Units in Amanzi#
Internally, Amanzi uses SI units for length, time, mass, and
concentration as indicated in the model_description/units
element. The only exception is the mol/L unit for the concentration.
Although, it will eventually support the automatic
translation of popular units in an input file to SI units internally,
at present, this translation is only supported by the unstructured component of Amanzi.
For example, the units for density, viscosity, pressure, hydraulic
conductivity, and permeability are kg/m3, Pa \(\cdot\)
s, Pa, m/s, and m2, respectively. Simulation outputs also
use these units.
Beyond dimensional units, the physical entities that are tracked in internal Amanzi calculations, and are often preferred for Amanzi input and output, may be less familiar to those accustomed to hydrologic applications with constant material properties. Specifically, to address the full range of multiphase-multicomponent reactive transport simulation capabilities specified in the Amanzi process models design document, Amanzi’s numerical calculations use the following physical parameters:
Absolute pressure \(P\) (Pa) rather than total/hydraulic head \(h = (P-P_{atm})/\rho g + z\) (m) or pressure head \((P-P_{atm})/\rho g\) (m)
Intrinsic permeability \(k\) (m2) instead of hydraulic conductivity \(K = \rho g k/\mu\) (m/s)
Mass flux \(\dot{m} = \rho \dot{q}\) (kg/m2s) instead of volumetric flux (Darcy velocity) \(\dot{q}\) (m3/m2s = m/s)
Pressure is preferred over head for variable fluid density because \(\rho\) is no longer a constant in the head expression \((P-P_{atm})/\rho g\). Hydraulic conductivity \(K = \rho g k/\mu\) (m/s) is a property of both the porous matrix material and the fluid. Permeability, which is strictly a material property, is a more fundamental entity. When the fluid properties are constant, hydraulic conductivity is proportional to intrinsic permeability and a convenient surrogate for the latter. However, permeability is preferred when fluid properties are variable to separate the effects of variations in the porous medium versus the fluid filling its pore space. Similarly, when fluid properties are constant, volumetric flux is proportional to mass flux and a convenient surrogate for the latter, but direct specification of mass flux is more convenient for varying fluid properties.