Experimental And Numerical Investigation On Flow Characteristics Of Offshore Wind Turbine With Tripod Foundation

With the development of offshore wind turbine from the offshore to deep sea,the tripod foundation has attracted more attention due to its good applicability to water depth,good stability and low construction cost.Compared with the large-diameter monopile foundation,the flow behavior and force dynamics for the tripod foundation are complicated.At some specific incident wave frequencies,the near trapping phenomenons occur in the the interior area of the column structures,leading to an abrupt increase of the wave surface,which has extremely adverse effects on the safety of the offshore tripod foundation.However,the mechanism of wake interaction among the three columns for the tripod foundation has not yet been explained,and the effect of spacing ratio on the near traping of waves among the three columns needs further study.Therefore,in this thesis,the flow interference and wave near trapping effect for the tripod foundation are investigated based on the experiment and numerical simulations.The details are described as followed:1.Partical image velocimetry technique(PIV)and OpenFOAM software are used to obtain the instantaneous and time-average flow field and dynamic response of three cylindrs in an equilateral-triangle configuration at difference spacing ratios and Reynolds numbers.The instantaneous and time-average vorticity contours,streamline topology,Reynolds stress,pressure distribution,force coefficients and vortex shedding frequency of three cylinders are analyzed.Depending on the spacing ratios and Reynolds numbers,five different flow regimes are identified:proximity with deflected flow(P-D),proximity with flip-flop flow(P-F),dual vortex shedding(DV),transition between dual vortex shedding and triple vortex shedding(DV-TV),triple vortex shedding(TV).And the critical spacing ratio of different flow regimes decreases with the increase of Reynolds number.With the increase of the spacing ratio or Reynolds number,the three-dimensionality of the wake flow becomes stronger.Due to the wake interference among the three cylinders,the force characteristics and vortex shedding frequencies are significantly different from those of the single cylinder.The mean lift coefficients of the lower side tandem cylinders are always smaller than the upper side cylinder,while the mean drag coefficients of the upstream cylinders are larger than the downstream cylinder.2.Three-dimensional numerical simulations on the near trapping phenomenon for the three cylinders in an equilateral-triangle configuration are performed using the open source software OpenFOAM and the mass source wave-making method.The effect of spacing ratio and wave parameters on the variations of wave run-up characteristics and force characteristics are analyzed.The results show that at small spacing ratio,the internal near trapping interference of the three cylinders is stronger,leading to the increasing of the wave propagation velocity and deviation of the propagation direction among the three cylinders,accompanied with the wave breaking phenomenon.As the steepness parameter increases,the nonlinear effect of the wave increases and the wave run-up height increases.With increasing of spacing ratio,the force characteristics of the three cylinders behave differently.Although the downstream cylinder has the highest wave run-up height,the maximum wave force not occurs on the downstream cylinder.