Setting Up Atmospheric Boundary Layers

To set up an atmospheric boundary layer (ABL), you specify the ground as a rough wall boundary. For the Standard K-Epsilon turbulence model, you can set inlet boundary profiles using the Atmospheric Boundary Layer model. If you want to use other turbulence models, you must specify the inlet values for the ABL (for example wind speed) by using the standard profiles, for example, field functions.

Mesh Considerations

From sensitivity studies [349], you are advised to use the following values when setting up the mesh for an ABL simulation:

Make sure $y > 0.2 r$ and $y \geq 10 y_{0}$ , where $y$ is the shortest distance from the wall to a cell centroid, $r$ is the roughness height, and $y_{0}$ is ground level.
Make sure $y^{+} \geq 30$ , in the logarithmic region of the boundary layer, where the Richards Hoxley model [349] applies.

To model ABL using the Standard K-Epsilon model:

For the physics continuum, deactivate Auto-select recommended models, and select the following models, in order:


Group Box	Model
Time	Steady
Material	Gas
Flow	Coupled Flow or Segregated Flow
Equation of State	Constant Density
Viscous Regime	Turbulent
Turbulence	Reynolds-Averaged Navier-Stokes
Reynolds-Averaged Turbulence	K-Epsilon Turbulence
K-Epsilon Turbulence Models	Standard K-Epsilon
K-Epsilon High y+ Wall Treatment	High y+ Wall Treatment
Optional Models	Atmospheric Boundary Layer

Select the Continua > [physics continuum] > Models > Atmospheric Boundary Layer node and set the following properties:


Property	Setting
Reference Speed	The wind speed at a given height, $u_{h}$ in Eqn. (1375). The default is 10.0 m/s.
Reference Height	The height at which the wind speed is taken, $h$ in Eqn. (1375). The default is 6.0 m.
Roughness Length	The size used to model obstacles to the flow at ground level, $y_{0}$ in Eqn. (1372). The default is 0.01 m.
von Karman Constant	A dimensionless coefficient $κ$ used in Eqn. (1374), the ABL velocity profile. The default is 0.42.

For more information, see Atmospheric Boundary Layer Model Reference.

Select the [physics continuum] > Models > Standard K-Epsilon node and set the following properties:


Property	Setting
Cmu	Set the value according to the inlet velocity profile. Typical values are between 0.03 and 0.11.
C1e	Set the value using the following equation: Figure 1. EQUATION_DISPLAY $c_{ϵ 1} = c_{ϵ 2} - \frac{κ^{2}}{\sqrt{c_{μ}} σ_{ϵ}}$ (258)

For more information, see K-Epsilon Models Reference.

To set the turbulence viscosity ratio, select the [physics continuum] > Initial Conditions > Turbulent Viscosity Ratio node, set Method to Constant, and set Value to 1E10.

Specify the ground as a rough wall boundary using the values from this example:

In a case with

y_{0} = 0.01 m

E = 8.75

C = 0.25

, and

y ≫ y_{0}

, the calculated roughness height

r

is 0.35 m.

Select the Regions > [region] > Boundaries > [ground boundary] node.

Set Type to Wall and set the following properties:


Node	Property	Setting
Physics Conditions
`Wall Surface Specification`	Method	Rough
Physics Values
Standard Wall Function	E	8.75
Wall Roughness Parameters	C	0.25
	RplusRough (R+)	90 or greater, so Eqn. (1377) is valid.
	Roughness Limiter	Deactivated, because the assumption $y ≫ y_{0}$ from Eqn. (1379) is valid.
Roughness Height	Value	0.35 m

Define the inflow boundary:

Select the Boundaries > [velocity inlet boundary] node and set Type to Velocity Inlet.

Edit the [velocity inlet boundary] node and set the following properties:


Node	Property	Setting
Physics Conditions
`Turbulence Specification`	Method	K+Epsilon
Physics Values
Velocity Magnitude	Method	Atmospheric Boundary Layer
Turbulence Dissipation Rate
Turbulence Kinetic Energy

The Atmospheric Boundary Layer model provides the fluid velocity

u

in Eqn. (1372), the turbulent dissipation rate

ε

in Eqn. (1374), and the turbulent kinetic energy

k

inEqn. (1373).

If you want use the Standard K-Omega turbulence model for your ABL simulation, select the [physics continuum] > Models > Standard K-Omega node and set the following properties:


Property	Setting
BetaStar	Set the value according to the inlet velocity profile, where $β^{*} \approx c_{μ}$ .
Beta	Set the value using the following equation: Figure 2. EQUATION_DISPLAY $β = \frac{β^{*}}{1.25}$ (259)
Alpha	Set the value using the following equation: Figure 3. EQUATION_DISPLAY $α = \frac{1}{1.25} - \frac{σ_{ω} κ^{2}}{\sqrt{β^{*}}}$ (260)
Sigma_w	0.5 (default)

For more information, see K-Omega Models Reference.