aerodynamics drag
TRANSCRIPT
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Aerodynamics II: Drag
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Viscous Drag
Reynolds number is a dimensionless parameter
important in determining the viscous drag on a body.
The units of viscosity are kg/m s
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The Boundary Layer
Increasing Reynolds number
The turbulent boundary layer has higher velocities, and is
rougher, but it also has more energy than the laminarboundary layer.
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Viscous Drag, cont.
Viscous drag is made up of skin friction drag and
pressure drag.
Skin friction is the frictional force associated with theair rubbing against the surface of the vehicle.
Generally, the skin friction component of the viscous
drag becomes larger at higher Reynolds number.This is because the turbulent boundary layer has
higher velocity and causes more vigorous rubbing
of the air on the surface.
Other contributors to high skin friction drag are:
Rough surface
Large surface area
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Viscous Drag, cont.
Pressure drag occurs when the pressures betweenthe fore and aft facing surfaces do not balance:
Generally, the pressure drag decreases as the
Reynolds number increases. This is because theturbulent boundary layer has more energy and can
remain attached to the body surface more easily than
the laminar boundary layer. Streamlining helps to reduce pressure drag.
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Pressure Drag vs Skin Friction Drag
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Aside: Separation
When a wing is at a high enough angle, the boundary layer cannot
hold on to the top surface any longer. When separation occurs,the wing is said to have stalled. A turbulent boundary layer can
remain attached more easily than a laminar one. Thus higher
Reynolds number leads to higher stall angle and, therefore, higher
maximum lift coefficient.
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Induced Drag
Net Aerodynamic ForceLift
Induced Drag
The action by the wing is actually down and slightly
forward, creating a reaction that is up (Lift) and
slightly rearward (Induced Drag). The amount of
induced drag depends on the amount of lift
generated.
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Estimating Induced Drag
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Exercise - Total Incompressible Drag
The Embraer Brasilia is flying at the conditions given
in the previous exercise. What is the total drag of theaircraft?
b = 19.78 m m = 11,000 kg S= 39.4 m2
= 0.771 kg/m3 CDP= .022
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Major Concepts
Incompressible Drag Parasite drag from viscous effects
Induced drag dependent on lift
Parasite Drag Skin friction (increases with Reynolds Number)
Pressure drag (decreases with Reynolds Number)
Reynolds Number is a dimensionless quantity
Low Reynolds number (< 500,000) laminar boundarylayer; high Reynolds number (>500,000) turbulentboundary layer
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Major Concepts, cont.
Wing terminology
AR = b2/S= Aspect Ratio
Induced Drag depends on amount of lift produced
Definition of drag coefficient: