Large-eddy simulation of astrophysical convection and acoustic emission

We summarize recent large-eddy simulations on compressible convection with particular emphasis on boundary conditions. The acoustic components of the flows are found to be sensitive to the choice of boundary conditions (BC's). Recent improvements in the method of characteristics as applied to dissipative flows allow us to make informed choices about appropriate BC's. As far as propagating acoustic modes are concerned, we show that coronal heating can be performed by acoustic modes in certain cool stars. The most suitable choice of boundary conditions in the astrophysical context, where solid boundaries are lacking, include layers of stable material above and below the convectively unstable layer. In this case, the convective flux exhibits cyclic behavior: energy transport cycles back and forth between convective and conductive modes, and there is no steady state. Cyclic convection in the Sun occurs on time-scales which vary with depth: at depths where acoustic emission is strongest, the cycle time-scales are 5-10 minutes. We suggest that the cyclic behavior explains both the observed granule life-times and also the observed peak in solar p-modes at periods around 5 minutes.