The behavior of the electric field together with the electron and ion densities in the vicinity of a nonemitting, plane anode is investigated. The selected approach involves non-linear analysis techniques on the continuum equations for steady-state, isothermal conditions where both ionization and two-body recombination are included. Ions, created through electron bombardment of neutral atoms, are repelled toward two stagnation regions: within or near the sheath boundary and near the plasma interface. These equilibria form as a result of the chemistry present: recombination establishes the latter while ionization stipulates the former. As presented, the sheath is fundamentally unstable - ions are driven toward the negative electrode. Using nitrogen data for a numeric example, the following observations are made: a sufficiently strong applied electric field pushes the ion density toward that ofthe electrons through a well - a constrictive phenomenon. Both a transition region, dominated by density gradients, and a diffusion-driven zone are found to move the system toward the plasma interface. The characteristics of this process are influenced by the applied electric field, but the instability of the chemistry-induced stagnation regions precludes numeric convergence. Insufficient dissipation may prevent the stability of the anode fall model as presented. Suggested improvements to the model descriptions include considering the effects of temperature gradients, magnetic fields, three-body recombination, diffusion written in terms of the electric field, multi-dimensionality and/or timedependencies^

Denna studie bestod i att undersöka situationen på marknaden angående framdriftsanläggningar för LNG-tankfartyg. Syftet med arbetet var att skapa en relevant bild av hur situationen ser ut idag och genom intervjuer ta reda på hur den kan komma att förändras. Genom en litteraturstudie undersöktes läget på marknaden och utifrån det kunde relevanta frågor utformas. Efter att frågorna hade utformats tog författarna kontakt med olika företag som var anknutna till marknaden för LNG-tankfartyg. Intervjuer med företagen genomfördes och dessa har gjort det möjligt att analysera hur läget kan komma att förändras. Studien har visat att det finns ett system, DFDE, som dominerar fartygsbeställningar i dagsläget. Det finns även nyare system som består av gasdrivna tvåtaktsmotorer som förväntas ta över en del av marknaden. Andra system såsom ånganläggningar och anläggningar med gasturbin finns tillgängliga men de ligger inte i fokus i dagsläget. / This study consisted of analysing the current situation on the market for propulsion systems for LNG-carriers. The purpose of the study was to create a relevant picture of what the situation looks like today and through interviews try to predict how the situation might come to change. By using already existing literature the authors reviewed the situation on the market as it is today and formed relevant questions. After the questions had been formed the authors contacted several companies associated with the LNG-carrier market. Interviews with these companies were carried out and it is through these interviews possible to analyse how the situation might come to change. The study has shown that there is one system that dominates the market for LNG-carrier propulsion, DFDE. There is however a new system, with dual-fuel two stroke engines that is expected to take over a part of the market in the near future. Other propulsion systems such as steam propulsion plant and a system with a gas turbine exist on the market but the interest in these systems is limited.

In SCUBA diving, the propulsive efficiency of a diver regulates, in part, his autonomy. An inefficient method of propulsion will increase the power output required and, therefore, the intake of oxygen and increase fatigue. Since the development of the SCUBA apparatus, fins have evolved based on the designer's intuition and knowledge of hydrodynamics. Some experimental work has been performed, but it is usually limited to studying the diver as whole and does not focus on the fin design. In this dissertation, a state-of-the-art fluid-structure interaction framework is developed and then used to study fin propulsion. This framework couples the structural dynamics of the fin with the fluid dynamics surrounding it using a modular framework. This way, mature state-of-the-art solvers can be used in each domain (structural and fluid). The flow field is solved using a computational fluid dynamics solver which resolves the Navier-Stokes equations. Coupled with these equations are a variety of turbulence models which can be used to resolve the turbulence in the flow. The CFD method is validated using a two-dimensional circular cylinder and a pitching and heaving airfoil, both immersed in a turbulent flow field. A commercial structural dynamics solver, is used to resolve the structural dynamics. The coupling of the two solvers is also described in detail. The basic design of a fin (a simple flat plate) is studied and modified in order to test the effect that altering key structural parameters has on the thrust, power and efficiency of the fin. The end result is a set of design recommendations which can be used to enhance the performance of a SCUBA fin. These recommendations are based on both quantitative and qualitative analysis of the performance characteristics of the fin.

