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We employed direct numerical simulations to estimate the error on chemical calculation in simulations with regional chemical-transport models induced by neglecting subgrid chemical segregation due to inefficient turbulent mixing in an urban boundary layer with strong and heterogeneously distributed surface emissions. In simulations of initially segregated reactive species with an entrainment-emission configuration with an A–B–C second-order chemical scheme, urban surface emission fluxes of the homogeneously emitted tracer A result in a very large segregation between the tracers and hence a very large overestimation of the effective chemical reaction rate in a complete-mixing model. This large effect can be indicated by a large Damköhler number (Da) of the limiting reactant. With heterogeneous surface emissions of the two reactants, the resultant normalised boundary-layer-averaged effective chemical reaction rate is found to be in a Gaussian function of Da, and it is increasingly overestimated by the imposed rate with an increased horizontal scale of emission heterogeneity. Coarse-grid models with resolutions commensurable to regional models give reduced yet still significant errors for all simulations with homogeneous emissions. Such model improvement is more sensitive to the increased vertical resolution. However, such improvement cannot be seen for simulations with heterogeneous emissions when the horizontal resolution of the model cannot resolve emission heterogeneity. This work highlights particular conditions in which the ability to resolve chemical segregation is especially important when modelling urban environments.

This paper presents a new approach for mitigating the unphysical delay in the Reynolds-averaged Navier–Stokes (RANS) to large-eddy simulation (LES) transition, often referred to as the gray area, which is a common issue for hybrid RANS–LES turbulence models such as delayed-detached eddy simulation. An existing methodology designed for improving the LES performance in complex flows is adapted and tested. This is based on reducing the numerical diffusion in critical areas for permitting a more accurate development of turbulence. The new formulation comprises both a two-dimensional sensitive velocity gradient model and an alternative definition of the subgrid length scale, which are tested both individually and in tandem, and compared with the other formulations commonly used for addressing the gray area. Four test cases are examined, a flat plate, two variants of the incompressible backwardfacing step, and an open jet compressible case, all of which are selected to expose the adverse impact of numerical diffusion that this study seeks to address. Furthermore, the proposed changes are implemented in two different codes for the purpose of cross-validation. Encouraging results are observed, supporting the suitability of the new approach as a candidate for addressing the gray area issue in flows of this kind. This work was financially supported by the Ministerio de Economía y Competitividad, Spain (No. ENE2017-88697-R). A.P.V. was supported by an FI-DGR 2016 predoctoral contract (No. 2018FI_ B2_00072) financed by Generalitat de Catalunya, Spain. F.X.T. was supported by a Ramón y Cajal postdoctoral contract (No. RYC-2012- 11996) financed by the Ministerio de Economía y Competitividad, Spain. A.D. and A.G. were supported by Moscow Center for Fundamental and Applied Mathematics, Agreement with the Ministry of Science and Higher Education of the Russian Federation, No. 075- 15-2019-1623. The NOISEtte computations were carried out using the equipment of the shared research facilities of HPC computing resources at Lomonosov Moscow State University and the computing resources of the federal collective usage center Complex for Simulation and Data Processing for Mega-science Facilities at NRC “Kurchatov Institute” (http://ckp.nrcki.ru/).

En este artículo se presenta el estudio numérico y comparativo del efecto de turbulencia en codos y dobleces para diámetros de una pulgada, mediante CFD y bajo las mismas condiciones de trabajo (velocidad, presión y temperatura), para determinar la fluctuación en la turbulencia de energía cinética entre estos dos accesorios variando los modelos de turbulencia. Se emplearon dos metodologías para esta investigación, kappa-épsilon (k- ε) y kappa-omega (k-ω). El método (k- ε) se divide en tres modelos: estándar, RNG y realizable, en los cuales se genera turbulencia de energía cinética y de disipación. El método (k-ω) también posee tres variantes: estándar, SST, BSL. El trabajo presenta una mayor turbulencia para el método de (k- ε) en energía cinética y de disipación bajo el modelo estándar tanto para codo como doblez, mientras que en el método (k-ω) se produce una mayor turbulencia de energía cinética en el modelo BSL para ambos accesorios, al igual que en el método (k- ε), el modelo estándar de (k-ω) representa una mayor turbulencia de frecuencia.

The study of complex turbulent flows by means of large-eddy simulation approaches has become increasingly popular in many scientific and engineering applications. The underlying filtering operation of the approach enables to significantly reduce the spatial and temporal resolution requirements by means of representing only large-scale motions. However, the small-scale stresses and their effects on the resolved flow field are not negligible, and therefore require additional modeling. As a consequence, the assumptions made in the closure formulations become potential sources of model-form uncertainty that can impact the quantities of interest. The objective of this work, thus, is to perform a model-form sensitivity analysis in large-eddy simulations of an axisymmetric turbulent jet following an eigenspace-based strategy recently proposed. The approach relies on introducing perturbations to the decomposed subgrid-scale stress tensor within a range of physically plausible values. These correspond to discrepancy in magnitude (trace), anisotropy (eigenvalues) and orientation (eigenvectors) of the normalized, small-scale stresses with respect to a given tensor state, such that propagation of their effects can be assessed. The generality of the framework with respect to the six degrees of freedom of the small-scale stress tensor makes it also suitable for its application within data-driven techniques for improved subgrid-scale modeling.

In this work, we aim to shed light to the following research question: can we find a nonlinear tensorial subgrid-scale (SGS) heat flux model with good physical and numerical properties, such that we can obtain satisfactory predictions for buoyancy-driven turbulent flows? This is motivated by our findings showing that the classical (linear) eddy-diffusivity assumption, q eddy ¿ ¿T, fails to provide a reasonable approximation for the actual SGS heat flux, q = uT - uT: namely, a priori analysis for airfilled Rayleigh-Bénard convection (RBC) clearly shows a strong misalignment. In the quest for more accurate models, we firstly study and confirm the suitability of the eddy-viscosity assumption for RBC carrying out a posteriori tests for different models at very low Prandtl numbers (liquid sodium, Pr = 0.005) where no heat flux SGS activity is expected. Then, different (nonlinear) tensor-diffusivity SGS heat flux models are studied a priori using DNS data of an air-filled (Pr = 0.7) RBC at Rayleigh numbers up to 1011. Apart from having good alignment trends with the actual SGS heat flux, we also restrict ourselves to models that are numerically stable per se and have the proper cubic near-wall behavior. This analysis leads to a new family of SGS heat flux models based on the symmetric positive semi-definite tensor GGT where G = ¿u, i.e. q ¿ GGT¿T, and the invariants of the GGT tensor. Finally, relevant numerical aspects regarding the implementation of this type of models are discussed in detail. A list of physical and numerical properties is identified and subsequently imposed, leading to a symmetrypreserving discretization that is based on discrete operators already available in any CFD code. F.X.T., F.D. and A.O. have been financially supported by the Ministerio de Economía y Competitividad, Spain, ANUMESOL project (ENE2017-88697-R). F.X.T. and A.O. are supported by the Generalitat de Catalunya RIS3CAT-FEDER, FusionCAT project (001-P-001722). F.D. is supported by the Austrian Federal Ministry for Digital and Economic Affairs, the National Foundation for Research, Technology and Development, and the K1MET center for metallurgical research in Austria (www.k1-met.com). D.S. is supported by an FI AGAUR-Generalitat de Catalunya fellowship (2020FI B 00839). Calculations have been performed on the IBM MareNostrum 4 supercomputer at the BSC (PRACE 15th Call, Ref. 2016163972, “Exploring new frontiers in Rayleigh-Bénard convection” and Tier-1 RES projects IM-2020-2-0029 & IM-2020-3-0030 “Direct and Large-Eddy Simulation of buoyancy-driven turbulence in liquid metals”). The authors thankfully acknowledge these institutions.

