Albert R. George

The three-dimensional interaction of a streamwise vortex with a large-chord lifting surface : theory and experiment

The three-dimensional vortex flow that develops around a close-coupled canard-wing configuration is characterized by a strong interaction between the vortex generated at the canard and the aircraft wing. In this paper, a theoretical potential flow model is devised to uncover the basic structure of the pressure and velocity distributions on the wing surface. The wing is modelled as a semi-infinite lifting-surface set at zero angle of attack. It is assumed that the vortex is a straight vortex filament, with constant strength, and lying in the freestream direction. The vortex filament is considered to be orthogonal to the leading-edge, passing a certain height over the surface. An incompressible and steady potential flow formulation is created based on the three-dimensional Laplaces equation for the velocity potential. The boundary-value problem is solved analytically using Fourier transforms and the Wiener-Hopf technique. A closed-form solution for the velocity potential is determined, from which the velocity and pressure distributions on the surface and a vortex path correction are obtained. The model predicts an anti-symmetric pressure distribution along the span in region near the leading-edge, and a symmetric pressure distribution downstream from it. The theory also predicts no vertical displacement of the vortex, but a significant lateral displacement. A set of experiments is carried out to study the main features of the flow and to test the theoretical model above. The experimental results include helium-soap bubble and oil-surface flow pattern visualization, as well as pressure measurements. The comparison shows good agreement only for a weak interaction case, whereas for the case where the interaction is strong, secondary boundary-layer separation and vortex breakdown are observed to occur, mainly owing to the strong vortex-boundary layer interaction. In such a case the model does not agree well with the experiments.

Flowfield and Acoustic Measurements from a Model Tiltrotor in Hover

Rotor ine ow aerodynamics and noise measurements were made on three cone gurations of an 8% scale model of the XV-15 tiltrotor in hover: 1 ) single isolated rotor, 2 ) two rotors with no fuselage, and 3 ) complete tiltrotor aircraft. For the tiltrotor aircraft cone guration and for the cone guration of two rotors without the fuselage, the mean ine ow velocity was higher at c = 270 deg compared with the rest of the rotor disk, leading to reduced blade angle of attack and blade loading in this region. This azimuthally varying blade loading caused an impulsive noise that radiated preferentially behind the model. For the complete tiltrotor cone guration, the turbulence ingested by the rotors was intermittent and depended on the instantaneous position of the fountain e ow, which shifted from side to side across the longitudinal plane of the model. The fountain turbulence had a higher velocity scale, smaller length scale, and was closer to isotropic than the ingested ambient turbulence. The tiltrotor cone guration radiated less harmonic noise, but more broadband noise than the cone guration with two rotors and no fuselage. Diagonal fences on the wings of the tiltrotor reduced the ine ow turbulence intensity in the fountain region by a factor of about 3, and reduced the noise by 4.1 dBA behind the model. Scaling relations were derived to extrapolate the model measurements to the full-scale XV-15.

Wake Displacement Modifications to Reduce Rotorcraft Blade-Vortex Intreaction Noise

Increasing the vertical separation between the rotor and its wake can reduce the strength of blade-vortex interaction (BVI) noise. A simple, first-order analytical model is developed to estimate the vertical distance between the rotor and the wake at any location of interest in the rotor disk. This model is used as an indicator to evaluate the potential effects of added forces, descent angle, advance ratio, and thrust coefficient on the separation between the wake and the rotor with the aim of identifying strategies that maximize this distance at a particular rotor disk location. The analysis focuses on landing approaches for helicopters and tiltrotors operating near helicopter configuration. The most appropriate strategies to increase this separation at the location of interest are found to depend on whether the wake is initially above or below the rotor

A survey of automobile wind noise.

The study of automobile aerodynamic noise is a field that has grown rapidly in the past decade. Automobile manufacturers and independent researchers now have a better understanding of the mechanisms of automobile aerodynamic noise. Methods of making wind noise measurements have been improved. Some fundamental wind noise reduction measures have been developed. In this paper, the basic mechanisms of automobile aerodynamic noise generation are explained. Current wind noise measurement instrumentation and techniques are reviewed. Some wind noise reduction measures found on today’s automobiles are discussed. Finally, a prediction method for an idealized case of automobile aerodynamic noise is shown. Knowledge of these advances will help automobile manufacturers to design quieter vehicles.