The Environmental Properties panel, shown in Figure 1, controls the atmospheric pressure, acceleration properties, standard conditions, and transition Reynolds number used to model the system.

Atmospheric Pressure

AFT Arrow 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.

Acceleration Properties

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.

You also can specify a rotational acceleration such as might be experienced in turbomachinery. You are prompted to enter a rotation rate in rpm (revolutions per minute). In this case, AFT Arrow interprets the junction elevation data as distance from the rotation centerline, and uses a varying acceleration body force on each individual pipe as determined by the laws of rotational dynamics.

Each individual pipe computing section will use the acceleration based on the arithmetic average over its length. You cannot specify both gravitational and rotational accelerations at the same time.

Standard Conditions (STP)

The standard temperature and pressure (STP) are used for defining common usage flow rate units including standard cubic feet per minute (scfm).

AFT Arrow uses a default standard pressure of 14.696 psia. This value is used consistently across industry, and the user likely will not need to change it. AFT Arrow uses a default standard temperature of 60 deg. F. This value changes across industries, and the user may wish to change it.

Another common usage flow rate is normal flow rate units. The definition of normal uses the same standard pressure as above, but 32 deg. F (0 deg. C) for temperature. The normal pressure and temperature can not be changed by the user.

Transition Reynolds Numbers

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