Initial conditions

The initial conditions are used to set values for turbulence fields, temperature and pressure at the beginning of the simulation. The full list of properties are provided below:

• Characteristic velocity: mean flow velocity, and is used to find values for initial turbulence fields. It should take a positive or a zero value. For dispersion scenarios, the value will automatically be pulled through from the Wind Wizard. For explosion scenarios, a value of 200 m/s is recommended for most scenarios.  
• Relative turbulence intensity: ratio between the isotropic fluctuating velocity and the mean flow velocity. A value of 0.3 is recommended for explosion scenarios. For dispersion scenarios a value between 0.01 and 0.1 is typical. 
• Turbulence length scale: typically 10% of the leak diameter for dispersion scenarios. A value of 0.05 is recommended for gas explosions. 
• Temperature
• Ambient pressure
• Ground height
• Ground roughness: aerodynamic roughness length, should not be greater than the control volume height close to the surface. Typical values can range between 0.0002 m (open water) and 1.0 m (regular large obstacles, e.g., forest) 
• Reference height: height above ground where the velocity is equal to the wind speed
• Canopy height: the height above the ground where the boundary layer starts due to vegetation or buildings. Typically left as 0. When a value is set for the canopy height, it should be about 2/3 of typical tree or building size. The canopy height must be equal or larger than zero and less than the reference height.
• Pasquill class: stability class to categorise the atmospheric turbulence present ranging from very unstable to stable.

The following properties are available upon selecting show advanced (via right-click):

• Up direction
• Gravity constant
• Air
• Latitude
• Surface heat 
• Mean surface heat flux