Piano Key Weir Characteristics The Piano Key Weir is a particular model of the labyrinth discharge spillway. It is composed of an alternation of reclined surfaces, one is the direction of the flow, the other in the opposite direction. Each of these slopes is separated by a vertical wall that follows the geometry of the shape. Seen from above, it is a series of rectangles cut in equally sized sections widthwise. For each of these sections one of the side perpendicular to the stream is pushed down, creating a slope on which the water can either flow or accumulate, depending on which side the section is lowered. This structure tops a smaller wall. The Piano Key Weir is generally used at the outbound of a dam, the idea being to dispose of a construction that is solid enough to resist the pressure created by a high quantity of water contained in a dam or a river during flood season, that can evacuate the overflow of water, and that is simple enough not to be too expensive. Usage of the Piano Key Weir The Piano Key Weir is a free flow discharge spillway. Its geometry allows for a significantly higher evacuation capacity. The studies conducted on this type of spillway have two aims: build solid and cheap discharge spillways, as well as reinforce the older structures. This enables engineers to avoid accidents of dams that break or that don’t have the capacity to discharge water correctly and in a controlled manner when there is a flood. Usually the weir is placed after the dam and before the cities so as to allow a controlled discharge of water. This type of discharge spillway presents several crucial advantages. First it is easy to install on structures that are already there, as opposed to classic labyrinth discharge spillways. Moreover, its shape that alternates between ascending and descending slopes allows the formation of two different flows depending on whether the water arrives on one or the other slope. When the water flows over the descending slope it forms a jet that goes towards the bottom of the dam, and when it is first contained by an ascending slope it forms a kind of film which then flows towards the jet below. This division of flow slows the stream down consequently in a much more efficient way than classic dams do. Simulation results In order to observe the effects of the Piano Key Weir we used FLOW-3D® to make a simulation displaying the weir in a stream. The model setup is the most basic one, given that it has been proved that it enabled the results to be as close to reality as possible. The simulation shows the characteristic flow of the water over the “keys” of the piano as well as the decrease of the flow rate after the weir. We can see that the error percentage between experimental results and the simulation using FLOW-3D is only between 2% and 4% on average. The only determining factor is the mesh resolution when it comes to the accuracy of the simulation, but it only changes the error of 3% or 4% maximum, which means that overall the simulation doesn’t go above an error of 6%. … [Read more...]
A dam failure 3D-Shallow Water hybrid simulation on a real topography
The water and environmental simulation of the effect of a dam failure on a real extent topography was in the past a very hard goal because of the too many cells needed for the calculation. Since v11.0 of FLOW-3D® it is possible to use a hybrid approach, coupling a complete 3D simulation in the zone around the dam, where the splashing effects are more important, to a shallow water approach in the zone far away, having a solution to fast simulate such kind of situation. Moreover, since v11.1 of FLOW-3D®, it is possible to easily import raster file containing the topography with just one click, making the setup phase a matter of only some minutes. https://www.youtube.com/watch?v=Q7x55ohyDxA Video 1 : Overall view In the simulation a real topography of a lake with mountains has been used, the extension of the computational domain is around 5’000 km2, and a hypothetical large Dam has been created inside the topography. Using all the physical models described before, and the general moving object to simulate the dam failure in a 3D accurate way, it was possible to predict the effect of the failure, the affected zone and the submerged zone in the topography. The simulation lasts for more of 35 minutes of real time, simulating in a transient way the impact of the water on the topography up to the empty of the Dam. The resulting flow-rate across the broken dam is an interesting output of the simulation. All the post processing makes use of lot of Flow-Sight new features: moving camera, different realistic coloration for topography and water, fine tuning of light transparencies and reflections to make the visualization more realistic as possible, the use of textures to represent the surface of the dam, moving camera to follow the fluid path, different plots and viewports to show in one only visualization all the critical aspect of the simulation. https://www.youtube.com/watch?v=xsKx4L9QThI Video 2 : flow depth Video 1 represent an overall view of the water flow with realistic colors. The video 2 instead is a more “scientific analysis”: the water is colored with the fluid depth, using a 20-colors color scale in order to highlight the depth difference also in small portion of the scale; the terrain is colored with the elevation. Finally, a plot of the flow rate across a baffle positioned in correspondence of the dam wall is also reported. In the images a set of realistic rendering are saved, some of them with an evolution over time. … [Read more...]
