TWIN IMPINGING JETS INLINE WITH A LOW-VELOCITY CROSSFLOW

Funder

Organizational Unit

Publications

Numerical Simulation of Twin Impinging Jets in Tandem Through a Crossflow

Publication . 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.

2015Journal article

Open access

Numerical Simulation of Twin Impinging jets in Tandem through a Crossflow

Publication . Vieira, Diana; Barata, Jorge M M; Neves, Fernando M. S. P.; Silva, André

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 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.

2015-01-03Journal article

Embargoed access

Twin impinging jets inline with a low-velocity crossflow

Publication . Vieira, Diana Filipa da Conceição; Barata, Jorge Manuel Martins

Vertical/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.

2017-01Doctoral thesis

Open access