The barriers created by dams can cause negative impacts to aquatic communities, and migratory fish species are directly affected. Fishways have been developed to allow the upstream passage of fishes through dams. In Brazil, after the implementation of environmental laws, these structures have been built based on European and American fishway designs. Studies have shown selectivity for different neotropical fishes in some Brazilian fishways, and the main challenge has been to promote upstream passage of a large number of diverse fish species. The patterns of flow circulation within the fish ladder may explain fish selectivity although few studies detail the fish response to hydraulic characteristics of fish ladder flow. This paper presents a laboratory study, where a vertical slot fishway was built in a hydraulic flume and the behavior of two neotropical fish species (Leporinus reinhardti and Pimelodus maculatus) were analyzed. The structure of flow was expressed in terms of mean velocity, Reynolds shear-stress and velocity fluctuation fields. The individuals of Leporinus reinhardti had higher passage success than Pimelodus maculatus in the laboratory flume. Both species preferred areas of low to zero Reynolds shear-stress values. In addition, different preferences were observed for these species concerning the horizontal components of velocity fluctuation.Interrupções em rios, como as causadas por barragens, provocam impactos negativos sobre as comunidades aquáticas e as espécies migratórias de peixes são diretamente afetadas. A fim de permitir a passagem de peixes, mecanismos de transposição são construídos junto a barramentos. No Brasil, após a implantação de leis ambientais, estas estruturas passaram a ser amplamente construídas e foram baseadas em projetos desenvolvidos na Europa e na América do Norte. Análises de mecanismos já construídos têm mostrado grande seletividade para espécies neotropicais e as demandas da diversa fauna de peixes neotropical são apresentadas como principal desafio ao projeto de passagens para peixes. Os padrões de escoamento dentro de escadas para peixes podem explicar a seletividade destes mecanismos, mas poucos estudos detalham a resposta dos peixes às características hidráulicas encontradas dentro das escadas. Este estudo apresenta uma análise realizada em um laboratório onde foi construída uma escada do tipo ranhura vertical e analisados os comportamentos de duas espécies neotropicais (Leporinus reinhardti e Pimelodus maculatus). O escoamento foi estudado considerando os campos de velocidade média, tensão cisalhante de Reynolds e flutuações de velocidades. Os indivíduos da espécie Leporinus reinhardti apresentaram maior sucesso de passagem do que os da espécie Pimelodus maculatus. Ambas as espécies preferiram áreas de tensão de Reynolds igual a zero. Além disso, diferentes preferências foram observadas para cada espécie em relação às componentes horizontais da flutuação da velocidade.

Hydropower plant (HPP) operation may influence downstream flow regimes, which can affect the fish movement. In South America, tailrace fisheries are often killed or injured when interacting with spillways and turbines. Hydrodynamic flow-pattern studies are essential to facilitate mitigation. We developed a computational fluid dynamics model to investigate flow downstream of Três Marias HPP (Brazil). Included in the model were the draft tubes, tailrace and a 3-km river reach. We simulated a common scenario consisting of three active turbines on the right side of the powerhouse (109.6, 108.0 and 108.0m3s–1) and three inactive turbines, by using Ansys Fluent (ver. 12). We identified a straight discharge plume from the right-most turbine that was constrained by the right-side wall. Further, there was the generation of significant plumes from Turbines 2 and 3. The maximum velocities in these plumes appears not to be a barrier for Pimelodus maculatus and Prochilodus costatus, because their prolonged swimming speeds for their maximum total length were higher than the modelled velocities. The results will support mitigation decisions such as fish passage and turbine-screen design in this particular HPP and may be a model for further studies in the South America.

Phase change computational simulations using a diffuse interface treatment for pressure were investigated in order to quantify the spurious currents and its consequences on the interface transport in the present paper. In addition, benchmarks were conducted with a sharp interface treatment for pressure. Namely, a Delta function method (Delta) was employed for the diffuse interface treatment and a ghost fluid method (GFM) for the sharp approach. An additional force term in the nondivergent form of the momentum equation is proposed for the first time in the literature, and its impact on interface motion during simulations of bubble growth by intense phase change has been quantified. In addition, the influence of recoil force on interface position was evaluated in simulations of water bubble condensation at near critical pressure. Finally, simulations of a complex industrial application were performed using the diffuse interface treatment, namely a case of film boiling with the development of Rayleigh-Taylor instability. Both interface treatments presented excellent results for the interface evolution in time. Even with the presence of some relevant spurious currents in the Delta method, the bubble evolution in time was accurately predicted. The sharp interface treatment potential was especially evident using a mass density flux of 1.0 kg/(m 2 s) or higher. Therefore, a diffuse interface treatment for pressure has been presented as an appropriate strategy for most phase change simulations since the presence of the spurious currents did not disturb the interface position, and its magnitude was low for even moderate phase change intensities. The inclusion of the source term due to the additional force in the non-divergent form of the momentum equation and the recoil force term was irrelevant in the cases tested. Lastly, the film boiling simulation using the diffuse interface treatment revealed the possibility of treating complex 3D cases for industrial applications with this method.

The computational fluid dynamics is an important methodology to study the characteristics of flows in nature and in a number of engineering applications. Modeling nonisothermal flows may be useful to predict the main flow behavior allowing the improvement of equipment and industrial processes. In addition, investigations using computational models may provide key information about the fundamental characteristics of flow, developing theoretical groundwork of physical processes. In the last years, the topic of phase change has been intensively studied using computational fluid dynamics due to the computational and numerical advances reported in the literature. Among several issues related to the phase change topic, direct contact condensation (DCC) is widely studied in the literature since it is part of a number of industrial applications. In the present work, DCC was studied using a mathematical and computational model with an Eulerian approach. The homemade code MFSim was used to run all the computational simulations in the cluster of the Fluid Mechanics Laboratory from the Federal University of Uberlandia (UFU). The computational model was validated and showed results with high accuracy and low differences compared to previous works in the literature. A complex case study of DCC with cross-flow was then studied and the computational model provided accurate results compared to experimental data from the literature. The jet centerline was well represented and the interface dynamic was accurately captured during all the simulation time. The investigation of the velocity field provided information about the deeply transient characteristic of this flow. The v-velocity component presented the more large variations in time since the standard deviation was subjected to a variation of about 45% compared to the temporal average. In addition, the time history of the maximum resultant velocities observed presented magnitude from 29 m/s to 73 m/s. The importance of modeling three-dimensional (3D) effects was confirmed with the relevance of the velocity magnitudes in the third axis component. Therefore, the Eulerian phase change model used in the present study indicated the possibility to model even complex phenomena using an Eulerian approach.

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