Traps are designed to allow gas penetration when the trap propellant is required. The entering gas is "trapped" either for the life of the mission or until it can be vented with a ventable trap (or start basket). Because the trap is sized by the volume required and not by the flow path needed, the flow losses and, more importantly, the flow transients are generally not significant. With low internal velocities, the basic bubble point equation above is sufficient to design a trap. The more interesting physics occurs within the trap where a gallery arm or vanes are required to access the propellant.
The main functionality of a trap is to provide a fixed volume of propellant. Traps are most often sized with a safety factor of two on volume to ensure proper operation and some off design capacity.
The entrance windows in traps are most often screen because screen wicks and can maintain a bubble point even with gas on both sides of the screen. Since gas will reside inside the trap and will contact the porous element on the outside, screen is preferred. However, it is not required if special measures are taken to ensure that the perforated sheet is always wetted. This can be done with fins inside the trap which always hold liquid against the perforated sheet.
For more information on the design, use, and physics of traps please see
AIAA 95-2531 Propellant Management Device Conceptual Design and Analysis: Traps & Troughs