The Experimental Verification of Numerical Models of Plunging Breakers

Abstract

Results of a WAVE-FOLLOWER EXPERIMENT are presented, in which a moving current meter entrained in the crest of a steep Stokes wave and a moving high-speed film camera follows the wave with its non-linear phase velocity. Measurements of wave particle velocities are then obtained both in non-breaking steep wave crests, and in breaking waves. The breaking waves in deep water conditions are obtained by the application of a non-linear sweep frequency modulation technique, and the Stokes wave becomes unstable due to interaction of 13 wave components focused into one single point in space and time, KJELDSEN 1982. The result of this interaction is a large freak wave, breaking as a plunging breaker in deep water. Measured crest particle velocities obtained with the current meter exceeded the phase velocity of this wave with 36 %. Digitalisation of the high-speed film showed that particle velocities at the very tip of the plunging jet obtained the value 2.65 times the linear phase velocity. These results are then compared with predictions obtained from numerical simulations by LONGUET-HIGGINS & COKELET 1976 and VINJE & BREVIG 1980.

Experimental research on kinematics of breaking waves

“…Z T c g π = (1) g = gravitational acceleration, 9.81ms -2 TZ = zero-downcrossing period, the time (seconds) taken for a successive wavelength, measured at the free surface.…”

“…Kjeldsen [1] created breaking waves via the superposition of 43 regular waves of carrying frequencies. High speed cameras were used for the capturing of the horizontal crest particle velocity.…”

On the Partition of Horizontal Momentum Between Velocity and Pressure Components Through the Transition Region of Breaking Waves

“…the partition of momentum changes through the transition. At the plunge point, we have taken the value of A£ the same as at the break point but increased A to over 2 as demonstrated numerically by Cokelet (1976) and experimentally by Kjeldsen (1984). The result is more velocity momentum with m* now being 5300 ft 3 /sec 2 (150 m 3 /s 2 ) or almost 89 percent of the total.…”

Wave Forces on Vertical Piles Caused by 2- And 3-Dimensional Breaking Waves

“…In addition the particle velocities within the crests of these transient plunging breakers in deep waters were measured. This was achieved with a newly developed wavefollower-system, consisting of a carriage with a high-speed cine camera and a current meter that followed one particular wave crest until it finally became unstable and developed into a plunging jet, see Kjeldsen (1984a). The very large particle velocities measured in the upper part of the crests suggest that local wave forces in breaking waves close to the free surface should be much higher than predictions of wave forces obtained from application of Stokes' higher order wave theories.…”