Forces

Table of Contents

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.

Usage

Basic operation of the forces function object comprises: 

forces1
{
    type            forces;
    libs            ("libforces.so");
    patches         (<list of patch names>);
}

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

forces1
{
    // Mandatory entries
    type            forces;
    libs            ("libforces.so");
    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
    binData
    {
        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

Optional storing and writing of force and moment fields:

  • forces: forces
  • moments: moments

Storing of integrated values for further post-processing:

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

Further information

Related:

Source code:

Example usage:

Would you like to suggest an improvement to this page? Create an issue

Copyright © 2016-2017 OpenCFD Ltd.

Licensed under the Creative Commons License BY-NC-ND Creative Commons License