Equations of State#
The density of water can be specified in many ways, depending upon phase and problem of interest. Options are available from the simplest (constant value) to functions of just temperature, temperature and pressure, and temperature/pressure/concentration (e.g. salinity).
Note that density includes both molar and mass-based values. Most density evaluators can provide either, or do provide both, the difference being simply a factor of the molecular mass of water.
Finally, nearly all (except the constant value) equations of state use a common set of models which accept all of temperature, pressure, and concentration. Many of these models ignore one or the other, so it should be preferred (but is not necessary) to choose the right model for a given evaluator. Choosing a model that uses fewer of these than the evaluator provides is valid, but it is inefficient. Choosing a model that uses more than the evaluator provides will result in an error.
Constant Value#
Like any quantity, a density can simply be a constant value, at which point it is not a secondary variable but an independent variable.
<ParameterList name="surface-molar_density_liquid" type="ParameterList">
<Parameter name="field evaluator type" type="string" value="independent variable constant" />
<Parameter name="value" type="double" value="55000" />
<Parameter name="units" type="string" value="mol m^-3" />
</ParameterList>
Standard Equation of State#
EOSEvaluator is the interface between state/data and the model, an EOS.