Fluid Mechanics and Hydraulics

Fluid mechanics is the study of liquids and gases at rest and in motion, and the forces they exert. It divides into fluid statics, which deals with fluids in equilibrium, and fluid dynamics, which describes how fluids flow. Key physical properties govern this behaviour: density measures mass per unit volume, viscosity describes a fluid's internal resistance to flow, and surface tension arises from cohesive forces at a liquid's surface. Together these properties determine how a fluid responds to pressure and motion.

In a fluid at rest, pressure increases with depth and acts equally in all directions at a given point. Pascal's law states that pressure applied to an enclosed fluid is transmitted undiminished throughout it, the principle that allows a hydraulic press or jack to multiply force: a small force on a small piston produces a large force on a larger one. Pressure is measured with instruments such as the manometer, and the same hydrostatic principles explain buoyancy and the design of dams and submerged structures.

For moving fluids, the conservation of mass requires that fluid speed up where a channel narrows, and Bernoulli's equation relates a fluid's pressure, speed and height along a streamline, showing that pressure falls where speed rises in an idealised, low-friction flow. Real flows involve viscosity and may become turbulent, while phenomena such as cavitation occur when local pressure drops enough for vapour bubbles to form. These ideas underpin hydraulic machinery, piping systems, pumps and the broader field of hydraulics.

Frequently asked questions

What does Pascal's law state?

Pressure applied to a confined fluid is transmitted equally throughout it, which lets hydraulic systems multiply force between pistons of different sizes.

What does Bernoulli's equation describe?

It relates a fluid's pressure, speed and height along a streamline, showing that in idealised flow pressure decreases where the fluid moves faster.

What is viscosity?

Viscosity is a fluid's internal resistance to flow; thick fluids like honey have high viscosity, while thin fluids like water have low viscosity.