Water entrainment due to spillway surface jets

Abstract

Strong flow entrainment has been observed downstream of spillways constructed with flow deflectors. This water entrainment has important environmental and ecological impacts because it improves the mixing of powerhouse and spillway flows, but may negatively impact fish migration or create adverse flow conditions.Most studies found in the literature attempt to explain this entrainment with turbulent mixing. Both reduced-scale hydraulic models and single-phase, isotropic RANS models grossly under-predict the degree of entrainment observed in prototypes. In this paper, an anisotropic model that accounts for the bubble volume fraction and attenuation of the normal velocity fluctuations at the free surface is presented. The model adequately predicts the main mechanisms causing water entrainment and compares well against experimental data for a round surface jet and for Brownlee Dam at model scale. It is shown that appropriate entrainment can only be captured if the turbulence anisotropy and the twophase nature of the flow are modelled.

A multiphase model for the hydrodynamics and total dissolved gas in tailraces

“…It is observed that surface jets attract water toward the jet region, a phenomenon called water entrainment. Turan et al (2007) described the main mechanisms causing water entrainment as acceleration of the surrounding fluid as the jets decelerates, surface currents, Coanda effect and the presence of bubbles.…”

“…Kinematic and dynamic boundary conditions as wells as turbulence quantities are programmed through UDFs. Details of the boundary conditions used for the Reynolds stress and velocity components are found in Turan et al (2007).…”

“…Earlier studies aimed at the prediction of the hydrodynamics in hydropower tailraces grossly underpredicted the water entrainment (Li and Weber, 2006). Turan et al (2007) used an anisotropic mixture model that accounts for the gas volume fraction and attenuation of normal fluctuations at the free surface. Although this model predicted considerably more water entrainment than the standard isotropic single-phase models, the degree of the entrainment obtained on prototype scale was still underpredicted.…”

Model development in OpenFOAM to predict spillway jet regimes

“…This phenomenon is called water entrainment and has been reported in both field (Schneider & Wihelms 1996) and model studies (Fuller 1997). Turan et al (2007) studied the main mechanism causing water entrainment and recognized that spillway jet regimes strongly influence lateral currents and TDG distribution in the tailrace.…”

Prediction of Total Dissolved Gas Exchange at Hydropower Dams

“…Urban et al (2008) presented a model to predict TDG saturation just downstream of a spillway based on physical processes of mass transfer. The effects of bubble size distribution (Politano et al, (2007(Politano et al, ( , 2005(Politano et al, ( , 2003 and bubble volume and normal velocity fluctuation attenuation (Turan et al, 2007) have been studied to better understand and model air entrainment. Physically-based models have incorporated geometrical aspects of the dams such as stilling basin and river depths (Geldert et al, 1998), spillway configurations and flow parameters (Hibbs and Gulliver, 1997), upstream TDG concentration (Roesner and Norton, 1971), and flood discharge characteristics, such as water depths and pressures (Ran et al, 2009), to predict gas transfer and downstream TDG levels.…”