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- Turbulent structure of the impact of a ground vortex flowPublication . Vieira, Diana Filipa da Conceição; Barata, Jorge Manuel MartinsTurbulent jets impinging on flat surfaces through a low-velocity crossflow are typical of the flow beneath a short/vertical take-off aircraft which is lifting off or landing with zero or small forward momentum1. Ground effect may occur and change the lift forces on the aircraft, cause reingestion of exhaust gases into the engine intake and raise fuselage skin temperatures. In this latter application the impingement of each downward-directed jet on the ground results in the formation of a wall jet which flows radially from the impinging point along the ground surface. The interaction of this wall jet with the free stream results in the formation of a “ground vortex” far upstream of the impinging jet, which has profound implications on the aircraft design. In addition the collision of the wall jets originates a fountain upwash flow, affecting the forces and moments induced in the aircraft when operating in ground effect. Improved knowledge of impinging flows is therefore necessary to avoid these effects and to be able to model a range of jet-impingement type of applications with practical interest. Recently there have been some reports of studies focused on one or multiple jets configurations to understand the phenomena associated with this type of complex flows, revealing this studies great importance to the Harrier/ AV-8B aircraft. This work is dedicated to a detailed analysis of the complex flow field beneath 2 impinging jets aligned with a low-velocity crossflow relevant for the future F-35 VSTOL configuration, and provides a quantitative picture of the main features of interest for impingement type of flows.
- Numerical Simulation of Twin Impinging Jets in Tandem Through a CrossflowPublication . Vieira, Diana; Barata, Jorge M M; Neves, Fernando M. S. P.; Silva, A. R. R.The flow field of ground vortex generated by twin impinging jets in tandem through a crossflow is numerically studied in detail. Numerical simulation and visualization are presented for two turbulent circular jets emerging into a low velocity cross stream, impinging after on a flat surface perpendicular to the geometrical jet nozzle axis. The numerical study is based in experimental studies done early, so all the features of the experimental flow were maintained when the numerical simulation was performed. The Reynolds number used was based on the jet exit conditions of 43.000-105.000, a jet to crossflow velocity ratio of 22.5-43.8 an impinging height of 20.1 jet diameters and an interject spacing’s of S = 5D and L = 6D. The analysis of the flow was extended to regions and flow conditions for which no measurements have been obtained in last experimental studies, i.e., for velocity ratios of 7.5-60. The numerical results show that for the smallest velocity ratios the jets initially do not mix but remain together in two layers. Three different types of flow regimes were identify, therefore when VSTOL aircrafts operating in ground vicinity, only the regime with strong impingement on ground and with a formation of a ground vortex is relevant. The numerical results allowed to extend the last experimental studies and prove that the deflection of the rear jet is due to the competing influences the wake, the shear layer, the downstream wall jet of the first jet and the crossflow.
- Twin Impinging Jets Through a CrossflowPublication . Barata, Jorge M M; Neves, Fernando M. S. P.; Vieira, Diana; Silva, AndréLaser Doppler measurements provide information on the flowfield created by twin impinging jets aligned with a low velocity crossflow. The experiments were carried out for a Reynolds number based on the jet exit conditions of Re j=4.3×104, an impingement height of 20.1 jet diameters and for a velocity ratio between the jet exit and the crossflow V R=Vj/Uo of 22.5, and an interjet spacing of S=6D. The results show a large penetration of the first (upstream) jet that is deflected by the crossflow and impinges on the ground, giving rise to a ground vortex due to the collision of the radial wall and the crossflow that wraps around the impinging point like a scarf. The second jet (located downstream) it is not so affected by the crossflow in terms of deflection, but due to the downstream wall jet that flows radially from the impinging point of the first jet it does not reach the ground. The results indicate a new flow pattern not yet reported so far, that for a VSTOL aircraft operating in ground vicinity with front wind or small forward movement may result in enhanced under pressures in the aft part of the aircraft causing a suction down force and a change of the pitching moment towards the ground.
