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Experimental Study of a Ground Vortex
No full textLaser-Doppler measurements of the mean and turbulent components of the velocity flowfield resulting from the frontal collision of a wall jet with a boundary layer are presented and discussed, together with the visualization of the flow with direct photography and digital film imaging. The experiments were carried out for boundary-layer-to-wall- jet velocity ratios of 0.5 and 0.58. The results revealed the existence of a small counter-rotating recirculating zone located upstream (in the sense of the wall-jet flow) of the separation point, not reported before for this type of flow. This secondary vortex has an oscillating behavior observed in other ground-vortex flows, similar to that which can be observed, namely, through the bimodal histogram of the horizontal velocity (parallel to the wall) measurements, although the power spectra measurements do not exhibit any particular peaks.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Modelling of Drop Deformation and Breakup
No full textThe present paper addresses the macroscopic atomization characteristics of liquid-fuel droplets when subjected to the infuence of a high velocity air crossfow. A breakup model is conceived by using a set of correlations available in the literature with the purpose of replicating such phenomena. The computational results are compared against experimental data to validate the model. The results show a reasonable agreement between measurements and predictions in both qualitative and quantitative outcomes evaluated, which sustain the mathematical formulation adopted. However, further improvements may be aspired given the fact that there is a lack of experimental data available when shearing effects come into play in the mechanisms occurring during the atomization process. On the other hand, the use of two fuels (diesel and bio-diesel) allowed to perceive a relevant impact of the liquid properties (particularly surface tension and viscosity) in the characteristics of the fragments resulting from the breakup event.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Influence of Trailing-Edge Shape on the Propulsive Performance of a Plunging Flat Plate
No full textUnsteady aerodynamics is the study of flows that pass an object subjected to oscillations, which has a strong association with bioinspired flows and rotor dynamics. The central objective of the paper is to study the propulsive characteristics of two plates at a Reynolds number of 3.1x10^3 with straight and sharp trailing edges. The reduced frequency is kept between 1.0 and 5.0 with a nondimensional amplitude ranging between 0.125 and 0.500. The problem was solved numerically using CFD (Computational Fluid Dynamics), and results show that differently from what is usually observed in airfoils, the mean thrust coefficient does not increase monotonously with the reduced frequency, having a dip around k = 3.0. The same is not verified for the mean power coefficient, which increases continuously with the reduced frequency. Two phenomenon responsible for the reduction of the mean thrust coefficient is identified. Through analysis of pressure contours around the flat plates, a low-pressure zone was seen near the trailing edge, creating a suction effect in that zone. The influence of k on the formation of the LEV is analyzed, via its approximate radius, evidencing a behavior similar to the mean Ct . Furthermore, the trailing-edge shape revealed to be an important factor on thrust reduction.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Cubic and multiparameter equations of state evaluation for supercritical flow modeling
No full textThe dawn of space exploration privatizing contributed to a boost and renewed interest in the modeling of vehicles used for such missions. However, the increase of temperature and pressure in combustion chambers, while enhancing fuel injection efficiency, makes fuel and oxidizer exceed their critical point entering a supercritical state. While cubic equations of state such as the Soave-Redlich-Kwong and Peng-Robinson are widely utilized due to their relative simplicity, recent formulations of multiparameter equations for transport properties based on the Helmholtz reduced energy are proving to be an alternative worth considering. The higher accuracy and improved performance of these equations in the critical region is a time-consuming enterprise, relying on large and accurate data sets. Therefore, this work aims at evaluating the influence of cubic and multiparameter transport equations of state in the numerical evolution of density and temperature inside a cryogenic liquid rocket combustion chamber at supercritical conditions.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Centennial 1922–2022: First Transoceanic Flight with Autonomous Aerial Navigation
No full textAlthough the Transoceanic Flights had already piqued the state's attention, the airplane's lack of aerial navigation equipment rendered it unsuitable for this new trend in 1919. Three years later, in 1922, Sacadura Cabral and Gago Coutinho conducted a Transoceanic Flight to demonstrate the autonomy and usefulness of the Sextant with an artificial horizon, in combination with novel air navigation instruments, all handled by a simplified short air navigation method. Unfortunately, due to errors induced by the vertical dynamics associated with the aircraft's speed, the sextant was constantly adjusted to the sea horizon rather than the perfect readings of the artificial horizon. This condition required a search for a suitably clear horizon line, forcing the seaplane's altitude to be often lowered. Furthermore, a few supplementary navigation devices risked their initial autonomous air navigation. Nonetheless, by measuring the size of the wingspan shadow reflected on the ocean's surface, the pilots managed to adjust their expertise to maintain the seaplane on the planned trajectory. Furthermore, this novel and reliable steering method allowed them to perform trigonometric calculations to estimate their height, which was critical in calculating the aircraft's positioning. Thus, the Journey was recognized as a significant milestone in aviation history, ushering the use of the sextant as a key means of air navigation and proving the effectiveness of a revolutionary principle of Coutinho's short methods, never before documented on Astronavigation. Along with the First Transoceanic Flight with Autonomous Aerial Navigation, one of the itineraries featured an 11 ½ hour crossing the Atlantic from Cape Verde to St. Peter and St. Paul Rocks. When it is remembered that they managed to reach this remote and a pretty small destination (a few hundred ft long and the highest point 60 ft above the water) after a flight of nearly 900 nautical miles over the Ocean, it exalts the remarkable feat of the airmen.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Design and Testing of a Wing with a Morphing Trailing Edge