While most propulsion systems which drive aquatic and aerial vehicles today are based on rotating blades or foils, there has recently been renewed interest in the use of oscillating foils for this purpose, similar to the fins or wings of biological swimmers and flyers. These propulsion systems offer the potential to achieve a much higher degree of manoeuvrability than what is possible with current man-made propulsion systems. There has been extensive research both on the theoretical aspects of oscillating-foil propulsion and the implementation of oscillating foils in practical vehicles, but the current understanding of the physics of oscillating foils is incomplete. In particular, questions remain about the selection of the appropriate structural properties for the use of flexible oscillating foils which, under suitable conditions, have been demonstrated to achieve better propulsive performance than rigid foils. This thesis investigates the effect of the foil inertia, stiffness, resonant frequency and oscillation kinematics on the thrust generation and efficiency of a flexible oscillating-foil propulsion system. The study is based on experimental measurements made by recording the applied forces while driving foil models submerged in a water tunnel in an oscillating motion using servo-motors. The design of the models allowed for the construction of foils with various levels of stiffness and inertia. High-speed photography was also used to observe the dynamic deformation of the flexible foils. The results show that the frequency ratio, or ratio of oscillation frequency to resonant frequency, is one of the main parameters which determines the propulsive efficiency since the phase of the deformation and overall amplitude of the motion of the bending foil depend on this ratio. When comparing foils of equivalent resonant frequency, heavier and stiffer foils were found to achieve greater thrust production than lighter and more flexible foils but the efficiency of each design was comparable. Through the development of a semi-empirical model of the foil structure, it was shown that the heavier foils have a lower damping ratio which allows for greater amplification of the input motion by the foil deformation. It is expected that the greater motion amplitude in turn leads to the improved propulsive performance. Changing the Reynolds number of the flow over the foils was found to have little effect on the relation between structural properties and propulsive performance. Conversely, increasing the amplitude of the driven oscillating motion was found to reduce the differences in performance between the various structural designs and also caused the peak efficiency to be achieved at lower frequency ratios. The semi-empirical model predicted a corresponding shift in the frequency ratio which results in the maximum amplification of the input motion and also predicted more rapid development of a phase lag between the deformation and the actuating motion at low frequency ratios. The shift in the location of the peak efficiency was attributed to these changes in the structural dynamics. When considering the form of the oscillating motion, foils driven in combined active rotation and translation motions were found to achieve greater efficiency but lower thrust production than foils which were driven in translation only. The peak efficiencies achieved by the different structural designs relative to each other also changed considerably when comparing the results of the combined motion trials to the translation-only cases. To complete the discussion of the results, the implications of all of these findings for the design of practical propulsion systems are examined. / Graduate / 0548

Wheelchair manufacturers design their products to provide the best possible experience to their users. The user effort and wheelchair mechanical efficiency during motion are highly affected by rolling resistance, which is a resistive force depending on many factors such as vehicle speed, tire/floor materials and tire inflation pressure. The study of this force over several conditions could lead to improvements in wheelchair design. The Anatomical Model Propulsion System (AMPS) is a robotic device to test manual wheelchairs with better repeatability than human subjects, helping to identify the effect of resistive forces. It uses electric motors to apply torque directly to the rear wheel handrims. In this thesis, a control system is developed to achieve a specified wheelchair trajectory and velocity profile, by controlling the input torque to the system. A kinetic and dynamic model is used to estimate this necessary input. Data collected from different experiments allows to determine rolling resistance under various conditions. The results show that rolling resistance also varies with acceleration, affecting the overall mechanical efficiency of a wheelchair following different velocity profiles.

