Characterization of the Nonaerated Flow Region in a Stepped Spillway by PIV

Abstract

ABSTRACT:The development of the roller compacted concrete as a technique of constructing dams and the stepped surface that results from the construction procedure opened a renewal interest in stepped spillways.Previous research has focused in studying the air-water flow down the stepped chute with objective of obtaining better design guidelines. The non aerated flow region enlarges as the flow rate increases and there is a lack of knowledge on the hydraulic performance of stepped spillways at high velocities that undermines its use in fear of cavitation damage. In the present study the developing flow region in a stepped channel with a slope 1v:0.8h is characterized using a particle image velocimetry technique. An expression for the growth of the boundary layer thickness is proposed based on the streamwise distance from the channel crest and the roughness height. The local flow resistance coefficient is calculated by application of the Von Karman's integral momentum equation.The shear strain, vorticity and swirling strength maps obtained from the mean velocity gradient tensor are presented. Also the fluctuating velocity field is assessed. The turbulent kinetic energy map indicates the region near the pseudo-bottom (imaginary line joining two adjacent step edges) as the most active in terms of Reynolds stresses. The turbulence was found to be very intense with maximum levels of turbulence intensity from 0.40 to 0.65 measured near the pseudo-bottom. Finally the quadrant analysis of the velocity fluctuations suggests the presence of strong outflows of fluid from the cavities as well as inflows into the cavities. It is conjectured that the mass transfer/exchange between cavities and main stream, play an important role in the high levels of turbulent energy observed.

Hydraulics of the Developing Flow Region of Stepped Spillways. II: Pressure and Velocity Fields

“…The observations were consistent with the velocity data of Amador et al (2006) and numerical modelling (Qian et al 2009). A close examination of Figure 6 showed longitudinal variations in pressure profiles across step cavities: for example, the step cavity 2-3 was generally governed by positive pressures, while a negative pressure core was observed in step cavity 3-4.…”

“…The best data fit yielded N = 4.5, with a normalised correlation coefficient r = 0.97. The value of N was close to that suggested by Chanson (2001) for a 1V:2H stepped model typical for embankment dams, but slightly different from N = 3.0 and 3.4 obtained by Amador et al (2006) and Meireles et al (2012) respectively. Figure 3B presents typical velocity contours for the entire flow between step edges 2 and 4, for d c /h = 1.5.…”

“…Following Pope (2000) and Amador et al (2006), an upper bound of the shear layer may be: y ub = y 0.9 where y 0.9 is the depth where V x = 0.9×V o . A typical evolution of the upper bound of the shear layer in the flow region surrounding step edge 3 is shown in Figure 4 for d c /h = 1.5, where k s = hcos is the step cavity height and L cav is the step cavity length: L cav = (h 2 +l 2 ) 1/2 .…”

Laboratory measurements and multi-block numerical simulations of the mean flow and turbulence in the non-aerated skimming flow region of steep stepped spillways

“…Empirical models have been developed for predicting the main air-water flow properties along the chute by Hager and Boes [45], Matos [57], Chanson [25], Boes and Hager [9,10], Meireles [65], Renna [71] and Ohtsu et al [67]. In spite of this considerable number of studies, and to the best of our knowledge, only Amador [2], Amador et al [3], Meireles et al [65], Gonzalez and Chanson [43], Carvalho and Amador [23] and Meireles and Matos [64] have focused on the flow properties of the non-aerated flow region of stepped spillways.…”

“…Although this is a strong reason to use stepped spillways, it was not until the improvement of roller compacted concrete (RCC) technology by the end of the twentieth century that the interest in stepped spillways was definitively renewed [3,27]. Currently, there is a considerable interest in evaluating the performance of stepped spillways over RCC dams for high specific discharges, in either the design of new spillways, or the re-analysis of existing spillways due to an update in the probable maximum flood.…”