No full textIn recent years, the interest in morphing technologies has been continuously growing. Many designs have been developed, but some are very complex to manufacture. The main goal of this paper is to present a wing design with a morphing trailing edge that is easy to manufacture and control while allowing the execution of appropriate aerodynamic studies in wind tunnel. Using computer-aided design and 3D printing, a wing that is easily controlled with easy-to-use hardware is developed, manufactured, and studied using flow visualization and force acquisition. Flow visualization showed that the presence of curvature delays stall occurrence when compared to the symmetrical configuration, where flow separation starts right at the leading edge. This is validated by force measurements where curvature plays an important role regarding lift enhancement. This design will allow for a wider range of studies, including unsteady conditions, which can offer insights concerning aerodynamic surfaces design.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
On the Modeling of a Spray Impingement Onto a Hot Surface
No full textThe present investigation addresses the impingement of a spray onto a heated solid surface under crossflow conditions at low pressure injection - typical of port-injection engines - during cold start. The characteristics of the initial spray are established by employing an empirical procedure, which relies on a comprehensive set of free spray measurements. This computational study considers the presence of a crossflow moving perpendicularly to an interposed surface and the formation of a dynamic liquid film over the impingement wall. Both previous conditions are often neglected in numerical simulations, despite their importance on the final outcome. Distinct wall and crossflow temperatures are analyzed systematically to evaluate the influence of droplets evaporation on the final outcome of spray impingement, and, particularly, on the distribution of the thin liquid film 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 Optimization of the Location of F1 CDG Wings
No full textIn October 2005, FIA (International Automobile Federation) proposed a set of rules, for the 2008/9 Formula 1 Championship, with the objective of decreasing the difficulties for overtaking due to the wake generated by the leading car. The proposal is based on the utilization of a new concept of a double rear wing "CDG-wing" (Central Downwash Generating Wing). The objectives of this work is to analyze the aerodynamic characteristics of the CDG wing proposed by FIA, and to investigate its better positioning relative to the rear wheel. The study was performed using three different geometries of the wing, and three different speeds, to reproduce three different curves of some race tracks used in Formula 1. The computational tests have shown that the location of the wings behind the rear tire increases the downforce.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Numerical Study of Cryogenic Jets Under Supercritical Conditions
No full textThe present work is devoted to the study of cryogenic nitrogen jets in sub- and supercritical conditions. A general trend to operate under increasingly higher combustor pressures is observed in rockets, gas turbines, and diesel engines, primarily as a result of enhanced effects on thrust, power, or efficiency. In these conditions the injected fluid(s) can experience ambient pressures exceeding the critical pressure(s) of the propellants, and recent experimental work of Chehroudi et al. showed a quantitative similarity to gas-jet-like behavior (Chehroudi, B., Conn, R., and Talley, D.. "Spray/Gas Behaviour of Cryogenic Fluids under Sub- and Supercritical Conditions," Paper. July 2000; Chehroudi. B., Cohn, R., and Talley, D.. "Initial Growth Rate and Visual Characteristics of a Round Jet into a Sub- to Supercritical Environment of Relevance to Rocket, Gas Turbine, and Diesel Engines," AIA A Paper 99-0206, Jan. 1999). This conclusion suggested that it would be expected that the mathematical models and numerical methods used for gaseous flows could also be used for supercritical flows. This paper reports an investigation, exploring this hypothesis, and aims to evaluate the capabilities and limitations of a computational method developed for incompressible but variable density flows when applied to supercritical conditions. The predicted initial jet growth rate was compared with available experimental data for liquid/gaseous jets and mixing layers and showed a good agreement for different supercritical density ratios. For subcritical conditions, when the flow deviates from the gaseous-like behavior, and approaches a more spray-like behavior the incompressible gaseous flow formulation was found inadequate.ICCTI of the Portuguese Ministry of Science and the French Embassyinfo:eu-repo/semantics/publishedVersio
On the Performance of the Spray Impingement Models
No full textThe present paper reports a numerical study of a spray impinging on a surface through a crossflow. This work is aimed at studying the fundamental aspects of the spray impingement phenomena in a three-dimensional configuration, which is relevant to practical engines. A computational study of the flow produced by a spray stream injected through a crossflow is reported and discussed in detail. Different impinging models were used and the results are compared with measurements and predictions of other authors. A new droplet impingement model was developed and tested. The results show a general good agreement with the experiments, and reveal that the splash droplets are very important for the spray development.info:eu-repo/semantics/publishedVersio