The aerodynamics of slender aircraft forebodies at high angle of attack.

Abstract

High angle-of-attack aerodynamics has been the focus of much attention as a result of the drive to increase combat aircraft manoeuverability and thereby improve survivability. A key factor in this regime is the behaviour of the complex vortical wake generated by the forebody. Even at zero yaw, the shear layers and vortex pairs which are symmetric at moderate angles of attack (alpha) can become highly asymmetric as alpha is increased. This can lead to large sideforces and yawing moments which may exceed available control power. Computationally simulating high-alpha forebody flowfields is known to be a challeng­ing problem. This thesis details the evaluation and enhancement of the CFD code NSMB with the objective of improving qualitative and quantitive predictions of the flowfield around fighter-type forebodies throughout the angle-of-attack range. Results on a tangent-ogive body confirmed that computing asymmetric flow required the introduction of a space- and time-fixed surface excrescence or the use of a non-symmetric solution algorithm to simulate flow instabilities via transient numerical error. Although solution stability problems were encountered, results with the non-symmetric algorithm showed promise. Suspected turbulence modelling issues were addressed by implementing the k-w family. Having established a methodology, solutions were obtained for the forebody of a cur­rent fighter aircraft, the Saab JAS-39 Gripen. The computed data shows excellent exper­imental agreement for 0° ≤ a ≤50° over the clean geometry but the inclusion of a nose pitot probe was seen to destabilise the calculation and prevent convergence. Finally, a single vertical nose strake or ‘rhino horn’ was added. When undeflected, this stabilised the flowfield, reduced solution oscillation and negated sideforce. Deflecting the hom pro­duced a stable flowfield with non-zero sideforce. Similar devices may be used, together with engine thrust-vectoring, in the next generation of combat aircraft and may also be added to existing airframes as a MLU.