Wind and current

Wind and current loads can be added to a model by defining:

  • wind/current areas

  • wind speed

  • wind direction

  • air density

  • current speed

  • current direction

  • water density

Wind and current areas are nodes. All the others are general settings of the Scene. directions is defined as going to in [deg] relative to the positive x-axis.

direction

Wind speed and direction are constant.

The force acts in the direction of the wind/current and is equal to:

\(Force = {1 \over 2} * \rho * Cd * A_e * V^2\)

Cd

Cd is the drag coefficient is a fixed coefficient and is user-provided. Typical values are 1.2 for a wire, 0.4 for a sphere, 2.0 for a flat plate perpendicular to the wind.

Effective Area

The effective area is a combination of an area (A) and the orientation of that area relative to the wind/current.

The area A [m2] is fixed and user-defined.

The effective area \(A_e\) is calculated from the area and its orientation relative to the wind/current. In general \(A_e = A_0 * |sin(\alpha)|\) where the term \(\sin(\alpha)\) accounts for the orientation of the area relative to the wind/current direction. It is 1 if the wind/current is perpendicular to the surface and 0 if it is parallel.

The orientation of the surface can be defined in three ways:

No orientation

The area is the same from any direction:

\[ A_w = A \]

This is the case for spheres.

Plane orientation

The area is a flat plane. The direction of the node is the normal of the plane:

\[ A_w = A * |d_{wind} . d_{plane}| \]

The area is zero if the wind/current is perpendicular to the defined direction.

Cylindrical orientation

The area is constant around one axis, but plane-like about an axis perpendicular to that. In this case the direction defines the axis about which the area is constant (ie: the center-axis of the cylinder).

\[ Aw = A * \sqrt{ 1 - (d_{wind} . d_{plane})^2 } \]

The effective area is zero if the wind/current is parallel to the defined direction.

direction

References:

  • DNV-RP-C205 : ENVIRONMENTAL CONDITIONS AND ENVIRONMENTAL LOADS