310 



vibration problem almost completely. It provided, 

 however, the risk that the laser might fail. 

 Fortunately, this did not happen. The laser, a 

 Coherent- Radiation {4 Watt) product, achieved the 

 same performance (900 mW) , to which it was adjusted 

 at the beginning, up to the end of the voyage with- 

 out any failure. The small vibration still observed 

 at the measuring point had no significant effect. 



6. RESULTS OF MEASUREMENTS 



Local Velocity 



It was mentioned already that the number of measure- 

 ments originally planned could not be carried out 

 due to lack of time. Thus, for instance, the size 

 and the direction of the local velocity could only 

 be determined at one measuring point. This measure- 

 ment took much time since there was no special 

 electronic device available. It was the first 

 measurement of this kind and it was included in the 

 program at a late date, which made it impossible 

 to establish a special measurement before the 

 departure. The measurement was, therefore, partly 

 performed with the electronic device which was also 

 used for the scattered light measurements, and with 

 some special interfaces. 



The velocity and one plane of the flow direction 

 at the place of the control volume could be deter- 

 mined for one velocity. At the ship's speed of 

 22 kn the velocity at the measuring point amounted 

 to 7.22 m/s and the direction was found at an angle 

 of 5° downward. The corresponding result from the 

 model test for the geometrically corresponding 

 position of the "Sydney Express" amoxinted to 7.47 

 m/s. This model test, however, was carried out for 

 the propeller plane of the towed model, without a 

 running propeller. - In full sale, on the other 

 hand, the plane formed by the flow direction and 

 by the optical axis of the reception device could 

 not be determined due to lack of time. 



t N_ 



,5-f 





\ = *15= 



5 5- 



^w^v^-n'^v^.. 



X = -5° 



.^w-.H'^-T'^-^ 



X = -15° 



150 ► 200 



T [/isecl 



FIGURE 14. Pulse width distribution and mean pulse 

 width dependent on the inclination of the flow. 



1Q0 



1.0 



01 



0.01-H 

 0.001 



-IVX- 



V- 



:A 



Test 47 



Measuring Time: 

 tm = 6. 8 sec 

 i^ = 37 N/cm3 



n =101.1 RPM 

 Vs=21.2kn 



60 



20 40 

 30-11 -1977, 

 FIGURE 15. Nuclei distribution 



80 100 /u. m 



— — Diameter 



Wind Force : 6 Beouf 



The ratio, local velocity to ship's speed, 7.22 

 m/s to 11.32 m/s, and which corresponds to the 

 local wake in the control volume for the ship at 

 22 kn, was applied for all nuclei concentration 

 measurements . The nuclei concentration was then 

 calculated from the recorded ship's velocity, the 

 measuring period, and the measuring cross section. 



Figure 14 shows examples of the velocity measure- 

 ment and also the change of the impulse width 

 distribution for the rotation of the rectangular 

 laser aperture. The value \ = 0° corresponds to 

 the horizontal plane. At 5° downward {A = -5°) the 

 mean impulse width, evaluated on the HP-computer, 

 reaches its minimum at 59.6 ysec. The large half- 

 width of the distribution curve results from tihe 

 turbulent flow. With a laminar flow the distribution 

 curve would be smaller. (See Figures A 2.2 and 

 A 2.3) . 



On the basis of these measurements a quantitative 

 statement about the turbulent degree of the flow 

 cannot yet be made. On the one hand we have no 

 experience witii this measuring technique, on the 

 other hand the ratio, length to width of the laser 

 beam cross-section, was too small at this measure- 

 ment (2:1). At high turbulent flow the corners of 

 the beam cross-section were dispersed by a relatively 

 high amount of nuclei which resulted in shorter 

 photomultiplier impulses than with nuclei running 

 through the middle of the beam. A higher ratio, 

 length to width, would be more favourable. 



The first practical experiences with this mea- 

 suring technique are so promising that its further 

 development is being promoted. The advantages which 

 this measuring procedure offers in connection with 

 the determination of the size of nuclei are quite 

 remarkable . 



Nuclei Spectra 



About one third of the spectra obtained between 30 

 November and 7 December 1977 are demonstrated in 

 Figures 15 through 24. The spectra contain the 

 respective sum of nuclei per cm for the respective 

 range of diameters. In t±ie diagrams one range of 

 diameter is marked by a horizontal line. The single 

 ranges of diameters do not have the same width. 

 The dissimilarity of tzhese spectra, which obviously 

 results from different conditions, will later be 

 described in detail. 



First, it has to be noticed that for all spectra 

 in the range of a bubble diameter from 20 to 40 ym 

 (micron) there is eitJier a relative maximum or an 

 absolute maximum of nuclei. The relative maximum 



