IS STELLAR GRANULATION TURBULENCE

We show that power spectra of granulation images or velocity fields ca nnot be compared meaningfully with spectra from theoretical models bas ed on turbulent cascades. The small scale power in these images is due almost entirely to the sharp edges between granules and intergranular lanes, not to turbulence in the usual sense. This is demonstrated wit h a number of experiments with result from numerical simulations, and with simpler synthetic data with power spectra similar to that of gran ulation. The reason for the seemingly laminar behavior of the granulat ion flow, in spite of the high Reynolds numbers involved, is the influ ence of stratification on the local ratio of turbulence to bulk flow. The rapid expansion of upflows, their deep origin and near-adiabatic s tratification lead to low levels of turbulence in the overturning flui d at the surface. Higher levels of turbulence are expected in the conv erging flows near downdrafts, but mostly at scales that are below curr ent observational resolution limits, and contributing relatively littl e to the total convective flux and to spectral line broadening.