1,726,825 research outputs found
Numerical investigation of buoyancy effects on non-premixed impinging jet flame
No full textIn this work, buoyancy effects on non-premixing impinging jet flames were studied using three-dimensional direct numerical simulation. The physical problem investigated is a methane jet issuing into an oxidant ambient environment of air with the Reynolds number of 2500 and a nozzle to the downstream impinging plate distance of 4 jet nozzle diameters. The reactive flow field is described by the compressible time-dependent Navier-Stokes equations in its non-dimensional form. A Cartesian grid system with approximately 134 million cells was used with uniform grid distributions in each direction. The comparisons between the buoyant flame and the non-buoyant flame revealed that the two flames have large differences at the primary jet shear layer and the impinging wall regions. Both velocity and temperature distributions demonstrate buoyancy effects on flame dynamics where the flow develops into large vortical structures. It has been found that the buoyant flames involve complex fluid dynamics interactions in addition to flow characteristics associated with the chemical heat release. The vortex dynamics including buoyancy instability under impinging conditions has been investigated
Ideal schedules in parallel machine settings
No full textAn ideal schedule is a schedule that simultaneously minimizes the two most popular scheduling objectives, namely the makespan and the total completion time. If a scheduling problem always has an ideal schedule, then the problem is called an ideal problem. We summarize ideal problem results of various scheduling problems in different machine environments and with job characteristics that include precedence constraints, release dates, processing times, eligibility constraints and preemptions. We present a comprehensive overview of ideal schedules including our new findings. (C) 2020 Elsevier B.V. All rights reserved.11Nsciescopu
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Direct numerical simulation of transitional noncircular buoyant reactive jets
No full textThe near field dynamics of transitional buoyant reactive jets established on noncircular geometries, including a rectangular nozzle with an aspect ratio of 2:1 and a square nozzle with the same cross-sectional area, are investigated by three-dimensional spatial direct numerical simulations. Without applying external perturbations at the inflow boundary, large vortical structures develop naturally in the flow field due to buoyancy effects. Simulation results and analysis describe the details and clarify mechanisms of vortex dynamics of the noncircular buoyant reactive jets. The interaction between density gradients and gravity initiates the flow vorticity. Among the major vorticity transport terms, the gravitational term mainly promotes flow vorticity in the cross-streamwise direction. For the baroclinic torque, it can either create or destroy flow vorticity depending on the local flow structure. The vortex stretching term has different effects on the streamwise and cross-streamwise vorticity. Streamwise vorticity is mainly created by vortex stretching, while this term can either create or destroy cross-streamwise vorticity. Under the coupling effects of buoyancy and noncircular nozzle geometry, three-dimensional vortex interactions lead to the transitional behavior of the reactive jets. Simulations also show that the rectangular jet is more vortical than the square jet. The rectangular jet has a stronger tendency of transition to turbulence at the downstream due to the aspect ratio effect. Mean flow property calculations show that the rectangular buoyant reactive jet has a higher entrainment rate than its square counterpart
Spatial DNS of flow transition of a rectangular buoyant reacting free jet (in special issue on Second International Symposium on Turbulence and Shear Flow Phenomena)
No full textThis paper describes a spatial direct numerical simulation (DNS) of the flow transition of a buoyant diffusion flame established on a rectangular nozzle with an aspect ratio of 2:1. Combustion is represented by a one-step finite-rate Arrhenius chemistry. Without applying external perturbations, large vortical structures develop naturally in the flow field due to buoyancy effects. The vortex dynamics of the rectangular buoyant reacting jet has been analysed. The interaction between density gradients and gravity initiates the flow vorticity in the cross-streamwise directions. The streamwise vorticity is mainly generated by the vortex stretching. Downstream of the reacting jet, a more disorganized flow regime characterized by small scales has been observed, following the breakdown of the large vortical structures due to three-dimensional vortex interactions. Analysis of the energy spectra shows that the spatially developing reacting jet has a tendency of transition to turbulence under the effects of combustion-induced buoyancy. Buoyancy effects are found to be very important to the formation, development, interaction and breakdown of vortices. In contrast with the relaminarization effects of chemical exothermicity on non-buoyant jet diffusion flames via volumetric expansion and viscous damping, the tendency towards transition to turbulence in buoyant reacting jets is greatly enhanced by the overwhelming buoyancy effects. Calculations of the mean flow property show that the rectangular buoyant reacting jet has a higher entrainment rate than its non-reacting counterpart
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