--- Welcome to the official ADCIRCWiki site! The site is currently under construction, with limited information. ---

Difference between revisions of "A00, B00, C00"

A00, B00, C00 are the weighting factors (at time levels k+1, k, k-1, respectively) for the free surface and boundary fluxes in the GWCE, and must sum to 1. Most critically, the weighting factors are used in the discretization of the linear gravity wave (pressure gradient) term and are responsible for determining the inherent implicity (impacting solution stability), in addition to order of accuracy and dispersive characteristics of the numerical method.

Typical Values

If the consistent mass-matrix solver is chosen (see IM parameter) then a semi-implicit method is possible and encouraged. In this case the most common choice for the weighting factors are:
$\displaystyle \mathrm{A00} = \mathrm{C00} = 0.35, \quad \mathrm{B00} = 0.30$

If the lumped mass-matrix solver is chosen then only an explicit method is possible (the weighting A00 must be zero as no matrix solve is conducted), and the weighting factors that are typically chosen become simply:
$\displaystyle \mathrm{A00} = \mathrm{C00} = 0, \quad \mathrm{B00} = 1$

Theory

Wave Continuity Equation

Theory is dominated by analysis of the Wave Continuity Equation (WCE), a special case of the Generalized Wave Continuity Equation (GWCE) where the TAU0 parameter is equal to the linear friction coefficient. In what has been determined to be a third-order accurate method centered in time, which was first introduced by Lynch and Gray (1979), the choice of A00, B00, C00 is reduced to depend on a single parameter, $\displaystyle \theta$ :

$\displaystyle \mathrm{A00} = \mathrm{C00} = 0.5\theta, \quad \mathrm{B00} = 1-\theta$

In other words, k+1 and k-1 weightings are chosen to be equal. It would however not appear that any restriction other than the requirement that A00, B00, C00 must sum to 1 is necessary to obtain second-order accuracy. Unconditional stability is achieved with the prescription of $\displaystyle \theta \geq 0.5$ . Likely because of this fact, the typical choice for ADCIRC has become $\displaystyle \theta = 0.7$ , i.e., $\displaystyle \mathrm{A00} = \mathrm{C00} = 0.35, \mathrm{B00} = 0.30$ as noted above. Different values of $\displaystyle \theta$ may be motivated by the following expression for optimal dispersive accuracy for the consistent mass-matrix solver:

$\displaystyle \theta = \frac{1}{6}\left(1 + \frac{1}{Cr^2}\right)$

where $\displaystyle Cr = \sqrt{gh}\Delta t/\Delta x$ is the Courant number based on the linear gravity wave speed.

A purely explicit method ($\displaystyle \theta = 0$ ) for the WCE is found to be stable under the following conditions:

• $\displaystyle Cr < 1$  : lumped mass-matrix solved in 1-D
• $\displaystyle Cr < \sqrt{3}/3$ : consistent mass-matrix solved in 1-D
• $\displaystyle Cr < \sqrt{2}/2$ : lumped mass-matrix solved in 2-D
• $\displaystyle Cr < \sqrt{6}/6$ : consistent mass-matrix solved in 2-D

These conditions are for linear finite-elements (ADCIRC uses these) with even node spacings and constant bathymetry. Other conditions for quadratic finite-elements, uneven node spacings, and non-constant bathymetry are shown in Kinnmark and Gray (1985).

In the explicit method case, stability (and optimal dispersive accuracy) is superior for the lumped mass-matrix solver versus the consistent mass-matrix solver, hence the lumped solver should always be chosen when employing an explicit method (see IM parameter for setting the solver type).

Generalized Wave Continuity Equation

For the theory based on the WCE, stability is shown to be independent of the choice of TAU0 ($\displaystyle \tau_0$ ). However, experience tell us that in the GWCE a larger value of $\displaystyle \tau_0$ tends to be more unstable than a smaller value. This makes sense since the behavior of the equations will become more and more similar to the Primitive Continuity Equation with greater $\displaystyle \tau_0$ , which is responsible for 2Δx instabilities - the motive for using the GWCE in the finite-element method.

Kinnmark's 1986 monograph does offer some analysis that backs up this empirical experience. For the consistent mass-matrix solver employing the centered $\displaystyle \theta$ scheme it can be shown that unconditional stability is only possible if $\displaystyle \theta \geq 0.5$ and,

$\displaystyle \tau_0 \leq \tau$

where $\displaystyle \tau$ is the linear friction coefficient. In the deep ocean when employing a quadratic drag law this could be a very restrictive requirement on $\displaystyle \tau_0$ (e.g., $\displaystyle \tau = C_d|u|/H \sim 10^{-7}$ assuming $\displaystyle C_d \sim 0.001$ , $\displaystyle |u| \sim 0.1$ m/s, $\displaystyle H \sim 1000$ m). In addition, it is in opposition to our desire to choose the recommended value, $\displaystyle \tau_0 \sim 1-10\tau_{max}$ , for good mass conservation and dispersive properties. An alternative way to view the criteria is to look for a Courant number which eliminates the restriction on the choice of $\displaystyle \tau_0$ . It can be shown that this is the case in 1-D when,

• $\displaystyle Cr < 2\sqrt{3}/3$ , when $\displaystyle \theta \geq 0.5$ (consistent mass-matrix)
• $\displaystyle Cr < \sqrt{3}/3$ , when $\displaystyle \theta = 0$ (consistent mass-matrix)
• $\displaystyle Cr < 2$ , when $\displaystyle \theta \geq 0.5$ (lumped mass-matrix)
• $\displaystyle Cr < 1$ , when $\displaystyle \theta = 0$ (lumped mass-matrix)

Assuming the same relationship between 1-D and 2-D as in the WCE, the conditions for 2-D can be recovered by multiplying by $\displaystyle \sqrt{2}/2$ . This means that the stability requirement for the semi-implicit scheme ($\displaystyle \theta \geq 0.5$ ) is two-fold less restrictive than the explicit scheme ($\displaystyle \theta = 0$ ), in which the latter has identical stability requirements as the WCE. Again, the only way to circumvent this stability requirement for the semi-implicit scheme in the GWCE is to choose a sufficiently small $\displaystyle \tau_0 \leq \tau$ .

Critique

One aspect missing from the theory above are different weighting schemes which are possibly non-centered, i.e., $\displaystyle \mathrm{A00} \neq \mathrm{C00}$ . Pringle et al. shows that stability of the GWCE is improved if,

$\displaystyle \mathrm{A00} = \mathrm{B00} = 0.5\theta, \quad \mathrm{C00} = 1-\theta$

This is unconditionally stable under the following conditions for the consistent mass-matrix solver,

$\displaystyle 2/3 \leq \theta \leq 1, \quad \tau_0\Delta t \leq (16/3)(3\theta/2-1)$

which removes the restrictive condition in the centered-scheme ($\displaystyle \tau_0 \leq \Delta t$ ), instead requiring only that $\displaystyle \tau_0$ is small enough for the given time step $\displaystyle \Delta t$ .

Moreover, the equations analyzed are always linearized (a requirement of the von Neumann stability analysis), thus stability may be more restricting in real-world problems, especially those with fine grid sizes and where nonlinearities are non-trivial. In such cases where it is not possible to achieve time steps more than twice that possible with an explicit method it becomes preferable to employ the explicit lumped mass-matrix solver since it is computationally twice as fast per time step solve.