Arnaud Prigent

Transition to turbulence in a tall annulus submitted to a radial temperature gradient

We have investigated the transition to turbulence in a water flow confined inside a tall vertical cylindrical annulus submitted to a radial temperature gradient using the space-time diagrams technique. As soon as a small radial temperature gradient is applied to the annular flow, the radial stratification of density induces a torque that produces a large convection cell. The first instability of this flow occurs via a supercritical bifurcation and gives rise to axisymmetric rolls localized in the middle of the system. Just above the onset, the pattern contains spatiotemporal defects. For large values of the control parameter, we have observed a coexistence of turbulent bursts and laminar domains. We have measured the turbulent fraction and have performed a statistical analysis of the laminar and turbulent zones, and have found that they bear the main characteristics of spatiotemporal intermittency.

Flow regimes in a vertical Taylor-Couette system with a radial thermal gradient

A rich variety of flow regimes in a Newtonian fluid inside a vertical large-aspect ratio and a wide-gap Taylor-Couette system with a radial temperature gradient has been determined in experiments and in direct numerical simulations (DNSs). Compared to previous experiments and numerical studies, a wider range of temperature differences (i.e., of the Grashof number Gr) and of the rotation rate (the Taylor number Ta) has been covered. The combined effect of rotation and of the radial temperature gradient is the occurrence of helicoidal vortices or modulated waves at the onset. Stationary axisymmetric vortices are found for very weak temperature differences. A good agreement was found for critical states between results from experiments, linear stability analysis, and DNS. Higher instability modes have been determined for a wide range of parameters and a state diagram of observable flow regimes has been established in the plane spanned by Gr and Ta. Some higher states observed in experiments were retrieved in...

Mixed Convection in a Differentially Rotating Annulus

We investigate experimentally and numerically the influence of a radial temperature gradient in a vertical rotating cylindrical annulus. The geometrical parameters are fixed with aspect ratio and radii ratio respectively equal to 114 and 0.8. After imposing a Grashof number, the convection cell is destabilized by inertial forces due to the increasing of the rotation of the inner cylinder. Above a critical value of the control parameter, the Reynolds number, a spiral pattern occurs giving rise to a finite extent propagating pattern through a supercritical Hopf bifurcation.Copyright

Destabilization of the Taylor Vortex Flow by a Radial Temperature Gradient

Both experimental and numerical studies have shown that the Taylor vortices are destabilized by a weak radial temperature gradient and transit to spiral vortices with a small inclination. For a large radial temperature gradient, from Taylor vortices emerges a disordered pattern with some windows of spiral vortices. Spatio-temporal characteristics of resulting pattern are presented.Copyright