1.4. Unit Convention

Aria makes no a priori specification concerning the units of each term. However, as with all engineering codes, errors associated with unit conversions are quite easy to make. To help avoid these errors, it is suggested to use fundamental SI units for all inputs (listed in Table 1.6 below).

Table 1.6 Fundamental SI units
Quantity	Name	Abbreviation
length	meter (metre)	m
mass	kilogram	kg
time	second	s
electric current	ampere	A
thermodynamic temperature	kelvin	K
amount of substance	kmole	kmol
luminous intensity	candela	cd

This ensures consistency across all calculations (including the mesh dimension). If another unit system (e.g. FPS) is used, care must be taken to ensure that it is dimensionally consistent so that

$1 \text{ [mass]} \times \left(1 \text{ [length]}\right)^2 \; / \; \left(1 \text{ [time]}\right)^3 = 1 \text{ [power]}$

For example if $ft$ is used as the [length] scale, $s$ is used as the [time] scale, and $lb_\mathrm{m}$ is used as the [mass] scale, then the [power] unit must be $lb_\mathrm{m} \cdot ft^2 /s^3$ NOT horsepower.

It is sometimes convenient to have access to quantities in different units. Rather than convert the entire input deck over, it is often sufficient (and is suggested) to simply provide the converted expression:

# Scope: Sierra
begin universal aria expressions
   # Convert temperature in Kelvin to Celcius:
   User Expression = scalar_string_function f = "temperature - 273.15" user_tag = tempC
end universal aria expressions

Similarly if the quantity is required only in the output file, a postprocessor can be used:

# Scope: Sierra > Procedure > Aria Region
# Convert a length from meters to feet:
postprocess value of function "length*3.28084" on all_blocks as lengthFt

See User Customization and Postprocessing for more information on creating custom expressions and postprocessors respectively.

In some situations, with just one or two driving forces playing a role in a calculation, nondimensionalization of the equations can lead to a simplification of the problem statement (and to increased solution robustness due to proper scaling of the terms in the equations). However, in complicated cases with multiple competing forces and rate constants, sticking to unit systems to specify all quantities frequently leads to less errors in engineering calculations, and also leads to the ability to incorporate third party library packages for specification of source terms and transport properties which necessarily presume to employ units systems in their application programming interfaces (API).

Attention

When only temperature difference is required and not absolute temperature, Celcius can be used in place of Kelvin (or similarly Fahrenheit can be used in place of Rankine). Special care must be taken when using a relative temperature. Common use cases that require absolute temperature include:

Chemistry
Radiation
Ideal Gas models