Experimental investigation of impact forces induced by plunging breakers on a vertical cylinder

A plunging breaker propagates with different stages, each of which induces a different force on structures. In this study, physical experiments of the different stages of a plunging breaker impacting a vertical cylinder are implemented in a wave flume. The concept of decomposing the measured forces into quasi-static forces and dynamic forces is adopted. The method proposed by Irschik et al. (2004) using an FFT filter and EMD (Empirical Mode Decomposition) is applied in the decomposition process. Then a new method with a continuous wavelet-based filter is used that produces a more reasonable result. Subsequently, the results of the measured forces, the quasi-static forces, and the dynamic forces are analyzed by considering the different stages of the plunging breaker. The results show the peak forces induced by the various stages of the plunging breaker are quite different. Prior to the overturning jet impinging the free surface, the quasi-static forces due to the various stages of the plunging breaker are approximately the same, while the dynamic forces contribute to different strengths of the force oscillations and exhibit a similar response. Following the mixture of water with air, the peak value of the quasi-static force is no longer invariant but declines sharply.