Control of Turbulent Wakes of 3D Bluff Bodies

Abstract

The aerodynamics of three-dimensional bluff bodies are dominated by complex turbulent wakes due to the massive flow separation at the rear end. The added complexity of the wake arises from the static symmetry-breaking mode which contributes towards drag. Turbulent wake control, using passive and active methods, induces flow field modifications to achieve base pressure recovery and thus mitigate the pressure drag. The present thesis explores the effect of active control on the wake of a flat-back Ahmed body which is a common academic representation of ground transport vehicles such as SUVs, vansandlorries. The wake is disturbed by blowing carried out from the center of the base using different scales of an unsteady oscillating jet and a steady jet. The wake is found to be completely symmetrized for the optimal blowing rate of a steady jet with the base drag reduced by around 9%. Different scales of actuation reveal a similar positive effect, where the optimal blowing coefficient is found to scale with the square root of bleed-to-base area ratio. The actuation impacts the shedding modes through a change in amplitude and frequency. The statistically converged mean wake, subject to steady jet actuation, is recovered using a stacked stereoscopic particle image velocimetry (SSPIV) technique. The reconstructed 3D flow field is used to estimate the different wake mass f luxes whose equilibrium determines the recirculation length. A favorable momentum regime is identified through an increased drag reduction concomitant with a strong decrease of wake asymmetry strength. This new regime is complementary to the mass regime reported in literature which presents an affine relationship between drag reduction and blowing coefficient. Proper Orthogonal Decomposition of the 3D wake field reveal the coherent structures related to the bi-stable flow. The wake asymmetric state presents an interaction of a major hairpin vortex with a secondary horseshoe structure, whereas the optimal blowing case comprises of a pair of horseshoe structures parallel to the base. The steady jet blowing also provides drag reduction under side-slip condition along with reduction of yaw induced asymmetry.

Date of Award	6 May 2024
Original language	American English
Supervisor	VLADIMIR Parezanovic (Supervisor)