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.
