*3.2. Computational Fluid Domain and Grid*

As shown in Figure 4a, the simulation used a rectangular computational fluid domain. The calculation domain had a width of 20D and a length of 30D. The middle position of the square was 10D away from the inlet. The boundary conditions used included the left speed inlet, the right pressure outlet, and the upper and lower sides' sliding wall surface. The center of the square column was the origin of the coordinate system; x and y respectively represent the along-wind and crosswind directions.

**Figure 4.** (**a**) Computational fluid domains; (**b**) Computational grid.

The model establishment and grid division were all carried out in the commercial software Ansys ICEM. The grids under different working conditions were all divided by non-uniform structured grids to ensure the accuracy of calculation and save a lot of computing resources. In computational fluid dynamics, the dynamic overset grid method has efficient dynamic grid processing capability and can guarantee the quality of the grid, so it has been widely used in unsteady flow simulation. Therefore, this research used overset grid technology; many concepts related to nested grid computing used today can be traced back to the breakthrough idea of Joseph Steger [29,30]. The overset grid method was used in Fluent; when FIM occurred, the constructed grid oscillated with the square column to ensure that the grid was not updated, which effectively avoided the negative element grid. The square coordinate grid (called the component grid) matched the square cylinder, and the background grid adopted a unified Cartesian grid. Details of the overset grid are shown in Figure 4b.

In order to make the grid height Y+ of the first layer of the wall satisfy the wall function, the height value was determined according to the required Reynolds number [31]. Enough densification was performed close to the wall, and at least 20 layers of wall surface grids were set with a rate of change of 1.2. The Y+ of the wall grids were all around 1.