Scramjet engines are the focus of considerable interest for propulsion in the hypersonic flow regime. One of the serious technical challenges for developing scramjets is reducing the skin friction drag on the engine. The combustion chamber, in particular, is a major contributor to the skin friction drag because of the high density of the flow through that region. This investigation focuses on reducing the combustion chamber skin friction drag by minimising the surface area and size of the combustion chamber and by employing a novel approach to accomplishing combustion. The first design criterion is addressed by using a single internal-combustor scramjet configuration, as opposed to multiple external combustors, and by injecting the fuel on the intake to reduce the mixing length required in the combustor. The second design criterion refers to the use of a new technique called radical farming. This uses the highly two-dimensional nature of the flow through the engine, which is created by deliberately ingesting the leading edge shocks, to achieve combustion at lower mean static pressures and temperatures than generally expected. A simplified approximate theoretical analysis of the radical farming concept is presented. Experiments were conducted in the T4 free-piston shock tunnel on a scramjet model with a single rectangular constant cross-sectional area combustion chamber. Pressure measurements were taken along the centreline of the intake, combustion chamber and thrust surface and across the model width at three locations. Gaseous hydrogen fuel was injected halfway along the intake at a range of equivalence ratios between zero and one. The combustion chamber height was varied from 20mm to 32mm, which varied the contraction ratio of the engine from 4.1 to 2.9. The experiments were conducted at a stagnation enthalpy of either 3MJ/kg or 4MJ/kg. The nominal 3MJ/kg condition corresponds to Mach 7.9 flight at an altitude of 24km. The majority of the 4MJ/kg experiments were conducted at a nominal condition corresponding to Mach 9.1 flight at an altitude of 32km. A small number of 4MJ/kg experiments were conducted at simulated flight altitudes of between 30 and 38km; the flight Mach number for these experiments was approximately 9.0. Thrust was calculated by integrating the centreline pressure distribution over the area of the thrust surface, assuming that the pressure at any axial location was constant across the engine width. These experimental thrust values were compared with theoretical estimates obtained using a one-dimensional analysis and a quasi-two-dimensional analysis. The comparison provided an indication of the level of completion of combustion in the experiments. The difference in thrust produced as a result of combusting fuel was examined by plotting the incremental specific impulse against equivalence ratio. Experimental and theoretical results agreed best at the higher equivalence ratios. Turbulent boundary layer separation correlations were used to provide reasonable estimates for the equivalence ratio at which the flow choked. The drag on the internal flowpath of the scramjet engine was estimated using the quasi-two-dimensional analysis. This drag estimate was combined with the experimental thrust measurements to provide estimates of the net specific impulse. Positive net specific impulse estimates were obtained above a certain minimum equivalence ratio, which depended on the contraction ratio and the test condition. The engine performance was observed to be highly dependent on the two-dimensional shock structure within the engine. Thrust and specific impulse were observed to decrease with increasing simulated flight altitude, as expected. Positive net specific impulse estimates were obtained at equivalence ratios of approximately one for simulated flight altitudes below 35km. Assuming complete combustion and that an equivalence ratio of one can be reached, the configuration considered in the present study can theoretically reach a net specific impulse of approximately 1000s at the 3MJ/kg condition and 500s at the 4MJ/kg condition. These numbers provide a promising testimonial for the use of this configuration, with modifications, as a more efficient alternative to rocket engines.

Thesis (M.S.)--Missouri University of Science and Technology, 2008. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed April 25, 2008) Includes bibliographical references (p. 57).

Thesis (Ph.D.)--University of Rhode Island, 2006. / Includes bibliographical references (leaves 130-134).

Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1990. / Vita. Abstract. Includes bibliographical references (leaves 70-86). Also available via the Internet.

Thesis (M.S.)--Oregon State University, 2010. / Printout. Includes bibliographical references (leaves 141-147). Also available on the World Wide Web.