Study On Wave-current Force Characteristics Of Single Pile And Pile Group Structure

Since the reform and opening up,China's transportation construction has made rapid progress due to economic development.As an infrastructure,the construction of cross-sea bridge has also started.The foundation of sea-crossing bridge not only bears the self-weight of superstructure and vehicle loads,but also is affected by the hydrodynamic force of sea water.Pile group and cap foundation is a common form of foundation for sea-crossing bridges,but there is no calculation method for wave-current force of pile group and cap foundation in relevant codes at present.Therefore,based on physical model tests and numerical simulation,the force characteristics of five-pile group with distance l/D=2 under wave and wave-current conditions are studied in this paper.Firstly,the physical model tests of single pile and pile group were designed.The waveinduced forces of structures were measured,which was caused by 9 non-linear wave cases with three different relative water depths and three different wave steepness.Also,the wave-current forces of structures were measured,which was caused by 27 wave-current cases with three different velocities combined with 9 wave parameters.Then a three-dimensional numerical flume was built by CFD software ANSYS Fluent,combined with velocity inlet wave-making method and momentum source wave-eliminating method.The effect of wave field around pile group and single pile were simulated.Then,the effect of wave propagation direction on the overall force of pile group under wave condition is discussed,and the force characteristics of each pile in 0° pile group are analyzed.And the detailed influence of pile group effect is analyzed by comparing with that of single pile.Finally,the relationship between force coefficients including wave direction force coefficient,transverse force coefficient and resultant force coefficient and KC is analyzed.Then the time-frequency domain characteristics of transverse force of single pile and pile group under wave and wave-current conditions are analyzed in detail.The main conclusions are as follows:(1)The wave direction force of pile group is calculated as a whole under wave condition.When KC > 7.5,the most disadvantageous calculation basis is the positive peak value of 0°-pile group.When the KC is 2.5-7.5,the negative peak value of 45°-pile group is the most disadvantageous basis for calculation.(2)Because of the influence of current,the discreteness of the ratio of wave direction force coefficient to resultant force coefficient and the discreteness of the ratio of transverse force coefficient to resultant force coefficient of single pile under the condition of wave-current are larger than those under wave condition;the proportion of wave direction force to resultant force of single pile under the condition of wave-current is smaller than that under wave condition,and the proportion of transverse force to resultant force is larger than that under wave condition.(3)The phase difference between the piles makes the positive and negative transverse forces of piles cancel each other out in pile group,which reduces the proportion of the transverse force to resultant force.Under wave with current condition,the effect of interaction between piles of pile group on the reduction of transverse force ratio to resultant force is more obvious.(4)The transverse force can not be neglected when calculating the resultant forces of single pile and pile group,whether under wave or wave-current conditions.(5)For a single pile,in the spectrum of wave-current transverse force,at the frequency which is 0.5 times of wave frequency and its integral multiple frequency,the proportion of corresponding amplitude in the spectrum is obviously larger than that of wave condition.The contribution of current to the wave-current transverse force spectrum of a single pile is mainly in the low frequency band,that is,in the spectrum of wave-current transverse force,current enlarges the proportion of corresponding amplitude of the low frequency components that also exist under the condition of wave or current.(6)The effect of current on wave-current transverse force spectrum of pile group is the same as that of single pile.But for pile group,regardless of wave or current conditions,when the velocity of water particle is high,the interaction of piles in pile group makes more high frequency components appear in the transverse force frequency spectrum of pile group.Therefore,for pile group,the distribution of frequency components in wave-current transverse forces spectrum of pile group are determined by the high frequency component caused by the interference of piles in pile group and the low frequency component of the flow contribution.