This simulation was inspired by The Slow Mo Guys’ video “90 ft. Vertical Spike Wave in Slow Mo” which shows the results of an experiment made by the FloWave Ocean Energy Research Facility. We use part of their video to compare results with our own simulation. https://www.youtube.com/watch?v=iWKFPTgkpXo&t=105s The Vertical Spike Wave The Vertical Spike Wave is the result of a concentric wave travelling towards its center. Depending on its velocity, the wave can collide in the middle and form a spike of water. This wave can be generated in a circular pool equipped of moving panels on the entire length of the perimeter that push in one coordinated motion the water towards the center. The pushers have to be activated simultaneously with the same motion in order to have the water travel in one single motion and collide at the same speed with the same energy. If the speed is high enough, the water will rise in the center of the pool in a high spike [Figure 1] and fall back splashing. Figure 1 - Vertical Spike Wave Model setup on FLOW-3D® FLOW-3D was used to setup the simulation of the Vertical Spike Wave, then the result was post-processed on FlowSight. In order to model the circular wave, the mesh was defined on cylindrical coordinates. That allowed to simulate only a part of the pool [Figure 2] then to duplicate it later on Flow Sight to make it look like the entire pool was modeled. Figure 2 - FLOW-3D setup Most of the measurements were given in The Slow Mo Guys’ video, so it was possible to give the simulation the real dimensions. The pool is about 50m wide and is equipped of 168 panels. We estimated that the pushers took 4s to motion back and forth once with an angle of 17.2°. To setup the simulation, a pool was created simply on FLOW-3D using the basic geometry provided by the software, then the panel was imported as an STL file. The pusher was setup as a moving object [Figure 3] to which a motion defined in advance was applied. The water is completely still at first, and a single push is sufficient to create the vertical spike wave. Figure 3 - Panels pushing the water Results The overall motion and energy correspond to the reality with a very good precision. Some differences can be seen only in the disrupted front of the spike: its effects are negligible for our comparison but could be taken into account with a complete 3D and 2-fluid simulation. Figure 4 - Experiment and Simulation side by side … [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...]
