Convert nodal coordinates field to local coordinate system

Currently, there is no native operator to get nodal coordinates in an Local Coordinate System (LCS). The operator rotate rotates the input field in Global Coordinate System (GCS) as per the input rotation matrix. So, if the LCS is at the same origin as the GCS, only one operation using the rotate operator give the desired output. However, if the aim is to obtain the LCS in a case where the LCS origin does not coincide with the GCS, a transformation is required after the rotation to get the correct coordinates in LCS.

The script below demonstrates the methodology using PyDPF.

# Import necessary modules
from ansys.dpf import core as dpf
from ansys.dpf.core import examples

Create a model object to establish a connection with an example result file:

model = dpf.Model(examples.download_hemisphere())

Get the property coordinates_field from nodes:

ncoord_f = model.metadata.meshed_region.nodes.coordinates_field

Get the rotation matrix of the LCS ID 12. The first 9 values in the cs output is the rotation matrix.

cs = model.operator(r"mapdl::rst::CS")
cs.inputs.cs_id.connect(12)
cs_rot_mat = cs.outputs.field.get_data().data.T[0:9]

Create a 3x3 rotation matrix field rot_mat_f:

rot_mat_f = dpf.fields_factory.create_scalar_field(1)
rot_mat_f.data = cs_rot_mat

Create a 3D vector field for the position vector of the LCS’s origin and rotate the origin as per the rotation matrix of the LCS. The last 3 entries of cs output is the LCS’s origin in GCS.

pos_vec = dpf.fields_factory.create_3d_vector_field(1)
pos_vec.data = cs.outputs.field.get_data().data.T[-3:]
pos_vec_rot = dpf.operators.geo.rotate(field=pos_vec, field_rotation_matrix=rot_mat_f)

Get rotated nodal coordinates field:

ncoord_rot_f = dpf.operators.geo.rotate(field=ncoord_f, field_rotation_matrix=rot_mat_f)

Transform rotated nodal coordinates field along rotated position vector pos_vec_rot:

pos_vec_rot_neg_f = dpf.operators.math.scale(field=pos_vec_rot, ponderation=-1.0)
pos_vec_rot_neg = pos_vec_rot_neg_f.outputs.field.get_data().data_as_list
ncoord_translate = dpf.operators.math.add_constant(field=ncoord_rot_f, ponderation=pos_vec_rot_neg)

Get the nodal coordinates field ncoord_lcs_f in LCS:

ncoord_lcs_f = ncoord_translate.outputs.field.get_data()

Coordinates of NID 1 in GCS

print(ncoord_f.get_entity_data_by_id(1))

[[-2.74445261 13.2627943   4.68419313]]

Coordinates of NID 1 in LCS

print(ncoord_lcs_f.get_entity_data_by_id(1))

[[11.71685114 -4.83846377 -2.35002614]]