Hydrodynamics

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.

Hydrogen plasmas were produced in an axially symmetric magnetic
field which was nearly uniform in the Lisitano coil
region. The field could be converted from a shallow mirror
to a deep mirror (R = 44), to a cusp geometry by varying
the current ratio in two sets of magnet coils. Unguarded
Langmuir probes and a periodic potential probe proved unsatisfactory
due to sheath growth. A gridded Langmuir probe
measured ion temperature of 26 eV and electron temperature
of 9 eV, with ion density 7 to 10x10^8 cm^-3. The magnetic
boundary value problem was solved analytically and comparison
with previous experiments at Florida Atlantic University
showed the electron temperature to depend on the magnetic
field gradient in the Lisitano coil. It was also seen that
high voltage (250 V) on the periodic probe was not ionizing
neutrals.

An investigation was conducted to determine the relationship
between hydrodynamic boundary layer parameters and biofouling
growth rates. A summary of previous investigations of hydrodynamic
effects on biofouling is presented. Wall shear stress
is shown to be an important parameter and is described in
detail. A submersible water tunnel was designed to allow
investigation of a flat plate subject to a uniform flow of
seawater. Parallel flow past a flat plate with a laminar
boundary layer was used to ensure that experimental conditions
existed in which a known wall shear stress distribution was
establised. Tests were conducted off Virginia Key in Miami,
Florida. The results of the experiments clearly indicate the
existance of a threshold value of shear stress which inhibits
the attachment of the macrofouler under study, the acorn
barnacle (Balanus spp.). Reported growth rates from other
investigations are presented to substantiate results. Recommendations
are made for additional hydrodynamic investigations
in dealing with biofouling.

Autonomous Surface Vehicle (ASV) research and development is inspired by the navigating and communicatiog challenges of Autonomous Underwater Vehicles (AUVs). The development objective is to provide real time positioning of and communication with AUVs through the air-sea interface. Despite extensive research on AUVs, the ASV has had limited research. The NAVY's desire to make AUV's defense capabilities realizable adds to the project's appeal. Guidance and control play an integral part in the ASV's success, motivating this thesis work. The overall vehicle dynamics were modeled and numerically simulated for 3 DOF lateral motion. These are development tools for the testing and tuning of PID and adaptive control algorithms. The results show the adaptive controller to be advantageous in terms of tuning, robustness and tracking performances. It uses a single layer neural network that bypasses the need for information about the system's dynamic structure and characteristics and provides portability.

A three-dimensional nonlinear time-dependent boundary-integral algorithm is developed to compute wave forces on an underwater vehicle. The effect of viscosity is neglected and the cases for which the effects could be important are discussed. The present algorithm is however an efficient tool to determine wave forces on a submerged body and can also be integrated into a viscous flow algorithm. A numerical wave tank is constructed for the simulation. A damping layer is introduced to minimize spurious reflection of scattered waves at the open boundary. A sinusoidal progressive pressure patch is used to generate incident waves. Wave forces are determined using four different methods: viz., (1) Froude-Krylov volume integration method, (2) Froude-Krylov surface pressure integration method, (3) Linear diffraction analysis and (4) Nonlinear diffraction analysis for a range of parameters including incident wavelength and wave height. Results are compared to quantify effects of nonlinearity and diffraction effect of the body.

The tasks Autonomous Underwater Vehicles (AUVs) are expected to perform are becoming more and more challenging. Thus, to be able to address such tasks, we implemented a high maneuverability propulsion system: a vectored thruster. The design of a vehicle equipped with such a propulsion system will be presented, from a mechanical, electronic and software point of view. The motion control of the resulting system is fairly complex, and no suitable controller is available in the literature. Accordingly, we will present the derivation of a novel tracking controller, whose adaptive properties will compensate for the lack of knowledge of the system's parameters. Computer simulations are provided and show the performance and robustness of the proposed control algorithm to external perturbations, unmodelled dynamics and dynamics variation. We finally illustrate the advantage of using an adaptive controller by comparing the presented controller to a Proportional Integral Derivative controller.

The inviscid hydrodynamic coefficients of an underwater vehicle (Ocean EXplorer), including the nonlinear effects of the wave surface, are computed using a boundary-integral method. A mixed Eulerian-Lagrangian formulation (Longuet-Higgins and Cokelet, 1976) is used for the treatment of nonlinear free-surface conditions. The algorithm is validated using the work-energy theorem (Yeung, 1982) and experimental data. Results, in the form of free-surface elevations and hydrodynamic coefficients, are obtained for a range of body geometries and maneuvers. The open-loop dynamics of underwater vehicles are then investigated by solving the 3DOF rigid-body equations of motion (OXY plane). The advantages and possible usage of the developed methodology for the design and control of underwater vehicles, as well as topics for further research, are addressed in the conclusion chapter of the thesis.

This thesis describes the determination of linear and nonlinear coefficients for the Morpheus vehicle. Added mass and nonlinear damping terms were obtained by strip-theory. These added mass coefficients were compared to the ones previously computed by boundary-integral method. Open-loop simulations were conducted using both sets of added-mass coefficients along with the damping terms, which were adjusted to fit at-sea data. A previously estimation technique for hydrodynamic coefficients has been applied to the Morpheus AUV using a Kalman filter. This technique based on linearized equations of motion was tested with linear and nonlinear data generated by simulation. Steering and diving motions were considered resulting in the estimation of different sets of coefficients. Results showed that the estimated values were able to reproduce accurately the vehicle motion in the linear as well as in the nonlinear case.

Boundary integral algorithms are developed to analyze three-dimensional inviscid fluid-body interactions, including the nonlinear free-surface effects. Hydrodynamic coefficients are computed for various body geometries, some corresponding to that of small underwater vehicles, in deep waters and near the free surface. The fully nonlinear unsteady wave-radiation problem corresponding to forced submerged-body oscillations and forward translation are solved using the mixed Eulerian-Lagrangian formulation (Longuet-Higgins and Cokelet, 1976). By implementing the leading-order free-surface conditions on the calm surface, linear time-domain solutions are also obtained. The nonlinear and linear results are compared to quantify the nonlinear free-surface effects. Linear frequency-domain analysis of the wave-body interactions is also carried out using a boundary-integral method based on the simple-source distribution (Yeung, 1974). The linear time-domain and the latter frequency-domain results are also compared for a validation of the algorithms.

A complete understanding of the gas-liquid two phase flow in bubble columns is
required for the development of reliable models for scale-up of these multiphase reactors.
Although several models have been proposed to describe the hydrodynamics.
lack of adequate experimental data has hindered meaningful evaluation of model parameters
and model predictions. The Computer Automated Radioactive Particle
Tracking ( CARPT) facility that was implemented earlier has provided interesting
results on the recirculation patterns of the liquid phase. The technique has provided
quantitative information on liquid velocities and turbulence parameters as well. In
addition to these hydrodynamic parameters the measurement of void fraction is also
important. To complement the capabilities of CARPT a scanner for ;-ray Computed
Tomography (CT) was implemented to quantify the local void fraction and
its distribution in two phase flow systems. The automated scanner is capable of
imaging flows in test sections between 0.02.5 m and 0..15 m in diameter at different
elevations above the distributor. The scanner makes use of the same detectors used
in the CARPT facility and with the use of a specially designed moving collimator provides a spatial resolution of about 5 mm. A non-conventional algorithm based
on maximum likelihood principles called the E-M algorithm was used for imagw reconstruction.
Long scanning times are required leading to time averaged density
profiles. Although the system is only capable of providing time averaged void fraction
distributions. it can provide unique information concerning the structure of two
phase flow. The system performance was evaluated by identifying the sources of
errors in measurement and their bounds. The capabilities of the scanner for imaging
void fraction distribution was demonstrated both qualitatively and quantitatively.
Fse of existing radiation detectors and the associated signal processing and data
acquisition system helped in reducing the cost of the system.
The scanner was utilized to quantify the local void fraction and its distribution
in bubble columns of five diameters (0.10. 0.14, 0.19, 0.26. 0.30 m internal diameter)
and at four superficial gas velocities. The effect of various operating parameters
such as column diameter, superficial gas velocity. the type of distributor. the static
liquid height and some changes in the physical properties of the liquid phase were
studied. For the first time a comprehensive characterization of the void fraction in
an air water bubble column using a non-invasive technique was achieved.
The experimental data obtained using CARPT and CT under identical operating
conditions was used for developing a methodology for scale-up of bubble columns
using a one dimensional model for liquid recirculation. Successful scale-up of liquid
hydrodynamics using a one dimensional model requires an adequate closure scheme
for the Reynolds shear stress. The existing correlations for the prescription of the
eddy viscosity or the mixing length scale are demonstrated to be applicable only for a limited range of conditions. and consequently cannot be used for scale-up predictions.
A method for estimating the mixing length scale has been explored and an attempt
at unifying a wide range of data available in the literature within the pun·iew of
the method has been made. The futility of such an attempt is attributed to the
non-reproducibility of the flow in different laboratories and the conasequent lack of
data obtained under identical conditions. It is demonstrated, however, that scale-up
based on the mixing length distribution is possible when it is obtained from a
consistent set of data for liquid velocity and gas void fraction profiles. Using the
present method for prescribing the mixing length scale. model predictions for scale-up
compare satisfactorily for the data that was obtained as part of this research. The
achievement was that the turbulence length scale estimated in one column diameter
was successfully used in predicting the liquid velocities in larger diameter columns.