Spatial Grids
The various GYRE frontends all discretize their equations on a spatial grid \(\{x_{1},x_{2},\ldots,x_{N}\}\) in the dimensionless radial coordinate \(x \equiv r/R\). The computational cost of a calculation scales with the total number of points \(N\) in this grid, while the grid’s resolution — i.e., the spacing between adjacent points — impacts both the accuracy of solutions, and in the case of the gyre frontend, the number of solutions that can be found. (The Limitations of the Numerical Method section discusses these behaviors in the context of the stretched string BVP).
Scaffold Grid
A fresh spatial grid is constructed for each iteration of the main
computation loop (see the flowcharts in the Frontends
chapter). This is done under the control of the &grid
namelist
groups; there must be at least one of these, subject to the tag
matching rules (see the Working With Tags chapter). If there is
more than one matching &grid
namelist group, then the final
one is used.
Each grid begins as a scaffold grid, comprising the following points:
an inner point \(\xin\);
an outer point \(\xout\);
the subset of points of the source grid satisfying \(\xin < x < \xout\)
The source grid can be either the input model grid, or a grid read
from file; this choice is determined by the scaffold_src
parameter of the &grid
namelist group. By default,
\(\xin\) and \(\xout\) are obtained from the source grid as
well (as its innermost and outermost point). However, either or
both can be overridden using the x_i
and x_o
parameters.
Iterative Refinement
Scaffold grids are refined via a sequence of iterations. During a given iteration, each subinterval \([x_{j},x_{j+1}]\) is assessed against various criteria (discussed in greater detail below). If any criteria match, then the subinterval is refined by bisection, inserting an additional point at the midpoint
The sequence terminates if no refinements occur during a given
iteration, or if the number of completed iterations equals the value
specified by the n_iter_max
parameter of the &grid
namelist group.
Mechanical Criterion
The wave criterion involves a local analysis of the mechanical parts of the oscillation equations, with the goal of improving resolution where the displacement perturbation \(\vxi\) is rapidly varying. Within the subinterval \([x_{j},x_{j+1}]\), the \(y_{1}\) and \(y_{2}\) solutions (see the Dimensionless Formulation section) take the approximate form
where \(\chi\) is one of the two eigenvalues of the mechanical (upperleft) \(2 \times 2\) submatrix of the full Jacobian matrix \(\mA\), evaluated at the midpoint \(x_{j+\half}\).
In propagation zones the imaginary part \(\chi_{\rm i}\) of the eigenvalue gives the local wavenumber in \(\ln x\) space, and \(2\pi \chi_{\rm i}^{1}\) the corresponding wavelength; while in evanescent zones the real part \(\chi_{\rm r}\) gives the local exponential growth/decay rate, and \(\chi_{\rm r}^{1}\) the corresponding efolding length.
Based on this analysis, the criterion for refinement of the subinterval is
where \(\wosc\) and \(\wexp\) are userdefinable weighting parameters. This causes refinement if the subinterval width (in \(\ln x\) space) exceeds \(\wosc^{1}\) times the local wavelength, or \(2\pi \wexp^{1}\) times the local efolding length.
Because there are two possible values for \(\chi\), the above refinement criterion is applied twice (once for each). Moreover, because \(\chi\) depends implicitly on the oscillation frequency, the criterion is applied for each frequency in the grid \(\{\omega_{1},\omega_{2},\ldots,\omega_{M}\}\) (see the Frequency Grids section).
Thermal Criterion
Similar to the wave criterion discussed above, the thermal criterion involves a local analysis of the energetic parts of the oscillation equation, with the goal of improving resolution where the thermal timescale is very long and perturbations are almost adiabatic. Within the subinterval \([x_{j},x_{j+1}]\), the \(y_{5}\) and \(y_{6}\) perturbation take the approximate form
where \(\pm\tau\) are the eigenvalues of the matrix formed from the energetic (bottomright) \(2 \times 2\) submatrix of the full Jacobian matrix \(\mA\), evaluated at the midpoint \(x_{j+\half}\).
Based on this analysis, the criterion for refinement of the subinterval is
where \(\wthm\) is a userdefinable weighting parameter.
Because \(\tau\) depends implicitly on the oscillation frequency, this criterion is applied for each frequency in the grid \(\{\omega_{1},\omega_{2},\ldots,\omega_{M}\}\).
Structural Criteria
The structural criteria have the goal of improving resolution where the stellar structure coefficients are changing rapidly. For a given coefficient \(C\), the criterion for refinement of the subinterval \([x_{j},x_{j+1}]\) is
where \(\wstr\) is a userdefinable weighting parameter. This criterion is applied separately to the \(V_2 \equiv V/x^{2}\), \(U\), \(A^{*}\), \(c_{1}\) and \(\Gamma_{1}\) coefficients (see the Structure Coefficients section).
Central Criteria
All of the above criteria depend on the logarithmic subinterval width
\((\ln x_{j+1}  \ln x_{j})\), and cannot be applied to the first
subinterval \([x_{1},x_{2}]\) if it extends to the center of the
star, \(x = 0\). In such cases, the resolve_ctr
parameter
of the &grid
namelist group determines whether the subinterval
is refined. If set to .FALSE.
, then no refinement occurs;
while if set to .TRUE.
, then the refinement criteria are
or
where \(\chi\) is the eigenvalue from the local analysis (see the Mechanical Criterion section) corresponding to the solution that remains wellbehaved at the origin, and \(w_{\rm ctr}\) is a userdefinable weighting parameter. The first criterion causes refinement if the subinterval is in a propagation zone, and the second if the solution slope \(\sderiv{y}{\ln x} \sim \chi_{\rm r}\) exceeds \(w_{\rm ctr}^{1}\).
Because \(\chi\) depends implicitly on the oscillation frequency, these criteria are applied for each frequency in the grid \(\{\omega_{1},\omega_{2},\ldots,\omega_{M}\}\).
Limiting Controls
A couple of additional controls affect the iterative refinement described above. Refinement of the \([x_{j},x_{j+1}]\) subinterval always occurs if
and never occurs if
where both \(\Delta x_{\rm max}\) and \(\Delta x_{\rm min}\) are userdefinable.
Namelist Parameters
The full set of parameters supported by the &grid
namelist
group is listed in the Grid Parameters section. However, the table
below summarizes the mapping between the userdefinable controls
appearing in the expressions above, and the corresponding namelist
parameters.
Symbol 
Parameter 

\(\wosc\) 

\(\wexp\) 

\(\wthm\) 

\(\wstr\) 

\(\wctr\) 

\(\Delta x_{\rm max}\) 

\(\Delta x_{\rm min}\) 

Recommended Values
While w_exp
, w_osc
and w_ctr
all default to zero, it is highly recommended to use nonzero values
for these parameters, to ensure adequate resolution of solutions
throughout the star. Reasonable starting choices are w_osc
= 10
, w_exp = 2
and w_ctr = 10
.