Proposal and testing for a fiber-optic-based measurement of flow vorticity

A fiber-optic arrangement is devised to measure the velocity difference, de ltav(l), down to small separation l. With two sets of optical fibers and co uplers the new technique becomes capable of measuring one component of the time- and space-resolved vorticity vector omega (r, t). The technique is te sted in a steady laminar flow, in which the velocity gradient (or flow vort icity) is known. The experiment verifies the working principle of the techn ique and demonstrates its applications. It is found that the new technique measures the velocity difference (and hence the velocity gradient when l is known) with the same high accuracy and high sampling rate as laser Doppler velocimetry does for the local velocity measurement. It is nonintrusive an d capable of measuring the velocity gradient with a spatial resolution as l ow as similar to 50 mum. The successful test of the fiber-optic technique i n the laminar flow with one optical channel is an important first step for the development of a two-channel fiber-optic vorticity probe, which has wid e use in the general area of fluid dynamics, especially in the study of tur bulent flows. (C) 2001 Optical Society of America.