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Browsing by Author "Nunes, Ricardo Bruno Freitas"

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  • Numerical study of the unsteadiness of a ground vortex
    Publication . Nunes, Ricardo Bruno Freitas; Barata, Jorge Manuel Martins
    Single impinging jets in a crossflow are typical in impingement cooling applications in industry, as well as of the flow beneath a V/STOL aircraft. In this latter application, a primary design consideration is the flow environment induced by the propulsion system during hover with zero or small forward momentum. Ground effect phenomena may occur and change the lift forces on the aircraft, cause re-ingestion of exhaust gases into the engine intake and raise fuselage skin temperatures. An important source of each is the ground vortex which forms far upstream of the impinging jet when the resulting radial wall jet meets a crossflow. Numerical and experimental studies have also been performed in this area. Some were dedicated to the study of the more fundamental configurations: single or multiple impinging jets through a crossflow. The present thesis extends the analysis of (Pandya, Murman, & Sankaran, 2003) to a wider range of velocity ratios, VR, from 0.065 to 0.2. The impact zone of a wall jet with a boundary layer was studied computationally using a Reynolds Averaged Navier-Stokes (RANS) approach with the “k-ε” turbulence model. The computational domain corresponds to complete experimental rig of (Cimbala, Billet, Gaublomme, & Oefelein, 1991) 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. This result confirms the observation of (Pandya, Murman, & Sankaran, 2003). The frequency of the “puffing” was found to compare well with the experimental results for VR=0.1, and the structure of the impact zone is similar. First, the wall jet fluid start to penetrate into the boundary layer side until a very small counterclockwise rotating vortex appears. Then it starts to grow blocking the passage of the clockwise rotating fluid of the wall jet, and a new small vortex appears, but now with clockwise vorticity. A particular result was obtained for VR=0.175. The flow exhibits a periodic behaviour, but no secondary vortexes are detected. Nevertheless, in this case the frequency was found to correlate well with the values obtained for VR=0.1, 0.125, and 0.15. For the case of very low velocity crossflow (small VR) the computations exhibit a stationary solution, which is in agreement with previous experimental results. For strong crossflows (large VR) the flow is also stationary, although there is a transition region of some unsteadiness without secondary vortexes present. The present work has shown that for a finite interval of velocity ratios between the impinging jet and the crossflow periodic oscillations of the ground vortex are observed. The results indicate a pattern similar to the “puffing” mechanism described by (Cimbala, Billet, Gaublomme, & Oefelein, 1991).
    2009Master thesis Open access Show more