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We show that high-frequency periodic jet blowing can be used to increase the base pressure of a bullet-shaped body with a turbulent axisymmetric wake by as much as 35%. A detailed investigation of the effects of forcing is made using random and phaselocked 2C PIV, and modal decomposition of dynamic pressure measurements on the base of the model. In contrast to other studies using periodic jet forcing, for example those discussed in [1], this control strategy does not target specific local or global wake instabilities. Instead, the high-frequency jet creates a row of closely spaced vortices which appear to act as a buffer between the wake and separating flow, thereby inhibiting the entrainment of fluid from the separating boundary. 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. We will show that the optimum forcing frequency is roughly six times the frequency of the shear layer mode, where excitation of the shear layer mode approaches unity gain. We also observe that despite being subject to an axisymmetric perturbation, the forced wake does not exhibit statistical axisymmetry.
Author(s):
Anthony Oxlade
Imperial College
United Kingdom
Jonathan Morrison
Imperial College
United Kingdom