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Numerical Simulation of Twin Impinging jets in Tandem through a Crossflow
No full textThe flow field of ground vortex generated by twin impinging jets in tandem through a crossflow is numerically studied in detail. Numerical simulation and visualization are presented for two turbulent circular jets emerging into a low velocity cross stream, impinging after on a flat surface perpendicular to the geometrical jet nozzle axis. The numerical study is based in experimental studies done early, so all the features of the experimental flow were maintained when the numerical simulation was performed. The Reynolds number used was based on the jet exit conditions of 43,000 to 105,000, a jet to crossflow velocity ratio of 22.5 to 43.8, an impinging height of 20.1 jet diameters and an interject spacing’s of S=5D and L=6D. The analysis of the flow was extended to regions and flow conditions for which no measurements have been obtained in last experimental studies, i.e., for velocity ratios of 15 to 90. The numerical results show that for the smallest velocity ratios the jets initially do not mix, but remain together in two layers. Three different types of flow regimes were identify, therefore when VSTOL aircrafts operating in ground vicinity, only the regime with strong impingement on ground and with a formation of a ground vortex is relevant. The numerical results allowed to extend the last experimental studies, and prove that the deflection of the rear jet is due to the competing influences the wake, the shear layer, the downstream wall jet of the first jet and the crossflow.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
The Turbulent Structure of a Ground Vortex
No full textLaser-Doppler measurements of the velocity characteristics of a ground vortex flow resulting from the collision of a wall jet with a boundary layer are presented and discussed together with flow visualization. In the present study a velocity ratio between the boundary layer and the wall jet of 0.5 was used, and the results confirmed the existence of a very complex vortex flow region in the collision zone near the ground wall. Vorticity values were calculated from the experimental data and revealed negative (clockwise) vorticity near the stagnation point which is an indication of the presence of a small secondary vortex, also detected from the visualization studies but not sufficiently identified from the LDA measurements. The analysis of turbulent energy equation terms using the measured data revealed that production by normal and shear stresses are both very important to the turbulent structure of the impact zone of the ground vortex.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Computational Modeling of Water Droplets Under Cooling and Freezing Conditions
No full textThe crystallization of water droplets is a ubiquitous phenomenon, which plays afundamental role in many natural and artificial processes. This phenomenon gainedparticular interest in the aeronautical industry due to ice accretion on lifting surfaces andengine intakes. A performance study is made of several models for predicting cooling and freezing phenomena, ranging from one- and two-way coupling to full-fledged four-stage freezing methodologies. First, a performance study is made of the one- and two-way coupling methodologies in the portrayal of the interaction of the particles and the surrounding gas on free-falling water droplets with diameters ranging from 3 to 6 mm and relative humidity ratios of 0.29, 0.36. 0.52 and 1.00. Then a generalized four-stage freezing model is implemented, where the temperature evolution of a suspended water droplet is tracked, adding to the cooling in the free-falling conditions, the recalescence and solidification stages. The results indicate that, for high relative humidity ratio, the interaction particles-medium is preponderant and needs to be accounted for, which does not take place for low relative humidity ratios, where one-way coupling seems to be suitable approximation. Lastly, the full stage freezing model was able to capture the experimental trend of recalescence and solidification stages.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Experimental and numerical study of single droplets impinging upon liquid films
No full textThe phenomenon of liquid drop impact onto liquid surfaces appears in a wide variety of engineering areas such as ink-jet printers, internal combustion engines, spray cooling, etc. The research of this study has been motivated by a need for better predictive capability in these industries. Many authors defined different outcomes regarding this phenomenon. Rioboo et al. [1] revealed more than six possible outcomes, which include deposition, stick, crown splash, prompt splash, among others. These different outcomes may vary depending on many factors such as the liquid drop parameters (diameter, impact velocity, viscosity, surface tension, etc.), the liquid film parameters (thickness, characteristics of the underlying surface, etc.) and the surrounding gas (velocity, pressure, etc.).Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Numerical and experimental study of a plunging airfoil
No full textCurrently existing air vehicles, although efficient in the cruise conditions, lose much of their performance in maneuvers and phases of the flight such as takeoff, landing, turn or climbing. On the other hand, the flight performed by some animals in Nature demonstrates an ideal adaptation and high efficiency for each flight phase. The production of lift and thrust is performed by the same structures, the wings. On the contrary, in nowadays vehicles, there is a separation of the lift force, produced by the wings, and thrust, produced by engines.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
