Random roughness of boundary increases the turbulent convection scaling exponent

The influence of the boundary layer properties on the heat transport in turbulent thermal convection is experimentally investigated in a cell with a rough bottom plate. It is shown that the standard 2/7 exponent of the convective heat flow dependence on the Rayleigh number, usually observed in a cell with smooth boundaries, increases if the roughness has power law distributed asperity heights and the thermal boundary layer thickness is smaller than the maximum asperity size. In contrast a periodic roughness does not influence the heat transport law exponent.