Chapter 1 – Static and Dynamic SACCON PIV Tests - Part II: Aft Flow Field

Abstract

The flow above delta wing shaped UCAV (Unmanned Combat Air Vehicle) configurations is often dominated by strong vortices, especially at moderate and high angle of attack. On delta wings with sharp leading edges, the flow separates at the leading edges already at small angles of attack. The shear layer rolls up and two vortices are formed on each side of the wing starting close to the wing apex and increasing in size towards the wing aft. The vortices produce strong suction peaks on the upper wing surface, so that additional lift is generated which is of benefit by performing low speed manoeuvres like take-off and landing. UCAV configurations often consist of rounded leading edges to improve their performance under cruise conditions. In this case the flow topology becomes more complicated because the flow separations at the leading edges are delayed to higher angles of attack and depend strongly on the Reynolds and Mach number. In regions of attached flow around blunt leading edges, other flow effects may produce additional vortices as observed on the VFE2 delta wing where vortices develop inboard the leading edge vortices caused by a kind of instability within the boundary layer. The onset of the leading edge vortices, their interactions and accompanying phenomena like vortex breakdown can cause adverse loading effects. To avoid such non-linear aerodynamic characteristics detailed flow investigations are of great importance during the design chain by improving the understanding of the aerodynamics. This all the more so if the dynamic behaviour of the vehicle has to be considered as the flow effects become even further complicated. Within the scope of the Task Group of AVT-161 (Chapter 1), Stereoscopic Particle Image Velocimetry (PIV) has been applied for flow field investigations on a SACCON (Stability And Control CONfiguration) wind tunnel model.