# Wetted Perimeter

Written by Jerry Ratzlaff on . Posted in Fluid Dynamics  Wetting perimeter, abbreviated as P_w, is the portion of the channel or pipe that is in contact with the fluid flowing. ## Wetting perimeter of a circular pipe formula

$$\large{ P_w = cos^{-1} \left( 1 - \frac{h}{r} \right) \; d }$$
Symbol English Metric
$$\large{ P_w }$$ = wetting perimeter $$\large{ in }$$ $$\large{ mm }$$
$$\large{ d }$$ = outside diameter $$\large{ in }$$ $$\large{ mm }$$
$$\large{ h }$$ = fluid depth $$\large{ in }$$ $$\large{ mm }$$
$$\large{ r }$$ = radius $$\large{ in }$$ $$\large{ mm }$$ ## Wetting perimeter of a Rectangular Channel formula

$$\large{ P_w = b + 2 \; h }$$
Symbol English Metric
$$\large{ P_w }$$ = wetting perimeter $$\large{ ft }$$ $$\large{ m }$$
$$\large{ b }$$ = bottom width $$\large{ ft }$$ $$\large{ m }$$
$$\large{ h }$$ = fluid depth $$\large{ ft }$$ $$\large{ m }$$ ## Wetting perimeter of a Trapezoidal Channel formula

$$\large{ P_w = b + 2 \; \left( \left( \frac{t \;-\;b}{2} \right)^2 + h^2 \right)^\frac{1}{2} }$$
Symbol English Metric
$$\large{ P_w }$$ = wetting perimeter $$\large{ ft }$$ $$\large{ m }$$
$$\large{ b }$$ = bottom width of liquid $$\large{ ft }$$ $$\large{ m }$$
$$\large{ t }$$ = top width of liquid $$\large{ ft }$$ $$\large{ m }$$
$$\large{ h }$$ = fluid depth $$\large{ ft }$$ $$\large{ m }$$ ## Wetting perimeter of a Triangle Channel formula

$$\large{ P_w = 2 \; h \; \left( 1 + z^2 \right)^\frac{1}{2} }$$
Symbol English Metric
$$\large{ P_w }$$ = wetting perimeter $$\large{ ft }$$ $$\large{ m }$$
$$\large{ h }$$ = fluid depth $$\large{ ft }$$ $$\large{ m }$$
$$\large{ z }$$ = horiz. side slope $$\large{ ft }$$ $$\large{ m }$$ Tags: Hydraulic Equations