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% $Header: /u/gcmpack/manual/part3/case_studies/rotating_tank/tank.tex,v 1.7 2004/07/26 21:09:47 afe Exp $ |
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% $Name: $ |
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\bodytext{bgcolor="#FFFFFFFF"} |
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%\begin{center} |
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%{\Large \bf Using MITgcm to Simulate a Rotating Tank in Cylindrical |
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%Coordinates} |
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% |
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%\vspace*{4mm} |
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% |
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%\vspace*{3mm} |
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%{\large May 2001} |
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%\end{center} |
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|
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\section{A Rotating Tank in Cylindrical Coordinates} |
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\label{sect:eg-tank} |
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\label{www:tutorials} |
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|
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This section illustrates an example of MITgcm simulating a laboratory |
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experiment on much smaller scales than those common to geophysical |
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fluid dynamics. |
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|
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\subsection{Overview} |
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\label{www:tutorials} |
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|
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|
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This example experiment demonstrates using the MITgcm to simulate |
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a laboratory experiment with a rotating tank of water with an ice |
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bucket in the center. The simulation is configured for a laboratory |
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scale on a |
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$3^{\circ}$ $\times$ 20cm |
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cyclindrical grid with twenty-nine vertical |
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levels. |
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\\ |
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|
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\subsection{Equations Solved} |
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\label{www:tutorials} |
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|
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|
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\subsection{Discrete Numerical Configuration} |
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\label{www:tutorials} |
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|
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The domain is discretised with |
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a uniform grid spacing in the horizontal set to |
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$\Delta x=\Delta y=20$~km, so |
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that there are sixty grid cells in the $x$ and $y$ directions. Vertically the |
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model is configured with a single layer with depth, $\Delta z$, of $5000$~m. |
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|
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\subsubsection{Numerical Stability Criteria} |
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\label{www:tutorials} |
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|
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The Laplacian dissipation coefficient, $A_{h}$, is set to $400 m s^{-1}$. |
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This value is chosen to yield a Munk layer width \cite{adcroft:95}, |
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|
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\begin{eqnarray} |
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\label{EQ:eg-baro-munk_layer} |
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M_{w} = \pi ( \frac { A_{h} }{ \beta } )^{\frac{1}{3}} |
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\end{eqnarray} |
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|
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\noindent of $\approx 100$km. This is greater than the model |
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resolution $\Delta x$, ensuring that the frictional boundary |
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layer is well resolved. |
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\\ |
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|
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\noindent The model is stepped forward with a |
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time step $\delta t=1200$secs. With this time step the stability |
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parameter to the horizontal Laplacian friction \cite{adcroft:95} |
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|
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|
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|
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\begin{eqnarray} |
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\label{EQ:eg-baro-laplacian_stability} |
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S_{l} = 4 \frac{A_{h} \delta t}{{\Delta x}^2} |
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\end{eqnarray} |
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|
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\noindent evaluates to 0.012, which is well below the 0.3 upper limit |
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for stability. |
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\\ |
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|
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\noindent The numerical stability for inertial oscillations |
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\cite{adcroft:95} |
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|
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\begin{eqnarray} |
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\label{EQ:eg-baro-inertial_stability} |
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S_{i} = f^{2} {\delta t}^2 |
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\end{eqnarray} |
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|
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\noindent evaluates to $0.0144$, which is well below the $0.5$ upper |
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limit for stability. |
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\\ |
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|
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\noindent The advective CFL \cite{adcroft:95} for an extreme maximum |
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horizontal flow speed of $ | \vec{u} | = 2 ms^{-1}$ |
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|
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\begin{eqnarray} |
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\label{EQ:eg-baro-cfl_stability} |
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S_{a} = \frac{| \vec{u} | \delta t}{ \Delta x} |
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\end{eqnarray} |
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|
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\noindent evaluates to 0.12. This is approaching the stability limit |
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of 0.5 and limits $\delta t$ to $1200s$. |
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|
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\subsection{Code Configuration} |
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\label{www:tutorials} |
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\label{SEC:eg-baro-code_config} |
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|
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The model configuration for this experiment resides under the |
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directory {\it verification/rotatingi\_tank/}. The experiment files |
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\begin{itemize} |
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\item {\it input/data} |
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\item {\it input/data.pkg} |
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\item {\it input/eedata}, |
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\item {\it input/bathyPol.bin}, |
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\item {\it input/thetaPol.bin}, |
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\item {\it code/CPP\_EEOPTIONS.h} |
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\item {\it code/CPP\_OPTIONS.h}, |
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\item {\it code/SIZE.h}. |
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\end{itemize} |
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|
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contain the code customizations and parameter settings for this |
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experiments. Below we describe the customizations |
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to these files associated with this experiment. |
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|
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\subsubsection{File {\it input/data}} |
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\label{www:tutorials} |
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|
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This file, reproduced completely below, specifies the main parameters |
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for the experiment. The parameters that are significant for this configuration |
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are |
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|
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\begin{itemize} |
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|
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\item Line X, \begin{verbatim} viscAh=5.0E-6, \end{verbatim} this line sets |
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the Laplacian friction coefficient to $0.000006 m^2s^{-1}$, which is ususally |
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low because of the small scale, presumably.... qqq |
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|
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\item Line X, \begin{verbatim}f0=0.5 , \end{verbatim} this line sets the |
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coriolis term, and represents a tank spinning at qqq |
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\item Line 10, \begin{verbatim} beta=1.E-11, \end{verbatim} this line sets |
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$\beta$ (the gradient of the coriolis parameter, $f$) to $10^{-11} s^{-1}m^{-1}$ |
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|
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\item Lines 15 and 16 |
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\begin{verbatim} |
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rigidLid=.TRUE., |
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implicitFreeSurface=.FALSE., |
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\end{verbatim} |
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|
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these lines do the opposite of the following: |
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suppress the rigid lid formulation of the surface |
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pressure inverter and activate the implicit free surface form |
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of the pressure inverter. |
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|
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\item Line 27, |
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\begin{verbatim} |
