Exploring the Regimes of Particle Behavior upon Impact via the Discrete Element Method.

Discrete element method simulations are conducted to probe the various regimes of post-impact behavior of particles with solid surfaces. The impacting particles are described as spherical agglomerates consisting of smaller constituent (or primary) particles held together via surface adhesion. Under the influence of a wide range of impact velocities and particle surface energies, five distinct behavioral regimes-rebounding, vibration, fragmentation, pancaking, and shattering-are identified, and force transmission patterns are linked to post-impact behavior. In the rebounding regime, the coefficient of restitution decreases linearly as impact velocity increases and the particle agglomerate experiences compaction. In the fragmentation regime, rebound velocity generally decreases with increasing fragment size. The rebound velocity of fragments decreases with time except for the smallest fragments, which can increase in velocity due to collisions with other fragments of high velocity. Particle breakage in the pancaking regime does not follow common mechanistic models of breakage.

Aerodynamic integration produces a vehicle shape with a negative drag coefficient.

Negative drag coefficients are normally associated with a vessel outfitted with a sail to extract energy from the wind and propel the vehicle forward. Therefore, the notion of a heavy vehicle, that is, a semi truck, that generates negative aerodynamic drag without a sail or any external appendages may seem implausible, especially given the fact that these vehicles have some of the largest drag coefficients on the road today. However, using both wind tunnel measurements and computational fluid dynamics simulations, we demonstrate aerodynamically integrated vehicle shapes that generate negative body-axis drag in a crosswind as a result of large negative frontal pressures that effectively "pull" the vehicle forward against the wind, much like a sailboat. While negative body-axis drag exists only for wind yaw angles above a certain analytical threshold, the negative frontal pressures exist at smaller yaw angles and subsequently produce body-axis drag coefficients that are significantly less than those of modern heavy vehicles. The application of this aerodynamic phenomenon to the heavy vehicle industry would produce sizable reductions in petroleum use throughout the United States.