Other Characteristics of Rotor Wakes

10.5.1 Periodicity versus Aperiodicity

In most cases, the rotor wake is deterministic, and the tip vortices generated by each blade will follow smooth curved and almost helical or epicycloidal paths. Also, under ideal circumstances, their spatial locations relative to the rotor will be periodic at the rotor rotational frequency. However, various types of aperiodic behavior of rotor wakes have been noticed in experiments. Aperiodicity can be defined as the random variations in the spatial locations of the vortex filaments from a mean position at a given wake age. In fixed – wing terminology, this phenomena is referred to as vortex wandering or meandering – see Devenport et al. (1996) and Leishman (1998). If aperiodicity occurs above some threshold, then measurements based on the assumption of a periodic flow will be biased because the small random displacements of the vortices essentially average out the flow field properties at a fixed measurement point.

For helicopter rotors, the available experimental evidence suggests that wake aperiodicity is a characteristic that is, in part, related to the nature of the rotor operating state; in hover the phenomenon is more likely to show than in forward flight. For example, measurements of the tip vortex locations in hover that are reconstructed on the basis of a series of images of the wake made at different wake ages may take the appearance of significant scatter, or even two possible geometries. Such a behavior can be traced to aperiodic flow effects and has been observed in several rotor experiments, including those of Landgrebe (1972), Norman & Light (1987), Bagai & Leishman (1992b), and Leishman (1998). Most of the available experimental evidence with rotors shows that aperiodicity is pronounced only at older wake ages (older than one or two complete rotor revolutions), at low thrust coefficients (where the slipstream convection velocities are low and the tip vortices remain close to the rotor plane), or after the first blade passage. At a minimum, appropriate allowance can be made when quoting measurement uncertainties and when comparing with computations of the wake topology. From a purely scientific perspective, the challenge is to understand whether the observed aperiodic behavior of rotor wakes is an inherent physical characteristic or if their source lies in small external flow disturbances that are produced within the ground test facility. However, the fact that rotor wakes disturbances occur under very controlled laboratory conditions suggests that they would also be expected to occur under the normal turbulence levels found in the atmosphere, especially closer to the ground.