On the Effect of Rotational Forces on Rotor Blade Boundary-Layer Transition

Laminar-turbulent boundary-layer transition is investigated on the suction side of Mach-scaled helicopter rotor blades in climb and analyzed in view of the effect of rotational forces. Transition positions are detected with temperature-sensitive paint and complemented by surface pressure measurements at two radial blade sections. The effect of rotational forces is investigated by systematic variation of the Rossby number from Ro = 4.76 to Ro = 6.95 at Re = 3.7 x 10^5 and M = 0.22. The findings do not show a significant effect on boundary-layer transition in the investigated parameter range and suggest predominantly two-dimensional flow behavior. The result is supported by subsequent validation with two-dimensional numerical tools. Based on quantitative agreement between measured and calculated surface pressures, measured transition positions are predicted to within ±4% chord if a critical amplification factor of N_{cr,MSES} = 5.6 is used in the coupled Euler/boundary-layer equation solver MSES for transition prediction in the rotor test facility of the DLR, German Aerospace Center in Göttingen, Germany. The measured transition onset positions are also correlated with integral growth rates, obtained from separate two-dimensional compressible boundary-layer computations and subsequent local linear stability analysis to get transition N factors of N_{cr} = 8.4 ± 0.5. Differences in N factors are discussed in view of the different approaches used. Overall, transition N-factor correlations are independent of relative chord Reynolds number and incompressible shape factor at the detected transition onset. The findings underline the capability of two-dimensional numerical techniques based on linear stability theory to model boundary-layer transition in the investigated parameter range