Pelton turbine simulation – starting transient up to regime
Pelton turbines (or Pelton wheel) are the most worlwide used type turbines for electricity generation in hydraylics powerplant, due to their high efficiency. Its design belongs to 1870 but, with some modifications, they are still the first choiche in modern powerplants. In a Pelton turbine the energy is extracted from the kinetic energy of the water, in contrast with other types of turbines where the hydrostatic pressure is used: the water, coming from an upper basin, is accelerated and ejected against the Pelton paddles. Paddle geometry is designed properly to absorb as much as possible the kinetic energy of the fluid, starting rotating. The rotational speed of the turbine is then converted to electric power through a conductive coil. The simulation analyse the initial transient of the turbine, where water at over 100 m/s impact on the Pelton’s paddle providing torque and angular acceleration. https://www.youtube.com/watch?v=lb2xEbHmWKw All geometries and data used in the simulation are realistic and coherent with the real phenomena: wheel geometry has real shape and mass property, fluid is water with a reasonable speed, and the nozzle contains a doble valve, used in real turbines to adjust water flow rate. Interesting is the plot of the angular speed of the wheel. For Pelton turbines, it is known that the top efficiency is reached when the peripheral speed of the wheel is about half the speed of the water at the nozzle. For this purpose, a probe has been located at the centre of the nozzle in order to monitor the fluid speed, while another probe has been attached to a wheel paddle, in order to catch the peripheral speed. The two quantities can be directly showed as output from the simulation. The videos make a large use of Flowsight features: transparency based on the value of the variables, moving camera, fine tuning of light and reflections, multi-plots and multi-viewport visualization. https://youtu.be/TddbeL1lK9I … [Read more...]
Success Criterion for Fish Passages
This article was contributed by Matthias Haselbauer, RMD Consult and Carlos Barreira Martinez, Federal University of Minas Gerais. In Brazil, the use of surface water has constantly increased during the past 150 years. To maintain navigability, to generate hydropower, and to defend against flooding, a large number of obstacles and diversions have been erected that interfere with natural flows. Fish and other small animals that inhabit the rivers suffer from these alterations. A massive decrease in the number of fish to the point of extinction of some species has been observed. With the simultaneous decrease in fish, bird, and mammal populations, the enormous human impact on the food chain has become obvious. In an attempt to keep rivers open for fish, a large number of fish passages have been built in Brazil, but their efficiency in respect to both their biological and technical aspects was often poor. The flow situations in the passages, often designed using one-dimensional and empirical assumptions, result in an excessive selectivity and in poor locations. In contrast to the traditional one-dimensional design of fish passages more appropriate tools are available today. With computational fluid dynamic (CFD) simulations, not only the mean velocity field can be investigated, but also transient flow effects, which have considerable influence on the usefulness of fish passages. To achieve optimum results a coupling of hydraulic and biological considerations is essential in the design process. In this work, turbulent coherent structures inside a periodic vertical sluice gate fish passage are discussed. Between two pools, with lengths of 4.50m and widths of 3.30 each, the flow has to pass a small vertical opening with an extension of 0.50m (Fig. 1). The CFD simulations were carried out with FLOW-3D. With periodic boundary conditions in the flow direction the achievable resolution was about 2.5cm. The level difference of the water surface Δh between the two pools was 20cm. Hence, the maximum of the absolute velocity is about 2 m/s ≈ Δh*2g. The entire potential energy is transformed into kinetic energy and later dissipated in the pool. Areas of high velocities form where jets are detached from the walls. By means of a Large Eddy Simulation (LES), a detailed analysis of the instantaneous flow regime was possible. The distribution of velocity and turbulence fields, as well as coherent turbulent structures within the pools allowed