Non-aerated Flow Region Research Articles

On air entrapment onset and surface velocity in high-speed turbulent prototype flows

In a free-surface spillway, the upstream flow is non-aerated and the flow becomes a strong air-water mix downstream of the onset location of air entrapment. Field observations were conducted over a steep spillway chute, and detailed quantitative measurements were undertaken in real-world high-speed flows with strong turbulence and very high Reynolds numbers within the range 10 7 to 10 8 . The data showed that the onset of air entrapment is a complicated transient three-dimensional process in high-speed strongly-turbulent flows. A robust optical flow (OF) technique was applied and provided physically-meaningful surface velocities in the non-aerated flow region. The streamwise velocities were reasonably close to ideal fluid flow calculations, with large streamwise surface velocity fluctuations, in the non-aerated flow region. Overall, the study demonstrated the application of optical techniques to prototype spillway flows, provided that some careful validation was undertaken. Optical Flow (OF) surface velocity on the Hinze dam spillway for Q = 150 m 3 /s, Re = 4.54 × 10 7 , θ = 51.3°, steps: h = 1.5 m, q 2/3 /(g 1/3 × h) = 1.58 - Comparison between centreline OF velocity and ideal fluid flow velocity (Left), with photograph of spillway chute flow corresponding to the data (Right). • Field observations were conducted in a high-velocity prototype chute. • Flow conditions corresponded to very-high Reynolds numbers within the range 10 7 to 10 8 . • The onset of air entrapment was complicated, transient and three-dimensional. • An optical flow (OF) technique was applied to video movies. • Physically meaningful OF surface velocity data were obtained in the non-aerated region.

Length prediction of non-aerated region flow at baffled chutes using intelligent nonlinear regression methods

Baffled chutes are used in irrigation systems, storm water systems, wastewater canal chutes, river training, and drop structures for energy dissipation. Two flow regions occur on the flow surface of baffled chutes. These are black water and white water regions. Knowing the location of the inception point where white water begins to appear on the surface is important for determination of the non-aerated flow region. Thus, cavitation damage can be prevented. In this study, 160 laboratory test results have been used for determining black water length (i.e., length of the non-aerated region) of baffled chutes with stepped, wedge, trapezoidal, and T-shaped baffle blocks. The obtained observation data have been analyzed by well-known soft computing methods such as artificial neural networks (ANN), curve fitting (CF), non-linear regression (NLR) and special extreme learning machine (ELM). The methods’ performance in mapping input data to the output were compared. The mean regression errors calculated by the curve fitting model, ANN, NLR and ELM are obtained as 2.5, 8.0, 11.25 and 0.8 %, respectively. The experimental results show that ELM’s nonlinear system modeling capability is superior to ANN, NLR, and CF.

Determination of flow characteristics of stepped spillways

Stepped spillways have become a popular method in recent times. Stepped spillways are used commonly for both energy dissipation and aeration. In this study, the energy dissipation ratio and inception point location of stepped spillways with and without end sills were studied using computational fluid dynamic (CFD) methods. The analyses were carried out for two sill types and three s/h ratios. Flow characteristics and air inception points were determined for slope angles of 30°, 40° and 50°. The results were compared and discussed with the other studies in the literature. The greatest energy dissipation ratio was obtained at the slope angle of 30° for type 2A (i.e. stepped sill and s/h ratio of 0·30). Moreover, the greatest energy dissipation ratio was shown at the slope angle of 50° for stepped spillways without end sills. It was found that the length of the non-aerated flow region was closely related to energy dissipation. Finally, the flow characteristics of the stepped spillways can be reliably determined by using CFD analysis.

Application of Displacement Height and Surface Roughness Length to Determination Boundary Layer Development Length over Stepped Spillway

One of the most uncertain parameters in stepped spillway design is the length (from the crest) of boundary layer development. The normal velocity profiles responding to the steps as bed roughness are investigated in the developing non-aerated flow region. A detailed analysis of the logarithmic vertical velocity profiles on stepped spillways is conducted through experimental data to verify the computational code and numerical experiments to expand the data available. To determine development length, the hydraulic roughness and displacement thickness, along with the shear velocity, are needed. This includes determining displacement height d and surface roughness length z0 and the relationship of d and z0 to the step geometry. The results show that the hydraulic roughness height ks is the primary factor on which d and z0 depend. In different step height, step width, discharge and intake Froude number, the relations d/ks = 0.22–0.27, z0/ks = 0.06–0.1 and d/z0 = 2.2–4 result in a good estimate. Using the computational code and numerical experiments, air inception will occur over stepped spillway flow as long as the Bauer-defined boundary layer thickness is between 0.72 and 0.79.

Study of the Inception Length of Flow over Stepped Spillway Models

This study is the modification of Bauer’s development length in order to determine the excess in his length which has not been investigated by earlier researchers. The observations were performed in the non-aerated flow region of wooden stepped spillway models fabricated and installed at the Hydraulics Laboratory of the Department of Water Resources and Environmental Engineering, Ahmadu Bello University, Zaria-Nigeria. A total of 40 experiments were conducted over three different chute geometry to determine Bauer’s development length which were plotted on a drawing paper to a scale of 1:1 to obtained the development length in this study. The results showed that the inception (development) length increases as the unit discharge increases and it decreases with an increase in both stepped roughness height and chute angle. The ratio of the development length, in this study, to that of Bauer’s was found to be 4:5. Finally, SMM-5 produced the least velocity of flow and the highest depth of inception; indicating that it could be employed optimally, considering this study, in curbing or preventing cavitation as a flood controls structure. http://dx.doi.org/10.4314/njt.v33i2.6

