▼ The purpose of this study was to experimentally elucidate the key physics of Coanda ejectors and to find a model that could both predict the performance of Coanda ejectors and aid in their design. Velocity profiles, turbulence intensities, turbulence spectrums, correlation functions and wall pressure distributions were measured inside two different Coanda ejectors at different operating conditions. Using laser anemometry, axial velocity profile measurements were done inside both ejectors. This was accomplished by angling the laser beams with respect to the axis of the ejectors. All velocity profiles measured had similar characteristics – a maximum near the ejector’s wall decreasing to a uniform and constant value in the center of the ejector. Based on the experimental results and our analysis of these results, we have postulated a mechanism for the operation of the Coanda ejector: The driving force for the ejector’s operation is the primary flow momentum. The turbulent mixing of the primary flow with the ambient air near the entrance of the ejector, transfers this momentum to the stagnant air and induces a secondary flow. This secondary flow is then dragged by turbulent shear forces downstream to the ejector exit while being mixed with the pr imary flow by the persistence of a large turbulent intensity throughout the ejector. With the key flow properties of the ejector elucidated by these experiments, we have examined several theoretical models to find a model that could predict the performance of these ejectors. Normally two approaches have been used: Control Volume Approach (CVA) and Physical Phenomena Approach (PPA). The results have shown that those models can predict the gross behavior of Coanda ejector. However, neither can predict the detailed performance of Coanda ejectors nor can they be used for design purposes. Due to this deficiency we have developed a new model. This model is based on the experimental results. Using this model, we have predicted the average velocities at any section of the Coanda ejector with the maximum difference of 9% between the predictions and measurements. Also, we have predicted the ratio of the Induced Flow Rate/Primary Flow Rate (I/P) with a maximum difference of 5%. Advisors/Committee Members: Dybbs, Alexander.

▼ This thesis extends the scaling analysis method to steady and oscillatory thermocapillary flows in cylindrical containers with unit aspect ratio. We investigate the hydrodynamic and thermal fields of the flows, which are induced by a constant radiative heat flux (the CF heating mode) or by a constant temperature difference (the CT heating mode). The scaling laws of the characteristic velocity, length and the Nusselt number are derived for the steady flows with each heating mode. These scaling laws cover much wider parametric ranges than those of past experiments on oscillatory thermocapillary flows. These scaling laws are compared with the numerical and experimental results, and very good agreement is obtained. The heating modes are found to have strong effects on the scaling laws and on the detailed flow features. Based on these results, the characteristics of oscillations and their physical mechanism are discussed in detail. And the important controlling parameters (the S parameters) of oscillations are derived for the flows under both CF and CT heating modes. Advisors/Committee Members: Kamotani, Yasuhiro.

▼ The low gravity environment provided by space flight has afforded the science community a unique arena for the study of fundamental and technological sciences. However, the dynamic environment observed on space shuttle flights and predicted for Space Station Freedom has complicated the analysis of prior "microgravity" experiments and prompted concern for the viability of proposed space experiments requiring long term, low gravity environments. Thus, isolation systems capable of providing significant improvements to this random environment have been developed. This dissertation deals with the design constraints imposed by acceleration sensitive, "microgravity" experiment payloads in the unique environment of space. A theoretical background for the inertial feedback and feedforward isolation of a payload was developed giving the basis for two experimental active inertial isolation systems developed for the demonstration of these advanced active isolation techniques. A prototype six degree of freedom digital active isolation system was designed and developed for the ground based testing of an actively isolated payload in three horizontal degrees of freedom. A second functionally equivalent system was built for the multi-dimensional testing of an active inertial isolation system in a reduce d gravity environment during low gravity aircraft trajectories. These multi-input multi-output control systems are discussed in detail with estimates on acceleration noise floor performance as well as the actual performance acceleration data. The attenuation performance is also given for both systems demonstrating the advantages between inertial and non-inertial control of a payload for both the ground base environment and the low gravity aircraft acceleration environment. A future goal for this area of research is to validate the technical approaches developed to the 0.01 Hz regime by demonstrating a functional active inertial feedforward/feedback isolation system during orbital flight. A NASA IN-STEP flight experiment has been proposed to accomplish this goal, and the expected selection for the IN-STEP program has been set for July of 1993. Advisors/Committee Members: Quinn, R.

▼ A theoretical model for the process of bubble and drop formation, which is applicable for both terrestrial and microgravity environments, has been developed by using a force balance. Two different flow systems are considered in which a nozzle is located either horizontally (co-flow system) or vertically (cross-flow system) in a flowing liquid through a pipe. The contact angle variation at the nozzle due to the inertia of the flowing liquid has been theoretically analyzed. The added mass coefficient of the bubble moving through a pipe is obtained analytically based on potential flow theory. The bubble formation process is assumed to take place in two stages, the expansion stage and the detachment stage. The important forces in a bubble formation process are nondimensionalized and the bubble motions during its expansion and detachment stages for the two different flow systems are analyzed by using nondimensional variables. The predictions of the present model are compared with available experimental results in the case of normal gravity and the agreement is satisfactory. The present model is applied for a gas-liquid system to estimate the bubble size for various conditions in microgravity. The effect of the nondimensional variables as well as the flow systems on the bubble size have been evaluated. The present model is also appl ied for the prediction of the flow pattern transition from bubble to slug flow in microgravity. The predictions are compared with the results of microgravity experiment and of ground-based experiment. It is indicated that flow patterns in microgravity are closely related to the bubble formation process as well as the entrance configuration. The predictions of the present model for the transition from bubble to slug flow show good qualitative agreement with experimental results. Advisors/Committee Members: Kamotani, Yasuhiro.

▼ In the enclosed environment of a manned space habitat, fire safety becomes a major concern. Though various combinations of pressure and oxygen can be used to support human habitation, the bulk of flammability testing for materials is done at standard atmospheric conditions. This study focuses on the flammability and flame spread in these potential space habitat environments. Pressures were kept below 1 atm, Oxgyen/Nitrogen mixtures between 21% and 30% O2, and gravity levels of 1 g/ge (Earth), 0.38 g/ge (Martian), and 0.16 g/ge (Lunar) were used. Two materials were examined. Part 1 of this report discusses Kimwipes®, a thin tissue paper, which was better suited for the reduced gravity experiments. A scaling relation that predicts upward spread rate as a function of pressure, gravity, and width, is introduced and experimentally verified. Nomex® fabric, a practical material commonly used in the space program, is the focus of Part 2. Nomex® has a lower oxygen limit for upward spread than for downward spread. A unique phenomenon was observed during experimentation, whereby the continually elongating flames broke off in the middle into two smaller upward propagating flames. One flame would then extinguish, and the remaining flame would repeat this cycle. The process was documented in detail and is believed to be a function of a two stage pyrolysis occurring in the fuel. Advisors/Committee Members: T'ien, James S.

▼ In this dissertation, three problems have been primarily studied in the area of robot control: (1) characterization of robot nonlinear dynamics, (2) force control of robot manipulators with unknown dynamics and disturbances, and (3) nonlinear impact force control. In order to identify the robot control problems, we studied the effects of plant dynamics, focusing on robot joint dynamics such as joint flexibility and friction. Extensive experimental studies have been done on robot transmissions. The experiments have included: transmission linearity, backlash, static and dynamic friction, and forward efficiency. In order to understand the influence of transmission properties on overall system performance, a comparative evaluation was performed on three competing transmission types: worm-gear drive, cone-drive and traction drive transmission. The results of experiments performed on worm-gear drive validated a load-dependent friction model, which was derived for feed-forward friction compensation in feedback control. With understanding of how transmission nonlinearities influence closed-loop and controlled behavior, a new controller has been developed by combining Natural Admittance Control with Time Delay Control. The propo sed nonlinear controller is not model based control. The only system parameter that must be estimated is inertia, rendering it easy to implement. It rejects unmodeled dynamics, nonlinearities, and disturbances without a difficult characterization process while preserving desired dynamics. The simulation results demonstrate not only good external disturbance rejection, robustness to parameter changes and insensitivity to noise, but also demonstrate good trajectory tracking providing good rejection of internal Coulomb friction. For stabilization of a robot manipulator upon collision with a stiff environment, we proposed a novel impact force control strategy which is developed based on the observation of human interactive behavior. It uses a robust natural admittance/time-delay control with an added negative force feedback to absorb impact force and stabilize the system. During the impact phase, this control input alternates with zero control input when no environment force is sensed. Simulation results show that this simple bang-bang control approach produces a stable interaction with a very stiff environment and its performance is comparable to the other existing impact force control techniques. Advisors/Committee Members: Loparo, Kenneth A.

