## 5 Results and Discussion
### Drag Force Sensitivity to Sampling Frequency and Period
This peak may represent the combined high-frequency components from the side and lower edges, which typically fall within the range of \(St=1.44\) to 4.5 (200-400 Hz). This frequency range accounts for approximately 4% of the total energy in the signal. In contrast, the body and nose pressure drag exhibit fewer dynamic structures, with significantly slower slopes compared to the rear fascia. The component at 349 Hz contains the largest portion of energy, indicating that dynamic components are more prominent at the rear fascia compared to the nose and body. The nose is primarily subject to stagnation, with minimal dynamic structures, and the energetic fluctuating components develop from areas near the edges toward the [PERSON] body. Additionally, the assigned body section contributes almost zero drag force; only the slits of the [PERSON] body generate pressure drag, while the remaining sections are perpendicular to the drag axis, playing a key role in lift and side forces, which are beyond the scope of this paper. Figure 6c,d, respectively, present the frequency spectrum and the cumulative sum of components for the friction drag (\(C_{D,f}\)) on the body and nose. The friction drag of the rear fascia is zero because the surface is normal to the drag force axis and is, therefore, excluded from the analysis. As expected, the body contributes more to friction drag compared to the nose. Although the peaks in the spectrum for low-frequency components are less prominent than those in the pressure signal, they are still present. The body friction drag reaches 90% of its energy within the range of \(St=0.60\) (approximately 84 Hz). Similar to the \(C_{D,p}\) spectrum in Figure 6a, a notable high-frequency structure at 349 Hz (\(St=2.51\)) was observed for \(C_{D,f}\) in Figure 6a, indicating a good correlation between pressure and stream-wise velocity in quasi-periodic structures. Similar to the \(C_{D,p}\) spectrum but with a wider range, (\(C_{D,f}\) maintains energy in the frequency range below 84 Hz (\(St=0.60\)), with a pronounced peak at \(St=2.51\) (\(f=349\) Hz). It is also worth noting that the \(C_{D,f}\) spectrum for the body shows a more gradual energy distribution compared to the nose. This is primarily due to the largely stagnant flow in the nose region. Finally, Figure 6e,f compare the pressure and friction drag for the full Ahmed body. The results show that the energy reduction slope for pressure drag is steeper than for friction drag, consistent with trends reported in the literature [48]. In order to capture all low-frequency dominant modes and ensure the convergence of \(C_{D}\), it is essential to identify a sufficiently large sampling time window. In order to ascertain an optimum time window, we calculated the dependence of the mean and RMS of the pressure and friction drag on the sampling periods, as shown in Figure 7. According to the literature [48; 50], for the flow over an Ahmed Body, the lowest oscillatory structure is \(St=0.08\), corresponding to \(f=11\) Hz for the present study. However, the spectral analysis of pressure and wall shear stress also revealed some dominant frequencies in the lower range, near \(St=0.036\) and \(0.018\). As discussed in previous sections, [PERSON] et al. [33] demonstrated that DMD can capture a mode with as little as one or even half a cycle, whereas classical FFT may require up to five cycles. Based on this, a sampling window of 0.2 s (\(T^{*}=28\)) should be sufficient to capture low-energy modes, such as the one at \(St=0.08\) (\(f_{s}=11.1\) Hz). Figure 7a shows that the full-body pressure drag requires approximately 2.0 s (\(T^{*}\approx 280\)) to reach a quasi-stationary trend, with the mean \(C_{D}\) converging within \(\pm 0.0001\). It is important to note that the pressure at the rear fascia exhibits bimodality, switching phases between positive and negative values every \(T^{*}=1000\), known as the scale of bistability [48]. The RMS of pressure drag coefficients also reflects these phase shifts, indicating their influence on the results (see Figure 7b).