Bunny Drill#
This example simulates the drilling motion of a bunny within a particle bed. It illustrates that the DEM module of Lethe can simulate complex moving objects and is a testament to our love of lagomorphs. Do not worry friend, no bunnies were hurt in the making of this example!
Features#
Solvers:
lethe-particlesFloating walls
GMSH grids
Insertion of particles from a plane
Files Used in this Example#
All files mentioned below are located in the example’s folder (examples/dem/3d-bunny-drill).
GMSH mesh of the bunny:
bunny-low-poly.mshgenerated using the corresponding STL filebunny-low-poly.stlParameter file used to load the particles:
bunny-drill-loading.prmParameter file used to simulate the bunny drill:
bunny-drill.prm
Description of the Case#
This simulation consists of two stages: filling (0-2 s) and bunny-drilling (2-4.75 s) of particles.
Parameter File#
Mesh#
The mesh is a cylinder generated using the deal.II grid generator.
subsection mesh
set type = cylinder
set grid type = balanced
set grid arguments = 6 : 0.10 : 0.25
set initial refinement = 2
end
Insertion Info#
An insertion plane is defined just below the bunny drill. The insertion plane is a useful mechanism to insert particles in a simulation in which the available volume is limited for rectangular box insertion. To ensure a more rapid insertion, we also give an initial downward velocity to the particles. We set insertion maximum offset = 0 so that the particles are inserted at the center of each cells being cut by the insertion plane.
subsection insertion info
set insertion method = plane
set inserted number of particles at each time step = 200
set insertion frequency = 4000
set insertion plane point = 0.025, 0, 0
set insertion plane normal vector = -1, 0, 0
set insertion maximum offset = 0
set insertion prn seed = 19
set initial velocity = -0.1, 0.0, 0.0
end
Lagrangian Physical Properties#
The total number of particles in this simulation is 8000. All particles have a diameter of 10 mm. The particles are relatively stiff, with a Young Modulus of \(E=10^7\) to ensure that the overlap between the bunny drill and the particles does not become sufficiently large to allow particles to jump through the bunny.
subsection lagrangian physical properties
set g = -9.81, 0, 0
set number of particle types = 1
subsection particle type 0
set size distribution type = uniform
set diameter = 0.01
set number of particles = 8000
set density particles = 2560
set young modulus particles = 1e7
set poisson ratio particles = 0.3
set restitution coefficient particles = 0.9
set friction coefficient particles = 0.2
set rolling friction particles = 0.3
end
set young modulus wall = 1e7
set poisson ratio wall = 0.2
set restitution coefficient wall = 0.9
set friction coefficient wall = 0.5
set rolling friction wall = 0.1
end
Simulation Control (Loading)#
The end time of the loading simulation is 2 seconds after which all the particles have been inserted.
subsection simulation control
set time step = 5e-6
set time end = 2
set log frequency = 1000
set output frequency = 1000
set output path = ./output/
set output boundaries = true
end
Simulation Control (Drilling)#
The end time end of the drilling simulation is 4.75 seconds after which the bunny has done one back-and-forth drilling motion.
subsection simulation control
set time step = 5e-6
set time end = 4.75
set log frequency = 1000
set output frequency = 1000
set output path = ./output/
set output boundaries = true
end
Solid Objects (Drilling)#
The bunny is defined using the solid surfaces feature of Lethe. The surface mesh of the bunny is a GMSH file. The translational velocity is defined to have a periodic motion along the axis of the cylinder and the bunny is rotating at a constant angular velocity once the particles have been loaded (\(t>2\text{s}\)) . This complex drilling motion is fully parametrized from the input file using the function parser of the translational and the angular velocity of the solid object.
subsection solid objects
subsection solid surfaces
set number of solids = 1
subsection solid object 0
subsection mesh
set type = gmsh
set file name = bunny-low-poly.msh
set simplex = true
set initial rotation axis = 0, 1, 0
set initial rotation angle = 1.5708 # pi/2
set initial translation = 0.05, 0, 0.035
end
subsection translational velocity
set Function expression = if (t>2,-0.27*sin(0.8*3.1416*(t-2)),0) ; 0 ; 0
end
subsection angular velocity
set Function expression = if (t>2,31.42,0) ; 0 ; 0
end
end
end
end
Running the Simulation#
The loading can be simulated using the following command:
Whereas the drilling is launched after the loading using:
Results#
As seen in the following two animations, the bunny drills into the particles which generates a complex motion within the particle bed. There is not much more to say here, it is a bunny drill!
The first animation displays the drill with the entirety of the particles. It is difficult to see the dynamics of the mighty bunny within these circumstances.
The following animation displays the drill with half of the particles clipped. Here we can clearly see the bunny in action.
Possibilities for Extension#
Use finer particles to see if the drilling dynamics are affected by the particle size.
Use an STL of an alternative animal. Although we believe lagomorphs are amazing, we are also fans of mustelidae (e.g., otters) and chinchillidae (e.g., chinchillas or, even better, viscachas). Feel free to replace the drill with your favorite animal and to send us your animation to lethe.cfd@gmail.com.