zCFD Functions

Several parameters in the zCFD control dictionary will accept functions as arguments. These functions are then evaluated by the solver when reqiured. This allows you to customise & tune to solver as it runs.

Scale Mesh Function

Description	Transform the location of a supplied point.
Valid for parameter	scale_mesh
Parameters	[x, y, z]: Point to transform
Returns	{‘coord’: [x, y, z]}: Transformed coordinates

Example:

# Custom function to scale z coordinate by 100
def my_scale_func(point):
    point[2] = point[2] * 100.0
    return {'coord' : point}

parameters = {
 ...
 'scale': my_scale_func
 ...
}

Ramp CFL Function

There may be cases where more detailed control of the CFL number is desired - for example in some aerodynamic simulations where a specific physical event occurs at a given time. The CFL Ramp function lets you do this.

The ramp function returns a single floating point number (the CFL number) and can also be used to update elements of the ‘cfl’ Python object such as cfl.min_cfl, cfl.max_cfl, cfl.current_cfl, cfl.coarse_cfl, cfl.transport_cfl, cfl.cfl_ramp, cfl.cfl_pmg, or cfl.transport_cfl_pmg.

Description	Provide a dynamic CFL number
Valid for parameter	ramp func
Parameters	solve_cycle, real_time_cycle, cfl
Returns	cfl: float

Example:

# Custom function to ramp CFL based on cycle, updates the cfl object variables and returns a CFL
def my_cfl_ramp(solve_cycle, real_time_cycle, cfl):
  cfl.min_cfl = 1.0
  cfl.max_cfl = 10.0
  cfl.cfl_ramp = 1.005
  cfl_transport = 5.0
  if solve_cycle < 500:
      cfl_new = min(cfl.min_cfl * cfl.cfl_ramp ** (max(0, solve_cycle - 1)), cfl.max_cfl)
      cfl.transport_cfl = cfl_transport * cfl_new / cfl.max_cfl
      return cfl_new

  elif solve_cycle < 1000:
      cfl.trasport_cfl = 7.5
      cfl.max_cfl = 15.0
      return 15.0

  else:
      cfl.transport_cfl = 10.0
      cfl.max_cfl = 30.0
      return 30.0

parameters = {
 ...
 'time marching': {
   ...
   'ramp func': my_cfl_ramp,
   ...
   }
 ...
}

Transform Function

A transformation function my be supplied to the force report block to transform the force into a different coordinate system. This is particularly useful for reporting lift and drag, which are often rotated from the solver coordinate system.

Description	Transform a supplied point - rotate, scale or translate.
Valid for parameter	Forces transform
Parameters	(x, y, z): floats
Returns	(x, y, z): floats

Example:

#zutil is a Zenotech python library providing various utilities, available within a zCFD installation
import zutil

# Angle of attack
alpha = 10.0
# Transform into wind axis
def my_transform(x,y,z):
    v = [x,y,z]
    v =  zutil.rotate_vector(v,alpha,0.0)
    return  {v[0], v[1], v[2]}

Initialisation Function

Description	Provide the initial flow field variables on a cell-by-cell basis
Valid for parameter	initial
Parameters	kwargs dictionary containing “pressure”, “temperature”, “velocity”, “wall_distance”, “location”
Returns	dictionary containing one or more of “pressure”, “temperature” or “velocity”

def my_initialisation(**kwargs):
  # Dimensional primitive variables
  pressure = kwargs['pressure']
  temperature = kwargs['temperature']
  velocity = kwargs['velocity']
  wall_distance = kwargs['wall_distance']
  # Cell centre localtion [X, Y, Z]
  location = kwargs['location']

  if location[0] > 10.0:
    velocity[0] *= 2.0

  # Return a dictionary with user defined quantity.
  return { 'velocity' : velocity }

parameters = {
 ...
 'initial': {
   ...
   'func': my_initialisation,
   ...
   }
 ...
}