Abstract
The torsion parameter β_0 (β_0=γ/(2δ)^<1/2.,γ:dimensionless torsion, δ:dimensionless curvature) is experimentally obtained for which value the critical Reynolds number Rec is smallest for helical pipe flows. The mean velocity distributions and the turbulent intensities in helical pipe flows are observed by use of a X-type hot-wire anemometer in the range of the Reynolds number from 200 to 13000. It is found that Rec first decreases as β_0 increases before β_0 reaches about 1.4, and then it increases as β_0 increases. The minimum of Rec is experimentally found to be about 400 at β_01.4.
