Analyzing the turbulent flow on steep open channels
TL;DR: In this article, the hydraulic variables including hydraulic depth and water surface profile are studied along the steep open channels, and it is shown that in the conditions in which the angle of the wave forehead is less than 35 degrees, the formation of roll waves is inevitable.
Abstract: In channels with steep slopes, the flow is supercritical and the formation of roll waves is expected. In order to evaluate the flow behaviors in steep open channels, the Azad dam spillway is simulated numerically and its flow characteristics are compared with the physical model tests. In this study, the hydraulic variables including hydraulic depth and water surface profile are studied along the steep open channels. The k- ɛ turbulence model is considered in this study. It is shown that in the conditions in which the angle of the wave forehead is less than 35 degrees, the formation of roll waves is inevitable.
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Abstract: Cavity length has been proven to have a significant effect on the air entrainment of an aerator in flood discharge engineering; however, the estimation of both the bottom and lateral cavity lengths downstream of a sudden fall-expansion aerator remain unclear. This research conducts a series of experiments involving various approach-flow conditions and geometric parameters of an aerator. An improved solution of the cavity length for the bottom and lateral cavities is established. The proposed equation was validated through the data of experiments and predecessor formulas, and exhibited a higher precision than other methods. Both the transverse turbulence and the axial dynamic pressure are found to be related to the formation of a lateral cavity. The present method involving the lateral cavity length was developed based on dimensional analysis and experimental test. The geometric morphology of a lateral cavity exhibits a parabolic shape, which is similar to that of the bottom cavity.
1 citations
TL;DR: In this paper, a series of physical experiments and numerical calculations were conducted to investigate the flow pattern, pressure distribution and cavitation downstream of a sudden fallexpansion aerator in a discharge tunnel.
Abstract: The mechanisms of cavitation damage in flood releasing tunnels remain unclear. In this study a series of physical experiments and numerical calculations were conducted to investigate the flow pattern, pressure distribution and cavitation downstream of a sudden fall-expansion aerator in a discharge tunnel. When the radial gate was partly open, the bottom cavity length reduced drastically, the lateral cavity disappeared, and the flow cavitation index near the sidewalls was less than 0.2. The pressure on the floor and sidewalls can be divided into four regimes: the cavity regime, the impact regime, the reflective regime and the stable regime. The time-average pressure is subject to a unimodal distribution when the gate is fully open, whereas a bimodal distribution is presented when the gate is partly open. The negative pressure regime presented an elliptic shape. Cavitation erosion occurred easily on lateral expansion sidewalls in the tunnel with the radial gate partly open.
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TL;DR: In this paper, the authors proposed a method that allows for the log-layer mismatch to be removed, thereby yielding accurately predicted skin friction, by considering the behavior of turbulence length scales near a wall.
Abstract: This paper addresses one of the most persistent errors in wall-modeled large eddy simulation: the inevitable presence of numerical and subgrid modeling errors in the first few grid points off the wall, which leads to the so-called “log-layer mismatch” with its associated 10-15% error in the predicted skin friction. By considering the behavior of turbulence length scales near a wall, the source of these errors is analyzed, and a method that allows for the log-layer mismatch to be removed, thereby yielding accurately predicted skin friction, is proposed.
438 citations
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29 Feb 1992
TL;DR: In this paper, a classical hydraulic jump in non-rectangular channel is described, followed by a submerged hydraulic jump using a bucket-type energy dissipator and a baffle rock.
Abstract: Part 1: Hydraulic Jump 1 Introduction 2 Classical Hydraulic Jump 3 Sloping Jump 4 Hydraulic Jump in Non-Rectangular Channel 5 Submerged Hydraulic Jump References Part 1 Notation Part 1 Part 2: Stilling Basins 6 Introduction 7 Steps and Sills 8 Baffle Rock 9 Effect of Roughness and Discharge 10 Expanding Channel 11 Bucket-type Energy Dissipator 12 Various Aspects of Stilling Basins 13 Types of Stilling Basins 14 Experiences with Stilling Basins References Part 2 Notation Part 2 Subject Index Author Index
269 citations
TL;DR: In this paper, the results of a comprehensive study addressing the non-aerated region of skimming flow in steep stepped spillways are presented and discuss the importance of this region in small dams and/or spillways with high specific discharges.
Abstract: We present and discuss the results of a comprehensive study addressing the non-aerated region of the skimming flow in steep stepped spillways. Although flows in stepped spillways are usually characterized by high air concentrations concomitant with high rates of energy dissipation, the non-aerated region becomes important in small dams and/or spillways with high specific discharges. A relatively large physical model of such spillway was used to acquire data on flow velocities and water levels and, then, well-resolved numerical simulations were performed with a commercial code to reproduce those experimental conditions. The numerical runs benefited from the ability of using multi-block grids in a Cartesian coordinate system, from capturing the free surface with the TruVOF method embedded in the code, and from the use of two turbulence models: the \({k{-}\varepsilon}\) and the RNG\({k{-}\varepsilon}\) models. Numerical results are in good agreement with the experimental data corresponding to three volumetric flow rates in terms of the time-averaged velocities measured at diverse steps in the spillway, and they are in very satisfactory agreement for water levels along the spillway. In addition, the numerical results provide information on the turbulence statistics of the flow. This work also discusses important aspects of the flow, such as the values of the exponents of the power-law velocity profiles, and the characteristics of the development of the boundary layer in the spillway.
118 citations
TL;DR: In this article, Zhang et al. showed that the dip in the spectrum of capillary-gravity waves is a result of blockage of both capillary gravity wind waves and parasitic capillary waves.
Abstract: Short water surface waves generated by wind in a water tunnel have been measured by an optical technique that provides a synoptic picture of the water surface gradient over an area of water surface (Zhang & Cox 1994). These images of the surface gradient can be integrated to recover the shape of the water surface and find the two-dimensional wavenumber spectrum. Waveforms and two-dimensional structures of short wind waves have many interesting features: short and steep waves featuring sharp troughs and flat crests are very commonly seen and most of the short waves are far less steep than the limiting wave forms ; waveforms that resemble capillary-gravity solitons are observed with a close match to the form theoretically predicted for potential flows (Longuet-Higgins 1989); capillaries are mainly found as parasitics on the downwind faces of gravity waves, and the longest wavelengths of those parasitic capillaries found are less than 1 cm; the phenomenon of capillary blockage (Phillips 1981) on dispersive freely travelling short waves is also observed. The spectra of short waves generated by low winds show a characteristic dip at the transition wavenumber between the gravity and capillary regimes, and the dip becomes filled in as the wind increases. The spectral cut-off at high wavenumbers shows a power law behaviour with an exponent of about minus four. The wavenumber of the transition from the dip to the cut-off is not sensitive to the change of wind speed. The minus fourth power law of the extreme capillary wind wave spectrum can be explained through a model of energy balances. The concept of an equilibrium spectrum is still useful. It is shown that the dip in the spectrum of capillary-gravity waves is a result of blockage of both capillary-gravity wind waves and parasitic capillary waves.
113 citations