The dynamic characteristics of the pressure and velocity fields of the unsteady incompressible laminar wake behind a circular cylinder are investigated numerically and analysed physically. The governing equations, written in a velocity-pressure formulation and in conservative form, are solved by a predictor-corrector pressure method, a finite-volume second-order-accurate scheme and an alternating-directionimplicit (ADI) procedure. The initiation mechanism for vortex shedding and the evaluation of the unsteady body forces are presented for Reynolds-number values of 100,200 and 1000.The vortex shedding is generated by a physical perturbation imposed numerically for a short time. The flow transition becomes periodic after a transient time interval. The frequency of the drag and lift oscillations agree well with the experimental data.The study of the interactions of the unsteady pressure and velocity fields shows the phase relations between the pressure and velocity, and the influence of different, factors : the strongly rotational viscous region, the convection of the eddies and the almost inviscid flow.The interactions among the different scales of structures in the near wake are also studied, and in particular the time-dependent evolution of the secondary eddies in relation to the fully developed primary ones is analysed.

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The transition and the development of turbulence in the near wake of a circular cylinder are investigated using hot-wire anemometry and flow visualization. The formation zone of the large regular vortices is studied in the subcritical regime (2000 < U0 D/v < 60000). with and without the introduction of a splitter plate. Two different regimes are identified in the interaction between the von Kármán vortices and those of the shear layer emerging from the separated boundary layer. Experimental evidence is given in support of the strong coupling at low Reynolds numbers characterized by phase modulations between the two types of structures. The interaction is weaker at high Reynolds numbers where the small-scale vortices are disconnected from the regular vortex shedding, giving rise to an intermittent pattern. Spectral properties are used to describe the different stages of the interaction between the shear-layer vortices and the alternating ones. Physical properties of the interaction are examined separately in a numerical simulation using a pressure-velocity formulation. Both unexcited and excited two-dimensional plane mixing layers are studied using streakline maps and time traces of the dynamical properties. The main features of the simulated vortex development are in agreement with the experimental results.

We investigate the stability of variable-density two-dimensional isolated vortices in the frame of incompressible mixing under negligible gravity. The focus on a single vortex flow stands as a first step towards vortex interactions and turbulent mixing. From heuristic arguments developed on a perturbed barotropic vortex, we find that high-density vortices are subject to a Rayleigh-Taylor instability. The basic mechanism relies on baroclinic vorticity generation when the density gradient is misaligned with the centripetal acceleration field. For Gaussian radial distributions of vorticity and density, the intensity of the baroclinic torque due to isopycnic deformation is shown to increase with the ratio δ/δ ρ of the vorticity radius to the density radius. Concentration of mass near the vortex core is confirmed to promote the instability by the use of an inviscid linear stability analysis. We measure the amplification rate for the favoured azimuthal wavenumbers m = 2, 3 on the whole range of positive density contrasts between the core and the surroundings. The separate influence of the density-contrast and the radius ratio is detailed for modes up to m = 6. For growing azimuthal wavenumbers the two-dimensional structure of the eigen mode concentrates on a ring of narrowing radial extent centered on the radius of maximum density gradient. The instability of the isolated high-density vortex is then explored beyond the linear stage based on high Reynolds number numerical simulations for modes m = 2, 3 and a moderate density contrast C ρ = 0.5. Secondary roll-ups are seen to emerge from the non-linear evolution of the vorticity and density fields. The transition towards m smaller vortices involves vorticity exchange between initially-rotating dense fluid particles and the irrotational less-dense medium. It is shown that baroclinic enstrophy production is associated with the centrifugal mass ejection away from the vortex center.