Reynolds Number Impact on Commercial Vehicle Aerodynamics and Performance

The impact of Reynolds number on the aerodynamics and operational performance of commercial vehicles is discussed. All supporting data has been obtained from published experimental and computational studies for complete vehicles and vehicle components. A review of Reynolds number effects on boundary layer state, unsteady and steady flow, time dependent wake structure, interacting shear layer and separated flows is presented. Reynolds number modeling and simulation criteria that impact aerodynamic characteristics and performance of a commercial vehicle are shown. The concepts of dimensional analysis and flow similarity are employed to show that aerodynamics of commercial ground vehicles is only dependent on Reynolds number. The terminology of Roshko is adopted for discussing the variation in drag with Reynolds number in which the subcritical, transitional and transcritical flow regimes are defined for commercial vehicles. Criteria for aerodynamic simulation as well as testing and design of commercial vehicles are defined and show a minimum transcritical Reynolds number value of 3 million is recommended for simulating the aerodynamics of full-scale commercial vehicles. Guidance is provided for aerodynamic design and analysis in the framework of Reynolds number and boundary layer flows as they relate to previous designs, current testing and analysis criteria, and the development of future vehicles. A discussion of aerodynamic tools is presented which call for the continued development of aerodynamic test and analysis tools capable of capturing Reynolds number sensitivities. Of particular importance is the maturation of computational simulation tools with improved friction modeling capability specifically developed for commercial ground vehicles.