The Dynamics of Thrown Granular Beds
Abstract: We shall discuss the behaviour of granular beds thrown from the surface of a vertically vibrating platform, when the bed is immersed in a fluid. Fluid damping reduces the granular temperature. There are a wide range of circumstances where this leads to a well defined gap between bed and platform, or between distinct beds, during bed flight. The effects of fluid forced through the bed and the existence of well-defined gaps lead to a fascinating range of behaviours. At lower frequencies, a bed may be unstable against Faraday tilting. Horizontal fluid flow during flight enhances tilt “construction”, while “destruction” processes occur close to the times of landing. Granular motion at the tilted upper surface is also strongly influenced by fluid flow. Behaviour relevant to the long-standing debate on the tilting mechanism will be illustrated by computer simulation and results from high-speed camera measurements. In the case of separated binary mixtures, we shall show that it is the shearing motion during the periods of flight when the gaps are open which enables separation to be maintained against the influences of convection. We will also describe how vibration-induced fluid flow leads to a range of oscillatory behaviours in separated binary mixtures, either in the form of tilt oscillations or in the form of travelling waves. These effects too will be illustrated with video clips.