Initial Studies
Experimental and numerical studies have been conducted to examine the effectiveness and feasibility of drag reduction using a wide variety of unventilated planar-sided cavity devices on the aft face of tractor-trailers. Qualitative numerical simulations have indicated a relative reduction in the pressure drag on the aft face of the trailer compared to the no device baseline. Wind tunnel parametric studies investigated cavity length, boat tail angle and inset from the trailer edge. Yaw angles up to 9 degrees have been examined. Device performance decreased with yaw angle, however reversal of this trend was observed at several of the largest yaw angles.

A full scale prototype utilizing rigid composite sides with a flexible top and bottom has been road tested, exhibiting cross-country road fuel savings of about 0.5 mpg, or 9%, over a 10000 mile trip. Estimated fuel savings for 120,000 miles per year traveled were 1500 gallons per truck. A third generation full scale composite prototype is being designed and initial road tests have been conducted in the summer of 2005 at Clarkson University.

Wind Tunnel Tests
Wind tunnel testing has been conducted on over 100 different design geometries using a 1:15 scale Peterbilt 379 tractor and 48 foot trailer. Cavity length, boat tail angle, and inset from the trailer edge were varied at yaw angles up to 9 degrees. Scale model drag increments, obtained at zero yaw and a Re of 2.3e105, based on trailer width, indicated drag coefficient reductions up to Cd = 0.10 to 0.12 or about 10% of the baseline model trailer drag for the four foot device with a boat tail angle of 10° and no inset. Removal of the top plate reduced the drag savings for each device by up to 45 percent and device performance decreased with yaw angle.

Recently, a new model has been constructed from an aluminum nylon sintered material and combined with the stereolithography capabilities at Clarkson, has enabled combined force balance and pressure measurements to be taken based on road tested geometries Wind tunnel improvements have also enabled a tripling of the Reynolds number available.

Full Scale Tests
A full-scale prototype utilizing rigid composite sides with a flexible top and bottom was road tested in 2000, exhibiting cross-country road fuel savings of about 0.5 mpg, or 8%, over a 10000 mile trip. Estimated fuel savings for a typical 120,000 miles per year traveled were 1500 gallons per truck.

Since the previous full scale testing involved only a single vehicle and no control, a new series of tests was initiated to optimize the full-scale geometry in 2005. Preliminary results from the configurations, three different sections of road, including interstate and secondary roadways, and 2500 miles of data collecting indicate the optimal geometry to provide more than 0.5 mpg savings. The plot illustrates a sample of the over the road averaged differences between the baseline and the optimum geometry for a series of 4 continuous runs between Potsdam and Canton. This configuration was found to provide the most benefit under normal operating conditions.

Collaborations and Partnerships
Clarkson is currently teamed with ATDynamics, in San Francisco, CA to work towards a viable commercial product of the open cavity design. Testing is continuing with LaValle Transport Inc. of Potsdam.

Funding for the project has been generously provided by the New York State Energy Research and Development Authority (NYSERDA), under the direction of Joseph R. Wagner, Sr. Project Manager.

Publications and Reports

Kehs, J., Visser,K., Grossmann, J., Horrell, C., and Smith, A., "Experimental and Full Scale Investigation of Base Cavity Drag Reduction Devices for Use on Ground Transport Vehicles" The Aerodynamics of Heavy Vehicles III: Trucks, Buses, and Trains, Lecture Notes in Applied and Computational Mechanics, 2010, submitted for publication.

Grover, K. and Visser, K.D., "Over-the-Road Tests of Sealed Aft Cavities on Tractor Trailers," SAE 2006 Transactions Journal of Commercial Vehicles, No. 2006-01-3529, pp.170, March 2007.

Coon, J.D, and Visser, K.D., "Drag Reduction of a Tractor-Trailer Using Planar Boat Tail Plates," The Aerodynamics of Heavy Vehicles: Trucks, Buses, and Trains, Lecture Notes in Applied and Computational Mechanics , Vol. 19, McCallen, Rose; Browand, Fred; Ross, James (Eds.), 2004, XII, 567 p. 378 illus. With CD-ROM., Hardcover, ISBN:3-540-22088-7

Maragno , D. and Visser, K.D., "The Effects of Aft Cavity Inset and Boat Tail Angle on Drag Reduction of Tractor Trailers" Report No. MAE-365, Department of Mechanical and Aeronautical Engineering, Clarkson University, May 2003.

Coon , J.D. and Visser, K.D., "The Effects of Non-Ventilated Plate-Cavity Devices on Drag Reduction of Tractor-Trailers" Report No. MAE-361, Department of Mechanical and Aeronautical Engineering, Clarkson University, June 2002