Analytical/experimental comparison of the axial velocity in trailing vortices

The axial velocity of a vortex is a parameter which is known to be strongly related to the vortex breakdown, yet to date a complete description of its physical origins has not been achieved. A series of experiments studying the vortex trailed from a NACA 0015 wing using stereoscopic particle image velocimetry is presented and the axial velocity studied in detail. The problem of centering the instantaneous vector fields is addressed showing a high sensitivity of the results from the centering method which is adopted. It is shown that a strong axial velocity excess exists and a linear relationship between the axial velocity and a circulation parameter of the vortex is shown. The experimental data are compared with the analytical descriptions of the velocity in the centre of a simplified vortex giving new insights of the viscous effects in the development of the axial flow

Vortex formation on squared and rounded tip

The vortical flow originated from the tip of a NACA 0012 rectangular wing is described in its initial formation and development over a rounded and a squared tip. Smoke visualizations show the rolling-up kinematic and evolution of the vortical systems moving the plane towards the trailing edge. The presence of intense secondary vortices affects the primary vortex unsteadiness and shape during the formation and in the early wake. Stereoscopic Particle Image Velocimetry is used to describe vorticity, axial velocity and turbulent kinetic energy distributions of the vortex during the formation and in the early wake at different angles of attack of the wing. The rolling-up of the vorticity sheet around the vortex system is strongly influenced by the vortex shape and the intensity of secondary vortices. Turbulence coming from secondary structures and shear layers is wrapped into the roll-up of the vortex and high levels of turbulence are measured in the vortex core. However, a laminar vortex core is observed for the lower angle of attack in the early wake. Comparing the meandering of the vortex for the two wingtip geometries, two different sources of the vortex fluctuation in the wake are identified: the interaction of secondary vortices moving around the primary vortex and the rolling-up of the vorticity sheet. Lastly, measurements in the wake of the wing at zero incidence are also presented showing a distinctive counter rotating vortex pair

Near field structure of wing tip vortices

High spatial resolution experiments in the near field of a trailing vortex using a Stereoscopic Particle Image Velocimetry technique have been carried out. A NACA 0015 model with flat tip has been tested for several Reynolds numbers and angles of attack. An axisymmetric meandering of the vortex is observed and a discussion on the aperiodicity correction method has identified the helicity peak as the most convenient indicator of the vortex centre. The axial velocity in the centre of the vortex has been recorded always as an excess except for low angle of attack cases where intermittent peaks of excess and deficit are superimposed on a large patch of deficit velocity. The double vortex structure and the consequent double inflection in the tangential velocity profiles is also studied with reference to a vortex age parameter. At already 2 chords of distance from the trailing edge the profiles exhibit axisymmetric behaviour. A spiral structure of the vortex core has been reported as effect of the early stage of the rolling up and considerations on the rotation confirmed the high dependency of the initial phase of the rolling up with the tip shape. The square tip produces a strong asymmetry of the vortex core and an intense secondary vortex. Good agreement of the tangential velocity and the circulation profiles between the experiments and analytical vortex expressions has been observed. The results confirm the existence of a three-part vortex structure, namely an inner, a logarithmic and an outer region of the vortex where the former is affected by the initial vortex structure and the latter is not universal but shows a dependence on the angle of attack

Rotor wake interactions with an obstacle on the ground

An investigation of the flow around an obstacle positioned within the wake of a rotor is described. A flow visualisation survey was performed using a smoke wand and particle image velocimetry, and surface pressure measurements on the obstacle were taken. The flow patterns were strongly dependent upon the rotor height above the ground and obstacle, and the relative position of the obstacle and rotor axis. High positive and suction pressures were measured on the obstacle surfaces, and these were unsteady in response to the passage of the vortex driven rotor wake over the surfaces. Integrated surface forces are of the order of the rotor thrust, and unsteady pressure information shows local unsteady loading of the same order as the mean loading. Rotor blade-tip vortex trajectories are responsible for the generation of these forces

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

Arianna: towards a new paradigm for assistive technology at home

Providing elderly and people with special needs to retain their independence as long as possible is one of the biggest challenges of the society of tomorrow. Teseo, a startup company spinoff from the University of Genoa, aims at accelerating the transition towards a sustainable healthcare system. Teseo's first concept and product, Arianna, allows for the automated recognition of activities of daily living at home and acts as a wellbeing and healthcare personalized assistant. This abstract outlines the main concepts underlying its features and capabilities.Comment: Paper accepted at the Eight Italian Forum on Ambient Assisted Living (ForItAAL 2017

Observations of the Vortex Ring State

This paper considers the vortex ring state, a flow condition usually associated with the descent of a rotor into its own wake. The phenomenon is investigated through experiments on simple rotor systems, and a comparison is then made with observations of a flow generated by a specially designed open core, annular jet that generates a mean flow velocity profile similar to the mean flow in a rotor wake in hover. In an experimentally simulated descent, the jet flow generates a flow state that shares many features of the rotor vortex ring state

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