The in-line and lift forces on vertical piles in side by side arrangement induced by both irregular waves and currents were investigated experimentally in this paper. The characteristics in both time and frequency domain of in-line, lift and resultant forces as well were analyzed. The grouping effect coefficients of inline and resultant forces on the piles related to KC number and relative spacing parameters are given. A comparison of the magnitude and direction of resultant forces on vertical piles in side by side arrangement with the corresponding values for a single cylinder is also made. The range of KC number tested is 6 ~ 60, the range of Reynolds number is (0.55~3.43) × 104.
The wave loading on a slender cylinder and groups of slender cylinders is one of the basic problems of hydrodynamics in offshore engineering. Since the development of Morison's Equation in 1950, the prediction of in-line force on a single cylinder becomes realistic. In spite of its shortcomings (unstrictness in viewpoint of theoretical analysis and sometimes large error may occur during the calculation), so far the Morison Equation is the only practical method to be used for engineering purposes. Except for some studies [Chakrabarti (1982). Gao and Sun (1987), Yu and Zhang (990)J concerning pure wave loading on pile groups, there is almost no report about the wave-current force on cylinders caused by irregular waves combined with currents. In fact, cylindrical pile groups are used more often than isolated cylinder in engineering application. This paper is part of the results of the authors" research on irregular wave - current forces on pile groups. Because of the complexity of the flow behaviour around cylinders, a more reliable method for this study is the physical model test.