Investigation of the Unsteady Wake behind a Generic Tractor-Trailer with Different Boundary Conditions

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Due to pressing legislation and customer requirements on low emissions and fuel consumption, the modern vehicle industry is faced by demanding challenges. A significant portion of emissions is contributed by long hauling heavy trucks. At cruising highway velocities most of the trucks fuel is consumed by overcoming its aerodynamic drag which largely originates from the wake region behind the trailer. Therefore, this region provides the most potential for reducing its overall drag when effectively controlled. Recently, alterations in the tractors geometry and the application of add-on devices have provided some drag improvements. However, most of the current research on passive and active flow control methods is considering an integral effect of these methods without detailed investigation of the underlying mechanisms. Also, little emphasis has yet been placed on the detailed investigation of the tractor-trailers wake without alterations. This region is fluid mechanically very complex and dominated by highly turbulent, non-stationary structures which remain a challenge in the field of bluff body aerodynamics. Only insufficient detail on the three-dimensional, time-dependent interaction of these structures is known to date. Especially for road vehicles an important aspect is the ground effect. Many studies consider a stationary ground or in some cases a moving belt. However, these experimental boundary conditions only simulate the conditions of a vehicle moving on a road. The impact of these differing boundary conditions has not been properly addressed yet. Also, experimental studies vary significantly in Reynolds number without sufficient emphasis on its effect. Considering the significance of this topic for the scientific community and industry, the proposed study is aimed at addressing some of these unanswered questions and inconsistencies. The wake of a generic 1/10th-scale tractor-trailer model will be examined in three different test facilities with different experimental boundary conditions and over a wide range of Reynolds numbers. A closed-loop wind tunnel at the TU Berlin will simulate the ground effect with a stationary ground, whereas a similarly sized wind tunnel at Chalmers Institute of Technology (Sweden) provides a moving ground. The third test facility is a water towing tank at the TU Berlin to study the effects of a moving model over a stationary ground. Time-resolved flow field measurements of the models wake structures will be analysed and compared for different boundary conditions. The increased knowledge of the interaction between the flow field in the wake and the bodys base pressure will form a cornerstone in passive (e.g., shaping) and active control for minimizing the wake size. In the proposed study, the extensive baseline investigation will aid the application of appropriate base flap geometries to the rear face of the model supported by active separation control through fluidic oscillators.