Flow visualization studies on the ITTC body of revolution

FLOW VISUALIZATION STUDIES ON THE ITTC BODY OF REVOLUTION

By Robert J. Etter January

1968

Prepared for Cavitation Committee of the

International Towing Tank Conference

Lab.

v. Scheepsbouwkunde..,

ARCHIEF

Technische Hogeschool

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INTRODUCTION

In ₁₉₆₅ the ITTC Cavitation Committee invited the partici-pation of several establishments having cavitation tunnels in a program of comparative testing of a body of revolution. The program called for determining the incipient cavitation number on a modificed ellipsoid (l)* at different water speeds and air content values. It was expected that a comparison of these results from different water tunnels would contribute

to an improved understanding of fundamental cavitation problems. A description of the test conditions and the observations made

are gfiven in (2).

HYDRONAUTICS, Incorporated participated in this program and issued a technical report (U) concerning the results of

sting the body of revolution in the HYDRONAUTICS' High Speed

Channel. A complete description of this channel is given in

(5). The modified ellipsoidal head was the same one tested

at the David Taylor Model Basin. The model was mounted on a sting which was attached to a strut from the top of the channel. Its position in the channel and a photograph of the head, sting and strut are shown in (U). The width of the channel test

sec-tion is 2U inches and the depth of flow during the tests was maintained at 20 inches.

To augment the information provided from the tests of cavitation inception, the Cavitation Committee has requested

*Numbers in parentheses refer to references at the end of the text.

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-2-HYDRONAUTICS to conduct a flow visualization study on the model under non-cavitating conditions to check for possible flow

separa-tion.

FLOW VISUALIZATION TECHNIQUE

The flow visualization technique used in this study was first developed by NASA

(6)

and consists of spreading a

suspen-sion of fluorescent pigment in oil over the surface of the model, allowing the flow to create a pattern on the model surface, and photographing this pattern under ultraviolet light.

The optical basis for the technique is as follows, Ultra-violet light, while beyond the range of the visible spectrum, will cause certain materials to fluoresce or emit light that; is within the visible spectrum. If ultraviolet light is projected on an object which has certain regions covered with a fluorescent substance, the relative brightness of various areas will indicate the distribution of the substance. A camera with a filter

designed to pass only visible light, will filter out reflected ultraviolet and record the pattern of the fluorescent substance only.

To apply this phenomenonto boundary layer flow visualiza-tion a commercial yellow fluorescent pigment is mixed with ordinary 2O-w motor oil. A small amount of dilute acetic acid aids the suspension. The details of the mixture are presented in Table 1. To apply the suspension to the surface of the model an ordinary paint brush is quite satisfactory. As the flow be-gins, most of the oil is wiped off the model leaving only a thin layer indicating the flow conditions very near the surface

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-3-(boundary layer). Relatively smooth, even patterns indicate laminar regions. Streaked, dark, or mottled areas indicate turbulent regions. Regions in which TTpoolshl of oil collect are regions of separation. In addition to the type of flow present, the streaks also indicate localized flow direction.

Two separate cameras were used in the flow visualization

study. A 35mm camera was used to take short exposure pictures

with high speed film. A 4 x 5 camera was used to take long exposure pictures with a slower Polaroid film.

The following list indicates the details of the equipment and camera settings used for the photographs:

Cameras

Nikon F 35 mm with 50 mm Nikkor lens manufactured by Nippon Kogaku, K. K., Tokyo, Japan.

Speed Graphic 4 x 5 w/FLL7, 127 mm lens manu factured by the Graflex Corporation, Rochester, N. Y., U.S.A.

Films

Eastman Kodak high speed ektachrome for color transparencies (ASA 160).

Polaroid Corporation Polacolor type 58 for 14 x 5 color prints (ASA 75)

Camera Settings

F2.0 to FLLO at - sec.

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-U-Ultraviolet Lamps

Blak-Ray Model B-100A long wave ultraviolet lamp manu-factured by Ultra Violet Products, Inc., San Gabriel, California (U lamps were used).

