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

Measurements of a rotor flow in ground effect and visualization of the brown-out phenomenon

Quantitative and qualitative results of a series of experiments conducted on a rotor in ground effect at low forward speeds are presented. The velocity over a wide area of the ground effect wake was measured using Particle Image Velocimetry (PIV), and the evolution of the flow is described as the forward speed increases. Helicopter brown-out was simulated through a series of flow visualisation experiments. The technique involved sprinkling a fine powder on the ground below and ahead of the rotor. This helps to validate the experimental simulation of the brown-out phenomenon. Larger dust clouds were observed at lower advance ratio, and the dust cloud penetrated into the areas of the flow including those where vorticity levels were of low or negligible magnitude

Characterisation of a horizontal axis wind turbine’s tip and root vortices

The vortical near wake of a model horizontal axis wind turbine has been investigated experimentally in a water channel. The objective of this work is to study vortex interaction and stability of the helical vortex filaments within a horizontal axis wind turbine wake. The experimental model is a geometrically scaled version of the Tjæreborg wind turbine, which existed in western Denmark in the late 1980s. Here, the turbine was tested in both the upwind and downwind configurations. Qualitative flow visualisations using hydrogen bubble, particle streakline and planar laser-induced fluorescence techniques were combined with quantitative data measurements taken using planar particle image velocimetry. Vortices were identified using velocity gradient tensor invariants. Parameters that describe the helical vortex wake, such as the helicoidal pitch, and vortex circulation, were determined for three tip speed ratios. Particular attention is given here to the root vortex, which has been investigated minimally to date. Signatures of the coherent tip vortices are seen throughout the measurement domain; however, the signature of the root vortex is only evident much closer to the rotor plane, irrespective of the turbine configuration. It is postulated that the root vortex diffuses rapidly due to the effects of the turbine support geometries

Analysis of Wake Vortices of a Medium Range Twin-Propeller Military Cargo Aircraft Using Statistically Designed Experiments

An experimental study was initiated to analyze the trajectories of the streamwise vortices behind the wing tip and flap of a medium range and propeller driven twin-engine military cargo aircraft. The model used for the experimental study was a generic, high wing and half model of a propeller driven aircraft and mounted within Old Dominion University\u27s Low Speed Wind Tunnel where the wind tunnel flow speed was set to constant value of 9 m/sec. The main purpose of the study was to reach regression models for the motion and vorticity strength of both vortices under varying factors such as angle of attack, flap angle, propeller pitch angle and downstream distance. Velocity measurements of the flow fields were accomplished using both Particle Image Velocimetry (PIV) and Hotwire Anemometry (HWA) to yield average velocities, turbulence levels, vorticity strengths and Reynolds shear stresses in the wake of the model. The results of measurements showed that the vertical motions, horizontal motions, and vorticity strengths of both vortices as well as the shortest distance between both vortices depend on the aforementioned factors and the interactions of some factors. It can be concluded that propeller pitch angle mainly affects the behaviors of the vortices as much as angle of attack to the extent that their second order terms take place in some of the regression models

Review of measurement techniques for unsteady helicopter rotor flows

The helicopter group at the DLR in Göttingen has been actively involved in the development of measurement techniques for unsteady flows, particularly as they apply to the problems found in unsteady rotor blade aerodynamics. This includes the development and validation of new techniques for the detection of dynamically moving boundary layer transition, and for the detection of dynamic stall and other transient flow separation events. These new techniques include pressure sensor analysis, differential infrared thermography, local infrared thermography and the automated analysis of hot-film data. Particle image velocimetry (PIV) and background oriented schlieren (BOS) have been used for the analysis of the unsteady off-body flow, and synchronised PIVBOS-pressure measurements have allowed direct comparisons between different methods. The Lagrangian volumetric PIV variant, shake-the-box, has been used to analyse secondary vortex structures in the vortex wake. This review article will give an overview of the advances in that group, as well as placing their activities in the context of international advances in these areas

Experimental Investigation of the Rotor-Wing Aerodynamic Interaction in a Tiltwing Aircraft in Hover

The hovering performance and the lifting capability of tiltrotor aircraft are strongly affected by the aerodynamic interaction between wing and rotors. The tiltwing concept represents a promising technology to increase the hover performance by reducing the wing-rotor interference. The present work describes an experimental activity carried out on a 1/4 scaled tiltwing aircraft half-model to achieve a detailed insight about the main issues characterising the aerodynamic interaction between wing and rotor in hover. The results of the experimental campaign, including both force measurements and Particle Image Velocimetry surveys, enabled to evaluate both the aircraft performance for different configurations of the tilting wing and to achieve a detailed insight about the flow physics of the rotor wake in the interaction with the wing. The test activity provided a comprehensive experimental database that was obtained over a not confidential aircraft configuration

FLOW ANALYSIS ON DELTA WING – A REVIEW

Many aircraft, like fighters and bombers, are delta-winged aircraft. These aircraft have the characteristics of producing a greater lift at very small angles of attack even at very low Reynold’s number. Highly swept-wing aircraft experience greater lift due to the generation of the leading-edge vortex at a high angle of attack flight. The phenomena of vortex breakdown are noticed which leads to the reduction of the vortex lift and leads to the instability of the aircraft. This paper deals with an analysis of the flow on the delta wing with the existing literature and compares its benefits over the conventional wings using experimental and computational methods. The critical angles, pressure, and velocity variations are observed along with chord and spanwise locations