Variable control of jet decay

The influence of blowing radially inward with a pair of control jets toward the centerline of and close to the exit from a circular main nozzle is investigated. Two regimes of control-jet influence exist, corresponding to conditions where the control jets are swept away in the turbulent mixing layer of the main jet or where they penetrate the central smooth-core flow. In the first regime, local velocity reductions in the main jet of up to 30% can be achieved with control-jet mass flows of 0.5% of that in the main-jet nozzle without severely distorting the main jet from its circular cross section. The action of the control jets is to promote more rapid mixing of the main-jet shear layer, and the response increases with the square of control-jet relative velocity and with the cube of its relative diameter. In the second regime, the main jet is strongly distorted from a circular cross section and local velocities in the main jet are less sensitive to control-jet velocity. The main-nozzle mass flow is influenced significantly in the second regime only when the control jets are very close to the main-nozzle lip.