Thrust vectoring

Countercurrent shear layer enhancement for fluidic thrust vector control

Thrust Vector Control or Thrust Vectoring is a technology that deflects the mean flow of an engine jet from the centerline in order to transfer some force to the aimed axis. By that imbalance, a momentum is created and used to control the change of attitude of the aircraft. Among other things, thrust vectoring greatly improves maneuverability, even at high angles of attack or low speeds where conventional aerodynamic control surfaces lose all effectiveness. Thrust Vector Control is currently achieved by complex arrays of mechanical actuators capable of modifying the geometry of the nozzle and thus defect the flow. This variable geometry greatly increases weight and maintenance to the engine, and therefore limits the benefits from vectoring the thrust.

Photo courtesy Pratt & Whitney, A United Technologies Company

F119 engine for F/A-22 Raptor showing the 2 extreme vectoring cases.

Fluidic Thrust Vector Control is a technology aiming at the above listed benefits by the use of fluidic means, implying less complexity and faster dynamic responses. Different concepts have been developed in the last decade to redirect the thrust without mechanical actuators. Induction to flow separation, countercurrent shear layer, synthetic pulses or skewing of the sonic line are some of the proven concepts.

Countercurrent shear flow control has been established as an effective method for fluidic thrust vector control. However, hardware integration issues exist and must be overcome in order to make a viable technology for future aircraft. Recent developments in fluidic thrust vector control have focused on nozzle interior methods that skew the throat of the nozzle using multiple transverse jets.

The aim of the present work is to combine this transverse jets approach with the countercurrent shear flow in order to enhance its control capacity inside the nozzle interior and achieve easily integrated fluidic thrust vector control. The increased turbulence and faster growth of the shear layer increase the pressure difference on the lower wall and in the outside of the jet respectively, thus modifying the angle.

CFD analysis of injection and suction coupling for thrust vector control achieving 9° thrust angle.

Streamlines and pressure contours.

Computational Fluid Dynamics (CFD) and Particle Image Velocimetry (PIV) have been used in the development and study of new configurations and their effect on the flow field. The experiments showed that the creation of a suction at the exit of the nozzle had an important effect on the flow field and thus on the vectoring angle. It was found that for various injection momentum ratios, the low pressure zone created by the suction had important changes in the flow field of the recirculation zone, main contributor to the vectoring of the jet. Along with a decrease of the recirculation height, the turbulence levels are increased in the shear layer region, and the turbulence gradients (important creators of negative pressure necessary for fluidic vectoring) near the lower wall are increased. As a result, an important response of the vectoring angle to the suction pressure was found for those momentum ratios.

The application of suction increases the turbulence gradients by decreasing the recirculation height (MR=0.2; ?P=0 and 1.4 respectively)

Streamlines and Reynolds stress contours

However, that turbulent enhancement that led to an increase of the vectoring capability of the system was limited to moderate momentum ratios in the studied configuration. It was noticed that, as the recirculation length grew out of the nozzle, the effects of suction on the recirculation zone and thus on the angle of the jet were greatly diminished. This was found to be linked to the capability of the suction slot to capture a streamline from the recirculation, giving it the control capability present in the lower injection rates.

At lower momentum ratios, the suction is capable of capturing the recirculation flow, enhancing the vectoring response. At higher momentum ratios the suction loses its effect

Streamlines and non-dimensional streamwise velocity contours

Bistability created by the attachment of the flow to the external hardware is an important issue in the integration of the fluidic systems as a viable technology for aircraft control. In the occurrence of attachment, the benefits of an important angle increase are overruled by the loss of control of the vectoring capability. In some of the tested configurations, attachment to the manifold was present, inducing jet angles ranging from 20° to 36°. That undesired behavior was however shown to disappear when a transverse jet was injected inside the nozzle.

PIV analysis of attached flow due to high suction levels

Streamlines and mean u-velocity contours.