Twenty-Second Symposium on Naval Hydrodynamics (1999) / Chapter Skim
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6 Frontier Experimental Techniques
6 Frontier Experimental Techniques
Pages 301-340
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From page 301... ...
With sequential images available, the magnitude and direction of the velocity vectors can be determined from the digital correlation method. The ACIA method thus preserves the merits of both PIV and DPIV methods by analyzing a m~tiexposure particle image using the digital correlation method of sequential images.
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From page 302... ...
For instances, by analyzing multiexposure particle image, the dynamic range of the velocity measurement of the PIV method is highly increased while the determination of flow direction is ambiguous and requires additional equipment such as the image shifting devices. The DPIV method determines the flow direction easily from a series of sequential images, allows denser seeding particles, but the dynamic range of velocity measurement is limited by the frame speed of the recording device.
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From page 303... ...
It should be emphasized that the time interval between the two separated sequential images is not limited by the frame speed of CCD camera but is determined from the frequency of the generated, alternating color laser sheet. Therefore the dynamic range of the velocity measurement of the ACIA method is much higher than that of the DPIV method.
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From page 304... ...
For a steady flow, a series of flow images can be captured at distinct time. Any pair of sequential images can be used to perform the correlation analysis.
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From page 305... ...
The CCD was triggered externally as the blue laser was detected by the fiber optic placed at the upstream of the laser sheet. The total exposure times of the alternating color particle image was determined from the frequency of the pulsed laser sheet and controlled by the shutter speed of the CCD camera.
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From page 306... ...
Figures 9 present the alternating color and its separated flow images of the two-dimensional channel flow acquired from a color CCD camera. Limited by the frame transfer rate of the black & white CCD camera, the velocity measured using the DPIV method can not be high.
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From page 307... ...
It is realized from paper review that, to increase the dynamic range of velocity measurement, analyzing a multi-exposure particle image is necessary, while to eliminate the directional ambiguity of a flow, determining its velocity vectors from sequential images is preferred. The proposed ACIA method successfully preserves these advantages by separating a multiexposure particle image into sequential images and analyzing them using the digital correlation method.
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From page 308... ...
13luc-lascr exposure (c) Green-laser exposure Figure 2 Four-time exposure, altemating color particle image of a rotating disk.
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From page 309... ...
-~P~;r ~ ~ Figure 3 The system setup of the ACIA method. Figure 4 Cross-correlation map of two sequential images.
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From page 310... ...
Act- ~ (a) Blue Image (be Gay image Figure 9 Alternating color and its separated images of a channel flow.
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From page 311... ...
(a) Four-time alternanng color particle image '' 4-~ .
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From page 312... ...
(a) Six-bme altcmanag color particle image \ (b)
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From page 313... ...
However, the velocity vectors are determined from the cross correlation analysis of two sequential images. If the time interval between two sequential images is short and particle density is high such that most of the particles were kept within the interrogating window of the two sequential images, the correlation analysis will result in a clear correlation peak.
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From page 314... ...
Fig. 1: Bursting Tip Vortex Cavitation in Full Scale ABSTRACT The prediction of propeller induced hull pressure pulses is - beside numerical calculations normally based on model tests carried out In a cav:tation tunnel.
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From page 315... ...
Different occurrences during the propeller revolution (propeller blade entry into the wake peak, vortex bursting etc.) create different peaks in the hull pressure time function.
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From page 316... ...
This individual treatment of course requires the previous identification of the effects leading to the different peaks in the hull pressure time function. Beside the measurement of the hull pressure with highest possible time solution, the simultaneous cavitation observation by means of a HSV system has been found to be a valuable tool for this purpose.
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From page 317... ...
The housing installed inside He ship model was linked with the trunk side wall by a rubber tube of 100 mat diameter to allow a water tight connection between camera and signal processor. For He propeller observation a perspex window was arranged on starboard side in the model hull above He propeller, as illustrated in Fig.
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From page 318... ...
6: Averaged Time Functions 318
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From page 319... ...
7: High Speed Video Recording of Tip Vortex Bursting 319
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From page 320... ...
also showed pronounced vortex bursting, it is therefore not astonishing that this former study already resulted in an optimum HYKAT gas content of 80 % saturation, although no detailed analysis of the pressure time functions had been made. At this phase of the propeller rotation, i.e.
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From page 321... ...
As expected, the 12 o'clock wake peak, which in case of single screw ships is mainly caused by the boundary layer of the ship hull, is considerably reduced in filll scale. To investigate the influence of this wake reduction on the second peak in the hull pressure time function, comparative numerical calculations were carried out by means of the method described in the foregoing section.
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From page 322... ...
But the creation of additional peaks resulting from the cavity volume expansion (and/or bursting tip vortex) may sometimes - depending on He location of these peaks on He time axis of He hull pressure time fi~nction - cause a decrease of individual components of the harmonic analysis.
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From page 323... ...
THE INFLUENCE OF GAS CONTENT In the foregoing section the perfect full scale similarity of the first pressure peak of He model hull pressure time function was emphasized. This good agreement was achieved by model tests carried out at He "normal" oxygen content of 80 % saturation in HYRAT.
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From page 324... ...
For this reason and as stated above, a correction method must deal with the hull pressure time functions instead of certain components of its harmonic analysis. When applying the method for other types of ships, especially for fast train screw ships, additional efforts are necessary for the treatment of the displacement effect.
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From page 325... ...
16: Model Hull Pressure Time Functions for Different Container Ships CONCLUSION Mainly financed by the German Ministry of Research and Technology extensive cavitation tests were carried out in HYKAT, HSVAs large Hydrodynamics and Cavitation Tunnel, to improve the reliability of full scale predictions for propeller induced hull pressure pulses. By means of High Speed Video records and simultaneous measurements of hull pressure time functions the hydrodynamic origin of the different peaks in these functions could be investigated.
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From page 326... ...
Friesch, J., Johannsen, C., Payer, H.G., "Correlation Studies on Propeller Cavitation Making Use of a Large Cavitation Tunnel", Allele, New York, 1992 4. Friesch, J., "HYKAT, the New Cavitation Test Facility of the Hamburg Ship Model Basin", H~91, Varna, 1991 5.
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From page 327... ...
Carderock Division, USA In this paper, the author has embarked on a laudable and ambitious objective to try to derive fluid mechanical connections between propeller inflow velocity patterns, cavity dynamics, and explanations of details of propeller-induced unsteady hull pressure time series characteristics. This kind of work could conceivably lead to improved predictions of full scale hull pressures based on a combination of model scale measurements and full scale calculated results.
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From page 328... ...
Furthermore, the time function peak correction proved its reliability for only one ship so far. Other stern arrangements, e.g., V-shaped hull forms as mentioned by Mike Wilson, will perhaps create hull pressure time fimctions of different character, requiring an additional treatment of the pressure peak created by He collapse of the cavity volume.
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From page 329... ...
To get a reliable full scale pressure pulse prediction from model tests, these occurrences, and not the components of the harmonic analysis, must be treated individually. To relate the time function pressure peaks to the different hydrodynamic occurrences, High Speed Video recordings are very valuable.
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From page 330... ...
INTRODUCTION A dye-streak tracer is under development as an inexpensive alternate to laser-based diagnostic tools used in water-flow studies. The tracer is effective in strongly eddying and ~bulent-flow situations encountered in water flow around objects and in He turbulent boundary layer.
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From page 331... ...
study of flow around shallow-immersion cylinders with a similar e~enment using the tracer, and find Cat Me streamlines of the tracer technique offer a virtually identical impression of the principal Bow characteristics when compared with the PIV velocity vectors. TRACER COMPOSITION The Lacer is a mixture of a surfactant, cetyltrime~ylammon~um salic~ylate (C~6TASal)
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From page 332... ...
The enhance conditions were adjusted very carefully using dye streaks to assure smooth entrance to the plate, and that the turbulent boundary layer began at the trip. The plate was smooth to the touch, before being spray-painted mall black Three-dimensional viewing was obtained for several test configurations by mounting a mirror at 45° above the channel.
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From page 333... ...
The major vortices are seen to form on the horizontal centerline behind the cylinder. The turbulent boundary layer does not seem to trigger the large Karman vortices, and only occasionally enters into Me vortex formation region.
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From page 334... ...
and others for smooth cylinders in the cntical-flow regime. Introducing dye into Me behind~linder region for the high Reynolds number flow shown in Figure 7 indicated that no organized activity exists in the near wake; it seems to be a high-pressure area accounting for the low drag.
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From page 335... ...
At a Reynolds Dumber of 5,000, the wake shows signs of Kirman vortex formation, but these vortices begin much Faber behind the Cylinder compared win Me smooth cylinder wake. At 15,000 Reynolds number, the flow appears completely detached from Me cylinder, as shown in Figure 12.
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From page 336... ...
The tracer is embedded in a turbulent boundary layer as it encounters the cylinder. lThe cylinder is 18.7 mm dia., Me vertical white line at the right of Me side view is 50 mm high, and at Mat point, the undisturbed boundary-layer thickness is 24 mm.
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From page 337... ...
We have repeated several of the flow geometries studied by Sheridan, et al. and find Hat the streaklines of the tracer provide a virtually identical impression of the pnucipal flow characteristics when compared with tile PIV velocity vectors.
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From page 338... ...
Apparently, the quasi-steady features of the flow are Pose detected by the flow tracer. Coherent Structures- Turbulent Boundary Layer A final area where Me Lacer has provided results of great interest is in visualizing large structures in the tudoulent boundary layer on a flat plate.
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From page 339... ...
12. Gursul, I., Lusseyran, D., and Rockwell, D., "On Interpretation of Flow Visualization of Unsteady Flows," Embedments in Fluids, Vol.
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From page 340... ...
"A Flow Visualization Study of Separated Flow over Circular, Elliptical and Square Cylinders" in M.V. Otugen, et al., eds.
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