The origin and effects of free-stream vortical disturbances on boundary layer transition in wind tunnels
Creators: name: Pook, David
Publisher: name: RMIT University
Year: 2024

description: Low-speed streaks are involved in bypass transition, yet they can interact with Tollmien-Schlichting waves to delay transition. Recent progress concerning streak stability has used the model of an Optimum streak computationally, or streaks from roughness elements experimentally. Questions remain concerning the characteristics of streaks which originate from the free-stream. This thesis provides insight into the origin of steady vortical free-stream disturbances, creation of streaks, and their influence on boundary layer transition. <br><br>Computational results are co-validated with published experimental observations of steady streaks, generated by the wake of a wire upstream. The largest streak occurs when the wire is placed upstream of the wind tunnel contraction. Flow through the contraction creates streamwise vorticity in the test-section via tilting and stretching of the normal vorticity in the wake. The computational results reinterpret the original experiment as a receptivity experiment, demonstrating the boundary layer is more receptive to streamwise vorticity. The sensitivity of normal vorticity receptivity to the leading edge attachment is also shown.<br><br>More than 30 years ago, Morkovin highlighted issues of spanwise variation (streaks) in wind tunnels and called for establishing the link with settling chamber screens. This thesis uses CFD to link screens with boundary layer streaks assuming steady, laminar flow. The screen is modelled by a zither of wires. The effects of zither open-area ratio and geometric imperfections are examined. The computational results mirror experiment with zithers of decreasing open-area ratio and greater imperfection creating larger streaks. For example, an imperfect zither with an open-area ratio of 50% can create a streak causing 80% variation of boundary layer thickness. The free-stream flow angle is only 0.1 degrees. The streaks have the same attributes as Klebanoff streaks. A PSE-3D analysis suggests the streaks will stabilise Tollmien-Schlichting waves.<br><br>Large streaks have been attributed to an “instability” downstream of screens that leads to “coalescence of jets”. Current results demonstrate and attribute streak formation to increasing drag variation across the zither. Drag variation modifies the wake decay rate, creating a stronger wake. The stronger wake passing through the contraction generates increased streamwise vorticity, creating larger streaks. A published analysis is tested for predicting zither wake strength and wavelength. New zither wake decay rates are derived based on drag variation. “Coalescence of jets” is considered to be a symptom rather than a cause.<br><br>Streaks are computationally generated near the leading edge using free-stream normal or streamwise vorticity. BiGlobal stability analysis in the early boundary layer shows normal vorticity streaks favour the growth of high frequency disturbances while streamwise vorticity streaks favour lower frequencies. Both streaks stabilise the boundary layer further downstream. A model using the spanwise mean and the first Fourier mode of the streak is used to investigate stability. Streaks can destabilise or stabilise depending on modification of the mean-flow wall-normal profile by the streak, the streak width relative to the boundary layer thickness, and the relative amplitude of the mean-flow modification. These characteristics depend on the free-stream source and nonlinear growth of the streak.<br>