1. Resistance in Fluid Systems

2. Objectives Define drag
Explain the difference between laminar and turbulent flow.
Explain the difference between frictional drag and pressure drag.
Define viscosity and explain how it can be measured.
Use Stoke’s law and Poiseuille’s Laws to solve problems involving fluid resistance.

3. Review
Frictional force – the opposing force resulting when one solid slides against another.
Drag – the opposing force of an object in a fluid…. or

4. Drag
Drag is defined as the force found when a solid object moves through a fluid, and there is a force that opposes the motion.
Example: When an airplane moves through air.
What is another example of drag?

5. Understanding Drag Thrust = Drag
Drag is an aerodynamic force that resists the motion of an object moving through a fluid.

6. Laminar Flow
Laminar or streamlined flow is a slow, smooth flow over a surface, in which the paths of individual particles do not cross. These particle follow theoretical layers of fluid.
Laminar Flow creates frictional drag
What is an example of Laminar Flow on an object?

7. Turbulent Flow
Turbulent flow is an irregular flow that is disrupted causing fluid to move in different directions.
Turbulence causes a wake behind a moving object.
Turbulent flow creates pressure drag
What is an example of Turbulent Flow on an object?

8. Frictional Drag
Frictional Drag is produced by the friction between successive or laminated layers of fluid, with increasing speed away from the surface.
When does Frictional Drag take place?

9. Pressure Drag
The pressure difference found in turbulent flow causes a force to act on the object in the direction opposite its relative velocity.
When does Pressure Drag take place?

10. Understanding Drag
Both Frictional and Pressure Drag increase as the speed of an object increases.
When turbulence is created, pressure drag increases more rapidly than friction drag.

11. Stokes’ Law
Irishmen George Stokes used viscosity and the equations of fluid flow to predict the drag force on a sphere moving through a fluid.
Stokes’ Law applies to objects moving at low enough speeds that the flow of fluid is streamlined or laminar.

12. Stokes’ Law Fdrag = 6rv 6 =Constant for sphere R = Radius of object
V = Speed of object
 = Fluid’s Viscosity

13. Stokes’ Law
Fdrag = 6rv
Example:

14. Poiseuille’s Law
Poiseuille was a physician, who experimented with flowing water, and learned that the rate at which fluid flows through a tube increases proportionately to the pressure applied the fourth power of the radius of the tube.
Poiseuille’s Law gives the volume flow rate of a fluid flowing through a tube or pipe.

15. 3 Factors of Resistance
In fluid flow resistance decreases the flow rate though a pipe. Poiseuille’s Law shows how this resistance depends on three factors:
Dependence on Radius
Dependence on Length
Dependence on Viscosity

16. Dependence on Radius
The larger the radius of a pipe, the greater volume of fluid per second
Fluid resistance decreases as pipe radius and cross-sectional area increase.

17. Dependence on Length Longer pipes have higher fluid resistance
Volume flow rate is inversely proportional to length

18. Dependence on Viscosity
Abrupt changes in the direction of fluid flow can cause turbulence and increase resistance
Fluid resistance increases as viscosity increases.

19. Poiseuille’s Law V = -  r4 8 L R = Radius of Pipe or Tube
 = P2 – P1
= Viscosity
L = Length of Pipe or Tube

20. Poiseuille’s Law
V = -  r4 
8 L
Example:

21. Viscosity
What is Viscosity??

22. Viscosity
Friction between two solid surfaces causes a resistance to movement between the surfaces.
Viscosity is the property of a fluid that describes its internal friction.

23. Viscosity
What are some items that might have a higher viscosity? A lower viscosity?

24. Viscosity
Different fluids resist motion differently, and therefore have different viscosities.
Gum and Molten Lava have high viscosities
Air and Water have a much lower viscosity

25. Viscosities of Common Fluids
Viscosity of most liquids decreases as temperature increases.
Viscosity of most gases increase with temperature
Example:
Cold honey is thick with a high viscosity
Hot honey is watery with a low viscosity
SAE (Society of Automotive Engineers) naming convention for motor oils:
10W – The viscosity of the oil when measured at 0 degrees F
(the W means winter grade)
30 – The viscosity of the oil when measured at 212 degrees F.
Physics in Context textbook
Pg. 188 Chapter 4, Table 4.2

26. Review Questions
What are the two types of drag and give an example of each?
What is the main difference between laminar and turbulent flow?
Explain viscosity and give an example.
Textbook pages 196 – 198, questions

27. Credits Basic presentation from Mr. Andracke