Convection is a ubiquitous heat transport mechanism in astrophysics and geophysics. In addition to transporting heat, convective motions drive a wide variety of interesting phenomena. In particular, the magnetic dynamos of the Earth and Sun are seated in the turbulent convective motions of their respective core and outer envelope. In addition to driving magnetism, convection is responsible for establishing observable mean flows, such as the Sun's differential rotation.The nonlinear nature of convection in the highly turbulent regime of geophysical and astrophysical settings makes its behavior difficult to predict, understand, and model.In this thesis, I conduct a series of experiments into the fundamental nature of convection.
The first two of these experiments examine numerical simulations of fully compressible convection in the context of stratified atmospheres. From these simulations, we have learned how to study increasingly turbulent, astrophysically-interesting convection while holding the Mach number and the Rossby number (which measures the importance of the Coriolis force) constant. In these studies, we have learned that the fundamental heat transport mechanisms at work in simulations of stratified convection are the same as those at work in more simple, incompressible convection.
The next two studies examine simulations of Boussinesq convection with a specific focus on understanding how simulations of convection relax over time from their initial conditions. In the first of these studies, we develop a simple procedure for coupling simulations with simple boundary value problems to rapidly accelerate the simulation relaxation process. We find very good agreement between accelerated simulations and simulations which traditionally timestep through their thermal relaxation process. In the second of these experiments, we show that the relaxation timescale is linked to the choice of thermal boundary conditions. We further show that thermal relaxation is akin to a walk through parameter space from a turbulent to more laminar regime.
In our final study, we examine the evolution of "thermals" in stratified atmospheres. Thermals are regions of buoyant (or dense) fluid which accelerate due to their buoyancy forces and may be the fundamental unit of convection in the Earth's atmosphere. We develop a theory describing the evolution of thermals in stratified domains. This theory describes how atmospheric stratification affects buoyant entrainment of atmospheric fluid. We test this theory with laminar 2D and 3D simulations, and discuss implications that this work may have for stellar envelope convection.
|Advisor:||Brown, Benjamin P|
|Commitee:||Rast, Mark P, Cranmer, Steven, Grooms, Ian, Julien, Keith, Hindman, Bradley|
|School:||University of Colorado at Boulder|
|Department:||Astrophysical and Planetary Sciences|
|School Location:||United States -- Colorado|
|Source:||DAI 81/11(E), Dissertation Abstracts International|
|Subjects:||Astrophysics, Fluid mechanics|
|Keywords:||Fluid dynamics, Numerical simulations, Stellar convection|
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