Tailgate: Up or Down?
Debate rages (OK, maybe rages is a little strong) amongst pickup truck drivers as to whether it is more efficient to drive with the tailgate up or down. The urban legend is that running a truck with the tailgate down reduces the truck's drag and therefore improves its fuel economy.
No higher an authority than the MythBusters have taken up the challenge (twice) to confirm or bust the myth. Add to this the latest findings from the paper "Experimental Study of a Pickup Truck Near Wake" (Journal of Wind Engineering and Industrial Aerodynamics) and you have yet another common held belief falling flat in the face of scientific evidence.
The two MythBusters (episodes 43 and 64) investigations and the paper found, counter to the legend, that the drag was lower with the tailgate up than down. As is often the case with fluid dynamics, our intuition is faulty - I think in part due to the invisible nature of air flow.
Making the velocity field visible, using streamlines for example, is something that Computational Fluid Dynamics (CFD) can accomplish with ease. Here's an example where a simple CFD simulation can immediately provide a visual cue to better guide our understanding of the mysteries of everyday airflow around us - exposing the legends before they become urban!