Active fluids exhibit complex turbulentlike flows at low Reynolds number. Recent work predicted that 2D active nematic turbulence follows scaling laws with universal exponents. However, experimentally testing these predictions is conditioned by the coupling to the 3D environment. Here, we measure the spectrum of the kinetic energy E(q) in an active nematic film in contact with a passive oil layer. At small and intermediate scales, we find the scaling regimes E(q)∼q^{-4} and E(q)∼q^{-1}, respectively, in agreement with the theoretical prediction for 2D active nematics. At large scales, however, we find a new scaling E(q)∼q, which emerges when the dissipation is dominated by the 3D oil layer. In addition, we derive an explicit expression for the spectrum that spans all length scales, thus explaining and connecting the different scaling regimes. This allows us to fit the data and extract the length scale that controls the crossover to the new large-scale regime, which we tune by varying the oil viscosity. Overall, our work experimentally demonstrates the emergence of scaling laws with universal exponents in active turbulence, and it establishes how the spectrum is affected by external dissipation.

Turbulence generally arises in shear flows if velocities and hence, inertial forces are sufficiently large. In striking contrast, viscoelastic fluids can exhibit disordered motion even at vanishing inertia. Intermediate between these cases, a state of chaotic motion, “elastoinertial turbulence” (EIT), has been observed in a narrow Reynolds number interval. We here determine the origin of EIT in experiments and show that characteristic EIT structures can be detected across an unexpectedly wide range of parameters. Close to onset, a pattern of chevron-shaped streaks emerges in qualitative agreement with linear and weakly nonlinear theory. However, in experiments, the dynamics remain weakly chaotic, and the instability can be traced to far lower Reynolds numbers than permitted by theory. For increasing inertia, the flow undergoes a transformation to a wall mode composed of inclined near-wall streaks and shear layers. This mode persists to what is known as the “maximum drag reduction limit,” and overall EIT is found to dominate viscoelastic flows across more than three orders of magnitude in Reynolds number.

This paper deals with the numerical analysis of the particle inertia and volume fraction effects on colliding particle-pair velocity correlation immersed in an unsteady isotropic homogeneous turbulent flow. Such correlation function is required to build reliable statistical models for inter-particle collisions, in the frame of the Euler&ndash, Lagrange approach, to be used in a broad range of two-phase flow applications. Computations of the turbulent flow have been carried out by means of Direct Numerical Simulation (DNS) by the Lattice Boltzmann Method (LBM). Moreover, the dependence of statistical properties of collisions on particle inertia and volumetric fraction is evaluated and quantified. It has been found that collision locations of particles of intermediate inertia, StK~1, occurs in regions where the fluid strain rate and dissipation are higher than the corresponding averaged values at particle positions. Connected with this fact, the average kinetic energy of colliding particles of intermediate inertia (i.e., Stokes number around 1) is lower than the value averaged over all particles. From the study of the particle-pair velocity correlation, it has been demonstrated that the colliding particle-pair velocity correlation function cannot be approximated by the Eulerian particle-pair correlation, obtained by theoretical approaches, as particle separation tends to zero, a fact related with the larger values of the relative radial velocity between colliding particles.

Atmospheric turbulence is an unsteady phenomenon found in nature and plays significance role in predicting natural events and life prediction of structures. In this work, turbulence in surface boundary layer has been studied through empirical methods. Computer simulation of Von Karman, Kaimal methods were evaluated for different surface roughness and for low (1%), medium (10%) and high (50%) turbulence intensities. Instantaneous values of one minute time series for longitudinal turbulent wind at mean wind speed of 12 m/s using both spectra showed strong correlation in validation trends. Influence of integral length scales on turbulence kinetic energy production at different heights is illustrated. Time series for mean wind speed of 12 m/s with surface roughness value of 0.05 m have shown that variance for longitudinal, lateral and vertical velocity components were different and found to be anisotropic. Wind speed power spectral density from Davenport and Simiu profiles have also been calculated at surface roughness of 0.05 m and compared with k−1 and k−3 slopes for Kolmogorov k−5/3 law in inertial sub-range and k−7 in viscous dissipation range. At high frequencies, logarithmic slope of Kolmogorov −5/3rd law agreed well with Davenport, Harris, Simiu and Solari spectra than at low frequencies. Resumo A turbulência atmosférica é um fenômeno instável encontrado na natureza e desempenha um papel significativo na previsão de eventos naturais e na previsão da vida de estruturas. Neste trabalho, a turbulência na camada limite superficial foi estudada por meio de métodos empíricos. Simulação computacional de Von Karman, métodos Kaimal foram avaliados para diferentes rugosidades superficiais e para intensidades de turbulência baixa (1%), média (10%) e alta (50%). Valores instantâneos de séries temporais de um minuto para vento turbulento longitudinal com velocidade média do vento de 12 m/s usando ambos os espectros mostraram forte correlação nas tendências de validação. A influência das escalas de comprimento integral na produção de energia cinética de turbulência em diferentes alturas é ilustrada. Séries temporais para velocidade média do vento de 12 m / s com rugosidade de superfície de 0,05 m mostraram que a variância para os componentes de velocidade longitudinal, lateral e vertical foram diferentes e considerados anisotrópicos. A densidade espectral da potência da velocidade do vento dos perfis de Davenport e Simiu também foi calculada na rugosidade da superfície de 0,05 m e comparada com inclinações k−1 e k−3 para a lei Kolmogorov k5/3 em subfaixa inercial e k−7 em viscoso faixa de dissipação. Em altas frequências, a inclinação logarítmica da lei Kolmogorov −5/3rd concordou bem com os espectros de Davenport, Harris, Simiu e Solari do que em baixas frequências.

