An easy way to represent a perforated plate or a baffle that has a known open (free) area is to use a free area ratio.
The free area ratio is the ratio of the open area to the total area of the plate or baffle:
To input a free area ratio:
- Select the desired direction and choose Free Area Ratio as the Variation Method.
- Enter the appropriate ratio in the Value field. A value of 1 indicates that the region is completely open, and the flow will encounter low resistance.
- The two normal directions are typically closed, so use a value of 0 to indicated a completely closed, high resistance condition.
Example showing creation of a Free Area Ratio Resistance
The relationship between loss coefficient, K, and free area ratio, FAR, is given as:
Note: This equation is valid for flow with Reynolds number greater than 105. The ratio of the flat portion of the hole length, l, to hydraulic diameter, Dh, is between 0 and 0.015:
