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.

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.