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15 mins
Transition and wavy walls: an experimental study
Robert Downs, Jens Fransson
Session: Instability and Transition 5
Session starts: Thursday 27 August, 10:30
Presentation starts: 11:45
Room: Room A

Robert Downs (KTH Mechanics)
Jens Fransson (KTH Mechanics)

A wide body of research exists which explores the effects of surface roughness or patterned wall shapes on instability growth and transition. Building on those works as well as recent experiments demonstrating passive laminar flow control using arrays of discrete roughness [3, 8], a set of spanwise-wavy walls is designed with the goal of suppressing instability growth in two-dimensional boundary layers. In a numerical investigation of Tollmien-Schlichting (TS) wave growth in the presence of streamwise boundary-layer streaks, Cossu and Brandt [1] found that stabilization of TS waves results from spanwise shear in the mean flow, which forms a negative contribution to production in the perturbation kinetic energy equation. Whereas previous efforts have employed streamwise vorticity developing in roughness wakes to provide the requisite mean-flow deformation, in this work stabilization is achieved through modulation of the no-slip surface. Miniature vortex generators (MVGs) have proven an effective means of producing streamwise streaks for transition delay [8], though relatively large streak amplitudes are necessary to counter their eventual decay through viscous dissipation. The notion motivating this work is that spanwise-wavy walls extended in the streamwise direction can produce a similar effect while avoiding bypass transition resulting from large-amplitude streamwise streaks. Toward that end, six wavy walls are used in a modular test model. When TS waves are excited upstream of the wavy walls, substantial delays in the onset of transition are observed for certain spanwise wavelengths compared with the flat-plate reference case.