- Experimental Study of Two Impinging Jets Aligned With a CrossflowPublication . Barata, Jorge M M; Neves, Fernando M. S. P.; Vieira, Diana; Silva, AndréLaser Doppler measurements provide information on the flowfield created by twin impinging jets aligned with a low velocity crossflow. The experiments were carried out for a Reynolds number based on the jet exit conditions of Rej = 4.3 × 104, an impingement height of 20.1 jet diameters and for a velocity ratio between the jet exit and the crossflow VR = Vj/Uo of 22.5, and an inter-jet spacing of S = 6D. The results show a large penetration of the first (upstream) jet that is deflected by the crossflow and impinges on the ground, giving rise to a ground vortex due to the collision of the radial wall and the crossflow that wraps around the impinging point like a scarf. The second jet (located downstream) is not so affected by the crossflow in terms of deflection, but due to the downstream wall jet that flows radially from the impinging point of the first jet it does not reach the ground. The results indicate a new flow pattern not yet reported so far, that is most relevant for a VSTOL aircraft operating in ground vicinity with front wind or small forward movement may result in enhanced under pressures in the aft part of the aircraft causing a suction down force and a change of the pitching moment towards the ground.
- Effect of the Impinging Height of Twin Jets in Tandem Through a CrossflowPublication . Barata, Jorge M M; Silva, A. R. R.; Vieira, DianaThe 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.
- Twin impinging jets inline with a low-velocity crossflowPublication . Vieira, Diana Filipa da Conceição; Barata, Jorge Manuel MartinsVertical/short take-off and landing aircrafts at their hovering phase of flight create a three dimensional flowfield between lift jet streams, the airframe surface and the ground. The flowfield surrounding the aircraft during transition from hover to wing borne flight is of particular importance. During the transitional flight phase, the jets in crossflow phenomenon represent the most relevant configuration due to the complex flowfield that is created beneath the aircraft. The wall jets created by the impingement on the ground of the individual turbulent jet flow meet at a stagnation line and form an upwards flowing “fountain” that interacts with the airframe. Sometimes the fountain can provide a beneficial lift – generating ground cushion. Although, in most of the cases the fountain flow created generates a variety of undesirable characteristics, such as, hot gas ingestion, pressure, thermal and acoustic loads, change of the lift forces, lifting losses and the fuselage skin raise. The wall jet created by the jets impingement on the ground interacting with the free stream, results in a formation of a ground vortex far upstream of the impingement jet. This resulting ground vortex shape is strongly affected and the corresponding induced suckdown effect tends to be reduced by the upload produced by the fountain. During the past three decades, the flowfield characteristics associated with this type of aircraft have been studied extensively. However, the complexity of the new VSTOL configurations with the very stringent requirements demands more investigation. The continued development of a VSTOL aircraft with an increasing reliance on computational design techniques is dependent on a better understanding of aerodynamics of the lift jets of an aircraft in ground effect. This work is dedicated to the continuation of the experimental study began during the master’s thesis, i.e., a detailed analysis of the complex flowfield of two in-line turbulent circular air jets with a low velocity crossflow impinging on a flat surface perpendicular to the geometrical jet nozzle axis. The jets exit conditions are changed along the study to provide a better understanding of the flowfield. To complete this analysis and in order to validate the experimental results a detailed numerical study is also presented, where all the features of the experimental flow are maintained. The numerical results extend the experimental study, revealing that the deflection of the rear jet is due to the competing influences of the wake, the shear layer, the downstream wall jet of the first jet and the crossflow. The first jet deflection and the location of the ground vortex depend on the velocity ratio between the jet exit and the crossflow as well as the impingement height used. Through the rear jet velocity change, it is possible to verify the quick deflection of the second jet, never reaching the ground directly, i.e., the downstream jet is entrained by the upstream jet and not by the crossflow itself. Through the impingement height change, it is possible to observe the absence of upwash fountain formation in the region between the impingement jets, as it was expected. In this region, it is unexpectedly observed the formation of a second ground vortex, something not yet reported in the literature.