Safety Goggles

Blak-Ray U.V. contrast control safety goggle Model UVC-303, U. V. Products, Inc., San Gabriel, California. Filter

Tifflin 1-A skylight filter (U.V.) Locations

Distance from camera to model about 2 feet. Distance from U.V. lamps to model - l- to 3 feet. Model viewed through 2 inches of plexiglas and 1 foot of water.

TABLE 1

Ingredients Used in Fluorescent-Oil Mixture

Ingredient Amount Details

20-w motor oil 5 acetic acid Fluorescent pigment 1/2 quart 1 teaspoon 3 table-spoons Ordinary vinegar is suitable Hi-Viz B-3539, lemon yellow pigment,

Lawter Chemicals, Inca, Chicago, Illinois

NOTE: Mixture requires frequent stirring to maintain

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-5-.

EXPERIIVNTAL PROCEDURE

Prior to the initial flow visualization tests, the model was throughly cleaned and polished using a commercial brass cleaner. The model was then evenly coated with a thin layer of the fluorescent-oil mixture, and lowered into the test

section of the high speed channel. The speed of flow ( at 20!!

depth) was steadily increased to a predetermined value. This speed was maintained for about one minute to allow complete de-velopment of the fluorescent-oil pattern. With the flow speed held constant, two photographs of the model under ultra-violet light were taken through the plexiglas window of the test

section. The flow speed was then reduced to zero, the model removed, wiped clean, and recoated with the fluorescent-oil mixture. The procedure was repeated for each flow speed of interest. All tests were conducted at atmospheric pressure with no cavitation present atm.>23>> a. = O.6215). The water temperature for all tests was a constant 72.5°F.

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-6

-RESULTS AND DISCUSSION

The lowest speed Polaroid photograph taken _{(5 fps) clearly} shows a separated region (bright band of fluorescent-oil)at about S/'D =

0.95

or between stations _{9 and 10 as defined by}

Rouse and McKnown (i). This is the location of the steepest

adverse pressure gradient for this body under non-cavitating conditions. (1) At 6 fps, the separated region is still clearly defined at the same location. At _7. fps the region is still

visible' but not as well defined. Slight separation is notice-able at8 fps but not discernnotice-able at ₉ fps.

The primary difference in oil patterns at higher veloc-ities (ii,

13, 20

and 25 fps) was a broadening of a dark band (no fluorescent-oil) inthe vicinity of the minimum pressure coefficient. The highest velocities (shears) at the model sur-face occur here and are probably responsible for the nearly

com-plete removal of the fluorescent-oil in this region.

The ₃₅ mm camera photos taken simultaneously (at

1/2

sec.

exposure as compared to

90

seconds for the Polaroid pictures) lead to a slightly different conslusion regarding separation. Photos with the Polaroid indicate no separation beyon _{9 fps.} However, the ₃₅ mm photos clearly show a separated region at 9 and at ]il fps. This region is not visible however at 13 fps. It is felt that the ₃₅ mm photos give a true indication of the flow conditions at these speeds. Although the region appears to be slightly separated, the long exposure time used in the Polaroid

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-7-photos allowed the flow to gradually extract the oil from the separated region before the film recorded the pattern.

From the above discussion we may conclude that (in the High Speed Channel) no separation occurs on the ITTC body of revolution at speeds greater than 12 fps or a Reynolds number greater than

3.5k x

1O5. The background turbulence level of the High Speed Channel however, is a significant factor not taken into account by the Reynolds number alone. One method of accounting for the difference is the application of a utur_ bulence factor" which, when applied to the Reynolds number of the flow under consideration, produces an effective Reynolds number in another flow which has been adopted as a standard. The turbulence factor of a flow is commonly defined as the ratio of the Reynolds number at which boundary layer tran-sition occurs on a sphere to the Reynolds number for this same phenomenon in an Initially turbulence-free flow as indicated by a rapid decrease in the magnitude of the drag coefficient. The value CD =

0.30

has been universally adopted as the drag at transition. This occurs at a Reynolds number of

_3.85

x lO in a turbulence-free flow.

