Chapter 2: Fluid Mechanics (Basics)

Description

Module 1: Introductory Concepts and Viscosity:

Fluid mechanics is the discipline within the broad field of applied mechanics. Fluid mechanics is concerned with the motion of gases and liquids. It covers a vast array of phenomena that occur in nature with or without human intervention. There are few aspects of our lives that do not involve fluids, either directly or indirectly.

Student Learning Outcomes:

After completing module 1, you should be able to:

1) List the dimensions and units of physical quantities.

2) Identify the key fluid properties used in the analysis of fluid behavior.

3) Calculate values for common fluid properties given appropriate information.

4) Apply the concepts of viscosity to various engineering applications.

5) Explain the effects of fluid compressibility and surface tension

Chapter Two: Fluid Mechanics

3.1 Introduction to Fluid Mechanics in Biology

Fluid mechanics is the study of how gases and liquids behave and interact with objects. In biology, this primarily involves how organisms move through fluids (air and water) and how fluids move within organisms (blood flow, air flow, etc.).

Organisms have evolved extraordinary adaptations to deal with the forces of fluid environments. Understanding these interactions is crucial to analyzing swimming, flying, breathing, circulation, and filter feeding.

3.2 Fundamental Concepts in Fluid Mechanics

Fluids

• Liquids (e.g., water, blood) – relatively incompressible
  
  

• Gases (e.g., air) – compressible and less dense
  
  

Key Properties of Fluids

• Viscosity (μ): Measure of a fluid’s resistance to deformation or flow (e.g., honey is more viscous than water).
  
  

• Density (ρ): Mass per unit volume of a fluid.
  
  

3.3 Forces in Fluids

Drag

• The resistance an object encounters while moving through a fluid.
  
  

  • Pressure Drag (form drag): From differences in pressure around an object.
    
    

  • Frictional Drag (skin friction): From fluid's viscosity acting on the surface.
    
    

Lift

• A force perpendicular to the direction of fluid flow, enabling flight or gliding (e.g., in bird wings, fish fins).
  
  

Buoyancy

• Upward force exerted by a fluid, enabling flotation.
  
  

  • Governed by Archimedes’ Principle: Buoyant force equals the weight of the fluid displaced.
    
    

3.4 Reynolds Number (Re)

The Reynolds number is a dimensionless quantity that predicts flow behavior:

Re = (ρvL) / μ

Where:

• ρ [rhoρ] = fluid density
  
  

• v = velocity
  
  

• L = characteristic length
  
  

• μ [mu] = dynamic viscosity
  
  

• Low Re (< 1): Viscous forces dominate → "sticky" world of microorganisms (laminar flow).
  
  

• High Re (> 1000): Inertial forces dominate → turbulent flow (e.g., fish, birds).
  
  

3.5 Biological Examples of Fluid Mechanics

Swimming

• Fish and aquatic organisms push against water using fins, undulations, or jets.
  
  

• Adaptations like streamlined bodies reduce drag.
  
  

Flying

• Birds and insects use wings to generate lift and control motion through air.
  
  

• Different flight modes: flapping (birds), gliding (flying squirrels), soaring (vultures), hovering (hummingbirds).
  
  

Internal Flows

• Blood flow in arteries and airflow in lungs depend on fluid principles.
  
  

• Cilia and flagella create flow at low Reynolds numbers in microorganisms.
  
  

3.6 Strategies to Control Fluid Forces

Organisms evolve to manipulate fluid mechanics:

• Streamlining to reduce drag (e.g., dolphins, penguins).
  
  

• Vortex generation to enhance propulsion (e.g., jellyfish, birds).
  
  

• Shape-shifting surfaces (e.g., tubercles on humpback whale fins improve lift and delay stall).
  
  

• Surface textures to influence boundary layer behavior (e.g., shark skin reduces drag).
  
  

3.7 Flow Visualization Techniques

Scientists use tools to study biological fluid mechanics:

• Flow tanks (wind tunnels or water channels)
  
  

• Dye tracing and particle image velocimetry (PIV) to observe flow patterns
  
  

• Computational fluid dynamics (CFD) for simulating flow around organisms