Investigations of boundary-layer transition and airloads on rotating blades

Kurzfassung

Measurements of surface pressure and boundary-layer transition on rotating blades is important for the validation of numerical tools, used e.g. to predict rotor efficiency in the design process. In this work, surface pressure distributions and boundary-layer transition were measured on a Mach-scaled helicopter rotor blade. Optical measurement techniques were used to obtain data at high spatial resolution and were complemented by integral thrust and local surface pressure tap measurements. Various collective pitch settings were investigated at tip chord Reynolds and Mach numbers of Re_{tip} = 4.6 - 9.3 x 10^5 and M_{tip} = 0.29 - 0.57. An optimized pressure-sensitive paint (PSP) system is presented, which allows omitting error-prone post-processing routines or laborious setups to eliminate artifacts originating from rotational image blur. The system was successfully applied for the first time to the investigated configuration. Boundary-layer transition positions were detected via temperature-sensitive paint (TSP) and for comparison also via infrared thermography. A data base was established, which is ready to use for validation purposes of numerical codes. For the first time, the effect of rotational forces on boundary-layer transition was systematically investigated. A rotational effect is found to be insignificant as the scaling parameter in terms of Rossby number Ro is varied from Ro = 6.95 to Ro = 4.76 at resulting Reynolds and Mach numbers of Re_{res} = 3.74 x 10^5 and M_{res} = 0.22. Measured surface pressure data at 77 % tip radius were compared to numerical solutions of a coupled two-dimensional Euler/ boundary-layer solver and the numerical solutions were used to determine critical N-factors based on two different approaches to the e^N-method, each based on two-dimensional flow assumptions only. Measured and calculated surface pressures are comparable within a difference of Delta c_p ≈ 0.02. Application of the determined N-factors yields a prediction capability of measured boundary-layer transition results of better than ± 5 % of the chord.