Regulation of the Aerodynamic Loads on a Circular Cylinder Using 2- and 3-D Coanda Wall Jet Arrays

Aerodynamic loads engendered by circulation control on a rotorcraft cylinder boom have been used to effect counter torque to replace the conventional tail rotor in NOTAR configurations. These loads that have been realized by the Coanda effect of a 2-D steady wall jet in the rotor downwash diminish with changes in the direction of the cross flow in sideward and forward flight and are typically augmented by an additional jet thruster. The present wind tunnel investigations explore the utility of segmented Coanda actuation on a circular cylinder model for generating streamwise and cross-stream loads that are less insensitive to the direction of the cross flow using spanwise arrays of 3-D fluidically oscillating wall jets with specific emphasis on the interaction of the control jets with the cross flow and their effects on the near wake. It is shown that for a common actuation momentum coefficient (and at a lower actuation power) the 3-D jets lead to markedly higher cross-stream loading than the 2-D jets and an extended azimuthal range of effective actuator positions. The lift increments of the 2- and 3-D jets are also accompanied by drag decrements of up to ~40%. The present investigations are conducted at subcritical Reynolds number (ReD = 2.1·10^5) and the effects of supercritical flow on the interaction of the 3-D jets with the cross flow were assessed by tripping the boundary layer upstream of the azimuthal position of the jet and taking load measurements coupled with planar PIV. When the actuation is applied upstream of its critical (highest lift) azimuthal position, the state of the boundary layer has minimal effect on the induced aerodynamic loads. However, near the critical actuation angle over the smooth cylinder perturbations in the tripped upstream boundary layer can strongly disrupt the interactions of the jets with the cross flow and significantly reduce the effected lift. These effects are investigated in the absence and presence of tripping using POD analysis of instantaneous PIV data.