DNS investigation of laminar-to-turbulent transition with favorable pressure gradient: effect of surface imperfections

Direct Numerical Simulations (DNS) have been performed of a) a flat-plate zero pressure gradient boundary layer and b) a flat-plate favorable pressure gradient boundary layer, using the numerical code NEK5000. The simulations are validated against analytical solutions and literature data in the laminar, transitional and turbulent regions. Surface imperfections in the form of a forward and backward facing step of varying heights have been implemented and simulated. Wind tunnel experiments have also been performed of a flat-plate zero pressure gradient boundary layer. Hot-wire measured velocity profiles indicate that the flow obeys the Blasius solution in the laminar region, and transition has been observed via interrogation of the hot-wire data. Both numerical and experimental results show that the introduction of step advances the location of transition onset. From the DNS, we find that for small step heights (less than 25\% of the inlet boundary layer height), increasing pressure gradient by increasing the Falkner-Skan wedge angle delays the the onset of transition. However, the presence of significant step (50\% of the inlet boundary layer height) negates the effect of pressure gradient, and the location of transition onset stagnates despite an increase in the Falkner-Skan parameter. Analysis of the longitudinal power spectrum shows that the growth of a sub-harmonic mode is enhanced with increasing step height.