The ddt2 function object computes the magnitude or magnitude squared of the Eulerian time derivative of an input volume field for time-variant simulations (not appropriate to steady-state simulations).

The result can be further used for determining e.g. variance or RMS values.


Operand Type Location
input vol<Type>Field $FOAM_CASE/<time>/<inpField>
output file - -
output field vol<Type>Field $FOAM_CASE/<time>/<outField>

where <Type>=Scalar/Vector/SphericalTensor/SymmTensor/Tensor.


Example of the ddt2 function object by using functions sub-dictionary in system/controlDict file:

    // Mandatory entries (unmodifiable)
    type            ddt2;
    libs            (fieldFunctionObjects);

    // Mandatory entries (runtime modifiable)
    fields          (<field1> <field2> ... <fieldN>);

    // Optional entries (unmodifiable)
    mag             false;

    // Optional entries (runtime modifiable)
    result          d@@dt2;

    // Optional (inherited) entries
    region          region0;
    enabled         true;
    log             true;
    timeStart       0;
    timeEnd         1000;
    executeControl  timeStep;
    executeInterval 1;
    writeControl    timeStep;
    writeInterval   1;

where the entries mean:

Property Description Type Required Default
type Type name: ddt2 word yes -
libs Library name: fieldFunctionObjects word yes -
fields Names of the operand fields wordList yes -
mag Compute ‘mag’ instead of ‘magSqr’ bool no false
result Name of results word no magSqr(ddt2(@@))

The inherited entries are elaborated in:

Usage by the postProcess utility is not available.

Notes on entries

A list of fields can contain exact names or regular expressions. The token \@\@ in the result name is replaced by the name of the source field. In the special case of a single source field (specified as a non-regex), the \@\@ token checking is suppressed.

The function object will skip over fields that appear to have already been processed (ie, their names are similar to the output names).

Further information


Source code: