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Fluids, Volume 3, Issue 4 (December 2018)

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Open AccessReview Acoustic Streaming and Its Applications
Received: 26 August 2018 / Revised: 29 September 2018 / Accepted: 7 December 2018 / Published: 18 December 2018
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Abstract
Broadly speaking, acoustic streaming is generated by a nonlinear acoustic wave with a finite amplitude propagating in a viscid fluid. The fluid volume elements of molecules, dV, are forced to oscillate at the same frequency as the incident acoustic wave. Due
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Broadly speaking, acoustic streaming is generated by a nonlinear acoustic wave with a finite amplitude propagating in a viscid fluid. The fluid volume elements of molecules, d V , are forced to oscillate at the same frequency as the incident acoustic wave. Due to the nature of the nonlinearity of the acoustic wave, the second-order effect of the wave propagation produces a time-independent flow velocity (DC flow) in addition to a regular oscillatory motion (AC motion). Consequently, the fluid moves in a certain direction, which depends on the geometry of the system and its boundary conditions, as well as the parameters of the incident acoustic wave. The small scale acoustic streaming in a fluid is called “microstreaming”. When it is associated with acoustic cavitation, which refers to activities of microbubbles in a general sense, it is often called “cavitation microstreaming”. For biomedical applications, microstreaming usually takes place in a boundary layer at proximity of a solid boundary, which could be the membrane of a cell or walls of a container. To satisfy the non-slip boundary condition, the flow motion at a solid boundary should be zero. The magnitude of the DC acoustic streaming velocity, as well as the oscillatory flow velocity near the boundary, drop drastically; consequently, the acoustic streaming velocity generates a DC velocity gradient and the oscillatory flow velocity gradient produces an AC velocity gradient; they both will produce shear stress. The former is a DC shear stress and the latter is AC shear stress. It was observed the DC shear stress plays the dominant role, which may enhance the permeability of molecules passing through the cell membrane. This phenomenon is called “sonoporation”. Sonoporation has shown a great potential for the targeted delivery of DNA, drugs, and macromolecules into a cell. Acoustic streaming has also been used in fluid mixing, boundary cooling, and many other applications. The goal of this work is to give a brief review of the basic mathematical theory for acoustic microstreaming related to the aforementioned applications. The emphasis will be on its applications in biotechnology. Full article
(This article belongs to the Special Issue Advances in Bubble Acoustics)
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Open AccessArticle Experimental Measurement of Steady and Transient Liquid Coiling with High-Speed Video and Digital Image Processing
Received: 19 October 2018 / Revised: 6 December 2018 / Accepted: 12 December 2018 / Published: 15 December 2018
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Abstract
Liquid coiling occurs as a viscous fluid flows into a stagnant reservoir causing a localized accumulation of settling material, which coils into a stack as it accumulates. These coiling flows are broadly characterized into three primary coiling regimes of viscous, gravitational, or inertial
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Liquid coiling occurs as a viscous fluid flows into a stagnant reservoir causing a localized accumulation of settling material, which coils into a stack as it accumulates. These coiling flows are broadly characterized into three primary coiling regimes of viscous, gravitational, or inertial coiling, based on the velocity of the falling fluid, the height of the fall, the radius of the fluid rope, the stack height, and the fluid properties including viscosity. A computer-controlled flow delivery apparatus was developed here to produce precisely controlled flow conditions to study steady and transitional coiling regimes with independently varied parameters. Data were recorded using high-speed digital video cameras and a purpose-built digital image processing routine to extract rope and stack dimensions as well as time-resolved coiling frequency. The precision of the setup and data analysis methods allowed a detailed study of the transition between gravitational and inertial flow regimes. The results show a smooth transition between the regimes, with no evidence of the inertial-gravitational regime. Unsteady coiling was able to be momentarily produced by applying a perturbation to the system, but the unstable regime quickly decayed to either the base inertial or gravitational regime. Full article
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Open AccessReview Rheological Behavior of Fresh Cement Pastes
Received: 7 November 2018 / Revised: 25 November 2018 / Accepted: 7 December 2018 / Published: 11 December 2018
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Abstract
Rheology of a concrete is mainly controlled by the rheological behavior of its cement paste. This is the main practical reason for the extensive research activity observed during 70 years in this research subfield. In this brief review, some areas of the research
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Rheology of a concrete is mainly controlled by the rheological behavior of its cement paste. This is the main practical reason for the extensive research activity observed during 70 years in this research subfield. In this brief review, some areas of the research on the rheological behavior of fresh cement pastes (mixture method influence, microstructure analysis, mineral additions influence, chemical additives influence, blended cements behavior, viscoelastic behavior, flow models, and flow behavior analysis with alternative methods) are examined. Full article
(This article belongs to the Special Issue Advances in Experimental and Computational Rheology)
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Open AccessArticle Numerical Study of Noise Barriers’ Side Edge Effects on Pollutant Dispersion near Roadside under Various Thermal Stability Conditions
Received: 26 October 2018 / Revised: 5 December 2018 / Accepted: 6 December 2018 / Published: 8 December 2018
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Abstract
Roadside noise barrier helps to reduce downwind pollutant concentrations from vehicle emission. This positive characteristic of the construction feature can be explained by its interaction with flow distribution and species dispersion. In this paper, a three-dimensional numerical model has been developed to simulate
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Roadside noise barrier helps to reduce downwind pollutant concentrations from vehicle emission. This positive characteristic of the construction feature can be explained by its interaction with flow distribution and species dispersion. In this paper, a three-dimensional numerical model has been developed to simulate highway pollutant dispersion—a realizable k-ε model was employed to model turbulent flow, and a non-reaction species dispersion model was applied to simulate species transport. First, numerical models were validated with experimental data, and good agreement was observed. Then, detailed simulations were conducted to study double barriers’ effects on highway pollutant dispersion under different settings: noise barriers with different heights, noise barriers with and without edge effects, and different atmospheric thermal boundary conditions. Results show that: (1) Noise barriers without edge effects cause bigger downwind velocity and turbulence intensity than noise barriers with edge effects. (2) At ground level, lower downwind pollutant concentration and higher pollutant concentration, near upwind barrier and between barriers, are observed for noise barriers without edge effect cases; higher on-road pollutant concentration can be seen near barrier side edges for cases with edge effect. (3) Downwind velocity and turbulence intensity increase as barrier height increases, which causes reduced downwind pollutant concentration. (4) With the same barrier height, under unstable atmospheric boundary condition, the lowest pollutant concentration can be found for both downwind and between barriers. Overall, these findings will provide valuable inputs to noise barrier design, so as to improve roadside neighborhood air quality. Full article
(This article belongs to the Special Issue Computational Simulation of Pollution Dispersion)
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Open AccessArticle Impact of Radiative Losses on Flame Acceleration and Deflagration to Detonation Transition of Lean Hydrogen-Air Mixtures in a Macro-Channel with Obstacles
Received: 12 November 2018 / Revised: 4 December 2018 / Accepted: 6 December 2018 / Published: 8 December 2018
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Abstract
While there has been some recognition regarding the impact of thermal boundary conditions (adiabatic versus isothermal) on premixed flame propagation mechanisms in micro-channels (hydraulic diameters <10 mm), their impact in macro-channels has often been overlooked due to small surface-area-to-volume ratios of the propagating
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While there has been some recognition regarding the impact of thermal boundary conditions (adiabatic versus isothermal) on premixed flame propagation mechanisms in micro-channels (hydraulic diameters <10 mm), their impact in macro-channels has often been overlooked due to small surface-area-to-volume ratios of the propagating combustion wave. Further, the impact of radiative losses has also been neglected due to its anticipated insignificance based on scaling analysis and the high computational cost associated with resolving it’s spatial, temporal, directional, and wavelength dependencies. However, when channel conditions promote flame acceleration and deflagration-to-detonation transitions (DDT), large pressures are encountered in the vicinity of the combustion wave, thereby increasing the magnitude of radiative losses which in turn can impact the strength and velocity of the combustion wave. This is demonstrated for the first time through simulations of lean (equivalence ratio: 0.5) hydrogen-air mixtures in a macro-channel (hydraulic diameter: 174 mm) with obstacles (Blockage ratio: 0.51). By employing Planck mean absorption coefficients in conjunction with the P-1 radiation model, radiative losses are shown to affect the run-up distances to DDT in a long channel (length: 11.878 m). As anticipated, the differences in run-up distances resulting from radiative losses only increased with system pressure. Full article
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Open AccessArticle Theoretical Modeling, Design and Simulation of an Innovative Diverting Valve Based on Coanda Effect
Received: 9 October 2018 / Revised: 23 November 2018 / Accepted: 24 November 2018 / Published: 6 December 2018
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Abstract
The present work is focused on the study of an innovative fluidic device. It consists of a two-ways diverter valve able to elaborate an inlet water flow and divert it through one of the two outlets without moving parts but as a result
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The present work is focused on the study of an innovative fluidic device. It consists of a two-ways diverter valve able to elaborate an inlet water flow and divert it through one of the two outlets without moving parts but as a result of a fluctuation of pressure induced by two actuation ports, or channels. Such apparatus is named Attachment Bi-Stable Diverter (ABD) and is able to work with the effect of the fluid adhesion to a convex wall adjacent to it, this phenomenon is known as Coanda Effect; it generates the force responsible for the fluid attachment and the consequent deviation. The main purpose of this work is to develop a knowhow for the design and development of such particular device. A mathematical model for the ABD has been developed and used to find the relationships between the geometrical parameters and the operative conditions. A configuration has been designed, simulated with a computational fluid dynamics approach. A prototype has been printed with and additive manufacturing printer and tested in laboratory to check the effective working point of the device. Full article
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Open AccessArticle Qualitative Effects of Hydraulic Conductivity Distribution on Groundwater Flow in Heterogeneous Soils
Received: 19 October 2018 / Revised: 23 November 2018 / Accepted: 29 November 2018 / Published: 5 December 2018
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Abstract
One of the most significant difficulties in subsurface hydrology is the considerable uncertainty in hydraulic conductivity values in the medium. This stimulates qualitative analysis of the effect of conductivity distribution on the solutions or on some components of the solutions of groundwater flow
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One of the most significant difficulties in subsurface hydrology is the considerable uncertainty in hydraulic conductivity values in the medium. This stimulates qualitative analysis of the effect of conductivity distribution on the solutions or on some components of the solutions of groundwater flow equations. This work is an attempt to develop a rigorous basis for deciding whether the solutions are monotonous with respect to hydraulic conductivity. Such monotonicity is analogous to the well-known comparison principles with respect to variations of initial data or external supplies. Some example problems are given in this paper, including a problem with a free boundary, in which the monotonous dependence of the solution on the conductivity distribution is proved rigorously. Examples are also given, in which monotonicity assumptions, despite being apparently obvious, are proved to be invalid. Full article
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Open AccessArticle A Method toward Real-Time CFD Modeling for Natural Ventilation
Received: 24 September 2018 / Revised: 19 November 2018 / Accepted: 26 November 2018 / Published: 1 December 2018
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Abstract
Natural ventilation is often used as a passive technology to reduce building energy consumption. To leverage the rule-based natural ventilation control to more advanced control at multiple spatial scales, mathematical modeling is needed to calculate the real-time ventilation rate, indoor air temperatures, and
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Natural ventilation is often used as a passive technology to reduce building energy consumption. To leverage the rule-based natural ventilation control to more advanced control at multiple spatial scales, mathematical modeling is needed to calculate the real-time ventilation rate, indoor air temperatures, and velocities at high spatial resolution. This study aims to develop a real-time mathematical modeling framework based on computational fluid dynamics (CFD). The real-time concept is implemented by using real-time sensor data, e.g., wall surface temperatures as boundary conditions, while data assimilation is employed to implement real-time self-calibration. The proof of concept is demonstrated by a case study using synthetic data. The results show that the modeling framework can adequately predict real-time ventilation rates and indoor air temperatures. The data assimilation method can nudge the simulated air velocities toward the observed values to continuously calibrate the model. The real-time CFD modeling framework will be further tested by the real-time sensor data once building construction is fully completed. Full article
(This article belongs to the Special Issue Ventilation and Passive Cooling for Healthy and Comfortable Buildings)
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Open AccessArticle Experimental Set-Up for the Investigation of Thermomechanical Oscillations of Thin Heaters in Air
Received: 5 September 2018 / Revised: 19 November 2018 / Accepted: 26 November 2018 / Published: 30 November 2018
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Abstract
The simultaneous visualization and characterization of heat transfer processes from hot vibrating objects is a challenging task. This article presents an experimental set-up for the investigation of thermomechanical oscillations in thin cylindrical heaters, allowing us to visualize convection processes using Schlieren photography, infrared
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The simultaneous visualization and characterization of heat transfer processes from hot vibrating objects is a challenging task. This article presents an experimental set-up for the investigation of thermomechanical oscillations in thin cylindrical heaters, allowing us to visualize convection processes using Schlieren photography, infrared photometry, and other methods. It is demonstrated that heat transfer considerably changes in the regions of parametric instability. Full article
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Open AccessFeature PaperArticle Time-Dependent Diffusion Coefficients for Chaotic Advection due to Fluctuations of Convective Rolls
Received: 16 October 2018 / Revised: 19 November 2018 / Accepted: 22 November 2018 / Published: 27 November 2018
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Abstract
The properties of chaotic advection arising from defect turbulence, that is, weak turbulence in the electroconvection of nematic liquid crystals, were experimentally investigated. Defect turbulence is a phenomenon in which fluctuations of convective rolls arise and are globally disturbed while maintaining convective rolls
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The properties of chaotic advection arising from defect turbulence, that is, weak turbulence in the electroconvection of nematic liquid crystals, were experimentally investigated. Defect turbulence is a phenomenon in which fluctuations of convective rolls arise and are globally disturbed while maintaining convective rolls locally. The time-dependent diffusion coefficient, as measured from the motion of a tagged particle driven by the turbulence, was used to clarify the dependence of the type of diffusion on coarse-graining time. The results showed that, as coarse-graining time increases, the type of diffusion changes from superdiffusion → subdiffusion → normal diffusion. The change in diffusive properties over the observed timescale reflects the coexistence of local order and global disorder in the defect turbulence. Full article
(This article belongs to the Special Issue Multiscale Turbulent Transport)
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Open AccessFeature PaperReview Application of Hydrodynamic Cavitation Reactors for Treatment of Wastewater Containing Organic Pollutants: Intensification Using Hybrid Approaches
Received: 20 September 2018 / Revised: 31 October 2018 / Accepted: 13 November 2018 / Published: 23 November 2018
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Abstract
The concentration of hazardous pollutants in the wastewater streams has to keep below a certain level in order to comply with the stringent environmental laws. The conventional technologies for wastewater treatment have drawbacks in terms of limited applicability and efficiency. Utilization of hydrodynamic
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The concentration of hazardous pollutants in the wastewater streams has to keep below a certain level in order to comply with the stringent environmental laws. The conventional technologies for wastewater treatment have drawbacks in terms of limited applicability and efficiency. Utilization of hydrodynamic cavitation (HC) reactors for the degradation of pollutants at large scale has shown considerable promise over last few years, due to higher energy efficiencies and low cost operation based on lower consumption of chemicals for the treatment. The present work overviews the degradation of different pollutants, such as pharmaceuticals, pesticide, phenolic derivatives and dyes, as well as the treatment of real industrial effluents using hybrid methods based on HC viz. HC/H2O2, HC/Ozone, HC/Fenton, HC/Ultraviolet irradiations (UV), and HC coupled with biological oxidation. Furthermore, based on the literature reports, recommendations for the selection of optimum operating parameters, such as inlet pressure, solution temperature, initial pH and initial pollutant concentration have been discussed in order to maximize the process intensification benefits. Moreover, hybrid methods based on HC has been demonstrated to show good synergism as compared to individual treatment approach. Overall, high energy efficient wastewater treatment can be achieved using a combined treatment approach based on HC under optimized conditions. Full article
