Investigation of orthogonal blade-vortex interaction using a particle image velocimetry technique
The complex flowfield which is associated with a rotor wake gives rise to the multitude of aerodynamic interactions that may occur during rotorcraft operation. These interactions may give rise to undesirable noise and lead to an unacceptable performance degradation, and as such the investigation of the fundamental mechanics of such interactions, that which occurs between the tail rotor and the trailing tip vortices shed from the main rotor assembly, is the focus of the current investigation. As the purpose of the tail rotor is to provide balance for the torque of the main rotor, these types of interaction will adversely impact on the overall rotorcraft performance. The basis of the present thesis has been an experimental investigation of the orthogonal BVI, in which the axis of the interacting vortex (in the plane of the vortex core axial flow) is nominally orthogonal to the interacting blade chordline, representing the tail rotor interaction. The tests have been conducted using a specifically designed facility at the University of Glasgow, with the flow interrogated using a Particle Image Velocimetry (PVI) technique. The PVI method allows global flowfield information to be obtained pertaining to the nature of the interaction. The methodology was benchmarked against synthetic flowfields, and with the accuracy of the flowfield measurements improved dramatically with the implementation of the Forward/Reverse Tile Test (FRTT), which improved the accuracy in the flowfields to 3% in two-dimensional interrigation, and 5% in three-dimensional. The interrogation of the flowfield around the representative tail rotor blade demonstrated that the characteristics imparted vortex due to the BVI event could be attributed to the manner in which the axial flow component of the vortex was affected by the interaction. The results for the isolated flow conditions agreed well with those from previous measurements of the vortical structure, and the post interaction structure clearly indicated distinct differences determined by the direction of the axial flow relative to the blade chordline. Initial testing indicated that the thickness ratio had a marked effect on the progression of the OBVI, and for a suitably high thickness ratio, there was little evidence to suggest that the vortex core axial flow is 'cut' by the interacting body in the manner observed for the lower thickness ratios. For lower thickness ratios, as the vortex core is blocked by the interacting blade surface, the retardation of the axial component on the blade lower surface leads to rapid redistribution of the fluid into the surrounding flow, and the corresponding enlargement and distortion to the vortex tangential velocity components promoted by the radial outflow. On the upper blade side, regions of negative axial flow velocity indicate the presence of some fluid passing down through the core towards the surface of the blade, which are accompanied by a split divergence pattern around the vortex core. The effects immediately behind the trailing edge continue to be of interest due to the manner in which the vortex might be regenerated after the interaction and before any subsequent interactions with following blades. A relative lack of distortion within the out-of-plane component indicates that a rapid regeneration of the axial flow component may occur once the vortex has passed over the trailing edge. The use of passive control techniques in reduction of the effects associated with the orthoganal BVI have also been addressed, considering the effect of a counter-rotating vortex pair on the progression of the interaction. Although the inclusion a notch in the leading edge and outboard sweep on the rotor blade producing the representative trailing tip vortex did produce a well defined inboard vortex structure, there is evidence to suggest that this structure is ingested into the outboard tip vortex, as there is no significant modification to the progression of the OBVI.