Boundary-Layer Control by Means of Pulsed Jets at Different Inclination Angles

Finite amounts of compressed air are impulsively emitted through a rectangular 20 x 0.5 mm outlet at an axial velocity of u(jet) approximate to 54 m/s. Their interaction with a steady crossflow with a zero-pressure-gradient turbulent boundary layer is studied on the basis of phase-locked particle image velocimetry measurements for a parameter space that is spanned by the jet inclination angle defined with respect to the surface tangent downstream of the outlet alpha approximate to (60,30) deg and the ratio between the axial jet velocity and the crossflow velocity r = 2.4 ... 11. Two types of vortex structures are observed. For alpha = 60 deg, r > 6.5, jets penetrate into the crossflow, and distinct, asymmetrical vortex rings are produced. For alpha = 60 deg, r < 3 as well as for alpha = 30 deg, r = 2.4 ... 11, starting jets attach to the wall as leading vortex half-rings are formed. The latter type is better suited to energize the boundary layer as low-momentum fluid is shifted away from the surface and high-momentum fluid from the freestream is entrained into the jet wake. In terms of the overall gain in streamwise momentum, the impulse provided due to overpressure during the rapid jet initiation is of major importance in the present study, and its exploitation may enable a significant enhancement in future flow control applications.