Investigation of the turbulent boundary layer in high Rayleigh number convection in air using long PIV sequences

This contribution reports on near-wall flow field measurements in turbulent Rayleigh-Bénard convection (RBC) in air at a fixed Prandtl number Pr = 0.7 and Rayleigh number Ra = 1.45 x 10^10. For the experiment the large scale convection (LSC) was confined to a rectangular box of 2.5 x 2.5 x 0.65 m^3 made of transparent acrylic sheets. Prior video-graphic visualizations of the bottom boundary layer flow by means of laser light sheet illumination of small particles indicated the presence of highly dynamic flow behaviour at flow conditions that classical stability analysis predict to still be in the laminar regime. While theory predicts a transition to turbulence at Reynolds numbers Re = 420 the present investigation exhibits highly unsteady flow at a much lower Reynolds number of Re = 260 based on boundary layer thickness. With the help of the PIV data it can be demonstrated that the entrainment of turbulent structures from the mean wind into the boundary layer acts, alongside with the destabilization due to inner shear, as a second mechanism on its path to turbulence. Both contributions must be considered when predicting the critical bound towards the ultimate regime of thermal convection. The measurements rely on the acquisition of long, continuous sequences of particle image velocimetry (PIV) data from which both statistical and spectral information can be retrieved. Contrary to conventional implementation of the PIV technique the field of view is restricted to a narrow strip, generally extending in wall-normal direction. In this way both the acquisition frequency and the total number images of the employed high speed camera are proportionally increased. The narrow field of view coupled with the increased sensitivity of modern high speed cameras allows the use of continuous wave (CW) lasers whose light is spread into a narrow light sheet. The temporally oversampled data allows the use of multi-frame PIV processing algorithms which reduces measurement uncertainties with respect to standard dual-frame analysis