T6: Propagating sound waves:
sound speed and interference

Before the tutorials:

Think about:

• What are waves?
• What is the difference between a longitudinal wave and a transversal wave?
• What is the sound speed of a medium?
• What is the Amplitude of a wave?
• What is the wavelength?

During the tutorials:

To run the experiment, we first need to

• Create a slab with uniform distribution of particles (say 200x10x10)
• Initialize a sound wave

Now we can perform the simulation and analyze it.

• When compiling the code, do not forget to switch on PERIODIC and NOGRAVITY again. Also you have to indicate the non quadratic form of the simulation domain by setting LONG_X=20, LONG_Y=1 and LONG_Z=1 in the Config.sh file before compiling.
• When setting proper values in the file box.param, do not forget to also set PeriodicBoundariesOn 1 again, before running the simulation.
• Verify that the perturbation propagates with the sound speed.
• Verify that the propagation speed does not depend on the amplitude of the wave.
• Check what happens if you increase the wavelength.

Programming goals for T6:

Again, we are producing different initial conditions and run simulations on non-cubic domains, especially we will

• glue together particle distributions to create a slab-like region
• set up some initial perturbation
• use MPI or OpenMP parallel features to speed up the code. Watch what the programs says at the beginning:
  Running on 1 MPI tasks
  Using 1 OpenMP threads
  you can change this by:
  • export OMP_NUM_THREADS=4
    which should then display like:
    Running on 1 MPI tasks
    Using 4 OpenMP threads
    
  • mpirun -np 4 OpenGadget3/P-Gadget3 box.param
    which should then display like:
    Running on 4 MPI tasks
    Using 1 OpenMP threads
    

Solutions

• Examples for a script to create a larger box of particles out of the small example patches from T03 and to place different perturbation in it.
  • setup_slab.pro in IDL
• Examples for a the initial velocity perturbation
  

• Examples for a script to show the time evolution
  • show_wave.pro in IDL
• Examples for the propagation of a longitudinal wave
  

• Examples for a transverse wave
  

• Examples comparing waves with different wavelength or amplitudes
  

• Example using Fortran and gnuplot:
  • ifort -g -traceback -check all -fpe0 -o slabsetup slabsetup.f90
    (this uses glass.txt from T04)
  • ./slabsetup
  • run Gadget using slab.ic as the initial conditions file and output as the snapshot file base in the parameter file
  • ifort -g -traceback -check all -fpe0 -o readsnap readsnap.f90
  • for file in output_???; do ./readsnap $file >$file.txt; done
  • gnuplot plot.plt
  • gv plot.ps