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Browsing by Issue Date, starting with "2017-01-05"

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  • Experimental Analysis of Forces During Take-Off of Birds
    Publication . Barata, Jorge M M; Silva, Telmo J.; Neves, Fernando M. S. P.; Silva, A. R. R.
    The increasing interest on Unmanned Air Vehicles (UAV’s) and their several utilities blended with the need of easy carrying and also the stealth, lead to the need to create the concept of Micro Air Vehicles (MAV’s) and the Nano Air Vehicles (NAV’s). Due to the current interest and the present lack of knowledge on the insect’s and bird’s flight, this study was intended to interpret the forces involved on the moment of the take-off of a bird, recurring to an experiment involving a fast data acquisition force sensor and high speed camera. It was found that the birds can produce special movements just before taking-off that will enhance the performance of the maneuver. In the present case, the bird stretches the neck forward and moves the head down, then stretches even more the neck and finally moves the head up while impelling himself into the air, resulting in an enhanced mechanical force against perch. The results have shown that Columba livia can generate about 4 times its weight of take-off force against perch.
    2017-01-05Journal article Restricted access Show more
  • Effect of the Impinging Height of Twin Jets in Tandem Through a Crossflow
    Publication . Barata, Jorge M M; Silva, A. R. R.; Vieira, Diana
    The complex flow field generated by the impact of twin impinging jets in tandem through a low velocity crossflow was numerically studied. The Reynolds number based on the jet exit conditions was 43,000, the jet-to-crossflow velocity ratio from 7.5 to 90, and an interject spacing of 6 diameters of the jet. The impingement heights used were 3 to 15. The mathematical model used is based on the solution of the continuity and momentum equations. A RANS formulation was adopted with the “k-ε” turbulent model to represent the turbulent stresses. The numerical results showed the influence of the impingement height on the ground vortex location, size and interaction with the surrounding flow, but new aspects of this type of flows were found for the present case of a tandem configuration. In the region between the jets the usual fountain upwash flow does not occur, but a second small ground vortex was detected for H/D≤10, due to the interaction between the wall jets of each impinging jet. To our knowledge this is a new phenomenon that is being reported in the literature for the very first time.
    2017-01-05Conference object Restricted access Show more
  • Spray-Wall Interactions Modeling
    Publication . Silva, A. R. R.; Rodrigues, Christian; Barata, Jorge M M
    Spray impingement is an important phenomenon affecting a wide variety of applications. The present work describes the developments that our research group that have been made to a basic spray impingement model to account for the influence of several parameters, such the liquid film deposited onto the surface, the wall temperature or the crossflow velocity. Numerical simulations are carried out for predicting the outcome of such flows that include liquid film formation, the droplet breakup and the spray evaporation. An empirical procedure is used to define the initial spray characteristics, which relies on a comprehensive set of free spray measurements. The computer model is evaluated by comparing the numerical simulations against experiments on spray impacting on a solid surface with the presence of a crossflow. The predicted results show a reasonable agreement with the measurements. Moreover, further results are presented on the formation droplets due to the deformation and breakup mechanisms.
    2017-01-05Conference object Restricted access Show more