Because of the complexity of turbulent flow, it is a tough issue to determine its friction factor.

鉴于紊流运动的复杂性，其沿程阻力系数确定起来较为困难。

youdao

Oh, mother, trickling flow of the river is always flowing your long hair, a long long mountain is the backbone of your tough.

哦，祖国，涓涓流贯的河川是你飘逸的长发，绵亘悠久长的山岳是你硬朗的脊梁。

youdao

Seriously in need of cash flow, Miles says he made the tough decision to sell at auction.

FORBES: On The Block

Mr. MARK WASSER (Attorney): It could substantially interfere with the ability of those courts to handle just the normal flow of litigation that they process, and so it's a tough question and elected officials in both state and perhaps the federal level are going to have to debate that.

NPR: Lawsuits Filed Over New Orleans Levee Breaks