The 1-D laser Doppler instrumentation

The development of reliable measurement of instantaneous velocity gradients by obtaining profiles of one component of the velocity vector instantly is presented. The instrument consists of the standard transmitting optics of a LDV system modified with cylindrical lenses to form laser sheets. The laser sheets cross and form a cylindrical LDV measurement volume. The fringes of the volume are oriented perpendicular to the velocity component of interest and scattered light from particles moving through the volume transmits the velocity information to the photodiode array. The signal from the photodiodes is processed in two different ways. The analog LDV-burst is stored on tape. Later on it is digitized and numerically analyzed to obtain the relevant velocity information. The second method of reducing the data involves optical frequency analysis. The laser Doppler signal detected by the photodiodes is high-pass filtered and amplified. This signal drives a Bragg-cell which acts as an optical frequency analyzer. The displacement of the first order of deflection of a laser beam passed through the Bragg-cell is proportional to the signal frequency. By feeding the laser Doppler bursts observed by different photodiodes to the Bragg-cell, and correlating the frequency proportional displacement of the analyzing laser beam with the location along the LDV-volume, a velocity profile is obtained. In order to obtain instantaneous velocity profiles, a time window is opened which is sufficiently short to time resolve the turbulence of the flow.