Hydraulics Characteristics Of Lateral Deflectors In Gentle-Slope Free-Surface Tunnels

The limited flood-discharge capacity of the dam body is a result of the complex hydrogeologic conditions of high head,large discharge and narrow valley that are typically encountered by high dams in southwest China,and the subsequent adoption of local material types.Because of this,the tunnel spillway is utilised in conjunction with other flood discharge structures,and the ’sagging dragon tail’ design is the most prevalent.There requires to be tight regulation of the flow pattern and velocity in the spillway tunnel to prevent any damage from cavitation or denudation.However,as the maximum operating head increases,the velocity of water flow in the tunnel becomes larger,increasing the risk of cavitation and necessitating the installation of aerators.Designing bottom aerators for gentle-slope tunnels is challenging due to cavity blockage and the requirement to protect the tunnel’s sidewalls against cavitation.Thus,lateral deflectors are often installed next to bottom aerators to further boost aeration efficiency,and they also help to dissipate some of the kinetic energy.Nevertheless,with such designs to deflect the flow laterally,water-wings and shock waves are typically induced by the lateral deflectors,thereby increasing the risks of cavitation and denudation.Existing researches into seeking solutions to these issues have largely concentrated on optimising the size and layout of the conventional triangular deflector,with only a small number of attempts found to alter the deflector geometry,along with energy dissipation calculation and detailed hydraulics of standalone lateral deflector.In this paper,we systematically explored the hydraulics of the standalone lateral deflector with close attention to the flow pattern and energy dissipation,to be specific,including comparative hydraulics of lateral deflectors with varying geometries,zoning approaches and energy dissipation characteristics,lateral cavity configuration and energy dissipation calculation,and the layout of the multistage deflectors.At first,an innovative lateral deflector able to effectively inhibit water-wings and shock waves was presented,and its hydraulics were investigated compared to those of more conventional designs.It has been found that the flow patterns behind the lateral deflector depended on the interaction of the lateral jet and the deflected flow: The triangular deflector was revealed to form a wide cavity due to the lateral jet advancement towards the middle of the tunnel,which allowed for the free rise up of the water-wings inside the cavity and continually rose over the sidewalls,leading to the development of a buddle-type shock wave.Whereas the two-arc deflector yielded a jet with a fluctuating surface to form smaller cavity and partially eliminate the lateral momentum,yet still inducing water-wings to intermittently jump over the sidewall,which further developed into diamond-type shock waves.The positive and negative arcs of the innovative deflector made the flow streamline more smooth,meanwhile the following straight line could stable and guide the detaching flow.By doing so,a stable and shorter cavity was formed behind the innovative deflector,thereby restricting the development of the rising flow and preventing the formation of water-wings and shock waves.Moreover,the two-arc deflector with a straight guiding line exhibited fast recovery of hydraulics,higher energy dissipation capacities and aeration efficiency because of the highly turbulent impact within the cavity.Secondly,using a novel zoning method tailored to the analysis of flow characteristics in the near-wall region,we conducted a deeper investigation into the process of energy dissipation by analyzing the spectrum of pressure fluctuations.While describing and analyzing near-wall hydraulics,we found that the flow pattern,velocity and pressure of the novel lateral deflector showed clear sectionalized distributions,segmented into contraction zones,surging zones,lateral cavities and impact zones,shock wave zones,and constant zones.The straight line’s regulating and stabilizing function was reconfirmed as the transverse velocity on the straight line segment rapidly decaying back to zero.The ranges of the shock wave and lateral cavity zone did not necessarily grow in tandem with increasing flow rates,but instead stabilized gradually.In addition,the surface turbulence of the novel deflector grew faster since the impact,along with a wider range of higher turbulence kinetic energy and dissipation rate generated from the bottom,indicating more kinetic energy was transferred and dissipated in the lateral cavity zone.The spectrum analysis also found that the marginal spectral energy reached the maximum near the impact,along with the increases of dominating frequency,the contribution of the high-frequency component,and the proportion of the overall variation,attributable to low-frequency large-scale turbulence vortices splitting into high-frequency small-scale ones and subsequently dissipating.Thirdly,the potential risks in the critical lateral cavity zone were outlined,hence an analysis of cavity length distribution based on the cavity configuration was provided,and ultimately,a method for calculating energy dissipation based on a generative model was proposed.Compared to the second type of water wing,which is formed by the lateral jet impacting on sidewalls,the first type caused by lateral contraction was shown to be more stable.Furthermore,the results showed that the second type was higher than the first type for conventional deflectors,but the reverse was true for the innovative deflector.However,there is still a risk of the second type water-wing overtopping with too small inlet water depth or too large flow rate and deflector contraction ratio.The water that gathered at the cavity end was made up of two parts: an upstream portion that was adhered to the sidewall and flowed back quickly;and a central portion that rose up as a result of the deflected flow.The backwater could cause scattering flow when returning to the ejection surface,which further possibly developed into discontinuous jet bandings on the lateral jet interface and choked the cavity,impacted the backwater and eventually destabilized the cavity.The boundary trajectory of the lateral cavity that was longer and wider on the top approximately exhibits 1/4 ellipse on the horizontal section,and the cavity length lengthening faster from the bottom conformed to the exponential distribution along the water depth.Increases in inlet velocity,contraction ratio,contraction angle,and decreases in inlet water depth resulted in a longer cavity,while variations in straight line length and bottom slope had no discernible effect on this relationship,expressed by a robust solution based on multivariate linear regression.The cross-section average velocity and the flux-averaged hydraulic head along the tunnel reflecting the change of energy increased with larger inlet velocity,water depth,and decrease with larger contraction ratio and angle of the deflector.The variation rule of the local head loss coefficient was on the opposite,but decreased when the water depth is too low or the contraction ratio is too large.The energy dissipation efficiency could not continue to be improved with mere increases to the deflector shrinkage parameter.When it comes to calculating energy dissipation,the results of gaussian mixture regression are preferable to those of multiple linear regression because of their greater precision and smaller deviation,thanks to the function’s consideration of the internal relationship of parameters.Finally,the influence of lateral deflector spacing on the effect of disturbance recovery and energy dissipation were investigated,and based on which three efficient multistage deflectors layouts were proposed and verified.When the findings are analyzed,it becomes clear that the flow behind the previous deflector does not stabilize again when the gap is too tiny.Thus,the water surface fluctuated vertically in M shape affected by water-wing,and the disturbance developing range extended horizontally due to shock wave.Under the circumstances,both sidewalls experienced white water crowns with substantial turbulence and dissipation rates,and the local head loss coefficient was significant.The influence of water-wing and shock wave were removed when the spacing reached 15 and 20 times the length of the deflector,respectively,and the turbulent kinetic energy and dissipation rate of the posterior deflector decreased to less than 50% and 25%,respectively.Two of the three recommended efficient layouts,the multistage mismatch approach and the staggered method,had obvious inapplicability,while the method to position the posterior deflector in advance before the disturbance completely dissipated,was the best option for this study.With this setup,energy is dissipated more efficiently,and flow patterns remain steady,because water is kept undisturbed between the disturbance ranges of the front and rear deflectors.The feasibility of this method was verified through five multistage deflectors and the results expressed the stable water-wing and impact disturbance,steadily improving energy dissipation efficiency of each deflector,and a progressively accelerating frictional head loss as the number of stages increased.