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TYCHO uses 2 modes of evolution (mode= 0,1), corresponding to explicit hydrodynamics (preserves causality), and implicit hydrodynamics (damping is added to maintain stability for timesteps exceeding the sound travel time across a zone , i.e., the Courant time). Strictly speaking, our "hydrostatics" uses optimal damping for hydrodynamic relaxation toward a hydrostatic state, and for timesteps much larger than the Courant time, gives results indistinguishable for conventionally defined "hydrostatics".
Our experience with implicit hydrodynamics (which is often used to allow timesteps to exceed the Courant time, while still purporting to describe hydrodynamic behavior), has been disappointing. If the problem at hand is really hydrodynamic, then explicit hydrodynamics gives more physically correct answers. Implicit hydrodynamics gives the illusion of plausible (converged) results (and gives them more quickly), but the damping often causes qualitatively incorrect behavior. This is the nastiest sort of error, which gives superficially plausible results while being qualitatively incorrect! These errors can be avoided by taking timesteps shorter than the Courant time, at which point the implicit algorithms have lost their computational advantage.
In the TYCHO control file, params.d, the variable "mapenv" not zero gives a mapping of the whole model, Henyey grid and envelope integration both, onto an enlarged grid which is appropriate for explicit hydrodynamics. After the mapping TYCHO produces a new model and stops. Maximum errors from a few test cases give a maximum fractional error in acceleration of about 0.001 of gravity (with most values lying below 1.0e-5); these are "low noise" models.
Sample versions of the initial model file "imodel" and the control file "params.d" are given in the template directory (imodel.dynam and params.dynam).
In the hydrodynamic mode (mode=0), convection is turned off. Later this will be updated to a more realistic algorithm; mixing-length convection has hydrostatic equilibrium built in which causes bizarre behavior.
For problems with little nuclear burning, turning off the burning (nburn = -1) saves computer resources and significantly speeds the computation.
E-mail:darnett@as.arizona.edu
TYCHO is not public domain software, but is freely available for non-commercial purposes. Absolutely no guarantee is given; use at your own risk...