How do I find the time varying center of gravity location of my body using DPF ?

Vishnu Member, Employee Posts: 214
Name Dropper First Anniversary Solution Developer Community of Practice Member First Comment
edited June 24 in Structures

The COG of a body might vary with time as the body deforms and you might be interested in knowing how to compute the time varying COG.

the example also highlights on how you can create your own mesh with DPF.

Example uses SOLID elements in this case to demonstrate and used DPF inside mechanical


  • Vishnu
    Vishnu Member, Employee Posts: 214
    Name Dropper First Anniversary Solution Developer Community of Practice Member First Comment
    edited June 26 Answer ✓

    NOTE : You will need 2024R1 version of Ansys for this to work

    time_point = 3
    named_selection_string = "MYBODY"
    #Import statements
    import mech_dpf
    import Ans.DataProcessing as dpf
    # Result Data
    analysis1 = ExtAPI.DataModel.Project.Model.Analyses[0]
    dataSource = dpf.DataSources(analysis1.Solution.ResultFilePath)
    # Get mesh
    my_model = dpf.Model(analysis1.Solution.ResultFilePath)
    my_mesh = my_model.Mesh
    # Time list
    timelist = dpf.operators.metadata.time_freq_provider(data_sources=dataSource).outputs.gettime_freq_support().TimeFreqs.Data
    time_index_list = [int(timelist.IndexOf(time_point)+1)]
    # Named Selection Mesh scoping
    ns_op = dpf.operators.scoping.on_named_selection(requested_location='Elemental', named_selection_name=named_selection_string, data_sources=dataSource)
    mymeshscoping = ns_op.outputs.getmesh_scoping()
    # Get Deformation
    u = dpf.operators.result.displacement(time_scoping=time_index_list, data_sources=dataSource)
    disp_field = u.outputs.fields_container.GetData()[0]
    # Create my own mesh
    created_mesh = dpf.MeshedRegion(my_mesh.Nodes.Count, 0)
    for node in my_mesh.Nodes:
        displacement = disp_field.GetEntityDataById(node.Id)
        created_mesh.AddNode(node.Id, [node.X + displacement[0], node.Y + displacement[1], node.Z + displacement[2]])
    createdmesh_nodeids = created_mesh.NodeIds
    for elem in my_mesh.Elements:
        created_mesh.AddSolidElement(elem.Id, list(map(lambda x: createdmesh_nodeids.IndexOf(x), elem.CornerNodeIds)))
    # Centroids
    centroids = dpf.Operator("compute_element_centroids")
    centroids.Connect(1, mymeshscoping)
    centroids.Connect(7, created_mesh)
    centroid = centroids.GetOutputAsField(0)
    def compute_elemental_mass():
        # List to store elemental densities
        all_mats = []
        mats = my_mesh.GetPropertyField("mat")
        mat_prop = my_model.CreateOperator("mapdl_material_properties")
        mat_field = mat_prop.outputs.properties_value.GetData()[0]
        for eid in my_mesh.ElementIds:
            mat_id = mats.GetEntityDataById(eid)
            density = mat_field.GetEntityDataById(mat_id[0])
        # Create Density Field
        density = dpf.FieldsFactory.CreateScalarField(numEntities=my_mesh.ElementCount,location=dpf.locations.elemental)
        density.ScopingIds = my_mesh.ElementIds
        density.Data = all_mats
        # Element volume
        el_vol = dpf.operators.geo.elements_volume(mesh=my_mesh)
        # Calculate elemental mass (density * volume)
        elemental_mass = dpf.operators.math.generalized_inner_product(fieldA=el_vol, fieldB=density).outputs.getfield()
        return elemental_mass
    elemental_mass = compute_elemental_mass()
    #Function to calculate mass-weighted centroid
    def calculate_mass_weighted_centroid(centroid, mass):
        num_points = centroid.ElementaryDataCount
        # Extract x, y, z components
        x_comp = dpf.operators.logic.component_selector(field=centroid, component_number=0).outputs.getfield().Data
        y_comp = dpf.operators.logic.component_selector(field=centroid, component_number=1).outputs.getfield().Data
        z_comp = dpf.operators.logic.component_selector(field=centroid, component_number=2).outputs.getfield().Data
        # Calculate mass-weighted sums
        sum_mass = sum(mass.Data)
        sum_x = sum(x * m for x, m in zip(x_comp, elemental_mass.Data))
        sum_y = sum(y * m for y, m in zip(y_comp, elemental_mass.Data))
        sum_z = sum(z * m for z, m in zip(z_comp, elemental_mass.Data))
        # Calculate mass-weighted centroid
        if sum_mass > 0:
            centroid_x = sum_x / sum_mass
            centroid_y = sum_y / sum_mass
            centroid_z = sum_z / sum_mass
            raise ValueError("Total mass is zero or negative")
        return (centroid_x, centroid_y, centroid_z)
    #Print mass-weighted centroid

  • Paul Profizi
    Paul Profizi Member, Employee Posts: 11
    5 Likes First Comment First Anniversary First Answer

    Hi @Vishnu, thank you for this example!

    One remark though:

    This is tagged as "PyDPF", yet as it is using the API for use within Mechanical, I am afraid this will create confusion for people trying to use this with PyDPF-Core/Post. Is there a better suited tag available? Or maybe we should create one specific for use of DPF from within Mechanical, be it in Python.

  • Vishnu
    Vishnu Member, Employee Posts: 214
    Name Dropper First Anniversary Solution Developer Community of Practice Member First Comment

    I think so too we may need to create one for DPF from within Mechanical

  • James Derrick
    James Derrick Administrator, Employee Posts: 248
    First Anniversary Solution Developer Community of Practice Member Ansys Employee 5 Up Votes

    Hey! I added the DPF tag to your post! Also can you confirm that COG = "Centre of Gravity"? I don't think you state it anywhere.