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Documentation

# Atmospheric Boundary Layer Inlet for Incompressible LBM

The Atmospheric Boundary Layer Inlet is an exclusive boundary condition for the Incompressible (LBM) simulation type. This article will go through the necessary details for the configuration of this boundary condition. Figure 1: Schematics of an atmospheric boundary layer at the inlet, highlighting the reference velocity and reference height.

## Overview

For external aerodynamic simulations, a common approach is to define a velocity profile at the inlet, where the velocity changes with the height. In Incompressible (LBM), there are two ways to achieve this result:

1. Using a table input with a Velocity inlet boundary condition
2. Using an Atmospheric boundary layer inlet boundary condition

With the first approach, the user needs to define a table with the velocity at each height of interest. The second option is a much more automated workflow, which will be explored in the next section.

## Atmospheric Boundary Layer Inlet

The setup of an atmospheric boundary layer inlet condition is simple. The figure below shows the configuration window: Figure 2: This configuration is available from the boundary condition window in Incompressible LBM analyses.

Having Figure 1 in mind, the boundary condition requires the following parameters:

• Reference velocity $$u_{ref}$$ of the velocity profile
• Reference height, often called $$z_{ref}$$ or $$h_{ref}$$, at which the reference velocity is measured. In case there is no terrain, the height is measured as the distance to the bottom of the External Flow Domain or any other no slip wall defined in the simulation setup. Alternatively, if there is a terrain intersecting with the inlet, the height above ground is measured against the terrain surface
• Ground roughness $$z_0$$, which represents the aerodynamic roughness length. The ground roughness is used to model the effects of vegetation and buildings upstream of the inlet. This documentation page shows the usual values for $$z_0$$.

## Formulation

The formulation described below controls the inlet profile for the velocity $$u$$, turbulent kinetic energy $$k$$, and specific dissipation rate $$\omega$$, based on the user-defined parameters. For organization purposes, a table containing the meaning of each parameter can be found at the end of this section.

The logarithmic velocity profile normal to the inlet has the following formulation:

$$u = \frac{u^{*}}{K} \cdot ln \left (\frac{z + z_{0}}{z_{0}} \right ) \tag{1}$$

The other two velocity components are null. The friction velocity $$u^*$$ is given by equation 2:

$$u^{*} = K \cdot \frac{u_{ref}} {ln \left (\frac{z_{ref} + z_{0}} {z_{0}} \right )} \tag{2}$$

Equation 3 shows the turbulent kinetic energy $$k$$ expression:

$$k = \frac{{u^{*}}^{2}} {\sqrt{c_{\mu}}} \tag{3}$$

Finally, the specific dissipation rate $$\omega$$ is given by the expression below:

$$\omega = \frac{u^{*}}{K \cdot \sqrt{c_{\mu}}} \cdot \frac{1}{z + z_{0}} \tag{4}$$

The table below contains a description of all parameters used in equations 1 through 4:

## Expected Result

As an example, let’s use the city block model from this incompressible LBM tutorial. The configuration of the boundary condition is straightforward: Figure 3: Example atmospheric boundary layer inlet definition for a site

In the example above, a Ground roughness of ‘2’ was defined, since the surroundings of the computational domain consist of a dense urban area.

When post-processing the simulation run, the resulting boundary layer is visible: Figure 4: Side view of the simulation results. The air flows from the left (inlet) to the right (outlet): the ABL profile is visible at the inlet.

This workflow allows the user to effortlessly define a velocity profile for the inlet, which is optimal for external aerodynamic applications.

Last updated: April 19th, 2022