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Effect of Freestream Turbulence on the Boundary Layer on a Swept Leading Edge

Fumarola, I. (2019). Effect of Freestream Turbulence on the Boundary Layer on a Swept Leading Edge. (Unpublished Doctoral thesis, City, University of London)

Abstract

An experimental study was carried out to understand the way the turbulence interacts with the boundary layer along the attachment-line of a swept wing, since it may have an important role in the receptivity process of cross-flow instability to freestream turbulence.

The work focused on the freestream turbulence amplification process approaching a leading edge, previously seen on two-dimensional bodies, but yet not investigated for swept models.

Two experimental investigations were carried out in two different wind tunnels using two different models. The first was carried out in the Gaster wind tunnel, characterised by very low turbulence intensity. The model consisted of a swept aluminium vertical flat plate inserted in a wooden fairing, similar to the unswept model used by Bearman (1972). The flow field was measured using a single hot-wire anemometer. In the low turbulent environment, no increase in the velocity fluctuations was observed as the wall was approached. Therefore, the freestream turbulence level was increased using, as a first attempt, a metallic string, crossing the entire wind tunnel section, placed at different orientations ahead of the model. It was found that the vertical string generated a localised freestream disturbance which was convected in the spanwise direction, following the streamlines, without influencing the level of fluctuations in the boundary layer. The horizontal string created a disturbance, distributed in the spanwise direction, that made the boundary layer turbulent at the attachment-line. A second attempt was carried out using a turbulent grid made of parallel rods, mounted either in a horizontal or vertical configuration. In both configurations, the grid had an effect similar to that of the horizontal string creating a turbulent boundary layer on the attachment-line. This effect may have been due to a contamination of turbulence from the root of the wing. In all the cases, the turbulent boundary layer presented an increase of fluctuations approaching the wall.

A second experiment was carried out in the T2 wind tunnel, characterised by a level of turbulence higher than that of the Gaster wind tunnel, on a circular cylinder model. In this case, a multi-component Laser Doppler anemometer was used, enabling simultaneous measurements of the three velocity components. The experiment focused on the flow approaching the stagnation point of a cylinder mounted in the unswept configuration and then at four different sweep angles (5°; 10°; 20°; 30°). The results achieved on the unswept configuration showed an amplification of the spanwise velocity fluctuations approaching the stagnation point with a maximum around the boundary layer edge, followed by decay as the wall was approached. The spanwise velocity fluctuation profiles were similar to those, based on hot-wire measurements, reported in the literature. In the swept configurations the increment of the spanwise velocity fluctuations was found to be still present and similar to the unswept case. At 30° sweep angle, the spanwise velocity fluctuations were observed to increasing right up to the measured point closest to wall. One of the effects of increasing sweep angle was to increase the frequency at which the turbulence was most amplified.

A number of new trends have been identified, confirming that, in general, the phenomena at the swept leading edge boundary layer cannot be explained using two-dimensional arguments.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TL Motor vehicles. Aeronautics. Astronautics
Departments: Doctoral Theses
Doctoral Theses > School of Mathematics, Computer Science and Engineering
School of Mathematics, Computer Science & Engineering
School of Mathematics, Computer Science & Engineering > Engineering > Mechanical Engineering & Aeronautics
URI: http://openaccess.city.ac.uk/id/eprint/22364
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