Rotor-Obstacle Aerodynamic Interaction in Hovering Flight: An Experimental Survey

Despite the presence of a fair number of numerical and experimental works on the rotor-obstacle interaction, a systematic study of the aerodynamic phenomena involved is lacking. In this paper a comprehensive experimental survey carried out at University of Glasgow is described, taking advantage of two different rotor rigs and several experimental techniques. Load measurements on the rotor were carried out in order to assess the rotor performance for different positions with respect to a cubic obstacle, thus simulating a set of possible hovering flight conditions around the obstacle. Laser Doppler Anemometry (LDA) measurements of the rotor inflow were used in order to investigate how the aerodynamic interaction affected the rotor performance. Eventually Stereoscopic Particle Image Velocimetry (SPIV) measurements in the region between the rotor and the obstacle were carried out in order to have a better insight of the interacting flow field

Investigation of the rotor–obstacle aerodynamic interaction in hovering flight

In this paper, a comprehensive experimental survey of the aerodynamic interaction of a hovering rotor in the proximity of a ground obstacle is described, taking advantage of multiple experimental techniques. Load measurements on the rotor were carried out to assess the change in the rotor performance for different positions with respect to the cubic obstacle, thus simulating a set of possible hovering flight conditions around the obstacle. Laser Doppler anemometry measurements of the rotor inflow were used to investigate how the aerodynamic interaction affected the rotor performance. Stereoscopic particle image velocimetry measurements in the region between the rotor and the obstacle were carried out to gain a better insight of the interacting flow field. The investigation showed two main regions of interest. The first region is the one above the edge of the obstacle, where the rotor experiences a gradual ground effect as it is positioned over the obstacle. The second region, probably of more interest, is the one just beside the obstacle where a recirculation region between the rotor and the obstacle develops, causing both a significant reduction in the thrust augmentation experienced in unobstructed hover in ground effect and significant pitching and rolling moments, due to the nonsymmetrical inflow pattern on the rotor

Formation and early development of wingtip vortices

Wingtip vortices are extremely important phenomena in fluid dynamics for their negative effects in many applications. Despite the many studies on this particular flow, the current understanding is still poor in providing a form base for the design of effective tip geometry modifications and vortex control devices. A rectangular wing with squared and rounded wingtips was tested in order to identify the main mechanisms involved in the formation of the vortex on the wing and in its early development in the wake. The complementarity of a number of experimental techniques adopted, such as surface flow visualizations, wall pressure measurements, smoke visualizations and stereoscopic particle image velocimetry (SPIV), gave a richer insight of the physics and the basic mechanisms of the vortex development. Furthermore, a large number of configurations were tested exploring the effects of several parameters such as wing chord, aspect ratio, wingtip geometry, angle of attack and Reynolds number. The development of the vortex along the wing showed the formation of several secondary vortices which interacted with the primary vortex generating low frequency fluctuations. The structure of the flow at this stage was analysed introducing a compact description through characteristic lines of the vortex system defined from the velocity vector field in the vicinity of the wing surface. The high spatial resolution achieved by the SPIV arrangement allowed a deeper understanding of the vortex structure in the early wake and the turbulence production and dissipation within the vortex core. The relaminarization process of the vortex core promoted by centrifugal motion was observed. The relation between vortex meandering, turbulence, secondary vortices and wake sheet was discussed. A comparison of different methods for the averaging of instantaneous planar vector fields was performed showing the effects and importance of the meandering. An axial acceleration of the flow within the vortex was observed and the formation of different axial flow distributions was discussed. A minimum wake-like flow of 0.62 and a maximum jet-like flow of 1.7 times the freestream velocity were measured and a linear relation between a vortex circulation parameter and the axial velocity peak was found

Forces on Obstacles in Rotor Wake – A GARTEUR Action Group

The paper describes the objectives and the structure of the GARTEUR Action Group HC/AG-22 project which deals with the basic research about the forces acting on obstacles when immersed in rotor wakes. The motivation started from the observation that there was a lack of experimental databases including the evaluation of the forces on obstacles in rotor wakes; and of both numerical and experimental investigations of the rotor downwash effects at medium-to-high separation distances from the rotor, in presence or without sling load. The four research centres: CIRA (I); DLR (D); NLR (NL); ONERA (F); and three universities: NTUA (GR); Politecnico di Milano (I); University of Glasgow (UK) created a team for the promotion of activities that could contribute to fill these gaps. In particular, both numerical and experimental investigations were proposed by the team to study, primarily, the effects of the confined area geometry on a hovering helicopter rotor, and, secondarily, the downwash and its influence on the forces acting on a load, loose or slung, at low to high separation distances from the rotor disc. The following activities were planned: a) application and possible improvement of computational tools for the study of helicopter rotor wake interactions with obstacles; b) set-up and performance of four cost-effective wind tunnel test campaigns aimed at producing a valuable experimental database for the validation of the numerical methodologies applied; c) final validation of the numerical methodologies. The project started in November 2014 and has a duration of three years

Rotor Blade Tip Vortices

No abstract available

Investigation of a trailing vortex near field by stereoscopic particle image velocimetry

High spatial resolution experiments in the near field of a trailing vortex using a SPIV technique have been carried out. A particular attention on the measurement technique is presented highlighting the importance of the laser pulses delay between the two frames in a double-frame/single-pulse method and of the images processing characteristics in the evaluation of velocity profiles and turbulence quantities. The number of samples needed for the statistical onvergence of the mean flow and the turbulent quantities has also been found. A study on the vortex aperiodicity correction method has revealed how the instantaneous vector fields centering affects the velocity profiles across the vortex. The importance in the choice of the vortex quantity which is used for the vortex centre detection is presented

Rotor-Obstacle Aerodynamic Interaction in Hovering Flight: An Experimental Survey

Despite the presence of a fair number of numerical and experimental works on the rotor-obstacle interaction, a systematic study of the aerodynamic phenomena involved is lacking. In this paper a comprehensive experimental survey carried out at University of Glasgow is described, taking advantage of two different rotor rigs and several experimental techniques. Load measurements on the rotor were carried out in order to assess the rotor performance for different positions with respect to a cubic obstacle, thus simulating a set of possible hovering flight conditions around the obstacle. Laser Doppler Anemometry (LDA) measurements of the rotor inflow were used in order to investigate how the aerodynamic interaction affected the rotor performance. Eventually Stereoscopic Particle Image Velocimetry (SPIV) measurements in the region between the rotor and the obstacle were carried out in order to have a better insight of the interacting flow field