Gravity - hyschive/gamer-fork GitHub Wiki

Related options: --gravity,   --pot_scheme,   --store_pot_ghost,   --unsplit_gravity,   --comoving  

Runtime parameters: Gravity

Other related parameters: DT__GRAVITY,   POT_GPU_NPGROUP  

Follow the steps below to add external acceleration with an analytical function. Get familiar with the general procedure of Adding Problem-specific Functionalities first and be aware that adding external acceleration/potential takes a few extra steps in order to utilize GPUs.

1. Enable OPT__EXT_ACC.
2. Go to your new test problem folder and copy the built-in point-mass acceleration routine as a template:

   cp ../../../SelfGravity/CPU_Gravity/CPU_ExtAcc_PointMass.cpp ExtAcc_NewProblem.cpp
   

3. Edit ExtAcc_NewProblem.cpp to
   • Replace the keyword PointMass by NewProblem (or whatever appropriate).
   • Edit the routine SetExtAccAuxArray_NewProblem() to set the auxiliary array AuxArray[], which will be automatically passed to ExtAcc_NewProblem() as the input array UserArray[]. The array size is set by EXT_ACC_NAUX_MAX in include/Macro.h (default 20).
   • Edit the routine ExtAcc_NewProblem(), which should return the external acceleration Acc[] at (x, y, z, Time).
4. Link ExtAcc_NewProblem.cpp to a corresponding CUDA source file:

   ln -s ExtAcc_NewProblem.cpp ExtAcc_NewProblem.cu
   

   This avoids writing redundant codes for CPU and GPU runs separately.
5. Edit your problem source file (e.g., Init_TestProb_Hydro_NewProblem.cpp) to
   • Declare a function prototype on top of the file:

     void Init_ExtAcc_NewProblem();

   • Set up a function pointer in your problem initialization function (e.g., Init_TestProb_Hydro_NewProblem()):

     # ifdef GRAVITY
       Init_ExtAcc_Ptr = Init_ExtAcc_NewProblem
     # endif

Adding external potential can be done in a very similar way. Enable OPT__EXT_POT=1 and then follow the procedure above and replace Acc by Pot. A point-mass example can be found at src/SelfGravity/CPU_Poisson/CPU_ExtPot_PointMass.cpp.

Caution: wave dark matter simulations (i.e., --model=ELBDM) does not support external acceleration. Just use external potential.

External potential can also be set via interpolating a table loaded from disk. Set OPT__EXT_POT=2 and all EXT_POT_TABLE_* parameters. The following C++ example creates such a table with a point-mass potential.

#include <cstdio>
#include <cmath>

typedef double real; // this must be consistent with the compilation option FLOAT8

int main()
{
// parameters
   const char  *Filename = "ExtPotTable";       // EXT_POT_TABLE_NAME
   const int    NX       = 131;                 // EXT_POT_TABLE_NPOINT_X
   const int    NY       = 131;                 // EXT_POT_TABLE_NPOINT_Y
   const int    NZ       = 131;                 // EXT_POT_TABLE_NPOINT_Z
   const double dh       = 0.0078125;           // EXT_POT_TABLE_NPOINT_DH
   const double EdgeL[3] = { -dh, -dh, -dh };   // EXT_POT_TABLE_EDGEL_X/Y/Z

// set the external potential array
   const double G  = 1.0;                       // gravitational constant
   const double M  = 1.0;                       // particle mass
   const double cx = EdgeL[0] + 0.5*(NX-1)*dh;  // centre
   const double cy = EdgeL[1] + 0.5*(NY-1)*dh;
   const double cz = EdgeL[2] + 0.5*(NZ-1)*dh;

   double dx, dy, dz, r;

   real (*ExtPot)[NY][NX] = new real [NZ][NY][NX];

   for (int k=0; k<NZ; k++)  {  dz = EdgeL[2] + k*dh - cz;
   for (int j=0; j<NY; j++)  {  dy = EdgeL[1] + j*dh - cy;
   for (int i=0; i<NX; i++)  {  dx = EdgeL[0] + i*dh - cx;

      r               = sqrt( dx*dx + dy*dy + dz*dz );
      ExtPot[k][j][i] = -G*M/r;

   }}}

// output
   FILE *File = fopen( Filename, "wb" );
   fwrite( ExtPot, sizeof(real), (long)NX*NY*NZ, File );
   fclose( File );

   delete [] ExtPot;
}

[Optional] To implement a user-specified interpolation routine, use src/SelfGravity/CPU_Poisson/CPU_ExtPot_Tabular.cpp as a template and follow the steps [2-5] in Using Analytical Function.

Caution: external acceleration does not support this functionality yet.

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