Geometry Interfaces in OpenMDAO (Working Document)

An OpenMDAO process model that integrates CFD & FEM based on a common geometry model into an optimization problem

Geometry Manipulation Interface

OpenMDAO Level

(Constructor)

The constructor instantiates the geometry manipulator as an OpenMDAO component. Several specific things have to be done after this component is instantiated but before it can be used in a process model:

• Instantiate the Geometry object
• Connect to the modeler
• Load the geometry
• Load the feature tree
• Query for list of all available parameters and suppression states
• Create OpenMDAO variable for each parameter using the appropriate traits
• Create OpenMDAO variable for each suppressible feature

(Setting Parameters and Suppression States)

Needed for requirements: Tools/Geometry/Interaction/01 and Tools/Geometry/Interaction/02

Parameters are set the same way as any other OpenMDAO input variable. For example, consider a cylinder with two parameters: radius and height. If we have a geometry manipulator called geo_manipulator, the geometry parameters can be directly set:

geo_manipulator.radius = 5
geo_manipulator.height = 15

More often, the geometry’s parameters will be set as part of an optimization problem, so they can be declared as design variables when an optimizer is added to a model.

# CONMIN Design Variables
self.driver.add_parameters([('geo_manipulator.radius', 3.0, 12.),
                            ('geo_manipulator.height', 6.5, 25.)])

Here, self is the top level assembly that contains an optimizer, the geometry manipulator, and some kind of process model such as the one pictured above.

The suppression of features (suppression states) can also be treated the same way at the component level. Here, the Boolean variable fillet1 is set to False to suppress the feature fillet1.

geo_manipulator.fillet1 = False

execute()

Regenerates model if any parameter or suppression state changes. Raises an exception if this process breaks associativity or causes incomplete regeneration of the model.

Note that if no parameters or suppression states change, there is no reason to regenerate the geometry or to invalidate any reference to this geometry object, which would trigger the execution of any components that depend on it (meshers, etc.)

Note also that if the geometry is capable of providing analytical sensitivities to the parameters, then these would be calculated here.

save_to_egg()

Saves current representation of the data model, including all parameters (both name and value), suppression states (likewise), and tags.

load()

Loads representation of the data model, including all parameters, suppression states, and tags, from a saved egg.

tag_volume(volume_label, tag_name, tag_description)

tag_face(face_label, tag_name, tag_description)

tag_edge(edge_label, tag_name, tag_description)

tag_node(node_label, tag_name, tag_description)

Needed for the requirement: Tools/Geometry/Interaction/07

Associates a geometric entity with some metadata. This is useful for marking an entity for later use by an analysis tool (e.g., marking loads and boundary conditions.) The most straightforward way to implement the tags’ storage would be to create each tag as an OpenMDAO variable, accessed via its tag_name.

(Visualization)

The requirements call for the ability to visualize the geometry. No interface for this has been worked out. The user must also be able to view the feature tree to choose parameters as design variables.

Python Component Level

At a lower level, the geometry manipulation component needs a set of functions to interact with the geometry object, making the above interface possible at the OpenMDAO level. These functions are used in the geometry manipulator and will not commonly be seen or used by users who build or run models.

status = initialize(modeler)

Performs any necessary action prior to loading a model. If the geometry is to be loaded and run in another process (or another server), some additional things might need to be passed. A status is returned indicating any error that occurs.

load_model(filename)

Loads the geometry from filename into the model.

parameters = get_parameters()

Returns a list containing a unique label and a value for all parameters in the model.

suppression_states = get_suppression_states()

Returns a list containing a unique label and a boolean for all suppression states in the model.

feature_tree = get_feature_tree()

Returns a data structure containing the feature tree. The format of this data structure is not currently known. This information can be used to give the component user a way to “visualize” the parametric model. Note that technically the parameters and suppression states can also be extracted from here instead of using the given functions above.

set_parameter(id, value)

Sets a new value for a parameter in the model. The model must be rebuilt for the effect of the new parameter to be realized.

set_suppression_state(id, boolean)

Sets a new value for a suppression state in the model. The model must be rebuilt for the effect of the new suppression state to be realized.

status = rebuild_model()

Rebuilds the model based on the current parameters and suppression states. A status is returned that indicates whether the regeneration was successful.

terminate()

Shuts down the geometry modeler and performs any necessary cleanup.