(This article belongs to the Special Issue Advances in Bubble Acoustics)
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Open AccessArticle Computing Functional Gains for Designing More Energy-Efficient Buildings Using a Model Reduction Framework
Received: 15 August 2018 / Revised: 17 October 2018 / Accepted: 20 November 2018 / Published: 23 November 2018
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Abstract
We discuss developing efficient reduced-order models (ROM) for designing energy-efficient buildings using computational fluid dynamics (CFD) simulations. This is often the first step in the reduce-then-control technique employed for flow control in various industrial and engineering problems. This approach computes the proper orthogonal
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We discuss developing efficient reduced-order models (ROM) for designing energy-efficient buildings using computational fluid dynamics (CFD) simulations. This is often the first step in the reduce-then-control technique employed for flow control in various industrial and engineering problems. This approach computes the proper orthogonal decomposition (POD) eigenfunctions from high-fidelity simulations data and then forms a ROM by projecting the Navier-Stokes equations onto these basic functions. In this study, we develop a linear quadratic regulator (LQR) control based on the ROM of flow in a room. We demonstrate these approaches on a one-room model, serving as a basic unit in a building. Furthermore, the ROM is used to compute feedback functional gains. These gains are in fact the spatial representation of the feedback control. Insight of these functional gains can be used for effective placement of sensors in the room. This research can further lead to developing mathematical tools for efficient design, optimization, and control in building management systems. Full article
(This article belongs to the Special Issue Reduced Order Modeling of Fluid Flows)
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Open AccessArticle The Effect of a Variable Background Density Stratification and Current on Oceanic Internal Solitary Waves
Received: 10 October 2018 / Revised: 6 November 2018 / Accepted: 13 November 2018 / Published: 20 November 2018
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Abstract
Large amplitude, horizontally propagating internal waves are commonly observed in the coastal ocean. They are often modelled by a variable-coefficient Korteweg–de Vries equation to take account of a horizontally varying background state. Although this equation is now well-known, a term representing non-conservative effects,
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Large amplitude, horizontally propagating internal waves are commonly observed in the coastal ocean. They are often modelled by a variable-coefficient Korteweg–de Vries equation to take account of a horizontally varying background state. Although this equation is now well-known, a term representing non-conservative effects, arising from horizontal variation in the underlying basic state density stratification and current, has often been omitted. In this paper, we examine the possible significance of this term using climatological data for several typical oceanic sites where internal waves have been observed. Full article
(This article belongs to the collection Geophysical Fluid Dynamics)
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Open AccessArticle Acoustics of Bubble Arrays: Role Played by the Dipole Response of Bubbles
Received: 3 October 2018 / Revised: 8 November 2018 / Accepted: 13 November 2018 / Published: 20 November 2018
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Abstract
A model for acoustic transmission through a 2D square crystal of R-radius bubbles with a lattice constant L was previously proposed. Assuming a purely monopole response of the bubbles, this model offers a simple analytical expression of the transmission. However, it is
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A model for acoustic transmission through a 2D square crystal of R-radius bubbles with a lattice constant L was previously proposed. Assuming a purely monopole response of the bubbles, this model offers a simple analytical expression of the transmission. However, it is not applicable when the bubbles are too close to each other (L/R < 5). This article proposes an extension of the model by including the dipole response of the bubbles. Comparisons with numerical and experimental results show that the new expression gives a good estimate of the concentration at which the monopole model is no longer valid, but fails at properly predicting the transmission. Full article
(This article belongs to the Special Issue Advances in Bubble Acoustics)
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Open AccessArticle Numerical Investigation of Steady and Harmonic Vortex Generator Jets Flow Separation Control
Received: 9 October 2018 / Revised: 28 October 2018 / Accepted: 14 November 2018 / Published: 17 November 2018
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Abstract
Active flow control of canonical laminar separation bubbles by steady and harmonic vortex generator jets (VGJs) was investigated using direct numerical simulations. Both control strategies were found to be effective in controlling the laminar boundary-layer separation. However, the present results indicate that using
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Active flow control of canonical laminar separation bubbles by steady and harmonic vortex generator jets (VGJs) was investigated using direct numerical simulations. Both control strategies were found to be effective in controlling the laminar boundary-layer separation. However, the present results indicate that using the same blowing amplitude, harmonic VGJs were more effective and efficient at reducing the separated region than the steady VGJs considering the fact that the harmonic VGJs use less momentum than the steady case. For steady VGJs, longitudinal structures forming immediately downstream of the injection location led to the formation of hairpin-type vortices, causing an earlier transition to turbulence. Symmetric hairpin vortices were shown to develop downstream of the forcing location for the harmonic VGJs, as well. However, the increased control effectiveness for harmonic VGJs’ flow control strategy is attributed to the fact that the shear-layer instability mechanism was exploited. As a result, disturbances introduced by VGJs were strongly amplified, leading to the development of large-scale coherent structures, which are very effective at increasing the momentum exchange, thus limiting the separated region. Full article
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Open AccessReview Microstreaming and Its Role in Applications: A Mini-Review
Received: 16 October 2018 / Revised: 11 November 2018 / Accepted: 14 November 2018 / Published: 17 November 2018
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Abstract
Acoustic streaming is the steady flow of a fluid that is caused by the propagation of sound through that fluid. The fluid flow in acoustic streaming is generated by a nonlinear, time-averaged effect that results from the spatial and temporal variations in a
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Acoustic streaming is the steady flow of a fluid that is caused by the propagation of sound through that fluid. The fluid flow in acoustic streaming is generated by a nonlinear, time-averaged effect that results from the spatial and temporal variations in a pressure field. When there is an oscillating body submerged in the fluid, such as a cavitation bubble, vorticity is generated on the boundary layer on its surface, resulting in microstreaming. Although the effects are generated at the microscale, microstreaming can have a profound influence on the fluid mechanics of ultrasound/acoustic processing systems, which are of high interest to sonochemistry, sonoprocessing, and acoustophoretic applications. The effects of microstreaming have been evaluated over the years using carefully controlled experiments that identify and quantify the fluid motion at a small scale. This mini-review article overviews the historical development of acoustic streaming, shows how microstreaming behaves, and provides an update on new numerical and experimental studies that seek to explore and improve our understanding of microstreaming. Full article
(This article belongs to the Special Issue Advances in Bubble Acoustics)
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Open AccessArticle Improving Ventilation Efficiency for a Highly Energy Efficient Indoor Swimming Pool Using CFD Simulations
Received: 26 September 2018 / Revised: 7 November 2018 / Accepted: 9 November 2018 / Published: 15 November 2018
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Abstract
The operation of a typical indoor swimming pool is very energy intensive. Previous studies have shown that high quality thermal building envelopes, i.e., with high levels of insulation and airtightness, make it possible to rethink conventional ventilation concepts. Due to the reduced condensation
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The operation of a typical indoor swimming pool is very energy intensive. Previous studies have shown that high quality thermal building envelopes, i.e., with high levels of insulation and airtightness, make it possible to rethink conventional ventilation concepts. Due to the reduced condensation risk in and on envelopes of high thermal quality, ventilation design can be optimized for indoor air quality rather than for averting condensation on the facade. This work investigates different air distribution concepts for an existing swimming pool via computational fluid dynamics (CFD) simulations to evaluate their ventilation efficiency. To reduce modelling and computational resources, the velocity and turbulence fields produced by the swirl-diffusers are determined in a set of separate CFD simulations and incorporated into the swimming pool models. The results show that the ventilation efficiency in the examined swimming pool could potentially be improved with various alternative air distribution concepts, therefore improving the indoor air quality. Although the results seem plausible and compare well with the limited measurement data of air humidity, a more formal experimental validation is still needed before generalizing recommendations. Full article
(This article belongs to the Special Issue Ventilation and Passive Cooling for Healthy and Comfortable Buildings)
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Open AccessFeature PaperArticle Role of Eddies in the Maintenance of Multiple Jets Embedded in Eastward and Westward Baroclinic Shears
Received: 6 October 2018 / Revised: 3 November 2018 / Accepted: 8 November 2018 / Published: 11 November 2018
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Abstract
Multiple zonal jets observed in many parts of the global ocean are often embedded in large-scale eastward and westward vertically sheared background flows. Properties of the jets and ambient eddies, as well as their dynamic interactions, are found to be different between eastward
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Multiple zonal jets observed in many parts of the global ocean are often embedded in large-scale eastward and westward vertically sheared background flows. Properties of the jets and ambient eddies, as well as their dynamic interactions, are found to be different between eastward and westward shears. However, the impact of these differences on overall eddy dynamics remains poorly understood and is the main subject of this study. The roles of eddy relative vorticity and buoyancy fluxes in the maintenance of oceanic zonal jets are studied in a two-layer quasigeostrophic model. Both eastward and westward uniform, zonal vertically sheared cases are considered in the study. It is shown that, despite the differences in eddy structure and local characteristics, the fundamental dynamics are essentially the same in both cases: the relative-vorticity fluxes force the jets in the entire fluid column, and the eddy-buoyancy fluxes transfer momentum from the top to the bottom layer, where it is balanced by bottom friction. It is also observed that the jets gain more energy via Reynolds stress work in the layer having a positive gradient in the background potential vorticity, and this is qualitatively explained by a simple reasoning based on Rossby wave group velocity. Full article
(This article belongs to the collection Geophysical Fluid Dynamics)
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Open AccessArticle Symmetry Approach in the Evaluation of the Effect of Boundary Proximity on Oscillation of Gas Bubbles
Received: 28 September 2018 / Revised: 22 October 2018 / Accepted: 1 November 2018 / Published: 10 November 2018
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Abstract
The purpose of the present review is to describe the effect of an interface between media with different mechanical properties on the acoustic response of a gas bubble. This is necessary to interpret sonar signals received from underwater gas seeps and mud volcanoes,
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The purpose of the present review is to describe the effect of an interface between media with different mechanical properties on the acoustic response of a gas bubble. This is necessary to interpret sonar signals received from underwater gas seeps and mud volcanoes, as well as in the case of acoustic studies on the Arctic shelf where rising gas bubbles accumulate at the lower boundary of the ice cover. The ability to describe the dynamics of constrained bubble by analytical methods is related to the presence of internal symmetry in the governing equations. This leads to the presence of specific (toroidal and bi-spherical) coordinate systems in which the variables are separated. The existence of symmetry properties is possible only under certain conditions. In particular, the characteristic wavelength should be larger than the bubble size and the distance to an interface. The derived analytical solution allows us to determine how the natural frequency, radiation damping, and bubble shape depend on the distance to the boundary and the material parameters of contacting media. Full article
(This article belongs to the Special Issue Advances in Bubble Acoustics)
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Open AccessTutorial Subcritical Instabilities in Neutral Fluids and Plasmas
Received: 28 August 2018 / Revised: 17 October 2018 / Accepted: 29 October 2018 / Published: 2 November 2018
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Abstract
In neutral fluids and plasmas, the analysis of perturbations often starts with an inventory of linearly unstable modes. Then, the nonlinear steady-state is analyzed or predicted based on these linear modes. A crude analogy would be to base the study of a chair
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In neutral fluids and plasmas, the analysis of perturbations often starts with an inventory of linearly unstable modes. Then, the nonlinear steady-state is analyzed or predicted based on these linear modes. A crude analogy would be to base the study of a chair on how it responds to infinitesimaly small perturbations. One would conclude that the chair is stable at all frequencies, and cannot fall down. Of course, a chair falls down if subjected to finite-amplitude perturbations. Similarly, waves and wave-like structures in neutral fluids and plasmas can be triggered even though they are linearly stable. These subcritical instabilities are dormant until an interaction, a drive, a forcing, or random noise pushes their amplitude above some threshold. Investigating their onset conditions requires nonlinear calculations. Subcritical instabilities are ubiquitous in neutral fluids and plasmas. In plasmas, subcritical instabilities have been investigated based on analytical models and numerical simulations since the 1960s. More recently, they have been measured in laboratory and space plasmas, albeit not always directly. The topic could benefit from the much longer and richer history of subcritical instability and transition to subcritical turbulence in neutral fluids. In this tutorial introduction, we describe the fundamental aspects of subcritical instabilities in plasmas, based on systems of increasing complexity, from simple examples of a point-mass in a potential well or a box on a table, to turbulence and instabilities in neutral fluids, and finally, to modern applications in magnetized toroidal fusion plasmas. Full article
(This article belongs to the Special Issue Modelling of Plasma Flow)
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Open AccessArticle Extreme Learning Machines as Encoders for Sparse Reconstruction
Received: 20 August 2018 / Revised: 18 October 2018 / Accepted: 27 October 2018 / Published: 1 November 2018
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Abstract
Reconstruction of fine-scale information from sparse data is often needed in practical fluid dynamics where the sensors are typically sparse and yet, one may need to learn the underlying flow structures or inform predictions through assimilation into data-driven models. Given that sparse reconstruction
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Reconstruction of fine-scale information from sparse data is often needed in practical fluid dynamics where the sensors are typically sparse and yet, one may need to learn the underlying flow structures or inform predictions through assimilation into data-driven models. Given that sparse reconstruction is inherently an ill-posed problem, the most successful approaches encode the physics into an underlying sparse basis space that spans the manifold to generate well-posedness. To achieve this, one commonly uses a generic orthogonal Fourier basis or a data specific proper orthogonal decomposition (POD) basis to reconstruct from sparse sensor information at chosen locations. Such a reconstruction problem is well-posed as long as the sensor locations are incoherent and can sample the key physical mechanisms. The resulting inverse problem is easily solved using l 2 minimization or if necessary, sparsity promoting l 1 minimization. Given the proliferation of machine learning and the need for robust reconstruction frameworks in the face of dynamically evolving flows, we explore in this study the suitability of non-orthogonal basis obtained from extreme learning machine (ELM) auto-encoders for sparse reconstruction. In particular, we assess the interplay between sensor quantity and sensor placement in a given system dimension for accurate reconstruction of canonical fluid flows in comparison to POD-based reconstruction. Full article
(This article belongs to the Special Issue Reduced Order Modeling of Fluid Flows)
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Open AccessArticle A Numerical Study of the Sound and Force Production of Flexible Insect Wings
Received: 17 September 2018 / Revised: 17 October 2018 / Accepted: 26 October 2018 / Published: 31 October 2018
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Abstract
We numerically solved the acoustic and flow field around cicada wing models with parametrically varied flexibility using the hydrodynamic/acoustic splitting method. We observed a gradual change of sound directivity with flexibility. We found that flexible wings generally produce lower sound due to reduced
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We numerically solved the acoustic and flow field around cicada wing models with parametrically varied flexibility using the hydrodynamic/acoustic splitting method. We observed a gradual change of sound directivity with flexibility. We found that flexible wings generally produce lower sound due to reduced aerodynamic forces, which were further found to scale with the dynamic pressure force defined as the integration of dynamic pressure over the wing area. Unlike conventional scaling where the incoming flow velocity is used as the reference to calculate the force coefficients, here only the normal component of the relative velocity of the wing to the flow was used to calculate the dynamic pressure, putting kinematic factors into the dynamic pressure force and leaving the more fundamental physics to the force coefficients. A high correlation was found between the aerodynamic forces and the dynamic pressure. The scaling is also supported by previously reported data of revolving wing experiments. Full article
(This article belongs to the Special Issue Bio-inspired Flow)
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Open AccessArticle A Hybrid Approach for Model Order Reduction of Barotropic Quasi-Geostrophic Turbulence
Received: 1 October 2018 / Revised: 24 October 2018 / Accepted: 26 October 2018 / Published: 31 October 2018
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Abstract
We put forth a robust reduced-order modeling approach for near real-time prediction of mesoscale flows. In our hybrid-modeling framework, we combine physics-based projection methods with neural network closures to account for truncated modes. We introduce a weighting parameter between the Galerkin projection and
