T10: Dynamical Friction

Before the tutorials:

Do, think about, and research the following:

• Bring your own program to the level of the example given for the last tutorial, so that you can start from a clean and working version.
• What is dynamical friction ?
• What is the approximate timescale of the process ? Hint: For a short introduction to the subject, see chapter 4.5 of the PhD thesis of Francesca Iannuzzi.

During the tutorials:

Program it!

• load the particles from the file plummer_1000.ascii or plummer_10000.ascii.gz or plummer_100000.ascii.gz
  Hint: Switch the internal units to
  m_unit = 1.989e43; // 1e10 Msol
  l_unit = 3.085678e21; // kpc
  v_unit = 1e5; // km/s
  t_unit = l_unit / v_unit;
• Add a particle with a twenty time larger mass on a circular orbit at a distance of five length units. Recall how to calculate the circular velocity of such particle.
• Try to plot the orbit of the test particle with time and compare to the expectation.

Example programs:

These are just examples.

• C/C++:
  • Current version (making extensively use of C++ features):
    example.cc (colorized), example.h (colorized), const.h (colorized), Makefile (colorized)
  • Example with all particles on individual timesteps (quite slow):
    example2.cc (colorized)
  • Example extended with particles on power of two timestep bins (enable the -DUSE_TIMESTEP_LEVELS in Makefile or give it as extra option when compiling):
    example3.cc (colorized)
• F90:
  • Simple version (this is sufficient for the moment, but it doesn’t hurt you to start improving it as suggested above):
    T10.f90 (colorized), Makefile (colorized) (rename to “Makefile” or use with “make -f Makefile_gfortran”)
  • More complex version:
    (To be added later. We will reorganize the program structure when we get to multiple particles; think about how you would do this on your own.)

Discussion of the results:

Plot your results with gnuplot and plot the behaviour of the systems. Can you explain what you see ?

Example plots:

Programming goals for T10:

Having many particles in our system and getting the program slow, we might think on a trivial parallelization. If you havn't done last time (T9), then think about:

• What independent within a independent loop ?
• What is the most time consuming loop where you should do something ?
• Which loop is difficult to be paralleled and why ?