A process is disclosed for reducing skin friction and inhibiting the effects of liquid turbulence in a system involving the flow of a liquid along the surface of a body, e.g., a marine vehicle. This process includes injecting a drag reducing polymer into the valleys of adjacent, evenly spaced, longitudinal grooves extending along the length of the surface of the body, so that the rate of diffusion of the polymer from individual grooves into the liquid flow is predictably controlled by the groove dimensions. When the polymer has diffused over the tips of the grooves into the near wall region of the boundary layer, the polymer effectively reduces the turbulent skin friction. A substantial drag reducing effect is achieved with less polymer than must be used to lower skin friction when the surface of the body is smooth

Bushnell, Dennis M.

Reed, Jason C.

English

NASA Technical Reports Server

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https://ntrs.nasa.gov/search.jsp?R=19910004245 2020-03-11T17:49:37+00:00Z
POLYMEWRIBLET COMBINATION FOR HYDRODYNAMIC 
SKIN FRICTION REDUCTION 
AWARDS ABSTRACT 
NASA Case No. LAR 14271 -1 -CU 
A process for reducing skin friction and inhibiting the effects of liquid turbulence in 
systems where a liquid flows along the surface of a body. 
Body 12 contains a multiplicity of adjacent, evenly spaced, longitudinal riblets 8. The 
dimensions of riblet 8 are scaled to the local viscous scaling, s+, which is a function of 
vehicle velocity and position of riblets 8 on the surface downstream from channels 2. 
Drag reducing polymer 10 is injected as deeply as possible into valleys 16 of riblets 8 
through channels 2. This injection is repeated at intervals downstream because the 
volume of riblets 8 is finite and drag reducing polymer 10 will become depleted as it is 
slowly pulled from riblets 8 by turbulence. As liquid 4 flows by riblets 8, drag reducing 
polymer slowly diffuses out of riblets 8 and over the tips of riblets 8 into the near wall 
region of the boundary layer where it effectively reduces the turbulent skin friction. 
The novelty of the invention resides in the use of precisely scaled grooves and multiple 
injection channels so that minimal amounts of drag reducing polymer need to be injected 
for large amounts of skin friction reduction. While riblets and drag reducing polymer have 
been used separately to reduce skin friction, they have not before been used together in 
a in such a way that the riblet geometry is specifically designed to release minimal 
amounts of polymer in a controlled and predictable fashion. 
Inventors: Dennis M. Bushnell Jason C. Reed 
Address: P.O. Box 176 40 Finns Pt. 
Haves,72 23669 SS#: 
Employer: NASA-Langley Old Dominion Univ. Research Foundation 
Evaluator: John 9. Anders 
Serial Number: 567,025 
Filed: August 14, 1990 
NASA LAR 14271 -1 -CU -1 - PATENT APPLICATION 
POLYMER/RIBLET COMBINATION FOR HYDRODYNAMIC 
SKIN FRICTION REDUCTION 
ORIGIN OF THE INVENTION 
5 The invention described herein was made jointly in the performance of 
work under a NASA contract and by an employee of the United States 
Government. In accordance with 35 U.S.C. 202, the contractor elected not to 
retain title. 
10 BACKGROUND OF THE INVENTION 
1. - Field of the Invention 
This invention relates generally to systems wherein a liquid flows along 
a surface of a body. It relates particularly to a process for reducing skin friction 
and inhibiting the effects of liquid turbulence. 
15 
2. - Description of Related Art 
Skin friction drag accounts for a sizable portion of the hull drag for both 
surface and fully submerged marine vehicles. Reducing this drag component 
would have the obvious advantages of increased speed and/or efficiency. One 
20 approach to skin friction drag reduction involves the use of long chain polymer 
molecules, which can reduce up to 80% of the skin friction under ideal 
circumstances. 
Although polymers generate large friction reductions, their use on marine 
vehicles has been limited by the large mass injection rate needed to maintain 
25 a sufficiently high polymer concentration close to the surface of the body. 
Turbulent diffusion in the boundary layer rapidly disperses polymer after it is 
injected from the body, causing much of the polymer to leave the boundary 
region close to the surface where the polymer drag reduction mechanism 
operates. Under these conditions, the amount of polymer that must be injected 
NASA LAR 14271 -1 -CU -2- PATENT APPLICATION 
is uneconomical when compared to the cost of fuel needed to overcome the 
drag. 
Wells in "An Analysis of Uniform Injection of a Drag-reducing Fluid into 
a Turbulent Boundary Layer," Viscous Drag Reduction (Proceedings of the 
5 Symposium on Viscous Drag Reduction), 1969, p. 361, has shown that a 
distributed injection of polymer over the entire surface rather than discrete 
injection of polymer from holes or a series of slots can produce local polymer 
concentration high enough to result in large drag reduction, but at less than 
one-tenth of the mass injection rate required by a discrete injection. While this 
10 approach allows the polymer injection rate to become economical compared to 
fuel costs, it also requires a uniformly porous surface over the entire body of 
the vehicle. This is difficult, if not totally unfeasible, to achieve on large vessels 
such as ships and submarines. Wells makes no mention of using the polymer 
in combination with a grooved surface to achieve better drag reduction. 
15 Walsh, US 4,706,910, discloses a method of reducing drag which uses 
micro-geometry longitudinal grooving of the flow surface. Walsh differs from 
the present invention by using the grooves alone to reduce surface drag, 
whereas the present invention uses surface grooves in combination with 
polymer to reduce the drag. 
20 Polymers have been combined with small longitudinal grooves (riblets) 
in two studies by the United States Navy, and by K.S. Choi, et al of British 
Maritime Technology. All the experimenters were looking for possible 
synergistic effects between the riblet and polymer mechanisms. Their results 
were as follows: 
25 (1) Both sets of Navy experimenters, Reidy and Anderson, "Drag 
Reduction for External and Internal Boundary Layers using Riblets and 
Polymers," AlAA Paper No. 88-031 8; and Beauchamp and Phillips, "Riblet and 
Polymer Drag Reduction on an Axisymetric Body," AlAA Paper No. 88-0138, 
used a "polymer ocean" in their experiments and did not inject the polymer from 
30 the test surface. They neither contemplated nor provided data on the use of 
NASA LAR 14271 -1 -CU -3- PATENT APPLICATION 
the riblet surface to control the turbulent diffusion of the polymer near the test 
surface. Instead, these studies only indicated that there were additive skin 
friction reductions for polymer and riblet combinations. The Navy experimenters 
also theorized only one injection point on the surface. As a result, large 
5 amounts of polymer would be required to overcome the rapid diffusion of 
polymer out of the boundary layer close to the surface. Nor did they examine 
the geometry of the riblets. The present invention, however, combines a study 
of the optimal groove measurements with multiple injection sites to minimize the 
amount of polymer needed to reduce drag effectively and economically by using 
10 the groove properties to limit the problem of polymer diffusion. 
(2) K.S. Choi, G.E. Gadd, H.H. Pearcey, A.M. Savill, and S. Svensson, 
"Drag Reduction With A Combined Use of Riblets and Polymer Coating," 
Additiveand Passive Devices: Combination Studies, 1 989, p. 271, coated the 
riblet surface with an ablative polymer material which slowly released polymer 
15 into the flow close to the surface. The slow release of the polymer from the 
surface was entirely a result of the nature of the polymer ablative coating, not 
the riblet geometry. Choi neither contemplated nor provided data pertaining to 
the use of the riblet geometry to control the polymer diffusion. Not only did this 
technique produce much less than optimum polymer drag reduction (10% vs. 
20 80% possible, indicating inefficient release and dispersion), but over time the 
sharp groove peaks will be worn down. As a result, the grooves will become 
much less effective in reducing drag. In the present invention, polymer is 
injected into the grooves, not coated on the surface. The present invention is 
specifically designed to use the riblet geometry to control release of polymer 
25 after it has been injected. 
SUMMARY OF THE INVENTION 
The present invention is a process for reducing skin friction and inhibiting 
the effects of liquid turbulence in systems involving liquid flow along a surface 
30 of a body. It has special utility in the development of marine vehicles. 
NASA LAR 14271 -1 -CU -4- PATENT APPLICATION 
According to the present invention, a drag reducing polymer is injected into 
longitudinal grooves on the surface of a hydrodynamic body. The polymer 
diffuses into the flow around the body at a rate controlled by the dimensions of 
the grooves. As the polymer reaches the near wall region of the boundary 
5 layer of the flow around the body at a point just above the grooved surface, the 
polymer begins to effectively reduce the turbulent skin friction. If the geometry 
and dimensions of the grooves are varied, the changes in the diffusion rate of 
the polymer produces different distributed injection characteristics. Similarly, 
varying the number of injection sites also changes the amount of polymer 
10 needed for effective skin friction reduction. 
BRIEF DESCRIPTION OF THE DRAWINGS 
For a more complete understanding of the present invention, including 
its primary object and attending benefits, reference should be made to the 
15 Description of the Preferred Embodiments, which is set forth below. This 
description should be read together with the accompanying drawings, wherein: 
FIG. 1 represents an optimized flow pattern for a diffusing polymer in 
accordance with the present invention; 
FIG. 2 is a cross sectional view of the optimized flow pattern of FIG. 1 
20 showing the polymer rising out of riblets 8; and 
FIG. 3 graphically represents the effect of variation in width of the 
grooves or valleys between riblets. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
25 Referring now to the drawings, FIG. 1 represents a side view of the 
optimal flow pattern generated by riblets 8 and drag reducing polymer 10. The 
surface of body 12 contains riblets 8. Drag reducing polymer 10 is injected into 
riblets 8 from body 12 through widely spaced channels 2. As liquid 4 flows by 
riblets 8, drag reducing polymer 10 slowly diffuses out of riblets 8 and into liquid 
30 4. 
NASA LAR 14271 -1 -CU -5- PATENT APPLICATION 
FIG. 2 represents a cross sectional view of FIG. 1 along line 2-2'. 
Arrows 14 representing low momentum flow demonstrate how drag reducing 
polymer 10 has risen out of riblets 8. 
FIG. 3 graphically represents how the size of riblets 8 influences the 
5 diffusion of dye, representing drag reducing polymer 8, from riblets 8. The 
viscous scaling, s+, is plotted against x/s, the number of riblet 8 widths that it 
takes until the dye has been completely diffused from riblets 8. The 
nondimensional units used demonstrate that as riblet 8 widths increase with the 
velocity of liquid 4 held constant, the viscous scaling increases and x/s 
10 decreases. Similarly, if the velocity of liquid 4 is increased and the riblet 8 
width is held constant, s+ will increase and x/s decrease. This occurs because 
the velocity of liquid 4 and the riblet 8 widths are both proportional to the 
viscous scaling. 
In the present invention, body 12 contains a multiplicity of adjacent, 
15 evenly spaced, longitudinal riblets 8. Riblets 8 can be a variety of geometries, 
such as triangular. See FIG. 2. Dimensions of riblets 8 must be scaled to the 
local viscous scaling, s+, which is a function of vehicle velocity and position of 
riblets 8 on the surface downstream from channels 2. Drag reducing polymer 
10 is injected into valleys 16 of riblets 8 through channels 2. See FIG. 2. Drag 
20 reducing polymer 10 can be, for example, POLYDOXB as marketed by Union 
Carbide Company, or any other high molecular weight, water soluble polymer 
suitable for the purpose. Channels 2 should be angled in a direction parallel 
to the flow of liquid 4 in order to put drag reducing polymer 10 as deeply as 
possible into valley 16 of riblet 8. Injection of drag reducing polymer 10 must 
25 be repeated at intervals along the length of body 12 because the volume of 
riblets 8 is finite and drag reducing polymer 10 will become depleted as it is 
slowly pulled from riblets 8 by turbulence. This is necessary for the function of 
the present invention because drag reducing polymer 10 must reach over the 
tips of riblets 8 into the near wall region of the boundary layer in order for drag 
NASA LAR 14271 -1 -CU -6- PATENT APPLICATION 
reducing polymer 10 to effectively reduce the turbulent skin friction. See FIG. 
1. 
Riblets 8 produce a local turbulence intensity smaller than that of a 
simple flat surface. The walls of riblets 8 impart a transverse frictional force to 
5 any liquid passing through them. See FIG. 2. This makes for a quiescent flow 
that lowers the diffusion rate of that liquid into the flow above riblets 8. To 
produce the most effective low momentum flow for the present invention, the 
s+ of riblets 8 should be less than 20. See Fig 3. Thus, drag reducing polymer 
10 inside of riblets 8 is also affected by this lower local turbulence, and diffuses 
10 more slowly into liquid 4 than when placed on a flat surface. By changing the 
dimensions of riblets 8, the local turbulence intensity can be varied. As a 
result, the rate at which drag reducing polymer 8 diffuses can be controlled, as 
shown by the dye in FIG. 3. Optimum riblet 8 scaling will produce an effect 
whereby drag reducing polymer 10 is slowly distributed over a predictable 
15 length at a predictable rate. Drag reducing polymer 8 can be injected into 
riblets 8 in much less quantity than needed for a comparably sized body with 
a flat surface, because the turbulence supplied by the flat body would more 
rapidly diffuse much more of drag reducing polymer 10 away from the near wall 
boundary layer where effective skin friction reduction occurs. Riblets 8 also act 
20 as a storage area for drag reducing polymer 10 until it is diffused. With correct 
scaling of riblets 8, a predictable amount of drag reducing polymer 10 slides 
down the length of valley 16 of riblet 8. This allows diffusion of equal amounts 
of drag reducing polymer 10 over a broad area of the surface. Therefore, the 
drag reducing polymer acts as if there was distributed injection over the entire 
25 length of body 12, without the requirement of a porous surface over the entire 
vessel. See FIG. 1. 
Alternate embodiments include varying the geometry of riblet 12 to 
produce different distributed injection characteristics for use in different types 
of turbulence. Moreover, any of a number of methods to place drag reducing 
30 polymer 12 in riblets 8 are feasible, including slots, several rows of small holes, 
NASA LAR 14271 -1 -CU -7- PATENT APPLICATION 
slots with a shroud to produce injections directed parallel to the surface, as well 
as channels 2. Riblets 8 can also contain a rough surface or small surface 
projections, such as baffles, which act to increase the frictional holding power 
of the riblets, allowing riblets 8 be deepened without a change in other 
5 characteristics. 
The present invention produces skin friction reductions as large as any 
polymer method applicable to marine vehicles. In addition, it is capable of 
achieving this drag reduction at mass injection requirements considerably less 
than that of normal methods. This approaches the efficiency of distributive 
10 injection without the need of a porous injection surface over the entire wetted 
area of the vehicle. 
It should become obvious to those skilled in the art that this invention is 
not limited to the preferred embodiments shown and described. 
What is claimed as new and desired to be secured by Letters Patent to 
15 the United States is: 
NASA LAR 14271 -1 -CU PATENT APPLICATION 
POLYMERIRIBLET COMBINATION FOR HYDRODYNAMIC 
SKIN FRICTION REDUCTION 
- 
- 
- 
ABSTRACT OF THE DISCLOSURE 
5 A process,for reducing skin friction and inhibiting the effects of liquid 
turbulence in a system involving the flow of a liquid along the surface of a body, 
. - - L r  P l r  I - - ? ,  . - 4 i c  i - r  i * 
e.g. a marine vehicle, a . mcludes injectikg a drag reducing polymer into the 
valleys of adjacent, evenly spaced, longitudinal grooves extending along the 
length of the surface of the body, so that the rate of diffusion of the polymer 
10 from individual grooves into the liquid flow is predictably controlled by the 
groove dimensions. When the polymer has diffused over the tips of the 
grooves into the near wall region of the boundary layer, the polymer effectively 
reduces the turbulent skin friction. A substantial drag reducing effect is 
achieved with less polymer than must be used to lower skin friction when the 
15 surface of the body is smooth. 
FIG. 1 
FIG. 2 
FIG.  3 


Polymer/riblet combination for hydrodynamic skin friction reduction

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