Interaction of a Propeller Slipstream with a Downstream Laminar Boundary Layer

Abstract

Over the past few decades, aircraft design priorities have shifted from maximising cruise speed to reducing fuel burn and emissions. Rising fuel costs, environmental regulations, and the emergence of electric and hybrid-electric propulsion have renewed interest in propeller-driven concepts, such as Distributed Electric Propulsion. These concepts often involve placing multiple propellers in proximity to each other or close to other aerodynamic surfaces. As such, there exists a need to better understand the behaviour of the boundary layer on downstream surfaces.

At the moderate chord Reynolds numbers relevant here, laminar–turbulent transition, boundary layer separation and the formation or suppression of LSBs can alter lift, drag, stall margins and noise. LSBs arise when a laminar layer separates, transitions within a free shear layer and then reattaches, resulting in a turbulent flow. A propeller modifies this flow field by generating a non-uniform, unsteady slipstream consisting of accelerated axial flow, swirl, and a helical system of tip and root vortices and blade wakes. The resulting spatially and temporally varying inflow alters local incidence, dynamic pressure and boundary layer dynamics over the wing. Existing studies rely primarily on time-averaged measurements and lack sufficient spatial and temporal resolution near the wall. As a result, the detailed mechanisms by which helical vortices and wake sheets interact with the downstream boundary layer, their spanwise variation within and around the slipstream core, and their influence on LSB dynamics remain only partially understood.

The present thesis aims to provide an experimental characterisation of how a propeller slipstream modifies a downstream laminar boundary layer behaviour. Two complementary campaigns have been used to answer the research objective: oil-flow visualisations and acoustic measurements on the surface of an airfoil placed under a propeller slipstream to understand the time-averaged boundary layer in a global sense, and a local, high-resolution investigation using phase-locked stereoscopic Particle Image Velocimetry (sPIV) to resolve the vortex structures within the slipstream and understand their effect on the boundary layer. Together, these experiments address the questions of (i) how the slipstream alters laminar–turbulent transition and LSB formation, and (ii) what role unsteady vortical structures within the slipstream play in triggering and modulating transition along the span.

The results show that the propeller slipstream strongly energises the downstream boundary layer and generally promotes earlier transition, but in a manner that is highly non-uniform in both span and blade-passage phase. The results indicate that under a propeller slipstream, transition is advanced and the boundary layer thickens on the inboard side, where inboard shearing and higher slipstream velocities amplify near-wall turbulence. The slipstream also modifies LSB behaviour, and depending upon slipstream amplitude and effective local incidence angle, the LSB may be suppressed entirely, shortened and displaced upstream, or persist within regions of weaker propeller loading. Phase-locked sPIV reveals that these behaviours are governed by an intrinsically three-dimensional, intermittent interaction between the boundary layer and a coupled system of primary tip vortices, secondary wall-originating vortices and blade wakes. Wake-impingement events generate short bursts of high shear and TKE separated by phases of weakly turbulent or nearly laminar near-wall flow. Comparable TKE levels for tripped and untripped cases at the same wall-normal position show that near-wall turbulence inside the slipstream is dominated by the imposed unsteady forcing rather than by the state of the boundary layer.