catchingthewave! - technical university of...
TRANSCRIPT
Catching the wave
Eline Dehandschoewercker, David Quéré, Christophe Clanet
Laboratoire d’Hydrodynamique de l’Ecole polytechnique (LadHyX), Ecole polytechnique, Palaiseau, France Physique et Mécanique des Milieux Hétérogènes (PMMH), ESPCI, Paris, France
Surfing is a free surface sport in which the athlete rides a wave standing on a board. We focus on the conditions needeed for a particle to catch the wave. We start with field observations. The surfer (a) waits for the approaching wave, (b) starts paddling, (c) takes off, (d) stands on the board and (e) rides the wave (Figure 1). The surfer catches the right wave at a specific position with an appropriate velocity. We have quantified his trajectory and velocity on the waves in order to determine the optimal conditions for surfing.
(a) (b) (c) (d) (e) Figure 1 : Images sequence of Alexandre De Sales (1.90 m) catching the wave at Uhabia surf spot in Bidart, France. Time between images is 1 second. To optimize surfing in the lab, we have developed an experimental setup that allows us to control the relevant physical parameters of this problem. A wave-‐maker generates propagating waves whose wavelength and amplitude are accurately controlled. Balsa boards with different aspect ratios and mass distributions are used as floating bodies with variable friction on the water waves. We study the motion of boards towed onto waves. First we show the existence of a critical initial velocity above which the board is captured by the wave and determine its dependence on the initial position of the board on the wave. We vary the main parameters that affect this capture threshold, specifically the slope of the wave and the friction coefficient of the board. Finally, we analyze the board motion in order to find the criteria that guarantee an efficient propulsion.