The present invention relates to fluid velocity measurement apparatuses and particularly an apparatus for determining the velocity of a fluid at any point in the fluid which utilizes Doppler-shift spectral analysis.
In recent years there has been increasing interest in the physics of fluid dynamics in turbulent systems, for example fluid flow through turbine engines and the aerodynamic properties of cars. One of the problems encountered in the study of fluid dynamics is that of establishing accurate measurements of the velocity of the fluid at any point in the fluid. Non-intrusive techniques have in general been based on single-point laser Doppler anenometry (IDA). Unfortunately, to date the results from using LDA have included undesirably large deviations owing to a number of factors affecting the size of the potential errors in the system. Moreover, such techniques require repeated spatial scanning cycles to obtain data over a two or three dimensional region and so are time consuming and are limited to steady state flow.
An alternative method of measuring velocities in a fluid is particle image velocimetry (PIV) but this method involves problems with detection and processing of the data because images are overlaid. Also, obtaining three dimensional information on a fluid flow is very difficult using PIV.
In U.S. Pat. No. 4,919,536 a real-time system for measuring velocities in a turbulent fluid system in a two dimensional region is described using a Doppler Global Velocimetry (DGV) technique. The system described has a laser which emits a beam which is passed through a lens system. The lens system expands the beam into a sheet of light incident on a region of the fluid for which velocity measurements are required. The fluid is seeded with particles, which are carried in the fluid, so that when the particles pass through the sheet of lights the particles scatter the light and the frequencies of the scattered light are Doppler-shifted in dependence on the individual velocities of the particles. In this way a two dimensional picture of the different velocities of the fluid in the illuminated region may be achieved. The scattered light is observed through a further lens system which focuses the scattered light and then splits the resultant beam in two. Half of the beam is directed to a reference image camera and half of the beam is directed through an iodine cell, functioning as a frequency-to-intensity converter, and then to a separate Doppler image camera. The laser is tuned to emit light at a frequency which corresponds to a 50% point on the transmission profile for iodine. In this way Doppler shifts in the frequency of the scattered light to both higher and lower frequencies can be detected as variations in the intensity of the scattered light emerging from the iodine cell. The system also includes control circuitry for synchronization and image capture of the images from the two cameras to enable the display of the measured velocity field in real-time.
Although the DGV technique described above enables measurement of flow velocities of a fluid in a region, problems remain in achieving accurate absolute measurements for the velocities and in reducing the sizes of the errors in such a system. For example, the light emerging from the laser has a tendency to drift from the desired frequency; the response of the iodine cell to the incident light and in particular its transmission characteristics can vary considerably with temperature; and aberrations in the lens system can be introduced for example by stresses applied to the optical elements as a result of the manner in which the optical elements are mounted and by crystal formation on the windows of the iodine cell. It is also essential that the two light paths of the scattered light beam are accurately aligned or the results from the two cameras will be misrepresentative of the scattered light intensity. Moreover, the imaging electronics for synchronizing the images from the two cameras and for image capture is both costly and complicated. The definition of the velocity field measurements may, as a result, be poor. In fact a deviation of .+-.7.5 ms.sup.-1 for calculated fluid velocities is normally allowed for.