Conventional second-order-accurate finite-difference schemes are much too dissipative for computations involving vortices that travel large distances (relative to some measure of the size of the vortex). This study uses a fifth-order-accurate upwind-biased scheme that preserves vortex structure much longer than second-order-accurate, central or upwind difference schemes. Vortex computations demonstrating this aspect of the fifth-order scheme are presented. The method is then applied to a blade-vortex interaction problem. The computed results are in good agreement with earlier numerical computations that required a priori information regarding vortex structure and position throughout the interaction period in order to sustain the vortex. High-order-accurate methods such as the one used in this study may prove essential to computing both nearfield aerodynamics and farfield acoustics associated with blade-vortex interactions as well as other acoustical phenomena.

Navier-Stokes Simulations of Blade-Vortex Interaction Using High-Order-Accurate Upwind Schemes

Computationally fast harmonic balance methods for unsteady aerodynamic predictions of helicopter rotors

Calcul numerique de l'interaction instationnaire d'un rotor d'helicoptere avec un troubillon pour des ecoulements subsoniques et transsoniques

Aerodynamics of two-dimensional blade-vortex interaction

Indicial approximations are derived for the lift on an airfoil penetrating a stationary sharp-edge gust in two-dimension al subsonic flow. Using an assumed exponential form, the approximations have been generalized in terms of Mach number alone by means of an optimization algorithm where certain coefficients of the approximations are free parameters. The optimization is subject to prescribed constraints in terms of the initial and asymptotic behavior of the gust response, and by requiring the response closely match the known exact solutions given by subsonic linear theory at earlier values of time. An alternative approximation is obtained by using results from a direct numerical simulation of the gust problem using computational fluid dynamcs (CFD). For an airfoil-vortex interaction problem, comparisons were made with experimental data and CFD results. Finally, the indicial method was integrated into a three-dimensional rotor simulation, and the near- and far-field acoustics were computed using the Ffowcs WilliamsHawkins equation. Good agreement was found with simultaneously measured airloads and acoustics data.

Subsonic unsteady aerodynamics caused by gusts using the indicial method

A two-dimensional, airfoil-vortex interaction experiment was conducted to obtain the aerodynamic behavior of the airfoil during a parallel interaction. The vortex was created by pitching a second airfoil placed upstream and parallel to an instrumented test airfoil. Hot-wire anemometer measurements were taken to determine the vortex velocity distribution. The interaction tests were conducted for a counterclockwise vortex passing above a symmetrical airfoil at zero angle of attack

Airfoil pressure measurements during a blade vortex interaction and a comparison with theory

A scheme for investigating the parallel blade vortex interaction (BVI) has been designed and tested. The scheme involves setting a vortex generator upstream of a nonlifting rotor so that the vortex interacts with the blade at the forward azimuth. The method has revealed two propagation mechanisms: a type C shock propagation from the leading edge induced by the vortex at high tip speeds, and a rapid but continuous pressure pulse associated with the proximity of the vortex to the leading edge. The latter is thought to be the more important source. The effects of Mach number and vortex proximity are discussed.

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F. X. Caradonna

Papers

An experimental investigation of the parallel blade-vortex interaction