Convention for Positive Flow Direction
Reference Positive Flow Direction
Next to each pipe's ID number is an arrow that indicates the reference positive flow direction for the pipe. AFT Fathom assigns a flow direction corresponding to the direction in which the pipe is drawn. You can reverse the reference positive flow direction by choosing Reverse Direction from the Arrange menu or selecting the reverse direction button on the Toolbar.
The reference positive flow direction is used for reference purposes only and need not be the actual flow direction. If the reference positive direction is the opposite of that obtained by the Solver (the part of AFT Fathom that applies the governing incompressible flow equations to obtain a solution to the pipe system), the output will show the flow rate as a negative number.
Convention for Flow Entering and Exiting
Many of the junction types permit fluid flow to come into or pass out of the system in a prescribed manner. The most obvious of these is the reservoir junction. Reservoirs maintain a constant pressure head at a node, and the flow adjusts according to the governing equations. While in principle, the fluid is stored or removed from the reservoir, AFT Fathom considers the fluid as coming into or going out of the system. The stored reservoir fluid is not part of the system.
The convention for defining inward or outward flow for the system or any AFT Fathom object is that flow in is positive (that is, the object/system gains the flow) and flow out is negative (the object/system loses the flow).