Plunging Flow Research Articles

PLUNGING AND STREAMING FLOWS IN POOL-AND-WEIR FISHWAYS

Fishways are river constructions which give a helping to fish migration. The rea re three types of flow formations in pool-and-weir fishways such as a plunging flow, streaming flow and intermixed flow of plunging and streaming flows. Rajaratnam et al. proposed a criterion formula that predicts the flow formations. However, this criterion formula has no physical meaning. Further the accuracy of prediction of their for mula is not high. In this study, the criterion formula, which can predict the flow formation in a pool-and-weir fishway, is derived semi-theoretically. The experiments were conducted with changing the aspect ratio and discharge in the pool-and-weir fishway. The water surface profiles were measured and also three components velocities were measured with a 3-D electromagnetic current meter. A new criterion formula, which is able to predict the flow formation, is proposed under the condition that the channel slope is 1 on 12.

HYDRAULIC CHARACTERISTICS OF ENERGY DISSIPATORS OF SLIT TYPE SABO DAMS

Recently, slit type Sabo dams have been constructed by considering river environments. But, there is little information on energy dissipator of the slit type Sabo dams. This paper shows a systematical investigation on energy dissipators of Sabo dams with a single slit. The flow conditions passing over the slit are similar to those in abruptly expanding channels, and both symmetric and asymmetric flows are existed. A systematical investigation yields that sub-dams downstream of the slit type Sabo dams are effective for the formation of symmetric flows in the stilling basin, and that the flow conditions can be classified into two types. Also, the hydraulic condition required to form each flow condition has been determined. Accordingly, a flow chart of hydraulic design for energy dissipator of Sabo dams with a single slit has been proposed. Further, it has been confirmed experimentally that even if the slit is closed due to debris flows, symmetric plunging flows can be formed in the designed stilling basin.

Stratified flow over topography: bifurcation fronts and transition to the uncontrolled state

A distinguishing feature of controlled stratified flows over topography is the formation of a wedge of partly mixed fluid downstream of a bifurcation or plunge point. We describe observations acquired over a sill in a coastal inlet under progressively increased tidal forcing. This wedge of partly mixed fluid is displaced downstream as the flow undergoes a continuous transition from control over the obstacle crest to an uncontrolled state. The effects of changing barotropic forcing and relative density difference between the plunging flow and partly mixed layer above combine to determine the fluid dynamical response. The relative density difference in turn is determined by the prior history of the flow as well as small–scale mixing. In general it decreases with time as denser fluid is entrained into the intermediate layer, thus increasing the effective forcing. For sufficiently strong tidal velocities and small relative density difference, the wedge of partly mixed fluid is displaced downstream of the crest and topographic control is lost. Such flows occur naturally in the ocean over sills and ridges, and in the atmosphere as severe downslope winds.

Mathematical simulation on accumulation process of a blooming plankton Peridinium bipes (Dinophyceae) in a dam reservoir

The accumulation process of the blooming plankton Peridinium bipes was studied in this paper using a mathematical model based on a horizontal one-dimensional multi-layered flow model coupled with a vertical migration model of the Peridinium bipes community. The estimated accumulation process consisted of three processes: upward swimming and dispersion of Peridinium bipes cells, advection of the cells with the countercurrent of surface water, and entrainment and effluent of the cells with the plunging flow of inflow river water. The numerical analysis has revealed that the accumulation process helps Peridinium bipes to avoid hydraulic washout and it contributes to a contributes to a concentration of Peridinium bipes cells on the upper side of the reservoir.

LAGRANGIAN APPROACH TO FLOW-VELOCITY FIELD UNDER THE EXISTENCE OF THE DRASTIC CHANGE OF WATER SURFACE

The Lagrangian concept of the discretization of the Navier-Stokes equation, or the MPS method, which is generally called the Particle Method, is applied to the unidirectional flow with the periodic boundaries at its upstream and downstream ends. The MPS method is less affected by the numerical diffusion due to the convection term than the Eulerian methods, namely it has a superiority in the accurate calculation of the complicated behavior of water surface. The characteristics of the pool-and-weir fishway is investigated by using MPS method as the typical flow situation with the complicated behavior of water surface and the applicability of the periodic boundary condition. The plunging flow and the streaming flow, which are known as the two sub-modes of the flow in the pool-and-weir fishway, are reproduced well. The complicated unsteady behavior of the flow under the plunging flow mode is numerically investigated by the present simulation.

Air entrainment at drops

This paper describes the results of a laboratory study of air entrainment below drops for a range of Drop numbers from 0.0007 to 0.0046. Characteristics of the plunging flow that occurs when the tailwaler depth yt is in the range of yt* . to h were studied where yt* . is the value of yt when a hydraulic jump is formed at the section where Ihe deflected stream below the drop becomes parallel to the bed and h is the height of the drop. Air entrainment below the drop was studied for the jump with yt = yt* as well as for the plunging flows and approximate expressions have been developed for their air entrainment rates. These results can contribute to the estimation of the reaeration of polluted streams below drops.

Tidal Intrusion Fronts

A tidal intrusion front forms as a dense seawater inflow plunges (subducts) beneath ambient estuarine water during flood tide. The associated foam lines and color changes have been observed on many smaller estuaries with constricted mouths. Internal hydraulic theory and laboratory experiments are reviewed and expressions are obtained for the position of plunging and the amount of associated mixing. The existence of a tidal intrusion front and its structure are discussed in terms of densimetric Froude numbers. These fronts are particularly important in smaller estuaries in which the intrusion process may dominate wind and tidal mixing and thus determine the overall stratification of the estuary. Three classes of three-dimensional plunging flow are identified and discussed. In particular, it is suggested that the peculiar, cursive V-shape plunge line is characteristic of strongly plunging flow.

Laboratory study of water destratification by a bubble plume

The artificial circulation and the mixing of temperature stratified water induced by an air bubble plume were studied in laboratory experiments. The time evolution of the spatial distribution of water temperatures was measured. The data were analyzed and gave information on the development of flow patterns and entrainment rates. Of particular interest were the development of the upwelling region, location of the plunge line, temperature structure in the farfield, and water entrainment by the bubble plume and by the plunging flow.

Two-layer hydraulics with comparable internal wave speeds

Two-layer hydraulics is developed for problems in which the moving layers can have stagnant layers above and below, the two internal wave modes can have comparable speeds and the total depth of the moving layers may vary. The general development allows both Boussinesq and non-Boussinesq problems to be studied. Solutions are presented in the Froude-number plane and the effect of different layer densities on the form of the solution space is shown. The theory is applied to two-layer plunging flows and a variety of controlled solutions are found. Solutions for the 2½-layer theory and the plunging flow theory are demonstrated experimentally. Shear instability is often observed in the divergent section of the channel.

Negatively Buoyant Flow in Diverging Channel, IV: Entrainment and Dilution

A negatively buoyant inflow to a lake or reservoir sinks to the bottom and forms an underflow. In this process, the “plunging flow” entrains ambient water and becomes diluted. The dilution alters the buoyancy and hence the flow behavior of the underflow. Since literature on entrainment by underflows does not include the “plunging process” itself, great uncertainty exists when the initial conditions of underflows have to be estimated. Therefore, the density driven plunging phenomenon is experimentally investigated in a horizontal diverging channel. Entrainment rates are determined from velocity and temperature measurements and related to channel diffuser angle, inflow densimetric Froude number, and inflow channel aspect ratio. The experiments are conducted at inflow channel Reynolds numbers larger than 10,000. Experimental diffuser angles are varied from 3° to 180°, inflow densimetric Froude numbers from 1.8 to 21 and channel aspect ratios from 0.5 to 1.8. The information is of use as input to one‐dimensional stratified lake or reservoir water quality models.

Plunging and Streaming Flows in Pool and Weir Fishways

This note presents the results of an experimental study on the hydraulics of pool and weir fishways. A simple weir-type flow analysis is found adequate to describe the plunging flow regime. A surface stream analysis is performed to predict the characteristics of the streaming flow. A parameter is developed to predict the transition from plunging to streaming flow.

Onset of Underflow in Slightly Diverging Channels

Experiments are conducted in a horizontal laboratory channel with slightly diverging walls to simulate buoyancy driven plunging flow. A temperature difference between inflow and downstream ambient provides the density differential that initiates the underflow. Temperature profiles and flow visualization are used to determine flow conditions in the plunge region. Channel divergence angles (half‐angles) from 1–15° are investigated. Volumetric flow rates in the sinking flow increase from 0–45% during the plunging process. Maximum entrainment rates are nearly linearly related to divergence angle. Depths of the underflow downstream from the plunge line are from 0.65–0.9 times the depth at the plunge line. Densimetric Froude numbers from 0.56–0.89 occur at the plunge line, depending on the channel divergence angle and the discharge densimetric Froude number. A flow analysis that explains and supports the experimental data is presented. Channel expansion affects the plunging process significantly, particularly e...