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The laminar-to-turbulent transition on high-speed rotating propeller blades is investigated in the present work experimentally. The analysis of limitations for optical skinfriction measurements by the oil-film interferometry, as well as of the results of feasibility experiments in the low-speed wind-tunnel BLSWT of AIRBUS Bremen at propeller rotation rates from 3000 up to 14400 rpm (50-240 Hz) and free-stream flow speeds between 30 and 70 m/s are included. The effects of Reynolds number and of the propeller advance ratio on the transition location and on flow topology are discussed. NomenclatureA variable of the thin-oil-film equation a acceleration a rot shear stress due to the centrifugal-force impact (a rot = ρ oil ω 2 R h/2) c chord length D propeller diameter g acceleration of gravity h film thickness J propeller advance ratio (J = πu ∞ /v rot = u ∞ /(nD)) k thermal conductivity M Mach number n rotation frequency (Hz) n r refraction coefficient P 0 total pressure p static pressure r recovery factor r cr critical onset transition (induced by leading-edge separation) radius r, θ polar coordinates R propeller radius ReReynolds number Re nD propeller Reynolds number (Re nD = (nD 2 )/ν) Re nr locally defined propeller Reynolds number (Re nr = (n4r 2 )/ν) Re nr,cr critical Reynolds number (Re nr,cr = (n4r 2 cr )/ν) t time T absolute temperature, or variable of the thin-oil-film equation T 0 total temperature

The Temperature-Sensitive Paint (TSP) method is used for surface-based flow visualizations on a swept-wing wind-tunnel model with a generic natural laminar-flow airfoil. Within the investigated parameter range the stationary crossflow instability is the dominating instability mechanism. Based on the TSP results the location of the laminar-turbulent transition and the most amplified wavenumber of the stationary crossflow instability are determined. The test is performed with three different conditions of the leading-edge surface: highly polished, unpolished, and highly polished with discrete roughness elements applied. The Temperature-Sensitive Paint method has proven to have sufficient spatial resolution and temperature sensitivity to resolve skin friction variations to detect the footprint of stationary crossflow vortices even inside of turbulent wedges. With the discrete roughness elements, i.e. cylindrical elements with micron-sized height, the transition could be delayed successfully for certain conditions. Local low-frequency movement of the beginning of turbulent wedges was detected for some data points with an unpolished leading edge.

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The application of Hybrid Laminar Flow Control (HLFC) by suction of the boundary layer has the potential to delay the location of laminar-turbulent transition and, hence, reduce drag caused by skin friction. Up to now most of the research on HLFC has been performed at low Reynolds and Mach numbers. To investigate the technology at flow conditions relevant for today's commercial flight additional studies at higher Reynolds numbers are required. At high Reynolds numbers the major challenge is an appropriate scaling of the perforation used for sucking the boundary layer, leading to very small perforation diameters of only a few microns. In this paper, the applicability of HLFC on a two-dimensional wind tunnel profile is investigated in the cryogenic Pilot European Transonic Windtunnel (PETW). The natural laminar-turbulent boundary-layer transition is visualized with temperature-sensitive paint (TSP). The transition locationobtained with an active HLFC system is compared to a reference case without HLFC for different angles of attack, Reynolds numbers, free stream Mach numbers and suction rates. Furthermore, the influence of the perforation diameter is also investigated. Results of an alternative approach, using a sintered porous material instead of a regular perforation for sucking the boundary layer, are also presented.