T6: Flow through a nozzle:
accelerating flows and boundaries

Before the tutorials:

Think about:

• What happens when a flow passes through a Nozzle?
  You might find this video from the 1960s useful, even more information can be found here.
• What was is the resolution in our SPH simulations (e.g., check the HSML from one of the previous tests and interpret it)
• What is now the minimum size of regions to be described consistently in the simulation?

During the tutorials:

You can run the experiment from above movie.

A flow through a nozzle

• Create a slab with uniform distribution of particles (say, 400×20×20).
• Assume a empty space of equal size to the left.
• Boundary particles can be defined by setting their id values to 0.
  Such particles will not feel any forces but keep their initial movement.
• Create a setup mimicking the vacuum pump and the nozzle from the simulation setup above.
  • How can you geometrically define a nozzle?
  • How can you set the id values for these regions to zero?
  • How can you make a wall move (e.g., setting the velocity in the x direction to a reasonable value, like −1)?
• Now create the setup:
  • First assume a wall at the right end of the slab.
  • Second create a wall on the left end of the slab, which is moving to the left.
  • Create a symmetric nozzle (i.e., a reduction of the area in the y direction) at the left side of the slab (but leave some place for the initial position of the moving wall).
  • Now move all more to the right side, so that the left wall can safely move to the left.
  • Hint: Here a is sketch of the geometry:

Now we can perform the simulation and analyze it.

• When compiling the code, do not forget to:
  • Insert the new settings SPH_BND_PARTICLES and SPH_BND_PARTICLES_ALLOW_DRIFT into the Config.sh
  • Do not forget to enable NOGRAVITY, TWODIMS and PERIODIC
  • Switch on LONG_X, LONG_Y and LONG_Z and set them to proper values.
  • Think which Kernel to use.
• Before running, edit the box.param file:
  • Choose resonable values for TimeMax and TimeBetSnapshot
  • Add ArtBulkViscConst and set it to 0.8
• some example initial conditions to start with:
  wget https://www.usm.uni-muenchen.de/~dolag/Lille2026/T06/box.ic
• Depending on your laptop we sugegst to use as much as possible CPUs for this experiment.
• Try to explain what is happening in the simulation.
• Is the gas at the beginning streaming through the nozzle (if not, why)?
• What happens to the gas on the opposite of the nozzle (and why)?
• Try to have a look at the temperature of the gas.
• What happens when the gas is finally flowing through the nozzle?
• As usual, you might increase resolution of the initial condition.

Programming goals for T6:

Producing initial condition with special boundary regions, especially we will

• learn how to create boundary regions in the initial conditions
• how to define special shapes and obstacles

Solution

As a 2D test case;

• Again, extending the setup script from previous experiments we can start with a slab with a base grid of 1000×50 sand the place the different obstacles in it.
  • wget https://www.usm.uni-muenchen.de/~dolag/Hydro/Hydro/Python/ic_header (if not already done before)
  • wget https://www.usm.uni-muenchen.de/~dolag/Lille2026/T06/setup_nozzle.py
  • python3 setup_nozzle.py
  you should get something like shown below, where the black and blue points indicate the defined boundary particles (black = stationary, blue = moving)
• Now vizualize the results after running the simulation
  
  • wget https://www.usm.uni-muenchen.de/~dolag/Lille2026/T06/show_nozzle.py
  • python3 show_nozzle.py
  you should be able to obtain an animation like this and identify all the phenomena discussed in the experiment from above

As a 3D test case (needs significant more computing power);

• wget https://www.usm.uni-muenchen.de/~dolag/Hydro/Hydro/glass_10x10x10
• wget https://www.usm.uni-muenchen.de/~dolag/Hydro/Hydro/Python/ic_header (if not already done before)
• wget https://www.usm.uni-muenchen.de/~dolag/Lille2026/T06/setup_nozzle_3d.py
• python3 setup_nozzle_3d.py

Useful commands

• More informations on the OpenGadget3 containers
• Start your docker session:docker run -it -v :/mnt --name opengadget3_container opengadget3:XXX
• Connect from a second shell to a running docker session:docker exec -it opengadget3_container bash
• Get the simulation code:cp -r ~/OpenGadget3 .
• Copy the config file: wget https://www.usm.uni-muenchen.de/~dolag/Hydro/Hydro/Config.sh
• Copy the parameter file: wget https://www.usm.uni-muenchen.de/~dolag/Hydro/Hydro/box.param
• Setup the environment for compiling the code: source Build/Container_build.sh
• Compile the code: make -j
• Run a simulation: mpiexec -np 2 OpenGadget3/OpenGadget3 box.param