ME 101: Fluids Engineering

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ME 101Fluids Engineering

• Chapter 6

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Two Areas for Mechanical Engineers

• Fluid Statics
• Deals with stationary objects
• Ships, Tanks, Dams
• Common calculations
• Pressure
• Buoyancy
• Fluid Dynamics
• Either fluid or object is in motion
• Calculations include
• Flow Rate, Velocity, Drag Force, Lift Force, etc.

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Mechanical Engineers

• Typical fluids
• Water, Air, Oil, Nitrogen, Coolants, etc.
• Why is it important?
• 98 of electricity in US is generated by some
  form of fluid process (hydroelectric, steam
  turbines, wind)
• Aeronautics
• Biomedical

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What is a Fluid?

• Substance unable to resist a shear force without
  moving
• Deforms continuously when subjected to a shear
  stress
• Motion continues until force is removed
• Flow Response of a fluid to shear stress
  that produces a continuous motion

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Two types of Fluids

• A liquid is an incompressible fluid
• Water, Oil, Coolants, Gasoline, etc.
• A gas can be easily compressed
• Air, Nitrogen, Propane, etc.

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Properties of Fluids

• What is a fluid shear force?
• Example Consider a deck of cards
• Top card moves the most, bottom card is
  stationary
• No-slip at solid-fluid boundary stationary
• Each layer moves at different speed

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Newtonian Fluid
Applied force balanced by shear stress exerted by
the fluid on the plate
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Viscosity
? - measure of friction or resistance to shear
force
Honey has higher viscosity than water
Often see cP (centipoise) Water 1cP at Room
Temperature
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What happens when fluids interact with solids?

• The forces created are known as buoyancy, drag,
  and lift
• Buoyancy is the force developed when a solid
  object is immersed in a fluid (no relative
  motion)
• Lift and Drag forces arise when fluids interact
  with a solid object (relative motion)

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Why Does Pressure Increase with Depth?
Pressure grows in direct proportion to the depth
and density of the fluid
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Buoyancy
Buoyancy force is related to the weight of the
fluid displaced
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Laminar and Turbulent Flows
Laminar Flow
Turbulent Flow
Irregular flow pattern fluid moving fast, flow
patterns break up, become random
Fluid flows smoothly associated with slow
moving fluids (relatively)
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What determines laminar or turbulent flow?

• Must consider the following
• Size of object moving through fluid (or size of
  pipe/duct fluid is flowing through)
• Speed of object (or of fluid)
• Density and viscosity of fluid
• Exact relationship among these variables
  discovered by British engineer Osborne Reynolds
• Reynolds number
• Dimensionless parameter describes that transition

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Reynolds Number

• l is a characteristic length pipe diameter,
  diameter of sphere, diameter of air duct, etc.
• ? is velocity
• ? is density
• µ is viscosity
• Ratio between the inertia (density related) and
  viscous forces (viscosity related) acting within
  a fluid
• When fluid moves quickly or is not very viscous
  or dense, Re large, inertia disrupts the flow
  turbulent
• When fluid is slow, very viscous, or very dense,
  Re is small, viscous effects stabilize the fluid
  laminar

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Reynolds Number
Flow is turbulent when Re gt 4000 Flow is laminar
when Relt2000
Experiments and detailed computer simulations
necessary to understand complexity of fluids
flowing in real hardware at real operating speeds
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Dimensionless Numbers

• Reynolds Number
• Poissons Ratio
• Mach Number

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Pipe Flow

• Fluids flow from high pressure to low pressure
• Flow develops shear stress at boundary
• Shear stresses balance pressure differential

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Laminar Pipe Flow

• Laminar velocity distribution for any point
  across the cross-section

Re lt 2000
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Pipe Flow

• Volumetric flow rate, q (volume/time)
• Often more interested in knowing the volume of
  fluid flowing through a pipe during a certain
  time interval
• For steady, incompressible, laminar flow, the
  volumetric flow rate in a pipe is

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Volumetric Flow Rate
Conservation of Mass Incompressible Fluid
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Aerodynamic Forces
For straight and level flight Lift
Weight Thrust Drag
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Drag Force

• Resists high-speed motion through fluid (air or
  water)
• CD quantifies how streamlined an object is
• Valid for any object or flow
• Drag force is parallel to direction of fluid flow

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Lift Force

• Lift due to pressure differences between upper
  and lower surfaces
• Lift force increases with increasing angle of
  attack
• Lift force is perpendicular to direction of fluid
  flow

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Airplane Wing Turbulent Flow
Stall Condition