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We put forth a robust reduced-order modeling approach for near real-time prediction of mesoscale flows. In our hybrid-modeling framework, we combine physics-based projection methods with neural network closures to account for truncated modes. We introduce a weighting parameter between the Galerkin projection and extreme learning machine models and explore its effectiveness, accuracy and generalizability. To illustrate the success of the proposed modeling paradigm, we predict both the mean flow pattern and the time series response of a single-layer quasi-geostrophic ocean model, which is a simplified prototype for wind-driven general circulation models. We demonstrate that our approach yields significant improvements over both the standard Galerkin projection and fully non-intrusive neural network methods with a negligible computational overhead. Full article
(This article belongs to the collection Geophysical Fluid Dynamics)
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Open AccessArticle Instability of a Diffusive Boundary Layer beneath a Capillary Transition Zone
Received: 18 October 2018 / Revised: 25 October 2018 / Accepted: 29 October 2018 / Published: 31 October 2018
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Abstract
Natural convection induced by carbon dioxide (CO2) dissolution from a gas cap into the resident formation brine of a deep saline aquifer in the presence of a capillary transition zone is an important phenomenon that can accelerate the dissolution process, reducing
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Natural convection induced by carbon dioxide (CO2) dissolution from a gas cap into the resident formation brine of a deep saline aquifer in the presence of a capillary transition zone is an important phenomenon that can accelerate the dissolution process, reducing the risk of CO2 leakage to the shallower formations. Majority of past investigations on the instability of the diffusive boundary layer assumed a sharp CO2–brine interface with constant CO2 concentration at the top of the aquifer, i.e., single-phase system. However, this assumption may lead to erroneous estimates of the onset of natural convection. The present study demonstrates the significant effect of the capillary transition zone on the onset of natural convection in a two-phase system in which a buoyant CO2 plume overlaid a water-saturated porous layer. Using the quasi-steady-state approximation (QSSA), we performed a linear stability analysis to assess critical times, critical wavenumbers, and neutral stability curves as a function of Bond number. We show that the capillary transition zone could potentially accelerate the evolution of the natural convection by sixfold. Furthermore, we characterized the instability problem for capillary-dominant, in-transition, and buoyancy-dominant systems. In the capillary-dominant systems, capillary transition zone has a strong role in destabilizing the diffusive boundary layer. In contrast, in the buoyancy-dominant systems, the buoyancy force is the sole cause of the instability, and the effect of the capillary transition zone can be ignored. Our findings provide further insight into the understanding of the natural convection in the two-phase CO2–brine system and the long-term fate of the injected CO2 in deep saline aquifers. Full article
(This article belongs to the Special Issue Fundamentals of CO2 Storage in Geological Formations)
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Open AccessArticle Evolve Filter Stabilization Reduced-Order Model for Stochastic Burgers Equation
Received: 6 August 2018 / Revised: 15 September 2018 / Accepted: 18 October 2018 / Published: 26 October 2018
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Abstract
In this paper, we introduce the evolve-then-filter (EF) regularization method for reduced order modeling of convection-dominated stochastic systems. The standard Galerkin projection reduced order model (G-ROM) yield numerical oscillations in a convection-dominated regime. The evolve-then-filter reduced order model (EF-ROM) aims at the numerical
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In this paper, we introduce the evolve-then-filter (EF) regularization method for reduced order modeling of convection-dominated stochastic systems. The standard Galerkin projection reduced order model (G-ROM) yield numerical oscillations in a convection-dominated regime. The evolve-then-filter reduced order model (EF-ROM) aims at the numerical stabilization of the standard G-ROM, which uses explicit ROM spatial filter to regularize various terms in the reduced order model (ROM). Our numerical results are based on a stochastic Burgers equation with linear multiplicative noise. The numerical result shows that the EF-ROM is significantly better than G-ROM. Full article
(This article belongs to the Special Issue Reduced Order Modeling of Fluid Flows)
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Open AccessArticle Differential Diffusivity Effects in Reactive Convective Dissolution
Received: 13 September 2018 / Revised: 10 October 2018 / Accepted: 16 October 2018 / Published: 26 October 2018
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Abstract
When a solute A dissolves into a host fluid containing a reactant B, an A + B → C reaction can influence the convection developing because of unstable density gradients in the gravity field. When A increases density and all three chemical species
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When a solute A dissolves into a host fluid containing a reactant B, an A + B → C reaction can influence the convection developing because of unstable density gradients in the gravity field. When A increases density and all three chemical species A, B and C diffuse at the same rate, the reactive case can lead to two different types of density profiles, i.e., a monotonically decreasing one from the interface to the bulk and a non-monotonic profile with a minimum. We study numerically here the nonlinear reactive convective dissolution dynamics in the more general case where the three solutes can diffuse at different rates. We show that differential diffusion can add new dynamic effects like the simultaneous presence of two different convection zones in the host phase when a non-monotonic profile with both a minimum and a maximum develops. Double diffusive instabilities can moreover affect the morphology of the convective fingers. Analysis of the mixing zone, the reaction rate, the total amount of stored A and the dissolution flux further shows that varying the diffusion coefficients of the various species has a quantitative effect on convection. Full article
(This article belongs to the Special Issue Fundamentals of CO2 Storage in Geological Formations)
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Open AccessReview On the Behaviour of Living Cells under the Influence of Ultrasound
