Simultaneous measurement of flow velocity variations at several points with the multi-point LDV

Variations of integral length scales and flow velocity distributions in a turbulent air jet are examined using a multipoint LDV system. The multipoint LDV contains a beam splitter, a phase diffraction grating, and lenses of special shape used for focusing the beams. The method for processing LDV signals detected by several photomultipliers is investigated using an electric scanner with light speed, a frequency counter, and a personal computer. It is observed that the intensity of the diffraction beams is three times greater when using the phase diffraction grating rather then the amplitude diffraction grating; the measurable maximum frequency of velocity variation is 7 kHz for the gate pulse width of 10 microsec, 3.6 kHz for 20 microsec, 1.4 kHz for 50 microsec, and 0.7 kHz for 100 microsec; and the variations of integral length scales during short period and flow velocity distributions can be measured directly.