# Atmosphere

The Atmosphere part deals with the wind and air parameters. The following parameters can be adjusted in this part:

## Air

### Density scheme

The density scheme enables you to choose how the air density is set.

Options:

SSL (default):

The SSL value for air density is used (i.e. at Standard sea level properties - SSL, 1.225 kg.m^3).

User defined:

Air density is set by you.

From temperature and altitude:

Air density is calculated based on the air temperature and altitude. The altitude used is the top point of the support structure.

### Air density

Density of the air, used to calculate the aerodynamic loads, as set by you.

• Default value: 1.225
• Unit: ${\text{kg}} \over {\text{m}^3}$
• Range: 0 — 2000

### Viscosity scheme

The dynamic viscosity is primarily used to calculate Reynolds number. The kinetic viscosity is the ratio of dynamic viscosity to density. The viscosity scheme enables you to chose how the dynamic viscosity is defined.

Options:

SSL (default):

The SSL value for dynamic viscosity is used (i.e. at Standard sea level properties - SSL, 1.81e-5 kg/(m s)).

User defined:

You set the value.

From temperature:

The value is calculated based on the temperature given by you.

### Dynamic viscosity (of air)

Dynamic viscosity of the air, mainly used to cpmpute the Reynolds number.

• Default value: 1.81e-05
• Unit: ${\text{kg}} \over {\text{m}\cdot\text{s}}$
• Range: 1e-06 — 0.0001

### Air temperature

The temperature of the air. The temperature is used to calculate density and/or dynamic viscosity if From temperature and altitude is chosen for Density scheme or From temperature is chosen for Viscosity scheme.

• Default value: 15
• Unit: $°\text{C}$
• Range: -50 — 50

### Atmospheric pressure scheme

The atmospheric pressure the wind turbine operates in. You can select from having it automatically calculated from air temperature, pressure at sea level and altitude or entering it manually.

Note: This is only used when aerodynamic loading is found using AeroDyn. When built-in BEM algorithms in Ashes are used, air density is used directly.

See:

Options:

SSL (default):

The SSL value for atmospheric pressure is used (i.e. Standard sea level conditions - SSL, 101.325 kPa).

User defined:

Atmospheric pressure is set by you. The Atmospheric pressure parameters becomes visible when this is chosen.

From temperature, altitude and pressure at sea level:

Atmospheric pressure is calculated based on the air temperature, altitude and the pressure at sea level. The altitude used is the top point of the support structure. The Atmospheric pressure at sea level becomes visible when this is chosen.

### Atmospheric pressure

Atmospheric pressure used when calculationg aerodynamic loading on the wind turbine. The default value is the standard atmospheric pressure at sealevel.

This parameter is only visible when the Atmospheric pressure scheme parameter is set to User defined.

• Default value: 101325
• Unit: $\text{Pa}$
• Range: 0 — 1e+06

### Atmospheric pressure at sea level

The atmospheric pressure at sea level, by default the SSL (Standard sea level) property, 101.325 kPa.

Only visible when the Atmospheric pressure scheme parameter is set to From temperature, altitude and pressure at sea level.

• Default value: 101325
• Unit: $\text{Pa}$
• Range: 0 — 1e+06

## Wind

### Type

Wind type enables you to model the wind in seven different ways

Options:

Live (default):

The wind is controlled by you and can be changed during the simulation.

Uniform:

The wind is simulated as constant, optionally with vertical shear. So, it's constant in time at any point in space. If vertical shear is applied it will make the velocity smaller closer to the ground, so it is not constant in the vertical direction.

IEC extreme:

The wind is simulated as one of the (deterministic) extreme wind conditions defined in IEC 64100-1.

Sine:

The wind speed is simulated as having a sine variation.

Linear:

The wind speed and angle changes linearly with time.

Stepwise:

The wind speed and angle are read from file or can be simulated with incremental changes

Turbulent:

The wind is simulated using a wind spectrum and is neither constant in time nor in space. Also called stochastic simulation.

For a more detailed descritpion of the types of wind, see the following sections: