308 



and the direction of the laser beam as well as the 

 optical axis of the scattered light receiving system 

 are standing vertically, one upon another. This is 

 optimal for the measuring technique used. 



The homogenization of the laser beam, i.e. the 

 conversion of the Gaussian intensity distribution 

 over the beam cross section into a rectangular 

 distribution, was made with a special filter. The 

 homogenous intensity distribution as well as the 

 shape of the laser beam {square or rectangular) 

 were maintained quite well by the very long focal 

 length of the laser system {about 6 m) . 



The control volume, optically defined, was 

 positioned in such a way that the stream line 

 through the control volume came into the range of 

 the propeller tip. The position of the control 

 volume in front of the propeller was determined by 

 the position of the reception window of the scattered 

 light between frames 12 and 13, i.e., 4.2 m in front 

 of the propeller plane. The additional geometrical 

 fixing of the control volume in the vertical direction 

 resulted from the laser window (located between 

 frames 13 and 14) with its horizontal beam outlet 

 into the water. Subsequently the positions for 

 the control volume was fixed as follows : 90 cm of 

 the ship's hull vertically downward and 145 cm from 

 midship on the port side between frames 12 and 13 

 (see Figure 2) . 



The Calibration Device 



The relationship between the photomultiplier impulse 

 amplitude and the size of nuclei was determined by 

 a calibration with latex spheres. For this purpose 

 a special device was put through an opening in the 

 ship's hull when the ship stopped in calm water. 

 With this device it was possible to maneuver a 

 fine nozzle near to the control volume and to inject 

 the latex spheres into the control volume. The 

 apparatus was operated by means of small hydraulic 

 elements from the inside of the ship (Figure 12) . 



For the calibration latex spheres of 45 and 25 

 ym were used. The corresponding photomultiplier 

 impulse amplitudes fit excellently to the theoretical 

 curve of the scattered light intensity. The measuring 

 range was set to 8-117 pm for the nuclei diameter. 



In addition to the scattered light intensity, 

 the dimensions of the control volume were important 

 data for the determination of nuclei spectra and 

 nuclei concentration. Since a direct measurement 

 or calculation of the cross section of the laser 

 beam in the control volume was not possible in this 

 case, a new method had to be applied to determine 

 the laser beam dimensions. By means of the above 

 mentioned hydraulic device a small rotating wheel 

 with thin platinum wires was adjusted in such a 

 way that the wires cut the laser beam vertically 

 at the location of the control volume. Thus the 

 light in the direction of the photomultiplier was 

 scattered. The dimensions could then be determined 

 from the width of the photomultiplier impulses, the 

 distance between the axis of the small wheel, and 

 the light point on the small platinum wires 

 {determined by crossed platinum wires) and the 

 revolution number of the small wheel. The diameter 

 (25 \lm) of the platinum wire had also to be con- 

 sidered. The exact knowledge of the control volume 

 dimensions was also important for the measurements 

 of the local velocity, as described below. 



The dimension of the control volume in the longi- 



Sluice Valve 



Micro - Hydraulic Device 



FIGURE 12. Calibration device and arrangement. 



tudinal direction of the laser beam was adjusted as 

 usual through the measuring slit in front of the 

 photomultiplier, after the enlargement factor of 

 the reception optic was determined. This again was 

 done by means of the hydraulic device with which 

 an object, whose dimensions were known, was placed 

 in the control volume; its picture was measured in 

 the plane of the measuring slit. 



For the nuclei measurement the dimensions of the 

 control volume were then fixed as follows: 0.86 mm 

 X 0.86 mm x 1.33 mm = 0.98 mm^ . The cross section 

 of the control volume rectangular to the flow 

 direction amounted to 0.86 mm x 1.33 mm = 1.14mm . 

 This detail was required for the determination of 

 the nuclei concentration. 



Measurement of Local Velocity 



When the cross section of the control volume and 

 the number of the nuclei measured per unit time , 

 were known, it was necessary in addition to know 

 the local flow velocity at the control volume in 

 order to determine the nuclei concentration. Since 

 the conversion of model test results from wake field 

 measurements to full scale appeared to be too in- 

 accurate for the determination of the local velocity 

 and because the measurement of the velocity with a 

 Prandtl tube, for instance, was not possible, a 

 new method was applied to measure the velocity and 



