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# Dynamic Pressure

Dynamic pressure is the kinetic energy of a flowing fluid - liquid or gas - per unit volume - and can be expressed as

p d = 1/2 ρ v 2 (1)

where

p d = dynamic pressure (N/m 2 (Pa), lb f /ft 2 (psf))

ρ = density of fluid (kg/m 3 , slugs /ft 3 )

v = velocity (m/s, ft/s)

### Dynamic Pressure Calculator - SI Units

The default values below are for water with density of 1000 kg/m 3 .

• 1 N/m 2 = 1 Pa = 1.4504x10 -4 lb f /in 2 (psi) = 0.02089 lb f /ft 2 (psf) = 1x10 -5 bar = 4.03x10 -3 in water = 0.336x10 -3 ft water = 0.1024 mm water = 0.295x10 -3 in mercury = 7.55x10 -3 mm mercury = 0.1024 kp/m 2 = 0.993x10 -5 atm

### Dynamic Pressure Calculator - Imperial Units

The default values below are for water with density of 1.940 slugs .

Some common densities at atmospheric pressure:

• Water - 0 o C - 1000 kg/m 3
• Water - 32 o F - 1.940 slugs/ft 3
• Air - 20 o C - 1.2 kg/m 3
• Air - 60 o F - 2.373 10 -3 slugs/ft 3

### Example - The Dynamic Pressure in a Water Flow

The dynamic pressure in water with - temperature 20 o C - density 1000 kg/m 3 and velocity 5 m/s - can be calculated as

p d = 1/2 (1000 kg/m 3 ) (5 m/s) 2

= 12500 Pa

= 12.5 kPa

### Example - Hurricane and Force acting on a Wall

The dynamic pressure in a hurricane with air temperature 20 o C, density of air 1.2 kg/m 3 and wind speed 37 m/s can be calculated as

p d = 1/2 (1.2 kg/m 3 ) (37 m/s) 2

= 821 Pa (N/m 2 )

The force acting directly on a wall with area 10 m 2 can be calculated as

F = p d A

= (821 N/m 2 ) (10 m 2 )

= 8210 N

= 8.2 kN

- almost the weight of a small car.

Note! - the real force acting on the wall - or an other obstruction in the wind - is in general more complicated to calculate due to drag , turbulence and other effects.

## Related Topics

### • Fluid Mechanics

The study of fluids - liquids and gases. Involving velocity, pressure, density and temperature as functions of space and time.

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Velocity plotted in time used diagram.

### Ducts Sizing - the Velocity Reduction Method

The velocity reduction method can be used when sizing air ducts.

### Kinetic Energy

Energy possessed by an object's motion is kinetic energy.

### Pitot Tubes

Pitot tubes can be used to measure fluid flow velocities by measuring the difference between static and dynamic pressure in the flow.

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