In case of the turbine is operated departing the optimum condition, the low presure and the vortex band will be established in the draft tube centre by the flow circulation.

水轮机在偏离最优工况运行时，水流的环量会使尾水管中心产生低压并形成涡带。

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Then comes The War, in this case Vietnam, and the band loses half its members to the draft.

FORBES: Behind The Music