Turbulent Structure of the Impact Zone of a Ground Vortex Flow

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Organizational Unit

Publications

Experimental Study of a Ground Vortex: the Effect of the Crossflow Velocity

Publication . Barata, Jorge M M; Bernardo, Nuno; Santos, Pedro José da Costa Teixeira; Silva, André

Laser-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.

2012-06-14Journal article

Embargoed access

Numerical study of the spray impingement onto a solid wall

Publication . Rodrigues, Christian Michel Gomes; Silva, André Resende Rodrigues da; Barata, Jorge Manuel Martins

The modelling of turbulent multiphase flows has been gathering high interest in the last decades in the scientific community due to its relevance in several applications, such as in industrial and environmental processes or for chemical and biomedical purposes. In fact, regarding the industrial applications, the impingement of liquid fuel sprays onto engine surfaces has become a subject of interest due to its influence on the mixture preparation prior to combustion and, consequently, engine performance and pollutants emission (Barata and Silva, 2005). However, there is still a lack of knowledge concerning the spray-wall interaction but also concerning the exact phenomenon occurring during the process. These gaps do not allow defining the most favourable conditions for the optimal engine performance. Hence, the main challenge for the investigators lies in attaining a much deeper understanding of the phenomena involved in the spray impingement process, through either theoretical analysis or experimental investigation. Meanwhile, the splash phenomenon has been the focus of many researchers due to its relevance in the combustion process of small-bore, direct-injected gasoline and diesel engines, as well as in a variety of other industrial devices in which sprays impinge on solid surfaces. Bai and Gosman (1995) developed a model to predict the outcomes of spray droplets impacting on a wall with temperatures below the fuel boiling point. This model, which has been formulated using a combination of simple theoretical analysis and experimental data from a wide variety of sources, was later improved (Bai et al., 2002) by refining the dissipation energy term and by enhancing the post-splashing characteristics. In fact, recently, significant attention has been given to this regime either through the definition of transition criteria that better fit specific conditions of the experimental configuration under study or by characterizing the behaviour of the drop during all stages of the regime (expansion of the lamella, crown formation and propagation, etc.) through both theoretical analyses and experimental data. Beyond the transition criteria, another aspect that controls the characteristics of the secondary droplets after the impacts is the energy dissipation term and thus, it is essential its proper definition for adequately modelling these multi-phase flows. However, contrary to spreading, there is little literature available related to this particular parameter and, more important than that is the fact that there is a certain ambiguity even for what it represents exactly. In addition, the majority of the dissipative energy loss relationships have been deduced for the spread regime, i.e., from the beginning of the expansion of the lamella until the drops reaches its maximum extent (without splashing). This situation can be overcome through some simplifying assumptions, which obviously carries inaccuracy. The present work is dedicated to the study of the sprays impingement onto a solid wall through a crossflow. The major purpose of the thesis is to improve the accuracy of the base model, which is the model of Bai et al. (2002), through the employment of both new correlations for the deposition/splash transition criteria and energy dissipation loss relationships available in the literature. The numerical predictions are then compared with the experimental data of Arcoumanis et al. (1997) for two crossflow rates ( and ). From the results, it can be concluded that the employment of different transition criteria can bring better results (see also Silva et al., 2011). On the other hand, no improvements were seen by the employment of the new energy dissipative loss relationships in the base models, which calls for further research in this particular matter.

2011Master thesis

Open access

Experimental Study of Two Impinging Jets Aligned With a Crossflow

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

Laser 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.

2014-10Journal 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

Numerical and Experimental Study of Two Impinging Jets in a Row through a Crossflow

Publication . Vieira, Diana; Silva, André; Neves, Fernando M. S. P.; Carvalho, Pedro dos Santos Duarte; Barata, Jorge M M

An 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.

2013-01-05Journal article

Embargoed access