ESTIMATES OF THE TRANSITION REYNOLDS-NUMBER IN A ROTATING-DISK BOUNDARY-LAYER STUDY

It is known that the flow over the surface of a rotating disc evolves from a laminar regime, around the centre of the disc, to a turbulent state, far from the rotating axis. Inflexional instabilities develop a vortex flow in an intermediate regime. On one hand, the transition from the laminar regime to the vortex flow begins to be well understood thanks of the linear stability theory. On the other hand, the transition from the vortex flow to turbulence is still an open problem. This paper is devoted to an original approach to this problem using wavelet analysis. From the wavelet transform of hot film signals in the boundary layer of the disc in the transitional regime to turbulence, local energy densities are derived. A simple study of these energy densities is then made to discriminate the turbulent regions from the vortex contributions. Estimates of the Reynolds number characterizing the transition process are then proposed. A (spectrum like) scale decomposition of the different parts of the signal is provided during the transition to turbulence.