DRAG REDUCTION IN INTERNAL FLOWS

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DRAG REDUCTION IN INTERNAL FLOWS
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FOR CENTURIES CONVENTIONAL WISDOM
BELIEVED THE SMOOTH SURFACE OFFERED LEAST
RESISTANCE
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FALSE
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Skin Friction Drag Reduction via riblets
smooth pipe
riblets
Ks/D ?/D 0.0726/12.7 0.006 Rohr Ks/D ?/D
0.1143/50.8 0.002 Liu
riblets
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Silky shark Caroharhinus falciformis
3M-riblets
Shark scales are 0.2 0.5 mm in height and are
aligned in flow direction ridge spacing may be
from 35 105 ?m
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Johnson (1970) Large drag measured - probably
due largely to the rough, sandpaper-like skin
of the shark
Bechert (1985) It appears that the fins might
have produced enormous parasitic drag and/or the
dead sharks may not have been towed at zero
angle of attack.
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s su/?
h hu/?
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My data
h hu/? s su/? h s
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h hu/? s su/? h s 13
k ?u/? k lt 4 hydraulically smooth 4 lt k lt
60 transitional regime k gt 60 fully rough (no ?
effect) White 1991 Viscous Fluid Flow
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1 -2 reduction in fuel if 70 of airplane
covered French Airbus
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HOW DO THEY WORK?
(BEATS ME, BUT ..)
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How do riblets provide reduction in skin
friction?
Hama (1957) noticed low speed dye streaks in the
sublayer of a turbulent boundary layer
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Free Stream Velocity 20.4 cm/s
Low speed streaks in viscous sublayer
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Smooth surface
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Less low-speed streak wavering flow within
riblets is slow and quiescent
effect of riblets is to modify and reduce the
momentum exchange properties caused by the
streamwise vortices developing near the
surface beneath a turbulent boundary layer
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AMAZING DRAG REDUCTION WITH POLYMERS
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PEO
Polymer drag reducing effect associated with
the stretching of the polymer molecules
thought to increase elongational viscosity
Used for fire fighting to reduce spray,
storm-sewer augmentation, pipelines, drag
reduction on ships
Fish also use polymer drag reduction
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Toms 1947 Friction reductions of up to 50
Forrest Grierson 1931, Friction Losses in
Cast Iron Pipe Carrying Paper Stock . Mysels
1949, Patent 2,492,173 Flow of Thickened Fluids
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In the early 60s oil companies began to notice
that the addition of guar gum (a plant
derivative), used to suspend sand in high
pressure sand-water mixtures greatly decreased
skin friction. This phenomena was brought to the
attention of the Navy.
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PEO (1963-Fabula, Hoyt Crawford) Only a few
mg/L necessary for significant drag
reduction. Molecular Weight 4 X 106 1 ppm
18 drag reduction Push transition to higher Re
numbers.
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The spectacular reduction of turbulent energy
losses by the addition of small amounts of
certain polymers is a phenomenon that is still
ill understood, in spite of the enormous
attention the subject has attracted over the
past few decades. Bonn et al. J. Phs. Cond.
Matter 2005 Vol. 17, S1195-S1202
Polymer drag reduction is a boundary layer effect
related to the increased elongational viscosity
(several orders of magnitude) of the mixture,
which interfere with bursting process.
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PEO
Polymer drag reducing effect associated with
the stretching of the polymer molecules
thought to increase elongational viscosity
Used for fire fighting to reduce spray,
storm-sewer augmentation, pipelines, drag
reduction on ships
Fish also use polymer drag reduction
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H2O
30 ppm PEO Jet (dyed) Bath (clear)
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water
¼
( nozzle exit)
water
water 50 ppm PEO
¼
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water
( 1 meter from nozzle exit )
water
water 200 ppm PEO
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( 1 meter from nozzle exit )
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THE END