Laser Doppler Velocimetry (LDV) is a technique that fluid mechanics researchers use to make instantaneous velocity measurements (magnitude and direction) of fluid flows. The technique is non-intrusive (no physical probe in the flow), can deliver measurements independent of ambient conditions, measures three directional components, can virtually access any flow region with the aid of fiber optics, and has a dynamic range from natural convection to supersonic velocities.

LDV makes use of the coherent wave nature of laser light. The crossing of two laser beams of the same wavelength produces areas of constructive and destructive interference patterns. The interference pattern, known as a 'fringe' pattern is composed of planar layers of high and low intensity light. Velocity measurements are made when particles 'seeded' in the flow pass through the fringe pattern created by the intersection of a pair of laser beams. These particles scatter light in all directions when going through the beam crossing. This scattered light is then collected by a stationary detector (receiving optics connected to a photomultiplier). The frequency of the scattered light is Doppler shifted and referred to as the Doppler frequency of the flow. This Doppler frequency is proportional to a component of the particles velocity which is perpendicular to the planar fringe pattern produced by the beam crossing. In order to obtain three components of velocity, three sets of fringe patterns need to be produced at the same region in space.

VIEW ANIMATION OF VELOCITY MAGNITUDE

This is an actual cross-sectional view of the beam crossing of two pairs of beams which illustrates a two component fringe pattern produced by an Argon-Ion Laser. The fringe patterns orientated vertically, scatter light proportional to the component of velocity normal to them, whereas the horizontal set scatters light proportional to the component of velocity normal to it. Adding a third pair of beams orthogonal to the first two pairs creates a third set of fringe patterns allowing for the measurement of the third component. When making velocity measurements, the fringe patterns are frequency shifted so they 'move' or cycle with a certain frequency. Frequency shifting is done with a device knows as a Bragg cell, which allows relative velocity measurements to be obtained.

Phase resolved LDV measurements of three components of velocity are carried out in motored transparent-cylinder engines as shown in the picture to the left. These measurements are used to calculate the turbulence, swirl, and tumble characteristics of piston/cylinder head arrangements which are critical to the mixing of the charge in IC engines. These ensemble averaged measurements are processed and animated on workstations. In this application, the speed of the signal processing equipment limits the resolution of the data to a few crank degrees, at a few thousand rpm. Turbulence measurements are processed by calculating the kinetic energy of the three fluctuating components of velocity via the standard deviation of the phase averaged measurements. Phase-averaged measurements are invaluable for studying the mixing qualities during intake and compression of a given intake geometry and as target data for computational models.