The influence of natural polymers on fluid friction and cavitation

TEE ThThUENCE OF NATURAL POLYMERS OÑ FLÚID FIcTION AND CAVITAT1ÖN

J. W. Hoyt

U. S. Naval Ordnance Test Station Pasadena, California, USA

For many years Nava], architects have regarded the ocean which their

ships traverse, as a non-living, inanimate substance having well-defined and

reproducible properties such as density and viscosity, which cotild be

pre-dicted in advance, and about which there was no question. Similar co±icepts were held regarding the water contained in towing-tanks and water-tunnels.

But to the student of the marine environment, the ocean is a "living broth" in which the total amount of photosynthesis equals that of the land, and the production of pròtein througn the food chain of the sea goes on continu-ously in a way so enormous as to be almost unimaginable.

Tb asse the great living processes of the ocean have no influence on man-made ships, other than perhaps biological fouling of hulls, has

lead, of course, to useful results But, there have been some puzzling anoma1es

Occasionally, the trials of supposedly identical sister-ships, tested, some months

apart, yield fairly vide variations in power required for a given speed, much

more than bould.. reasonably be expected fröm. shipyard tolerance, deviations.

On a. smaller scale, the resistance of ship-models measured In

towing-tanks sometimes seems to vary with no explanation other than a "change in the resistance quality of the water " Occasionally, there have been periods in

some towing-tanks in which resistance "storms" occurred when the apparent friction was'greatly decreased.fora length of time extending to months.

The recent discovery that'dilute high molecular weight polymer solu-tions may have unusually low turbulent-flow friction coefficients has provided the key to making progress in understanding these anomalies It nov appears

that many marine organisms are capable of secreting high-polymer substances into the sea (or into a towing-tank, for fresh-water organisms) which are effective in changing the turbulent-friction resistance of immersed bodies The presence of these naturally-produced materials can thus greatly change frictional

resis-tance determinations made in the towing-tank and at sea. . .

Further, it seems not unreasonable to suspect that these substances which affect friction so markedly may also play some role in changïng cavitation

2

Friction Reduction of Microscopic Algae

'By flowing cultures of living phy-toplankton through a small pipe at

a given rate, and comparing the pressure drop in a known length with that ob-tained under the same conditions with water, evidence of frictional change due to the influence of the algae can readily be shown.

In this way, cultures reprèséntig all the main classes of microsco-pic algae have been found to exude substances which reduce the friction.

In briefest terminology, the algae may bó divided Into groups which

generally are separable by color:

Blue-i'een' Green

Red

-Brown.'

Yellow-brown

Microscopic or single celi marine, forms maybe found in

all

of these

grôups, but in the ocean, the most prominent are the yellow-brown diatoms. Also very important are another large category not identifiable by color, the dino-flaéllates or' unleellular swimming forms.

Severa], representatives of all of these great classes of marine

organisms have demonstrated the capability of secreting high-polymer substances

In laboratory culture. Many of the dirioflagellates in particular are fitted with special' pores or ducts fór' secreting the material.1 Exactly why this is

done is not.known, but presumably there is a protective Or possibly

anti-bacter-ial

action 'in. the materials. However,: the main intei-est to the Naval architect

lies in the fact that often the substances are soluble, high weight and apparently linear molecules which, in süff±cient ôoncentration, vill'reduce the friction..

As an example f the dinò-lageUatês, the very conmon Prorocentrum

micans, found In both Atlantic arid Pacific waters, is an excellent friction

reducer ±n laboratory culture as shown in Figure 1. In this species, the.

fric-tion-reducing materials are liberated toward the end of the active growth period,

while with-the diatom Chaetoceros d.idyinus .(Figure 2), liberation of the

friction-reducing substances accompanies the périod of rapid growth. With both orgánisms

substantial (above 2.5%) friction reductions are readily obtained. These data were taken at

pipe

flow Reynolds numbei-s basêd on pure water of lO,OOO-lL,OOO.

The cells of the algae play no part in the effect; removing the cells by

centri-fuging does not: change the observed friction-reduction. Although there is great

1These have been demonstrated in beautiful detail by Jean Dragesco,

"Etude Cytolçgiq.Ue de Quelques Flagellés Msopsammiques," Cahiers de Biologie Marine, VI,

1965, pp.

83-115.

3

scatter in the data.shown on Figures 1 and 2, the general trend cf the friction-reduction with time was established by following the progress of individual

cultures, and this trend is represented by the curves.

r 32 28

z

024

I-20 k 'u X

.z'2

_o

I-u

_X4

u-.

