Small- and medium-scale propellers are likely to operate at chord-based Reynolds numbers where the boundary layer is transitional rather than fully turbulent. This complicates both the prediction and the wind-tunnel scaling of their performance. This thesis investigates how the laminar-to-turbulent transition location affects the aerodynamic performance of a model-scale propeller, and whether forcing transition with passive trip devices can bring the boundary-layer state closer to that of a full-scale, fully turbulent blade.

An experimental campaign was conducted on the non-swept TUD-XPROP propeller in the TU Delft Low Turbulence Tunnel. This combines quantitative performance measurements from a rotating shaft balance with qualitative infrared thermography images of the blade suction side. A boundary-layer analysis with XFOIL resulted in four adhesive-vinyl trip-device configurations: a dart, a dotted, and two continuous line trips at different chord-wise locations. These trip devices are manufactured, applied, and verified geometrically with a laser surface scanner before testing against a clean, free-transition baseline.

The results show that forced transition does not uniformly improve or degrade propeller efficiency, its effect depends on the combination between trip geometry, chord-wise position, blade loading, and Reynolds and Mach number. The continuous line trip consistently outperformed the discrete configurations, and its optimal chord-wise location over the largest operating domain for the roughness height available is further aft from the leading-edge. Forcing transition flattened the thrust and power curves and reduced the influence of the Reynolds and Mach number on the efficiency. This resulted in an increased efficiency at low Reynolds and Mach numbers, but at higher the efficiency is reduced compared to the clean blade. The infrared measurements did not resolve a distinct transition front, but consistently captured a line over the radius of reduced intensity compared to reference cases. For increased loading, the line moves upstream, especially at the radial location where the angle of attack increases most. Due to the low resolution, it is difficult to observe the influences of the Mach and Reynolds number. And its presence is suppressed once transition is forced upstream of it. The observed trends of this line match with the behaviour of a laminar separation bubble.

Together, the performance and thermal datasets demonstrate the potential of infrared thermography to link changes in boundary-layer state to measured changes in propeller performance. Despite its limited spatial and thermal resolution when applied to a fast-rotating model, infrared thermography provides useful insight into the development of the boundary layer. The results further indicate that effective forced-transition designs should be tailored to the intended operating conditions rather than apply it as a fixed, one-size-fits-all solution. ... Read more