Turbulence characteristics in skimming flows on stepped spillways G. Carosi and H. Chanson Abstract: The stepped spillway design is characterized by an increase in the rate of energy dissipation on the chute associ- ated with a reduction of the size of the downstream energy dissipation system. This study presents a thorough investigation of the air–water flow properties in skimming flows with a focus on the turbulent characteristics. New measurements were conducted in a large-size facility (q = 228; step height, h = 0.1 m) with several phase-detection intrusive probes. Correla- tion analyses were applied to estimate the integral turbulent length and time scales. The skimming flow properties pre- sented some basic characteristics that were qualitatively and quantitatively in agreement with previous air–water flow measurements in skimming flows. Present measurements showed some relatively good correlation between turbulence in- tensities T u and turbulent length and time scales. These measurements also illustrated large turbulence levels and large tur- bulent time and length scales in the intermediate region between the spray and bubbly flow regions. Key words: turbulence, stepped spillways, skimming flows, turbulent energy dissipation. Re ´sume ´: Les e ´vacuateurs de crues en marches d’escalier sont caracte ´rise ´s par une taux important de dissipation d’e ´nergie cine ´tique sur le coursier, et donc, une re ´duction de la taille du bassin de dissipation aval. Dans cette e ´tude, on pre ´sente des se ´ries de mesures de ´taille ´es dans l’e ´coulement diphasique eau–air, avec de nouvelles mesures des proprie ´te ´s turbulentes. Ce travail a e ´te ´ re ´alise ´ dans une mode `le physique de grande taille (q = 228, h = 0,1 m) avec plusieurs sondes de mesures intrusives. L’application d’analyses corre ´latives fournit des mesures de longueur et temps integrale turbulent. Les re ´sultats de l’e ´tude sont en accord qualitatifs et quantitatifs avec des e ´tudes pre ´ce ´dentes en e ´coulements extre `mement turbulents (« skimming flows »). On montre une corre ´lation relativement bonne entre les intensite ´s turbulentes T u et les e ´chelles inte ´grales turbulentes de longueur et de temps. Les re ´sultats sugge `rent un me ´canisme de dissipation turbulente dans la re ´gion interme ´daire entre la re ´gion d’e ´coulement a ` bulles et la re ´gion d’e ´coulement a ` gouttes. Mots-cle ´s : turbulence, coursier en marches d’escalier, e ´coulement extre `mement turbulent, dissipation d’e ´nergie. Introduction Stepped spillways have been used for many centuries (Chanson 1995b, 2000, 2001a). The stepped design in- creases the rate of energy dissipation on the chute and re- duces the size of the downstream energy dissipation system (Fig. 1). Figure 1 shows two recent reinforced cement con- crete (RCC) dam stepped spillways with small stilling ba- sins. For the last 20 years, research in the hydraulics of stepped spillways has been active (Chanson 1995a, 2001a). On a stepped spillway, the waters flow as a succession of free-falling nappes (nappe flow regime) at small discharges (Chamani and Rajaratnam 1994; Chanson 1994a; Toombes 2002; El-Kamash et al. 2005). For a range of intermediate flow rates, a transition flow regime is observed (Ohtsu and Yasuda 1997; Chanson 2001b; Chanson and Toombes 2004). Modern stepped spillways are typically designed for large discharge capacities corresponding to a skimming flow regime (Rajaratnam 1990; Chanson 1994b; Chamani and Rajaratnam 1999). In a skimming flow, the flow is nonaerated at the upstream end of the chute. Free-surface aeration occurs when the turbulent shear next to the free sur- face becomes larger than the bubble resistance offered by surface tension and buoyancy. Downstream of the inception point of free-surface aeration, some strong air–water mixing takes place. Large amounts of air are entrained and very strong interactions between main stream turbulence, step- cavity recirculation zones, and free surface associated with strong energy dissipation and flow resistance are observed (Chanson and Toombes 2002a; Kokpinar 2005). The flow resistance is primarily a form drag in skimming flows (Rajaratnam 1990; Chanson et al. 2002). At each step, the cavity flow is driven by the developing shear layer and the transfer of momentum across it (Gonzalez and Chanson 2004). The energy dissipation mechanisms include cavity re- circulation, momentum exchange with the free stream, and interactions between free-surface and mainstream turbulence (Fig. 2). The interactions between mixing layer and horizon- tal step face, and the skin friction at the step faces, may con- tribute to further energy dissipation, in particular on moderate slopes. At each step edge, highly coherent small- scale vortices are formed abruptly at the step corner because of the large gradient of vorticity at the corner (Fig. 2). The initial region of the mixing layer is dominated by a train of Received 26 September 2006. Revision accepted 20 February 2008. Published on the NRC Research Press Web site at cjce.nrc.ca on 12 August 2008. G. Carosi and H. Chanson. 1 Division of Civil Engineering, The University of Queensland, Brisbane QLD 4072, Australia. Written discussion of this article is welcomed and will be received by the Editor until 31 January 2009. 1 Corresponding author (e-mail: h.chanson@uq.edu.au). 865 Can. J. Civ. Eng. 35: 865–880 (2008) doi:10.1139/L08-030 # 2008 NRC Canada