Rotor Wake Investigation Using the Smoke Flow Visualisation Technique

Experiments were carried out on a small four bladed rotor at different climb rates. The smoke-filament technique was used to visualise the rotor tip vortices; video equipment was then used to record the images produced. The recorded images were digitised and enhanced to assist in the identification of the vortex location. The vortex trajectories were compared with established data and with the paths generated by the general tip vortex path equations. It was found that these equations oversimplified the vortex trajectories, modelling neither the interaction between vortices nor their meandering. The wakes studied showed evidence of vortex interaction. It was found that vortices close to one another combined in pairs and spun about a common centre as they moved downstream. A new mean path equation was thus defined which could model this type of behaviour. An exponential equation was selected to model the mean path. To assist in the determination of the order which best suited the curve, the least squares method was used. The meandering of the vortex trajectory about its mean path was studied and the types of instability present were determined. Three types of instability were found in tip vortices; short-wave, mutual-inductance and longwave. It was discovered that the helical path followed by a hovering rotor’s tip vortex was unstable under most flight conditions and that fluid damping suppressed the magnitude of these instabilities.