We derive the exact relation for the energy transfer in three-dimensional compressible two-fluid plasma turbulence. In the long-time limit, we obtain an exact law which expresses the scale-to-scale average energy flux rate in terms of two point increments of the fluid variables of each species, electric and magnetic field and current density, and puts a strong constraint on the turbulent dynamics. The incompressible single fluid and two-fluid limits and the compressible single fluid limit are recovered under appropriate assumption. In the single fluid limits, analyses are done with and without neglecting the electron mass thereby making the exact relation suitable for a broader range of application. In the compressible two-fluid regime, the total energy flux rate, unlike the single fluid case, is found to be unaltered by the presence of a background magnetic field. The exact relation provides a way to test whether a range of scales in a plasma is inertial or dissipative and is essential to understand the nonlinear nature of both space and dilute astrophysical plasmas. Fil: Banerjee, Supratik. Indian Institute of Technology Kanpur; India Fil: Andrés, Nahuel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

The study of thermal radiation interacting with particle-laden turbulence is of great importance in a wide range of scientific and engineering applications. The computational study of such systems is challenging as a result of the large number of thermo-fluid mechanisms governing the underlying physics. To build confidence and improve the prediction accuracy of such simulations, the impact of uncertainties on the quantities of interest must be measured. This, however, requires a computational budget that is typically a large multiple of the cost of a single calculation, and thus may become infeasible for expensive simulation models featuring a large number of uncertain inputs and highly nonlinear behavior. In this regard, multifidelity methods have become increasingly popular in recent years as acceleration strategies to reduce the computational cost. These methods are based on a hierarchy of generalized numerical resolutions, or model fidelities, and attempt to leverage the correlation between high- and low-fidelity models to obtain a more accurate statistical estimator with a relatively small number of high-fidelity calculations. In this work, the performance of a collection of different multifidelity strategies and modeling approaches is assessed to propagate the uncertainties encountered in the simulation of irradiated particle-laden turbulence relevant to volumetric solar energy receivers. The results obtained indicate that multifidelity methods provide speedups on the order of 10-1000x with respect to straightforward Monte Carlo approaches, resulting in remarkable reductions in computational cost.

The study of thermal radiation interacting with particle-laden turbulence is of great importance in a wide range of scientific and engineering applications. The fundamental and applied study of such systems is challenging as a result of the large number of thermo-fluid mechanisms governing the underlying physics. This complexity is significantly reduced by transforming the problem of interest into its scale-free form by means of dimensional analysis techniques. However, the theoretical framework of classical dimensional analysis presents the limitations of not providing a unique set of dimensionless groups, and no support for measuring the relative importance between them. In the interest of addressing these shortfalls for multiphysics turbulent flow applications, we present a semi-empirical dimensional analysis approach to efficiently extract important dimensionless groups from data obtained by means of computational (or laboratory) experiments. The methodology presented is then used to characterize important dimensionless groups in irradiated particle-laden turbulence. The study concludes that two dimensionless groups are responsible for most of the variation in the system’s thermal response, with the absorption of radiation by particles, the radiative energy deposition rate and the turbulent flow mixing the most important thermo-fluid mechanisms. The generality of the results obtained can be leveraged to effectively reduce the dimensionality of irradiated particle-laden turbulent flows in research studies and in the design and optimization of similar systems.

The traditional approach for the hydrodynamic characterization of the flow down stepped spillways is through physical modeling, which is susceptible to scale effects and has limitations related to experimental apparatus, laboratory space and the spatial discretization of data collection. Computational fluid dynamics (CFD) is an important tool for hydrodynamic analysis because, if used properly, it presents great potential for application in hydraulics. In this work, CFD was used to model the skimming flow down a stepped spillway to investigate the effects of possible pressure measurement errors due to uncertainties in the position of the sensors within the steps. The numerical model was validated through literature velocity profiles and pressure experimental data. The results showed that the best values of water fraction (α) to define free surface are α = 0.30 in the nonaerated region and α = 0.10 in the aerated region. Statistical parameters were calculated using experimental data to estimate extreme pressures. These parameters and the simulation results were used to determine that the extreme maximum and minimum pressures occur, respectively, in the region of 0.81 < x/l < 0.98, in the horizontal faces, and in the region of 0.93 < y/h < 0.98, in the vertical faces. RESUMO A abordagem tradicional para a caracterização hidrodinâmica do escoamento sobre vertedouros em degraus é feita através da modelagem física, a qual é suscetível aos efeitos de escala e possui limitações relacionadas ao aparato experimental, espaço físico de laboratório e discretização espacial da obtenção de dados. A fluidodinâmica computacional é uma importante ferramenta de análise dos escoamentos, pois, se utilizada corretamente, apresenta grande potencial de aplicação na engenharia hidráulica. Neste trabalho, foi utilizada fluidodinâmica computacional para modelar o escoamento deslizante sobre turbilhões em um vertedouro em degraus. Buscou-se investigar os efeitos de possíveis erros de medição de pressão devido às incertezas na posição dos medidores dentro dos degraus. O modelo numérico foi validado através de dados de pressão obtidos experimentalmente e dados de velocidade encontrados na literatura. Os resultados mostram que os melhores valores de fração de água (α) que definem a superfície livre são α = 0,30 para a região não aerada e α = 0,10 para a região aerada. Utilizando-se dados experimentais, foram calculados parâmetros estatísticos para estimar pressões extremas. O uso destes parâmetros, em conjunto com os resultados das simulações, permite definir que as pressões extremas positivas ocorrem na região 0,81 < x/l < 0,98 nos patamares e as mínimas na região 0,93 < y/h < 0,98 nos espelhos.

Turbulent superstructures, i.e. large-scale flow structures in turbulent flows, play a crucial role in many geo- and astrophysical settings. In turbulent Rayleigh–Bénard convection, for example, horizontally extended coherent large-scale convection rolls emerge. Currently, a detailed understanding of the interplay of small-scale turbulent fluctuations and large-scale coherent structures is missing. Here, we investigate the resolved kinetic energy and temperature variance budgets by applying a filtering approach to direct numerical simulations of Rayleigh–Bénard convection at high aspect ratio. In particular, we focus on the energy transfer rate between large-scale flow structures and small-scale fluctuations. We show that the small scales primarily act as a dissipation for the superstructures. However, we find that the height-dependent energy transfer rate has a complex structure with distinct bulk and boundary layer features. Additionally, we observe that the heat transfer between scales mainly occurs close to the thermal boundary layer. Our results clarify the interplay of superstructures and turbulent fluctuations and may help to guide the development of an effective description of large-scale flow features in terms of reduced-order models.

The smallest hydrodynamic length scales in two-phase turbulence are located at the interface between phases, or fluids, as a result of two-way coupling phenomena. Typically, these interface-generated scales are several times smaller than the dissipative scales in the surrounding bulk flow identified by Kolmogorov’s 1941 theory. Consequently, to properly capture these interface-generated small scales with sufficiently fine resolutions, the computational cost of performing large-eddy simulations of two-phase turbulent flow increases significantly from its (single-phase) theoretical optimum and toward values on the order of the direct numerical simulation of turbulence. Therefore, to maintain the cost of scale- resolving approaches linear with respect to the Reynolds number, this work investigates the modeling of the small-scale fluid motions in the vicinity of the viscous near-interface region of two-phase turbulent flows. Given the resemblance between the flow structures in the near-interface regions and those found in the boundary layers of turbulent wall-bounded flow, the modeling methodology proposed is inspired by ideas developed for turbulent flows interacting with solid walls, but modified to capture slip-velocity effects between phases. The performance of the approach is a priori assessed by utilizing data from direct numerical simulations of decaying isotropic turbulence laden with droplets of super-Kolmogorov size, demonstrating its computational feasibility and potential for reducing the cost of large-eddy simulation studies of two-phase turbulence.

CFD, procesamiEl procesamiento de gas natural requiere la aplicación de nuevas tecnologías en un contexto de aumento de la demanda en todo el mundo. La separación de líquidos de gas natural (LGN) utilizando dispositivos supersónicos, es una novedosa y eficiente manera de reducir el volumen de equipos instalados y costos, utilizando los efectos de los flujos circulares altamente turbulentos. En esta investigación se implementa dinámica computacional de fluidos (CFD) para mejorar la eficiencia del proceso típico de recuperación de LGN utilizando el enfoque supersónico. También se puso en práctica un nuevo enfoque de modelado de la turbulencia con el objetivo de minimizar el tiempo de procesamiento, los resultados obtenidos fueron validados con datos experimentales disponibles. Esta investigación se basa en el modelo llamado k-épsilon RNG modificado para flujo remolino; este modelo no se ha utilizado ni validado antes en sistemas de flujo altamente compresibles, turbulentos y circulares. La eficiencia del proceso fue mejorada en un 11% en comparación con la eficiencia reportada en investigaciones previas, y el tiempo de procesamiento para el modelado redujo 40% con el enfoque turbulencia propuesto y ajustado. Durante la validación del modelo k-épsilon RNG modificado para flujo remolino el factor de remolino, parte del modelo de turbulencia, se ajustó a un valor óptimo para sistemas de flujo compresibles, turbulentos y circulares que participan en procesos de separación de NGL supersónico, lo que permite obtener resultados más precisos y con un menor tiempo de procesamiento en comparación con otros enfoques típicos y comunes como RSM y LES.

The topology of the fine-scale motions in decaying isotropic turbulence laden with droplets of super-Kolmogorov size is investigated using results from direct numerical simulations. The invariants of the velocity-gradient, rate-of-strain, and rate-of-rotation tensors are computed in the carrier phase. The joint probability density functions of the invariants are calculated and conditioned on different distances from the droplet surface. The results show that outside the viscous region near the interface, the flow topologies favor stable focus/stretching and unstable node/saddle/saddle structures, which is in agreement with those found in canonical homogeneous isotropic turbulence. Inside the viscous layer at the droplet surface, the flow topologies shift from a preference for high-enstrophy/low-dissipation motions to favoring low-enstrophy/high-dissipation. At the droplet surface, there is a strong tendency for boundary-layer-like and vortex-sheet flow topologies in which the strain and rotation rates are positively correlated. An interesting observation is that the shapes of the invariant distributions at the droplet surface are remarkably similar to those reported in the viscous sublayer of turbulent wall-bounded flows. Also, the results show that the smallest hydrodynamic length scale of the carrier fluid turbulence is located at the droplet interface and that this length scale is two to three times smaller than that of the surrounding bulk flow. From a computational viewpoint, this suggests a more stringent spatial resolution requirement for the direct numerical simulation of finite-size droplets in isotropic turbulence than its single-phase counterpart.

Producción Científica, This work analyzes the influence of the mixer configuration on the mixing efficiency in the process of biomass ultrafast hydrolysis by supercritical water. The results of the CFD simulations of a horizontal tee, a vertical tee and a mixing cross, selected as the optimum mixing configurations, together with the experimental results obtained in our hydrolysis plant, are the base to determine the configuration which provides the best mixing performance. Although slightly higher conversions are obtained in those experiments performed with a horizontal tee, the small differences between the results demand a theoretical analysis. Therefore, according to the CFD simulation results, since the mixing cross provides the best flow distribution and temperature homogenization at the outlet of the mixers and because of the great similarity between the residence time distribution curves of the mixers, the mixing cross is selected as the optimum geometry to perform the mixing., Ministerio de Economía, Industria y Competitividad - FEDER (Projects CTQ2013-44143-R and CTQ2016-79777-R)

Vertical slot fishways allow energy dissipation as a function of the pool, longitudinal slope, baffle and vertical slot design. The mean and turbulent flow patterns in these structures must be compatible with the fish target. The design of these structures is commonly based on previous successful fishways as well as simplified theoretical equations and empirical relationships. To aid in the design of these structures, a three-dimensional hydrodynamic model was used to simulate the flow, and experimental studies were used to validate the model. The mean velocities, pressures and parameters indicative of turbulence were analyzed. The maximum flow velocities were up to 32% higher than the values obtained using a simplified theoretical equation. The evaluation of the volumetric dissipated power indicated that the mean value for the pool was lower than 150 W/m3; however, analysis of the spatial distribution showed that in some areas, the values can exceed 1000 W/m3. The results indicate that the numerical simulation was able to adequately represent the flow considering the computational cost involved. Accordingly, it can be used as a complementary tool for the design of new fishways and for the analysis of modifications in existing ones. RESUMO As escadas para peixes com ranhuras verticais permitem que ocorra a dissipação da energia do escoamento em função da forma da bacia, dos defletores, da declividade longitudinal e da largura da ranhura vertical. Estas estruturas devem apresentar padrões médios e turbulentos do escoamento compatíveis com os peixes que irão transpô-la. Tipicamente a concepção das escadas para peixes é baseada na utilização de geometrias empregadas com sucesso em situações anteriores e a estimativa das características do escoamento através de equações teóricas simplificadas e relações empíricas. Para contribuir na concepção dessas estruturas, foi utilizado um modelo hidrodinâmico tridimensional para simular o escoamento e estudos experimentais para validar o modelo. Velocidades médias, pressões e parâmetros indicativos da turbulência foram analisados. As velocidades máximas do escoamento foram até 32% superiores aos valores obtidos utilizando uma equação teórica simplificada. A avaliação da potência dissipada por unidade de volume indica que o valor médio para o tanque é inferior a 150 W/m3, no entanto, a análise da distribuição espacial desta grandeza no tanque, mostra que valores pontuais podem ultrapassar 1000 W/m3. Os resultados indicam que a simulação numérica foi capaz de representar de forma adequada o escoamento, considerando o custo computacional envolvido e pode ser utilizada como uma ferramenta complementar para o projeto de novas escadas para peixes e para a análise de modificações em estruturas existentes.

The drag of turbulent flows can be drastically decreased by addition of small amounts of high molecular weight polymers. While drag reduction initially increases with polymer concentration, it eventually saturates to what is known as the maximum drag reduction (MDR) asymptote; this asymptote is generally attributed to the dynamics being reduced to a marginal yet persistent state of subdued turbulent motion. Contrary to this accepted view we will show in the following that for an appropriate choice of parameters polymers can reduce the drag beyond the suggested asymptotic limit, eliminating turbulence and giving way to laminar flow. However at higher polymer concentrations the laminar state becomes unstable, resulting in a fluctuating flow with the characteristic drag of the MDR asymptote. The asymptotic state is hence dynamically disconnected from ordinary turbulence., Comment: 6 pages, 6 figures

International audience; Helicity is the scalar product between velocity and vorticity and, just like energy, its integral is an in-viscid invariant of the three-dimensional incompressible Navier-Stokes equations. However, space-and time-discretization methods typically corrupt this property, leading to violation of the inviscid conservation principles. This work investigates the discrete helicity conservation properties of spectral and finite-differencing methods, in relation to the form employed for the convective term. Effects due to Runge-Kutta time-advancement schemes are also taken into consideration in the analysis. The theoretical results are proved against inviscid numerical simulations, while a scale-dependent analysis of energy, helicity and their non-linear transfers is performed to further characterize the discretization errors of the different forms in forced helical turbulence simulations.

