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One of the largest-scale unstructured Large Eddy Simulation (LES) of flow around a full-scale road vehicle is conducted on the Earth Simulator in Japan. The main objective of our study is to look into the validity of LES for the assessment of vehicle aerodynamics, especially in the context of its possibility for unsteady or transient aerodynamic forces. Firstly, the aerodynamic LES proposed is quantitatively validated on the ASMO simplified model by comparing the mean pressure distributions on the vehicle surface with those obtained by a conventional Reynolds-Averaged Navier-Stokes simulation (RANS) or a wind tunnel measurement. Then, the method is applied to the full-scale vehicle with complicated geometry to qualitatively investigate the capability of capturing organized flow structures around the vehicle. Finally, unsteady aerodynamic forces acting on the vehicle in transient yawing-angle change are estimated and relationship between the flow structures and the transient aerodynamic forces is mentioned. As a result, it is demonstrated that LES will be a powerful tool for the vehicle aerodynamic assessment in the foreseeable future, because it can provide precious aerodynamic data which conventional wind tunnel tests or RANS simulations are difficult to provide.

The influence of transient flows on vehicle stability was investigated by large eddy simulation. To consider the dynamic response of a vehicle to real-life transient aerodynamics, a dimensionless parameter that quantifies the amount of aerodynamic damping for vehicle subjects to pitching oscillation is proposed. Two vehicle models with different stability characteristics were created to verify the parameter. For idealized notchback models, underbody has the highest contribution to the total aerodynamic damping, which was up to 69%. However, the difference between the aerodynamic damping of models with distinct A-and C-pillar configurations mainly depends on the trunk-deck contribution. Comparison between dynamically obtained phase-averaged pitching moment with quasisteady values shows totally different aerodynamic behaviors.

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Conventionally, the pitching instability of road vehicles has been controlled mechanically through the application of suspension systems. The present study demonstrates how unsteady aerodynamics can be exploited for such control by properly configuring vehicle body shapes. To discern the effect of unsteady aerodynamics on road vehicle stability, large eddy simulation has been conducted to simulate the flow past simplified vehicle models. Forced-sinusoidal-pitching oscillation was imposed on the models during the simulation to probe their dynamic responses. Numerical results were compared with wind tunnel measurements for validation, and good agreement is attained. Unsteady flow structures above the rear section of the vehicles were found to significantly affect their pitching stability. Depending on the vehicle body shape configurations, the induced aerodynamic force tended to either enhance or restrain the vehicles' pitching instability.

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In recent years, the dielectric barrier discharge plasma actuator (DBD-PA), which is a fluid control device, has been investigated for achieving both high aerodynamic performance and pleasing styling of transportation equipment. In this study, the authors installed a DBD-PA system on a simplified three-dimensional bluff automobile body to reduce the aerodynamic drag. In particular, the authors focused on the sides of the rear end of the body, where the local shape has high sensitivity regarding both styling and aerodynamic drag. At the rear sides of the automobile-like bluff body, a sharp edge rather than a smooth rounded corner often reduces the aerodynamic drag by promoting airflow separation. Therefore, the authors aimed to reduce the aerodynamic drag by using a DBD-PA system to promote flow separation at the rear end while retaining its rounded shape. Aerodynamic measurements using a one-fifth scale simplified automobile model were conducted in a wind tunnel. Preliminary investigation of the aerodynamic effect at the rear clarified how the longitudinal vortices from the rear pillar and the side edge of the trunk deck cause the drag increase at the rear-end corners. Two parallel DBD-PAs were installed on the rear surface to shift these vortices away from the corners by promoting flow separation. The drag reduction rate reached 3% at the highest applied voltage using the DBD-PA system on a rounded shape, and it achieved approximately half the effect of the sharp-edged shape. The longitudinal vortices were successfully kept away from the rear-end corners by the DBD-PAs. The surface pressure increased with the displacement of the vortices, which led to the drag reduction observed.