defined the piston bottom as a rigid body, the udf program is:
#include<stdio.h>
#include<math.h>
#include"udf.h"
DEFINE_CG_MOTION(piston, dt, vel, omega, time, dtime)
{
const PI=3.1415926;
vel[0]=-0.1;
vel[2]=-((0.771*sin(50*PI*time))/(1-0.0085*cos(50*PI*time))/(1-0.0085*cos(50*PI*time)));
}
while vel[2]defining the piston bottom moves along Z axis, vel[0] describing piston body move along X axis.
the piston cylndrical surface is defined as deforming, the geometry of which is defined as cylinder.

Secondly, for the piston movement, which is defined in "cell zone conditions" the X axis translational velocity is defined as -0.1m/s, the same as written in udf.

However, preview mesh motion shows negative cell volume.
What's the reason?

P.S.
If only dynamic mesh is used, no problem.
If only mesh motion is used, no problem.
But, if two of them are used, negative cell volume occurs.

[john Lu]

It seems that, once the piston cylndrical surface is defined as deforming-cylinder, which would not move though the piston movement is defined in "cell zone contions- mesh motion".