The Limits of Drag Behavior for Two Bluff Bodies in Tandem

We demonstrate the interaction of two truck shapes in tandem. Both trucks experience a decreased drag coefficient from the interaction. The degree of drag saving depends strongly upon the drag coefficients of the model trucks in isolation, and upon how the two trucks are arranged. For the two simplest shapes-parallelepipeds with or without partial leading-edge rounding-the total drag saving can range from 10 percent to 40% at a spacing of 2√A (approximately 18 feet at full scale) depending upon whether the lead or the trail parallelepiped has rounding. These two shapes-blunt and rounded-have drag coefficients in isolation of 0.94 and 0.51 respectively, and probably bracket the savings to be obtained for all real truck geometries.

Our realistic model trucks (with wheels and a gap between tractor and trailer serving to distribute the source of drag along the length of the truck) have drag coefficients in the range C_D = 0.5-0.7, and the total drag saving is a more modest 15-20% at 2√A.