6. Using BOND & RIGD

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Bonding Layers/Elements

Bond and glue are fantastically useful commands in OnScale, especially in creating refined grids. For CMUT models they facilitate the use of 'fake' continuum elements to be used to represent the dielectric properties of Shell elements (which have mechanical, but no electric properties). 
  
To compensate, users can create a single line of elements with zero stiffness & mass but contain correct dielectric properties for a specific layer. To prevent a discontinuity in stiffness in the model, these elements can be effectively 'bonded closed' be joining the top nodes to the bottom nodes via bond or glue
  
As the Shell elements are defined along a set of nodes in this 'fake' material, then the dielectric properties of this layer are now effectively transferred to the Shell elements and electrical Degrees of Freedom remain un-bonded.

/* Bond the shell nodes to the bottom of the non-mechanical dielectric layer
/* gives the electrical properties but none of the mechanical
bond
    defn b1 nchk
    cors $i1 $i3 $j4 $j4
    fine $i1 $i3 $j3 $j3
    end 

Kinematic Boundary Conditions

Rigd is typically thought of as a command to define Rigid substructures and 'fix' various Degrees of Freedom in nodes. However, it is actually much more versatile than this for several reasons:

  • It can be used to apply force or velocity functions to nodes in any of 6 DOF
  • The Boundary Conditions applied can be changed during a simulation, therefore users can turn BCs on and off at specific times, and/or set fixed or free conditions depending on what is required at that point in the simulation.
  • It can be used internally in a model, not just the outside boundary edges
  • It can be used to apply pre-stress forces/velocities in CMUTs and other systems, providing the user has knowledge of the pre-stress levels.

In the case of CMUT models, rigd is helpful in constraining the membrane at the outer edges (where it would be fixed to the substrate) and also for defining a fixed substrate region. Additional driving functions can easily be defined to allow unique time-histories to be used for each BC or drive in the model.

/* Use RIGD to fix the ends of the membrane 
rigd
    defn rig1
    fix x y z rx ry rz
    node $i2 $i3 $j4 $j4 /* must match the CORS nodes in BOND
end
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