Received: 19 September 2018 / Revised: 11 October 2018 / Accepted: 22 October 2018 / Published: 26 October 2018
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Abstract
Medical ultrasound technology is available, affordable, and non-invasive. It is used to detect, quantify, and heat tissue structures. This review article gives a concise overview of the types of behaviour that biological cells experience under the influence of ultrasound only, i.e., without the
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Medical ultrasound technology is available, affordable, and non-invasive. It is used to detect, quantify, and heat tissue structures. This review article gives a concise overview of the types of behaviour that biological cells experience under the influence of ultrasound only, i.e., without the presence of microbubbles. The phenomena are discussed from a physics and engineering perspective. They include proliferation, translation, apoptosis, lysis, transient membrane permeation, and oscillation. The ultimate goal of cellular acoustics is the detection, quantification, manipulation and eradication of individual cells. Full article
(This article belongs to the Special Issue Advances in Bubble Acoustics)
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Open AccessArticle Quasi-Steady versus Navier–Stokes Solutions of Flapping Wing Aerodynamics
Received: 20 September 2018 / Revised: 16 October 2018 / Accepted: 21 October 2018 / Published: 24 October 2018
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Abstract
Various tools have been developed to model the aerodynamics of flapping wings. In particular, quasi-steady models, which are considerably faster and easier to solve than the Navier–Stokes equations, are often utilized in the study of flight dynamics of flapping wing flyers. However, the
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Various tools have been developed to model the aerodynamics of flapping wings. In particular, quasi-steady models, which are considerably faster and easier to solve than the Navier–Stokes equations, are often utilized in the study of flight dynamics of flapping wing flyers. However, the accuracy of the quasi-steady models has not been properly documented. The objective of this study is to assess the accuracy of a quasi-steady model by comparing the resulting aerodynamic forces against three-dimensional (3D) Navier–Stokes solutions. The same wing motion is prescribed at a fruit fly scale. The pitching amplitude, axis, and duration are varied. Comparison of the aerodynamic force coefficients suggests that the quasi-steady model shows significant discrepancies under extreme pitching motions, i.e., the pitching motion is large, quick, and occurs about the leading or trailing edge. The differences are as large as 1.7 in the cycle-averaged lift coefficient. The quasi-steady model performs well when the kinematics are mild, i.e., the pitching motion is small, long, and occurs near the mid-chord with a small difference in the lift coefficient of 0.01. Our analysis suggests that the main source for the error is the inaccuracy of the rotational lift term and the inability to model the wing-wake interaction in the quasi-steady model. Full article
(This article belongs to the Special Issue Bio-inspired Flow)
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Open AccessArticle Modeling of Carbon Dioxide Leakage from Storage Aquifers
Received: 16 July 2018 / Revised: 14 October 2018 / Accepted: 18 October 2018 / Published: 23 October 2018
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Abstract
Long-term geological storage of CO2 in deep saline aquifers offers the possibility of sustaining access to fossil fuels while reducing emissions. However, prior to implementation, associated risks of CO2 leakage need to be carefully addressed to ensure safety of storage. CO
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Long-term geological storage of CO2 in deep saline aquifers offers the possibility of sustaining access to fossil fuels while reducing emissions. However, prior to implementation, associated risks of CO2 leakage need to be carefully addressed to ensure safety of storage. CO2 storage takes place by several trapping mechanisms that are active on different time scales. The injected CO2 may be trapped under an impermeable rock due to structural trapping. Over time, the contribution of capillary, solubility, and mineral trapping mechanisms come into play. Leaky faults and fractures provide pathways for CO2 to migrate upward toward shallower depths and reduce the effectiveness of storage. Therefore, understanding the transport processes and the impact of various forces such as viscous, capillary and gravity is necessary. In this study, a mechanistic model is developed to investigate the influence of the driving forces on CO2 migration through a water saturated leakage pathway. The developed numerical model is used to determine leakage characteristics for different rock formations from a potential CO2 storage site in central Alberta, Canada. The model allows for preliminary analysis of CO2 leakage and finds applications in screening and site selection for geological storage of CO2 in deep saline aquifers. Full article
(This article belongs to the Special Issue Fundamentals of CO2 Storage in Geological Formations)
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Open AccessArticle CFD Analysis of Twin Turbulent Impinging Round Jets at Different Impingement Angles
Received: 23 August 2018 / Revised: 16 October 2018 / Accepted: 17 October 2018 / Published: 23 October 2018
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Abstract
Round jets impinging at multiple impingement angles were considered for this study to gain better understanding of the parameters affecting resultant jet growth and velocity distribution. Work done by the authors previously on single jet has helped to establish that the SST (Shear
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Round jets impinging at multiple impingement angles were considered for this study to gain better understanding of the parameters affecting resultant jet growth and velocity distribution. Work done by the authors previously on single jet has helped to establish that the SST (Shear Stress Transport) k-ω model is the ideal turbulence model for predicting flow characteristics of jets exiting a fully developed pipe at low Reynolds number. Hence, for the study of impinging jets, SST k-ω turbulence model was used to study the velocity and jet growth characteristics. Based on the mesh obtained from the grid sensitivity study, jets impinging at 30, 45 and 60 degrees at Reynolds number of 7500 were numerically analyzed. It was observed that the profile of the resultant jet closely matched with the prediction of elliptical profile predicted by past researchers. In addition, it was seen that higher jet growth was predicted in the case of jets impinging at a higher impingement angle. Full article
(This article belongs to the Special Issue Turbulence and Trasitional Modeling of Aerodynamic Flows)
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