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startTime=0, |
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\end{verbatim} |
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this line indicates that the experiment should start from $t=0$ |
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and implicitly suppresses searching for checkpoint files associated |
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with restarting an numerical integration from a previously saved state. |
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|
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\item Line 30, |
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\begin{verbatim} |
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deltaT=0.1, |
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\end{verbatim} |
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This line sets the integration timestep to $0.1s$. This is an unsually |
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small value among the examples due to the small physical scale of the |
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experiment. |
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|
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\item Line 39, |
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\begin{verbatim} |
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usingCylindricalGrid=.TRUE., |
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\end{verbatim} |
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This line requests that the simulation be performed in a |
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cylindrical coordinate system. |
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|
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\item Line qqq, |
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\begin{verbatim} |
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dXspacing=3, |
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\end{verbatim} |
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This line sets the azimuthal grid spacing between each x-coordinate line |
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in the discrete grid. The syntax indicates that the discrete grid |
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should be comprise of $120$ grid lines each separated by $3^{\circ}$. |
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|
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|
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|
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\item Line qqq, |
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\begin{verbatim} |
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dYspacing=0.01, |
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\end{verbatim} |
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This line sets the radial grid spacing between each $\rho$-coordinate line |
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in the discrete grid to $1cm$. |
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|
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\item Line 43, |
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\begin{verbatim} |
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delZ=29*0.005, |
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\end{verbatim} |
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This line sets the vertical grid spacing between each z-coordinate line |
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in the discrete grid to $5000m$ ($5$~km). |
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|
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\item Line 46, |
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\begin{verbatim} |
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bathyFile='bathyPol.bin', |
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\end{verbatim} |
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This line specifies the name of the file from which the domain |
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``bathymetry'' (tank depth) is read. This file is a two-dimensional |
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($x,y$) map of |
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depths. This file is assumed to contain 64-bit binary numbers |
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giving the depth of the model at each grid cell, ordered with the $x$ |
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coordinate varying fastest. The points are ordered from low coordinate |
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to high coordinate for both axes. The units and orientation of the |
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depths in this file are the same as used in the MITgcm code. In this |
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experiment, a depth of $0m$ indicates an area outside of the tank |
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and a depth |
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f $-0.145m$ indicates the tank itself. |
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|
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\item Line 49, |
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\begin{verbatim} |
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hydrogThetaFile='thetaPol.bin', |
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\end{verbatim} |
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This line specifies the name of the file from which the initial values |
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of $\theta$ |
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are read. This file is a three-dimensional |
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($x,y,z$) map and is enumerated and formatted in the same manner as the |
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bathymetry file. |
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|
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\item Line qqq |
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\begin{verbatim} |
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tCyl = 0 |
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\end{verbatim} |
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This line specifies the temperature in degrees Celsius of the interior |
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wall of the tank -- usually a bucket of ice water. |
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|
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|
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\end{itemize} |
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|
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\noindent other lines in the file {\it input/data} are standard values |
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that are described in the MITgcm Getting Started and MITgcm Parameters |
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notes. |
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|
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\begin{small} |
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\input{part3/case_studies/rotating_tank/input/data} |
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\end{small} |
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|
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\subsubsection{File {\it input/data.pkg}} |
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\label{www:tutorials} |
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|
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This file uses standard default values and does not contain |
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customizations for this experiment. |
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|
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\subsubsection{File {\it input/eedata}} |
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\label{www:tutorials} |
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|
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This file uses standard default values and does not contain |
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customizations for this experiment. |
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|
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\subsubsection{File {\it input/thetaPol.bin}} |
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\label{www:tutorials} |
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|
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The {\it input/thetaPol.bin} file specifies a three-dimensional ($x,y,z$) |
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map of initial values of $\theta$ in degrees Celsius. |
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|
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\subsubsection{File {\it input/bathyPol.bin}} |
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\label{www:tutorials} |
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|
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|
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The {\it input/bathyPol.bin} file specifies a two-dimensional ($x,y$) |
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map of depth values. For this experiment values are either |
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$0m$ or {\bf -delZ}m, corresponding respectively to outside or inside of |
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the tank. The file contains a raw binary stream of data that is enumerated |
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in the same way as standard MITgcm two-dimensional, horizontal arrays. |
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|
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\subsubsection{File {\it code/SIZE.h}} |
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\label{www:tutorials} |
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|
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Two lines are customized in this file for the current experiment |
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|
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\begin{itemize} |
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|
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\item Line 39, |
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\begin{verbatim} sNx=120, \end{verbatim} this line sets |
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the lateral domain extent in grid points for the |
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axis aligned with the x-coordinate. |
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|
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\item Line 40, |
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\begin{verbatim} sNy=31, \end{verbatim} this line sets |
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the lateral domain extent in grid points for the |
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axis aligned with the y-coordinate. |
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|
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\end{itemize} |
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|
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\begin{small} |
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\input{part3/case_studies/rotating_tank/code/SIZE.h} |
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\end{small} |
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|
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\subsubsection{File {\it code/CPP\_OPTIONS.h}} |
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\label{www:tutorials} |
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|
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This file uses standard default values and does not contain |
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customizations for this experiment. |
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|
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|
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\subsubsection{File {\it code/CPP\_EEOPTIONS.h}} |
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\label{www:tutorials} |
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|
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This file uses standard default values and does not contain |
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customizations for this experiment. |
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