0 8 16243240485664728088

CULTURE AGE, DAYS

Figure 1.. Friction reduction obtained with cultures of the dlnofla-geliate Prorocentrum micans, and a sketch of the organism.

o 40 o 30 z 020 8 16 24 32 40 48 56

CULTURE AGE, DAYS

Figure 2. Friction rednetlon obtained with cultures of the diatom toceros didymus,and a sketch of the organaim.

While .some algae secretions decay in friction-reducing effectiveness w.th time, others resist breakdown by bacteria and. retain ful]. friction-reducing ability, for long periods of time. Initially, the. resistance-changing substance

concentration parallels cell growth, as shown in Figure ₃ for a marine

Porphyridium species. However, after cell division ceases, the substance retains its effectiveness foa year or more in laboratory cultures.

70

60

Io

2The possibility of resistance changes in towing tanks was first put forth in published form by R. N. Newton, "Standard Model Technique at Admiralty Experiment Works, Haslar," RINA, i,

July 1960, pp. 435-11.70.

4 6 8' 12 20 30 40

CULTURE AGE.DAYS

Figure

3.

FÍictiOn reduction obtained. With cultures of Pbrpbyridiu sp.

Since various species of diatoms, dinoflage].lates and other algae

occur in dense "blooms" in the surface water of the ocean from time to time,

usii.l ly in the Spring, but occasionally at alaost any season, it certainly

seems possible that ships could be operated in an ocean area containing suffi-cient high-polymer exudate to change the skin friction. If this were to occur

on the triai, trip of a new ship, the instrumentation aboard would indicate a

very pleasing reSult to the Naval architect; the propulsive power would be

lower than pred.icted

Now there are also a vast number of microscopiò fresh water algae,

many of whom also exude high polymer, and thus, could change the resistance. 1f

they wére present in a towing tank2 or water tunnel As examples, the following 50 z o 40 s-o o w o 30 o 20.

5

Hence, algae should be carefully excluded from towing tanks. The Anabaena

listed above was one of the organisms identified in the towing tank at ft. Steyne in which drag measurements were seriously affected by substances traceable to biological origins

.3

Recent exper1ments' 5 with synthetic high polymers deliberately dïssolved in towing tank water leave little doubt that the drag may be greatly reduced by this means; as little asl parts per million by weight of the synthetic polymer poly(ethylene oxide) reduced the drag of a flat plate by

almost ¿i.O% in the Aerojet towing tank. Friction Reduction of Larger PJ.gae

Many of the larger algae or "seaweeds" are harvested commercially

for extraction of their water-soluble materia], which is used as thickeners and viscosity builders in food. and industrial applications. These thickening

agents are also friction reducers in dilute solution. Figure Ii. shows the results of tests of carrageenan, the commercial extractive of Chondrus chrispus, a common

red seaweed.

During

1966,

approximately 65 species of fresh seaweed were gathered,

principally from the West Coast of the United States, but also from Japan and

the Mediterranean. 0f these,

50

gave a reduced friction solution when extracted with hot sea water or fresh water. The liberation of organic materials into the sea by large algae is currently the topic of much marine botanical research;

clearly, there is ,a possibility that the larger sea plants can affect the

resis-tance characteristics of the nearby water. This is probably evident to those who have seen the changed appearance of the water surface in the region of

large "kelp" beds offshore, and Figure ₅ shows the frictional reduction obtained from a sample of a common large algae.

The important concern of all this to those connected with sea trials of ships is to avoid making measurements in areas where large growths of

seaweed occur.

3Barnaby, K. C. and A. L. Dorey, "A Towing Tank Storm," RINA, 107,

1965, p. 265.

)lExnerSon _{A., "Model Experiments}

_Using

Dilute Polymer Solutions Instead of Water," North East Coast Institution of Engineers & Shipbuilders, 81, February

1965, pp. 201-20L

5Ievy, J. and S. Davis, "Drag Measurements on a Thin Plate in Dilute Polymer Solutions," International Shipbuilding Progress, April

_1967.

results were obtained: Drag Reduction %

Anabaena floss-aquae (blue-green)

_59.5

Ch].amydomonas peterfii (green)

29.0

6

k)

a.

-Figure ii. Friction reduction obtained with solutions of caxrageenan, and

sketch of the source plant, Chondrus criapus.

c.. -

---

-S

h

-.e

S

I

Figure 5. Friction reduction of Macrocystis and its surface appearance.

Cavitation Inception

In 1965,

Lt, R. J. Prather, working at the U. S. Nava]. Ordnance Test

Station, observed that the inception point of a cavitating fluid jet was consider-ably reduced when the high-polymers poly(ethylene oxide) and Guar were added to

the test solution. In particular, he found that the cavitation index, sigma, defined as: throat AA T 2 -

pv

was reduced from 0.Ii.9 for a water jet to approxImately

0.3

for a 50 ppm

poly(ethylene oxide) solution, and for a 500 ppm Guar solution.

Recently Professor A. T. Ellis at the California Institute of Tech-nology has studied cavitation inception on a hemisphere-nosed cylindrical body6

in a blow-down water tunnel. Using a

0.635

cm dia stainless steel test body, cavitation inception was detected in two ways. A laser beam was adjusted to

just graze the surface of the hemisphere nose in the region where cavitation

first appears. As shown in Figure

6,

light scattered p the cavitation bubbles was detected by a photo cell sensing light at about 90 from the laser beam

direction. This method of cavitation detection was checked by acoustic

obser-vation, and extremely close agreement was obtained.

FLOW

:5,

Figure 6. Diaram of cavitation detection arrangement.

e

p

6Barnard, B. J. S., A. T. Ellis, and M. E. Slater, "The Unsteady

Flow Cavitation Tunnel at the California Institute of Technology,t' Cavitation

Tests were made with water (passed

through a O.4.

filter), 20, 50 and

loo

pm poly(ethylene oxide) and a suspension of Porphyridium aerugineurn.

The test fluid for the algae case consisted of 15 31 liters of culture having a frïötion réduction of

_29.3%,

diluted with a further ₅₆₅ liters of water.

AU tests in this series vere made with water containing about

17

ppm dissolved air as measured with a Van Slyke apparatus. Typically, the polymer .o1utions havé a.reduced surfacé tension; measurements of a

50 ppm

poly(éthylene oxide) sol'ition indicate a surfacé-tension of

62.2

dynes/cm;

100 ppm gave

61.4

dynes/cm.

The following inception data were obtained (averaeé of 4 runs):

Thus, it can be seen that the polymer

content

of the water has a large effect on the cavitation inception point. This may be a factor in ex-plaining the large differences in inception index in tests of the same body shape in water tunnels throughout the world

.7

Observations of Cavitation Appearance

There seéms to be

_a

noticeable difference in the appearance of steady-state cavities in flOws containing high-polymer in solutions compared

with observations at the same. cavitation index in pure water. Figure 7 shows

the comparison for a tripped cavity on a body of revOlution between water and a

50 ppm

poly(ethylene oxide) solution, both at a cavitation index of 0.22. The

polymer Solution cavity is more striated, and appears to collapse less violently than the water cavity; high frequency respons pressure measurements confirm the diminishing of the intensity of fluctuations.0

7H. Lïnd.gren and C. A. Johnsson, "Caifitation Inception on Head

Forms - ITC Comparative Eicperiments," Appendix V of.Report of Cavitation Committee, llthITTC,

1966.

J. W. Hoyt, "Effects of High-Polymer SolutiOns ori a Cavitating Body," Proc. 11th Iiternationa3. Towing Tank: Conference, Tokyo, October

1966.

Tunnel Velocity, rn/s Inception Cavitation Index Water

12.55

_.73

20 ppm Polyox

13.40

.50

50 ppm

Polyox

14.18

_.39

loo ppm

Polyox 13.70 .41 Algae

12.88

.66

9

FIgure

7.

_{Appearance of fully-developed cavitation on body} of revolution in water and in 50 ppn poly(ethy].ene oxide) solution.

The initial apDearance of the cavitation bubble is also changed by

the presence of high-polymer. In the experiments at the California Institute of Technology mentioned earlier, cavitation inception over the hemisphere nose,

cylindrical body was detected by scattering of a laser beam; a photocell detecting the scattcrin then triggering (with or without a time delay) a photo flash and

camera. Photos taken in this way differ markedly when poly(ethylene oxide) _or algae are added to the water as shown in Figure

_8.

Thus, a change in external appearance of cavitation may be exoected when known (or unknown) contamination of the water tunnel by high-polymer substances occurs.

Cavitation Erosion

Although an e8rly report9 indicated lowered cavitation erosion in both rotating disk and magnetostrictive types of cavitation _{damage apoaratus} with polymer solutions, Professor Milton Plesset of the California Institute of Technology has been unable to confin this, at least with _{a magnetostrictive} device in the polymer solution concentrations of most interest in friction re-duction. _{Using a standardized technique-0 with dished aluminum samples at a}

vibration rate of 111- _{kc/sec., Professor Plesset and Mr. Devine of Cal Tech}

ob-tained the date shown in Figure 9 for solutions of guar, poly(ethylene oxide) and a friction-reducing algae suspension. _{In this method of testing at least,} there seems to be little change in erosion damage with the _{polymers present.}

9U. S. Naval Applied Science Laboratory Technical _{Memorandum No.}

_5,

"Report of InvestIgation of the _{Effect of Non-Newtonian Fluids on Cavitation}

Erosion Damage of Materials," 26 May _196Li..

10M. S. Plesset and R. E. Devine, "Effect of Exposure Time _on Cavi-tation Damage," Journal of Basic Engineering, December

_1966.

iNCEPTION

DEVELOPED

FULLY

DEVELOPED

ALGAE

WATER

POLYOX

Figure 8. Appearance of cavitation on hin1phere-nose bod7 in water, algae, aM poly(etzylene oxide) solution.

However, the magnetostrictive technique is not necessarily representative of cavitation erosion in flow situations and further study is needed to clarify whether or not erosion is affected by the use of polymers in a flowing stream.

Other Manifestations of Algal Activity in the Ocean

Since the high polym.ers released by microscopic and large algae are both turbulence supressing and. surface tension lowering, it is not surprising that where large quantities of algae are present, the surface appearance of the

ocean is changed. Slicks have been reported from "blooms" or dense concentra-tions oÍ' microscopic algae of several kinds.11

10

11J. N. McSieburth and J. T. Conover, "Slicks Associated with

25 o

u

FRICTION

SUBSTANCE

REDUCTION, %

WATER

-20 -. 1,000 ppm GUAR

53.8

D 100 ppm POLYOX

53.4

* P. AERUGINEUM

30.1

15-

(CULTURE) O)

GUAR SOLUTION

O

R AERUGINEUM

'5-

_{POLYOX SOLUTION} DISTILLED WATER

G. Sturdy and W. H. Fischer, "Surface Tension of Slick Patches

Near Kelp Beds, Nature,

211, 1966, p. 951-952.

13T. Hidaka and R. Baudoin, "] Fucus et la Formation de l'Ecume Marine," C. R. Academy of Science, Paris,

260, 1965, 5861-58614.

o 10

20

30

40

CAVITATION EXPOSURE TIME, MIN

Figure 9. Cavitation erosion data using magnetostrictive technique.

It is further apparent that very often "slicks" accompany the presence of larger algae (Figure

5)

and it has been established that the sur-face tension in these patches of water is greatly reduced. It has also been

demonstrated13 that the algae of rocky shores liberate surface tension reducing materials and are responsible for the increased appearance of foam on the water

in such areas.

Of course, color changes of ail kinds are further indications of

the presence oÍ' algal activity. Patches of colored water have been noticed in

the ocean since earliest times; these patches may be of almost any color: white, grey, brown, yellow or red.

How then is the Naval architect to be assured that his ship trials are not affected by biological activity in the ocean? I think the only answer is to test the water. It seems possible that portable turbulent-flow friction measuring devices can be constructed for use on shipboard, and one way of doing

PRESSURE TAPS TEST PIPE. .023 IN. INNER DIAM. FILLING CUP THREE-WAY VALVE 5 CC HYPODERMIC SYRINGE VARIABLE-SPEED MOTOR

this Is Shown in Figure 10. The results

shown in Figures 1 and. 2 were obtained

on an instrument of this type, with distilled water used as the reference

standard.

For the towing tank or water tunnel, a larger, more robust and

re-liable instxument of thee same general

type has been deve1oped and used as a standard for evaluating samples of water and candidate friction-reducing polymer

solutions. The data shown on Figure 1# vere taken in such a device..

Since there is as yet an un-certa±n translation of friction coeffi-cient in pipe s at one Reynolds. number .to flat-plate coefficient at another, de-partures from the pure water values ob-tained with these instruments should F1gue 10. Sketch of friction-testing apparatus.

probably be used as warnings, in the ship-trial, towing-tank and vater-tunnel cases, rather than attempting quantitative corrections to the resistance under

measurement.

Concluding Remarks

it ha's been shown 'that the :i1uence 'of natural products in the sea and in shore-based hydrodynaznic facilities should not be disregarded in

measure-ments involving resistance and cavitation. Further study of the occurence and role of these large organic molecules in natural and man-made bodies of water is

sure to lead, tò many neî, ixnportant, and interesting discoveries.

Acknowlêdgement. '..

In addition to the help of many co-wOrkers at the U. S.. Naval

Ordnance Test Station, I would like to acknowledge the assistance of Professors A. T. Ellis and Milton Plesset of the Calïfornia Institute of Technology in

releasing .unpüblished information for this paper. These studies were aided by support from the Fluid Dynamics and Microbiology Branches of the Office of Naval

Research, United States Navy.

GEAR BOX

J. W. Hoyt, A Turbulent-Flow Rheometer," ASI Symposium on