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

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

Experiments on the Helicopter-Obstacle Aerodynamic Interference in Absence of External Wind

The rotor-obstacle interaction has become a challenging research topic in the last few years. In the present paper a comprehensive experimental survey carried out at University of Glasgow is described, taking advantage of two di↵erent rotor rigs and several experimental techniques. The results are then compared with those already obtained for a similar investigation at Politecnico di Milano. The experimental database comprises load measurements on the rotor (in order to assess the rotor performance for di↵erent positions with respect to a cubic obstacle), Laser Doppler Anemometry (LDA) measurements of the rotor inflow and Stereoscopic Particle Image Velocimetry (SPIV) measurements in the region between the rotor and the obstacle. Despite a few slight di↵erences in geometry and test conditions, the two databases show several similarities that are analysed in the paper

Experiments on the Helicopter-Obstacle Aerodynamic Interference in Absence of External Wind

The rotor-obstacle interaction has become a challenging research topic in the last few years. In the present paper a comprehensive experimental survey carried out at University of Glasgow is described, taking advantage of two di↵erent rotor rigs and several experimental techniques. The results are then compared with those already obtained for a similar investigation at Politecnico di Milano. The experimental database comprises load measurements on the rotor (in order to assess the rotor performance for di↵erent positions with respect to a cubic obstacle), Laser Doppler Anemometry (LDA) measurements of the rotor inflow and Stereoscopic Particle Image Velocimetry (SPIV) measurements in the region between the rotor and the obstacle. Despite a few slight di↵erences in geometry and test conditions, the two databases show several similarities that are analysed in the paper

Experiments on the Helicopter-Obstacle Aerodynamic Interference in Absence of External Wind

The rotor-obstacle interaction has become a challenging research topic in the last few years. In the present paper a comprehensive experimental survey carried out at University of Glasgow is described, taking advantage of two different rotor rigs and several experimental techniques. The results are then compared with those already obtained for a similar investigation at Politecnico di Milano. The experimental database comprises load measurements on the rotor (in order to assess the rotor performance for different positions with respect to a cubic obstacle), Laser Doppler Anemometry (LDA) measurements of the rotor in ow and Stereoscopic Particle Image Velocimetry (SPIV) measurements in the region between the rotor and the obstacle. Despite a few slight differences in geometry and test conditions, the two databases show several similarities that are analysed in the paper

The Clean Sky ‘Card’ Project: Wind Tunnel Measurements of a Model Helicopter Rotor and Fuselage Drag

Results of a wind tunnel test campaign to assess technologies for improvement of helicopter fuselage and rotor system drag are described in this paper. The work done was part of the EU funded Clean Sky CARD project (Contribution to Analysis of Rotor-hub Drag reduction). Wind tunnel tests of an accurate 1/4 scale, Airbus Helicopters H155 were performed in a variety of advance ratios, pitch angles and side-slip angles to simulate cruise and approach conditions. 1/3rd of the rotor diameter was represented in the model, and the rotor was otherwise rigid and un-actuated (fixed blade pitch around the azimuth). Rotor system forces and moments were measured independently of the fuselage forces and moments, thus allowing the various contributions to the total drag to be evaluated. The paper describes elements of the model design and data analysis for drag and power evaluation, and presents sample results