Stilling Basin Research Articles

Comparison of Mixture and VOF Models for Numerical Simulation of Air–entrainment in Skimming Flow over Stepped Spillways

Because stepped spillways can significantly reduce the depth and size needed for a stilling basin at the toe of a dam and lead to great economic benefit, these spillways are increasingly attractive in hydraulic engineering. In this paper, for investigation of Mixture and VOF method's ability, Air-Entrainment in skimming flow over the stepped spillways is simulated by using fluent software. The numerical simulation results of free surface, velocity components and air concentration in water and circumstance of air entry into the water have been compared with the experimental results. It was found, downstream of the inception point of free-surface aeration where rapid free-surface aeration is observed, free surface is simulated better by Mixture method.

Pressões Médias e Flutuantes Devidas ao Escoamento em uma Curva de Concordância Vertical entre Vertedouro e Canal Horizontal ? Estudo Experimental

A spillway invert vertical transition aims to redirect the spillway flow from two distinct channel slopes, downstream and upstream, in such a way as to minimize the hydraulic forces which would occur if the change in slopes was abrupt. In a dam discharge structure these transitions present as a solution to redirect the high speed flow, guiding it to the horizontal slope of the stilling basin. An evaluation of mean and fluctuating pressures caused by the vertical transition curves is shown in the present work, regarding the centrifugal acceleration generated by the bend, as well as the propagation of these mean effects downstream in the dissipation structure

Application and Comparsion of Mathematical and Physical Models on Inspecting Slab of Stilling Basin Floor under Static and Dynamic Forces

To protect downstream of hydraulic structures against erosion and degradation, must dipress water energy. one of the most comon methods to achive this purpose is constructing the stilling basin at the downstream of such structures. In stilling basin, the water energy dissipated by taking place of hydraulic jump. When a hydraulic jump occurs, the hydrodynamic and hydrostatic forces effect on the stilling basin floor slab. These forces include the force due to water weight and the up lift force affected the slab below which these forces are classified in hydrostatic. also the other force is hydrodynamic force due to pressure fluctions below the hydraulic jump. To keep the stilling basin stabillity from liffting up and destruction must the resistant forces be equal to destroyer forces. In this study after discution about the hydraulic jump and its resultant forces, a equation has been offerd to determine the thickness of stilling basin floor slab using buckingham theory andsoftware DATAFIT. Finaly the slab which evaluated by physical model, simulated using mathematical model(software ANSYS 10) and studied. The stress and strain diagrams were extracted. Results showed that physical and mathematical models were matched very good.

Safety modifications at Lower Laithe reservoir

Lower Laithe reservoir is located near Stanbury in West Yorkshire. The reservoir retains a volume of 1 275 000 m3 of water when full to its top water level, and as such falls under the legal obligations of the Reservoirs Act 1975. A statutory inspection of Lower Laithe reservoir was carried out in 2008 in accordance with the Act. This inspection revealed potential inadequacies with the ability of the existing overflow arrangement safely to convey the design flood. Mott MacDonald Bentley has subsequently been employed by the reservoir owner to investigate these inadequacies. A flood study and physical model have shown that during the design flood, spill flows in excess of 72 m3/s could be expected, with velocities exceeding 19 m/s. Flows of this magnitude would overtop the walls of the spillway and stilling basin and would be likely to cause extensive damage to the existing masonry lined channel. Remedial works have subsequently taken place at Lower Laithe to protect the overflow arrangement from the design flood. Improvement works to the existing spillway have included lining the structure with concrete and raising the masonry walls. Innovative modifications have taken place in the stilling basin to direct flows to the river channel, thereby protecting the toe of the embankment. In addition it has been necessary to raise the level of the clay core above the design flood. This paper describes these safety improvement works, as well as the stringent flood contingency measures which were required to maintain reservoir safety while completing works on an existing spillway structure.