This study treats with the influence of a symmetry-breaking transversal magnetic field on the nonlinear dynamics of ferrofluidic Taylor-Couette flow – flow confined between two concentric independently rotating cylinders. We detected alternating ‘flip’ solutions which are flow states featuring typical characteristics of slow-fast-dynamics in dynamical systems. The flip corresponds to a temporal change in the axial wavenumber and we find them to appear either as pure 2-fold axisymmetric (due to the symmetry-breaking nature of the applied transversal magnetic field) or involving non-axisymmetric, helical modes in its interim solution. The latter ones show features of typical ribbon solutions. In any case the flip solutions have a preferential first axial wavenumber which corresponds to the more stable state (slow dynamics) and second axial wavenumber, corresponding to the short appearing more unstable state (fast dynamics). However, in both cases the flip time grows exponential with increasing the magnetic field strength before the flip solutions, living on 2-tori invariant manifolds, cease to exist, with lifetime going to infinity. Further we show that ferrofluidic flow turbulence differ from the classical, ordinary (usually at high Reynolds number) turbulence. The applied magnetic field hinders the free motion of ferrofluid partials and therefore smoothen typical turbulent quantities and features so that speaking of mildly chaotic dynamics seems to be a more appropriate expression for the observed motion.

The present work assesses the limits of the Oberbeck–Boussinesq (OB) approximation for the resolution of turbulent fluid flow and heat transfer inside a tall differentially heated cavity of aspect ratio Γ = 6.67 filled with water ( Pr = 3.27, Ra = 2.12 × 10 11 ). The cavity models the integrated solar collector-storage element installed on an advanced facade. The implications of the Oberbeck–Boussinesq approximation is submitted to investigation by means of direct numerical simulations (DNS) carried out for a wide range of temperature differences. Non-Oberbeck–Boussinesq (NOB) effects are found to be relevant, especially beyond the temperature difference of 30 °C, in the estimation of heat transfer, stratification, and flow configuration.

Wind energy is an important component of renewable energy sources. Studying of turbulent wakes for wind turbines and interactions with Atmospheric Boundary Layer is an urgent problem. The theory of chaos and fractal geometry with some research in fluid dynamics will change profoundly the scope of complex scale to scale transport in theory of turbulence, in particular when body forces, such as stratification and rotation are important. For this reason, we present some preliminary ideas and some results of the analysis of the structure function and multi-fractal behaviour of the evolution of turbulence in the wake of a single wind turbine through the scaling analysis using the program ImaCalc. The analysis of fractal dimension was performed on the results of a numerical simulation consisting on a Large-Eddy Simulation and Lagrangian Dynamic Smagorinsky Mode (LDSM)l for the wind turbine wakes in the Atmospheric Boundary Layer by OpenFoam software. The role of intermittency in wind turbine technology in the stratified Atmosphere is also discussed.

Oscillating-grid turbulence (OGT) in a homogeneous fluid has been widely investigated. In a recent paper Pujol et al. (2010) used an oscillating grid to mimic wind-generated turbulence. They quantified the vertical distribution of turbulent kinetic energy (TKE) above and within different types of vegetation, which were simulated by constructing two different canopies at the bottom of a water tank. The first canopy, made of PVC cylinders, was rigid and the second, made of nylon threads, was semi-rigid. We propose a k-ε turbulence model to simulate the results obtained in their paper. Two source terms (~(TKE) 3/2) were added to a classical k-ε turbulence formulation to account for canopy dissipation. Results of the model show good agreement with experimental data This work was supported by the Spanish government (MCYT) through the CGL2010-17289 project

The goal of this work is to propose a new methodology to simulate turbulent thermal wall flows using the classical κ - ε model. The focus of this approach is based on the manner used to implement heat flux boundary conditions on the solid walls. In order to explain and to validate this new algorithm, several test cases are presented, testing a great range of flows in order to analyze the numerical response on different physical aspects of the fluid flow. The proposed approach uses simultaneously a thermal wall law, an analogy between fluid friction and heat transfer and an interpolating polynomial relation that is constructed with a data base generated on experimental research and numerical simulation. The algorithm used to execute the numerical simulations applies the classical κ - ε model with a consolidate Reynolds and Favre averaging process for the turbulent variables. The turbulent inner layer can be modeled by four distinct velocity wall laws and by one temperature wall law. Spacial discretization is done by P1 and P1/isoP2 finite elements and the temporal discretization is implemented using a semi-implicit sequential scheme of finite differences. The pressure-velocity coupling is numerically solved by a variation of Uzawa's algorithm. To filter the numerical noises, originated by the symmetric treatment of the convective fluxes, it is adopted a balance dissipation method. The remaining non-linearities, due to explicit calculations of boundary conditions by wall laws, are treated by a minimal residual method.

Direct numerical simulations of the turbulent temporally evolving cloud-top mixing layer are used to investigate the role of evaporative cooling by isobaric mixing locally at the stratocumulus top. It is shown that the system develops a horizontal layered structure whose evolution is determined by molecular transport. A relatively thin inversion with a constant thickness h = κ/we is formed on top and travels upwards at a mean velocity we ≃ 0.1(κ |bs|χc2)1/3, where κ is the mixture-fraction diffusivity, bs < 0 is the buoyancy anomaly at saturation conditions χs and χc is the cross-over mixture fraction defining the interval of buoyancy reversing mixtures. A turbulent convection layer develops below and continuously broadens into the cloud (the lower saturated fluid). This turbulent layer approaches a self-preserving state that is characterized by the convection scales constructed from a constant reference buoyancy flux Bs = |bs|we/χs. Right underneath the inversion base, a transition or buffer zone is defined based on a strong local conversion of vertical to horizontal motion that leads to a cellular pattern and sheet-like plumes, as observed in cloud measurements and reported in other free-convection problems. The fluctuating saturation surface (instantaneous cloud top) is contained inside this intermediate region. Results show that the inversion is not broken due to the turbulent convection generated by the evaporative cooling, and the upward mean entrainment velocity we is negligibly small compared to the convection velocity scale w* of the turbulent layer and the corresponding growth rate into the cloud.

The diffusion of astrophysical magnetic fields in conducting fluids in the presence of turbulence depends on whether magnetic fields can change their topology via reconnection in highly conducting media. Recent progress in understanding fast magnetic reconnection in the presence of turbulence is reassuring that the magnetic field behavior in computer simulations and turbulent astrophysical environments is similar, as far as magnetic reconnection is concerned. Our studies of magnetic field diffusion in turbulent medium reveal interesting new phenomena. In the presence of gravity and turbulence, our 3D simulations show the decrease of the magnetic flux-to-mass ratio as the gaseous density at the center of the gravitational potential increases. We observe this effect both in the situations when we start with equilibrium distributions of gas and magnetic field and when we follow the evolution of collapsing dynamically unstable configurations. Thus the process of turbulent magnetic field removal should be applicable both to quasi-static subcritical molecular clouds and cores and violently collapsing supercritical entities. The increase of the gravitational potential as well as the magnetization of the gas increases the segregation of the mass and magnetic flux in the saturated final state of the simulations, supporting the notion that the reconnection-enabled diffusivity relaxes the magnetic field + gas system in the gravitational field to its minimal energy state. This effect is expected to play an important role in star formation, from its initial stages of concentrating interstellar gas to the final stages of the accretion to the forming protostar., 22 pages, 16 figures. Accepted for publication in the ApJ.

