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Converting Nodal Coordinates Field to Local Coordinate System

| 09.30.2022

Today’s Script Tip comes from Ayush Kumar, Senior Applications Engineer at Ansys.

GCS: Global Coordinate System

LCS: Local Coordinate System

In 3D simulations, local coordinate systems play a crucial role. Therefore, conversion of nodal coordinates from GCS to LCS is often needed.

The operator

:class: `rotate <ansys.dpf.core.operators.geo.rotate.rotate>`

rotates the input field in GCS as per the input rotation matrix. If the LCS is at the same origin as the GCS, only one operation using the `rotate` operator gives the desired output. But for an LCS with a transformed origin, after rotation, a transformation along the rotated position vector of the LCS’s origin is also needed to get the correct coordinates in LCS. Below is a PyDPF example to achieve that.



# 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(r"\Path\to\my\file.rst") 
############################################################################### 
# Get the property `coordinates_field` from :class: nodes ansys.dpf.core.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 
nccord_translated = 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 = nccord_translated.outputs.field.get_data() 
############################################################################### 
# Coordinates of NID 1 in GCS 

print(ncoord_f.get_entity_data_by_id(1)) 
############################################################################### 
# Coordinates of NID 1 in LCS 

print(ncoord_lcs_f.get_entity_data_by_id(1))