Drag Characteristics of Two-Dimensional Sills in Forced Hydraulic Jumps

AbstractSills and other appurtenances are often used to aid the formation of a hydraulic jump in a stilling basin. The drag force acting on a sill in forced hydraulic jump depends on the initial Froude number F1, the height of the sill S, and its distance xs, measured from the toe of the jump. The forward flow in the main body of the hydraulic jump behaves like a wall jet. Therefore, based on observed data, simple relationships were developed to predict the maximum velocity Um and the half-width b of a free hydraulic jump. These are coupled with a dynamic pressure model that expresses the local drag coefficient Cdδ of a sill in a plane turbulent wall jet. The drag coefficient for the sill in a forced hydraulic jump Cd was obtained by a simple transformation involving the wall jet parameters.

Experimental Study on Characteristic of Pressure Distribution about the Falling-sill and Expanding Bottom-flow Energy Dissipater

Based on the method of hydraulic test the influence of expansion ratio,depth of the falling-sill and the flowing-in Froude number on the hydro-dynamic pressure in impact area of the stilling basin bottom is analyzed emphatically.The research results show choosing the appropriate expansion ratio,depth of the falling-sill and the flowing-in Froude number can lower the hydraulics indicators of the stilling basin bottom and the sidewall effectively in the falling-sill and expanding bottom-flow energy dissipater.Under the optimum combination they can not only keep the energy dissipate ratio higher but also can keep the hydro-dynamic pressure lower to avert the bottom plates from levee slipping in impact area of the stilling basin bottom and the sidewall.

The Effect of Netting Dissipaters on Increasing the Efficiency of Energy Dissipation in Vertical Drops

Abrupt changes of the channel bed elevation in hydraulic systems, e.g. in chutes, drops and steeped spillways, results to create a sever flow kinematic energy. This excess energy, can tend to different phenomena, such as tremendous forces, scouring and degrading the channel bed, resulting to destruction of the downstream hydraulic structures. The most important source to cause this phenomena is the the existence of sequent vertical drops along the the channel. In this study, the energy dissipation efficiency was increased by installing a new type of dissipaters namely netting dissipater on the crest of the vertical drops based on model experimtation. Also, the features of hydraulic jump, created in the stilling basin, were compared to those of a simple vertical drop. By assembling the above mentioned structure to the top of the stilling basin, the jet flow direction is changed, the degree of turbulence is increased and as the results, the kinematic energy loss increases and the length of the hydraulic jump decreses. The enhanced efficiency of the proposed structure was quantified based on the achieved experimental data.

Prediction of turbulence energy dissipation in flexible apron of barrages using numerical method

SUMMARYBarrages form an important component of hydraulic structures constructed to intercept and divert part of the available flow of a river and use it for purposes like irrigation, hydropower and domestic or industrial water supply. A flexible apron is a protection system provided in the barrage to prevent the possibility of the scour hole traveling close to the stilling basin. The turbulence kinetic energy is predicted using a 3D VOF (volume of fluid) realizable k −ϵ model for both horizontal and depressed flexible aprons placed downstream of the stilling basin of the barrage. The turbulence kinetic energy at the downstream end of the inclined flexible apron is comparably less compared with the horizontal flexible apron for all the cases. Copyright © 2011 John Wiley & Sons, Ltd.

Stepped and smooth spillways: resistance effects on stilling basin lengths

By ANDRE LUIZ ANDRADE SIMOES, HARRY EDMAR SCHULZ, and RODORIGO DE MELO PORTO, Journal of Hydraulic Research Vol. 48, No. 3 (2010), pp. 329–337 The authors investigated the gradually-varied flow equ...

Scour profiles at downstream of cascades

A study was conducted to simplify and broaden the procedures for estimating scour dimensions at the downstream of cascades. There are many studies about energy dissipation, aeration and oxygen transfer in stepped-channel chutes and cascades. There is little attention paid to the scour downstream of the stepped chutes and their downstream scour hole geometry. Measurements were taken to obtain a scour hole profile in the equilibrium state of the scour, the value of the maximum depth of the scour, the location of the maximum scour depth and the length of the scour hole. It was also found that step geometry, stepped chute angle, downstream water level and sill types of a stilling basin are very important parameters for the geometry of the scour hole.