Air–water flows and free-surface profiles on a non-uniform stepped chute

Comparative experiments were conducted between uniform and non-uniform stepped spillway profiles in a large-size laboratory facility. For each stepped configuration, the air concentration distributions matched the advective diffusion equation and the interfacial velocity was well correlated with a power law. A comparison of the air–water flow properties showed small differences in terms of number of entrained air bubbles, chord sizes and turbulence characteristics between the different configurations. For the non-uniform steps, larger flow instabilities and stronger variation in the air–water flow properties were observed. Further some non-intrusive measurements were performed with acoustic displacement metres to characterize the free-surface profiles, free-surface fluctuations and free-surface wave celerity in both non-aerated and aerated flow regions. The experiments highlighted a close agreement between experimental data and theoretical predictions in the non-aerated flows and with conductivity probe data in the aerated flows. The non-intrusive technique was suitable for measuring the free-surface characteristics on stepped chutes, especially in the non-aerated flow region.

Investigation of pressure variations over stepped spillways using smooth particle hydrodynamics

Abstract In this paper the smoothed particle hydrodynamics, (SPH), technique is used to investigate the pressure distribution on steps located in the non-aerated flow region of a stepped spillway for different discharges typical of skimming flow conditions. The open source code 2D SPHysics has been employed after being validated against the laboratory model studies of flow over broad crested weirs and flow over stepped spillways. The numerical results, in terms of the water surface and velocity profiles at different sections, are in good agreement with the corresponding experimental results. The code is then applied to determine the pressure distribution on the vertical and horizontal step faces. Also, the aspects of the pressure pattern are described and the positions/magnitudes of the maximum and minimum pressure values are presented.

Experimental study on time-averaged pressures in stepped spillway

Steps located in the non-aerated flow region of a chute are prone to cavitation damage. Two models operated with various Froude numbers and bottom slopes were employed to measure the pressure on the steps. The minimum and maximum pressures on the horizontal step surfaces occurred at the chute beginning and at the chute end, respectively. These characteristics were also found in the fully-developed flow region. The time-averaged pressure on the horizontal step surfaces is negative at the chute start, and then becomes positive along the flow direction in the non-uniform flow region. On the vertical step surfaces, the pressure is always negative at the top and remains negative along the entire vertical surface if the unit discharges are high enough in the non-uniform flow region. These results differ from the pressure characteristics in the uniform aerated flow region.

Skimming Flow in the Nonaerated Region of Stepped Spillways over Embankment Dams

Traditionally, research on stepped spillway hydraulics has been focused on the air-water flow region but for the hydraulic design of small embankment dams experiencing relatively large overtopping flows, the nonaerated region can be very important. Empirical formulas are presented for predicting skimming flow properties upstream of the point of inception of air entrainment for 1V:2H sloping stepped spillways, and the location and flow depth at the point of inception. Particular emphasis is placed on the clear-water depth, velocity distribution, and the energy dissipation characteristics in the developing nonaerated flow region. The velocity distribution is well described by a power law. The normalized clear-water depth and the normalized specific energy varied with the relative distance along the spillway and the effect of the normalized critical depth was negligible. Finally, the rate of energy dissipation was small, which has direct implications for the design of the downstream energy dissipator.

Developing Flow Region and Pressure Fluctuations on Steeply Sloping Stepped Spillways

The hydrodynamic pressure field is important for the design and safety of steeply sloping stepped spillways, which are typically designed for considerably lower maximum specific discharges than smooth spillways. The hydraulic performance of stepped spillways at high velocities may compromise its use due to major concern with safety against cavitation damage. Hydraulic model investigations were conducted in different large-size stepped chutes to characterize the nonaerated flow region which is potentially prone to cavitation damage and the pressure field acting on the step faces. The clear water depths and energy dissipation in the developing flow region are described in terms of integral measures of the turbulent boundary layer. Expressions for the location of and the flow depth at the inception point of air entrainment are derived. Pressure distributions on the horizontal and vertical faces of the step along the spillway are presented. Measurements indicated a different behavior of the pressure field in the aerated and nonaerated flow region. The mean and fluctuating pressure coefficients along the spillway are approximated by a regression function. The vertical face near the outer step edge close to the inception point of air entrainment is identified as a critical region for predicting cavitation inception in flow over stepped spillways. From the analysis of the pressure fluctuations in that region a maximum velocity of 15 m/s is proposed as a criterion to avoid extreme negative pressures in typical prototype steeply sloping stepped spillways, eventually leading to the occurrence of cavitation in the nonaerated flow.

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

The development of the roller-compacted concrete (RCC) as a technique of constructing dams and the stepped surface that results from the construction procedure opened a renewed interest in stepped spillways. Previous research has focused on studying the air-water flow down the stepped chute with the objective of obtaining better design guidelines. The nonaerated 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 Kármán 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 pseudobottom (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 pseudobottom. 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.

An experimental investigation of aeration performance in stepped spillways

AbstractDissolved oxygen content is a prime indicator of water quality. The oxygen transfer across the air–water interface at a hydraulic structure, such as a weir or spillway, occurs by self‐aeration along the chute and by flow aeration in the hydraulic jump at the downstream end of the structure. Despite increased research activities in the field of stepped spillways, the aeration efficiency of stepped spillways is not yet known. This paper investigates the aeration efficiency of stepped spillways, in particular the effects of varying chute angle and step height. Empirical correlations predicting length of the nonaerated flow region and aeration efficiency were developed. The results indicate that stepped spillways are effective for oxygen transfer.