Free stream turbulence induced disturbances in boundary layers with wall suction

Authors: Yoshioka, S., Fransson, J. H. M., Alfredsson, P.H.
Document Type: Article
Pubstate: Published
Journal: Phys. Fluids
Volume: 16   3530-3539
Year: 2004


An experimental investigation of free stream turbulence (FST) induced disturbances in asymptotic suction boundary layers (ASBL) has been performed. In the present study four different suction rates are used and the highest is 0.40% of the free stream velocity, together with three different FST levels (Tu= 1.6, 2.0 and 2.3%). A turbulence generating grid of active type is used and offers the possibility to vary the Tu-level while the scales of the turbulence remains almost constant. It is known that FST induces elongated disturbances consisting of high and low velocity regions, usually denoted streaky structures, into the boundary layer. The experiments show that wall suction suppresses the disturbance growth and may significantly delay or inhibit the breakdown to turbulence. Two-point correlation measurements in the spanwise direction show that the averaged streak spacing decreases with increasing FST-level, whereas the spacing in the ASBL is more or less constant in the downstream direction. This is in contrast to what is observed in a Blasius boundary layer where streaks develop and adapt their spanwise scale close to the boundary layer thickness. A hypothesis for the present observations is that the observed scale reflects the disturbances that are fed from the leading edge part into the boundary layer whereafter the structures only passively are convected downstream.