Other
Scientific paper
Jun 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt........21r&link_type=abstract
Ph.D. Thesis Colorado Univ., Boulder.
Other
4
Boundary Layer Stability, Compressible Flow, Convection, Convective Flow, Heat Transfer, Hydrogen, Ionization, Transport Properties, Acoustic Excitation, Enthalpy, Ideal Gas, Internal Energy, Particle Energy, Plumes, Solar Granulation, Specific Heat, Thermal Boundary Layer
Scientific paper
The influence of non-ideal effects associated with ionization upon the dynamics and thermodynamics of compressible convection is studied. Ionization causes changes in the particle number density, internal energy, specific heat and opacity of a fluid. The effects of the first three of these are studied with a simplified model involving pure hydrogen fluid. Linear and finite-amplitude analyses and fully nonlinear two-dimensional simulations are undertaken, with the numerical simulations performed on the massively parallel Connection Machine CM-2. Opacity effects are examined by analyzing solutions obtained from more realistic three-dimensional simulations of solar granulation carried out by Nordlund and Stein. Ionization effects are expected to influence the dynamics of granulation and acoustic mode excitation in the Sun and other stars, and likewise the coupling of convection with pulsations that occurs in stars such as white dwarfs and Cepheid variables. Both the global transport properties and the local dynamical properties of convective flows are affected by ionization. In regions of partial ionization, the enthalpy flux is dominated by latent heat transport. Strong temperature fluctuations and corresponding buoyancy forces develop wherever rapid changes in ionization state occur. These can result in narrow regions of intense vertical flow. The flow asymmetries reported in compressible ideal gas convection can either be enhanced or diminished depending on the vertical positioning of the partially ionized region within the domain. Ionization-induced variations in the radiative properties of a convecting fluid can result in thermal boundary layer instability and plume formation. The interval between plume formation events depends on the growth rate of the instability, the scale of the underlying convective motions and the phase speed of the perturbation. As plumes formed in the upper boundary layer descend, buoyancy forces remain significant as long as heat exchange and compressional heating result primarily in ionization of the fluid rather than in temperature equilibration. This can lead to supersonic vertical flows in an otherwise subsonic flow field, and can serve to excite acoustic oscillations, the phase of which can be abruptly altered by subsequent plume events. In the three-dimensional simulations, significant entrainment of surrounding fluid with depth gradually weakens the sheets and plumes of fast downflow.
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