[FLASH-USERS] How can I add density property to the particle

[Tristan]

Hello, everyone:
I am doing a simulation about the characteristic of the high mach turbulence. I want to add some passive Lagrangian particles into the simulation. But the default properties of particles include velocity and position, not the density. I want to know the density of the trajectory the particles draw.
Maybe use the Runge-Kutta method.

I have reading the guide and add the two line to the ParticlesMain unit Config :

```
PARTICLEPROP dens REAL
PARTICLEMAP TO dens FROM VARIABLE dens
```

I can get the particles output file, it includes the density property, but all the values are  -1. I guess the density is initialized but not updated each step.

Do I need to change the  pt_advanceRK.F90 in the ParticlesMain/passive/RungeKutta directory? If someone have done the same thing. Please give me some advice. 
Thanks!
Best wishes!