- Laser Doppler Measurements of Twin Impinging Jets Aligned with a CrossflowPublication . Barata, Jorge M M; Carvalho, Pedro dos Santos Duarte; Neves, Fernando M. S. P.; Silva, André; Vieira, Diana; Durão, DiamantinoThis paper presents a detailed analysis of the complex flow beneath two impinging jets aligned with a low-velocity crossflow which is relevant for the future F-35 VSTOL configuration, and provides a quantitative picture of the main features of interest for impingement type of flows. The experiments were carried out for a Reynolds number based on the jet exit conditions of Rej = 4.3 104, an impingement height of 20.1 jet diameters and for a velocity ratio between the jet exit and the crossflow VR = Vj/Uo of 22.5. The rear jet is located at S = 6 D downstream of the first jet. The results show a large penetration of the first (upstream) jet that is deflected by the crossflow and impinges on the ground, giving rise to a ground vortex due to the collision of the radial wall and the crossflow that wraps around the impinging point like a scarf. The rear jet (located downstream) it is not so affected by the crossflow in terms of deflection, but due to the downstream wall jet that flows radially from the impinging point of the first jet it does not reach the ground. The results indicate a new flow pattern not yet reported so far, that for a VSTOL aircraft operating in ground vicinity with front wind or small forward movement may result in enhanced under pressures in the aft part of the aircraft causing a suction down force and a change of the pitching moment towards the ground.
- Numerical Simulation of Twin Impinging jets in Tandem through a CrossflowPublication . Vieira, Diana; Barata, Jorge M M; Neves, Fernando M. S. P.; Silva, AndréThe flow field of ground vortex generated by twin impinging jets in tandem through a crossflow is numerically studied in detail. Numerical simulation and visualization are presented for two turbulent circular jets emerging into a low velocity cross stream, impinging after on a flat surface perpendicular to the geometrical jet nozzle axis. The numerical study is based in experimental studies done early, so all the features of the experimental flow were maintained when the numerical simulation was performed. The Reynolds number used was based on the jet exit conditions of 43,000 to 105,000, a jet to crossflow velocity ratio of 22.5 to 43.8, an impinging height of 20.1 jet diameters and an interject spacing’s of S=5D and L=6D. The analysis of the flow was extended to regions and flow conditions for which no measurements have been obtained in last experimental studies, i.e., for velocity ratios of 15 to 90. The numerical results show that for the smallest velocity ratios the jets initially do not mix, but remain together in two layers. Three different types of flow regimes were identify, therefore when VSTOL aircrafts operating in ground vicinity, only the regime with strong impingement on ground and with a formation of a ground vortex is relevant. The numerical results allowed to extend the last experimental studies, and prove that the deflection of the rear jet is due to the competing influences the wake, the shear layer, the downstream wall jet of the first jet and the crossflow.
- Numerical and Experimental Study of Two Impinging Jets in a Row through a CrossflowPublication . Vieira, Diana; Silva, André; Neves, Fernando M. S. P.; Carvalho, Pedro dos Santos Duarte; Barata, Jorge M MAn experimental and numerical study is carried out to investigate the flowfield created by twin impinging jets aligned with a low velocity crossflow. The mean velocity, velocity fluctuation and visualization in the impingement region were obtained for a Reynolds number based on the jet exit conditions of Rej=4.3x104, an impingement height of 20.1 diameter and for a velocity ratios between the jet exit and the crossflow VR=Vj/Uo of 22.5, 33.7 and 43.8 with interject spacing, S of 6D. The experimental results show a large penetration of the first (upstream) impinging jet, giving rise to a ground vortex due to the collision of the radial wall jet and the crossflow that wraps around the impinging point like a scarf. The second jet (located downstream) it is not so affected by the crossflow in terms of deflection, but it does not reach the ground due to the downstream wall jet that flows radially from the impinging point of the first jet along the ground. The results indicate a new flow pattern not yet reported so far, in which a VSTOL aircraft operating in ground vicinity with front wind or small forward movement may result in enhanced negative pressures in the underside of the aircraft causing a pitching moment and a suction down force towards the ground. To extend the experimental results, the flow was also numerically simulated using a Reynolds Averaged Navier-Stokes formulation with the "k-ε" turbulence model.