The results of a combined experimental and numerical study of the flow in slowly diverging pipes are presented. Interestingly, an axisymmetric conical recirculation cell has been observed. The conditions for its existence and the length of the cell are simulated for a range of diverging angles and expansion ratios. There is a critical velocity for the appearance of this state. When the flow rate increases further, a subcritical transition for localized turbulence arises. The transition and relaminarization experiments described here quantify the extent of turbulence. The findings suggest that the transition scenario in slowly diverging pipes is a combination of stages similar to those observed in sudden expansions and in straight circular pipe flow.Comment: 8 pages, 5 figure

A thermal Taylor-Couette facility (THETACO) has been designed to investigate turbulent flows generated by differential rotation and radial temperature gradient. It consists of a cylindrical annulus with a rotating inner cylinder and fixed outer cylinder. The electric heating system is installed inside the inner cylinder and the annulus is immersed in a large cylindrical container filled with cooling fluid.Temperature regulators control independently the temperature of the inner surface of the inner cylinder and that of the cooling fluid. The facility allows to reach values of the Reynolds number ( ~ 5 × 10 5 ) and of the Rayleigh number ( ~ 3 × 10 6 ) for water as working fluid. The facility provides torque measurements, a full optical access at the side and from the bottom for velocity measurements using PIV (2D, stereoscopic and tomographic). Temperature measurements in the flow can be performed by thermochromic liquid crystals or laser induced fluorescence.