6.2+Fluid+mechanics

Fluid Mechanics and Biomechanics

Fluid Mechanics

  • Study of fluids (liquids and gases) in motion and at rest.

Outline of Fluid Mechanics

  • Fluid force

  • Drag force

  • Lift force

Fluid Definition

  • Fluid: Any substance that can flow and change shape quickly.

  • Includes:

    • Liquids

    • Gases

  • Key focus: Water and air.

  • Fluid exerts a force on objects placed in or moving relative to it.

Fluid Force

  • Definition: Force exerted on an object by the fluid.

  • Two types of fluid forces:

    • Buoyant force (Buoyancy): Upward force on objects in a fluid.

    • Dynamic fluid force: Acts on objects with relative motion between object and fluid.

      • Examples: Objects traveling through fluid, or fluid moving around objects such as a wind.

Buoyant Force

  • Magnitude is equal to the weight of the fluid displaced by the object.

  • Factors affecting buoyant force:

    • Greater volume and density = Greater buoyant force.

  • Weight equation: Weight = Volume x Density.

Vertical Forces on Objects

  • Net force = Weight of the object + Buoyant force.

  • Object behavior:

    • Floats: Weight = Buoyant force.

    • Sinks: Weight > Buoyant force.

Application in Scuba Diving

  • Divers control body movement through inhalation/exhalation:

    • Inhale increases chest volume and buoyant force, causing the body to float up.

    • Exhale decreases chest volume and buoyant force, causing the body to sink down.

Fluid Movement and Boundary Layer

  • Relative motion: Object traveling through the fluid or fluid moving around an object.

Boundary Layer

  • Definition: Thin layer of fluid adjacent to an object's surface.

  • Velocity dynamics within layer:

    • Closest fluid moves with the object.

    • Fluid farther from object moves with surrounding fluid.

  • Velocity difference creates friction in the boundary layer.

Flow Types

  • Laminar Flow: Smooth flow following object's contour, eventually separating.

  • Turbulent Flow: Irregular, chaotic fluid movement, creating wake behind the object.

Dynamic Fluid Force Components

  • Drag Force: Acts parallel to fluid direction, resisting motion.

  • Lift Force: Acts perpendicular, changing object's path.

Impact of Drag Force

  • Drag resists motion; crucial in speed sports.

Impact of Lift Force

  • Can redirect an object's path.

  • Not always results in lifting the object upward.

Factors Affecting Drag Force

  • Sources:

    • Surface drag: Friction at the fluid-object interface.

    • Pressure drag: Pressure differences between the object's front and back.

Pressure Drag Dynamics

  • High pressure at the front and low pressure at the back create resistance.

Factors Determining Drag Magnitude

  1. Relative velocity of object to fluid.

  2. Object characteristics (size, shape, surface smoothness).

  3. Fluid characteristics (density and viscosity).

Relative Velocity Calculations

  • Different relative velocities between fluid and object affect drag:

    • Increased headwind increases resistance.

Object Size Influence on Drag

  • Larger size correlates with greater pressure drag.

Projected Frontal Area Effect

  • Larger cross-sectional area leads to increased pressure drag.

Object Shape Influence

  • Smooth shapes minimize turbulence, reducing drag.

  • Streamlined shapes improve movement efficiency.

Surface Smoothness Impact

  • Rough surfaces increase friction in the boundary layer, leading to higher drag forces.

Density Effects

  • Higher fluid density results in increased pressure and surface drag.

  • Examples:

    • Water density: 1000 kg/m³

    • Air density: 1.2 kg/m³

Viscosity Effects

  • Viscosity measures resistance to flow, affecting drag.

  • Higher viscosity results in greater drag; lower viscosity easier flow.

Drag Coefficient

  • Drag coefficient (CD): Measures resistance of an object in fluid.

  • Lower CD indicates an aerodynamic advantage.

Optimizing Drag in Sports

  • Drafting: Technique to reduce drag by trailing close to another rider.

Summary of Drag Force

  • Drag defined by motion resistance, surface friction, and pressure differences affecting fluid dynamics.

Lift Force Production

  • Lift generated by changing the direction of fluid flow around an object.

  • When fluid is displaced in one direction, the object is pushed in the opposite direction.

Magnus Effect

  • A lift force caused by spinning objects, affecting trajectories in sports.

Summary of Lift Force

  • Lift is perpendicular to fluid flow and influenced by similar factors affecting drag force.

  • Magnus effect enhances influence based on spin and velocity.