Estuda-se, via modelagens física e matemática, os efeitos da velocidade de lingotamento; geometria do molde; profundidade de imersão; geometria e inclinação dos furos da SEN (Submerged Entry Nozzle) sobre a estabilidade do fluxo de aço na região do menisco do molde de LC e suas possíveis influências sobre a emulsificação metal-escória. Dados experimentais obtidos, via modelagem física, são agrupados por meio do parâmetro F, definido a partir da velocidade de lingotamento, da profundidade de impacto do jato contra a face estreita do molde e do ângulo de saída do jato. Simulações numéricas via programa CFX, permitem estimar a velocidade subsuperficial para as condições ensaiadas no modelo. A análise dos resultados de modelagem física e matemática permitem identificar as condições de lingotamento ideais, para duas configurações de válvulas submersas. The likely influence of casting velocity, mold geometry and depth of immersion of the SEN, internal geometry and inclination of the ports of the SEN on the stability of the steel flow in the mold of a continuous casting machine have been assessed through mathematical and physical modeling. Experimental data obtained through physical modeling are grouped by means of the F parameter, defined from the values of casting speed, the depth of the impact point of the steel jet against the narrow face and angle of the steel jet. Numerical simulations allow to estimate the subsurface velocity for the test conditions used in the model. The results of physical and mathematical modeling allow the identification of the ideal casting conditions for two nozzle configurations.

Diffusion and scaling of the velocity and vorticity in a thermoelectric driven heating and cooling experimental device is presented in order to map the different patterns and transitions between two and three dimensional convection in an enclosure with complex driven flows. The size of the water tank is of 0.2 x 0.2 x 0.1 m and the heat sources or sinks can be regulated both in power and sign [1-3]. The thermal convective driven flows are generated by means of Peltier effects in 4 wall extended positions of 0.05 x 0.05 cm each. The parameter range of convective cell array varies strongly with the Topology of the boundary conditions. Side heat and momentum fluxes are a function of Rayleigh, Peclet and Nusselt numbers, [4-6] Visualizations are performed by PIV, Particle tracking and shadowgraph. The structure of the flow is shown by setting up a convective flow generated by buoyant heat fluxes. The experiments described here investigate high Prandtl number mixing using brine and fresh water in order to form a density interface and low Prandtl number mixing with temperature gradients. The evolution of the mixing fronts are compared and the topological characteristics of the merging of the convective structures are examined for different configurations. Based on two dimensional Vorticity spectral analysis, new techniques can be very useful to determine the evolution of scales considering the multi-fractal structure of the convective flows.

The transition to turbulence in a stably stratified flow is a problem of considerable interest in fluid dynamics with applications in both geophysical sciences and engineering. This transition is controlled by competition between the vertical shear of the base flow and the buoyancy forces due to the density stratification. The present work investigates numerically the effect of stable stratification on the development of a Kelvin-Helmholtz (KH) instability and the formation of streamwise vortices, which are developed after the saturation of the primary billows of KH. The Direct Numerical Simulation (DNS) technique was used to solve the complete Navier-Stokes equations, in the Boussinesq approximation. Numerical tests were done with different Richardson numbers and forced initial conditions for velocity fluctuations. The results showed that high stratification inhibits the pairing process, reduces the buoyancy flux, weakens the vertical motions, decreases the thickness of the mixing layer and affects the formation of streamwise vortices. The three-dimensional results demonstrated that the streamwise vortices are clearly formed in non-stratified cases. In the stratified cases, on the other hand, the streamwise vortices are weakened, due to the streamwise density gradient, which decrease the levels of vorticity in the billows of KH, while increase in the braid zone.

The effect of collisions on the particle behavior in a fully developed turbulent flow in a straight square duct at Re-r = 300 is numerically investigated. The hydrodynamic modeling of the fluid phase is based on direct numerical simulation. The kinematics and trajectory of the particles as well as the collisions are described by the discrete element method. Three sizes of particles are considered with diameters equal to 50 mu m, 100 mu m and 500 mu m. Firstly, the particle transportation by turbulent flow is studied in the absence of the gravitational effect. It is found that the collisions play an important role in the particle distribution especially in the near-wall regions. The inter-particle collisions enhance the particle diffusion in the direction perpendicular to streamwise flow and make the particles distribute more uniformly near the wall. Then, the particle deposition is studied under the effect of the wall-normal gravity force in which the influence of collisions on the particle resuspension rate and the final stage of particle distribution on the duct floor are discussed, respectively. The collisions are found to have influence on the particle resuspension rate near the duct floor whereas hardly affect the particle behavior near the duct center. Under the gravitational effect the 50 mu m particles deposit more efficiently near the side walls but the 100 mu m and 500 mu m particles preferentially deposit near the center of the duct floor. Moreover, all the sizes of particles tend to concentrate near the center of the duct floor at the final stage of the particle deposition when the inter-particle collisions are considered. This work has been financially supported by the Ministerio de Ciencia e Innovación, Spain (ENE2010-17801). Hao Zhang would like to acknowledge the FI-AGAUR doctorate scholarship granted by the Secretaria d'Universitats i Recerca (SUR) del Departament d'Economia i Coneixement (ECO) de la Generalitat de Catalunya, and by the European Social Fund. F. Xavier Trias would like to thank the financial support by the Ramón y Cajal postdoctoral contracts (RYC-2012- 11996) by the Ministerio de Ciencia e Innovación. Yuanqiang Tan would like to thank the Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province [2012] 318. The calculations have been performed on the IBM MareNostrum supercomputer at the Barcelona Supercomputing Center. The authors thankfully acknowledge these institutions. We also thank the anonymous reviewers for their comments and remarks which helped to improve the quality of this work.

Published under licence in IOP Conference Series: Materials Science and Engineering by IOP Publishing Ltd. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. The aim of this paper is to carry out a group of numerical experiments over the fluid flow through a valve reed, using the CFD&HT code TermoFluids, an unstructured and parallel object-oriented CFD code for accurate and reliable solving of industrial flows. Turbulent flow and its solution is a very complex problem due to there is a non-lineal interaction between viscous and inertial effects further complicated by their rotational nature, together with the three-dimensionality inherent in these types of flow and the non-steady state solutions. In this work, different meshes, geometrical conditions and LES turbulence models (WALE, VMS, QR and SIGMA) are tested and results compared. On the other hand, the fluid flow boundary conditions are obtained by means of the numerical simulation model of hermetic reciprocating compressors tool, NEST-compressor code. The numerical results presented are based on a specific geometry, where the valve gap opening percentage is 11% of hole diameter and Reynolds numbers given by the one-dimensional model is 4.22 × 105, with density meshes of approximately 8 million CVs. Geometrical aspects related with the orifice's shape and its influence on fluid flow behaviour and pressure drop are analysed in detail, furthermore, flow results for different valve openings are also studied.

