# INTERPOLATED_MOTION_SURFACE

Specifies a surface that is used to scale displacements of mesh nodes.

AcuSolve Command

## Syntax

INTERPOLATED_MOTION_SURFACE ("name") {parameters...}

User-given name.

## Parameters

shape (enumerated) [no default]
Shape of the surfaces in this set.
three_node_triangle or tri3
Three-node triangle.
six_node_triangle or tri6
Six-node triangle.
element_set or elem_set (string) [no default]
User-given name of the parent element set.
surfaces (array) [no default]
List of element faces.
surface_sets (list) [={}]
List of surface set names (strings) to use in this command. When using this option, the connectivity, shape, and parent element of the surfaces are provided by the surface set container and it is unnecessary to specify the shape, element_set and surfaces parameters directly to the INTERPOLATED_MOTION_SURFACE command. This option is used in place of directly specifying these parameters. In the event that both of the surface_sets and surfaces parameters are provided, the full collection of surface elements is read and a warning message is issued. The surface_sets option is the preferred method to specify the surface elements. This option provides support for mixed element topologies and simplifies pre-processing and post-processing.
type (enumerated) [=faceted]
Type of the interpolated motion surface.
faceted
Interpolated motion surface is made up of the discrete points and faces contained within the input surfaces.

## Description

This command specifies a faceted surface (a set of element faces) to be used in conjunction with a MESH_MOTION of type = interpolated. When using interpolated motion, each INTERPOLATED_MOTION_SURFACE associated with a given MESH_MOTION command is collected into a single set. All nodes falling within the boundaries of that set are then interpolated to determine their weighted displacement based on the distance from their surrounding "driving" surfaces. For example, consider a simple 2D application consisting of a rigid cylinder embedded in a square domain with fictitious springs attached to it. As the flow passes over the cylinder, the forces cause the cylinder to translate in the stream wise and cross stream directions. Assigning a MESH_MOTION of type = interpolated to the nodes in the volume will linearly scale the displacement of the surrounding nodes as a function of distance between INTERPOLATED_MESH_MOTION surfaces. For example:
MESH_MOTION( "vibrating cylinder" ) {
type                          = rigid_body_dynamic
rigid_body_center             = { 0.05, 0.05, 0.00125; }
rigid_body_mass               = 2.35619449e-007
rigid_body_stiffness          = { 0.000654446111, 5.2355689e-005, 0.0, 0.0, 0.0, 0.0; }
rigid_body_surface_outputs    = { "cylinder surface" }
}
MESH_MOTION( "Interpolated Motion" ) {
type                          = interpolated_motion
interpolated_motion_surfaces  = { "cylinder surface", "box walls"}
}
INTERPOLATED_MOTION_SURFACE( "cylinder surface" ) {
element_set                   = "Air"
type                          = faceted
}
INTERPOLATED_MOTION_SURFACE( "box walls" ) {
element_set                   = "Air"
type                          = faceted
}
ELEMENT_SET( "Air" ) {
...
mesh_motion                   = "Interpolated Motion"
mesh_motion_precedence        = 1
...
}
There are two main forms of this command. The legacy version (or single topology version) of the command relies on the use of the surfaces parameter to define the surfaces. When using this form of the command, all surfaces within a given set must have the same shape, and it is necessary to include both the element_set and shape parameters in the command. shape specifies the shape of the surface. This shape must be compatible with the shape of the "parent" element set whose user-given name is provided by element_set. The element set shape is specified by the shape parameter of the ELEMENT_SET command. The compatible shapes are:
Element Shape
Surface Shape
four_node_tet
three_node_triangle
five_node_pyramid
three_node_triangle
five_node_pyramid
six_node_wedge
three_node_triangle
six_node_wedge
eight_node_brick
ten_node_tet
six_node_triangle

The surfaces parameter contains the faces of the element set. This parameter is a multi-column array. The number of columns depends on the shape of the surface. For three_node_triangle, this parameter has five columns, corresponding to the element number (of the parent element set), a unique (within this set) surface number, and the three nodes of the element face. For four_node_quad, surfaces has six columns, corresponding to the element number, a surface number, and the four nodes of the element face. For six_node_triangle, surfaces has eight columns, corresponding to the element number, a surface number, and the six nodes of the element face. One row per surface must be given. The three, four, or six nodes of the surface may be in any arbitrary order, since they are reordered internally based on the parent element definition.

The surfaces may be read from a file. For the above example, the surfaces may be placed in a file, such as walls.ebc:
4 41 2 5 6
5 51 5 6 3
INTERPOLATED_MOTION_SURFACE( "walls" ) {
shape        = three_node_triangle
element_set  = "flow_elements"
...
}
The mixed topology form of the INTERPOLATED_MOTION_SURFACE command provides a more powerful and flexible mechanism for defining the surfaces. Using this form of the command, it is possible to define a collection of surfaces that contains different element shapes. This is accomplished through the use of the surface_sets parameter. The element faces are first created in the input file using the SURFACE_SET command, and are then referred to by the INTERPOLATED_MOTION_SURFACE command. For example, a collection of triangular and quadrilateral element faces can be defined using the following SURFACE_SET commands.
SURFACE_SET( "tri faces" ) {
surfaces    = { 1, 1, 1, 2, 4 ;
2, 2, 3, 4, 6 ;
3, 3, 5, 6, 8 ; }
shape       = three_node_triangle
volume_set  = "tetrahedra"
}
surfaces    = { 1, 1, 1, 2, 4, 9 ;
2, 2, 3, 4, 6, 12 ;
3, 3, 5, 6, 8, 15 ; }
volume_set  = "prisms"
Then, a single INTERPOLATED_MOTION_SURFACE command is defined that contains the tri and quad faces as follows:
INTERPOLATED_MOTION_SURFACE ( "walls" ) {
...
}
The list of surface sets can also be placed in a file, such as surface_sets.srfst:
tri faces
quad faces

INTERPOLATED_MOTION_SURFACE ( "walls" ) {
}