  By tailoring the geometry of the upper boundary in turbulent
Rayleigh-B\'enard convection we manipulate the boundary layer -- interior flow
interaction, and examine the heat transport using the Lattice Boltzmann method.
For fixed amplitude and varying boundary wavelength $\lambda$, we find that the
exponent $\beta$ in the Nusselt-Rayleigh scaling relation, $Nu-1 \propto
Ra^\beta$, is maximized at $\lambda \equiv \lambda_{\text{max}} \approx (2
\pi)^{-1}$, but decays to the planar value in both the large ($\lambda \gg
\lambda_{\text{max}}$) and small ($\lambda \ll \lambda_{\text{max}}$)
wavelength limits. The changes in the exponent originate in the nature of the
coupling between the boundary layer and the interior flow. We present a simple
scaling argument embodying this coupling, which describes the maximal
convective heat flux.
