Supersonic Coanda Jets For Flight Control Effectors

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Abstract

Traditional control surfaces used on aircraft for high lift and manoeuvre control are complex systems requiring robust mechanical design to achieve the desired movement. Circulation control (CC) offers the potential to achieve similar or greater control gains by injecting air along the trailing edge of the wing with reduced mechanical complexity. A jet of air is blown over a rounded trailing edge surface, moving the aft stagnation point along the curved surface. The effectiveness of CC devices has been shown to be sensitive to geometry i.e. slot height and Coanda surface radius and the freestream conditions. Moreover, lift augmentation has been shown to reduce with increasing freestream velocity. To maintain the velocity ratio at higher freestream velocities the jet velocity has to be increased to and beyond critical condi- tion. Previous work suggests the shock structure produced by a convergent nozzle operated at supercritical pressures causes shock induced jet separation, leading to a sudden loss in CC performance. In the field of fluidic thrust vectoring (FTV) increased jet attachment at supercritical pressure ratios has been achieved through the use of convergent-divergent nozzles. Other studies sug- gest the use of a sheared velocity profile improves the attachment of supersonic jets to curved surfaces. The outcomes from the FTV research are applied to CC in an attempt to extend the performance of trailing edge fluidic devices in high subsonic/low transonic freestream condi- tions. The work presented is an experimental study of jet attachment and effectiveness of supercriti- cal circulation control devices. The first experiment consisted of a quiescent rig used to study on- and off-design attachment qualities of uniform and sheared velocity profiles for a CC con- figuration with a slot height to radius ratio of 0.1. The test nozzles are designed for pressure ratios of 4 and 7. For each nozzle pressure ratio (NPR) design point, both uniform and sheared velocity profiles showed full jet attachment for overexpanded and on-design conditions, with jet separation occurring in under-expanded conditions. The sheared velocity profile nozzles showed improved jet attachment in under-expanded conditions compared to the uniform veloc- ity profile nozzles. Wind tunnel experiments were carried out in a transonic wind tunnel to assess the effective- ness of supercritical CC devices in the presence of external freestream flow. Freestream Mach number was varied between 0.19 and 0.54. Test pieces included a convergent only nozzle, a convergent-stepped geometry (1:1 exit to step), NPR 4 uniform nozzle and NPR4 sheared velocity profile nozzle. The convergent-only geometry showed the smallest range of jet at- tachment, separating at a pressure ratio of 3.5. The convergent stepped nozzle showed a larger operational range, albeit for reduced performance. The uniform and sheared velocity pro- file convergent-divergent nozzles exhibited the same behaviour as the quiescent tests over the range of freestream conditions, where the sheared velocity profile remained attached at a higher pressure ratio. Lift augmentation factors around 10 were shown for the convergent-divergent nozzles at the highest freestream Mach number.

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Keyword(s)

Circulation Control; Coanda Jet; Fluidic Controls; High speed subsonic; Supersonic Coanda

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