On the Modelling of Evaporating Sprays Impinging onto Solid Surfaces
No full textThe present paper presents a numerical study on evaporating droplets impinging onto a solid surface through a crossflow. The characteristics of the initial spray are established by employing an empirical procedure, which relies on a comprehensive set of free spray measurements. Distinct wall and crossflow temperatures are analysed systematically to evaluate the in fluence of droplets evaporation on the nal outcome of spray impingement, and, particularly, on the distribution of the thin liquid lm over the surface. The present computational model already proved to deliver accurate predictions of the spray/wall interactions under different conditions. In this work, the conditions are extrapolated to a heated environment, which reproduce more adequately what is found in in-cylinder situations. The computational model is adapted to meet the new requirements and perform within the range of conditions for which it is now formulated. The analysis shows that higher temperatures lead to smaller impinging droplets, an increase in the number of depositing droplets but a decrease in the fraction of mass of particles contributing to the liquid film; and a more uniform distribution of the liquid layer over the surface.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Numerical Modeling Of Freezing Water Droplets
No full textThis work enhances the understanding of droplet dynamics under cryogenics conditions through an in-house developed analytical tool used to predict the supercooling of water droplets. The physical process is based on a full-scale four-stage supercooling process in which the recalescence stage is assumed instantaneous and the crystallization kinematics of the droplet neglected. The transition temperature of each stage is obtained, resorting to a theoretical balance for the droplets internal energy against the heat loss to the environment. In this way, the representation for the droplets temperature curves alongside the time. The droplets are considered spherical, and the internal motion is so vigorous that complete mixing occurs. The droplets convective effects are accounted for using the Ranz-Marshall classical formulation.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Effects of a Dynamic Leading Edge on a Plunging Airfoil
No full textThe dynamics of oscillating airfoils are of great interest in many research areas such as rotor dynamics and biomimetics. The results reported in this research provide an insight into the mechanics of birds’ leading edge and how the dynamic curvature of the airfoil can highly benefit the aerodynamic and propulsive performance, especially at high angles of attack. The main goal of the current work is to numerically investigate the influence of a deflecting leading edge on the propulsive coefficients and flowfield created by a plunging airfoil at a Reynolds number of 1.4 × 104 and a constant Strouhal number of 0.15 with different ( k, ℎ) combinations. Employing a RANS approach with the proposed NACA0012-IK30 airfoil, results show that dynamically deflecting the leading edge significantly improves the propulsive efficiency of the airfoil by either reducing the required power or improving the thrust production. The outcomes regarding the propulsive efficiency show a considerable increase of up to 92% when the higher nondimensional amplitude was considered.Fundação para a Ciência e Tecnologia (FCT), Santander-UBI BID/FE/2019, Brazilian’s National Council for Scientific and Technological Development, São Paulo State Research Foundation-info:eu-repo/semantics/publishedVersio
Theoretical and Numerical Analysis of Oscillating Airfoil Including Viscous Effects
No full textUnsteady airfoils are the way to explore new aerodynamic phenomena which do not appear in ordinarily aeronautical applications. At lower Reynolds numbers, unsteady flow can make way to newer technologies to be implemented in areas such as extraterrestrial exploration. In the present work, the Theodorsen classical unsteady theory and a viscous extension to this model are implemented. Results from those computations are then compared with high fidelity CFD simulations using laminar and turbulent assumptions. Results obtained by using the viscous model indicate a good agreement with CFD data, although there are still some discrepancies at the trailing edge.Fundação para a Ciência e a Tecnologia, National Council for Scientific and Technological Development – CNPq, São Paulo State Research Agency – FAPESPinfo:eu-repo/semantics/publishedVersio
Wing Design and Analysis for Micro Air Vehicle Development
No full textNatural flight was always a source of inspiration to human beings, and by observing it, new technologies emerged, especially in bioinspired aerial vehicle design. The main objective of the present work is to experimentally evaluate the influence of the wing shape on their behavior and power consumption. To accomplish that goal, four wings were selected and an experimental rig was developed. Airflow velocities tested ranged from 0 up to 4 m s−1 with four different throttle positions. Flapping frequency and amplitude were obtained from high-speed camera tests and the power consumption of the wings tested was measured. It is observed that amplitude is between 0.12 m and 0.25 m, frequency between 4 and 15 Hz. Regarding power consumption, the design wings presented lower averaged power-to-weight ratio values when compared to BionicBird wings.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio