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FREE STREAM TURBULENCE INDUCED DISTURBANCES IN A UNIFORM SUCTION BOUNDARY LAYER

Jens H. M. Fransson
Linne Flow Centre Department of Mechanics KTH–Royal Institute of Technology SE–100 44 Stockholm, Sweden

P. Henrik Alfredsson
Liné Flow Centre, KTH Mechanics, Osquars Backe 18, SE-100 44 Stockholm, Sweden

Résumé

An experimental study on the effect of boundary layer suction on the development of boundary layer disturbances introduced by free stream turbulence has been 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. 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 not observed 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 is only slightly decreased.