The present invention is directed to a method and apparatus for use in Laser Doppler anemometry for obtaining substantial spacial coincidence of the intersection volumes of two pairs of intersecting laser beams in a fluid flow conduit to provide a better means for measuring characteristics important in studying fluid flow properties.
Studies on the properties of fluid flow mechanics including the development of turbulent flow models generally require an accurate determination of the Reynolds shear stress for a variety of flow geometries. The Reynolds stress elements are obtained from simultaneous measurement of two components of the flow velocity at the same point in space. For example, one element of the Reynolds shear stress is .tau.=-.rho.&lt;.DELTA.u.DELTA.v&gt;, wherein .rho. is the fluid density, .DELTA.u and .DELTA.v are the fluctuating parts of the two velocity components u and v respectively and &lt;&gt; means a time or ensemble average. The Reynolds stress is then determined and defined at a point in space within the fluid flow region.
Recently, Laser Doppler anemometry (LDA) has been developed as a method for measuring fluid velocity non-invasively. The LDA method requires that a pair of thin coherent light beams (laser beams) of the same frequency intersect within the traveling fluid of a conduit as shown in FIG. 1. Direct measurement of the Reynolds shear stress by the LDA method requires simultaneous measurements with two pairs of beams that intersect at the same spacial location within the fluid flow conduit. That is, it is necessary that the two intersection volumes of the two pairs of beams attain a spacial coincidence within the fluid flow conduit. Thus a two-channel or two-color LDA system is utilized to measure correlations between mutually perpendicular components of the fluid velocity.
However, in cylindrical conduits this spacial coincidence rarely occurs due to severe refraction effects at the cylindrical interfaces. This problem in obtaining spacial coincidence is not as great for rectangular fluid conduits because spacial coincidence of two pairs of intersecting beams can effectively occur anywhere within the conduit. But in the cylindrical type of conduits, such spacial coincidence is difficult to attain and rarely occurs.