Systems for 2D and 3D measurements
If the direction of speed is not known, it is necessary to measure two components if the motion is 2D, or three components if the motion is 3D. If the flow pattern is constant in time, the various components of velocity can be measured by rotating the optical emission of the LDA, so that the bisector of the angle formed by the interfering beams is normal to the direction of the velocity component to be measured.
If the motion is unsteady, this technique cannot be used because the measurements of different components of velocity would be made at different times: in turbulent flow it is mandatory to measure all components simultaneously. With the same device (Figure 4.17), two components of velocity can be measured in the plane perpendicular to the optical axis, using an argon ion laser: the beam is split into two beams by a beam splitter, the green and blue colors are separated by a prism, pairs
Four beams LDA for the measurement of two components. Backscatter configuration
of each color beams are focused in two perpendicular directions in the same measurement volume.
The light scattered from particles contains both colors; two photomultipliers, each preceded by an appropriate interferometric filter, green or blue, respectively, will measure only one of the two components of velocity.
If a third component of velocity has to be measured, another separate LDA is needed with an optical axis normal to that of the 2D system (Figure 4.18) which uses a third color, such as the purple of an argon ion laser.