Turbulent axisymmetric base flows: Symmetry and long-term behavior

This thesis deals with the flow around truncated bodies of revolution. Such flows are encountered in a variety of engineering applications relevant to the aerospace transportation industry, notably to space launcher vehicles. The work focuses on the unsteady behavior of the wake and particularly on the dynamics of the recirculation region behind the base.
The manuscript starts with a survey of the past literature on the topic of turbulent axisymmetric wake flows. Salient aspects are discussed mainly in relation to flow topology and dynamical behavior. The vortex shedding process is examined along with the associated instabilities, namely the large-scale wake oscillations, the backflow azimuthal meandering and the transition scenarios exhibited by the wake across the different flow regimes.
Chapter 3 illustrates the current methodology of investigation. The flow facility and the geometrical models used in the experiments are described. The operating principles of the Particle Image Velocimetry (PIV) technique are summarized. The main contributions of uncertainty affecting the present results are defined. Details are provided of the Proper Orthogonal Decomposition (POD) procedure adopted in the analysis of the large-scale fluctuations.
The influence of base geometry and symmetry on the behavior of a turbulent incompressible reattaching flow is addressed in Chapter 4. Afterbody geometries with varying diameter ratios are discussed as to model axisymmetric backward facing step (BFS) flows of varying step heights. Any increase in the afterbody diameter induces earlier shear layer reattachment and inhibits the large-scale shear layer fluctuations. Comparison with equivalent planar BFS flows reveals an opposite scaling of the reattachment distance for the axisymmetric and the two-dimensional flow case, with convergence towards small values of the step height.
The large-scale fluctuations of the turbulent wake behind a circular base are spatio-temporally characterized in chapter 5. It is found that the wake dynamics is dominated by very-low-frequency backflow fluctuations in proximity of the stagnation point on the base, while it undergoes a global radial displacement closer to the rear-stagnation point.
The very-low-frequency turbulent wake unsteadiness is examined in chapter 6 under the effects of a small pitch angle. It is found that the reversed-flow region tends to stabilize away from the body axis of symmetry with increasing angles between the body and the freestream flow. Analysis of the instantaneous velocity field and POD of the velocity fluctuations gives evidence of a backflow large-scale unsteadiness only within 0.1° deviations from axisymmetric inflow conditions.
The near-wake azimuthal organization in presence of an afterbody is analyzed in chapter 7 within different azimuthal-radial planes behind the base and for different diameter ratios. The afterbody is found not to alter the shear layer behavior significantly, but it interferes with the inner backflow meandering. It is shown that the wake unsteadiness of an afterbody flow is dominated by the shear layer development.
The main findings from the preceding chapters are summarized at the end of the manuscript. The conclusions of the present research are drawn and possible directions for future research on the topic of turbulent wake dynamics are outlined.