Spectral analysis of boundary layers in Rayleigh-Bénard convection

A combined experimental and numerical study of the boundary layer in a 4:1 aspect-ratio Rayleigh-B\'enard cell over a four-decade range of Rayleigh numbers has been undertaken aimed at gaining a better insight into the character of the boundary layers. The experiments involved the simultaneous laser Doppler anemometry measurements of fluid velocity at two locations, i.e., in the boundary layer and far away from it in the bulk, for Rayleigh numbers varying between $1.6\ifmmode\times\else\texttimes\fi{}{10}^{7}$ and $2.4\ifmmode\times\else\texttimes\fi{}{10}^{9}$. In parallel, direct numerical simulations have been performed for the same configuration for Rayleigh numbers between $7.0\ifmmode\times\else\texttimes\fi{}{10}^{4}$ and $7.7\ifmmode\times\else\texttimes\fi{}{10}^{7}$. The temperature and velocity probability density functions and the power spectra of the horizontal velocity fluctuations measured in the boundary layer and in the bulk flow are found to be practically identical. Except for the smallest Rayleigh numbers, the spectra in the boundary layer and in the bulk central region are continuous and have a wide range of active scales. This indicates that both the bulk and the boundary layers are turbulent in the Ra number range considered. However, molecular effects can still be observed and the boundary layer does not behave like a classical shear-driven turbulent boundary layer.