The Development of Multi - Point Laser Doppler Anemometry ( MPLDA ) : A New Method to Estimate Fluid Flow Statistics

In this paper we describe a method to measure fluid flow fields that we call Multi-Point Laser Doppler Anemometry (MPLDA). Like Doppler Global Velocimetry (DGV) and single point Laser Doppler Anemometry (LDA), the technique derives information from the Doppler shift observed when light is scattered from a moving particle and exploits the advantages of these techniques. The method retains the elegant simplicity of DGV, but uses coherent heterodyne detection as in LDA rather than applying an absorption edge to demodulate the Doppler signal (as in DGV). This makes MPLDA suitable for the measurement of flow fields throughout the velocity range. In contrast with pulsed laser imaging techniques, such as Particle Image Velocimetry (PIV) and its derivatives, MPLDA does not require individual seeding particles to be resolved as is the case with LDA and DGV. We believe that this makes MPLDA unique in its ability to gain whole field measurements in large scale, low speed wind tunnel facilities. In essence MPLDA utilises a streak camera and coherent detection to record a short segments of the Doppler signals collected from many points within a seeded flow field. The streak image is then analysed to estimate the Doppler frequency at each point in the flow. In our original studies we used reflection from a galvanometer mirror to implement streak imaging. Using this technology we were able to prove the principles of MPLDA by measuring the surface velocity of a spinning disk. In this case the disk velocity was low (1/20 rev/sec) and consequently the measured Doppler frequencies were in the range of 0-10kHz. These initial results clearly demonstrated the potential of MPLDA, however, the scale and speed of the experiment are several orders of magnitude less than that which would be expected in a wind tunnel. In addition the reference beam necessary for coherent detection did not include a frequency shift and as such the measurements were subject to directional ambiguity. More recently we have conducted investigations into the use of electronic vacuum tube technology as a means to implement the streak imaging requirement of MPLDA. A streak tube of this kind is similar in principle to an image intensifier and uses deflection plates to displace electrons en route to an output phosphor screen. In this way pico second resolution is possible. In what follows, a streak tube in conjunction with a Peltier cooled CCD camera is used to provide a flexible MPLDA streak system for both low and high speed wind tunnel facilities. We demonstrate the use of this system as a means to measure the velocity of spray particles within the measurement volume of a basic, single component, LDA system.