Large Trucks Drag Reduction Using Active Flow Control

Aerodynamic drag is the cause for more than two-thirds of the fuel consumption of large trucks at highway speeds. Due to functionality considerations, the aerodynamic efficiency of the aft regions of large trucks was traditionally sacrificed. This leads to massively separated flow at the lee side of truck trailers, with an associated drag penalty: roughly a third of the total aerodynamic drag. Active Flow Control (AFC), the capability to alter the flow behavior using small, unsteady, localized energy injection, can very effectively delay boundary layer separation. By attaching a compact and relatively inexpensive "add-on" AFC device to the back side of truck trailers (or by modifying it when possible) the flow separating from the truck trailer could be redirected to turn into the lee side of the truck, increasing the back pressure, thus significantly reducing drag. A comprehensive and aggressive research plan that combines actuator development, computational fluid dynamics and bench-top as well as wind tunnel testing was performed. The research uses an array of 15 newly developed suction and oscillatory blowing actuators housed inside a circular cylinder attached to the aft edges of a generic 2D truck model. Preliminary results indicate that a net drag reduction of 10% on full-scale trucks is achievable. 1. Scientific background Active flow control (AFC) is a fast-growing, multidisciplinary science and technology thrust aimed at altering a natural flow state to a more desired flow state (or path). Flow control was simultaneously introduced with the boundary layer concept by Prandtl [1] at the turn of the 20th century. In the period leading to and during WW II, as well as in the Cold War era, flow control was extensively studied and applied mainly to military fluid related systems. Though the fluid mechanics aspect can be robust, steadystate flow control methods were proven to be of inherently marginal power efficiency, and therefore limited the implementation of the resulting systems. Unsteady flow control using periodic excitation and utilizing flow instability phenomena (such as the control of flow separation [2]) has the potential of overcoming the efficiency barrier. Separation control using periodic excitation at a reduced frequency of the same order, but higher than the natural vortex shedding frequency of bluff bodies (such as an airfoil in post-stall or a circular cylinder), can save 90% to 99% of the momentum required to obtain similar gains in performance compared to the classical method of steady tangential blowing. The feasibility of increasing the efficiency and simplifying fluid related systems (e.g., for high-lift, Refs. 7-8) is very appealing. This becomes even more appealing if one considers that savings of 1% in the fuel consumption of the US fleet of large trucks (about 5 million trucks) is worth about $1.5 billion per year, while the environmental impact is proportional to the fuel savings. The political implications clearly exist but are difficult to quantify. The progress in the development of actuators, sensors, simulation techniques and system integration and miniaturization enables using wide bandwidth unsteady flow control methods in a closed-loop AFC (CLAFC) manner. (See Ref. 3 for a comprehensive review of the subject.) Experimental demonstrations are required to close the gap between the current theoretical understanding, the computational capabilities and real-world problems. The study described brings together AFC expertise, specifically actuator development and implementation for separation control, closer to real-life industrial applications. The essential know-how and components of an active separation control system, packaged as an "add-on" device that will be attached to the rear end of large truck trailers, in order to reduce the aerodynamic drag by about 20%, were recently assembled and will be presented in the following sections. At highway speeds, the aerodynamic drag is responsible for roughly 65% of the fuel consumption,