When fluid velocity increases, its pressure tends to decrease

 When fluid velocity increases, its pressure tends to decrease (for the ideal-flow conditions under which Bernoulli’s equation applies).

P = pressure
ρ = density of fluid
v = velocity
g = acceleration due to gravity
h = height above a reference level

This is the Bernoulli principle, which is very important in fluid mechanics and HVAC/MEP systems.

Bernoulli’s Equation

$$P + \frac{1}{2}\rho v^2 + \rho gh = \text{constant}$$

It means that, along a streamline for ideal, steady, incompressible flow:

• P = Static pressure
• ρ = Fluid density
• v = Fluid velocity
• g = Acceleration due to gravity
• h = Height above the reference level

Simple understanding

If the velocity of the fluid increases, its pressure tends to decrease, provided the other conditions remain appropriate.

For example, when water flows through a smaller pipe section, its velocity increases. To maintain the energy balance, the static pressure generally decreases.

Example:
A pipe changes from a large diameter to a smaller diameter:

Large pipe → Smaller pipe
Velocity ↓ → Velocity ↑
Static pressure ↑ → Static pressure ↓

$\rho = \frac{m}{V}$

$\rho=\frac{\text{12}\,\mathrm{kg}}{\text{5}\,\mathrm{L}}=\text{2.4}\,\mathrm{kg/L}$

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Important: Bernoulli's equation applies to idealized flow. In real piping systems, friction, fittings, valves, pumps, and other losses must also be considered.