13. Fluid Loads

Normally in OnScale fluid loads present no difficult in simulation set-up. Users simply allocate a region of the grid to have fluid properties and the solver can handle it without the need for interfaces layers and the like. 
  
Due to the Large Deformation analysis and Dynamic Relaxation requirements for typical CMUT models, problems can arise in the grid and at absorbing boundaries for fluids. 
  
To prevent this, users can implement a method to remove the fluid load from the model during the problematic stages of the model, i.e. the static phase, then re-insert the load for the dynamic phase.

1. Make a copy of the of the structural and material arrays (data copy)
2. Delete the selected materials with mods command for modification of the model.
3. Be sure to use the 'skip' option in mods optn to ensure plod surface remains at membrane
4. Run the appropriate number of timesteps for the static phase
5. Return material from temporary array with data copy
6. regrid the fluid to ensure mesh is not overly distorted before dynamic phase
7. See MEMBRANE ONLY below for more instructions about modifications required for no ESTA, receive models.

Note: This procedure can only be applied to SKEWED regions of the grid.

This procedure works very well to ensure model stability and efficiency as the fields in the fluids will be all be zero until the onset of the dynamic loading phase.

MEMBRANE ONLY: Be careful with application of plod loads anywhere in the Fluid region. When these nodes are deactivated, plod location moves 'through' the eroded nodes to find the next continuum elements and loads them.

For example, applying plod at the top of a model for a receive calculation then using mods will cause the plod command to move through the eroded fluid and be applied at the surface of the CMUT.

This can be avoided by applying the plod command after the dynamic relaxation phase has been completed and the fluid elements have been placed back into the model.

/* Make a copy of internal element material tables for model including load
data
    copy pflp pflp:b
    copy sitp sitp:b

/* Remove the load by using the 'mods' command group
/* 'mods' allows elements in the model to be turned into 'void' elements
/* Allows for quick dynamic relaxation process
mods
    prnt on
    optn cont dact * skip /* set option to deactivate continuum elements

    /* User should make sure that the indices bound the load region of the model
    erod $i1 $i3 $j4 $j5 /* erode (remove) load elements
    end

After the Static Phase has completed:

/* Replace load material
data
    clos sitp
    clos void
    copy pflp:b pflp
    copy sitp:b sitp
    end

/* Regrid load material to ensure good mesh for analysis
regrid
    fill j $i1 $i3 $j4 $j5 /* j is the thickness direction
    end