HYDRODYNAMICS OF SEMICONVECTION

Two-dimensional numerical simulations of semiconvection have been performed under conditions resembling those in the deep interior of a 30 M(.) star. As predicted by linear theory, the instability in its early stages consists of amplified internal gravity waves having wavelengths of about 500 km. As the simulation progresses, dramatically different outcomes are realized, depending on how strongly the instability is driven. For strong driving, the initial disturbances evolve into large-amplitude standing waves, which break and locally mix. Relatively rapid global mixing ensues, primarily due to localized regions of overturning whose vertical extent increases with time. In the more weakly driven simulations, such dramatic overturning does not occur. Instead, motions with larger wavelengths organize themselves into horizontally propagating structures resembling solitary waves. Intermittent weak overturning prevents further growth. The results are discussed in the context of various proposed treatments of semiconvection in stellar models. The dynamics and stability of freely overturning layers such as accompany the analogous thermohaline instability in laboratory mixtures of water and salt are also examined. The results suggest that such structures may be less stable when the Prandtl number is small, as in stars, than when it is large, as in water.