The wake bi-stability behind notchback Ahmed bodies is investigated by performing wind tunnel experiments and large eddy simulations (LESs). The focus of this study is on the suppression of bi-stable wakes achieved by rounding the roof's trailing edge of the body. The suppression effect is found to depend on the Reynolds number ($Re$ ). The wake behind a sharp edge remains bi-stable for all tested $Re$ . However, for a rounded edge with small radius, wake bi-stability at $Re=0.5\times {10}^5$ and wake symmetrization with $0.75\times {10}^5\le Re\le 1.5\times {10}^5$ are observed. Increasing $Re$ with $Re\ge 1.75\times {10}^5$ , the wake returns to the bi-stable state. Particularly, with $Re\ge 2\times {10}^5$ , a stable asymmetric wake state with no switches is observed for long periods. Performing LES confirms the expected wake asymmetry at $Re=0.5\times {10}^5$ and symmetry at $Re=1\times {10}^5$ for the case of rounded edge with a small radius. Besides, another wake symmetry is observed for the rounded edge with a large radius at $Re=0.5\times {10}^5$ . For the two wake symmetries shown in the LES results, the symmetrization is attributed to wake suppression in the notchback region, forcing the flow separation from the rear roof to attach to the slant on both sides of the body.

The expected flow asymmetry behind a three-dimensional notchback Ahmed body is numerically investigated using large-eddy simulations with different blockage ratios of $0$ %, $5\%$ , $10\%$ , and $20\%$ . The focus of the study is on the natural bi-stable flow influenced by the blockage ratio. Although the wake asymmetry can be observed under the blockage ratio within $0\%-10\%$ , the significant blockage influence on the sensitive bi-stability is indicated by the wake structures, pressure gradients, and wake dynamics achieved with the proper orthogonal decomposition. The higher blockage ratio increases the turbulence kinetic energy, velocity, and negative pressure in the near-wake region, resulting in the decrease in the asymmetry degree. The consistency of the asymmetric wake is found with blockage ratios between $0$ % and $5\%$ . However, a $20\%$ blockage ratio symmetrizes the bi-stable wake. Several existing drag corrections for the blockage influence are discussed. The wind tunnel experiment verifies the bi-stable flow with low blockage ratios and the wake symmetrization with a $20\%$ blockage ratio.

The wake dynamics of two referenced variations of the squareback Windsor model with and without wheels is numerically studied by performing improved delayed detached eddy simulation. Numerical assessments are validated against publicly available experimental data. The focus of this study is on the wake states influenced by the wheels and the thick oncoming floor boundary layer. Results show that the addition of the wheels significantly changes the aerodynamic forces, the underbody flow, and the wake topology. The wake bi-stability is also enhanced with wheels in place due to the increased curvature of lateral shear layers in the near wake. However, the bi-stable behavior is largely suppressed when immersed in a thick boundary layer. These alterations depend on the degree of interaction between the wake recirculation and the bottom flow, and such degree is strongly affected by the underbody flow momentum. The evolution of low-order flow organizations and complementary spectral analysis highlight the differences in the coherent dynamics of the wake. The finding of this present work suggests that the wake bi-stability behind the squareback body can exist not only for a simplified geometry but also for a more realistic car with wheels in real-world upstream conditions.

This paper numerically studies the steady Coanda effect for drag reduction and airwake manipulations on the Chalmers ship model (CSM) using large eddy simulation with wall-adapting local-eddy viscosity model. Numerical methods are validated by experimental data acquired from the baseline CSM. In creating the flow control model, the hanger base of the baseline CSM is modified with Coanda surfaces and injection slots along its roof edge and two side edges. Four representative cases are studied: a no-jet case and three cases with the same momentum coefficient of the jet flow activated at different locations (roof, sides, and combined). The results show that the four cases have various performances in drag reduction and vortex structures on the deck. They are also different in mean and turbulent quantities as well as POD (proper orthogonal decomposition) modes in their airwake. It is found that the roof-jet has a stronger Coanda effect and is more vectored toward the low-speed area (LSA) on the deck than the side-jets that detach earlier from the Coanda surface. The energization process is, therefore, different where the roof-jet is more effective that directly brings high momentum to LSA and side-jets manipulate shear layers for mixing enhancement. The cases with roof-jet achieve better mitigation of flow re-circulation and higher recovery of streamwise velocity with lower turbulent fluctuation in the airwake. POD analysis suggests that the roof-jet can stabilize the wake.