The wake dynamics of a rectangular flat-backed bluff body is studied using both the wind tunnel experiment and Improved Delayed Detached Eddy Simulation (IDDES) at Re = 9.2 × 10⁴. Both approaches are systematically investigated in order to provide a quantitative comparison. The wake barycenter deficit and base pressure gradient dynamics are first investigated to characterize the wake states. Second, the known global dynamics such as long-term bi-stability, vortex shedding, and wake pumping are analyzed using proper orthogonal decomposition. It is found that the wake dynamics is globally well captured by the IDDES, but with a more intense activity due to the absence of the fore-body separations observed in the experiment. The coupling of these global dynamics is explored by utilizing low-order modeling in cross planes and elaborating the evolution of the three-dimensional (3D) instantaneous wake flow from IDDES. The shedding of a large-scale hairpin vortex from the horizontal shear layer is closely associated with the pumping motion during wake switchings or switching attempts. A concept model is proposed for 3D bi-stable wake topology, which attempts to elucidate both asymmetric and symmetric wake configurations.

The flow around a seal-vibrissa-shaped cylinder (SVSC) is numerically investigated using the large eddy simulation framework at a Reynolds number of 20 000. Compared with a circular cylinder (CC), the wake of the SVSC presents more stable three-dimensional separation, a longer vortex formation length, and a weaker vortex strength. The mean drag and fluctuation of the lift coefficient are 59.5% and 87.7% lower than those of the CC, respectively. Three-dimensional spectral proper orthogonal decomposition (SPOD) is used to investigate the turbulent flow around these two types of cylinders in terms of the spatial modes, mode energy, mode coefficients, and reconstructed flow by a reduced-order modeling. Four typical vortex shedding patterns are first extracted by SPOD for the SVSC, producing crescent-, twist-, branch-, and knot-shaped vortices. A concept model is proposed for the wake dynamics of the SVSC, allowing the formation and transformation of these modes to be elucidated. Detailed analysis of the impact of the flow pattern on the associated forces indicates that the dominant out-phase vortex shedding at the upper and lower saddle planes makes a significant contribution to the reduction in lift fluctuations.

In this work, the impact of the isentropic and Kantrowitz limits on the aerodynamic behavior of evacuated tube transportation (ETT) was numerically explored. Two tube train systems with different blockage ratios (β), that is, β = 0.09 and β = 0.25, were employed for the comparative study of aerodynamic drag and flow structure. The results revealed three distinct aerodynamic behaviors, corresponding to the three speed regions separated by the two critical Mach numbers. Furthermore, the influence of head and tail lengths on drag reduction was investigated in these three speed ranges. An increase in head length appeared to be more sensitive to drag reduction at a speed of 600 m/s, while a long tail was found to induce a pronounced drag reduction at 200 m/s. In addition, the combined effect of the head and tail lengths on drag reduction was close to the superposition of their individual optimization effect. Based on the results, this study concludes that the individual designs of the head and tail of ETT systems may be rather demanding to achieve the desired optimization when considering distinct cruising speeds.

We propose a novel reduced-order model and examine its applicability to the complex three-dimensional turbulent wake of a generic square-backed bluff body called the Ahmed body at Reynolds number Re_H =U_∞H/ν =9.2×10⁴ (where U_∞ is free-stream velocity, H the height of the body and ν viscosity). Training datasets are obtained by large eddy simulation (LES). The model reduction method consists of two components, a VGG-based hierarchical autoencoder (H-VGG-AE) and a temporal convolutional neural network (TCN). The first step is to map the high-dimensional flow attributes into low-dimensional features, namely latent modes, which are employed as the input for the second step. The TCN is then trained to predict the low-dimensional features in a time series. We compare this method with a TCN based on proper orthogonal decomposition (POD), which utilizes time coefficients as the input in the second part. It turns out that the H-VGG-AE has a lower reconstruction error than POD when the number of latent modes is relatively small in the first part. As the number of latent modes increases, POD exceeds in the performance of model reduction. However, the H-VGG-AE-based TCN is still more effective in terms of spatio-temporal predictions because it has a lower prediction error and costs much less time.

Through comparing with the tandem circular and elliptic cylinders with the same characteristic dimensions, the behavior of flow and flow-induced noise generated around the tandem seal-vibrissa-shaped cylinder is studied based on delayed detached-eddy simulation model and acoustic analogy approach. The co-shedding pattern of flow developed around the tandem cylindrical-like bars is investigated. The spatial modes, mode energy, and mode coefficients of turbulent flow around the geometries are analyzed through spectral proper orthogonal decomposition. Results show that the lift fluctuations of downstream bar are stronger than those of upstream bar, and more aerodynamic noise is radiated from the downstream bar than from the upstream bar. The alternative arrangement of nodal and saddle planes of seal-vibrissa-shaped cylinder introduces three-dimensional flow separations and suppresses the shear layer interactions, inhibiting the regular vortex shedding of Kármán vortex street occurring in the tandem cylinder wake. The reversed vortex shedding generated by two adjacent saddle surfaces in the wake of seal-vibrissa-shaped cylinder balances the lateral force and mitigates the lift fluctuations greatly, thereafter reduces the aerodynamic noise generated by wall pressure fluctuations introduced by unsteady fluctuating forces exerting on the surfaces of geometries. Compared to the tandem circular and elliptic cylinders, the good noise reduction effect with sound pressure level reduced at main frequency range has been achieved from the tandem seal-vibrissa-shaped cylinder. The calculated spectra and amplitude levels of aerodynamic noise agree well with the experimental measurements from the anechoic wind tunnel, verifying the accuracy of the numerical simulations.