Active flow control methods for aerodynamic applications

The cylinder in cross flow has been the subject of many numerical and experimental studies since it provides a deep insight of the physical phenomena occurring in a wide range of flow regimes. Despite a number of investigations at Reynolds number (Re = 3900), there has been a constant debate on the important aspects of the flow such as spanwise resolutions, lateral domain extent, convergence of turbulent statistics in the near wake, the so called U-V streamwise velocity profiles at x = 1D, where D is the cylinder diameter, and the critical Re for the onset of shear layer instability together with its characterization. In this thesis, an attempt has been made to address some of these issues and report new results through Direct numerical simulations (DNS) by employing spanwise domain extents i.e. Lz = 1.5D; 2D; 2.5D; pD at the moderate flow regime i.e. Re = 2000, where boundary layer is still laminar while the near wake has gone fully turbulent. Intermittent bursts of shear layer instability have been spotted at this Re indicating the signs of the incipient laminar to turbulent transition in the separating shear layer. It is further confirmed that the secondary instability develops in the regions between the opposite sign large scale spanwise vortices and features a phase lag of 135 degree. Pseudo-Floquet analysis gives a good prediction of fastest growing mode consistent with the reported numerical and experimental measurements. In the second part of the thesis, active flow control (AFC) past circular cylinder has been thoroughly investigated with the aid of parametric analysis at the same Re. We applied spanwise-dependent fluidic actuation, both steady and time-dependent, on the flow past a circular cylinder at Re = 2000. The actuation takes place in two configurations: in-phase blowing and suction from the slits located at ±90 degree (top and bottom) with respect to the upstream stagnation point for both steady and time periodic actuation, and blowing and suction from the top and bottom slits traveling oppositely with respect to each other in the spanwise direction. Optimal forcing amplitude and wavelength are obtained by sweeping across the parametric space. Spanwise-dependent time-independent forcing with wavelength ?z = 2D has been found the optimal one in terms of drag reduction and attenuation in lift fluctuations. The time-dependent forcing with sinusoids travelling oppositely with respect to each other along the span produced significant reduction in drag force and lift fluctuations, however, the in-phase time periodic actuation with forcing frequency four times the natural vortex shedding frequency resulted in significant increased drag and lift fluctuation, signalling to a potential candidate for the energy harvesting applications. Finally, in the last part of the thesis, time-dependence of flow inside novel laminar-fluidic-oscillator has been analyzed using DNS. Again, pseudoFloquet stability analysis has been utilized to predict the fastest growing Fourier modes along the homogeneous direction. Supplementary three-dimensional numerical study has also been conducted for the suitable cases at various Re. It has been found that steady flow inside fluidic oscillator’s cavity bifurcates from steady state to time-periodic state through supercritical Hopf bifurcation. The secondary transition inside fluidic oscillator’s cavity occurs through the breaking of flow symmetry about the cavity axis by pitchfork supercritical bifurcation.