Enhancing Active Flow Control Strategies for Delaying 2D Boundary Layer Transition

Through research studies conducted over the years, it has been demonstrated that suction is a highly effective active flow control (AFC) technique exploited for laminar-turbulent transition delaying. The primary aim of employing active flow techniques for transition control is the maintenance of laminar flow over a region whose extension needs to be maximized. Considering that half of the total drag stems from skin friction(Marec, J. P., 2001, Drag reduction: a major task for research.), this strategy diminishes the overall friction drag experienced by an object immersed in a fluid. This thesis focuses on utilizing a numerical framework to determine the optimal suction configuration, intending to maximize the extension of the laminar region within a steady, incompressible, and two-dimensional flat plate boundary layer.The computational approach involves integrating the boundary layer equations using an internally developed code (IBL), with suction effects introduced through the wall boundary condition. Validation is conducted by comparing IBL computed velocity field with the Blasius and openFOAM solutions. Linear Stability theory is used to asses the modal and local stability of the boundary layer flow. The Companion Matrix Method is used to solve the Orr-Sommerfeld equation and obtain the spatial evolution of the perturbation modes (Bridges, T. J., Morris, P. J., 1984, Differential eigenvalue problems in which the parameter appears nonlinearly), while the eN method is employed for transition modeling (Van Ingen, J., 2008, The eN method for transition prediction. Historical review of work at TU Delft. ). In order to maximize the transition delay, an optimization of the flow control parameters (suction strength and location) was carried out by means of the Bayesian optimization approach. The effect of suction configuration is evaluated both versus the transition position and the evolution of the skin friction coefficient over the flat plate. Results indicate a narrow range of optimal mass flow conducive to effective and efficient transition control, with the optimal suction region extension ranging from 20% and 80% of the plate total length. Moreover, the location of the suction region significantly influences transition. It is shown that shifting the suction location, while fixing the mass flow, results in a displacement of transition. A clear relationship between suction configuration and transition position was found. From this, a predictive AFC model is proposed. The output of the proposed model, tested against the results of the IBL-LST framework, agree within 1% accuracy. Further experimental validation of the derived observations is sought after. Towards this goal a wind tunnel model was designed and the experimental tests are planned. Finally, future development of the optimization framework could include a non uniform suction velocity profile.