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Licentiate seminar

Investigations of the asymptotic suction boundary layer


Defendant Main Advisor Extra Advisor Date
Jens Fransson Henrik Alfredsson 2001-09-27

Opponent
Håkan Gustavsson, LTU

Evaluation committee

Abstract

An experimental and theoretical study on the effect of boundary layer suction on the laminar-turbulent transition process has been carried out. Both experiments on the development of Tollmien-Schlichting (TS) waves and boundary layer disturbances introduced by free stream turbulence were carried out. In the study an asymptotic suction boundary layer was established in a wind tunnel. Uniform suction was applied over a large area and the boundary layer was nearly constant over a length of 1800 mm. Measurements were made both with and without suction so comparisons between the two cases could easily be made. Measurements of the development of the mean velocity distribution showed good agreement with theory. In connection to the TS-wave experiments spatial linear stability calculations, where the wall-normal velocity component is accounted for, were carried out for comparison with the experiments. This comparison shows satisfactory agreement even though the stability of the asymptotic suction profile is somewhat overpredicted by the theory. Free stream turbulence was generated by three different grids giving turbulence intensities at the leading edge of the plate between 1.4 % and 4.0 %. The free stream turbulence induces disturbances into the boundary layer and it was shown that for the suction case the disturbance level inside the boundary layer saturates at a level which is proportional to the free stream turbulence intensity. In all cases transition was prevented when suction was applied although without suction the two highest levels of grid turbulence gave rise to transition. Despite a twofold reduction in the boundary layer thickness in the suction case compared to the no suction case the spanwise scale of the streaky structures was almost constant.
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