318 



.v'^^'i 



Laser 



Rectangular 

 aperture 



? Measuring 

 ' slit 



FIGURE A2.1. Principle of velocity measurement. 



FIGURE A2.3. Sketch of inclined control volume and 

 flow direction. 



± 



A. 



FIGURE A2.2. Sketch of inclined control volume and 

 received photomultiplier signals. 



In example II Figure A 2.2 the main axis of the 

 rectangular aperture is positioned parallel to the 

 projection of the flow direction versus the plane 

 vertical to the optical axis of the laser, and 

 consequently the peak of the pulse width distribution 

 is at a maximum value of t. 



In example III in Figure A 2.3 the direction of 

 the measuring slit is also parallel to the projection 

 of the flow direction versus the plane vertical to 

 the optical axis of the photomultiplier, so that 

 all dimensions of the measuring volume in the 

 direction of the flow are the same, and the pulse 

 width distribution therefore shows its most narrow 

 shape. The peak of the distribution indicates the 

 velocity in the main direction, whilst the shape of 

 the curve is a measure of the turbulence level . 



The direction of flow can now be determined by 

 the position of the rectangular aperture and the 

 measuring slit. They each define a plane containing 

 the corresponding optical axis, whereby the line 

 of intersection represents the direction of the 

 main flow in this region. 



