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EVOLUTION OF DISTURBANCES IN BOUNDARY LAYERS WITH WALL SUCTION

Shuya Yoshioka
Department of Mechanical Engineering, Keio University, 3-14-1, Hiyoshi, Kouhoku-ku, Yokohama, 223-8522, Japan; KTH Mechanics 5-100 44 Stockholm, Sweden

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 investigation of free stream turbulence (FST) induced transition in asymptotic suction boundary layers (ASBL) has been performed. It is known that FST induces elongated disturbances consisting of high and low velocity regions, denoted streaky structure, into the boundary layer. Emphasis is here placed on the disturbance growth and the modification of the streaky structure when subjected to various uniform wall suction rates. The most salient feature of the ASBL is its constant boundary layer thickness along the plate which experimentally is reached after some entry length when uniform suction is applied over a large area. This makes it possible to change the Reynolds number and the boundary layer thickness independetly, which is unique for the ASBL case. Three different FST levels (Tu = 1.6−2.3%) have been investigated with an active turbulence generating grid. This grid offers the possiblities to vary the Tu-level while keeping the FST characteristic scales.
Wall suction is shown to suppress the disturbance growth and delay or inhibit the breakdown to turbulence depending on the strength of suction and the level of Tu. Two-point correlation measurements in the spanwise direction reveal that the averaged streak spacing is affected by the boundary layer thickness (with Re = const.) and the Tu-level, but is independent of the Reynolds number (with boundary layer thickness / Re = const.). The streaky structure in a Blasius boundary layer is known to take the aspect ratio of around unity and in this investigation we are able to show how this structure is flattened with increasing suction. Furthermore, by keeping the Reynolds number constant while varying the boundary layer thickness we provide the result of a linear relationship between the streak spacing and the boundary layer thickness.