# Drag Force

Drag is a force opposite to the relative velocity of a moving object inside a fluid.

The drag force, D, is applied to the middle of the exposed surface on all types of surfaces for all collision geometries, according to the following formula: where:

• d is the drag coefficient provided by the user
• ρ is the fluid density
• v is the object's velocity relative to the fluid
• A is the cross-sectional area perpendicular to the relative velocity
Note
The drag coefficient is not necessarily the same in each direction. For this reason, the user can specify a different drag coefficient value along each local axis of the collision geometry, which is specified as a three dimensional vector.
For example, a box shape can have three different drag coefficients along its X-, Y-, and Z-axes. For a sphere, only the X-component of the vector is used as the uniform drag coefficient in all directions. However, for a cylinder and capsule, the drag coefficient for the faces (cylinder) or caps (capsule) is given by the Z-component, while the drag coefficient for the body is given by the X-component only. In Vortex®, drag is set up for a collision geometry by modifying the drag coefficients along the respective translation and rotation axes of the volume. The drag force is applied considering the linear and the angular velocities of the geometry and the linear fluid flow velocity. The latter can be specified in the fluid state. If a mesh is used to represent the body's shape in the interaction with the fluid, the body's surface is discretized into small surface patches and the contributions to the total drag force of every patch are computed and integrated. It is recommended to use a fine triangular mesh (ideally with equilateral triangles) to represent rigid bodies. If you need to change the value of the drag coefficient, you can set it in the Editor or Vx::VxCollisionGeometry::FluidInteractionData::setDragCoefficient.

For information about how to set the drag force in the Editor, see Enabling the Drag Force.