A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Experimental Investigations of Capillary Effects on Nonlinear Free-Surface Waves(2010) Diorio, James Daniel; Duncan, James H; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis presents the results of three experiments on various aspects of the effects of surface tension on nonlinear free-surface waves. The first two experiments focus on capillary effects on the breaking of short-wavelength gravity waves, a problem of interest in areas of physical oceanography and remote sensing. The third experiment is concerned with the bifurcation of solitary capillary-gravity waves, a problem that is relevant in the study of nonlinear, dispersive wave systems. In the first set of experiments, streamwise profile measurements were made of spilling breakers at the point of incipient breaking. Both wind-waves and mechanically generated waves were investigated in this study, with gravity wavelengths in the range of 10--120 cm. Although it has been previously argued that the crest shape is dependent only on the surface tension, the results reported herein are to the contrary as several geometrical parameters used to describe the crest change significantly with the wavelength. However, the non-dimensional crest shape is self-similar, with two-shape parameters that depend on a measure of the local wave slope. This self-similarity persists over the entire range of wavelengths and breaker conditions measured, indicating a universal behavior in the near-crest dynamics that is independent of the method used to generate the wave. The measured wave slope is found to be related to the wave growth rate and phase-speed prior to breaking, a result that contributes towards the development of a breaking criterion for unsteady capillary-gravity waves. The second set of experiments examines the cross-stream surface structure in the turbulent breaking zone generated by short-wavelength breakers. Waves in this study were generated using a mechanical wedge and ranged in wavelength from 80--120 cm. To isolate the effects of surface tension on the flow, the important experimental parameters were adjusted to produce Froude-scaled, dispersively-focused wave packets. The results show the development of ``quasi''-2D streamwise ripples along with smaller cross-stream ripples that grow as breaking develops and can become comparable in amplitude to the streamwise ripples for larger breakers. It is found that the amplitude of the cross-stream surface ripples scale as $\bar{&lambda}^3$, where $\bar{&lambda}$ is the average wavelength of the wave packet. The cross-stream ripple activity appears to be highest in the ``troughs'' of the larger streamwise ripples, with the appearance of persistent ``scar''-like features. Based on these observations, a simple model for the coupling between the vorticity and capillary structure in the breaking zone is conjectured. The third set of experiments focuses on the generation of capillary-gravity waves by a pressure source moving near the minimum phase speed cmin. Near this minimum, nonlinear capillary-gravity solitary waves, or ``lumps'', have been shown to exist theoretically. We identify an abrupt transition to a wave-like state that features a localized solitary wave that trails the pressure forcing. This trailing wave is steady, fully localized in 3D, elongated in the cross-stream relative to the streamwise direction, and has a one-to-one relationship between height and phase speed. All of these characterisitics are commensurate with the freely propogating ``lumps'' computed by previous authors, and a quantitative comparison between these previous numerical calculations and the current experiments is presented. At speeds closer to cmin, a new time-dependent state is observed that can qualitatively be described by the shedding of solitary depressions from the tips of a ``V''-shaped pattern. These results are discussed in conjunction with a new theoretical model for these waves that employs nonlinear and viscous effects, both of which are crucial in capturing the salient features of the surface response. While discussed in the context of water waves, these results have applicaiton to other wave systems where nonlinear and dispersive effects are important.Item Numerical Investigations of Gaseous Spherical Diffusion Flames(2009) Lecoustre, Vivien Renaud Francis; Sunderland, Peter B.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Spherical diffusion flames have several unique characteristics that make them attractive from experimental and theoretical perspectives. They can be modeled with one spatial dimension, which frees computational resources for detailed chemistry, transport, and radiative loss models. This dissertation is a numerical study of two classes of spherical diffusion flames: hydrogen micro-diffusion flames, emphasizing kinetic extinction, and ethylene diffusion flames, emphasizing sooting limits. The flames were modeled using a one-dimensional, time-accurate diffusion flame code with detailed chemistry and transport. Radiative losses from products were modeled using a detailed absorption/emission statistical narrow band model and the discrete ordinates method. During this work the code has been enhanced by the implementation of a soot formation/oxidation model using the method of moments. Hydrogen micro-diffusion flames were studied experimentally and numerically. The experiments involved gas jets of hydrogen. At their quenching limits, these flames had heat release rates of 0.46 and 0.25 W in air and in oxygen, respectively. These are the weakest flames ever observed. The modeling results confirmed the quenching limits and revealed high rates of reactant leakage near the limits. The effects of the burner size and mass flow rate were predicted to have a significant impact on the flame chemistry and species distribution profiles, favoring kinetic extinction. Spherical ethylene diffusion flames at their sooting limits were also examined. Seventeen normal and inverse spherical flames were considered. Initially sooty, these flames were experimentally observed to reach their sooting limits 2 s after