21.1. Units Syntax

Dimensional quantities are defined in units or a combination of units. For example, mass can have units of [kg], [g] or [lb] (and many others); pressure can have units of [atm], [N m^-2] and [Pa] (and many others).

The general units syntax in CFX is defined as [multiplier|unit|^power] where multiplier is a multiplying quantity (such as mega, pico, centi, and so on), unit is the unit string (kg, m, J, and so on), and power is the power to which the unit is raised. When typing units in expression, they must be enclosed by square brackets, [...]. You will usually not see the brackets when selecting units from a list of commonly used units. In general, units declarations must obey the following rules:

  • A units string consists of one or more units quantities, each with an optional multiplier and optional power. Each separate units quantity is separated by one or more spaces.

  • Short forms of the multiplier are usually used. n stands for nano, μ stands for micro, c for centi, k for kilo, m for milli, M for mega and G for giga.

  • Powers are denoted by the ^ (caret) symbol. A power of 1 is assumed if no power is given.

    Note:  The / operator is not supported, so a negative power is used for unit division (for example, [kg m^-3] corresponds to kilograms per cubic meter).

  • If you enter units that are inconsistent with the physical quantity being described, then a dialog box will appear informing you of the error, and the units will revert to the previous units.

  • Units do not have to be given in terms of the fundamental units (mass, length, time, temperature, angle and solid angle). For instance, Pa (Pascals) and J (Joules) are both acceptable as parts of unit strings.

  • Units strings are case sensitive; for example, Kg and KG are both invalid parts of units strings.

    To give the units of dynamic viscosity, which has the dimensions of Mass Length-1 Time-1, the unit string [kg m^-1 s^-1] (or [lb ft^-1 hr^-1]) is valid.

    Note:  The following unit strings are invalid:

    [kg/(metre sec)][kg/(ms)]
    [kg/m/s][kg/(m.s)]
    [kg/(m s)][kg/(m*s)]
    [kg/(m sec)][lb/(ft hr)]