  Using high-resolution direct numerical simulation and arguments based on the
kinetic energy flux $\Pi_u$, we demonstrate that for stably stratified flows,
the kinetic energy spectrum $E_u(k) \sim k^{-11/5}$, the entropy spectrum
$E_\theta(k) \sim k^{-7/5}$, and $\Pi_u(k) \sim k^{-4/5}$, consistent with the
Bolgiano-Obukhov scaling. This scaling arises due to the conversion of kinetic
energy to the potential energy by buoyancy. For weaker buoyancy, this
conversion is weak, hence $E_u(k)$ follows Kolmogorov's spectrum with a
constant energy flux. For Rayleigh B\'{e}nard convection, we show that the
energy supply rate by buoyancy is positive, which leads to an increasing
$\Pi_u(k)$ with $k$, thus ruling out Bolgiano-Obukhov scaling for the
convective turbulence. Our numerical results show that convective turbulence
for unit Prandt number exhibits a constant $\Pi_u(k)$ and $E_u(k) \sim
k^{-5/3}$ for a narrow band of wavenumbers.
