# Spectral Characteristics of Fluctuating Aerodynamic Forces Acting on Rectangular Prisms
## 3 Results and Discussion
### Spectral Characteristics of Typical Pressure Taps
The intrinsic relationship between flow phenomena like turbulence, separation, and reattachment flow, and vortex shedding and aerodynamic forces, could not be visually investigated in this study due to the limitations of the experimental conditions; instead, this was primarily hypothesized and summarized through pressure measurement tests. The flow around the regime and properties of the aerodynamic spectrum becomes more complex as the magnitude of the lateral spreading continues to grow. In Figure 7, the aerodynamic mechanism and energy transfer of the rectangular section with large side ratios are first examined using the rectangular prism with \(D/B=3\) and \(1/3\) in the \(BL1\) flow as an example. The spectral characteristics of the fluctuating wind pressure at the windward pressure taps are basically the same as those of the other rectangular prisms (Figure 7a), with the same fluid separation and vortex shedding at the windward corner, which is accompanied by a reduction in the low-frequency value, and an increase in the high-frequency value of the power spectrum. Given the lateral side's extensive spreading length, it is split into two pieces, starting from a location close to the potential point of reattachment (\(x/B=1.54\)) (see Figure 7b,c). This portion is in the inner zone, before the separation flow is reattached (Figure 7b). Moreover, separation bubbles present in this area are attached to the lateral side at the inner zone's midpoint, which is also the location of the lateral side's highest negative wind pressure (\(x/B=0.82\)). Although the high-frequency value steadily rises, and the spectrum peak at the vortex shedding frequency gradually reaches its maximum, the low-frequency value of the power spectrum declines and reaches a minimum near the midpoint of the inner area. It should be noted that the inner zone's fluctuating pressure spectrum properties differ dramatically from those of the apparent vortex shedding. These phenomena are particularly prominent in high-level turbulence intensity flows (\(BL2\)), where incoming turbulence causes the large-scale vortices in the separated flow to be squeezed and stretched, prompting the energy to be redistributed to a wider spectral bandwidth. The spectral peak value drops by approximately 75% when the side ratio is raised from 1 to 3, although less than 15% of the energy of the fluctuating wind pressure is lost overall. The low-frequency values of the spectrum begin to pick up again between the point that is halfway downstream of the inner zone and the reattachment point, whereas the high-frequency values remained mostly unchanged. In terms of energy change, the variable wind pressure spectrum in this area has a rising energy that peaks nearby to the reattachment point (\(x/B=1.54\)). This finding shows that the high-frequency, small-scale vortices dissipate and are less affected by it; rather, they are primarily controlled by the larger-scale, low-frequency vortices, due to the increase in the curvature of the separated shear layer and the continuous absorption of energy from turbulent fluctuations [3; 6; 7; 9; 11; 18; 19; 20]. The low-frequency values of the spectrum degrade quickly in the region downstream of the reattachment point (Figure 7c), whereas the high-frequency variations gradually rise. These phenomena are clear examples of separation shear flow reattachment, wherein energy is transferred from low-frequency to high-frequency wind pressure fluctuations. The spectral peak of the fluctuating wind pressure sharply rises as it approaches the corner point of the tail (\(x/B=3.16\to 3.43\)), suggesting that the reattachment fluid may become separated once more. According to Figure 7d, the results for the pressure taps on the leeward side are mostly influenced by the development of distinctive turbulence, caused by secondary separation; this is comparable to the spectrum features for \(D/B=2\).