Maintaining laminar flow on large swept surfaces of subsonic transport aircraft, i.e. the wings and the stabilisers, is currently posing a considerable challenge for aerodynamic design. Improving the efficiency of aircraft by delaying or removing the laminar-to-turbulent transition process over the wing and tail parts can substantially reduce contaminant emissions. The dominant flow instability causing laminar-turbulent transition of swept-wing flow is the so-called crossflow instability (CFI). Ongoing research at TU Delft has shown potential to delay transition by use of passive mechanisms. As such, a framework has been designed to numerically compute crossflow development and transition to turbulence on swept wings. Through the use of experimental data acquired in wind-tunnel measurements at TU Delft, the CFI development and transition process on swept wings has been modelled numerically by means of Direct Numerical Simulation (DNS). Based on a DNS laminar flow field generated from the pressure distribution along the model surface, a numerical primary CFI mode in good agreement with the experiment was obtained through Non-linear Parabolized Stability Equations (NPSE). Following this steady flow field analysis, the simulation was made unsteady by the implementation of numerical free-stream turbulence. This novel method resulted in unprecedented modelling of the receptivity mechanisms of transition in three-dimensional crossflow cases, overcoming ad-hoc treatments. Both experimental and numerical flow fields indicated a Type-I dominant secondary CFI (i.e. K-H type of response in the laterally inclined shear layer of the stationary crossflow vortex), which consequently carries the formation of near-wall hairpins and ultimately turbulence. Crossflow vortex frequency content also agrees well in the low-frequency band (450Hz ≤ 𝑓 ≤ 3000Hz), whilst the numerical high-frequency content (3500Hz ≤ 𝑓 ≤ 9000𝐻𝑧) does show a distinct delay in amplitude growth throughout the majority of the transition region. Contradicting the promising qualitative analysis of the free-stream turbulence methodology, this discrepancy in the frequency spectrum indicates the major shortcoming in the numerical setup, which was shown to be biased towards introducing more low-frequency disturbances at the inflow boundary.

The flow of air over a swept wing initially starts in a smooth laminar state (referred to as base flow) and entrains disturbances which subsequently grow and transition the flow to a chaotic, turbulent state. Active efforts have been made to study and control these disturbances, which manifest as stationary crossflow modes. Excrescences in the form of a forward-facing step (FFS) impose a base flow deformation in the form of a rapid near-wall pressure change, flow separation, and strong upwash. This modifies the behaviour of stationary crossflow instability and subsequently leads to an upstream or downstream shift of transition location depending on FFS height. The mechanisms responsible for the above behavioural modification are unknown, motivating the current thesis. The evolution of primary stationary crossflow instability, in its linear growth phase, is studied through a spanwise invariant, synthetic, and idealized rapid base flow deformation imposed by changing the near-wall pressure distribution of a clean swept flat plate (possessing a favourable pressure gradient) via Gaussian-like pressure variations. An energy balance framework is developed that identifies the production term's behaviour as the differentiator between regions of perturbation growth and decay. The behaviour of the production term is described by two competing mechanisms, the first controlled by wall-tangential base flow shear and the second controlled by wall-tangential base flow acceleration or deceleration. The balance of these mechanisms shows that perturbations grow faster than the clean case in regions of wall-tangential base flow deceleration and slower than the clean case in regions of wall-tangential base flow acceleration. Perturbations are even found to attenuate in some cases when a region of wall-tangential base flow acceleration follows a region of wall-tangential base flow deceleration. The modification of energy transfer mechanisms brings into question whether the initial modal stationary crossflow mode deviates from modal character on interacting with the base flow deformation. The Orr mechanism is shown to identify differences in perturbation behaviour from local modal character. However, criteria from the literature hint towards an absence of any non-modal effects. Finally, the extent to which a synthetic idealized base flow deformation mimics the effects of an FFS on deforming the base flow and changing trends of stationary crossflow instability evolution is tested to show the applicability of methods developed in the thesis to instances of natural base flow deformation. The progress in understanding mechanisms by which a deformed base flow affects the linear phase of primary stationary crossflow instability growth leads to suggestions on devices that can be tested to delay this phase of instability growth. These devices could potentially also delay subsequent stages of instability growth and hopefully lead to the development of novel transition delay techniques.