An exhaustive study, based on numerical three-dimensional simulations, of the Boussinesq thermal convection of a fluid confined in a rotating spherical shell is presented. A moderately low Prandtl number fluid (ro = 0.1) bounded by differentially-heated solid spherical shells is mainly considered. Asymptotic power laws for the mean physical properties of the flows are obtained in the limit of low Rossby number and compared with laboratory experiments and with previous numerical results computed by taking either stress-free boundary conditions or quasi-geostrophic restrictions, and with geodynamo models. Finally, using parameters as close as possible to those of the Earth's outer core, some estimations of the characteristic time and length scales of convection are given. © 2014 Elsevier B.V.

It is well known that the flow past a circular cylinder at critical Reynolds number combines flow separation, turbulence transition, reattachment of the flow, and further turbulent separation of the boundary layer. The transition to turbulence in the boundary layer causes the delaying of the separation point and an important reduction of the drag force on the cylinder surface known as the drag crisis. In the present work, large-eddy simulations of the flow past a cylinder at Reynolds numbers in the range 2.5 × 105-6.5 × 105 are performed. It is shown how the pressure distribution changes as the Reynolds number increases in an asymmetric manner, occurring first on one side of the cylinder and then on the other side to complete the drop in the drag up to 0.23 at Re = 6.5 × 105. These variations in the pressure profile are accompanied by the presence of a small recirculation bubble, observed as a small plateau in the pressure, and located around ¿ = 105° (measured from the stagnation point). This small recirculation bubble anticipated by the experimental measurements is here well captured by the present computations and its position and size measured at every Reynolds number. The changes in the wake configuration as the Reynolds number increases are also shown and their relation to the increase in the vortex shedding frequency is discussed. The power spectra for the velocity fluctuations are computed. The analysis of the resulting spectrum showed the footprint of Kelvin-Helmholtz instabilities in the whole range. It is found that the ratio of these instabilities frequency to the primary vortex shedding frequency matches quite well the scaling proposed by Prasad and Williamson [“The instability of the separated shear layer from a bluff body,” Phys. Fluids 8, 1347 (1996); “The instability of the shear layer separating from a bluff body,” J. Fluid Mech. 333, 375–492 (1997)] (f KH /fvs ¿ Re 0.67).

The present contribution describes three-dimensional Euler=Lagrange calculations of confined horizontal gas-particle flows emphasizing the importance of elementary processes, such as particle collisions with rough walls and interparticle collisions, on the predicted two-phase flow variables and pressure drop along the duct. In the chosen configuration the pneumatic conveying of spherical particles along a 6 m long horizontal channel with rectangular cross section is described from a numerical perspective. Calculations were carried out for spherical glass beads of different diameters (130 and 195 lm) with a mass loading of 1.0 (kg particles=kg gas). Additionally, different wall roughnesses were considered. In the experiments, the air volume flow rate was constant to maintain a fixed gas average velocity of 20 m=s. The numerical computations were performed by the Euler=Lagrange approach in connection with a Reynolds stress turbulence model accounting for two-way coupling and interparticle collisions. For the calculation of particle motion all relevant forces (i.e., drag, slip-shear and slip-rotational lift, and gravity), interparticle collisions and particle-rough wall collisions were considered. The agreement of the computations with the experiments of Sommerfeld and Kussin (2004) was found to be satisfactory for pressure drop and mean and fluctuating velocities of both phases as well as for the normalized particle mass flux

Natural convection in a differentially heated cavity is characterized by different phenomena such as laminar to turbulent flow transition in the boundary layer, turbulent mixing, and thermal stratification in the core of the cavity. In order to predict the thermal and fluid dynamic behavior of the flow in these cavities, the location of transition to turbulence should be accurately determined. In this work, the performance of three subgrid-scale (SGS) models is submitted to investigation in a water-filled cavity of aspect ratio 5 at Rayleigh number Ra = 3 × 10 11 . To do so, the models are compared with the solution obtained by means of direct numerical simulation. The models tested are: (i) the wall-adapting local-eddy viscosity (WALE) model, (ii) the QR model, (iii) the WALE model within a variational multiscale framework (VMS-WALE). It has been shown that the VMS-WALE and WALE models perform better in estimating the location of transition to turbulence, and thus their overall behavior is more accurate than the QR model. The results have also revealed that the use of SGS models is justified in this flow as the transition location and consequently the flow structure cannot be captured properly if no model is used for the tested spatial resolution.

Parabolic trough solar collectors are currently one of the most mature and prominent solar technology for the production of electricity. These systems are usually located in an open terrain where strong winds may be found, and could affect their stability and optical performance, as well as the heat exchange between the solar receiver and the ambient air. In this context, a wind flow analysis around a parabolic trough solar collector under real working conditions is performed. A numerical aerodynamic and heat transfer study based on Large Eddy Simulations is carried out to characterise the wind loads and heat transfer coefficients. After the study carried out by the authors in an earlier work (Hachicha et al. 2013) at ReW1=3.9e5, computations are performed at a higher Reynolds number of ReW2=1e6, and for various pitch angles. The effects of wind speed and pitch angle on the averaged and instantaneous flow are assessed. The aerodynamic coefficients are calculated around the solar collector and validated with measurements performed in wind tunnel tests. The variation of the heat transfer coefficient around the heat collector element with the Reynolds number is presented and compared to the circular cylinder in cross-flow. The unsteady flow is studied for three pitch angles: 0 ; 45 and 90 and different structures and recirculation regions are identified. A spectral analysis around the parabola and its receiver is also carried out in order to detect the most relevant frequencies related to the vortex shedding mechanism which affects the stability of the collector.

For a molecular cloud clump to form stars some transport of magnetic flux is required from the denser, inner regions to the outer regions of the cloud, otherwise this can prevent the collapse. Fast magnetic reconnection which takes place in the presence of turbulence can induce a process of reconnection diffusion (RD). Extending earlier numerical studies of reconnection diffusion in cylindrical clouds, we consider more realistic clouds with spherical gravitational potentials and also account for the effects of the gas self-gravity. We demonstrate that within our setup RD is efficient. We have also identified the conditions under which RD becomes strong enough to make an initially subcritical cloud clump supercritical and induce its collapse. Our results indicate that the formation of a supercritical core is regulated by a complex interplay between gravity, self-gravity, the magnetic field strength and nearly transonic and trans-Alfv\'enic turbulence, confirming that RD is able to remove magnetic flux from collapsing clumps, but only a few of them become nearly critical or supercritical, sub-Alfv\'enic cores, which is consistent with the observations. Besides, we have found that the supercritical cores built up in our simulations develop a predominantly helical magnetic field geometry which is also consistent with observations. Finally, we have evaluated the effective values of the turbulent reconnection diffusion coefficient and found that they are much larger than the numerical diffusion, especially for initially trans-Alfv\'enic clouds, ensuring that the detected magnetic flux removal is due to to the action of the RD rather than to numerical diffusivity., Comment: 24 pages, 18 figures, accepted for publication in the ApJ

Recent technology advancements in Particle Image Velocimetry (PIV) technique offers a new possibility to study the complex turbulent flows over a wide range of scales. It can be used for the estimation of the turbulent kinetic energy dissipation rate including spatial derivatives of the velocity fluctuation components. In this study the flow characteristics of an external gear pump were studied with time-resolved Particle Image Velocimetry. Measurements were performed for two different spatial resolution configurations and various interrogation areas to estimate the dissipation rate of the turbulent kinetic energy. Results show that the spatial resolution is a critical factor in the accuracy of the measurements in the dissipation rate estimations. The optimal results can be achieved if the interrogation area is neither too large (sampling phenomenon) nor too small (noise in the measurement data). The effect of number of samples employed in the ensemble-averaging was considered as well.

