Author(s): Masayuki Takahashi; Youichi Yasuda; Iwao Ohtsu
Linked Author(s): Masayuki Takahashi, Iwao Ohtsu, Youichi Yasuda
Keywords: Erated flow; Stepped channel; Energy head; Air-concentration; Aerated flow velocity; Skimming flow; Transition flow; Nappe flow
Abstract: Stepped channels are effective for dissipating the energy of supercritical flow that occurs at steep channels. Generally, the flow conditions in stepped channels have been classified into skimming flow, transition flow, and nappe flow. Also, stepped channel flows have been characterized as aerated flows. For design purposes, it is important to predict the aerated flow velocity, the air-concentration, and the energy head of aerated flows in skimming, transition, and nappe flows. The authors have proposed a method for estimating the energy head of aerated flows in skimming flows by considering the air-concentration C and the mean velocity of aerated flows u (Ohtsu et al. 2004; Ohtsu et al. 2005), while the energy head of aerated flows in transition and nappe flows has not been shown. In this paper, aerated flow characteristics of skimming, transition, and nappe flows have been investigated for a wide range of relative step height S / dc (S = step height, dc = critical depth) under a given channel slope θ = 19°. It was found that the velocity and air-concentration profiles characterize each flow condition. The energy head of aerated flows E in transition and nappe flows is determined from the air-concentration C and the aerated flow velocity u. By using a clear-water depth dw and the averaged velocity Vw (= qw / dw; qw: water discharge per unit width), the energy head of aerated flows E in transition and nappe flows can be expressed as E = Cpdw+ CvVw 2/2g (g = acceleration of gravity) with the correction coefficients Cp and Cv obtained from C and u. If the energy head of clear water is defined as Ew, the ratio of E to Ew is found to be 1. 1 (E/Ew ≒ 1. 1) for skimming, transition, and nappe flows under θ = 19° and 0. 5 ≤ S / dc ≤ 5. 5.
Year: 2007