Turbulent Structure of the Impact Zone of Two Opposed Wall Flows

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Publication . Barata, Jorge M M; Ribeiro, Samuel; Santos, Pedro José da Costa Teixeira; Silva, André

Laser-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.

2009-05-22Journal article

Embargoed access

Aerodynamic control of the mixing of confined, plane and co-axial jets

Publication . Neves, Fernando Manuel da Silva Pereira das; Barata, Jorge Manuel Martins

Turbulent mixing of confined coaxial jets is a complex dynamic process with many practical applications such as ejectors, jet pumps, industrial burners, jet engine combustion chambers, gaseous nuclear rockets, mixing chambers, afterburners, and turbofan engine mixing chambers. In this latter applications the confinement is also of basic interest because it involves a certain interacting turbulent flow phenomena, the details of which are not yet fully understood quantitatively (Ahmed and Sharma, 2000). There have been studies of different geometric and operating parameters on the turbulent mixing process of confined coaxial jets with different motivations. While a wide range of diameter ratios has been investigated in the past, no work was reported on diameter ratios below 2, which is the case with many low bypass turbofan engines with bypass ratios of 0.3 and less. Only Ahmed and Sharma (2000) reported recently detailed LDV measurements of mean and turbulence components of velocity as well as the total and static pressures over a range of velocity ratios with relevance in the present context. In their work they analyse the influence of velocity ratio on the turbulent mixing process, but there are many other parameters which influence the turbulent mixing process. This research is dedicated to the study of the turbulent mixing of parietal jets without any generated effect of swirl and without the presence of bluff bodies. The major objective was to design mixing control strategies using purely aerodynamic tools. Following a work on axisymmetric geometries, a two-dimensional geometry was adopted to study the influence of geometric and initial parameters to control the flow: confinement, inclination angle and turbulence intensity of the outer flow.

2012Doctoral thesis

Open access

The Turbulent Structure of a Ground Vortex

Publication . Barata, Jorge M M; Santos, Pedro; 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 a velocity ratio between the boundary layer and the wall jet of 0.5 was used, and the results confirmed the existence of a very complex vortex flow region in the collision zone near the ground wall. Vorticity values were calculated from the experimental data and revealed negative (clockwise) vorticity near the stagnation point which is an indication of the presence of a small secondary vortex, also detected from the visualization studies but not sufficiently identified from the LDA measurements. 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.

2012-06-25Journal article

Embargoed access

Laser-Doppler Analysis of the Separation Zone of the Ground Vortex Flow

Publication . Silva, André; Durão, Diamantino; Barata, Jorge M M; Santos, Pedro José da Costa Teixeira; Ribeiro, Samuel

Laser 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.

2009-06Journal article

Embargoed access