Flow visualization

Micromixer is one of the most significant components of microfluidic systems,
which manifest essential applications in the field of chemistry and biochemistry. Achieving
complete mixing performance at the shortest micro channel length is essential for a
successful micromixer design. We have developed five novel micromixers which have
advantages of high efficiency, simple fabrication, easy integration and ease for mass
production. The design principle is based on the concept of splitting-recombination and
chaotic advection. Numerical models of these micromixers are developed to characterize
the mixing performance. Experiments are also carried out to fabricate the micromixers
and evaluate the mixing performance. Numerical simulation for different parameters such
as fluids properties, inlet velocities and microchannel cross sectional sizes are also
conducted to investigate their effects on the mixing performance. The results show that
critical inlet velocities can be predicted for normal fluid flow in the micromixers. When the inlet velocity is smaller than the critical value, the fluids mixing is dominated by
mechanism of splitting-recombination, otherwise, it is dominated by chaotic advection. If
the micromixer can tolerate higher inlet velocity, the complete mixing length can be further
reduced. Our simulation results will provide valuable information for engineers to design
a micromixer by choosing appropriate geometry to boost mixing performance and broaden
implicational range to fit their specific needs. Accurate and complicated fluidic control,
such as flow mixing or reaction, solution preparation, large scale combination of different
reagents is also important for bio-application of microfluidics. A proposal microfluidic
system is capable of creating 1024 kinds of combination mixtures. The system is composed
of a high density integrated microfluidic chip and control system. The high density
microfluidic chip, which is simply fabricated through soft lithography technique, contains
a pair of 32 flow channels that can be specifically addressed by each 10 actuation channels
based on principle of multiplexor in electronic circuits. The corresponding hardware and
software compose the control system, which can be easy fabricated and modified,
especially for prototype machine developing. Moreover, the control system has general
application. Experiments are conducted to verify the feasibility of this microfluidic system
for multi-optional solution combination.

Particle image velocimetry and flow visualization are used to characterize the wake of a heaving airfoil in a set of two experiments. In the first experiment a tandem airfoil configuration is used, with a stationary airfoil downstream of a heaving airfoil (modified Schmidt wave-propeller). Several vortex structures are identified for a forced Strouhal number (St)---based on airfoil chord-length, forcing frequency, and free-steam velocity---for 0.1 < St < 0.7. An asymmetric average velocity profile is measured in the upper St range. In the second experiment, the wake behind a single heaving airfoil is further inspected, with the purpose of highlighting the asymmetric wake, for 0.1 < St < 1.0. A maximum wake excursion of 18 degrees is measured at St = 0.6, and a minimum excursion of 5.7 degrees occurs at St = 0.9. Using averaged velocity profiles, a virtual origin of the wake excursion is also calculated.

When a boundary-layer flow, either laminar or turbulent, encounters a hemispherical body extending from a surface, a horseshoe-shaped vortex forms at the juncture. In this thesis, we study the evolution of this vortex using a numerical inviscid model and laboratory experiments. The numerical model is based on determining the evolution of the filament using the cut-off method. The assumption is that although the generation of the vortex depends on viscous effects, the dynamic evolution is well described by inviscid equations of motion. It is found that the vortex filament is fairly steady on the upstream side but on the downstream side, travelling waves appear on it which cannot be suppressed through evolution. For a range of Reynolds number, steady horseshoe-shaped vortex was obtained in the experiments, revealing the shape past the hemisphere. This is compared with the numerical results.

A study of two-layer quasi-geostrophic vortex flow is performed to determine the effect of a current difference between the layers on a vortex initially extending through both the layers. In particular, the conditions under which the current difference can 'tear' the vortex are examined. In the first set of flows studied, the current difference is generated by a (stronger) third vortex in the upper layer located at a large distance from the (weaker) vortex under study. A set of flows are also considered in which an ambient geostrophic current difference is produced by a non-uniform background potential vorticity field. The results of the study will be useful in determining the conditions under which large geophysical vortex structures, such as cyclones and ocean rings, can extend to large depths even though the mean currents in the ambient flow change significantly along the vortex length.

This study pertaining to the settling of fine particles is developed using various models and formulas. The model considers two layers, i.e, the suspension zone dominated by perikinetic flocculation and the settling zone governed by gravitational force. In the suspension zone, floc formation of fine particles is simulated by the maximum chain model in which floc parameters and fractal dimension are compared with existing data. In addition, fractal dimension is compared with that of the hierarchical model. The main assumption of the model is that any floc having sixteen particles outweighs Brownian force, and thus the floc starts falling down into the settling zone. The flocs moving from the suspension zone are considered as nonspherical particles in the settling zone. The study uses a dimensionless settling velocity, omega*, for estimation of the sedimentation of flocs. Settling causes aggregation of the depositing flocs. The form of these aggregates is analyzed by the fractal relationship P ~ L delta.

The transition of two dimensional flow within a crack
with oscillating wall is studied experimentally to establish
the role of the parameters involved. Multiple instabilities
in the fluid are produced by the motion of the oscillating
wall. For a better understanding of the flow structure and
displaying in the whole flow field at a time, flow
visualization was used. Frequency, amplitude, and crack
width were systematically varied. The present study shows
that, in general, the frequency and crack width play a role
in the transition process at low frequencies of
oscillations. However, amplitude becomes progressively
important at its higher range. Results also show that the
flow changes its character approximately after a Reynolds
number 11420. After that the transition process continues
until the Reynolds number 34322 which is the upper limit of
variation of frequency (Reynolds number) in the present
study.