ABSTRACT
The phenomenon of downhill skiing or snowboarding, in its simplest form, refers to the motion of a human being sliding down an inclined plane on a porous medium (snow layer), as shown in Figure 1. Much of the classic literature treating the science of skiing and snowboarding focuses on the micron-thick water film that is formed on the underside of the ski or snowboard due to frictional heating (Colbeck, 1991; 1992; 1994a; 1994b; 1995). Recently, Feng and Weinbaum (2000) developed a new lubrication theory for highly compressible porous media which demonstrated that the excess pore pressure generated by a planing surface (ski/snowboard) moving on a compressible porous layer (snow) scales as α2 h2/K, where h is the layer thickness and K is the Darcy permeability; and that α is of order 102 or larger for humans skiing, thus, the lift forces generated can be four or more orders of magnitude greater than classical lubrication theory. The huge enhancement in the lift arises from the fact that as the porous medium (snow) compresses there is a dramatic increase in the lubrication pressure because of the marked increase in the hydraulic resistance that the fluid (air) encounters as it tries to escape from the confining boundaries through the compressed porous layer. At typical skiing velocities of 10 to 30 m/s the duration of contact of a ski or snowboard with the snow will vary from 0.05 to 0.2 s depending on the length of the planing surface and its speed. More recently, Wu et al. (2004a; 2004b; 2005b; 2005c) developed a novel experimental and theoretical approach to examine the excess pore pressure generation inside a snow layer on the time scale of skiing or snowboarding, which for the first time, qualitatively verified Feng and Weinbaum’s theory. The fundamental insights gained from these studies on the lift generation in porous media provide a new perspective for understanding the phenomenon of skiing and have further led to
the first realistic model for the lift mechanics of downhill skiing and snowboarding developed by Wu et al. (2006a; 2006b). This new skiing mechanics theory incorporates lift contribution from both the transiently trapped air and the compressed ice crystals. It captures the key physics of the stability and control during skiing or snowboarding and realistically predicts the performance of skiing or snowboarding as a function of the skier’s velocity, the sliding friction between the planing surface and snow, various snow types as well as the geometry of a ski or snowboard.
