Since O. Reynolds' experiment revealed the transition from laminar state to turbulence in pipe flow (1883), numerous efforts had been exerted in order to understand the mechanism. Linear hydrodynamic stability theory was established in the first half ofthe twentieth century. This theory predicts the growth of small disturbances during the initial stage of transition. As an important achievement, the Tollmien-Schlichting (T-S) instability, which represents the initial evolution of disturbances in two dimensional shear flows, has been described and verified in boundary layer experiments. Once the enonnous benefits from the delay of flow transition (and then turbulence) were acknowledged, diversities of flow control techniques have been motivated. In fact, some relevant techniques have already been demonstrated, such as the drag reduction oflarge aeroplane through laminar flow control. At the same time, the development of control theory makes it possible to implement deliberately designed systems to control transitional flows. After decades of advances with passive control, the concept of active control became widely accepted in fluid mechanics in the early 1980s. . The study in this thesis attempts to find an efficient closed-loop active control scheme to cancel the unstable Tollmien-Schlichting waves in a transitional boundary layer flow. The T-S instabilities was simulated by numerical solutions of the linearised Navier-Stokes equation system. Controllers were designed according to the frequency characteristics ofthe disturbed flowfield and implemented on the basis of the classical control theory in frequency domain. The perfonnance of controllers in the disturbed boundary layer flow along a flat plate were evaluated through numerical simulations and then validated by experiments.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:504556 |
| Date | January 2008 |
| Creators | Zhang, Zhenyu |
| Publisher | Queen Mary, University of London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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