In this paper the results from physical experiments with an instrumented cylinder conducted in laboratory environments are presented. The primary aim of the study has been to investigate the effect from wave directionality on the local and depth integrated maximum wave forces on a smooth vertical cylinder. The experiments were carried out using a newly designed cylinder instrumented with miniature pressure transducers. As expected the experiments show a significant reduction of the in-line and resultant wave forces in directional waves compared to unidirectional waves with equal spectral properties. The observed reductions are nearly constant below mean water level but increase rapidly above mean water level. The transverse wave forces are generally increased in directional waves.
It is a generally adopted fact that the wave forces m irregular directional (short crested) waves are smaller than in a unidirectional waves with equal spectral properties the effect is a consequence of the directional spreading of energy m a short crested sea. Consider a simple example where two wavelets are travelling in perpendicular directions Let the wavelets have equal phase, frequency and amplitude Measured by a wave gauge the surface formed by the two wavelets will be identical to the surface formed by one regular wave with twice the amplitude but phase and frequency as one of the wavelets By superposition of the kinematics in the two wavelets. It is evident that the horizontal particle velocity and acceleration will be equal to 1/√2 times those in this corresponding regular wave. The force on a cylinder calculated by the Monson equation will therefore be reduced by the factor 1/√2 for the inertia term and 1/√2 for the quadratic drag term When a cos2 spreading is applied to the wavelets the reductions will be 85% and 72%, respectively, whereas the reductions will be 93% and 87%, respectively, if a cos6 spreading is applied.