The Geometry Object and its Query Interface

OpenMDAO provides query access to the geometry object at the Python component level.

Topology Access

Needed for requirement: Tools/Geometry/Interaction/07

Geometry access for query includes entity query and evaluation, traversal of topology, and tag query.

The following functions comprise traversal of the Boundary Representation topology.

(point) = GetNode(vol, node)

Returns the (x,y,z) coordinate for a node in the volume.

(trange, nodes) = GetEdge(vol, edge)

Returns the nodes associated with an edge in a given volume; trange returns the parameterization t in terms of the original curve coordinate (t).

(urange, nloop, loops, edges) = GetFace(vol, face)

Returns the edges associated with a face in a given volume. nloop is the number of loops, loops is the edge loop lengths, and edges contains the edge indices and orientation.

(nface, faces, name) = GetBoundary(vol, bound)

Returns the faces associated with a boundary in a given volume. Also returns a boundary name.

(nnode, nedge, nface, nbound, name) = GetVolume(vol)

Returns basic information for a volume, including its name and the number of nodes, edges, faces, and boundaries that it includes.

Mesh Generation

Needed for requirement: Tools/Geometry/Grid Generation/01

(point, d1, d2) = PointOnEdge(vol, edge, t, req_derivative)

Returns the Cartesian coordinate of a point on an edge given the point’s coordinate in the local (t) parameter space used to mesh the edge. First and second derivatives can also be returned if available using the req_derivative parameter (0 = no derivatives, 1 = 1st order, 2 = 2nd order).

(point, du, dv, duu, duv, dvv) = PointOnFace(vol, face, uv, req_derivative)

Returns the Cartesian coordinate of a point on a face given the point’s coordinate in the local (u,v) parameter space used to mesh the face. First and second derivatives can also be returned if available using the req_derivative parameter (0 = no derivatives, 1 = 1st order, 2 = 2nd order.)

(point_on_edge, t) = NearestOnEdge(vol, edge, coor, point, t_guess)

Returns the coordinate of the point on a given edge that lies the closest to the input point. The associated t parameter is also returned. Some kernels require an initial guess in the form of a point on the edge that is nearby.

(point_on_face, uv) = NearestOnFace(vol, face, coor, point, uv_guess)

Returns the coordinate of the point on a given edge that lies the closest to the input point. The associated t parameter is also returned. Some kernels require an initial guess in the form of a point on the face that is nearby.

status = InEdge(vol, edge, point)

Returns True if the given point lies on the edge.

status = InFace(vol, face, point)

Returns True if the given point lies on the face.

Measurements

Needed for all requirements in: Tools/Geometry/Interaction/08

(length) = LengthOfEdge(vol, edge, t1, t2)

Returns the arc length for an edge. Parameters t1 and t2 can be used to return the length of a subsection of the edge.

(arc_length, centroid, bounding_box) = EdgeProperties(vol, edge)

Returns the arc length (for the full edge) and centroid coordinate for an edge in a given volume.

(area, centroid, inertia_matrix, bounding_box) = FaceProperties(vol, face)

Returns the area, centroid coordinate, and inertia matrix for a face in a given volume.

(mass_properties, bounding_box) = VolumeProperties(vol)

Returns the mass properties for a given volume. These include volume, wetted area, center of gravity, inertia matrix, and mass.

Planar Cuts

Needed for requirements in: Tools/Geometry/Interaction/10

(nsec, ivec, data) = VolumeSection(vol, face, type, isvec, idata)

A basic interface would require a volume label, a set of face labels, and a vector that defines the normal of the cutting plane. CAPRI suggests a more detailed interface that allows a few different types of cuts.

Tesselation

While not spelled out directly in the requirements, it is often useful to have the capability to generate a watertight descretized representation of the geometry for performing sanity checks.

[Needs Functions]

The Mesh Object

Use Cases

An OpenMDAO process model that shows how multiple geometry manipulators are used to provide derived geometries based on the original geometry