Active and Passive Flow Control of an Incompressible Axisymmetric Jet

Abstract

An experimental investigation was carried out to compare active and passive flow control of an incompressible axisymmetric jet. For active flow control, the lip of the circular jet was equipped with a single small flap deflected away from the jet stream at an angle of 30°. The flap incorporated a flow control slot through which steady suction and oscillatory suction were implemented. For passive flow control, the lip of the circular jet was equipped with a single small triangular tab deflected into the jet stream at an angle of 30°. Both the flap and triangular tab chord lengths were one sixth of the jet diameter. The momentum of jet increased in the case of active flow control by entraining ambient fluid, whereas momentum decreased in the case of passive flow control. The effect of steady suction saturated for volumetric suction coefficient values greater than approximately 0.82%. The strength of the streamwise vortices generated by active flow control flaps were greater than those generated by the passive triangular tab. Steady suction produced positive pressures just downstream of the flow control slot in the central portion of the flap and negative pressures at the flap edges. Oscillatory suction was highly dependent on dimensionless frequency (F+) based on flap-length; the pressures on the central portion of the flap increased for F+≤0.11 and then decreased for greater F+; finally attaining negative pressures at F+ = 0.44. The increase in jet momentum, combined with the generation of strong streamwise vortices makes a strong case for improvements in propulsion efficiency and jet noise reduction.