Optical fiber velocimetry: a technique for measuring velocity in two-dimensional flows

In this article we describe a new technique, optical fiber velocimetry (OFV) to instantaneously measure two components of velocity at a point in freely suspended flowing liquid films. The technique relies on the measurement of displacements of an optical fiber tip which is coupled to the flowing film. The deflection of the fiber tip is proportional to the velocity and it behaves as a simple harmonic oscillator. Thus the low frequency response of the fiber gibes direct measurements of the flow velocity. A statistical test using data acquired simultaneously by the fiber and by a laser Doppler velocimeter shows good agreement between the two techniques. Velocity power spectra measured in the wake of a von Karman street and in two-dimensional (2D) grid turbulence using the OFV also compare favorably with the laser Doppler velocimeter. The OFV technique is simple and robust, allowing it to be used in a wide variety of flows that have strong 2D characteristics. New prospects of using multiple fibers to measure circulation and velocity correlations at several separated spatial points are discussed.