def excitation_force(w, z1, z2, diameter, Cm=2):
"""Calculate the excitation force. Specific for now to vertical
cylinders of varying diameter.
``z1``, ``z2``: coordinates of ends of element
``diameter``: array (2 x Npoints) diameter at each z value
"""
config = {
'end1': [0, 0, z1],
'end2': [0, 0, z2],
'diameter': diameter,
'strip width': 1.0,
}
Morison_model = ViscousDragModel({
'inertia coefficient': Cm,
'members': [config],
})
X = Morison_model.Morison_inertial_force(w)
return X
