# Lift Force

The lift is a force orthogonal to the object velocity and is applied to the middle of the surface. The lift force is described by the following formula:

where:

• l is the lift coefficient provided by the user
• ρ is the fluid density
• v is the object velocity
• n is the surface normal
• A is the cross-sectional area perpendicular to the relative velocity
Note
The lift coefficient is not necessarily the same in each direction. For this reason, the user can specify a different lift 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 lift coefficients along its X-, Y-, and Z-axes. For a sphere, only the X-component of the vector is used as the uniform lift coefficient in all directions. However, for a cylinder and capsule, the lift coefficient for the faces (cylinder) or caps (capsule) is given by the Z-component, while the lift coefficient for the body is given by the X-component only.

If the normal surface of the collision geometry is collinear or perpendicular to the relative object velocity, there is no lift applied on this surface. Also, there is no lift if objects are symmetric about the direction of movement.

The force depends on fluid density, projected area along the path, velocity, and angle of attack. The lift coefficient is a dimensionless value that depends on the desired angle of attack, Mach number, and Reynolds number. In Vortex®, lift force is set for a geometry by modifying the lift translation coefficients along its respective axes.

It is recommended to use a fine triangular mesh (ideally with equilateral triangles) to represent rigid bodies. You can set the coefficient in the Editor or via Vx::VxCollisionGeometry::FluidInteractionData::setLiftCoefficient.

For information about how to enable lift in the Editor, see Enabling the Lift Force.