Su, Tsung-Chow

The drag reduction by vortex fusion was investigated. A comparison of flow over a bundle of cylinders in uniform and in disturbed currents was performed in a water channel. The model was subjected to cross flow. A thin cylindrical wire located nearby upstream and leveled at half the height of the test model was used as a source of disturbance. A hydrogen bubble technique was utilized to observe the flow pattern. The accumulation of vortices at stagnating regions in front of a bundle of cylinders transformed into a counter-rotated curl at leading edges of each leading cylinder in the bundle. Measurements were carried out by a computerized data acquisition system. Drag coefficient measurements, digital spectral and fourier analyses were also performed. Results have shown that a drag reduction can be obtained by introducing a thin cylindrical wire in front of the stagnation.

This thesis addresses the groundwater contamination problem from a probabilistic point of view. The objective of this study is to obtain analytical solutions for the backward type Kolmogorov equations governing certain crucial variables in the case of a two-dimensional medium. These include the rate at which pollutants enter the protected zone and the length of time that a pollutant particle takes to travel from a given location to the boundary of the protected zone. It is assumed that the groundwater flow is horizontal and uniform, and that a protected zone is located downstream. Analytical solutions are obtained by using the techniques of Laplace transform and separation of variables. Numerical results are given for two cases, one with flow coming from a boundary which is imperious to the pollutant, and another with flow coming from infinity.

Wind loads on a bridge may be classified into two types: the buffeting loads and the self-excited loads. The research reported in this thesis is concerned with experimental determination of the self-excited loads in the frequency domain, in particular, their non-dimensional coefficients, called flutter derivatives. The experiments were conducted in a water channel with water substituting for air. Five bridge-section models of different shapes were tested, each of which was driven to move harmonically by linkages, and the forces on the linkages were measured to determine the fluid loads. A thin-plate model, simulating an airfoil, was also tested and the results were compared with those obtained from the thin airfoil theory. The setup of the experiments and data acquisition, processing and analysis are presented herein.

The discrete vortex method was applied to the calculation of separation flow past NACA 4412 airfoil: Vortex panel was used to represent the body surface and discrete vortices were used to model the wake. Generally the uniform upstream condition is used in the calculation of separation flow. But actually an airfoil could move in wake of other airfoils or disturbed fluid. In this thesis, discrete vortices were used to model the upstream disturbance to investigate the effect of upstream disturbance to the pressure, lift and drag coefficients around the airfoil. Also the animation of separation flow was made on HP workstation using "Starbase" computer graphic package to study the separation process.

It is empirically observed and broadly used in the semiconductor manufacturing industry that the clean environment of in-vacuum processing can be enhanced through heating from outside the clean chamber. This paper reports an analytical investigation of this problem and presents the results that confirm the speeding effects of changing the temperature gradient at one end of the clean chamber on venting the aerosol particles from inside the clean chamber. First, a mathematical frame for the basic problem is formulated and then various analytical techniques are employed to reduce a set of coupled nonlinear partial differential equations to linear equations, and finally, numerical results are obtained from explicit, analytical formulas.

Search and rescue of crafts in distress on the sea surface require the capability to accurately predict the drift of target craft in a short time. A nonlinear mathematical model (original model) has been developed as a tool for the study of the boat drift. Wind load computation considers the wind gradient and the vessel geometry. The current loads include form and friction drag. From the numerical evaluations, the effects of various environmental and vehical parameters on drift are properly accounted for. Simplification of the original model results in a model which is computationally 60 times faster, so-called version 1, without losing much accuracy. In order to meet the needs of two minute prediction for practical implementation, a further simplified model, version 2, was developed from version 1. It is found that the computing speed can be improved one hundred times, and this will facilitate the practical implementation of FAU model in search planning.

This thesis addresses the issue of flow development near the boundary of a work table within a clean room. The flow is subjected to periodic external disturbance, either through the pulsation of a source of mass at fixed location, or a moving vortex. The global system consists of a flat table in a parallel oncoming laminar flow. The source of the disturbance is located in the vicinity of the boundary layer. The strength of disturbance is limited in order to apply the quasi-steady boundary layer theory. Based on a quasi-steady assumption, a Thwaytes type integration was performed in order to evaluate the displacement thickness and the shear stress on the wall. A parametric study based upon the change of the pulsation, the location and the strength of the disturbance was included in the study. Thwaytes's deviation applied to unsteady cases proved to be successful, and worth being used in further developments.

The behavior of a swirling buoyant turbulent jet in a cross
flow has been studied. Dimensional analysis has been
carried out to obtain asymptotic relations for the jet
trajectory and dilution. Experiments were carried out to
ascertain the validity of the relations obtained and to
evaluate the constants arising in the analysis. While
photographic observations were made to study the jet
trajectory, concentration measurements were made using a
light probe. Measurements of the spreading angle of the jet
were also made. The study indicated that swirl caused an
increase in spreading angle of the jet, and a great
improvement in jet dilution.

The plume resulting from a subsea well blowout alters the
environmental loading on a floating platform which is
directly above the well. Horizontal ocean currents deflect
the plume and may create a more severe loading on the
structure. During the early stages of a blowout the
"starting plume" induces forces of a different magnitude on
the structure. A preliminary experimental study,
investigating the transient loads induced by starting plumes
and the effects of currents on blowout plumes, was
conducted. The results of the experiments showed that the
forces involve significant fluctuations which are quite
different from the average values. The normal forces induced
by starting plumes were slightly greater. Horizontal
currents caused an increase in the moment acting on the
structure. This is of considerable importance as moments,
when coupled with the situation of flooded decks and
compartments, may cause the structure to sink.

A numerical algorithm based on the Volume of Fluid
technique, is used to study the impact loading of liquid
sloshing in partially filled enclosed prismatic tanks
subject to large amplitude excitation. A moving coordinate
system fixed to the tank is introduced. Based on the
Navier-Stokes equations, finite difference equations are
solved. The entire flow configuration is advanced through
a finite increment of time. In transporting the fluid
through the mesh, an improved donor-acceptor method which
takes into account surface orientation and transports
trapezoidal shapes from cell to cell is used. The results
obtained using the simple and improved schemes are compared.
It is shown that a long time simulation can be achieved for
both shallow and deep liquid sloshing.