Moving Body¶

Introduction¶

This option creates a new moving body parent. The property grid of the parents defines the nature of the boundary condition, as discussed below. All moving body parents require a child geometry.

Users have three options for adding child geometry: (1) via the Add Geometry form, which opens automatically when a new moving body parent is created; (2) by importing geometry through the Assembly Explorer; or (3) by importing geometry from a spreadsheet. See the Geometry page for more details.

For an already-existing parent component, you can add children via the Add Geometry command found on the Context Specific Toolbar. Children geometry can also be extracted from an assembly file.

Positioning Moving Bodies¶

Parent

Once you have imported a moving body, select the parent moving geometry in the model tree. Then select the type of transformation (move, rotate, scale, etc.) from the Context Specific Toolbar. A transformation to the parent affects all children. The position and orientation of the parent (relative to the simulation basis) is reported in the property grid.

Children

Children can be rotated, translated, scaled, and colored independently of any parents and siblings. The position and orientation of the child (relative to the simulation frame) is reported in the property grid. The position of all children is defined relative to the global origin.

See Importing CAD geometry tutorial.

Defining Motion Vectors¶

Moving bodies are characterized by a mount point and a motion vector. The mount point defines the initial position of the object in space. The motion vector defines how the object moves relative to this mount point (e.g., for a spinning object the vector defines the axis of rotation, for a translating object the vector defines the direction of translation, etc.). The mount point defines the root of the motion vector.

When defining parametric geometry, the mount point is defined automatically as the centroid of the child geometry with a motion vector aligned with the +Y axis. The position and orientation of the object can be modified on the Define Axis form, which is accessed from the Moving Body Context Specific Tool Bar.

Whenever importing child geometry from a file, the Define Axis form is launched automatically, and users will be prompted to enter a mount point and a motion vector. Again, the root and unit vector characterize the dynamics of the moving object. For example:

When importing a spinning impeller, the axis root should be positioned at the center of the impeller hub, while the axis unit vector should point along the axis of rotation.
When importing a translating object, the axis root should be the center of the object, and the axis unit vector should point in the direction of motion.
For flux surfaces, the axis root should be the center of the object, and the axis unit vector should point in the direction of imposed flow.

Once defined, the axis of rotation will commute with any additional impeller rotation and/or translations applied to the moving object parent.

Important

It is very helpful to center any moving object files about their local origin prior to importing them into the model. This positioning greatly simplifies selection of the axis root.
For rotating objects, align the rotation axis directly with the local x, y, or z-axis prior to import. This alignment selection greatly simplifies specification of the axis vector.
By default, the GUI will attempt to place the axis root at the volume centroid of the imported file. If the volume centroid cannot be calculated, the default location will be the global domain origin.

Note

If users import a moving object that contains multiple co-moving features (e.g., a single shaft with multiple impellers attached to it), the force and torque values reported for this moving object will represent those for the entire assembly. The power number will also be reported for rotating equipment, although the value will be calculated using the maximum diameter of the assembly.
Choosing to import multiple objects as part of a single assembly, or choosing to load in objects individually has no net effect on the fluid flow. The only effect will be in how the model discriminates between individual objects and reports individual forces and torques. This logic conveys to individual blades, which can be imported individually. In this way, users can extract time-dependant forces and torques on individual blades.

Property Grid¶

`General`¶

Motion Axis Point

meters | This parameter defines the location of the axis point of the moving object inside the bounding geometry in model units. This is presented for reference and is read-only here. To edit this value, open the Define Axis form from the context specific menu.

Motion Axis Vector

This parameter defines the direction of the vector motion. This is presented for reference and is read-only here. To edit this value, open the Define Axis form from the context specific menu.

Motion Type

There are nine options for defining motion in a moving body:

Rotation: Object spins with a user-defined rate about a fixed axis of rotation.
Translation Cyclic: Object oscillates at a user-defined frequency along a fixed translational axis.
Flux Surface: Flux surface imposes a velocity boundary condition on the fluid.
Translation: Object moves at a user-defined set velocity.
Ball Joint: Object pivots in a manner similar to a ball joint where the pivot is defined by a vector whose direction changes over time.
Driven: Object’s motion is based on the forces acting on it from the fluid.
Deforming: Object motion is the most flexible and requires a user-defined function or a series of imported STL files.
Deforming UDF: Object motion is the most flexible and requires a user-defined function.
Motion Table Displacement: Object’s displacement is computed from a user-provided motion data table.
Motion Table Rotation: Object’s rotation is computed from a user-provided motion data table.
Motion Table Displacement with Rotation: Object’s displacement with rotation is computed from a user-provided motion data table.

`Advanced`¶

Method Type

This setting specifies the algorithm used to model interactions between the fluid and the moving wall. Two options are available: Immersed Boundary and Voxelized.

Immersed Boundary: The Immersed Boundary Method represents immersed structures using discrete points within a Lagrangian framework, interacting with the surrounding fluid through force terms that enforce no-slip boundary conditions. This approach efficiently handles complex and moving geometries without the need for re-meshing, simplifying simulations of fluid-structure interactions. However, this approach does not explicitly account for the volume of fluid within immersed structures and may not accurately capture pressure waves within the fluid domain, which can be a limitation in scenarios where internal fluid dynamics are significant or species/fluid transport across the interface must be prohibited.
Voxelized: The Voxelized method represents walls and boundaries by assigning a local solid volume fraction to each voxel in the computational grid. As the wall moves, the solid volume fractions of nearby voxels evolve over time, accurately modeling the displacement and re-occupation of fluid. This method explicitly accounts for the fluid volume within the simulation domain and effectively captures pressure waves. This makes is suitable for applications where precise fluid volume representation, wave propagation, and species transport are crucial. However, the voxelized approach often requires higher simulation resolutions to achieve accurate results, which can increase computational costs.

Ref Diameter

meters | This value characterizes the diameter of the moving object. It is computed automatically when setting the motion axis unit vector. This radius defines a control volume around the impeller and is used to compute various statistics at runtime including power number. This is also used to estimate an associated Reynolds number for the component which is used to automatically compute a time step for the system.

Ignore Free Surface Interface

Applies no force to the free surface interface cells.

Off

`Display Attributes`¶

Visible

Display reference axis attributes.

Hidden: Hide the reference axis.
Shown: Show the reference axis.

Mode

Offers two options to view the reference axis.

Wire: Wire frame.
Shaded: 3D shaded mode.

Color

Customize the color of the reference axis.

Width

Customize the width of the reference axis.

Moving Body Toolbar¶

Context-Specific Toolbar Forms	Description
Add Geometry	The Add Geometry form adds child geometry by importing from external CAD files, extracting from external CAD assemblies, or defining internally using built-in parametric geometry.
Move	The Move form enables three-dimensional rigid body transform of object through free drag or point-to-point snapping.
Rotate	The Rotate form enables three-dimensional rotation of geometry.
Scale	The Scale form enables volumetric scaling of a geometry about a set anchor point.
Mate	The Mate form allows surface-to-surface mating and alignment.
Edit Mesh	The Edit Mesh form modifies the resolution of the solid body surface mesh used in the simulation.
Define Axis	The Define Axis form specifies the motion axis mount point and motion vector.
Moments	The Moments form calculates the moments of inertia of a moving object.
Deformation	The Deformation form imports a series of deforming meshes.
Help	The Help command launches the M-Star reference documentation in your web browser.

Moving Body Child Geometry Toolbar¶

Context-Specific Toolbar Forms	Description
Move	The Move form enables three-dimensional rigid body transform of object through free drag or point-to-point snapping.
Rotate	The Rotate form enables three-dimensional rotation of geometry.
Scale	The Scale form enables volumetric scaling of a geometry about a set anchor point.
Mate	The Mate form allows surface-to-surface mating and alignment.
Create Inlet	The Create Inlet form offers a link to the Inlet/Outlet Setup form directly from a child geometry.
Diagnostics	The Diagnostics form reports the position, orientation, and moments of inertia associated with a static body.
Reset	The Reset form moves the object back to its world origin or aligns the object to the world-aligned axis.
Help	The Help command launches the M-Star reference documentation in your web browser.

For a full description of each option, see Context-Specific Toolbar selections.