Local Rayleigh-number closure
= Local Rayleigh-number closure
{c}
A local Rayleigh-number closure estimates the thickness $\delta$ of a turbulent <thermal boundary layer> by setting
$$
\operatorname{Ra}_\delta
=\frac{g\Delta\rho\,\delta^3}{\rho\nu\kappa}
=\operatorname{Ra}_c,
$$
where $\operatorname{Ra}_c$ is an order-one or geometry-dependent critical value. Combining this with <Fourier's law> converts the boundary-layer temperature and density differences into a <heat flux> law.