Evolution of disturbances in boundary layers with wall suction

Authors: Yoshioka, S., Fransson, J. H. M., Alfredsson, P.H.
Document Type: Conference
Pubstate: Published
Journal: Turbulence and Shear Flow Phenomena - 3
Volume: 1   489-494
Year: 2003


An experimental investiagion 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 structures, 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 lenth when uniform suction is applied over a large area. This makes it possible to change the Reynolds number and the boundary layer thickness independently, 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 possiblitiy 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 flattend 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.