The present research analyzes the asymmetry in the rolling up shear layers behind the blunt trailing edge of an airfoil 4412 with a miniflap acting as active flow control device and its wake organization. Experimental investigations relating the asymmetry of the vortex flow in the near wake region, able to distort the flow increasing the downwash of an airfoil, have been performed. All of these in a free upstream turbulent flow (1.8% intensity). We examine the near wake region characteristics of a wing model with a 4412 airfoil without and with a rotating miniflap located on the lower surface, near the trailing edge. The flow in the near wake, for 3 x-positions (along chord line) and 20 vertical points in each x-position, was explored, for three different rotating frequencies, in order to identify signs of asymmetry of the initial counter rotating vortex structures. Experimental evidence is presented showing that for typical lifting conditions the shear layer rollup process within the near wake is different for the upper and lower vortices: the shear layer separating from the pressure side of the airfoil begins its rollup immediately behind the trailing edge, creating a stronger vortex while the shear layer from the suction side begins its rollup more downstream creating a weaker vortex. The experimental data were processed by classical statistics methods. Aspects of a mechanism connecting the different evolution and pattern of these initial vortex structures with lift changes and wake alleviating processes, due to these miniflaps, will be studied in future works., Laboratorio de Capa Límite y Fluidodinámica Ambiental

The Prandtl mixing length model has been used to take into account the turbulent effects in a 30 A high-energy density cutting torch model. In particular, the model requires the introduction of only one adjustable coefficient c corresponding to the length of action of the turbulence. It is shown that the c value has little effect on the plasma temperature profiles outside the nozzle (the differences being less than 10 %), but severely affects the plasma velocity distribution, with differences reaching about 100 % at the middle of the nozzle-anode gap. Within the experimental uncertainties it was also found that the value c = 0.08 allows to reproduce both, the experimental data of velocity and temperature. Fil: Mancinelli, Beatriz Rosa. Universidad Tecnológica Nacional. Facultad Reg.venado Tuerto. Departamento de Ing.electromecanica. Laboratorio de Descargas Eléctricas; Argentina Fil: Minotti, Fernando Oscar. Consejo Nacional de Investigaciones Científicas y Técnicas. Universidad de Buenos Aires; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física del Plasma. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física del Plasma; Argentina Fil: Kelly, Hector Juan. Consejo Nacional de Investigaciones Científicas y Técnicas. Universidad de Buenos Aires; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física del Plasma. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física del Plasma; Argentina

Time-Resolved Particle Image Velocimetry (TRPIV) has been used to investigate the flow inside the suction chamber of an external gear pump where the movement of the fluid through the pump is maintained by the rotation of the gears. The main purpose of this paper is to study the characteristics of the complex flow pattern of this pump system in order to help in improving its total performance. The applied experimental techniques establish a method that allows visualising the flow inside the gear pump with a high rotational velocity system. Small micro air bubbles have been used as flow seeding. The images have been processed using domestic PIV software that uses specific aspects of the TRPIV technique. Instantaneous and phase-locked ensemble average fluid motions have been obtained for different gear pump rotational velocities to investigate the turbulence effect of rotating gears to the system.

By simulations of the Barkley model, action of uniform periodic nonresonant forcing on scroll rings and wave turbulence in three-dimensional excitable media is investigated. Sufficiently strong rapid forcing converts expanding scroll rings into the collapsing ones and suppresses the Winfree turbulence caused by the negative tension of wave filaments. Slow strong forcing has an opposite effect, leading to expansion of scroll rings and induction of the turbulence. These effects are explained in the framework of the phenomenological kinematic theory of scroll waves.

Rotating scroll waves are dynamical spatiotemporal structures characteristic of three-dimensional activemedia. It is well known that, under low excitability conditions, scroll waves develop an intrinsically unstabledynamical regime that leads to a highly disorganized pattern of wave propagation. Such a “turbulent” statebears some resemblance to fibrillation states in cardiac tissue. We show here that this unstable regime can becontrolled by using a spatially distributed random forcing superimposed on a control parameter of the system.Our results are obtained from numerical simulations but an explicit analytical argument that rationalizes ourobservations is also presented.DOI: 10.1103/PhysRevE.70.067201 PACS number(s): 82.40.Bj, 05.40.Ca, 47.54.1r

Winfree turbulence of scroll waves is a special kind of spatiotemporal chaos that exists exclusively in three-dimensional excitable media and is currently considered one of the principal mechanisms of cardiac fibrillation. A chaotic wave pattern develops through the negative-tension instability of vortex filaments, which tend to spontaneously stretch, bend, loop, and produce an expanding tangle that fills up the volume. We demonstrate that such turbulence can readily be controlled by weak nonresonant modulation of the medium excitability. Depending on the forcing frequency, either suppression or induction of turbulence can be achieved.

The transition to three-dimensional and unsteady flow in an annulus with a discrete heat source on the inner cylinder is studied numerically. For large applied heat flux through the heater (large Grashof number Gr), there is a strong wall plume originating at the heater that reaches the top and forms a large scale axisymmetric wavy structure along the top. For Gr approximate to 6 x 109, this wavy structure becomes unstable to three-dimensional instabilities with high azimuthal wavenumbers m similar to 30, influenced by mode competition within an Eckhaus band of wavenumbers. Coexisting with some of these steady three-dimensional states, solution branches with localized defects break parity and result in spatio-temporal dynamics. We have identified two such time dependent states. One is a limit cycle that while breaking spatial parity, retains spatio-temporal parity. The other branch corresponds to quasi-periodic states that have globally broken parity. (C) 2014 AIP Publishing LLC.

From the conservation equation of the quantity of movement and the kinetic energy of the studied turbulence, the dynamics of flat and turbulent path in a non comprehensive liquid environment. In the case of a liquid flow of a path, it is proved that the considered models is well formulated, and the asymptotic properties of the solution are studied. A partir de la ecuación de la conservación de la cantidad de movimiento y de la energía cinética de la turbulencia estudiada, la dinámica de una huella plana y turbulenta en un medio líquido incomprensible. Para el caso de una corriente líquida de una huella lejana se demuestra que el modelo considerado está bien planteado, y como se estudian las propiedades asintóticas de la solución.