Toru Koso

Mixing of Matter by a Rising Bubble Near a Wall

The turbulent mixing of liquid mass caused by an air bubble rising near a wall in a still liquid in a pipe is investigated experimentally using a photochromic dye. A part of the liquid is activated by UV light and subjected to the fluid motion caused by a zigzag rising bubble of which Reynolds number is 214. The visualized mixing patterns showed that the dye is mixed by vortex motions in the bubble wake that is similar to the case of a bubble rising in the center of the pipe. The concentration distributions were deduced from the dye images using Lambert-Beer’s law and the turbulent diffusion coefficient (TDC) was evaluated from the temporal changes in the mass dispersion. The TDCs showed that a near-wall bubble generates stronger mixing than for a bubble in the center of the pipe. This stronger mixing can be attributed to the large-scale vortices observed for a near-wall bubble, which remains active for a longer time due to the lack of oppositely rotating vortices and mixes more fluids.Copyright

Study on the Flow Characteristics of Synthetic Jets Near a Rigid Plane Boundary

The jet flows have been applied to various fields to control the flow separation. Over the last decade, several studies have investigated synthetic jets. However, there are still many clarifications needed, including details of the structure and Coanda effect of synthetic jets. The present study clarifies some fundamental flow characteristics of free synthetic jets and synthetic jets near a rigid boundary by conducting an experiment and numerical simulations. As the main results, it is found that the velocity distribution of free synthetic jets depends on K = Re/S2 (the ratio of the Reynolds number to the square of the Stokes number) and can be identified by the maximum velocity at the centerline and the jet half-width. Flow visualization is carried out applying the smoke wire method. In addition, it is confirmed that the flow characteristics of the synthetic jet near a rigid boundary and re-attachment length of the synthetic jet are determined not only by H1 /b0 (normalized step heights) but also K.Copyright

Agitated Turbulent Flowfield of a Circular Jet With an Annular Synthetic Jet Actuator

An agitated circular turbulent jet with a coaxial annular synthetic jet actuator is investigated experimentally. The mean and turbulent velocities are measured by a hot-wire anemometer at a jet Reynolds number of 6400. The measured iso-velocity contour maps indicate that the width of the circular jet is increased even when the intensity of the alternating flow is too weak not to generate a synthetic jet. This can be attributed to the periodic disturbances by the actuator, which agitates the instability in the initial shear layer and moves the shear layer growth to upstream. With a strong agitation so as to generate a strong synthetic jet by itself, the agitated jet has the higher velocity and the larger width than those of unagitated jet. With this agitation, the momentum of the agitated jet is increased and its increment is equal to the momentum of the synthetic jet. Smoke visualization shows clearly that the actuator enhances the dispersion of primary jet fluids. These results are compared with those of a jet at a low Reynolds number of 3088 (Koso, 2005) and it is found that the mixing enhancement is qualitatively same for both Reynolds numbers, but the weak agitation is more effective in the low Reynolds number jet.Copyright

Measurement of Turbulent Mass Mixing Caused by a Particle Wake

When a particle moves in a fluid, the fluid is disturbed by the particle and the fluid mass is mixed. This phenomenon is commonly observed in particle-laden flows and dispersed bubble flows. This mass mixing can be composed of two mechanisms. One is the mass transfer by convective flows that are induced by the reaction of the particle drag and the other is the turbulent mass mixing in the turbulent wake of the particle. The effect of the former one can be evaluated using the previous studies on the particle drag. However, the effect of the turbulent mixing is little understood. The turbulent mixing caused by a particle wake is investigated by visualization and noninvasive concentration measurement using a photochromic dye. A sphere brass particle of 5mm in diameter is dropped in kerosene filled in a vertical pipe and the mixing of dye is visualized. The photochromic dye, which is activated by an ultraviolet light, keeps its color in a few minutes after the activation. A part of the fluid is activated without disturbances and is subjected to the mixing by the particle wake. The visualized dye patterns indicate the dye is mixed isotropically by large-scale vortex motions when the particle sheds the vortices. Furthermore, the photochromic concentration measuring (PCM) technique is developed to obtain the concentration of the mixed dye. This PCM technique is based on the Lambert-Beer’s law for light adsorption and provides the average dye concentration along the light path. The measured concentration distribution shows rather isotropic mixing in longitudinal direction. The turbulent diffusion coefficient (TDC) is calculated from the temporal changes in the measured concentration distributions. The evaluated TDC shows strong time-dependency, which is attributed to the change in scale and strength of wake vortices. The TDC is about 104 times larger than the molecular diffusion coefficient at its maximum. The effect of particle Reynolds number on the turbulent mixing is also investigated for the Reynolds number range from 263 to 3290. The observed mixing patterns show a drastic change at the critical Reynolds number on the vortex shedding from the particle. The Reynolds number dependency on the non-dimensional TDC and mixing time are discussed.Copyright

Applicability of Image Pattern Correlation Velocimetry

The particle imaging velocimetry (PIV) based on the pattern correlation method has an inherent problem of erroneous velocity vectors due to erroneous correlation. Two types of images were examined; one was the image of air flow visualized by a smoke tracer method and the other was the image of absolutely static water taken by a rotating video camera. The gray level difference method was also applied and the results were compared with those of the pattern correlation method. The probability of erroneous velocity vectors is strongly affected by the correlation matrix size as well as by the type of image.

Visualization of Mixing Process in a Cell Mixer Agitated by a Piezoelectric Vibrating Stirrer

The mixing process in a cell mixer by a piezoelectric vibrating actuator as a stirrer was visualized by means of a dye-tracer method and was pictured using a video camera. The local concentration of dye was transformed to a gray scale using an image processing technique. The quantitative information on 2D mixing process as well as the flow pattern were deduced and discussed by the image processing. The time variation in the probability density function of gray scale for the whole area as well as the time variation in the local gray scale were also discussed.