The forces function object generates aerodynamic force and moment data for surfaces and porous regions, with optional

  • inclusion of porous drag
  • local co-ordinate system
  • data binning

Forces comprise normal pressure

\[\vec{F}_p = \sum_i \rho_i \vec{s}_{f,i} \left(p_i - p_{\ref}\right)\]

and tangential viscous contributions:

\[\vec{F}_v = \sum_i s_{f,i} \dprod \left(\mu \tensor{R_{\mathrm{dev}}} \right)\]

Where \(\rho\) is the density, \(\vec{s}_{f,i}\) the face area vector, \(p\) the pressure\(\mu\) the dynamic viscosity and \(\tensor{R_{\mathrm{dev}}}\) the deviatoric stress tensor.


Basic operation of the forces function object comprises:

    type            forces;
    libs            ("");
    patches         (<list of patch names>);

For more complete control, the full set of input entries includes:

    // Mandatory entries
    type            forces;
    libs            ("");
    patches         (<list of patch names>);

    // Optional entries

    // Field names
    p               p;
    U               U;
    rho             rho;

    // Reference pressure [Pa]
    pRef            0;

    // Include porosity effects?
    porosity        no;

    // Store and write volume field representations of forces and moments
    writeFields     yes;

    // Centre of rotation for moment calculations
    CofR            (0 0 0);

    // Spatial data binning
    // - extents given by the bounds of the input geometry
        nBin        20;
        direction   (1 0 0);
        cumulative  yes;

Default behaviour assumes that the case is compressible. For incompressible cases, i.e. solved using the kinematic pressure the rho entry can be used to set the free stream density:

rho         rhoInf;
rhoInf      100000;

Sample output

Text reporting:

  • integrated force and moment data divided into total, pressure, viscous and porous contributions

    Sum of forces
        Total    : xxx
        Pressure : xxx
        Viscous  : xxx
        Porous   : xxx
    Sum of moments
        Total    : xxx
        Pressure : xxx
        Viscous  : xxx
        Porous   : xxx

Stored properties

Reduced data

The following data are stored by the function object, and can be used by other function objects:

  • normalForce : total normal force
  • tangentialForce : total tangential force
  • porousForce : total porous force
  • normalMoment : total normal moment
  • tangentialMoment : total tangential moment
  • porousMoment : total porous moment


Optional storing and writing of force and moment fields:

  • forces : forces
  • moments : moments

Further information


Source code:


Example usage: