Chapter 6. Fluid Mechanics

While we stand on solid ground, we largely live in a world of fluids (liquids and gases). We walk around in a mass of air. We breathe it in our lungs and feel its pressure on our bodies. Blood flows through our body, keeping us nourished and healthy. We drink lots of water and occasionally swim in it. In the mechanical world, we use fluids like hydraulic oil to move things or we force things like cars to move through the atmosphere. We might wonder why boats and birds are streamlined, why golf balls are dimpled, what drag is, and how it’s possible to stop a car with a touch of the brake pedal. Designers frequently work with wide-ranging fluids, including water, air, oil, and blood. Their difference in behavior can be surprising.

The study of fluid behavior is broken down into two disciplines. The first is the study of fluids at rest, called fluid statics. And second is fluid dynamics, which describes the nature of moving fluids.

Fluid Behavior

Solids differ from fluids in that a fluid cannot permanently resist shear stresses. Therefore, where a solid will deform (strain) a limited amount when a force is applied, a fluid will not. The rate of movement is dictated by the force but not the total amount of movement—it just keeps on flowing. A fluid eliminates any shear stresses simply by flowing.

The resistance to flow (technically, shearing force) is called viscosity. High viscosity indicates the fluid moves slowly and takes a long time to remove shear ...

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