7: Processing the Model

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PRCS Command

Before running the model, you should always issue the prcs command. In preparation for processing the time-step information, it computes the model time step for stability and opens the necessary arrays for field-data storage.

prcs

Although the prcs command is automatically entered with the first exec command if it has not already been so, it is good practice to enter it before beginning. Once it is entered, no alterations to model geometry, material properties, and so on are permitted.

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Although 80% is default, it can be on the conservative side for some models. Users can adjust this upwards using the time command (must be issued before the prcs command) to help:

  1. Reduce simulation time (larger timestep means less steps for a set simulation time)
  2. Increase accuracy (reducing the amount of sampling in the time domain)

However, it is important to note than increasing it too much will eventually cause the model to become unstable, and care should be taken. A good value to try initially is 0.95:

time * * 0.95 

Increasing the TSF is also useful if you are running a model that propagates over longer distances (>25 wavelengths). By reducing the effect of sampling in the time domain, the solution at each timestep is more accurate, and therefore the cumulative effects of numerical dispersion can be lessened somewhat.

Model Check

Typically after the prcs command, certain aspects of the model can be plotted and to check if they have been set correctly:

  • model Boundaries
grph
    plot boun_type    /* plot model boundaries 
  • defined electrodes with poling arrows
grph
    arrow pole    /* turn on material poling arrows
    plot piez    /* plot defined electrodes with poling arrows 
  • plot material geometry with electrodes and poling arrows
grph
    arrow pole    /* turn on material poling arrows
    plot matr piez    /* plot defined electrodes over model geometry with poling arrows  
  • plot material geometry with electrodes and poling arrows
grph
    arrow cstm 10 5 xvflow yvflow    /* set up custom vector arrows for flow velocities
    plot xvflow yvflow    /* plot combination of x and y flow fields with velocity arrows  

For any piezoelectric model, it important to check if material poling has been set correctly otherwise, the device may not perform as expected. For most piezoelectric transducer applications, the poling should be in parallel with the thickness direction of the PZT. For our PZT example, this should be along the Y-axis. To change the poling direction of any material, we must access the it in our material file and locate the matr axis command for the PZT material we are using:

...
     elec pmt3 $aepxx $aepyy $aepzz

     piez pmt3 1 1 $ex1 1 2 $ex2 1 3 $ex3 1 4 $ex4 1 5 $ex5 1 6 $ex6 &
     2 1 $ey1 2 2 $ey2 2 3 $ey3 2 4 $ey4 2 5 $ey5 2 6 $ey6 &
     3 1 $ez1 3 2 $ez2 3 3 $ez3 3 4 $ez4 3 5 $ez5 3 6 $ez6

     rdmp pmt3 $freqdamp q $qdmp $qsdmp $freqloss 1.0

     axis pmt3 posy /* relate materials local system to global system
...  

Tip: by default, there are material axis definitions which can be used to control the material poling along each of the XYZ axes. Use the desired axis definition through its name: posx, negx, posy, negy, posz and negz.

For our PZT example we will set up the plotting with the following code:

grph
     nvew 2 1
     line on	            /* turn on mesh lines
     arrow pole          /* turn on poling arrows
     plot piez	          /* plot created electrode
     plot matr piez	     /* plot created electrode on top of model
     end
term     /* pause model to allow check

Tutorial Tasks

Within the PZT2D.flxinp file:

  1. Issue the prcs command
  2. Using the grph command:
    • set up 2 views using the nvew subcommand
    • turn on mesh lines with the line subcommand
    • set poling arrow on (pole) with the arrow subcommand
    • plot electrodes (piezplot subcommand
    • plot geometry with electrodes (matr piezplot subcommand
    • Insert term command to pause model for viewing
  3. Run Model and check the electrode placement and material poling are correct. If not, modify the axis command to set the poling direction in the thickness direction of the piezoelectric material.

Progress to tutorial section: Model Execution

prcs /* check models, sets up required data arrays, calculates stable timestep for solver to run model

grph
     nvew 2 1
     line on	            /* turn on mesh lines
     arrow pole          /* turn on poling arrows
     plot piez	          /* plot created electrode
     plot matr piez	     /* plot created electrode on top of model
     end
term     /* pause model to allow check
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