This study quantifies the viscous interaction between propeller tip vortices and a turbulent boundary layer developing over a semi-elliptic leading-edge plate, located downstream. The experimental wind-tunnel set-up is designed to be representative of the tractor-propeller-wing configuration. Using stereoscopic particle image velocimetry and static wall-pressure measurements, the near-wall flow topology is resolved over the plate, semi-immersed in the propeller slipstream. The results show that the interaction exhibits high spatio-temporal coherence and is dominated by a coupling between primary and secondary vortical structures. Two distinct interaction regions are identified relative to the tip-vortex core: on the inboard side, towards the slipstream interior, the boundary-layer flow experiences strong velocity gradient transitions and amplified near-wall vorticity. The flow on the outboard side, moving out of the slipstream, exhibits wall-parallel velocity deficits and vorticity lift-up consistent with unsteady vortex-induced separation mechanisms. Spanwise velocity induced by the wall-normal component of the primary vortex connects these two regions, with the secondary vortex structure identified as enhancing boundary-layer lift-up on the outboard side. Although no local flow reversal occurs under the tested conditions, localised shear amplification and vorticity roll-up indicative of separation-like behaviour were observed. These findings advance the understanding of viscous slipstream-boundary-layer interaction and its implications for tractor-propeller-wing integration.

The present work details the steady and unsteady flow topology in the vicinity of an array of periodically spaced super-critical (i.e. causing flow tripping) discrete roughness elements (DRE) applied in a swept wing boundary layer. The stationary flow field is acquired by means of high-magnification dual-pulse tomographic particle tracking velocimetry (3D-PTV), while the unsteady instabilities are investigated through high-resolution hot wire anemometry (HWA). The 3D-PTV time-averaged velocity fields, indicate that the near-element flow region is dominated by the alternation of high- and low-speed streaks. A high-speed region substitutes the wake development shortly downstream of the DRE location, due to the high-speed streaks merging. This initiates a region of strong unsteady fluctuations that expands in the spanwise and wall-normal directions, ultimately leading to the boundary layer transition to turbulence. The spectral content of the stationary flow structures is investigated through a spanwise spatial Fourier transform. The extracted spectra and instability amplitudes, indicate the presence of non-modal mechanisms in the near-element stationary wake region. Nonetheless, the temporal spectral analysis of the HWA velocity signal, identifies the presence of strongly tonal shedding mechanisms initiating and the unsteady instabilities the element vicinity. Their rapid downstream growth and evolution retains a fundamental role in the transitional process.