The measurement of velocity fields of a plunging wave impacting on a structure in a two-dimensional wave tank was investigated experimentally. As the wave impinged and overtopped the structure, a large highly aerated region was created in front of the structure and on top of the structure. The broken wave in front of the structure and associated greenwater on top of the structure are highly aerated containing not only a large number of bubbles but also very large sizes of bubbles. The highly aerated bubbly flow caused the traditional particle image velocimetry (PIV) technique to fail due to the uncontrollable scattering of laser light. A modified PIV method, called bubble image velocimetry (BIV), was introduced by directly using bubbles as the tracer and measuring the bubble velocity by correlating the ‘texture’ of the bubble images. No laser light sheet was needed while the depth of field was limited to minimize the error. Velocity measurements using BIV and fibre optic reflectometer were compared to validate the BIV technique. While the fluid velocity in the region where no or few bubbles exist can be successfully obtained using PIV, the velocity in the high void fraction region can be measured using BIV. Therefore, BIV can be seen as a complementary technique for PIV. The use of BIV is essential in the studied problem here due to the fact that in the vicinity of the structure the flow is almost entirely bubbly flow. From both the PIV and BIV measurements, it was found that the maximum fluid particle velocity as well as the bubble velocity in front of the structure during the impinging process is about 1.5 times the phase speed of the waves.

Reply to "Comment on 'Fiber optic reflectometer for velocity and fraction ratio measurements in multiphase flows' " [Rev. Sci. Instrum. 74, 3559 (2003)] Rev. Sci. Instrum. 75, 286 (2004); 10.1063/1.1634361Comments on "Fiber optic reflectometer for velocity and fraction ratio measurements in multiphase flows" [Rev.A technique based on the coherent mixing of scattered signal with Fresnel reflection signal from the tip of an optical fiber is used to demonstrate the feasibility of measuring the velocity and fraction ratio of solid particles and gas bubbles or liquid droplets in a liquid or gas flow. If the liquid or gas flow is seeded with small neutrally buoyant particles, the technique is then capable of measuring the velocity as well as the fraction ratio of all three phases of the flow at a given point. The method is briefly described as follows. An optical signal derived from a diode laser driven by a constant current is launched into a single-mode optical fiber and transmitted, through a fiber coupler, to the signal fiber inserted into the test fluid. The diode laser used is a multilongitudinal mode device that has a low coherence length of about 200 m. The coherently mixed signal propagates back to the signal fiber, through the fiber coupler, and detected by a detector. By analyzing the signal, the velocity and fraction ratio of each phase can be obtained. Using water seeded with small solid particles and air bubbles, it is demonstrated that the technique is capable of measuring the velocity in the direction parallel to the fiber. Since the only intrusion to the fluid is the tiny fiber probe ͑a dimension of 125 m in diameter͒, the disturbance to most fluid flows is negligible, therefore, the technique is nearly nonintrusive.

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A fiber optic reflectometer (FOR) technique featuring a single fiber probe is investigated for its feasibility of measuring the bubble velocity, diameter, and void fraction in a multiphase flow. The method is based on the interference of the scattered signal from the bubble surface with the Fresnel reflection signal from the tip of the optical fiber. Void fraction is obtained with a high accuracy if an appropriate correction is applied to compensate the underestimated measurement value. Velocity information is accurately obtained from the reflected signals before the fiber tip touches the bubble surface so that several factors affecting the traditional dual-tip probes such as blinding, crawling, and drifting effects due to the interaction between the probe and bubbles can be prevented. The coherent signals reflected from both the front and rear ends of a bubble can provide velocity information. Deceleration of rising bubbles and particles due to the presence of the fiber probe is observed when they are very close to the fiber tip. With the residence time obtained, the bubble chord length can be determined by analyzing the coherent signal for velocity determination before the deceleration starts. The bubble diameters are directly obtained from analyzing the signals of the bubbles that contain velocity information. The chord lengths of these bubbles measured by FOR represent the bubble diameters when the bubble shape is spherical or represent the minor axes when the bubble shape is ellipsoidal. The velocity and size of bubbles obtained from the FOR measurements are compared with those obtained simultaneously using a high speed camera.

A new and simple normalization technique that greatly enhances the measurement resolution of conventional fibre-optic reflectometry based on Fresnel reflection from the tip of a fibre is used for demonstrating the feasibility of measuring solute concentrations and index changes in fluids to very high precision. The amplitude of pulses originating from reflection from the fibre–fluid interface is compared in real time with the amplitude of reference pulses from a fibre–air interface such that errors caused by pulse amplitude fluctuations and slightly varying detector responses are corrected. Using solutions of sodium chloride and water, it is demonstrated that the technique is capable of measuring index changes of 2 × 10−5 corresponding to a NaCl concentration of 0.02%.