The effect of angular misalignment on the low-frequency dynamics of the near wake of a blunt-based axisymmetric body is investigated at a Reynolds number of . While for axisymmetric boundary conditions all azimuthal orientations of the wake are explored with equal probability, resulting in a statistical axisymmetry, an angular offset as small as is found to suppress the low-frequency large-scale behaviour that is associated with the erratic meandering of the region of reversed flow. As a result of the misalignment, the centroid of this backflow region is displaced from the model axis and remains confined around an average off-centre position. Spectral and modal analysis provides evidence that the erratic backflow behaviour occurs within a narrow angular range of deviations from axisymmetric conditions.

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.

The so-called very-low-frequency (VLF) azimuthal meandering of the reversed-flow region has been shown to contribute significantly to the unsteadiness of the turbulent wake flow past a bluff body of revolution (Rigas et al. 2014; Grandemange et al. 2014). Such an erratic behavior causes a continuous change in the wake topology and, particularly due to its slow nature, has been linked with the pronounced sensitivity that turbulent wake flows typically display with respect to the boundary conditions (Klei 2012; Wolf et al. 2013; Grandemange et al. 2012). Currently, the existence of such an instability is attributed to the persistence at high Reynolds numbers of the reflectional symmetry breaking mode (RSB) at laminar regime (Fabre et al. 2008; Bury and Jardin 2012). Despite the numerous investigations, some even attempting its theoretical modeling (Rigas et al. 2015), the relation of such an instability with the main vortex shedding process has not been characterized yet. Moreover, the backflow meandering reflects a condition of indifferent equilibrium in the azimuthalradial plane, which is ultimately dictated by the axial symmetry of the flow. Such a very-low frequency dynamics however, still needs to be examined under the influence of off-nominal (i.e. asymmetric) inflow conditions. Scope of the present work is to examine how the backflow unsteadiness evolves moving away from separation and additionally, to assess how it is affected by asymmetric inflow conditions. For this purpose time-resolved stereoscopic Particle Image Velocimetry (PIV) measurements are carried in the turbulent near-wake of an ogive-cylinder at different stations downstream of the base and for varying pitch angles, whereas the velocity fluctuations are examined using a snapshot POD approach.

Quantification of surface pressure is critical for the efficient design of aerospace structures. One way of measuring pressure is PIV/PTV-based pressure reconstruction [1]. In this approach, PIV/PTV data are used to determine the material acceleration and subsequently pressure via the momentum equation. In recent years, the technique has become increasingly feasible and appealing due to the development of (time-resolved) volumetric diagnostic capabilities, such as tomographic PIV [2] and Lagrangian particle tracking [3]. The performance of a variety of state-of-the-art techniques was recently assessed for the case of a transonic base flow within the collaborative European framework programs 'NIOPLEX' [4]. Since the NIOPLEX test case considers a simulated experiment, it does not necessarily demonstrate the actual capabilities of PIV/PTV-based pressure reconstruction techniques for realistic measurement conditions.
The present study overcomes this limitation by reconstructing pressure from actual PIV/PTV measurements of a flow that is similar to the NIOPLEX test case, i.e. an axisymmetric step albeit in a low-speed flow, facilitating comparison. Reference measurements are obtained using microphones and static pressure sensors to provide a source for comparison

The turbulent wake past an axisymmetric body is investigated with time-resolved stereoscopic particle image velocimetry (PIV) at a Reynolds number ReD = 6.7 × 104 based on the object diameter. The azimuthal organization of the near-wake is studied at different locations downstream of the trailing edge. The time-averaged velocity field features a circular shear layer bounding a region of recirculating flow. Inspection of instantaneous PIV snapshots reveals azimuthal meandering of the reverse flow region with a significant radial offset with respect to the time-averaged position. The backflow meandering appears as the major contribution to the nearwake dynamics in proximity of the base, whereas closer to the rear-stagnation point, the shear layer fluctuations become important. For x/D ≤ 0.75, the time-history and probability distributions of the backflow centroid position allow to identify this motion with an irregular precession about the model symmetry axis occurring at time scales in the order of 103 D/U∞ or higher. The first two modes obtained by snapshot proper orthogonal decomposition of the velocity fluctuations can be related to an antisymmetric mode of azimuthal wave-number m = 1 reflecting a radial displacement of the separated flow region, while the third and fourth proper orthogonal decomposition modes are identified with a second mode pair m = 2 and are representing wake ovalization. Close to the base, a third axisymmetric mode m = 0 is identified, corresponding to a streamwise pulsation of the reverse flow region. Based on the analysis of the spatial eigen-functions and frequency spectra of the time-coefficients, it is concluded that the anti-symmetric mode m = 1 is associated with the backflow instability in the very-low frequency range StD = 10−4/10−3 close to separation, whereas more downstream it reflects the fluctuations related to the shear layer development.

The backflow instability in the wake past a cylindrical blunt-based body in pitch is investigated at a Reynolds number ReD = 6.7 · 104 based on the cylinder diameter. Time-resolved stereoscopic particle image velocimetry measurements have been performed in a cross-flow plane located 0.3 D downstream of the model base. An increasing displacement of the backflow region from the body centerline with increasing pitch angles is observed in the long-time average of the velocity field, with the emergence of a preferred orientation of the wake. The time history of the backflow centroid position shows a progressive reduction of both amplitude and time scales of the fluctuations, reflecting the transition from large-scale azimuthal meandering to a more stable confinement at an off-center position. Proper Orthogonal Decomposition of the velocity fluctuations reveals a reduction by approximately 80% in the contribution of the first two modes for angles increasing up to 1°, accompanied by a distortion of the dipolar distribution typically associated with backflow meandering, for misalignments from 0.3° and higher. The frequency spectra of the POD time-coefficients display a very-lowfrequency peak near StD ~ 10-3 only within 0.1° deviations from axisymmetric inflow conditions, thus endorsing the hypothesis that the long-term backflow instability only survives within small deviations from axisymmetric inflow conditions.