Experiments on Three-Dimensional Disturbance Interactions Leading to Boundary-Layer Transition

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PROPOSAL NO.: CTS-0245253PROPOSAL TYPE: INVESTIGATOR INITIATEDPRINCIPAL INVESTIGATOR: EDWARD B. WHITEINSTITUTION: CASE WESTERN RESERVE UNIVERSITYEXPERIMENTS ON THREE-DIMENSIONAL DISTURBANCE INTERACTIONS LEADING TO BOUNDARY LAYER TRANSITIONThe laminar-to-turbulent transition of boundary layers is a complicated process that can proceed via a number of scenarios depending on fluid velocity, surface geometry, environmental disturbances, and other factors. In spite of its complicated nature, past research on the subject of transition has tended to focus on single disturbance-growth mechanisms in isolation from all other competing mechanisms. For Blasius boundary layers, the growth of streamwise-traveling, spanwise-invariant disturbances known as Tollmien-Schlichting (T-S) waves is one such mechanism that has been extensively studied. A newly discovered mechanism known as transient growth, which favors stationary, spanwise-varying disturbances, has also received significant attention. Although much progress has been made in understanding these mechanisms in isolation, little progress has been made in predicting or controlling transition onset in real systems in which several competing mechanisms can exist. To address the lack of useful data on realistic transition scenarios involving competing disturbances, a wind-tunnel investigation of the interactions of T-S waves and transient disturbances in a Blasius boundary layer will be undertaken. The proposed experiments will provide a controlled and systematic investigation of how T-S waves, transient disturbances, and the secondary instabilities of both compete and interact to bring about transition. The input disturbances will be varied over wide parameter ranges and will be designed to take full advantage of signal-analysis techniques suitable for extracting very low amplitude disturbance signals from a noisy background. The results will provide data necessary for transition prediction and control efforts. Beyond the intrinsic scientific value of the project, the proposed work will also contribute to the improvement of important technological systems and to the promotion of the role of research in teaching at Case Western Reserve University. Transition affects the performance and efficiency of a broad range of vehicles and industrial systems because laminar boundary layers result in low vehicle drag, whereas turbulent boundary layers are often desired for complete and efficient fluid mixing. It would be of great benefit if designers were capable of predicting and controlling the conditions under which laminar flows become turbulent. Accurate prediction would shorten design cycles and reduce the over design typical of systems sensitive to transition, and flow control would permit the design of radically more efficient systems. More immediately, this project will enhance the role of research in teaching by supporting a vigorous experimental program in a wind-tunnel laboratory that is accessible to undergraduates and by incorporating the work performed in the tunnel into experimental-methods and fluid mechanics instruction.