Third-moment closure of turbulence for predictions of separating and reattaching shear flows: A study of Reynolds-stress closure model

A numerical study of computations in backward-facing steps with flow separation and reattachment, using the Reynolds stress closure is presented. The highlight of this study is the improvement of the Reynold-stress model (RSM) by modifying the diffusive transport of the Reynolds stresses through the formulation, solution and subsequent incorporation of the transport equations of the third moments, bar-u(i)u(j)u(k), into the turbulence model. The diffusive transport of the Reynolds stresses, represented by the gradients of the third moments, attains greater significance in recirculating flows. The third moments evaluated by the development and solution of the complete transport equations are superior to those obtained by existing algebraic correlations. A low-Reynolds number model for the transport equations of the third moments is developed and considerable improvement in the near-wall profiles of the third moments is observed. The values of the empirical constants utilized in the development of the model are recommended. The Reynolds-stress closure is consolidated by incorporating the equations of k and e, containing the modified diffusion coefficients, and the transport equations of the third moments into the Reynolds stress equations. Computational results obtained by the original k-e model, the original RSM and the consolidated and modified RSM are compared with experimental data. Overall improvement in the predictions is seen by consolidation of the RMS and a marked improvement in the profiles of bar-u(i)u(j)u(k) is obtained around the reattachment region

Improvement of the second- and third-moment modeling of turbulence: A study of Reynolds-stress closure model

Four parts of the Reynolds-stress closure modeling are reported: (1) improvement of the k and epsilon equaitons; (2) development of the third-moment transport equation; (3) formulation of the diffusion coefficient of the momentum equation by using the algebraic-stress model of turbulence; and (4) the application of the Reynolds-stress model to a heat exchanger problem. It was demonstrated that the third-moment transport model improved the prediction of the triple-velocity products in the recirculating and reattaching flow regions in comparison with the existing algebraic models for the triple-velocity products. Optimum values for empirical coefficients are obtained for the prediction of the backward-facing step flows. A functional expression is derived for the coefficient of the momentum diffusion by employing the algebraic-stress model. The second-moment closure is applied to a heat transfer problem. The computations for the flow in a corrugated-wall channel show that the second-moment closure improves the prediction of the heat transfer rates by 30% over the k - epsilon model

A study of Reynolds-stress closure model

A hybrid model of the Reynolds stress closure was developed. This model was tested for various sizes of step flow, and the computed Reynolds stress behavior was compared with experimental data. The third order closure model was reviewed. Transport equations for the triple velocity correlation were developed and implemented in a numerical code to evaluate the behavior of the triple velocity products in various regions of the flow field including recirculating, reattaching, and redeveloping flow domains

Numerical study of a separating and reattaching flow by using Reynolds-stress tubulence closure

The numerical study of the Reynolds-stress turbulence closure for separating, reattaching, recirculating and redeveloping flow is summarized. The calculations were made for two different closure models of pressure - strain correlation. The results were compared with the experimental data. Furthermore, these results were compared with the computations made by using the one layer and three layer treatment of k-epsilon turbulence model which were developed. Generally the computations by the Reynolds-stress model show better results than those by the k-epsilon model, in particular, some improvement was noticed in the redeveloping region of the separating and reattaching flow in a pipe with sudden expansion

A transport model of the turbulent scalar-velocity

Performance tests of the third-order turbulence closure for predictions of separating and recirculating flows in backward-facing steps were studied. Computations of the momentum and temperature fields in the flow domain being considered entail the solution of time-averaged transport equations containing the second-order turbulent fluctuating products. The triple products, which are responsible for the diffusive transport of the second-order products, attain greater significance in separating and reattaching flows. The computations are compared with several algebraic models and with the experimental data. The prediction was improved considerably, particularly in the separated shear layer. Computations are further made for the temperature-velocity double products and triple products. Finally, several advantages were observed in the usage of the transport equations for the evaluation of the turbulence triple products; one of the most important features is that the transport model can always take the effects of convection and diffusion into account in strong convective shear flows such as reattaching separated layers while conventional algebraic models cannot account for these effects in the evaluation of turbulence variables

Turbulent transport modeling of shear flows around an aerodynamic wing. Development of turbulent near-wall model and its application to recirculating flows

Progress in implementing and refining two near-wall turbulence models in which the near-wall region is divided into either two or three zones is outlined. These models were successfully applied to the computation of recirculating flows. The research was further extended to obtaining experimental results of two different recirculating flow conditions in order to check the validity of the present models. Two different experimental apparatuses were set up: axisymmetric turbulent impinging jets on a flat plate, and turbulent flows in a circular pipe with a abrupt pipe expansion. It is shown that generally better results are obtained by using the present near-wall models, and among the models the three-zone model is superior to the two-zone model

Investigation of Flow Behavior around Corotating Blades in a Double-Spindle Lawn Mower Deck

When the airflow patterns inside a lawn mower deck are understood, the deck can be redesigned to be efficient and have an increased cutting ability. To learn more, a combination of computational and experimental studies was performed to investigate the effects of blade and housing designs on a flow pattern inside a 1.1m wide corotating double-spindle lawn mower deck with side discharge. For the experimental portion of the study, air velocities inside the deck were measured using a laser Doppler velocimetry (LDV) system. A high-speed video camera was used to observe the flow pattern. Furthermore, noise levels were measured using a sound level meter. For the computational fluid dynamics (CFD) work, several arbitrary radial sections of a two-dimensional blade were selected to study flow computations. A three-dimensional, full deck model was also developed for realistic flow analysis. The computational results were then compared with the experimental results.</p

The importance of globalisation in driving the introduction and establishment of alien species in Europe

Understanding the role of globalisation in promoting introduction and establishment of alien species is an important step towards successful management of biological invasions. We aimed to quantify the taxon-dependent association of globalisation with the introduction and establishment of alien species in Europe. The availability of the KOF Index of Globalisation that measures all economic, social and political dimensions of global connectivity enables a study of this factor. Based on an extensive database of alien species, we used model selections based on the Akaike Information Criterion and hierarchical partitioning to identify the importance of globalisation in predicting the number of all introduced species and established species of ten mainly terrestrial taxa in countries across Europe. The association of globalisation with alien species establishment varied depending on taxon type. While the gross domestic product (GDP) of countries was a strong predictor for all but one taxon, globalisation was also found to be an important predictor for three taxa including those of high (e.g. insects) and low mobility (e.g. magnoliophyta). Globalisation explained 3.1 to 22 % independently, and 5.5 to 35 % jointly with other variables, of among-country variations in the number of established alien species. The effect of globalisation on the distribution of all introduced species is not substantially different from that on the established alien species. This study highlights how globalisation among habitat availability and environmental conditions can determine the patterns of alien species introduction and establishment across Europe. The results also emphasise the varying degree of importance between different taxa. Knowledge of the relative significance of various pathways with regard to different taxa is important for correctly focusing efforts to reduce the spread of these species.T.A. is financially supported by the European Commission’s Marie Curie International Incoming Fellowship Programme (PIIF-GA-2011-303221). K.S.-H.P. is supported by the IFLS Fellowship at the University of Southampton.This is the author accepted manuscript. The final version is available from Wiley via http://dx.doi.org/10.1111/ecog.0189

Reducing Resonant Vibration of a Rotor by Tuning the Gap between a Superconducting Bulk and a Permanent Magnet

AbstractThis study investigated passing through a critical speed of a rotor supported by a superconductor with an electromagnet. Here we adopted the idea that the gap between the superconductor and the rotor can be tuned variably by using electromagnetic force of the electromagnet so that the natural frequency or the stiffness can be changed. By using this method, it can be expected that resonant vibration be reduced. We developed an analytical model and then carried out numerical simulation. Numerical results show that considerable reduction of the resonant amplitude can be achieved by proper tuning of switching the electromagnet

Relativistic Electron Shock Drift Acceleration in Low Mach Number Galaxy Cluster Shocks

An extreme case of electron shock drift acceleration in low Mach number collisionless shocks is investigated as a plausible mechanism of initial acceleration of relativistic electrons in large-scale shocks in galaxy clusters where upstream plasma temperature is of the order of 10 keV and a degree of magnetization is not too small. One-dimensional electromagnetic full particle simulations reveal that, even though a shock is rather moderate, a part of thermal incoming electrons are accelerated and reflected through relativistic shock drift acceleration and form a local nonthermal population just upstream of the shock. The accelerated electrons can self-generate local coherent waves and further be back-scattered toward the shock by those waves. This may be a scenario for the first stage of the electron shock acceleration occurring at the large-scale shocks in galaxy clusters such as CIZA J2242.8+5301 which has well defined radio relics.Comment: 26 pages, 10 figures, accepted for publication in Ap