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. Spatially highly resolved data and precisely detected transition positions are obtained by means of temperature-sensitive paint and accompanied by local surface pressure measurements at two radial blade sections. The effect of rotational forces is investigated by systematic variation of Rossby number from Ro = 4.76 to 6.95 at Re = 3.7x10^5 and M = 0.22. The findings do not show a measurable effect of rotational forces on boundary-layer transition in the investigated parameter range and suggest predominantly two-dimensional (2D) flow behaviour, which is confirmed by subsequent validation with 2D numerical tools. Based on quantitative agreement between measured and calculated surface pressures, it is shown that experimentally detected transition positions are predicted to within +-4 % chord if a critical amplification factor of N_cr,MSES = 5.6 is used in MSES for transition prediction in the rotor test facility of the DLR Göttingen. The measured transition onset positions are also correlated with the obtained integral growth rates from 2D compressible local linear stability theory to get transition N-factors of N_cr = 8.4 + 0.5 . Correlations are shown to be independent of relative chord Reynolds number and incompressible shape factor at the detected transition onset underlining the capability of 2D numerical techniques based on linear stability theory to model boundary-layer transition in the investigated parameter range