4. Model Outputs

Prior to executing the model, we must define the exact outputs we are interested in which is required in both modes of operation.

Calculated Properties

By default OnScale will calculate the velocity arrays for a simulation. Anything that is required beyond these arrays must be requested explicitly in order to be calculated and stored. The most common calculated data arrays are:

• Pressure: pres & aprs
• Displacement: disp (accessed through: xdsp, ydsp, zdsp)
• Stresses: strs (all 6 components accessed through the sg arrays e.g. sgxx sgyy...)
• Strain: strn (all 6 components accessed through ep arrays e.g. epxx epyy...)
• Loss: loss
• Maximum data arrays: max

Designer Mode

The user is only required to request the data Array Type from the drop down menu when a new output is created in the model:

Analyst Mode

Any requested property can be included using the calc command like so:

calc
    pres aprs /* calculate pressure and acoustic pressure
    disp /* calculate displacement
    strn /* calculate strain
    strs /* calculate stress
    loss /* calculate losses
    max aprs pmin pmax /* capture minimum & maximum acoustic pressure and store in 'pmin' & 'pmax' array
    end

Output Types

Time Histories

Time Histories in OnScale are simply data fields captured over time for specific nodes/elements in the model.

As a time-domain solver, OnScale can generate any of the calculated data arrays as a set of time histories. All that is required is to request the data at specific locations on the IJK grid or by their predefined reference.

Designer Mode

Time histories can selected as an output option when a new output is created in the model tree:

Location and Array Type properties for a time history can be modified in the properties section.

Analyst Mode

Time histories are defined using the pout and typically, with the hist or histname subcommands:

pout
    hist func /* time function - does not require ijk location
    hist pres $i1 $i1 1 $j3 $j3 1 /* pressure for a single element
    hist pres $i1 $i2 1 $j3 $j3 1 /* pressure for every elements in from i1 and i2
    hist pres $i1 $i2 2 $j3 $j3 1 /* pressure for every 2nd element from i1 and i2
    histname electrode vqi all /* voltage, charge & current data for all electrodes defined
    histname electrode vq top /* voltage & charge data for 'top' electrode
    histname avrg a all /* store all requested averages
    histname avrg a avepres /* store the average with name 'avepres'
    end

All time histories are given an array number in the sequence they are defined. The array numbers gives you access to the data array and can be used for plotting purposes if we want to visualise these signals during the model execution stage. This will be demonstrated later on in the tutorial section.

Mode Shapes

Mode shapes are extracted through Harmonic Analysis methods. In OnScale, we can extract the harmonic behaviour of a system by performing DFTs at specified frequencies of interest. This can also be performed for specific data fields of interest, for example pressure, x-displacement and many others. It is a powerful tool as it can provide insights into system behaviour that may be difficult to achieve in a other ways.

The advantages of performing harmonic analysis in this manner, rather than with a modal solver, is that this method takes into account damping and mode-coupling in a more effective manner. It will therefore correspond more directly with experimental measurements from laser vibrometers taken in a similar manner i.e. driving a system at a constant frequency.

Designer Mode

Mode shapes are created in the Output section of the Model Tree. Mode shapes can be added via the plus button next to the Output dropdown and characterized in the properties window. For example:

Analyst Mode

Mode shapes can be created via the shap command. This allows the user to specify the harmonic frequencies  they are interested in along with any additional data fields  they wish to extract, for example:

shap
    data pres /* pressure data to be extracted with mode shapes
    data xdsp /* x displacement data to be extracted with mode shapes
    freq 1.5e6 /* mode shape '1'
    freq 3.e6 /* mode shape '2'
    end

Extrapolation

OnScale works most efficiently when models are on the order of tens of wavelengths in size along each axis. Medical and Sonar devices, however, commonly require predictions of pressure amplitudes at distances many hundreds of wavelengths from the source. These are either impractical or inefficient to model fully. The best approach in such cases is to extrapolate.

The Kirchoff extrapolation technique is a powerful tool incorporated into OnScale. It uses analytic techniques (essentially a combination of Green's functions and Huygen's principle) to take pressure/time data from the finite-element model and extrapolate to a full pressure/time response at any arbitrary location or locations.

The technique is valid in both near and far fields, but makes several assumptions:

1. Propagating medium is homogenous, i.e., no intervening layers
2. Propagating medium is fluid, i.e. no shear waves
3. Propagating medium is either lossless or very low loss
4. Response is linear

Designer Mode

Extrapolation boundaries can be setup very easily and visualised within the tool:

Analyst Mode

The setup process is to create a boundary around or over your device to capture the propagating pressure wave.

The extr command is the required command and here is a simple example of a typical extrapolation boundary setup:

extr 
    defn kirc         /* define type extrapolation boundary type - kirchoff
    ref in $x1 $y2  /* reference point to determine extrapolation vector 
    node $i1 $indgrd-1 $j3+5 $j3+5  /* extrapolation boundary at the defined nodal indices  
    driv func         /* storing time function to allow tvr calculation in toolkit   end

Data Fields

Data Fields are usually snapshots of the entire model with data array overlaid on top of the geometric model. By capturing a series of snapshots over time, animations can also be created.

Designer Mode

The properties currently only allow the extraction of maximum and minimum data fields:

The analysis of all the mentioned outputs will be discussed in more detail in the Post Processor section.

Analyst Mode

The data field snapshots can be captured using the data out command:

 data
     out modl      /* output model geometry
     out pmax      /* output pressure maximum
     out shap/all  /* output all requested mode shape data
     out ydsp      /* out y-displacement snapshot
     end