Efficient thermal management is critical for future electric aircraft. An innovative approach (Outer Mold Line cooling) leverages the aircraft’s aerodynamic surfaces for heat dissipation, heating the adjacent boundary layer. It is known that wall-heating can have detrimental effects on laminar–turbulent transition driven by stationary crossflow instabilities (S-CFI). However, its impact remains largely unclear due to a lack of experimental studies. This work investigates the effect of a heated wall across all stages of S-CFI development, spanning linear growth, non-linear saturation, and laminar-turbulent transition. This is achieved through comprehensive wind-tunnel experiments, complemented by comparisons with Compressible Nonlinear Parabolized Stability Equations (CNPSE) results. Results show that wall-heating increases the boundary-layer momentum deficit and the crossflow velocity component, leading to increased destabilization of S-CFI. In turn, higher S-CFI amplitudes promote an earlier onset of non-linear interactions between stationary, traveling, and secondary instability modes. Notably, under equivalent freestream turbulence (Tu[jls-end-space/]) levels, wall-heating results in a higher S-CFI saturation amplitude compared to adiabatic conditions. In addition, spectral analysis reveals substantial amplification of unsteady perturbations with wall-heating. A key finding of this work is the strong destabilization of traveling CFI under wall-heating, which persists into the nonlinear regime and yields highly amplified type-III instabilities. One possible implication of the strong destabilization of T-CFI and type-III instabilities is that at sufficiently high wall-to-freestream temperature ratios, T-CFI could dominate the transition process, potentially leading to a transition scenario similar to that observed under high levels of freestream turbulence.

The interaction between a forward-facing step (FFS) and single-frequency Tollmien–Schlichting (TS) waves is investigated with experiments and two-dimensional (2-D) direct numerical simulations (DNS). Dedicated hot-wire anemometry and particle image velocimetry measurements in the vicinity of the FFS provide characterisation of the perturbation field, as well as validation of the DNS results. Comparison between experiments, 2-D DNS, and linear parabolised stability equations confirm the 2-D nature of the flow and the linearity of the instability mechanisms around the FFS. Upstream of the step, TS waves are gradually amplified by the increasing adverse pressure gradient. In the step vicinity, both mean flow and perturbation field exhibit abrupt distortion, with decoupling of the base flow-oriented growth rate components indicating significant non-modal evolution. Downstream of the step, the mean flow recovers to baseline conditions, but the perturbation field remains highly distorted. Linear stability theory results suggest the presence of superimposed modes on the original TS mode in this region. Despite their decay in the streamwise direction, their presence imprints modifications in the TS wave growth and shape, manifested as the tilting of the perturbation structure in and against the mean flow shear direction. This initiates a reversed Orr mechanism, characterised by a region of stabilisation followed by destabilisation further downstream. Eventually, the TS waves realign to their asymptotic (modal) behaviour. Overall, the FFS destabilises the TS wave far downstream. However, the streamwise extent and magnitude of the stabilisation downstream of the FFS remain significant.

A new experimental facility named Swept Transition Experimental Platform (STEP) has been designed and built for detailed studies of crossflow instability and its interaction with surface irregularities and varying wall temperature conditions. The STEP is designed for use in the anechoic low-turbulence wind tunnel facility at the Delft University of Technology (TU Delft). The new facility consists of a swept flat-plate model with a movable leading edge capable of precisely translating to create forward/backward-facing step irregularities. In addition, the plate’s wall temperature can be adjusted to study the potential of thermal laminar flow control. An adjustable pressure body provides the favorable pressure distribution required to enhance the development of crossflow instability. Static pressure measurements are conducted to characterize the nominal pressure distribution. In addition, detailed hot-wire measurements and theoretical stability calculations reveal that the combination of discrete roughness elements, pressure distribution, and experimental facility allows for a detailed study of the development of crossflow instability in the linear and non-linear growth regime. Consequently, the STEP enables further fundamental research on laminar flow control at TU Delft.

One of the most critical technological challenges embedded in the electrification of future aircraft revolves around the thermal management of batteries and fuel cells. An innovative idea involves using the aircraft’s aerodynamic surfaces to dissipate the extra heat, thereby reducing the impact that traditional thermal management systems (e.g. ram air heat exchanger) have on the overall aerodynamic efficiency of the aircraft. However, the limited experimental research addressing the influence of a heated surface on the stability and transition of the crossflow instability (CFI) hinders the assessment of the aerodynamic impact of this technology for future aircraft, where swept wings are ubiquitous. Thus, the objective of this work is to experimentally study the effect of a heated wall on the stability and final breakdown of CF vortices. To do so, experiments are conducted on a 45◦ swept flat plate wind tunnel model, where the surface temperature is increased by means of a surface-embedded electrical heater, yielding a mean wall-temperature ratio of T _w/T _∞ = 1.055. Overall, the experimental (i.e. HWA) and numerical (i.e. CLST) results show that wall heating leads to significant destabilization of the stationary CFI. Interestingly, a spectral analysis of the HWA signal reveals substantial amplification of the traveling CF mode under wall-heating conditions, which in turn appears significantly more destabilized than the stationary CF mode. Additionally, inspection of the high-frequency content in the HWA measurements indicates premature breakdown of the CF vortices and advancement of the laminar-turbulent transition by Δ _x/c _x = 6.3% with wall heating. The results presented in this work render a first insight into the impact of a non-adiabatic wall on the development of the crossflow instability and subsequent breakdown to turbulence.

This work presents an experimental and numerical investigation jointly conducted by TU Delft and DLR on Tollmien-Schlichting (TS) waves interaction with a Forward-Facing Step (FFS). Experiments are conducted at the TU Delft low-turbulence anechoic wind tunnel on an unswept flat plate model. Single-frequency disturbances are introduced using controlled acoustic excitation. The temporal response of the flow in the vicinity of the step is measured using Hot-Wire Anemometry (HWA). In addition, the global effect of the step on laminar-turbulent transition is captured using Infrared Thermography (IR). Two-dimensional (2-D) Direct Numerical Simulations (DNS) performed at DLR provide detailed information at the step. Experimental and DNS results in clean and step case conditions present very good agreement. Both methods predict large distortion of the TS waves downstream of the step, where DNS results present different growth trends between |û| and |(equation presented)| components of the TS waves. Furthermore, negative and positive regions of the production term are observed to correlate with streamwise positions where the disturbances appear tilted in and against the mean shear, respectively. These findings point towards the presence of different growth mechanisms triggered by the step which could modify the level of amplification of disturbances far downstream.