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Title: High-frequency pulsed jet forcing of an axisymmetric bluff body wake
Author: Oxlade, Anthony
ISNI:       0000 0005 0732 7265
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2014
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The turbulent wake of a bullet-shaped axisymmetric bluff body is forced using a periodic pulsed jet located immediately beneath the point of separation. The Reynolds number based on body diameter is Re_D = 188,000, the ratio of body length to diameter is L/D = 6.48, and the ratio of body diameter to boundary layer momentum thickness at separation is D/θ = 92. A parametric study of the response of the mean base pressure to the governing variables of forcing frequency and forcing amplitude is performed. At high forcing frequencies (several times that of the shear-layer frequency) the area-weighted mean base pressure is increased by as much as 33%. A detailed investigation of the forced and unforced wake is made using random and phase-locked two-component PIV, and modal decomposition of pressure fluctuations on the base of the body. In addition to the well known shear-layer and vortex shedding wake structures, a dominant very-low-frequency mode with azimuthal wavenumber m=±1 and Strouhal number St_D ≈ 0.0015 is identified in the forced and unforced wake. This mode spatially modulates the coherent and incoherent wake oscillations in an orbit around the central axis of the body. Statistical axisymmetry is recovered by the random variations in radius and azimuthal angle of this orbit. This feature of the turbulent wake appears to be an unsteady manifestation of the SS regime of the laminar wake (the latter is a steady state with reflectional symmetry). Although the pulsed jet provides zero-net-mass flux, a non-linear interaction with the wake creates a finite momentum flux. The pressure on the base of the body (and hence drag) can be increased or decreased dependent upon the forcing frequency and amplitude. Increasing the base pressure by forcing at high frequency is a unique form of direct wake control in that it does not target the dominant flow instabilities. Instead, the jet introduces a row of closely spaced single-sign vortices that advect within the separating shear layer, remaining coherent for less than half a body diameter. In the time average these vortices generate a narrow region of large enstrophy bounded on each side by a strong shear layer; the latter being associated with high dissipation and non-local pressure recovery. The magnitude of the base pressure recovery is shown to be proportional to the strength of the jet vortices and is accompanied by broadband suppression of energy across all azimuthal wavenumbers with no preferential mode selection. The pressure recovery is proportional to forcing frequency, reaching saturation at approximately 5 times the natural frequency of the shear layer. This frequency dependence is a direct result of progressively reduced coupling between the jet perturbation and both the shear layer, and the vortex shedding wake instabilities. The latter are responsible for enhancing entrainment and turbulence production, thereby competing directly with the pressure recovery mechanism.
Supervisor: Morrison, Jonathan Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral