## 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](images/wind_current_direction.png)
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](images/wind_area_orientations.gif)
### Notebooks
{doc}`Wind demonstration`
### References:
- DNV-RP-C205 : ENVIRONMENTAL CONDITIONS AND ENVIRONMENTAL LOADS