Experimental investigation on the effects of abrupt wind forcing on deep water gravity waves

Abstract:

Experiments were performed to study statistical properties of nonlinear gravity waves in the presence of abrupt winds in the 300-meter-long Multiple Function Towing Tank at Shanghai Jiao Tong University. Wind turbines were installed at the one-third the length of the wave flume away from the wavemakers with the maximum wind speed up to 7 m/s, and an absorbing beach at the end of the flume. Waves were generated by wave makers of the flap type and were set to encounter the wind forcing after they went past wind turbines. Apparently the wind strength experienced a gradual decay along the flume. Wave elevation was measured by probes set within the wind field (fixed rigidly enough to minimize its vibration) and another place downstream that could be considered free from wind effects. Three sets of waves generated from the JONSWAP spectrum characterized by different values of steepness and peak period, and repeated in the flume with and without wind. Results show that the presence of abrupt strong winds greatly enhances the ‘fat tail’ effect, and the kurtosis of the wave elevation experiences a noticeable increase under the effect of abrupt wind forcing. On the other hand, significant wave heights for all three sets of measurement in the windy conditions are slightly reduced. It is suggested that gusty conditions may enhance the possibility of rogue wave occurrence by diverting the nonlinear wave system from its equilibrium state.