Force: GRADCV

Model ElementThe Force: GRADCV element defines a contact force between a deformable curve and graphic.

Description

The deformable curve is defined using the Reference_DeformCurve element. The graphic is defined using the Post_Graphic element. When the distance between the graphic and the curve becomes less than the zero, a repulsive contact force is applied. Two types of contact force models are supported: Linear and Poisson.

Format

<Force_GRADCV
      id                    = "integer"    
    [ label                 = "string" ]      
      graphics_id           = "integer"    
      dcurve_id             = "integer"
      radius_start          = "real"
      radius_end            = "real"
   { 
      force_model           = "LINEAR"      
      [ stiffness           = "real" ]
      [ damping             = "real" ]   
    | 
      force_model           = "POISSON"      
      [ penalty             = "real" ]
      [ restitution_coef    = "real" ]

   }

/> 

Attributes

id
Element identification number (integer>0). This number is unique among all Force_GRADCV elements.
label
The name of the Force_GRADCV element.
graphics_id
Specifies a Post_Graphic used to define a geometry. This graphic is used to detect contact with the deformable curve.
dcurve_id
Specifies the deformable curve used to define the Force_GRADCV element.
radius_start
Radius at the start of the deformable curve, Use a negative radius to define a pipe.
Default = 0.0.
radius_end
Radius at the end of the deformable curve, Use a negative radius to define a pipe.
Default = 0.0.
force_model
Select from LINEAR or POISSON.
stiffness
Stiffness coefficient for the contact force model. Only used when the force_model is set to LINEAR. The stiffness must be positive.

Default = 0.0

damping
Damping coefficient for the contact force model. Only used when the force_model is set to LINEAR. The damping must be positive.
Default = 0.0
penalty
Specifies the stiffness coefficient that is to be used for calculating the spring force. A large value for penalty permits only a small penetration between the two contacting geometries; a small value permits a larger penetration. Hertzian contact theory may be used to estimate the local stiffness. See The Roark's Formulas for Stress and Strain for more information on calculating Hertzian contact stiffness parameters. Only used when the force_model is set to POISSON. The penalty must be positive.
Default = 0.0
restitution_coef
Defines the coefficient of restitution (COR) between the contacting bodies. A value of zero specifies perfectly plastic contact meaning that the two bodies coalesce after contact. A value of one specifies perfectly elastic contact. No energy is lost in the collision and the relative velocity of separation equals the relative velocity of approach. Only used when the force_model is set to POISSON. The restitution_coef must lie between 0.0 and 1.0.
Default = 1.0

Example 1

<Reference_DeformCurve
     id                  = "1"
     label               = "DCurve/1"
     end_type_left       = "NATURAL"
     end_type_right      = "NATURAL"
     u_span              = "1."
     tension             = "0."
     is_u_closed         = "FALSE"
     num_marker_id       = "5">
1 2 3 4 5
</Reference_DeformCurve>
<Post_Graphic
     id                  = "90000"
     type                = "Cylinder"
     center_marker_id    = "30102030"
     radius              = "5."
     length              = "400."
     ends_type           = "CLOSED"
     refinement_level    = "4"
     is_material_inside  = "TRUE"
/>

<Force_GRADCV
     id                  = "1"
     graphics_id         = "90000"
     dcurve_id           = "1"
     radius_start        = "5."
     radius_end          = "5."
     force_model         = "LINEAR"
     stiffness           = "100."
     damping             = "0.01"
/>

<Post_Graphic
     id                  = "1"
     label               = "Deformable Curve"
     type                = "DeformCurve"
     curve_id            = "1"
     nseg                = "100"
/>

Comments

  1. The Force_GRADCV element can have only one unique contact point at any time during the simulation.
  2. To correctly model a Force_GRADCV element, you must ensure that the contact point stays within the deformable curve throughout the simulation. MotionSolve calculates a (u) value for the point's current position with respect to the deformable curve. This (u) value must always stay within range of the defined u_span. For example:

    - u_span/2 ≤ u ≤ + u_span/2

    If the current u value goes out of this range, MotionSolve will not calculate a force for that element. However, this may lead to incorrect results. You are encouraged to check the results for correctness in such a situation. For more information, please refer to the Reference: Deformable Curve model element.

  3. The Force_GRADCV element permits lift-off. As long as the contact point remains within the "u" range, the normal distance between the graphics and the deformable curve is allowed to be greater than the value specified in the attribute radius.