Open-Loop Control of a Turbulent Axisymmetric Wake

A high-frequency periodic jet, issuing parallel to the freestream from immediately below the point of separation, is used to force the turbulent wake of a bluff axisymmetric body. It is shown that, at the optimum jet forcing frequency and momentum flux coefficient, the time-averaged area-weighted base pressure increases by as much as 33 %. A detailed investigation of the effects of forcing is made using modal decomposition of pressure fluctuations on the base of the model and phase-locked two-component PIV. The high-frequency jet creates a row of closely spaced vortices, immediately adjacent to which are regions of large irrotational dissipation on each side. These shear layers inhibit the entrainment of fluid. The resulting pressure recovery is proportional to the strength of the vortices produced by the jet, and is accompanied by a broadband suppression of base pressure fluctuations associated with all modes. The optimum forcing frequency, at which amplification of the shear layer approaches unity gain, is roughly five times the shear-layer frequency.