Browsing by Author "Santos, Pedro José da Costa Teixeira"
Now showing 1 - 6 of 6
Results Per Page
Sort Options
- Experimental Study of a Ground VortexPublication . Barata, Jorge M M; Ribeiro, Samuel; Santos, Pedro José da Costa Teixeira; Silva, AndréLaser-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.
- Experimental Study of a Ground Vortex: The Effect of the Crossflow VelocityPublication . Barata, Jorge M M; Bernardo, Nuno; Santos, Pedro José da Costa Teixeira; Silva, AndréThe study of the complex flowfield produced by a single jet impinging on a wall through a crossflow has been carried out over the past several decades with many different motivations and applications. This type of flow is most relevant to the successful design and operation of vertical and/or short take-off and landing aircrafts. The increased momentum of the wall jet is consistent with the movement of the separation point in the X direction, but the region of higher mean vertical velocity spreads in the wall direction. Nevertheless, their results did not exhibit any bimodal LDV histograms of discrete frequencies that could be associated with any instabilities or oscillations. To avoid the influence of the impinging region, a plane wall jet is produced independently. The wall jet collides with the boundary layer produced using a conventional wind tunnel giving rise to a highly curved region, which can be studied for different velocity ratios between the wall jet and crossflow.
- Experimental Study of a Ground Vortex: the Effect of the Crossflow VelocityPublication . Barata, Jorge M M; Bernardo, Nuno; Santos, Pedro José da Costa Teixeira; Silva, AndréLaser-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 velocity ratios between the boundary layer and the wall jet of 0.5 and 0.62 were considered. Two secondary vortices were found upstream the separation point for the highest velocity ratio. The angle of deflection of the upwash flow was found to decrease with the velocity ratio, while the wall jet penetration increases.
- Laser-Doppler Analysis of the Separation Zone of the Ground Vortex FlowPublication . Silva, André; Durão, Diamantino; Barata, Jorge M M; Santos, Pedro José da Costa Teixeira; Ribeiro, SamuelLaser Doppler measurements are presented for a highly curved flow generated by the collision of plane wall turbulent jet with a low-velocity boundary layer. The experiments were performed for a wall jet-to-boundary layer velocity ratio of 2, and include mean and turbulent velocity characteristics along the two normal directions contained in planes parallel to the nozzle axis. The results, which have relevance to flows encountered by powered-lift aircraft operating in ground effect, quantify the structure of the complex ground vortex flow resulting from the collision of a wall jet with a boundary layer. The results revealed the existence of a very low-frequency instability. The source of this low frequency unsteadiness is probably associated with a small vortex located near the separation point. In the central zone of the upwash flow where the maximum values of the vertical velocity component occurs, additional distinct high frequency peaks were also identified.
- Turbulent Energy Budgets of a Ground Vortex FlowPublication . Barata, Jorge M M; Santos, Pedro José da Costa Teixeira; Silva, André; Durão, DiamantinoTurbulent kinetic energy budgets are presented for a highly curved flow generated by the collision of plane wall turbulent jet with a low-velocity boundary layer. The different terms are obtained in the vertical plane of symmetry by quadratic interpolation of the LDV (Laser Doppler Velocimetry) measurements, for a wall jet-to-boundary layer velocity ratio of 2. The results, which have relevance to flows encountered in powered-lift aircraft operating in ground effect, quantify the structure of the complex ground vortex flow. 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. This is an indication that the modeling of turbulence of a ground vortex requires a good representation of the production by normal stresses which is most important in the collision zone.
- Vorticity, kinetic energy and momentum analysis of the collision zone between a plane wall jet and a crossflowPublication . Santos, Pedro José da Costa Teixeira; Barata, Jorge Manuel Martins; Silva, André Resende Rodrigues daWhen a Short/Vertical Take-off and Landing (V/STOL) aircraft is lifting off or landing with zero or small forward momentum, a complex flow can be found under of aircraft. The lifting jets impinging on the ground giving rise to wall jets that interact between them forming an upwash or that can collide with any crosswind due to the presence of wind or due to the movement of aircraft. These type of flows have profound influences in aircraft performance, such as: lift losses; enhanced entrainment close to the ground (suckdown); engine thrust losses and re-ingestion of the exhaust gases; and, also, possible aerodynamic instabilities caused by the fountain impingement on aircraft underside. The impingements of a lift jet on the ground origins a wall jet that flows radially from the impinging point and along the ground surface. When this wall jet meets a freestream flowing parallel to the wall in the opposite sense, the crossflow, there is the formation of a highly curved flow far upstream of the impinging jet from the perspective of the crossflow. This highly curved flow is named by ground vortex, and has profound influences on the flow development. Measurements of these types of flows are very scarce in the literature, and are reported as a secondary flow within the impinging jet flow problem, and are, also, dispersed among many different configurations and operating conditions. The present work is included in a research program dedicated to the identification of the parameters and regimes associated with instabilities, and other secondary effects of this ground vortex flow. It is presented a detailed analysis of a ground vortex resulting from the collision between a wall jet and a boundary layer, and follows the previous study of Barata et al. (2005), which detected a small recirculation zone located upstream the separation point and not yet reported. To avoid the influence of the impinging region, created by the lift jet, a plane turbulent wall jet is produced independently using a configurations inspired in a previous study about bi-dimensional upwash flows. The wall jet collides with the boundary layer produced by a conventional wind tunnel creating the ground vortex. The experimental facility used in this work permits to study different velocity ratios between the boundary layer and the wall jet. Laser Doppler measurements are presented for a velocity ratio between boundary layer and wall jet of 0.5, and include mean and turbulent velocity characteristics along the two normal directions in the plane of symmetry of the flow. Vorticity, turbulent kinetic energy balances and momentum balances were determined to understand the complex flow in the collision zone near the ground wall, which is characterized by the turbulent structures that change their size and shape with time. The results revealed that the modeling of turbulence of this flow may require an adequate treatment of production of turbulent kinetic energy by normal stresses, which are predominant in the collision zone. This work aims to improve the understanding of the essential dynamics of ground vortex flows with application to the V/STOL aircrafts.