Lay Language Papers 2008

High-speed Jet Formation after Solid Object Impact

When dropping a stone into water, a thin and very fast water jet can shoot out from the surface, a familiar and simple everyday phenomenon which any child has tried himself or herself and which is common in nature and technology. However, only a closer look with high-speed cameras can reveal the rich and complex underlying dynamics. The initial downward motion of the impacting object is turned into an upward motion of the liquid jet. During impact the intruding object creates an air-filled cavity in its wake. The subsequent violent collapse of this cavity due to hydrostatic pressure then leads to the formation of the high-speed liquid jet. In this talk we illustrate how the collapsing cavity wall squeezes out the jet very much like the squeezing a tube of toothpaste -- only much faster.

To study the impact process in a very controlled fashion, we use a circular disc which is pulled through the liquid surface by a linear motor, allowing us to prescribe the impact speed and keep it constant throughout the experiment. High-speed video images taken at up to 30,000 frames per second illustrate the formation of the air cavity, its collapse, and the resulting jet formation. Accompanying computer simulations, which are shown to be in excellent agreement with the experimental images, allow us to study in detail the flows that create the high-speed liquid jet. It turns out that the inertial energy focusing during cavity collapse makes the present phenomenon reminiscent of the very violent jet of fluidized metal observed during the collapse of "lined cavities" in military and mining operations.

References

• Physics of Fluids, University of Twente, The Netherlands
• Grupo de Mecánica de Fluidos, Universidad de Sevilla, Spain

Correspondence: s.gekle@tnw.utwente.nl

Fig. 1: The impact of a disc (a, b) entrains a giant bubble which upon collapse (c) forms a violent jet shooting upwards (d)

Fig. 2: Close up of the high-speed jet.