Sequent Depth Ratio of a B-Jump

A B-jump is defined as the jump having the toe section located on a positively sloping upstream channel and the roller end on a downstream horizontal channel. This jump often occurs in the stilling basins with a horizontal bottom and located downstream of a steep channel. For a B-jump, a completely theoretical approach is not sufficient to solve the momentum equation and to establish the sequent depth ratio. In this paper, by using the laboratory measurements carried out in this investigation, some available empirical relationships useful for estimating the sequent depth ratio are tested. Then, by using the Π theorem of the dimensional analysis and the incomplete self-similarity theory, a generalized functional relationship for estimating the sequent depth ratio for different types of jumps is deduced. The estimate of the coefficient appearing in this relationship is dependent on the particular type of jump. In conclusion, the analysis established that the sequent depth ratio for a B-jump depends on a par...

The effect of tailwater depth on the local scour downstream of stepped-chutes

This paper reports on an experimental study of the effect of tailwater depth on local scour downstream of stepped-chutes. Different tailwater conditions were tested to study the form of the scour for two gradations of non-cohesive bed material, various flow rates, chute angles, sill heights and step heights. Measurements were taken to obtain the scour hole profile in the equilibrium state of scour, the value of the maximum depth of scour, the location of the maximum scour depth, and the length of the scour hole. The present results indicate that maximum depth and area of scour are highly dependent on tailwater depth. Moreover, the maximum scour depth increases with increases in the discharge and the tailwater depth. It was also found that step geometry, stepped chute angle and the sill types of the stilling basin are very important parameters for scour hole and geometry. A comparison was made between the scour characteristics produced by different tailwater depths. Empirical correlations predicting maximum scour depth were developed. Hence, accurate equations for the maximum scour depth are introduced.

Design of a detention reservoir

Design of a detention reservoir The contribution contains the reasons for and the methods of flood protection for villages in small valleys attacked by flash floods, which cause high economic, cultural and social damage. One of the possible solutions is to build a detention reservoir, which has to retain the flood wave and prevent an odd amount of water from flowing out of the river bed. In this paper a model example is given. The aim of the example is to show (according to hydrological data) how to design the dimension parameters of the dam, i.e. the height of the dam, the outlet structure capacity, the spillway capacity, the stilling basin and the necessary river bed lining. Attention is paid to stability problems, as well.

Design of Stilling Basins using Artificial Roughness

The stilling basins are commonly used to dissipate the energy and protect the downstream floor from erosion. The aim of the present experimental work is to improve the roughened stilling basin using T-shape roughness instead of the regular cubic one and design this new shape. As a result of the present work the best intensity and the best roughness length are identified. Also, it is found that the T-shape roughness save materials and reduce the jump length compared to the cubic one. Sensitivity analysis was performed and it was noticed that the change in the length of jump is more sensitive to the change in roughness length than the change in intensity.

Time-dependent local scour downstream of positive-step stilling basins

Local scour downstream of spillways is an important problem in hydraulic engineering. The objective of this experimental study was to investigate the temporal evolution of local scour downstream of a spillway followed by a positive-step stilling basin. Experiments were conducted in a rectangular channel using four different sediments for the mobile bed. Additional tests concerned the incipient motion of sediment particles. The main parameters governing the scour process were found to be the tailwater densimetric Froude number and the relative tailwater depth. No experimental evidence was found on similar scour profiles. Scour starts at a much reduced tailwater densimetric Froude number when compared with Shields. The temporal variation of the scour hole geometry was estimated using simple power-law equations relating the scour geometry to the controlling scour parameters.

