Dynamic Pressure

p
stat
p
tot
p
dyn
p
tot
p
stat
p
stat
p
tot
Q
p
d d
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2. Performance curves
2.2 Pressure
Pressure (p) is an expression of force per unit area and is split into static and
dynamic pressure. The sum of the two pressures is the total pressure:
Pa p p p
dyn
(2.1)
stat tot

where
p
tot
= Total pressure [Pa]
p
stat
= Static pressure [Pa]
p
dyn
= Dynamic pressure [Pa]
Static pressure is measured with a pressure gauge, and the measurement of
static pressure must always be done in static fluid or through a pressure tap
mounted perpendicular to the flow direction, see figure 2.3.
Total pressure can be measured through a pressure tap with the opening
facing the flow direction, see figure 2.3. The dynamic pressure can be found
measuring the pressure diference between total pressure and static pressure.
Such a combined pressure measurement can be performed using a pitot tube.
Dynamic pressure is a function of the fluid velocity. The dynamic pressure can
be calculated with the following formula,where the velocity (V) is measured
and the fluid density () is know:
(2.2) Pa V
2
1
p
2
dyn
ρ
where
V = Velocity [m/s]
= Density [kg/m
3
]
Dynamic pressure can be transformed to static pressure and vice versa. Flow
through a pipe where the pipe diameter is increased converts dynamic pressure
to static pressure, see figure 2.4. The flow through a pipe is called a pipe flow, and
the part of the pipe where the diameter is increasing is called a difusor.
Figure 2.4: Example of conversion of
dynamic pressure to static pressure in
a difusor.
Figure 2.3: This is how static pressure p
stat
,
total pressure p
tot
and dynamic pressure
p
dyn
are measured.