The required power of a helicopter rotor is significantly influenced by the drag due to skin friction on the rotor blades. The level of skin friction strongly depends on the flow state near the blade surface, which can be laminar, turbulent or transitional. Areas of laminar flow generate considerably lower levels of skin friction than turbulent areas. Thus the capability to predict the extent of laminar flow represents a key component for accurate performance computations of helicopter rotors.
In general the stability of the laminar boundary layer and its transition from laminar to turbulent state are affected by various flow parameters like e.g. the Mach and Reynolds number, the freestream turbulence level, the characteristic of adverse pressure gradients or the occurrence of yawed flow. Helicopter rotors typically operate in a complex flow field with large variations of the above mentioned flow parameters due to strong vortex structures generated at the blade tips and the superposition of rotational and translational velocity components. Due to the large variations of the rotor flow conditions a spectrum of different instability mechanisms can influence the laminar boundary layer and cause the laminar-turbulent transition onset. Possible instability mechanisms are:
Laminar-turbulent transition onset on the blade of a Bo 105 helicopter in hover;
top: flight test
bottom: numerical simulation with the DLR FLOWer code
For the simulation of the complex flow field around a helicopter rotor, computational in-house tools based on the Navier-Stokes equations are used. The investigation of the laminar-turbulent flow transition on helicopter rotors focuses on the following research activities: