Vortex Wake of Coaxial Rotors in Hover

An extensive sets of experiments are carried out regarding the stability characteristics of helical vortex filaments o a coaxial rotor model. A coaxial rotor model consisting of two 26-cm (10.25-in) diameter counter-rotating rotors with a root cut-out of 3.2 cm (25%) is employed in these experiments in one- and two-bladed configurations. Axial rotor separation varied from 25% of the rotor radius to one rotor radius. Rotor speed ranged from 2 to 8 rps at each rotor spacing. Dye flow visualization, particle flow visualization and particle image velocimetry are employed in order to take measurements of the flow field. For one-bladed tests, helical tip vortex filament trailed from the lower rotor blade is observed to develop strong deformations soon after its formation at the rotor spacing of a quarter rotor radius as opposed to that of upper rotor. This deformation is less pronounced at the rotor spacing of 0:8R. Upper and lower rotor vortices are seen to develop long-wave vortex pairing instabilities where they orbit around each other as they travel downstream, particularly at smaller rotor spacing, which is also confirmed in PIV results. Successive turns of the upper rotor helices appear to be stable as opposed to unstable nature of lower rotor helix in both rotor separations. At H=R = 0:8, the upper rotor wake contraction is markedly higher than that of the lower rotor. Vortex filaments develop short-wave instabilities at 2 & 4 rps just immediately after their inceptions in both rotor separations. In two-bladed configurations, in all cases tip vortices trailed from the lower rotor experience extreme distortions, resulting in the loss of their orderly helical shapes, which is a clear manifestation of the effect of the upper rotor on the lower one. At smaller rotor spacings up to 50% radius, lower rotor filaments develop long-wave vortex pairing instabilities in similar ways to what is found in single rotor wakes although they differ to the extent lower vortex filaments undergo deformations. At this rotor spacing range, upper rotor filaments does not have sufficient space between the rotors to develop mutual interactions before they reach the plane of the lower rotor and interact with lower rotor blades, i.e. interrotor blade-vortex interactions (BVIs). As the rotor spacing is further increased up to a rotor radius, upper rotor filaments are seen to develop long-wave pairing instabilities similar to what is observed in the wakes of single rotor runs. As in the one-bladed configurations, upper rotor filaments possess a more stable nature whereas those from lower rotor becomes unstable within first rotor revolution beneath the lower rotor. At rotor spacing of 50% radius, hairpin vortices are observed to form beneath the lower rotor during the long-wave instability mode developed by the lower rotor filaments where streamwise elongation of the vortex filament from a particular blade of the lower rotor in each rotor revolution is observed. At other rotor spacings, similar deformation of the vortex filaments in the streamwise direction is seen to occur however distinct hairpin vortex structures are only seen at the rotor spacing of 0:5R. Upper rotor wakes contract to more inward than those of the lower rotor wakes for all the rotor separation distances tested, which is also confirmed from the PIV measurements and particle visualization results. Increasing rotor spacing changes the wake contraction of the upper rotor at the plane of the lower rotor where it reaches a more fully-developed state towards the rotor spacing of one radius. This is further confirmed from the streamline patterns from the PIV results. Regardless of the rotor spacing, short-wave instabilities formed along the helical filaments at 2 & 4 rps cases from their generation whereas at 6 & 8 rps filaments are only observed to develop these types of instabilities long after their formation and not in an explicit manner since they become superimposed with long-wave instability mode where filaments develop strong distortions beneath the lower rotor.

Advisor: Savas, Omer