def prepare_materials(self, field, velo=1.0, diffusion=0.1, penalty=100):
"""
Crates material objects with data attribute, containing properly shaped
data to pass to terms
:param field: DGField
:param velo: optional values for velocity a
:param diffusion: optional value for diffusion tensor D
:param penalty: optional value for diffusion penalty Cw
:return: a, D, Cw
"""
a = Material('a', val=[velo])
a.data = nm.ones((field.n_cell, 1)) * velo
D = Material('D', val=[diffusion])
D.data = nm.ones((field.n_cell, 1, 1)) * diffusion
Cw = Material("Cw", values={".val": penalty})
Cw.data = penalty
return a, D, Cw
