Evaporative cooling by water spray is increasingly used as an efficient and environmentally-friendly approach to enhance thermal comfort in built environments. The complex two-phase flow in a water ...spray system is influenced by many factors such as continuous phase velocity, temperature and relative humidity patterns, droplet characteristics and continuous phase–droplet and droplet–droplet interactions. Computational Fluid Dynamics (CFD) can be a valuable tool for assessing the potential and performance of evaporative cooling by water spray systems in outdoor and indoor urban environments. This paper presents a systematic evaluation of the Lagrangian–Eulerian approach for evaporative cooling provided by the use of a water spray system with a hollow-cone nozzle configuration. The evaluation is based on grid-sensitivity analysis and validated using wind-tunnel measurements. This paper also presents a sensitivity analysis focused on the impact of the turbulence model for the continuous phase, the drag coefficient model, the number of particle streams for the discrete phase and the nozzle spray angle. The results show that CFD simulation of evaporation by the Lagrangian–Eulerian (3D steady RANS) approach, in spite of its limitations, can accurately predict the evaporation process, with local deviations from the wind-tunnel measurements within 10% for dry bulb temperature, 5% for wet bulb temperature and 7% for the specific enthalpy. The average deviations for all three variables are less than 3% in absolute values. The results of this paper are intended to support future CFD studies of evaporative cooling by water spray systems in outdoor and indoor urban environments.
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•CFD simulation of evaporative cooling by water spray systems.•Grid-sensitivity analysis and validation with wind-tunnel measurements.•Lagrangian–Eulerian approach can accurately simulate the evaporation process.•Maximum local deviations: 10% for dry bulb temperature, 5% for wet bulb temperature and 7% for specific enthalpy.
Topology is introducing new tools for the study of fluid waves. The existence of unidirectional Yanai and Kelvin equatorial waves has been related to a topological invariant, the Chern number, that ...describes the winding of
$f$
-plane shallow water eigenmodes around band-crossing points in parameter space. In this previous study, the topological invariant was a property of the interface between two hemispheres. Here we ask whether a topological index can be assigned to each hemisphere. We show that this can be done if the shallow water model in the
$f$
-plane geometry is regularized by an additional odd-viscosity term. We then compute the spectrum of a shallow water model with a sharp equator separating two flat hemispheres, and recover the Kelvin and Yanai waves as two exponentially trapped waves along the equator, with all the other modes delocalized into the bulk. This model provides an exactly solvable example of bulk-interface correspondence in a flow with a sharp interface, and offers a topological interpretation for some of the transition modes described by Iga (J. Fluid Mech., vol. 294, 1995, pp. 367–390). It also paves the way towards a topological interpretation of coastal Kelvin waves along a boundary and, more generally, to an understanding of bulk-boundary correspondence in continuous media.
The position of window openings and roof inclination are important parameters determining the effectiveness of wind-driven cross-ventilation in buildings. Many studies on natural ventilation have ...been performed in the past, however, a detailed review of the literature indicates that the majority of these studies focused on flat roofs with symmetric opening positions. There is a lack of research that analyzes the impact of asymmetric opening positions and roof inclination on natural ventilation potential. This paper presents Computational Fluid Dynamics (CFD) simulations to analyze the natural ventilation flow in a generic isolated building with different vertical positions of the outlet opening – yielding asymmetric opening positions – and five different roof inclination angles. The simulations are performed using the 3D steady Reynolds-Averaged Navier–Stokes (RANS) equations. They are based on a grid-sensitivity analysis and on validation with previously published wind-tunnel measurements using Particle Image Velocimetry. The results show that the shear-stress transport (SST) k-ω and the Renormalization-group (RNG) k-ε turbulence models provide the best agreement with the experimental data. It is also shown that the roof inclination angle has a significant effect on the ventilation flow; the volume flow rate increases by more than 22%. The maximum local indoor air velocity increases considerably when the inclination angle is increased, however, the differences in the average velocity in the occupied zone are only around 7%. The vertical position of the outlet opening has a relatively small impact on the volume flow rate (less than 4%), and a small influence on the average velocity in the occupied zone (<5%).
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•CFD simulations of cross ventilation with 3D steady RANS and various turbulence models.•Influence of roof inclination angle and outlet opening position.•Grid-sensitivity analysis and validation with PIV measurements.•SST k-ω model and RNG k-ε model show best performance.•Roof inclination angle has much larger effect than position of outlet opening.
A new model implemented in the neptune_cfd code is used to study steam condensation in the presence of air in a mini-channel of opening 100µm. This model is the Full Generalized Large Interface ...Model. It combines a sub-grid approach and an interface capturing approach. The sub-grid approach is used to compute the small flow structures, which can be small bubbles or droplets, while the interface capturing approach is used to compute the large flow structures, which can be gas or liquid films, large bubbles or large drops. It is conceived to simulate a wide range of flow configurations, without the need for the grid cells to be smaller than the smallest flow structures. The results show that the model is able to reproduce all the flow patterns experimentally observed in mini-channels in the analyzed range of control parameters. Both the pressure drop and the Nusselt number (dimensionless wall heat flux) are found to be in good agreement with experimental correlations from the literature.
