Fluid Mechanics (General Engineering) (SSC JE)

Stability of a submerged body–
• The body remains in stable equilibrium when G lies below B
• The body remains in unstable equilibrium when G lies above B
• The body remains in neutral equilibrium when G coincides with B
where, G = centre of gravity
B = centre of buoyancy

Stream lines are tangential curves which are tangential to the velocity vector of the flow. These lines depict the path through which massless fluid elements travel with any point of time. Therefore, it is a line tangent to which is in the direction of velocity vector at every point.

Net Positive Suction Head (NPSH)–
The term NPSH is very commonly used in the pump industry. The net positive suction head is defined as the absolute pressure head at the inlet to the pump minus the vapour pressure head plus the velocity head.

Stability of a submerged body
The stability of a submerged body such as a balloon submerged in air or a submarine submerged in water is determined by the relative position of the center of gravity G and center of buoyancy B of the body. The conditions for stability of a submerged body are listed below.
• The body remains in stable equilibrium when G lies below B
• The body remains in unstable equilibrium when G lies above B
• The body remains in neutral equilibrium when G coincides.

In impulse-reaction turbines, the pressure drops both fixed and moving blades. In reaction turbine both the nozzles and blades act as expanding nozzles. So the static pressure decreases in both the fixed and moving blades.