DRAG REDUCTION IN INTERNAL FLOWS
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DRAG REDUCTION IN INTERNAL FLOWS. FOR CENTURIES CONVENTIONAL WISDOM. BELIEVED THE SMOOTH SURFACE OFFERED LEAST RESISTANCE. FALSE. Skin Friction Drag Reduction via riblets. “smooth pipe”. riblets. K s /D = /D = 0.0726/12.7 = 0.006; Rohr K s /D = /D = 0.1143/50.8 = 0.002: Liu.

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FOR CENTURIES CONVENTIONAL WISDOM

BELIEVED THE SMOOTH SURFACE OFFERED LEAST RESISTANCE



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


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.”


s+ = su*/

h+ = hu*/


“My” data

h+ = hu*/

s+ = su*/

h = s


h+ = hu*/

s+ = su*/

h+ = s+ ~ 13

k* = u*/

k* < 4: hydraulically smooth

4 < k*< 60 transitional regime

k* > 60 fully rough (no  effect)

White 1991 – Viscous Fluid Flow


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



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”



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): the addition of guar gum (a plant derivative),

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.


“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 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.”

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


H 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.” 2O

30 ppm PEO

Jet (dyed)

Bath (clear)


water 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.”

¼”

( nozzle exit)

water

water + 50 ppm PEO

¼”


water 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.”

( 1 meter from nozzle exit )

water

water + 200 ppm PEO


( 1 meter from nozzle exit ) 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.”


THE END 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.”


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