In addition to the rapid change in CD at the critical Reynolds number, a corresponding change in the differential

pressure between the rear of the sphere and the stagnation point also occurs. Dryden (6) and Hoerner

₍₈₎

have studied the relation-ship between this pressure differential and the drag coefficient. Using spheres of slightly different design, they respectively

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-8-obtained results of Ap = 1.22 q and p = 1.18 q corresponding to =

0.3.

A turbulence sphere of the design used by Hoerner has been used to measure the turbulence factor of the High Speed Channel (9). At speeds of 13 to 15 fps, the value obtained was

1.90.

A turbulence factor of

1.9

corresponds to a turbulent intensity of:

0.9

< <

"1 u'2

U

Most research wind tunnels have turbulence factor levels below 1.5. However, a few (10) have factors as high or higher than 2.0.

If the turbulence factor is taken into account one may conclude that the critical speed below which laminar separation occurs may be considerably higher than 12 fps in a turbulence free flow (such as in a towing tank). In fact if the T.F. is applied directly to the critical speed (a procedure open to some criticism) one obtains a value of 22.8 fps or RN =

6,73 x 10.

Reference (ii) contains a flow visualization study of the ITTC body of revolution using an ink-flow technique. Enclosure ₉ of that reference shows the flow patterns at ₅ meters/sec and

10 meters/sec. It is the present author's interpretation that separation is present at 5 but not at 10 meters/sec in these photographs. Converting the speed for separation corrected for turbulence level in the present study to metric units one obtains 7.k8 meters/sec. The turbulence level of the ITTC Tunnel is not indicated in (11), however, assuming it to be lower than that of

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-9-the HYDRONAUTICS High Speed Channel, no conflict on -9-the speed at which separation occurs is obtained.

Reference (11) indicates the location of the separation zone at S/ =

0.77.

This is slightly foreward. of the location obtained in the present study (S/t =

0.95).

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-10-REFERENCES

Rouse, H., and McNown, J. S., "Cavitation and Pressure Distribution. Head Forms at Zero Angle of Yaw," State University of Iowa, Studies in Engineering Bulletin 32, 19k8.

Lindgren, H., and Johnsson, C. A., "A Proposed Program

for Comparative Cavitation Tests," Statens Skeppsprovnings.-anstalt P.M. K67-1A; Göteborg, Sweden; Nov. 196k.

Lindgren, H., and Johnsson, C. A., "Cavitation Inception on Head Forms - ITTC Comparative Tests," SSPA Publication No. 58, 1966 (also presented at the 11th ITTC Conference, Tokyo, 1966).

k. Contractor, D. N., "Cavitation Inception Studies _{on the} ITTC Body of Revolution," HYDRONAUTICS, Incorporated Technical Report ITTC-1, March 1966.

Johnson, V. E., and Goodman A., "The HYDRONAUTICS,

Incorporated Free Surface, High-Speed Channel," Cavitation Research Facilities and Techniques, 196k, ASME, New York,

196k, _p. 6k.

Loving, D. L., and Katzof, S., "The Fluorescent-Oil Film Method and Other Techniques for Boundary-Layer Flow Visuali-zation," NASA Memorandum 3-17-59L, March 1959.

Dryden, H. L., Schubauer, Mock, and Skramstad, "Measurements àf the Intensity and Scale of Wind Tunnel Turbulence _and their Relation to the Critical Reynolds Number of Spheres," NACA Technical Report 581, 1937.

Hoerner, S. F., Fluid Dynamic Drag, 2nd Ed., New York, 1958. Etter, R. J., and Huang, T.T., "An Experimental Investigation

of the Static Hydrodynamic Characteristics of Several Simu-lated Faired Cables Having Symmetrical NACA Airfoil Sections," HYDRONAUTICS, Incorporated Technical Report 530-1, July 1967.

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-11-Platt, R. C., !!Turbulence Factors of NACA Wind Tunnels as Determined by Sphere Tests," NACA Tech. Report _558, 1936. Johnsson, C. A., "Pressure Distribution, Streamlines, and Cavitation Inception Tests on a Modified Ellipsoid Head Form (ITTC Head Form)," SSPA Pm BK2Ll.l, December

1967.