Form Drag and Separation

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Form Drag and Separation
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Overview
Total drag Friction drag Form drag
Friction drag is important for slender bodies
aircraft wings and fuselage, ships hulls
Form drag is important for bluff bodies
lorries, oil platform legs, bridge piers
Form drag occurs when a separated wake is formed.
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Separation
u-component
Streamlines
Consider b.l flow over a flat plate in the
presence of an adverse pressure gradient
U(x)
u0
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Flow around a cylinder
U?
?
Streamlines closer together indicates higher
velocity Higher velocity means lower pressure
(Bernoulli)
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Flow around a cylinder
At the front of the cylinder
No separation occurs (note x is measured around
cylinder)
At the rear of the cylinder
Separation occurs leading to vortex shedding
s
Larger wake
s
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Cylinder at Re9.6
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Relt5
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Cylinder at Re24.3
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Regt10
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Cylinder R2000
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Regt50
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Vortex shedding

• Depending on the Reynolds number, the wake can
  consist of
• trapped vortices
• alternate shedding (a vortex street)
• complete turbulence
• Separation may be induced by sharp edges
• vehicles no longer have guttering
• fitting ... improves stability and handling

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Separation points Re20
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Separation Points (Regt200)
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Separation points Re500
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Cylinder Wake Relt1
Flow is symmetric - downstream same as
upstream Friction drag dominates
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Cylinder Wake Regt5
Appearance of trapped vortices in lee of
cylinder. Friction drag still dominant, but some
contribution from wake.
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Cylinder Wake Regt50
Vortex shedding downstream (von Karman
street). Very large wake. Low pressure in lee
gives dominant form drag. Separation points
fluctuating gives severe time-dependent lift
forces (Strouhal Number)
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Cylinder Wake 103-105
Shed vortices decay immediately. Drag
coefficient constant over this range. Separation
points at 900 ensures maximum form drag.
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Cylinder Wake Regt106
Boundary layer turbulent separates later. Much
lower drag coefficient than high Re laminar flows.
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Cylinder Drag Coefficient
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Streamlining
Small increase in back pressure cannot overcome
b.l. momentum
s
?
Increasing the angle of attack, ?, increases the
expansion over the upper surface leading to a
rapid increase in back pressure causing
separation and stall.
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Streamlined Aerofoil
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Aerofoil separation
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Preventing separation

• The max angle of attack of a flat plate 10o
• Turbulent boundary layers carry more momentum
  (use upstream trigger)
• Inject air to increase b.l. momentum
• Remove slow part of b.l. (suction)
• Can increase angle of attack to 20o

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Laminar v. Turbulent
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Modified aerofoil
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Form drag

• Form drag is dependent on the width of the wake
  and the consequent loss of momentum
• Form drag is generally complex to determine and
  should be measured in a wind tunnel.
• Form drag may be reduced significantly by
  appropriate streamlining