Dimples are shallow, indented surfaces that attempt to reduce drag in turbulent boundary layer flows. However, the underlying effect of the dimples on the drag is not entirely understood. This thesis sets out to expand our understanding; a numerical investigation of turbulent boundary layer flow over dimpled surfaces is conducted. This research aims to verify the drag results from recent experimental studies and investigate the possible drag-reducing mechanism of a dimpled plate. The research considers shallow, rounded-edge dimples with a staggered layout. Implicit large eddy simulation (ILES) is carried out with the cell spacing close to direct numerical simulations (DNS). Simulation outputs conclude that the dimple plate causes a total drag increase of approximately 1% compared to a smooth plate. This value confirms the results of Spalart et al. and recent wind tunnel measurements within the Aerodynamics group. The turbulent coherent structures are further investigated by performing hole-filtering sampling to the quadrant events. Results suggest that the dimple plate induces a more intense turbulent activity in the buffer layer. The increased occurrence contributes to a higher Reynolds shear stress. The development of quadrant events is further analysed using a Variable-Interval Time-Averaging (VITA) technique. It reveals that the averaged quadrant event development between two plates is almost the same. However, a more extended sweep development is found in the wake region. Given such a mild even evolution, the resulting Reynolds shear stress generation remains the same. Lastly, the coherent structure response is linked to the skin friction response through the Fukagata-Iwamoto-Kasagi (FIK) identity. The resulting decomposition using the FIK identity reveals that dimples contribute to higher total drag due to increased Reynolds shear stress. On the other hand, the observed skin friction reduction seems relative to the mean flow convection.

The location of laminar to turbulent transition is an important consideration as turbulent flow is associated with higher skin friction drag and, by extension, lower fuel economy. In unswept boundary layers, natural transition proceeds by the amplification of Tollmien–Schlichting waves. Tollmien–Schlichting waves are convective instabilities with spanwise oriented vorticity. The amplification of these instabilities/perturbations is sensitive to roughness elements, such as forward-facing steps. These surface imperfections are inevitable as steps, gaps, and humps are a byproduct of mismatch between panels of a wing. However, their interaction with Tollmien–Schlichting waves is not very well understood. Direct numerical simulation of the flow field around forward facing steps has been performed in this thesis to gain an in-depth understanding of the particular flow features that stabilise or destabilise the incoming Tollmien–Schlichting wave, with respect to a flat plate zero pressure gradient flow. The forward facing step is found to significantly distort the base flow, its effect scaling with the roughness Reynolds number in the upstream regime. This distortion of the base flow is observed to amplify the incoming instability, both upstream and far downstream. At the step location, however, stabilisation or destabilisation can be observed, depending upon the height of the step. The step causes the incoming Tollmien–Schlichting wave to split into two, just upstream of the step, and leads to two counter-rotating structures at the step location. The interaction of these structures influences downstream growth. Localised stabilisation is observed, at the step location, for step heights that are smaller than the boundary layer displacement thickness. Destabilisation is observed for larger step heights. The upstream base flow distortion is due to an adverse pressure gradient imposed by the forward facing step. The magnitude of the pressure gradient is found to scale with the roughness Reynolds number. The upstream amplification is due to the Tollmien–Schlichting wave encountering the distorted base flow. The response of the Tollmien–Schlichting wave to the distorted base flow is observed to scale with its wavelength. The ratio of the roughness Reynolds number to the wavelength ($\gls{Rehh}/\lambda$) is found to be the governing parameter for the upstream interaction of the step with the instability.