for a better understanding of fish behavior. Turbulent pressure fluctuations The instantaneous velocity or pressure fields can be divided into the mean values and corresponding fluctuations. The respective equation for the fluctuating pressure is: An examination of the turbulent pressure field shows, that the turbulent pressure inside of vortices is negative. The local minimum values of the turbulent pressure indicates cores of large scale vortices, as shown in Figure 2. In the fish passage, several horizontal rollers can be observed. The vortices are formed inside the shear layer of the sluice. With increasing running distance of the vertices, the turbulent pressure inside the rollers increases due to the increasing vortex diameter and the decreasing turbulent pressure amplitude. Analysis of the turbulent pressure in open channel flows in relation to coherent structures is quite difficult. Large scale vortices can rarely be detected by direct observation. This is due to the fluctuations of the water surface and the related pressure fluctuations inside the entire current. The pressure fluctuations invoked by surface waves decrease with the water depth z by the following exponential law [Kundu, 2004]: The superposition of different pressure fluctuations makes it difficult to detect large scale coherent structures near the surface. Q-Criterion Another tool for vortex detection was proposed by Dubrief (2000) and Hunt (1988), who compared isosurfaces of the pressure, of the vorticity and of the Q-criterion. Read more... … [Read more...]
Interaction Between Waves and Breakwaters
This article is an adapted version of an article published in the journal of the Engineering Association for Offshore and Marine in Italy by Fabio Dentale, E. Pugliese Carratelli, S.D. Russo, and Stefano Mascetti. The first three authors are users at the University of Salerno; Mr. Mascetti is an engineer at XC Engineering, Flow Science’s associate for Italy and France. The design of breakwaters must be based on the full understanding of the interaction of a complex natural system (the sea and shores) with artificial structures (breakwaters). Typically, design work entails extensive physical modelling, which can be quite expensive and time-consuming. Until recently, the complex aspects of breakwater behavior were considered too challenging for detailed numerical simulations. This is especially the case for breakwaters consisting of rubble mounds composed of blocks of concrete or rocks in which water flows through complex paths with unsteady motion. The gap between numerical and physical, investigations, has narrowed, thanks to the advancement of computing technology. It is now possible to accurately represent a solid structure consisting of individual blocks which interacts with the flow, so as to create a numerical flow domain within the empty spaces between the blocks. This enables the evaluation of the effect of the full hydrodynamic behavior, including convective terms, and the effects of turbulence, which cannot be taken into account with the classical Darcy scheme in which the breakwaters are approximated as homogeneous porous media. Modeling Rubble Mound Breakwaters The following examples describe cases where rubble mound breakwaters are modelled on the basis of their real geometry, taking into account the hydrodynamic interactions with the wave motion. Figure 1: Artificial blocksFigure 2a: Submerged BreakwatersFigures 2b and 2c: Emerged Breakwater – Accropode regular & Accropode irregular The work takes into consideration a schematic representation of a natural stone mound, reproduced as a set of spheres, and was further developed to consider commonly-used artificial blocks such as the cube, the modified cube, the antifer, the tetrapod, the accropode, the accropode II, the coreloc, the xbloc,and the xbloc base (Fig. 1). Breakwaters, both submerged and emerged, were sized by making use of standard empirical formulas as available in the literature and numerically constructed by overlapping individual blocks following real geometric patterns, modelling the structure as in the full size construction and in the physical modelling (Fig. 2). In order to validate the quality of the proposed procedure, three different geometries were considered for the submerged breakwater: solid, porous, solid-porous (Fig. 2a), while for the emerged breakwater, two different geometries were used, according to the shape of the elements: regular and random (Fig. 2b – 2c). Read more... … [Read more...]
