Vorticity, kinetic energy and momentum analysis of the collision zone between a plane wall jet and a crossflow

When 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