Fluid Mechanics

FLUID MECHANICS

John Petrizzo, PT, DPT, CSCS

Overview of Fluid Mechanics

  • Fluid Mechanics: Study of the behavior of fluids (liquids and gases) in motion and at rest.

  • Key Points:

    • The major external forces that influence human movement are related to immersion in or flow of fluids around the body.

    • The two most commonly encountered fluids in human movement are air and water.

    • Fluid forces often cause considerable resistance during high-velocity movements.

    • Sports techniques and equipment have been designed specifically to reduce fluid resistance.

Fluid Forces in Human Motion

  • Fluid Forces: Can be harnessed to create movement, including the application of spin on projectiles.

  • Definition of Fluids:

    • Mechanically defined as substances that flow or continuously deform when acted upon by shear forces.

Classification of Fluid Forces

  • Types of Fluid Forces Affecting Motion:

    1. Buoyancy

    2. Lift

    3. Drag

  • Individuals typically have a significant degree of control over factors affecting these fluid forces.

Buoyancy

  • Definition: The upward, supporting force exerted by a fluid.

  • Impact on Objects:

    • When an object is placed in a fluid, the combination of gravitational force and buoyant force determines whether the object will float or sink.

  • Archimedes' Principle:

    • States that the buoyant force acting on a submerged object is equal to the weight of the fluid displaced by the object.

Human Body and Buoyancy

  • The density of the human body is nearly equal to that of water due to the high water content in tissues.

    • Lean tissue (muscle & bone) has higher densities than water.

    • Body fat tends to have lower density than water.

Variations in Buoyant Force

  • The buoyant force on a swimmer will change with varying body composition:

    • Buoyant forces fluctuate when the swimmer inhales or exhales.

    • Inhalation: Increases body volume leading to an increase in buoyancy.

    • Center of Buoyancy (COB): Acts upward at the centroid of displaced fluid volume, typically located at the trunk's center, influencing buoyancy.

    • Comparison to Center of Gravity (COG): The movement of the body has more influence on the COG than on the COB.

Hydrotherapy

  • Involves therapeutic exercises conducted in water that utilize buoyancy to assist with unloading the lower extremities.

  • The degree of unloading can be controlled by adjusting the level of body submersion in water.

  • Fluid flow generates lift and drag forces that can enhance muscle activation during exercises.

Drag

  • Definition: The fluid force that opposes the motion of an object through a fluid, acting in line with fluid current and opposing the motion of the object.

  • Characteristics of Drag Forces:

    • Acts parallel to the flow of fluid relative to the object, similar to friction.

Factors Affecting Drag

  • Drag is affected by several factors, including:

    1. Fluid Density

    2. Projected Frontal Area of the object in relation to fluid flow.

    3. Relative Velocity: This is the most significant factor influencing drag.

Types of Drag Forces

  • Sources of Drag Force:

    1. Surface Drag:

    • Comparable to fluid friction.

    • Arises from friction between fluid molecules and the object surface.

    • Viscosity: The internal resistance of a fluid:

      • Air (low viscosity) < Water < Maple Syrup (high viscosity).

    1. Pressure Drag:

    • Major contributor to fluid resistance, arising due to pressure differentials when fluid flows past an object.

    1. Wave Drag:

    • Occurs when disturbances create waves in the fluid that obstruct the object's motion. More prominent in swimming.

Understanding Surface Drag

  • Boundary Layer:

    • The area of fluid close to the object that moves slower due to viscous forces, impacting the overall drag experienced by the object.

  • Modification of Surface Drag:

    • While viscosity cannot be altered, an athlete can reduce surface drag by adjusting the smoothness of their skin or gear (e.g., waxing surfboards and skis, swimmers shaving body hair or wearing sleek swimsuits).

Addressing Pressure Drag

  • Techniques include:

    • Decrease Frontal Area: Making the body or equipment more streamlined is essential.

    • Streamlining: Adopting streamlined positions in sports, such as in swimming and cycling, where preserving low drag is critical to performance.

Addressing Wave Drag

  • Impact of Swimming Environments:

    • Swimmers experience less wave drag in enclosed pools where wave dampening systems (like lane markers) are present.

    • Triathletes in open water contend with waves generated by wind and other swimmers, increasing wave drag.

Lift

  • Definition: A fluid force acting perpendicular to the flow of the fluid. It is influenced by the shape, speed, and rotation of an object.

  • Application Examples:

    • Wings on race cars generate downward lift for stability.

    • Used in various propulsion techniques across different sports.

Bernoulli’s Principle

  • This principle explains that the pressure in a fluid decreases as the velocity increases:

    • Pressure-Velocity Relationship: Higher fluid velocity results in lower pressure exerted by the fluid.

Lift Versus Drag

  • Fluid flow complexity creates difficulty in determining which fluid forces are more influential in propulsion strategies across different sports.

The Magnus Effect

  • Definition: Lift forces generated by spin imparted to spherical projectiles, causing curved flight paths due to pressure differences.

  • Notable in sports, affecting various balls.

Exceptions to Lift Influence

  • Certain projections with minimal spin, such as a knuckleball in baseball or a floater in volleyball, exhibit erratic trajectories due to unpredictable airflow, despite similar forces acting upon them.

Comparing Fastball and Knuckleball

  • Fastballs exert uniform lift due to spin, while knuckleballs create unpredictable flight paths due to lack of consistent spin.

Principle of Spin

  • Fluid forces significantly influence projectile motion; thus, modifying lift via spin can improve performance in sporting activities.

  • Backspin Example:

    • Increases ball trajectory in golf, helps basketball shots maintain proximity to the hoop due to enhanced bounce off the rim.

  • Trade-off: Increasing spin may reduce the projectile's linear speed.

Importance of Recognizing Spin Benefits

  • Understanding when and how to utilize spin effectively while maintaining communication through technical cues is crucial to athletic performance.

    • Cues: Should be clear and relate directly to the target or technique without overly technical jargon.

Summary of Key Concepts

  • Fluid forces, derived from air and water, profoundly influence human movement.

  • The three primary fluid forces are:

    1. Buoyancy: The upward force that fluids exert on submerged objects.

    2. Drag: The opposing force acting in the same direction as the fluid flow relative to an object.

    3. Lift: The fluid force acting perpendicular to the flow direction.

  • The Principle of Spin enables performers to manipulate spin on projectiles, helping create desired trajectories and enhancing performance in various sports.