Children and adults often ask us the question, “If I jump from a bridge into water can I throw an object at the water before I land and reduce the impact force?” We tested this theory by dropping two spheres, axially aligned but distance separated, into a quiescent pool of water. The first sphere creates a cavity through which the second sphere enters. We show that the high peak force at the first few moments of the impact can be dramatically reduced under the right conditions. What about oblique entry? We show that elastomeric silicone rubber spheres ricochet from a water surface under a much broader range of conditions when compared to rigid spheres and disks (or skipping stones), but why? High speed cameras allow us to see that these unique spheres deform significantly as they impact the water surface, flattening into oblate shapes at impact and then returning to spherical shapes. We present a regime diagram which enables the prediction of ricochet from sphere impact conditions. Our experiments and mathematical models show that these deformable spheres skip more readily because deformation momentarily increases cross-sectional area and produces an attack angle with the water which is favorable to skipping. Predictions from our mathematical model agree strongly with observations from experiments. While studying multiple impact events in an outdoor setting, we discovered a previously unidentified means of skipping (namely walking), which is unique to deformable spheres.
Tadd Truscott’s current research interests are in fluid dynamics, novel imaging and experimental methods. By merging different areas of research, he works on problems such as three-dimensional flow-field dynamics of rising and falling objects in water, acceleration induced cavitation, the transport of water through desert plants and the collective behavior of pelotons and fish. Tadd received his B.S in mechanical engineering from the University of Utah, and then attended Massachusetts Institute of Technology for his Ph.D.09’ in Ocean and Mechanical Engineering. He is currently an associate professor at Utah State University.
Host: Professor Randy Ewoldt