Study On The Interaction Between Multi-directional Irregular Waves And Pile Groups

Pile group is one of the main offshore foundation structural forms.The wave forces on different piles are different due to the interaction between piles in the pile groups,which is usually described by the pile group coefficient.The pile group coefficients of three piles in sideby-side and tandem arrangements under the action of uni-directional waves are given in relevant codes.In fact,the pile group structure is often a multi-piles structure.At the same time,the ocean waves are multi-directional irregular waves,and the wave directionality has an important impact on the wave loads on ocean engineering structures.Therefore,exploring the wave force characteristics of pile groups under the action of multi-directional irregular waves are of great theoretical value and can satisfy the needs of relevant engineering structural design at present.This thesis carry out a physical model test study on the effects of multi-directional irregular waves on four kinds of pile groups,including the pile group in a side-by-side arrangement,the pile group in a tandem arrangement,the pile group in double-row side-by-side arrangement and the pile group in a square arrangement.The wave force characteristics and pile group effects of the pile groups are systematically analyzed.The results show that when the number of piles in the pile group is large,the influence of relative pile diameter D/L1/3 on the force of the pile groups is remarkable.Therefore,considering comprehensively the influence of KC1/3 number and relative pile diameter D/L1/3,a new parameter KCLD1/3-KC1/3×L1/3/D is proposed.It is found that KCLD1/3 number is better than KC1/3 number in describing the variation of pile group coefficient.It can be used as a parameter to describe the change of pile group effect.Firstly,the statistical characteristics and probability distribution characteristics of wave forces on different pile groups are analyzed.The results show that in the range of KCLD1/3 from 0 to 1200,the pile group coefficient of inline force of the piles arranged in side-by-side increases,and the pile group coefficient of transverse force increases and then decreases with KCLD1/3 number increasing.The pile group coefficient of inline force on the last pile in the tandem pile group is the largest.The wave force characteristics of the pile in double-row sideby-side and square arrangement include the wave force characteristics of pile groups in the sideby-side and tandem arrangement.The group coefficients of multi-row side-by-side pile groups are smaller than that of the pile group in a side-by-side arrangement.When the KCLD113 number is large,?? in the range of 12.73°?24.98°,the inline force of the pile under the action of multidirectional waves is greater than that under the action of uni-directional waves.The transverse force increases as the directional spreading width increasing.Considering the influence of wave directionality,the influence factors of wave forces are proposed.The statistical results further show that Weibull distribution can well describe the probability distribution of wave force of pile group.The results of probability statistics show that the probability of exceeding large inline and transverse forces increases as KCLD1/3 increasing.The influence of directional distribution on the probability distribution of transverse force is stronger than that of inline force.Secondly,the frequency characteristics of wave forces on pile groups are analyzed.The results show that the spectra of the inline force on the pile groups and that on the single pile are single peak,and the spectral peak frequency is the same as that of waves.The high-frequency"energy" of the transverse force spectra decreases and the "energy" at the peak frequency increases as the wave directional spreading widens.When the piles arranged in tandem,the high-frequency "energy" in the inline force is large than those of the pile in side-by-side arrangement,and the high-frequency "energy"of the inline force of the last pile is the largest.Lastly,the numerical wave tank using moving boundary to simulate wave board motion based on the second order wave-maker theory is established by using Fluent to solve N-S equation,and a numerical wave tank which can simulate multi-directional irregular waves is extended.Furthermore,the numerical model of the interaction between waves and pile groups is established.The numerical simulations of wave forces on the pile groups in side-by-side,tandem and square arrangements under the action of regular waves and wave forces on the sideby-side pile group under the action of multi-directional irregular waves are carried out.The distribution characteristics and mechanism of wave force on the pile groups are analyzed.The results show that when the inline force of the side-by-side pile group reaches the maximum,the velocity distribution on the back of the edge pile is asymmetric.This makes the isobaric of the back of the pile deflection appear.The closer the pile is to the inner pile,the weaker the isobaric deflection is.It explains why the inline force of the edge pile is smaller than that of the other piles.When the piles are arranged in tandem,the pressure behind the last pile does not increase.Since there is no other pile behind,the velocity near the last pile can be further increased.The static pressure decreases as the flow velocity increases,which further increases the pressure difference in the direction of the inline force.It explains why the inline force of the last pile is the largest.The flow field of the side-by-side pile group under the action of multi-directional waves is different from that under the action of uni-directional waves due to the influence of wave directionality.The acceleration effect of the fluid passing through the gap between the two piles is weak,and the velocity difference between the two sides of the pile is relatively small.The component of the wave forces generated by multi-directional waves in the direction of the transverse force is the main component of the transverse force.The reason why the frequency of transverse force decreases with mult-idirectional waves is explained.