Browsing by Issue Date, starting with "2012-06-15"
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- Unsteadiness of a Ground Vortex FlowPublication . Silva, André; Barata, Jorge M M; Santos, Pedro J. T. C; Nunes, Ricardo F.; Durão, DiamantinoThe impact zone of a wall jet with a boundary layer is studied computationally using a Reynolds averaged Navier-Stokes (RANS) approach with the "κ-ε" turbulence model. The computational domain corresponds to complete experimental rig and the measured boundary conditions were used. It was found that the gross features of the flow are well predicted, and the fluctuations of the flowfield around the ground vortex occur in a very small region near the wall where the impact between each flow occurs. The computational results showed a cyclic formation of two small secondary vortexes that appear and disappear cyclically around the separation and maximum penetration points of the ground vortex.
- Experimental Study of the Collision Zone of a Boundary Layer with a Wall JetPublication . Barata, Jorge M M; Ribeiro, Samuel Dinis Galvão; Santos, Pedro; Silva, AndréLaser-Doppler measurements are presented for a highly curved flow resulting from the collision of plane turbulent wall jet with a boundary layer. This configuration can be found in the case of impinging jets of a V/STOL aircraft operating in ground vicinity. The experiments were carried out for boundary layer to wall jet velocity ratio of 0.5. The results include mean and turbulent velocity characteristics of the flow field as well as the characterization of the vorticity. The results revealed the existence of very small recirculation zones located upstream the separation point that co-exist and interact with the very well known VSTOL ground vortex. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc.
- Numerical Study of Mixing of Co-Axial JetsPublication . Silva, André; Neves, Fernando M. S. P.; Barata, Jorge M MThis paper presents a numerical study of the effect of the turbulence intensity in the turbulent mixing of coaxial confined jets. The objective of this work is to study the effect of the initial levels of turbulence of each coaxial jet in the process of turbulent mixing for diameter ratios less than 2, which is the case of very low bypass turbofan engines. The computational method was tested using the available experimental data, and revealed good capabilities for the simulation of this kind of flows. The effect of the velocity ratio of the two streams is presented for velocity ratios of the two streams of 0.3, 1.5, and 6, and turbulence intensity ratios between 10-5 and 10 5. The results revealed that for relatively high turbulence intensities of the outer jet a vortex is always present for velocity ratios of 6 or larger, and this is independent of the turbulence intensity ratio. When the turbulence intensity of the inner jet is kept constant and the ratio of turbulence intensities is increased, the recirculation zone decreases in size, its center moves further upstream until it disappears completely. The existence of a favorable pressure gradient as a similar effect, revealing that this is an additional parameter to be taken into consideration together with the velocity ratio and turbulence intensity ratio as far as the recirculation zone is concerned.
- A Comparison of Different Gas Turbine Engines Ground Vortex FlowsPublication . Barata, Jorge M M; Manquinho, Pedro; Silva, AndréA computational method was used to predict and compare the ground vortices forming between the ground and a 1/1th scale model intake of different gas turbine engines. Calculations have been performed for the case of an irrotational crossflow mode of formation. Two different kinds of vortices appear around the intake: an inlet vortex and a trailing vortex. Away from the inlet plane other types of complex vortical structures were also identified. The computational method was found to describe all the main features of the flow for full scale geometry.
- Experimental Study of Instabilities and Secondary Effects of a Ground Vortex FlowPublication . Barata, Jorge M M; Ribeiro, Samuel Dinis Galvão; Santos, Pedro; Silva, André; Silvestre, M.A.R.Laser-Doppler measurements of velocity characteristics of the flowfield resulting from the collision of a wall jet with a boundary layer are presented and discussed together with visualization of the flow. The experiments have been carried out for boundary layer to wall jet velocity ratios of 0.5 and 0.58. Direct photography, digital film imaging, and mean and turbulent laser Doppler measurements were obtained to characterize this type of flows. For the present conditions the results revealed the existence of a small counter-rotating recirculating zone located upstream the separation point not yet reported before for this type of flows. The results show that this secondary vortex has a similar oscillating behavior observed in some configurations leading to ground vortex flows, where they are also probably present, but due the geometric and characteristic parameters could not be identified separately. A particular ordered sequence was recognized for the small recirculation zone, and can be confirmed by the bimodal histogram of the horizontal velocity measurements obtained. In spite of the apparent organized sequence of the turbulent structure of the collision zone, the power spectra measurements did not exhibit any evident particular peaks.
- Numerical Study of Impinging Droplets Through a CrossflowPublication . Barata, Jorge M M; Silva, AndréThe present paper reports a numerical study of a spray impinging on a surface through a crossflow. This work is aimed to study the fundamental aspects of the spray impingement phenomena in a three-dimensional configuration, which is relevant to practical engines. The work describes a computational study of the flow produced by a spray stream injected through a crossflow. Different impinging models were tested, including a variant of the Grover and Assanis model developed in the present study, and the results are compared with measurements and predictions of other authors. The predicted results show in general reasonable agreement with the experimental data. The performance of droplet impinging models depends strongly on the conditions of pre-impingement.
- First Flight From Europe to the South Atlantic - Gago Coutinho and Sacadura CabralPublication . Silva, André; Barata, Jorge M M; Morgado, Cândido M. P.; Neves, Fernando M. S. P.The history of the transatlantic flights goes back to 1919, and began with a flight between Newfoundland and Lisbon, and another between Newfoundland and Ireland. Gago Coutinho and Sacadura Cabral crossed the South Atlantic Ocean by air in 1922. This last flight was the first that used only internal means of navigation, and a new instrument that consisted in a type of sextant incorporating two spirit levels to provide an artificial horizon was invented. The flight began with a Fairey hydroplane that did not have enough fuel capacity to make the entire trip so some stops were made along the way. Several problems had to be solved and near the Brazilian coast he aviators were forced to stay some time in the ocean water, having been rescued by a Bristish ship called "Paris City". Finally, the 8,300 km trip was concluded, only with internal means of navigation, after 62 hours of flight using another plane, with a most enthusiastic reception at Rio de Janeiro, and the new artificial horizon sextant had proven itself.
- Flow Characteristics of Twin Impinging JetsPublication . Barata, Jorge M M; Sousa, Nuno M. S.; Silva, André; Meireles, Pedro M. S.This work is devoted to the visualization and characterization of a twin jet flow when impinging vertically to a solid surface, which is of primary importance to the understanding of the phenomena relevant to V/STOL type of aircraft performance and stability when operating in ground vicinity, during short take-off and landing. The visualization of a distinctive fan-shaped flow that is formed in the collision zone (or fountain formation zone) is the major concern in this work. Variables analyzed in this work are the separation distance, height and diameter of the jets. It was found that the fountain flow involves complex flow structures, quite sensitive to small imbalances between the jets. It was also observed that significant changes in the flow pattern and properties in the fountain formation zone occur with according changes in the parameters studied.