docs/pnmhd/6.txt File Reference

Detailed Description

Modifying Viscosity
Version 1 Created by Jonathan M. on 30 May 2009.
BTW,

I forgot to mention how to modify the viscosity in case that’s needed.

1)

In global.h there is also:

turn on/off different terms (1 or 0)

#define VISCE11 1

#define VISCE22 1

#define VISCE33 1

#define VISCE12 1

#define VISCE21 VISCE12

#define VISCE13 1

#define VISCE31 VISCE13

#define VISCE23 1

#define VISCE32 VISCE23

These turn on/off each component of the viscous stress tensor, where in spherical polar coordinates 1=r 2=theta 3=phi. Often people only use VISCE13=r\phi. I tried both all and only r\phi as in my paper.

2) Next, in init.c there is a function called init_visc(void). This sets the functional form of the viscosity, where “vreal” determines which one is used. You are using vreal==4. Make sure you understand whether the GM, etc. are correctly chosen.

3) Next, in init.c there is a parameter called alpha_real0 that sets the coefficient of the viscosity. Also see in stepgen.c where alpha_real itself is set (no 0 at end) based upon the time of the simulation. This turns on/off viscosity. It’s often useful to avoid having viscosity turned on right at t=0 due to the initial conditions often having very sharp edges that the viscosity tensor triggers too strongly on.

4) Finally, see in stepgen.c how nu_real is finally computed from alpha_real and nu_fact in the function void nu_compute(void). Since you are using vreal==4, this gives $\nu = \alpha c_s^2/\Omega_{Keplerian}$. So that means the thing in nu_fact was $1/\Omega_K$. Make sure it is correct!

Cheers!

Jon

Definition in file 6.txt.