BIOD43H3 Lecture Notes - Lecture 10: Tetraodontidae, Parasitic Drag, Flow Separation
BIOD43 Lecture 10 Notes: Movement in Fluid Part 2
Key Points
Steady Flow-Drag and Streamlines
• Faster a fluid moves the greater the pressure, creating a pressure gradient and pressure
drag. (problem for high Re numbers-turbulence)
• Reynold number (Re) is used in fluid mechanics to help predict flow patterns in different
fluid flow situations.
-When Re is high pressure drag is the main component of drag
Q: How to deal with increased Pressure Drag?
A: Streamlining, which makes water path easier by keeping flow attached throughout the
animal. Streamlined designs reduce pressure drag by discouraging flow separation or
causing it to occur further behind animal.
Ex. Reef Puffer Fish have a round shape, and swim slowly. They do not need to move
quickly because they have other defenses such as the spikes on their body. If Reef Puffer
Fish swam fast they would experience high levels of pressure drag, because they do not
have streamline shape. In contrast, Tuna fish are streamlined and are able to swim very
quickly, combating pressure drag.
• Fluids resist shear
• Boundary layer does not move
• Fluid velocity speeds up as we move away from the surface of the object. Therefore, near
fluid layers shear past each other imposing viscous drag.
• At higher Re, structure of shear vortices can breakdown allowing flow to become
turbulent, leading to greater energy lost!
• Bernoulli’s Principle----Pressure decrease occurs as speed of fluid increases.
(Increase in Fluid SPEED = Decrease in Fluid Potential Energy)
Ex. Velocity is faster through a narrow tube compared to a wide tube
• Lift Vector is PERPENDICULAR to direction of flow
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Document Summary
Biod43 lecture 10 notes: movement in fluid part 2. When re is high pressure drag is the main component of drag. A: streamlining, which makes water path easier by keeping flow attached throughout the animal. Streamlined designs reduce pressure drag by discouraging flow separation or causing it to occur further behind animal. Reef puffer fish have a round shape, and swim slowly. They do not need to move quickly because they have other defenses such as the spikes on their body. Fish swam fast they would experience high levels of pressure drag, because they do not have streamline shape. In contrast, tuna fish are streamlined and are able to swim very quickly, combating pressure drag: fluids resist shear, boundary layer does not move, fluid velocity speeds up as we move away from the surface of the object. Velocity is faster through a narrow tube compared to a wide tube: lift vector is perpendicular to direction of flow.
