Part 2: Flow Around Obstacles: Geometry, Mesh, and Boundary Conditions
The flow around an obstacle is involved in many engineering applications, such as wind engineering, aerodynamics in automotive, airfoils, and other fields. It refers to the behavior of fluid dynamics that occurs when a fluid flows around an obstacle: cylinder, square, triangle, NACA, or angle member. To explore more about the key aerodynamic parameters, please check our previous Newsletter's article.
To accurately describe the flow around an obstacle, it is essential to provide optimal geometry dimensions. For example, when using a small computational domain height around the obstacle, this can lead to a loss of information on fluid detachment. A wider domain with optimal height prevents the blockage effect due to the upper and lower boundaries. An optimal distance should be chosen ahead of the obstacle to ensure a smooth and continuous flow of fluid and to accurately capture fluid blockage upon impact with the obstacle. Similarly, the distance after the obstacle should be sufficient to capture the fluid wake. For 3D geometries, it is recommended to use at least π *D for the domain width. Hereafter is an example of a computational domain chosen by Trias et al. to describe a flow around a square. They explored different dimensions, and it was observed that the size of the domain in the cross-stream direction has a relevant influence on the results probably due to some blockage effects.
2. Mesh
In order to obtain accurate results, a high-quality mesh is essential. For flow around obstacles, the literature explores both structured and unstructured meshes. While a structured mesh is recommended for optimal CFD simulations, it is only feasible for simple geometries such as squares and cylinders. For more complex geometries like motorbikes or cars, a structured mesh can be challenging to construct and may not be the best option. Hybrid grids present an alternative option as they can combine an unstructured grid in complex zones with a structured grid in the remaining domain. This approach can offer a good compromise between accuracy and feasibility for complex geometries like motorbikes or cars. Huang et al. conducted a study to investigate the impact of grid type on simulation results, using different grids (see Figure 2 and Figure 3). The resulting aerodynamic coefficients and flow behavior were found to be similar for both grids.
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3. Boundary conditions
Accurate and appropriate boundary conditions are essential for accurately describing the conditions applied to the computational domain boundaries. In their study of flow around an angle member, Huang et al. employed the boundary conditions shown in Figure 4. The upper and lower walls of the computational domain were assigned as symmetry boundaries to prevent any blockage effect of the fluid at these boundaries and so to indicate that the domain is open from the top and bottom. A velocity inlet was chosen to specify the fluid flow velocity and direction, while an outlet boundary condition was used to drive the fluid out of the computational domain by pressure. The validation of boundary conditions is essential to ensure that they accurately reflect the physical conditions of the system and result in reliable and meaningful simulation results.
4. Conclusion
Flow around an obstacle has a wide range of applications. In order to accurately simulate this flow and obtain reliable and meaningful simulation results, it is essential to carefully select the computational domain geometry and mesh, as well as appropriate boundary conditions. The literature provides numerous recommendations for optimizing these factors. Additionally, the specificities of the physical problem being studied should be taken into consideration when selecting boundary conditions.
5. References