Concept of Static Pressure

Concept of Static Pressure

• The thermodynamic or hydrostatic pressure caused by molecular collisions is known as static pressure in a fluid flow and is usually referred to as the pressure p.
  
• When the fluid is at rest, this pressure p is the same in all directions and is categorically known as the hydrostatic pressure.
  
• For the flow of a real and Stoksian fluid (the fluid which obeys Stoke’s law) the static or thermodynamic pressure becomes equal to the arithmetic average of the normal stresses at a point. The static pressure is a parameter to describe the state of a flowing fluid.
  
• Let us consider the flow of a fluid through a closed passage as shown in Fig. 16.1a.

Fig 16.1   Measurement of Static pressure
(a)  Single Wall tap     (b)  Multiple Wall Tap 

• If a hole is made at the wall and is connected to any pressure measuring device, it will then sense the static pressure at the wall. This type of hole at the wall is known as a wall tap.
  
• The fact that a wall tap actually senses the static pressure can be appreciated by noticing that there is no component of velocity along the axis of the hole.
  
• In most circumstances, for example, in case of parallel flows, the static pressure at a cross-section remains the same. The wall tap under this situation registers the static pressure at that cross-section.
  
• In practice, instead of a single wall tap, a number of taps along the periphery of the wall are made and are mutually connected by flexible tubes (Fig. 16.1b) in order to register the static pressure more accurately.