•A new model has been implemented in the neptune_cfd code.•It is dedicated to flows with small droplets, small bubbles and large interfaces.•It combines a sub-grid approach and an interface capturing approach.•Its ability to predict steam condensation in a mini-channel is assessed.•The results are found to be consistent with correlations from the literature.
Where two quantum fluids meet Yoon, Taehyun; Kim, Na Young
Nature photonics,
07/2019, Letnik:
13, Številka:
7
Journal Article
Recenzirano
Odprti dostop
Josephson vortices are observed at the boundary between two exciton-polariton condensates, with lasers used to create the required local phase twist. The finding opens new opportunities for exploring ...fundamental physics and engineering novel quantum devices.
Dilute suspensions of repulsive particles exhibit a Newtonian response to flow that can be accurately predicted by the particle volume fraction and the viscosity of the suspending fluid. However, ...such a description fails when the particles are weakly attractive. In a simple shear flow, suspensions of attractive particles exhibit complex, anisotropic microstructures and flow instabilities that are poorly understood and plague industrial processes. One such phenomenon, the formation of log-rolling flocs, which is ubiquitously observed in suspensions of attractive particles that are sheared while confined between parallel plates, is an exemplar of this phenomenology. Combining experiments and discrete element simulations, we demonstrate that this shear-induced structuring is driven by hydrodynamic coupling between the flocs and the confining boundaries. Clusters of particles trigger the formation of viscous eddies that are spaced periodically and whose centers act as stable regions where particles aggregate to form flocs spanning the vorticity direction. Simulation results for the wavelength of the periodic pattern of stripes formed by the logs and for the log diameter are in quantitative agreement with experimental observations on both colloidal and noncolloidal suspensions. Numerical and experimental results are successfully combined by means of rescaling in terms of a Mason number that describes the strength of the shear flow relative to the rupture force between contacting particles in the flocs. The introduction of this dimensionless group leads to a universal stability diagram for the log-rolling structures and allows for application of shear-induced structuring as a tool for assembling and patterning suspensions of attractive particles.
A walk-in type .sup.222Rn calibration chamber (~ 22 m.sup.3) is established at the Centre for Advanced Research in Environmental Radioactivity (CARER), Mangalore University, India which is being used ...by research groups working on .sup.222Rn in India and other countries as well. In recent times, computational fluid dynamics (CFD) technique is opted as an alternative approach for the prediction of .sup.222Rn concentration profile in the closed domain. CFD simulations were carried out to study the transient build-up and spatial behavior of .sup.222Rn concentration in the calibration chamber. Measurements were performed using active .sup.222Rn measuring devices and results of the CFD predictions and direct measurements were compared. A good agreement was observed between the simulated and experimental results with deviation between the two entities being ~ 3% in the case of transient build up and ~ 8% in the case of spatial distribution of .sup.222Rn concentration.
The topological equivalent aerodynamic method is introduced to design a funnel-shape galloping energy harvester with wide working wind-speed range and high normalized harvesting power. The funnel ...shape bluff body is designed from the square bluff body to avoid the vortex reattachment, enhance the structural non-stream fluid flow and allow the pressure direction along the lift force. This design enlarges the aerodynamic force and thus improves the energy harvesting efficiency. To analyze the harvesting performance, the extended Hamilton principle and Gauss law are employed to derive the electro-mechanical coupled governing equations. The Galerkin procedure and equivalent structure method are then used to calculate the expressions of the onset galloping wind speed and harvested power density. Three energy harvesters of the square, triangular and funnel-shaped bluff bodies are tested in a closed direct-flow wind tunnel. The maximal experimental power densities for the funnel-shape, triangle and square bluff bodies are respectively 2.34 mW/cm3, 1.56 mW/cm3 and 0.207 mW/cm3. The corresponding experimental onset galloping wind speeds are 7 m/s, 9 m/s and 13 m/s. The good agreement between the theoretical prediction and experimental result demonstrates the accuracy of the mathematical model. The parameter analysis using the analytical model indicates that the energy harvester with funnel-shape bluff body always maintains the smallest onset speed and the highest power density. Besides, compared with previous studies of energy harvesting from the wind, the proposed energy harvester has the largest normalized power density. To better understand the fluid dynamics, two-dimensional unsteady numerical simulation is employed to show that the funnel shape has the largest lift coefficient compared to the other two cases. The velocity and pressure contours explain the physical cause that the flow pattern of the funnel shape can maintain the longest vortex region and highest vortex intensity.
•Topological equivalent aerodynamics is used to improve galloping energy harvester.•Proposed fluid-electro-mechanical coupled model is validated by wind-tunnel tests.•Funnel-shape bluff body maintains smallest onset speed and highest power density.•Unsteady simulation shows high pressure difference and strong vortex intensity.
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