309 



flow direction. If the dimensions of the control 

 volume in the flow direction are known, the velocity 

 can be determined from the measured impulse width. 

 In order to estimate the local flow direction at 

 the control volume, an aperture is put into the 

 beam path of the laser (Figure 13) , which gives 

 the laser ray a rectangular shape. This aperture 

 is turned until the photomultiplier impulses have 

 reached a maximum. Then it is possible to determine 

 from the position of the aperture the position of 

 the plane, formed by the flow direction and the 

 laser beam. If now the measuring slit is turned 

 until the half width of the distribution of the 

 impulse width has reached a minimum, it is possible 

 to read - from the position of the measuring slit - 

 the plane which is formed by the flow direction 

 and the optical axis of the reception system. The 

 flow direction in the volume results from the inter- 

 section of the two determined planes ; the impulse 

 width gives the flow velocity, and the impulse 

 width spectra provides information on the degree 

 of the turbulence flow. 



In this way flow characteristics can be determined, 

 undisturbedly and locally, with one measurement; 

 otherwise they could only be determined with a 

 three-component measurement. Furthermore, the 

 control volume simultaneously reaches the optimum 

 inclination for the measurement of the nuclei size. 

 Thus one signal provides data about the distribution 

 of nuclei size and about the flow field. 



Large particles or bubbles require a longer 

 period to completely cross the control volume than 

 smaller particles at the same speed. This means 

 that besides the larger impulse amplitude there is 

 also a larger impulse width. These facts have to 

 be considered in the measurement of the velocity. 

 Therefore, a single-channel discriminator is inserted 

 into the impulse processing electronics. The 

 discriminator choses for the measurement only 

 impulses of the amplitude or a strongly limited 

 range of amplitudes. Thus it is possible to draw 

 a clear conclusion from the measured impulse width 

 on the speed of the particles in the control volume . 



The new technique to measure the velocity is 

 illustrated in the Appendix. A rectangular beam 

 cross section whose breadth is the vertical to the 

 flow direction, has proved to be the optimum for 

 the measurement of velocity and the determination 

 of the flow direction. 



General Remarks 



Originally it was planned to shift the height of 

 the measuring point on the optical axis of the 

 reception system by different laser beam directions. 

 In addition, this axis should be shifted laterally 

 through two additional observation windows between 

 the frames 12 and 13. This would make it possible 

 to measure at several points in the plane between 

 the frames 12 and 13. Unfortunately, this could 

 not be realized due to lack of time, because the 

 installation of the measuring equipment at the 

 beginning of the voyage had taken too much time . 



It is not intended to describe all the diffi- 

 culties which occurred at the installation of the 

 equipment. The problem of vibration, however, must 

 be mentioned. 



To protect the laser, vibration damping should 

 be guaranteed as far as possible. It was, however, 

 observed during the outward voyage that the pneu- 

 matic vibration isolation, which had a resonant 

 frequency of f^ = 1.8 up to 3.0 Hz, could not be 

 used, - even if the exciting blade frequency of the 

 propeller was within the range of 8 and 9 Hz. 

 Excitations occurred, of course, also at a propeller 

 speed of f = 1.8 Hz and due to seaway frequencies . 

 When it was obvious that different damper devices 

 also did not help, the support, on which the laser 

 and the photomultiplier were fixed, had to be 

 stiffly connected with the steel construction of 

 the after peak. This labor and the laser adjust- 

 ments required more than half the time of the voyage 

 to Australia during difficult climatic conditions. 

 The laser adjustment was carried out mainly when 

 the ship was stopped. The calibration of the 

 nuclei impulses and the determination of the control 

 volume, in which the nuclei were measured, were 

 also carried out during these periods. These were 

 kindly granted by the captain and his officers and 

 had to be regarded as a special concession since 

 the "Sydney Express" was on a fixed schedule. In 

 this connection it must also be mentioned that the 

 calibrations and later the measurements , made on 

 the return voyage, could only be carried out after 

 dark. For this reason, extra maneuvering watches 

 had to be set in the engine control room, usually 

 while the ship had a "16-hours-unattended-machinery- 

 space" . 



The above mentioned stiff support solved the 



13 



V 



UJ 



1 Laser 



2 Beam Expander 



3 Aperture 



'Idealized Photomultiplier 

 signal from scattering 

 objects 

 ^ Idealized signals with grey 

 wedge filter(5) in rays. 



Slope indicates direction 



k Spectral Filter 



5 Grey Wedge Filter 



6 Rectangular Aperture 



7 Lens 



8 Microscope Lens 



9 Flow Section 



10 Control Volume 



11 Receiving Lenses 



12 Measuring Slit 



13 Photomultiplier 



FIGURE 13. Principle of LSL-measurements . 



