The Environmental Properties panel, shown in Figure 1, controls the definition of the atmospheric pressure, gravitational acceleration, and transition Reynolds number used to model the system.

Environmental Properties

Specify the ambient pressure and gravitational acceleration values for the solver to use in the calculations. By default, these values are 1 atm and 1 g, respectively.

Ambient Pressure

AFT Fathom uses a default atmospheric pressure of 1 standard earth atmosphere, or 14.696 psia. This pressure is also used to convert absolute pressures to gauge pressures. Keep in mind that atmospheric pressure varies with altitude. A project at Sea Level should not have the same atmospheric pressure as a project at 8500 feet. Fathom can automatically adjust atmospheric pressure using the Ambient Pressure vs. Elevation Table found in the Analysis Setup window. Here, users can input a pressure profile that will change the ambient pressure depending on a junction's elevation.

Gravitational Acceleration

The default for gravitational acceleration is 1 standard earth gravitational acceleration, or 32.174 feet/sec2. You can change this to a multiple of standard earth accelerations (that is, number of g's) according to the design environment for your system. In principle, the body force on a fluid system does not have to be due to gravity.

Transition Reynolds Number

AFT Fathom allows you to modify the transition Reynolds numbers. The default values assume that a Reynolds number of less than 2300 is laminar and a Reynolds number of greater than 4000 is turbulent. In the transition zone, a linear interpolation is assumed. This applies for friction factors and convection heat transfer.

You can modify these transition Reynolds numbers if you choose by manually typing in the values on the Environmental Properties panel in the Analysis Setup window.