Abstract

This paper has the objective to analyze passive flow control technologies that can reduce the shock wave intensity and thus the interference drag in the strut-wing junction, in transonic flight. Two possible flow control devices were investigated by means of CFD, Kuchemann Carrot (KC) and shock control bumps (SCB). Also a combination of these two configurations was studied. Drag and lift coefficients were monitored for each component and the angle of attack was modified to match the lift of the baseline configuration while comparing the drag. The mesh is based on hexahedral cells using a cut-cell approach and y+ approx. 1 near the walls. This mesh strategy was adopted to ensure constant mesh distribution from one configuration to the other. All the simulations were performed using the k-ω SST with Intermittency Transition model.
Anti-shock bodies have been used for civil and military aircrafts in order to improve drag performance in transonic conditions. A particular case of anti-shock bodies used at the junction of the vertical and horizontal tail to improve cross section area variation is the KC. The KC shape used in this study was limited to the pressure side of the wing and extended only beyond the leading edge. It contained a “fuselage waisting” at the location of the maximum thickness of the airfoil in the vertical strut. The KC showed a decrease of the angle of attack to match the baseline lift and reduced total drag. The drag reduction compared to baseline although less than 1% was due to the pressure drag mitigation and displayed an adverse effect for the
viscous drag; justified by the increase in wetted area. The pressure drag effect is
explained through the reduction of the shockwave in the vicinity of the wing-strut
junction.
The second option for passive flow control devices used in transonic flows is the
SCB. Classically they are presented in the literature for controlling flow on the
suction side of a wing at transonic Mach numbers that generate an acceleration
above Mach number 1.3. In the case of the wing-strut junction, the flow is
accelerated on the pressure side of the wing to Mach numbers of that magnitude;
this fact alone suggest the possibility to use SCB as drag reduction devices. The SCB
has a 3D wedge type geometry with rounded sides. The bumps are aligned with the
flow and placed at 0.25 m distance from each other on both the suction side of the
strut and the pressure side of the wing. The SCB have an extended tail, flat top, a
width to height ratio of approx. 9 and a length to width ratio of 4. The height of the
bump differs from wing to strut, and are determined from 2D analysis at three span
wise locations of 15, 15.5 and 16m. The height of the bump on the wing is roughly
70% of the boundary layer thickness while on the strut is around 95%. The height is
kept almost constant in the span wise direction on the strut and also on the wing.
The SCB showed an increase of the angle of attack to match the baseline lift and
reduced total drag. The drag reduction compared to baseline, although almost
identical to the KC, was due to the pressure drag mitigation and displayed a positive
effect for the viscous drag also.
When compared to the baseline, both KC and SCB were able to reduce the total
drag but with complementary effects. Firstly, in order to match the lift, the KC
generates a decrease in incidence that is roughly half of the increase in incidence
needed for the SCB to match the lift. Secondly, the KC affects the reduction of
maximum Mach number at the junction stronger, while the SCB has a more span
wise distributed effect. The pressure drag reduction for the KC is almost double, for
lift matching; while at the same angle of attack, 1 deg., the opposite is observed. In
order to investigate further the benefits of both passive flow control devices by
combining them, several configurations have been analyzed, and abbreviated by
KC_SCB.
The first configuration of the KC_SCB was the combination of the separate versions
of them. This version showed the worst performance in terms of drag reduction
due to large flow separation at the junction. By removing the first three SCB, located
near the junction, the performance improved from the previous version, but remained worst that the baseline, again due to separated flow induced by the SCB. The third version kept the number of SCB and their span wise locations, but moved them in the stream wise direction with respect to the new shock position. The drag performance improved again, but the separated flow region persisted and induced a drag higher than the baseline. Although this mix of KC and SCB didn’t produce an improvement, the drag reduction trend was positive and it is expected that when using an optimization process will generate a superior solution. Considering that for both KC and SCB no 3D optimization, only design based on literature review was used, can be argued that the performance can be further improved.