ignition. Structure of the flames at 2 s was considered, with an emphasis on the relationships among local temperature, carbon to oxygen atom ratio (C/O), and scalar dissipation rate. A critical C/O ratio was identified, along with two different sooting limit regimes. Diffusion flames with local scalar dissipation rates below 2 1/s were found to have temperatures near 1410 K at the location of the critical C/O ratio, whereas flames with greater local scalar dissipation rate exhibited increased temperatures. The present work sheds light on important combustion phenomenon related to flame extinction and soot formation. Applications to energy efficiency, pollutant reduction, and fire safety are expected.Item EFFICIENT SIMULATION OF ELECTRON TRAPPING IN LASER AND PLASMA WAKEFIELD ACCELERATION(2009) Morshed, Sepehr; Antonsen, Thomas M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Plasma based laser Wakefield accelerators (LWFA) have been a subject of interest in the plasma community for many years. In LWFA schemes the laser pulse must propagate several centimeters and maintain its coherence over this distance, which corresponds to many Rayleigh lengths. These Wakefields and their effect on the laser can be simulated in the quasistatic approximation. The 2D, cylindrically symmetric, quasistatic simulation code, WAKE is an efficient tool for the modeling of short-pulse laser propagation in under dense plasmas [P. Mora & T.M. Antonsen Phys. Plasmas 4, 1997]. The quasistatic approximation, which assumes that the driver and its wakefields are undisturbed during the transit time of plasma electrons, through the pulse, cannot, however, treat electron trapping and beam loading. Here we modify WAKE to include the effects of electron trapping and beam loading by introducing a population of beam electrons. Background plasma electrons that are beginning to start their oscillation around the radial axis and have energy above some threshold are removed from the background plasma and promoted to "beam" electrons. The population of beam electrons which are no longer subject to the quasistatic approximation, are treated without approximation and provide their own electromagnetic field that acts upon the background plasma. The algorithm is benchmarked to OSIRIS (a standard particle in cell code) simulations which makes no quasistatic approximation. We also have done simulation and comparison of results for centimeter scale GeV electron accelerator experiments from LBNL. These modifications to WAKE provide a tool for simulating GeV laser or plasma wakefield acceleration on desktop computers.Item STUDY OF LONGITUDINAL SPACE CHARGE WAVES IN SPACE-CHARGE DOMINATED BEAMS(2009) Thangaraj, Jayakar Charles Tobin; O'Shea, Patrick; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Future x-ray free electron lasers will probe matter at the atomic scale with femtosecond time resolution. Such x-ray sources require a high current electron beam with very low emittance and energy spread. Any density fluctuation in an intense beam can launch space charge waves that lead to energy modulation. The energy modulations may cause further density modulations in any dispersive element and can, for example, excite the microbunching instability in x-ray free electron lasers. Hence, it is important to understand and control the evolution of density modulations on an intense beam. This dissertation focuses on long path-length experimental study of intense beams with density perturbations. The experimental results are compared with theory and computer simulation. We took advantage of the multi-turn operation of the University of Maryland Electron Ring (UMER), to carry out long path-length (100 m) experimental studies of space-charge-dominated beams with density perturbations. First, a single density perturbation is introduced on a space-charge dominated electron beam using photoemission from a laser. The perturbation splits and propagates as a fast and a slow wave on the beam. The speed of the space charge waves is measured experimentally as a function of beam current and perturbation strength. The results are in good agreement with Particle-in-cell (PIC) simulation and 1-D cold fluid theory in the linear regime. We then show that linear space-charge waves can be used as non-interceptive transverse beam diagnostics in UMER. Using time-resolved imaging techniques, we report the transverse effects of a longitudinal perturbation in a circular machine. We introduce multiple perturbations on the beam and show that the fast and the slow waves superpose and cross each other. We then present experimental results on the beam response from introducing a controlled energy modulation on the density modulated beam and compare them with the theory. In the non-linear regime, where the strength of the perturbation is large (>25% compared to the beam current), we report, for the first time, a wave train formation of the space charge waves. Finally, experimental observation of a photo-emitted beam pulse splitting into sub-pulses under high laser power is presented and compared with 1-D virtual cathode theory. From this work, we conclude that density modulations on an intense beam produce fast and slow waves, which, in the linear regime at least, can be controlled through energy modulation. Moreover, a large amplitude density modulation, when allowed to propagate, can break into sub-pulses, causing energy modulation. Hence, a density modulation should not be allowed to grow and must be controlled as soon as possible.Item Dynamics of Erythrocytes and Microcapsules(2008-04-25) Dodson, Walter; Dimitrakopoulos, Panagiotis; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The erythrocytes are the primary carriers of oxygen and carbon dioxide to and from the systemic tissue. The ability of these cells to deform and navigate through the capillary beds is of fundamental importance for proper functioning of the cardiovascular transport system. The erythrocyte is essentially a capsule, and flow-induced erythrocyte deformation involves the interfacial dynamics of a membrane-enclosed fluid volume stressed in a viscous flow. Elastic capsule