This example is based on the atmospheric boundary layer (ABL) test from the document of the German WTG: Fact Sheet of Committee 3 - Numerical simulation of wind flows, Chapter 9.1 (see references). Before each numerical simulation, the user should check whether the atmospheric boundary layer defined at the inflow reaches the structure by testing its development in an empty tunnel. This affects not only the distribution of the velocities, but also the turbulent quantities. The test must be carried out for both steady (RANS) and transient (URANS, LES) calculations. In the following article, the development of a velocity field, turbulence kinetic energy field, and turbulence dissipation rate field is shown for the four terrain categories I to IV defined in the EN 1991-1-4. A vertically anisotropic turbulence acc. to Chapter 6.3.1 and the RANS k-ω SST turbulence model is used.
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Conclusion on the ABL example in chapter 9.1 of WTG :
The differences in results observed in RWIND, compared to the ABL example in WTG, can be attributed to several important factors related to input definition and boundary conditions:
- Inlet profiles: The input profiles in WTG are not clearly specified and ESDU information for roughness classes I to IV are too general description. This leads to deviations from the intended Atmospheric Boundary Layer (ABL) characteristics.
- Unclear boundary conditions: The example does not clearly specify the boundary conditions, which are critical for accurate reproduction of the inlet flow.
- Tunnel geometry: The width of the tunnel is not provided. This raises uncertainty about whether the simulation is a full 3D domain or a simplified 2D case, which directly affects flow behavior and turbulence development.
- Mesh resolution: Details regarding mesh density (including y+ values) are missing, making it difficult to assess near-wall modeling accuracy and the quality of turbulence resolution.
- Turbulence modeling: The turbulence kinetic energy (k) is treated as a variable, contrary to the fixed value prescribed in WTG (Equation 27). This inconsistency further contributes to deviations in the velocity and turbulence profiles.
In RWIND, additional differences in inlet profile preservation are caused by current limitations in boundary condition implementations:
- Top boundary: A slip condition is used instead of applying a constant shear stress or fixed values for velocity, k, ε, or ω. Although this has minimal impact in the area around the building. This issue can be addressed with a relatively simple update in RWIND 4.
- Near the tunnel bottom: No-slip boundary condition is applied without modeling aerodynamic surface roughness of the tunnel bottom (in this VE in RWIND). This is sufficient except for the simulations focused on pedestrian wind comfort and longer approach fetches. In the RWIND default tunnels for determining wind loads for static analysis of structures, the approach fetches are too short for the profiles to dissipate, and the deviations in the near ground region do not significantly influence the wind load results.