DRAG REDUCTION IN INTERNAL FLOWS

FOR CENTURIES CONVENTIONAL WISDOM BELIEVED THE SMOOTH SURFACE OFFERED LEAST RESISTANCE

FALSE

“smooth pipe” K s /D =  /D = /12.7 = 0.006; Rohr K s /D =  /D = /50.8 = 0.002: Liu riblets Skin Friction Drag Reduction via riblets

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

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.”

h + = hu * / s + = su * /

h + = hu * / s + = su * / h = s “My” data

k* =  u * / k * < 4: hydraulically smooth 4 < k * < 60 transitional regime k * > 60 fully rough (no  effect) White 1991 – Viscous Fluid Flow h + = hu * / s + = su * / h + = s + ~ 13

1% -2% reduction in fuel if 70% of airplane covered French Airbus

HOW DO THEY WORK? (BEATS ME, BUT …..)

How do riblets provide reduction in skin friction? Hama (1957) noticed low speed dye streaks in the sublayer of a turbulent boundary layer

Free Stream Velocity = 20.4 cm/s Low speed streaks in viscous sublayer

Smooth surface

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”

AMAZING DRAG REDUCTION WITH POLYMERS

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

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

In the early ’60’s 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.

PEO (1963-Fabula, Hoyt & Crawford): Only a few mg/L necessary for significant drag reduction. Molecular Weight ~ 4 X 10 6 ; 1 ppm ~ 18% drag reduction Push transition to higher Re numbers.

“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.

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

H2OH2O 30 ppm PEO Jet (dyed) Bath (clear)

water + 50 ppm PEO water ( nozzle exit) ¼”

water ( 1 meter from nozzle exit ) water ppm PEO water

( 1 meter from nozzle exit )

THE END