Improving Concrete Containment Structures Associated with Fixed-Cone Valves

Fixed-Cone valves are often used to dissipate energy and regulate flow at the low level outlet works of dams. Fixed-Cone valves, also known as Howell-Bunger valves, create an expanding conical jet allowing the energy of the water to dissipate over a large area. However, in many applications constructing the large stilling basin necessary for these valves is either not possible or not feasible. In order to reduce the relative size of the stilling basin, hoods or concrete containment structures have been used in conjunction with Fixed-Cone valves. This paper discusses the use of baffles in concrete containment structures in order to dissipate energy in a considerably confined space. It was determined that using baffles, in place of a deflector ring and end sill (Used in traditional containment structures.), significantly improves the function of containment structures by reducing downstream flow velocities and improving flow patterns and stability. This information will be useful to engineers allowing them to minimize scour and erosion associated with concrete containment structures.

Hydraulic Performance of Mandali Dam Spillway in Iraq

A physical model was designed and constructed with a scale of 1:50 to simulate Mandali Dam spillway and its approaches. Twenty six measuring were carried out with different discharges that cover the range of expected discharges. Analysis of the collected data showed that the discharge coefficient, Cd, of the spillway was 1.7 at low discharges and it was 2.05 in case of high discharges. The model showed that the relative losses of energy dissipated through the stilling basin were varied inversely with the discharge between 71.7% and 64.6%. The measurements of the three piezometers sets located on the spillway indicated that all the measured pressures along the weir surface are positive for full range of discharges. The hydraulic model confirmed that the approach flow to the spillway inlet was generally smooth and without disturbances.

Effect of Stilling Basin Geometry on Clear Water Scour Morphology Downstream of a Block Ramp

The scour mechanism downstream of a block ramp in clear water conditions is quite a complex phenomenon that depends on several parameters. Majors role are played by ramp configuration, hydraulic conditions (downstream tailwater level and discharge), material granulometry, and stilling basin geometry. Previous studies analyzed both the scour phenomenon and the effects of all the parameters involved except the last one (i.e., the case in which the ramp has the same width as the downstream stilling basin) and the case of symmetrically expanding stilling basins. This last basin configuration represents an optimal equilibrium between the necessity to dissipate energy and to create a natural pool for the biological species, thus it has a prominent ecological value. The present paper aims to assess the effect of both the width and length of the downstream stilling basin on scour features and flow pattern in clear water conditions. Different scour morphology types are distinguished and classified according to hyd...

Scale effects of air-water flows in stilling basin of multi-horizontal submerged jets

A series of experiments were carried out on multi-horizontal submerged jets with four different model scales of 1:36, 1:57, 1:80, 1:200. In routine tests, scale effects have to be considered, due to complex vortex structure and strong air entrainment in stilling basin. Our focus was laid on measuring and analyzing the time-averaged pressure distribution, water depth and closed-to-bed velocity in the stilling basin of multi-horizontal submerged jets. The experiments results show that the model scale has but a slight effect on the time-averaged hydraulic characteristics in the stilling basin of multi-horizontal submerged jets, which indicates that the results of time-averaged hydraulic characteristics for a normal pressure model are reliable. However, the scale effects of air entrainment, fluctuation pressure and vortex structure call for further investigations in order to make the results of experiments serve as scientific references for practical engineering.

Effect of stilling basin shape on the hydraulic characteristics of the flow downstream radial gates

Barrage regulators are considered as one of the most important projects in the Egyptian irrigation practice, which is obvious by its controlling of the released discharge and both of upstream and downstream water levels. In the present time, the ministry of water resources and irrigation starts to construct new barrages on the River Nile instead of the oldest ones, which are not able to withstand the requirements of increasing head difference between the upstream and downstream water levels upon the gates. The present study was focused on investigating the effect of different shapes of stilling basins of regulator on the length of the submerged hydraulic jump, velocity profiles along the apron, and local scour downstream regulator floor. Extensive experimental program was conducted on a re-circulating flume with 1.0 m wide, 26.0 m long and 1.2 m deep, with discharges range from 40 to 190 l/s. The relative velocity near bed, and shear Reynolds number were studied to fix the best shape of stilling basin that could reduce both of length of submerged hydraulic jump and local scour downstream stilling basin. Statistical equation was developed to correlate the length of submerged jump with the other independent parameters. Finally, clear matching of results from the length of jump and velocity analysis was investigated.