▼ An investigation of the structure and development of streamwise vortices embedded in a turbulent boundary layer was conducted in the test facility CW-22 at NASA Lewis Research Center. The vortices were generated by a single spanwise row of rectangular vortex generator blades. A single embedded vortex was examined, as well as arrays of embedded counter-rotating vortices produced by equally spaced vortex generators. Measurements of the secondary velocity field in the crossplane provided the basis for characterization of vortex structure. Vortex structure was characterized by four descriptors. The center of each vortex core was located at the spanwise and normal position of peak streamwise vorticity. Vortex concentration was characterized by the magnitude of the peak streamwise vorticity, and the vortex strength by its circulation. Measurements of the secondary velocity field were conducted at two crossplane locations to examine the streamwise development of the vortex arrays. Large initial spacings of the vortex generators produced pairs of strong vortices which tended to move away from the wall region while smaller spacings produced tight arrays of weak vortices close to the wall. The crossplane structure of embedded vortices is observed to be very s imilar to that exhibited by the two dimensional Oseen vortex with matching descriptors. Quantitative comparisons are established. A model of vortex interaction and development is constructed using the experimental results. The model is based on the structure of the Oseen vortex. Vortex trajectories are successfully modelled by including the convective effects of neighbors, and images to represent the wall. The streamwise decay of circulation is successfully modelled for the single vortex, and for large initial spacings, by accounting for the effects of wall friction. An additional mechanism associated with the turbulent stress field in the near vicinity of the vortex cores is postulated to explain the large losses in circulation obtained for the smaller initial spacings. The streamwise decay of vortex circulation at the smaller spacings is successfully modelled by summing wall friction losses and "proximity" losses. These proximity losses are found to be proportional to the gradient in streamwise vorticity occurring between an embedded vortex and its adjacent counter-rotating neighbors. Advisors/Committee Members: Greber, Isaac.

▼ Unmanned aircraft vehicles have inspired the creation of smaller, lighter aircraft, capable of the same mission requirements. To aid these micro air vehicles in maintaining a stable flight, a flexible fabric is used on the wing to absorb gust loads and provide passive adaptive washout. While these features are desirable for longer, more stable flights, a performance evaluation would provide information for an improved micro air vehicle platform. This inspired the testing of thin wings, and aircraft to determine the aerodynamic characteristics of lift, drag and aerodynamic pitching moment, and then evaluate the options for improving aircraft performance. This thesis describes the designs and methods used to capture these characteristics, quantifies the data and error in the setup, and evaluates the best options for a micro air vehicle based upon the wings and aircraft tested. Advisors/Committee Members: Quinn, Roger.

▼ Mathematical and physical models are powerful tools for researchers and engineers to investigate and design complex structures. A great deal of attention has been focused on finding more accurate and efficient methods to generate models, especially to generate dynamic models. In this study, the modified modal superposition method is developed. This method uses participation factors to identify the natural modes which have significant contribution to the vibration of a structure. As a result, perhaps only a few modes may be selected to generate the mathematical model of a structure. On the other hand, another modeling method is developed for the purpose of designing a dynamically scaled physical model of a complex structure. This method is based upon the finite element method and the model is generated at the element level using dimensional analysis. Hence, the explicit equations of motion are not required to find the scale relations between parameters. As examples, the modified modal superposition method is applied to the vibration analysis and acoustic radiation of a cylindrical shell, and vibration analysis of Space Station Freedom. The dynamically scaled modeling method is used to develop a small scale model of Space Station Freedom without changing its dynamic behavior. In this study, related topics su ch as transient acoustic radiation and active suspension systems of the laboratory model of Space Station Freedom are discussed. Advisors/Committee Members: Quinn, Roger D.