# Time-stepping and the fractional step method

With the pressure decomposition as discussed, the momentum evolves via:

$$$\begin{equation} \label{eq:momentum-time-derivative} \partial_t \boldsymbol{v} = \boldsymbol{G}_{\boldsymbol{v}} - \boldsymbol{\nabla} p_{\rm{non}} \, , \end{equation}$$$

where, e.g., for the non-hydrostatic model (ignoring background velocities and surface-wave effects)

$$$\boldsymbol{G}_{\boldsymbol{v}} \equiv - \boldsymbol{\nabla}_h p_{\rm{hyd}} - \left ( \boldsymbol{v} \boldsymbol{\cdot} \boldsymbol{\nabla} \right ) \boldsymbol{v} - \boldsymbol{f} \times \boldsymbol{v} + \boldsymbol{\nabla} \boldsymbol{\cdot} \boldsymbol{\tau} + \boldsymbol{F}_{\boldsymbol{v}}$$$

collects all terms on the right side of the momentum equation \eqref{eq:momentum-time-derivative}, except the contribution of non-hydrostatic pressure $\boldsymbol{\nabla} p_{\rm{non}}$.

The time-integral of the momentum equation \eqref{eq:momentum-time-derivative} from time step $n$ at $t = t_n$ to time step $n+1$ at $t_{n+1}$ is:

$$$\begin{equation} \label{eq:momentum-time-integral} \boldsymbol{v}^{n+1} - \boldsymbol{v}^n = \int_{t_n}^{t_{n+1}} \Big [ - \boldsymbol{\nabla} p_{\rm{non}} + \boldsymbol{G}_{\boldsymbol{v}} \Big ] \, \mathrm{d} t \, , \end{equation}$$$

where the superscript $n$ and $n+1$ imply evaluation at $t_n$ and $t_{n+1}$, such that $\boldsymbol{v}^n \equiv \boldsymbol{v}(t=t_n)$. The crux of the fractional step method is to treat the pressure term $\boldsymbol{\nabla} p_{\rm{non}}$ implicitly using the approximation

\begin{align} \label{eq:pnon_implicit} \int_{t_n}^{t_{n+1}} \boldsymbol{\nabla} p_{\rm{non}} \, \mathrm{d} t \approx \Delta t \boldsymbol{\nabla} p_{\rm{non}}^{n+1} \, , \end{align}

while treating the rest of the terms on the right hand side of \eqref{eq:momentum-time-integral} explicitly. The implicit treatment of pressure ensures that the velocity field obtained at time step $n+1$ is divergence-free.

To effect such a fractional step method, we define an intermediate velocity field $\boldsymbol{v}^\star$ such that

$$$\begin{equation} \label{eq:intermediate-velocity-field} \boldsymbol{v}^\star - \boldsymbol{v}^n = \int_{t_n}^{t_{n+1}} \boldsymbol{G}_{\boldsymbol{v}} \, \mathrm{d} t \, , \end{equation}$$$

The integral on the right of the equation for $\boldsymbol{v}^\star$ may be approximated by a variety of explicit methods. For example, a forward Euler method approximates the integral via

$$$\begin{equation} \int_{t_n}^{t_{n+1}} G \, \mathrm{d} t \approx \Delta t G^n \, , \label{eq:forward-euler} \end{equation}$$$

for any time-dependent function $G(t)$, while a second-order Adams-Bashforth method uses the approximation

$$$\begin{equation} \label{eq:adams-bashforth} \int_{t_n}^{t_{n+1}} G \, \mathrm{d} t \approx \Delta t \left [ \left ( \tfrac{3}{2} + \chi \right ) G^n - \left ( \tfrac{1}{2} + \chi \right ) G^{n-1} \right ] \, , \end{equation}$$$

where $\chi$ is a parameter. U. Ascher, S. Ruuth, B. Wetton (1995) claim that $\chi = \tfrac{1}{8}$ is optimal; $\chi = -\tfrac{1}{2}$ yields the forward Euler scheme.

Combining the equation \eqref{eq:intermediate-velocity-field} for $\boldsymbol{v}^\star$ and the time integral of the non-hydrostatic pressure \eqref{eq:pnon_implicit} yields

$$$\begin{equation} \label{eq:fractional-step} \boldsymbol{v}^{n+1} - \boldsymbol{v}^\star = - \Delta t \boldsymbol{\nabla} p_{\rm{non}}^{n+1} \, . \end{equation}$$$

Taking the divergence of fractional step equation \eqref{eq:fractional-step} and requiring that $\boldsymbol{\nabla} \boldsymbol{\cdot} \boldsymbol{v}^{n+1} = 0$ yields a Poisson equation for the kinematic pressure $p_{\rm{non}}$ at time-step $n+1$:

$$$\begin{equation} \label{eq:pressure-poisson} \nabla^2 p_{\rm{non}}^{n+1} = \frac{\boldsymbol{\nabla} \boldsymbol{\cdot} \boldsymbol{v}^{\star}}{\Delta t} \, . \end{equation}$$$

With $\boldsymbol{v}^\star$ in hand we can invert \eqref{eq:pressure-poisson} to get $p_{\rm{non}}^{n+1}$ and then $\boldsymbol{v}^{n+1}$ is computed via the fractional step equation \eqref{eq:fractional-step}.

Tracers are stepped forward explicitly via

$$$\begin{equation} \label{eq:tracer-timestep} c^{n+1} - c^n = \int_{t_n}^{t_{n+1}} G_c \, \mathrm{d} t \, , \end{equation}$$$

where

$$$\begin{equation} G_c \equiv - \boldsymbol{\nabla} \boldsymbol{\cdot} \left ( \boldsymbol{v} c \right ) - \boldsymbol{\nabla} \boldsymbol{\cdot} \boldsymbol{q}_c + F_c \, , \end{equation}$$$

and the same forward Euler or Adams-Bashforth scheme as for the explicit evaluation of the time-integral of $\boldsymbol{G}_u$ is used to evaluate the integral of $G_c$.