# Flow past a circular cylinder using open boundary conditions¶

This example demonstrates the API of inlet and outlet boundary conditions in PySPH. The flow past a circular cylinder is an example which uses both inlet and outlet boundary conditions. To run it one may do:

$pysph run flow_past_cylinder_2d  There are many command line options that this example provides, check them out with: $ pysph run flow_past_cylinder_2d -h


In this example, we have a wind tunnel with two bounding slip walls on the top and bottom of the tunnel. The inlet is on the left and the outlet is on the right. In order to perform the simulation five particle arrays, solid, fluid, wall, inlet and outlet are generated. fluid, solid and wall has to solved using edac scheme, whereas inlet and outlet are solved according to the equations provided by the Inlet Outlet Manager (IOM). The example source can be seen at flow_past_cylinder_2d.py.

This example demonstrates:

• Setting up a wind tunnel kind of simulation.
• Setting up inlet and outlet boundary condition
• Force evaluation on the solid body of interest

The IOM is created in the Application instance however, it is passed to a Scheme instance and most of its methods are called in the scheme only. We discuss the implementation in the EDAC Scheme in Writing inlet oulet manager. The IOM has the following functions:

• Creation of ghost particle arrays
• Creation of inlet outlet stepper
• Creation of inlet outlet equations
• Creation of inlet outlet updater

The following are discussed in detail:

• Construction of IOM
• Passing IOM to the scheme
• Creating ghost particles
• Creating updater
• Overall setup
• Evaluating forces on solid

## Construction of IOM¶

def _get_io_info(self):
from pysph.sph.bc.hybrid.inlet import Inlet
from pysph.sph.bc.hybrid.outlet import Outlet
from pysph.sph.bc.hybrid.simple_inlet_outlet import (
SimpleInletOutlet)
i_update_cls = Inlet
o_update_cls = Outlet
o_has_ghost = False
manager = SimpleInletOutlet
props_to_copy += ['uta', 'pta', 'u0', 'v0', 'w0', 'p0']

inlet_info = InletInfo(
pa_name='inlet', normal=[-1.0, 0.0, 0.0],
refpoint=[0.0, 0.0, 0.0], equations=inleteqns,
has_ghost=i_has_ghost, update_cls=i_update_cls,
umax=umax
)

outlet_info = OutletInfo(
pa_name='outlet', normal=[1.0, 0.0, 0.0],
refpoint=[self.Lt, 0.0, 0.0], has_ghost=o_has_ghost,
update_cls=o_update_cls, equations=None,
props_to_copy=props_to_copy
)

return inlet_info, outlet_info, manager

def _create_inlet_outlet_manager(self):
inlet_info, outlet_info, manager = self._get_io_info()
iom = manager(
fluid_arrays=['fluid'], inletinfo=[inlet_info],
outletinfo=[outlet_info]
)
return iom


In the function _get_io_info the inlet_info and outlet_info are created, and manager class are returned. The inlet_info and outlet_info info contains specific information about inlet and outlet that enables IOM to create equations, stepper and updater. In _create_inlet_outlet_manager the IOM is created using the info objects.

Note that the extra properties required by the equations are also passed by the IOM.

## Passing IOM to scheme¶

def configure_scheme(self):
scheme = self.scheme
self.iom = self._create_inlet_outlet_manager()
scheme.inlet_outlet_manager = self.iom
pfreq = 100
kernel = QuinticSpline(dim=2)
self.iom.update_dx(self.dx)
scheme.configure(h=self.h, nu=self.nu)

scheme.configure_solver(kernel=kernel, tf=self.tf, dt=self.dt,
pfreq=pfreq, n_damp=0)


The IOM object of the application is initialized in the method configure_scheme of the Application class. All the post-initialization method which require data from user could be called here e.g. update_dx.

## Creating ghost particles¶

def create_particles(self):
fluid = self._create_fluid()
solid = self._create_solid()
outlet = self._create_outlet()
inlet = self._create_inlet()
wall = self._create_wall()

ghost_inlet = self.iom.create_ghost(inlet, inlet=True)
ghost_outlet = self.iom.create_ghost(outlet, inlet=False)

particles = [fluid, inlet, outlet, solid, wall]
if ghost_inlet:
particles.append(ghost_inlet)
if ghost_outlet:
particles.append(ghost_outlet)

self.scheme.setup_properties(particles)
self._set_wall_normal(wall)

if self.io_method == 'hybrid':
fluid.uag[:] = umax
fluid.uta[:] = umax
outlet.uta[:] = umax

return particles


The particle arrays ghost_inlet and ghost_outlet are generated by the IOM depending upon the type of IOM subclass used. The properties $$uag$$, $$uta$$ are the time average and velocity array in $$x$$ direction at t=0.

## Creating updater¶

The purpose of the updater is to remove particle from inlet and add them to fluid whenever a particle crosses the inlet-outlet interface. Similarly, it is done in case of the oulet. It also adds new particle to inlet as required and remove a particle from the simulation when they flow past outlet.

def create_inlet_outlet(self, particle_arrays):
iom = self.iom
io = iom.get_inlet_outlet(particle_arrays)
return io


the function create_inlet_outlet takes the updater io created by the IOM and plugs it into the update routine of the application class automatically.

## Overall setup¶

In order to run the simulation, the IOM object must be passed to the scheme. In the scheme, the IOM object must be implemented in the manner as described in Writing inlet oulet manager.

A few points to note while dealing with inlet outlet boundary condition,

1. Construction of the IOM happens after the scheme is created with a void IOM.

def create_scheme(self):
h = nu = None
s = EDACScheme(
['fluid'], ['solid'], dim=2, rho0=rho, c0=c0, h=h, pb=p0,
nu=nu, inlet_outlet_manager=None,
inviscid_solids=['wall']
)
return s

2. The IOM must be configured in the configure_scheme function.

3. In case you change the integrator of the function, make sure the updater io is updating in the appropriate stage. For example, in case of a PECIntegrator class of integrator, the particles integrated half step in stage 1 and finally advected in stage 2 then io updates the particle arrays after stage 2 is complete. In case one wants to do the update in stage 1 (while using another integrator) the arguments must be passed to the updater appropriately.

## Evaluating forces on solid¶

The force on the fluid particles is evaluated using

$a = \frac{-\nabla{p}}{\rho} + \nu \nabla^{2} \mathbf{u}$

In order to evaluate the forces, the solid is considered as fluid and force is evaluated by solving the following equations

equations = [
Group(
equations=[
SummationDensity(dest='fluid', sources=['fluid', 'solid']),
SummationDensity(dest='solid', sources=['fluid', 'solid']),
SetWallVelocity(dest='solid', sources=['fluid']),
], real=False),
Group(
equations=[
dest='solid', sources=['fluid'], pb=p0),
SolidWallNoSlipBCReverse(
dest='solid', sources=['fluid'], nu=self.nu),
], real=True),
]


The equations are solved on the output saved as *.npz files. In the equation SolidWallNoSlipBCReverse we are just reversing the sign of the velocity difference unlike the usual equation where $$u - u_g$$ is used. The total force is evaluated by multiplying the acceleration with the mass of the solid particles

fxp = sum(solid.m*solid.au)
fyp = sum(solid.m*solid.av)
fxf = sum(solid.m*solid.auf)
fyf = sum(solid.m*solid.avf)
fx = fxf + fxp
fy = fyf + fyp


Here, the au is acceleration due to pressure and auf is due to shear stress. The force fx provides the drag force and fy provides the lift force.