Kuang-An Chang

Vortex generation and evolution in water waves propagating over a submerged rectangular obstacle. Part II: Cnoidal waves

Vortex generation and evolution due to flow separation around a submerged rectangular obstacle under incoming cnoidal waves is investigated both experimentally and numerically. The Particle Image Velocimetry (PIV) technique is used in the measurement. Based on the PIV data, a characteristic velocity, phrased in terms of incoming wave height, phase speed, dimension of the obstacle, and a local Reynolds number are proposed to describe the intensity of vortex. The numerical model, which solves the two dimensional Reynolds Averaged Navier Stokes (RANS) equations, is used to further study the effects of wave period on the vortex intensity. Measurements for the mean and turbulent velocity fields further indicate that the time history of the intensity of fluid turbulence is closely related to that of the vortex intensity.

Velocity, acceleration and vorticity under a breaking wave

The fluid particle velocities in the overturning jet of a breaking wave have been measured by the Particle Image Velocimetry (PIV) technique. Monochromatic waves with wave height of 14.5 cm and wavelength of 121 cm were generated in the water depth of 20 cm. The measured fluid particle velocity at the tip of the overturning jet reached 1.68 times of the phase velocity calculated from the linear wave theory. Fluid particle accelerations were estimated from the velocity data with the following results: The overturning jet enters the horizontal water surface with an acceleration of 1.1 g at an angle of 88° downward. The PIV technique was also used to measure the instantaneous vertical vorticities generated by breaking waves. The number and locations of the vortices on the horizontal plane appear to be random. The maximum instantaneous vorticity was in the order of magnitude of 20–30 s−1, whereas the ensemble-averaged vorticity was quite small.

Experimental investigation of turbulence generated by breaking waves in water of intermediate depth

This paper reports a set of laboratory data for breaking waves in the water of intermediate depth. A monochromatic wave train with a wave height of 14.5 cm and a wavelength of 121 cm was generated in a water depth, h, of 20 cm. The wave train breaks consistently at a distance of about 2h from the wave generator. The instantaneous velocity fields under the breaking waves on a two-dimensional vertical plane were measured by using the particle image velocimetry (PIV) technique. By repeating the same experiments twenty times and performing the ensemble average, mean velocity, mean vorticity, turbulence intensity, and other flow properties such as the Reynolds stress and the mean strain rate were calculated. Outside the aerated region, where the density of air bubbles is high, the experimental data show that the mean vorticity was of the same order of magnitude as (C/h) (≈6 s−1) with C being the phase speed. The maximum turbulence intensity outside the aerated region was in the order of magnitude of 0.1 C (≈11...

Electrospinning of silica nanochannels for single molecule detection

We have fabricated silica nanochannels with inner diameter as small as 20nm using a scanned coaxial electrospinning and demonstrated their application for single molecule detection. A coaxial jet, with the use of motor oil as the core and silica sol-gel solution as the shell, is extruded through a coaxial source and deposited on the rotating collector as oriented nanofibers. They are then annealed to cross-link silica and eliminate motor oil, thereby forming nanochannels. Subsequently, a fluorescent dye was injected into the individual nanochannels via a capillary force and single molecule detection was performed by monitoring the photon signals from 5-Iodoacetamidofluorescein.

Fiber optic reflectometer for velocity and fraction ratio measurements in multiphase flows

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 μm. 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 μm in diameter), the disturbance to most fluid flows is negligible, therefore, the technique is nearly nonintrusive.

A fibre optic Fresnel ratio meter for measurements of solute concentration and refractive index change in fluids

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%.

Experimental study on plunging breaking waves in deep water

This study presents a unique data set that combines measurements of velocities and void fraction under an unsteady deep water plunging breaker in a laboratory. Flow properties in the aerated crest region of the breaking wave were measured using modified particle image velocimetry (PIV) and bubble image velocimetry (BIV). Results show that the maximum velocity in the plunging breaker reached 1.68C at the first impingement of the overturning water jet with C being the phase speed of the primary breaking wave, while the maximum velocity reached 2.14C at the beginning of the first splash-up. A similarity profile of void fraction was found in the successive impinging and splash-up rollers. In the highly foamy splashing roller, the increase of turbulent level and vorticity level were strongly correlated with the increase of void fraction when the range of void fraction was between 0 and 0.4 (from the trough level to approximately the center of the roller). The levels became constant when void fraction was greater than 0.5. The mass flux, momentum flux, kinetic energy, potential energy, and total energy were computed and compared with and without the void fraction being accounted for. The results show that all the mean and turbulence properties related to the air-water mixture are considerably overestimated unless void fraction is considered. When including the density variation due to the air bubbles, the wave energy dissipated exponentially a short distance after breaking; about 54% and 85% of the total energy dissipated within one and two wavelengths beyond the breaking wave impingement point, respectively.

Experimental study on mean velocity characteristics of flow over vertical drop

The characteristics of flows over a vertical drop were investigated experimentally using laser Doppler velocimetry for detailed quantitative velocity measurements, and a flow visualization technique for qualitative study of flow pattern. A range of velocity and depth of the subcritical approaching flows was tested to understand the flow structure at the regions of the falling jet, the sliding jet, and the energy dissipating pool. Using the measured velocity, four similarity profiles of the mean velocity at different locations were obtained: the jet velocity at the intersection of the falling jet and the sliding jet, the jet velocity along the free surface of the sliding jet, the maximum negative velocity and the mean horizontal velocity of the deflected wall jet in the pool.Variation trends of several important characteristic velocity and length scales of the deflected wall jet in the pool are also discussed

Bubble velocity, diameter, and void fraction measurements in a multiphase flow using fiber optic reflectometer.

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.

Mean and Turbulence Properties of a Neutrally Buoyant Round Jet in a Wave Environment

An experimental study of a turbulent round jet discharged into a regular wave field is presented. The particle image velocimetry (PIV) technique was employed to measure instantaneous velocity fields of the flow. Quantitative mean and turbulence properties were obtained using ensemble- and phase-averaged methods. Both the jet potential core region and self-similar region were measured. Three different wave heights were used to examine the effect of wave steepness on the jet properties. Measurements were also taken at three different phases to examine the wave phase effect. Experimental results demonstrate that the mean jet width, turbulence intensity, and Reynolds stress increased significantly when the jet was acted on by the waves. In addition, the jet mean kinetic energy decreased whereas the turbulent kinetic energy increased when the jet was under the waves, indicating an increase in turbulence. The wave phase has an insignificant effect on the both the mean and turbulence properties. The turbulent kinetic energy budget in the self-similar region at different wave conditions was also examined. It was found that the turbulent production, advection, and dissipation terms all increase with the increase of the wave height.