ARCHIEF
Experiments El] have demonstrated that a vibrated cavitation test specimen will, after initial erosive roughening of the surface, exhibit a steadier and much reduced rate of erosion. This reduced rate of erosion is shown [1] to occur Whether the surface is initially roughened by Cavitation or is artificially roughened prior to cavitation exposure. It is also reported to occur for cavitation erosion in both Venturi and rota-ting disk types of test facilities [2]. Plesset and Devine
[3]
have shown that the roughness, whether caused by erosion or by machining many small cylindri-cal depressions, greatly reduced the visible evidence of cavity fOrmation under Vibratory exposurecondi-tions. They suggest that the decreased damage rate is an incidental hydrodynamic effect due to the surface
contour. Eisenberg, et.al. [1] speculate that the re-duced rate is due either to deaeration of the liquid in the roughened area or to air accumulations. They Suggest that deaeration is more probable.
The writer in an earlier paper [4] presented evi-dence Suggesting that water boundaries exposed to pul-sating pressures will outgas from tiny crevices due to a rectified diffusion of the gasses in solution in the
water. Hughes and Nyborg [5] have also given consid-eration to outgassing from a pitted vibrating surface.'
This paper describeS exploratory studies which attempt to more conclusively extend the various obser-vations noted above. In these experiments, two
5/8-inch diameter test specimens were made of aluminum, Type 6061-T-561, and surface polished with a No. 600paper. One specimen
was
left smooth-and the other was indented with the pattern of 69 depressions on 0.05-inch centers, as shown inFig. 1(a).
The conical in-dentations Were madeby
pressing a 1/5 darner's needle into the surface approximately 0.05 inches. This sur-face was again made plane by polishing with No. 600paper. Each of these specimens was then exposed to oscillation in an AS ME type vibratory apparatus at a
frequency of about 6180 cps and 'a double amplitude, Of 0.002 inches.
The erosion patterns resulting from 285 minutes of vibratory exposure are shown in Fig. 1, and the cumulative erosion-time curves are shown in Fig.
2.
The significant finding of this evidence is that the total erosive weight lops from the indented surface is only about 40 per cent of that from the smooth surface. The exact nature of the beneficial mechanist was not clearly shown by this evidence. It was conjectured, however, that the presence of self-generated gas, either within the depressions or issuing from the de
-pressions, served to attenuate either the formation or the collapse pressure of vaporous cavitation.
it was attempted to gain additional insight into the mechanism by visually and photographically examin-ing the outgassexamin-ing and collapse from a side view usexamin-ing
a transparent plexiglass specimen. This specimen was indented in the same manner as the aluminum specimen but with only two depressions, one being located at
Lab. v. Scheepsbouwkunde
Technische Hogeschool
ST ANTHONY FALLS HYDRAULIC LABORATORYDelft
FURTHER OBSERVATIONS ON SURFACE OUTGASSING AS AN
INFLUENCE IN CAVITATION DAMAGE'
By John F. Ripken Professor of Hydromechanics St. Anthony Falls Hydraulic Laboratory
University of Minnesota
11
TECHNICAL PAPER NO. 34 SERIES A
the center and the other about
1/8
inch radiallyout-ward. Visual observations of this vibrated specimen showed a gas pocket with interface to exist at the apex of the depressions. This interface experienced active oscillation within the depression and occasion-al release of gas from the meuth of the depression.
Figure 3 shows two random unsynchronized photos taken with an exposure of about one micro-second. These photos are not cleat enough to show the gas in-terface within the depression, but do indicate that outgassing from the left depression supplies, nuclei for cavities Which stream radially inward from the left depression toward the right depression, which is at the center Of the specimen. In Fig. 3(a) the cavi-ties appear to be near full development, while in Fig. 3(b) collapse appears to have taken place, leaving only gaseous debris. In Fig. 3(a) the size of the cavities appears to increase from left to right in what is probably an indication of the pressure grading toward lower values near the center. The radial in-ward movement of the gas was also indicated by subse-.quent observations of the pattern of erosive etching. on the face of the specimen between the holes. This
showed an etch trail which was narrow at the outer hole and widened toward the center hole. This col-lapse trail is also indicated radially inward from the outer holes in Fig. 1(a).
The foregoing cycle of outgassing appeared to have a frequency which was much less than the
oscilla-tory frequency. This indicates the possibility that a complete cycle involves a long period of gaseous re-charging within the cavity, followed by a brief,
occa-sional dumping of this gas. The observed reduction in erosive weight loss is presumed to be a result of.el-aetic damping in the freed gas. Whether the effect-ive gas is that within the depression or that spilled from the depression it not established;
The writer gratefully acknowledges the support of the Office of Naval Research, U.S. Department of the Navy, and the contributions of his colleague, Dr. R.M.
Olson.
Eisenberg, P., Preieer, H.S., and Thiruvengadam.,
A., "On the Mechanism of Cavitation Damage and Methods of Protection," Society of
Naval_Archi-tects and Marine Engineers, Paper No. 6, Annual Meeting, New York, 1965.
Eisenberg, P., in the Discussion of Ref. [3].
Pleeset, M.S. and Devine, R.E., "Effect of Expo-sure Time on Cavitation Damage," Trans. of ASME, Journal of Basic Engineering, December 1966. Ripken, J.F., "Surface Outgassing as an Influence in Cavitation Damage," Cavitation Forum,ASME,1966. Hughes, D.E. and Nyborg, W.L., "Cell Disruption by ' Ultrasound," Science, Vol.
138,
October 12, 1962.Printed in U.S.A.
(a) Indented
(b) Plane
Fig.
1- Cavitation Erosion of Aluminum After 285 Minutes of Vibratory Exposure
(Spherical Depressions near the Edge are Hardness Test Marks)
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Fig. 2 - Cavitation Weight Loss Measurements
for Specimen of Fig.
1Reprinted from:
1968 CAVITATION FORUM
Published by:
The American Society of Mechanical Engineers
345 East 47th Street, New York, N.Y. 10017
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St. Anthony Falls .Hydraulic Laboratory
University of Minnesota
20. GROUP3. fICCPC.,RT TITLE
'
FURIBM OBSERVATIONS ON SURFACE OUTGASSING AS AN INFLUENCE .IN
CAVITATION DANACI
...
4. DeiCRIPTIVE NOTES
(Type of report andInetasive dote.)
Technical Report - May 1967 to May 1968
---S.AU-0:0711111 (First name, middle
Male,
Mat name);,. .
John F. Ripken
, . . ... ,...w....a. REPORT DATE
May 1968
7B. TOTAL NO. OF PAGES lb. NO. OF REFS
2
5
ea. 06.;;1RACTOR GRANT-NO.
..
:FC0014-674-0113-0031 KR 062-369/3.4..66
b. PAOJECT NO.
.
d. .
DB. ORIGINATOR'S REPORT MIJOADIERCB) .
.
Technical Paper No. 34g Series A.
75707r4ER REPORT ROM (Any other numbers Oat ma, tee ooOtarst
dila report)
.
10. DISTRIBUTION STATEMENT
, .
Distribution of this document is unlimited.
re
It. SUPPLEMENTARY NOTBS 12. sPONBORiNG AnLITARY ACTIVITY
Office of Naval Research
Fluid Dynamics Branch
4 I i . AESTRACT
.
.
. .
Vibratory cavitation erosion tests show that -a spec
surface
.,
..imenprovided with many small holes experienced mach less erosive damage than
OX untreated specimen.
Observations .indicated that gas generated_
within the holes served to attenuate the erosive action of the
cavitation.
. . . . . . . . . . . . .,
. ... . . . ' ' - ..--
