浮游式黑潮渦輪發電機於垂向剪切流場下之流體動力分析

Abstract

由於實際海域洋流流動並非均勻流，通常隨著水深會有垂向的流速差異，亦即垂向剪切流(vertical shear flow)。當全尺寸浮游式黑潮渦輪發電機(FKT)在垂向剪切流中運作時，轉子葉片所受的流體動力將會隨旋轉在不同位置而變動，亦即會是時變動力。如果此變動振幅過大，不僅可能因結構共振而發生破壞，也可能因長期的振動引發疲勞破壞，因此有必要對垂向剪切流引致的時變流體動力進行探討與掌握。 本研究使用計算流體力學軟體ANSYS-FLUENT，將流速分布簡化成線性垂向剪切流，對轉子葉片直徑5 m的20kW FKT 示範機組進行此議題的探討。首先以穩態(steady)運算進行單一轉子，以及FKT全機組於垂向剪切流下的流體動力模擬計算，同時探討不同紊流強度對單一轉子流體動力的影響，最後以非穩態(unsteady)運算探討單一轉子於垂向剪切流下的流體動力，以及葉片受力和葉片根部彎矩隨時間的變動。 計算結果發現，單一轉子以及FKT全機組的流體動力及其性能受剪切流的影響並不明顯，但葉片根部負荷受剪切流的影響則頗為顯著，且會隨速度梯度的增高，受力變動振幅就越大，亦即葉片受反覆振動的流體動力影響越大，葉片結構承受共振破壞或疲勞破壞的風險也會隨之增加，而此問題會隨著葉片尺寸的增大而更形顯著。

In general a marine current flow is not uniform. Therefore, when a Floating Kuroshio Turbine (FKT) operates in it, not only the hydrodynamic performance may be affected, but also the hydrodynamic loads acting on the blade may vary with time. In this way, the hydrodynamic loads may induce damage to blades due to resonance or fatigue failure. Therefore, hydrodynamic loads acting on blades operating in vertical shear flow were investigated in the present study. In this present study, the CFD software ANSYS-FLUENT is applied to deal with the hydrodynamic problems. To simulate the velocity distribution of Kuroshio Current, linear vertical shear flows are adopted for simplification. First of all, steady-state simulation is applied to analysis the hydrodynamics for single turbine and full FKT in vertical shear flow. Meanwhile, in this simulation, the effects of turbulent intensity to the hydrodynamics of single turbine are also discussed. Finally, unsteady-state simulation is applied to deal with the hydrodynamics for single turbine operating in vertical shear flow, the time varying hydrodynamic forces and moments acting on the blade root are investigated. The numerical results show that the hydrodynamic performance of single turbine or full FKT is not affected significantly by vertical shear in the flow. However, the amplitudes of time varying hydrodynamic forces and moments acting on the blade root become more significant with the increase of velocity gradient of the vertical shear in the flow. It means that these hydrodynamic loads should be taken into consideration in blade structure design.