The transition behaviour of a boundary layer with zero pressure gradient in a low-subsonic freestream is examined for a set of wall temperatures below the freestream temperature. Solutions of the boundary layer provided by both an incompressible and a compressible numerical flow solver are compared to wind tunnel measurements on a uniformly cooled section of a flat plate. The compressible solver features a temperature dependence of the fluid properties, which proves to be crucial for modeling the stability of a thermal boundary layer. Thermal images of the surface show that the spanwise-averaged transition front moves downstream with decreasing wall-to-freestream temperature ratio. At the streamwise station where no frictional heating due to turbulent structures is observed, T-S waves are detected in the measured velocity fluctuation profiles. The experimental data show good correspondence with the numerical linear stability predictions here. Further downstream, where streaks of transition occur, T-S waves can no longer be distinguished, and it is likely that non-linear effects have overtaken the boundary layer flow here. The absolute amplitude and the amplitude growth rate of the velocity perturbations both decrease with the wall temperature in the region where T-S waves are seen, and even more in the non-linear region. It is concluded that uniform surface-cooling stabilizes the boundary layer on a flat plate in an incompressible freestream, but a variable-fluid property solver is required to model the stability characteristics.

To achieve delay of laminar-to-turbulent transition of boundary layers, the application of acoustic metamaterial concepts is investigated. This is done analytically, numerically and experimentally by studying the interaction between a Helmholtz resonator and a TS wave. A Helmholtz resonator can serve as a meta-atom in a metamaterial. The desired wall controller property of a metamaterial to suppress TS waves is evaluated. This study shows that TS wave attenuation is achieved if the wall-normal velocity from a metamaterial is in-phase with the TS wave pressure. TS wave amplification is obtained if the pressure and velocity are more than (approximately) 90 degrees out of phase. The p-v phase relation corresponding to a Helmholtz resonator only remains in the amplification regime. Therefore, the conclusion is drawn that a single Helmholtz resonator can only amplify TS waves. If an acoustic metamaterial concept is found where pressure and wall-normal velocity are in-phase, this can be useful in boundary layer instability control.

This thesis concerns a numerical investigation for the stabilization (delay of laminar-to-turbulent transition) of ows over a at plate with a meta-unit of a possible metamaterial. The type of instability studied is that due to small disturbances in an incompressible, two-dimensional boundary layer. The linear growth of the disturbance in the form of a wave (also called the Tollmien-Schlichting wave) was of particular interest. There is a newly emerging class of structures called "metamaterials" which are capable of controlling the properties of classical waves (electromagnetic/optical/acoustic). This thesis investigates whether or not one of these materials are capable of attenuating TS waves. A literature review was conducted on three types of metamaterials, namely those based on Helmholtz-resonators, membranes structures and phononic crystals. The mechanisms for which the three aforementioned metamaterials attenuate acoustic waves are resonance, anti-resonance and Bragg's scattering respectively. Based on the likelihood of successfully achieving damping of TS waves, the phononic crystal was chosen to be further studied…