Highly swept leading edge extensions(LEX) applied to delta wings have greatly improved the subsonic maneuverability of contemporary fighters. The LEX vortices generated at high angles of attack improve the maximum lift capability of the delta wing by way of induced suction pressure over the inboard surfaces directly under the vortices and additionally through beneficial interaction with the separating flow outboard on the wing.
In this study, systematic approach by PIV experimental method within a circulating water channel was adopted to study the fundamental characteristics of induced vortex generation, development and its breakdown appearing on a delta wing model with or without LEX in terms of four angles of attacks(15°, 20°, 25°, 30°) and six measuring sections of chord length(30%, 40%, 50%, 60%, 70%, 80%). Sideslip effect in case of the LEX was also studied for two sideslip(yaw) angles(5°, 10°) at one angle of attack(20°)
High resolution digital CCD camera(1K x 1K) and its synchronizing system with a dual pulse illuminating laser(Nd-Yag, 100mJ) was adopted to obtain the reliable PIV data. Distribution of time-averaged velocity vectors and vorticity over the delta wing model were compared along the chord length direction. Quantitative comparison of the maximum vorticity featuring the induced pressure distribution were also conducted to clarify the significance of the LEX existence.
In case of the LEX-on, the vortex breakdown is delayed at higher chord length, resulting in lift increase. Furthermore, double vortices at each side were detected indicating the effect of the LEX while the delta wing without LEX gave single vortex formation and its early breakdown along the chord length. Animation presentation in velocity distribution was also implemented to clarify the effect of LEX with LEX/wing vortex interaction.
The present 3-D stereo PIV includes the Identification of 2-D cross-correlation equation, stereo matching of 2-D velocity vectors of two cameras, accurate calculation of 3-D velocity vectors by homogeneous coordinate system, removal of error vectors by a statistical method followed by a continuity equation criterior and so on.
The present dynamic stereo PIV represents the complicated vortex behavior, especially, in terms of time-dependent characteristics of the vortices at given measuring sections. Quantities such as three velocity vector components, vorticity and other flow information can be easily visualized via the 3D time-resolved post-processing to make the easy understanding of the LEX effect or vortex emerging and collapse which are important phenomena occurring in the field of delta wing aerodynamics.
Computational fluid dynamics study using CFX, one of the commercial software, was carried out to compare with the result of PIV method.