dynamics is a complicated problem involving the coupling of fluid and membrane forces; it is also found in a variety of scientific and engineering applications. In this work, we investigate the dynamics of elastic capsules and erythrocytes using the Spectral Boundary Element (SBE) method, a high-order / high-accuracy method for capsule and cellular dynamics. For strain-hardening Skalak elastic capsules in an extensional flow, our investigations demonstrate a shape transition in accordance with experimental observations to a cusped conformation at high flow rates, which allows the capsule to withstand the increased hydrodynamic forces. Our computational methodology reveals a region of bifurcation, in which both spindled and cusped steady-state geometries coexist for a single flow rate. The method is also used to investigate the dynamics of strain-softening Neohookean capsules in the same flow pattern. The strain-softening capsules become highly extended at weaker flow rates than strain-hardening capsules, and do not form steady-state cusped shapes. The SBE method has been extended to model the erythrocyte by using a biconcave disc reference geometry and adaptive prestress to enforce area incompressibility. The method accurately reproduces experimental data from erythrocyte ektacytometry, but allows examination of the erythrocyte dynamics beyond the geometric constraints inherent in ektacytometry and other experimental techniques, including observation of the three-dimensional oscillatory behavior over a range of capillary numbers and viscosity ratios. Our results support a prediction by Fischer, Skalak, and coworkers that the erythrocyte shear modulus decreases at small shear deformations. Our work also suggests that cellular deformation is largely independent of the flow pattern, consistent with the findings of experimental investigators.Item TOMOGRAPHIC MEASUREMENT OF THE PHASE-SPACE DISTRIBUTION OF A SPACE-CHARGE-DOMINATED BEAM(2008-04-24) Stratakis, Diktys; O'Shea, Patrick G; Kishek, Rami A; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Many applications of accelerators, such as free electron lasers, pulsed neutron sources, and heavy ion fusion, require a good quality beam with high intensity. In practice, the achievable intensity is often limited by the dynamics at the low-energy, space-charge dominated end of the machine. Because low-energy beams can have complex distribution functions, a good understanding of their detailed evolution is needed. To address this issue, we have developed a simple and accurate tomographic method to map the beam phase using quadrupole magnets, which includes the effects from space charge. We extend this technique to use also solenoidal magnets which are commonly used at low energies, especially in photoinjectors, thus making the diagnostic applicable to most machines. We simulate our technique using a particle in cell code (PIC), to ascertain accuracy of the reconstruction. Using this diagnostic we report a number of experiments to study and optimize injection, transport and acceleration of intense space charge dominated beams. We examine phase mixing, by studying the phase-space evolution of an intense beam with a transversely nonuniform initial density distribution. Experimental measurements, theoretical predictions and PIC simulations are in good agreement each other. Finally, we generate a parabolic beam pulse to model those beams from photoinjectors, and combine tomography with fast imaging techniques to investigate the time-sliced parameters of beam current, size, energy spread and transverse emittance. We found significant differences between the slice emittance profiles and slice orientation as the beam propagates downstream. The combined effect of longitudinal nonuniform profiles and fast imaging of the transverse phase space provided us with information about correlations between longitudinal and transverse dynamics that we report within this dissertation.Item Large-Eddy Simulation of High Reynolds Number Flows in Complex Geometries(2007-12-14) radhakrishnan, senthilkumaran; Piomelli, Ugo; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Large-eddy simulation (LES) of wall-bounded flows is limited to moderate Reynolds number flows due to the high computational cost required to resolve the near wall eddies. LES can be extended to high Reynolds number flows by using wall-layer models which bypass the near-wall region and model its effect on the outer region. Wall-layer models based on equilibrium laws yield poor prediction in non-equilibrium flows, in which Wall-Modeled LES (WMLES) that model the near wall region by Reynolds-Averaged Navier-Stokes (RANS) equation and the outer region by LES, has the potential to yield better results. However, in attached equilibrium flows, WMLES under-predicts the skin friction due to slow generation of resolved eddies at the RANS/LES interface; application of stochastic forcing results in faster generation of resolved eddies and improved predictions. In this work, wall-layer models based on equilibrium laws and WMLES are tested for four non-equilibrium flows. Flow over a contoured ramp, with a shallow separation followed by a recovery region, was studied. LES using equilibrium laws was unable to resolve the shallow separation. WMLES predicted the mean velocity reasonably well but over-predicted the Reynolds stresses in the separation and recovery region; application of the stochastic forcing corrected this error. Next, the flow past a two-dimensional bump, in which curvature and pressure-gradient effects dictate the flow development, was studied. WMLES predicted the mean velocity accurately but over-predicted the Reynolds stresses in the adverse pressure gradient region; application of the stochastic forcing also corrected this error. Same trends were seen in a three-dimensional flow studied. A turbulent oscillating boundary layer was also investigated. WMLES was found to be excessively dissipative, which resulted in incorrect prediction of the flow development. LES calculation based on equilibrium laws and dynamic models predicted the flow development correctly. In summary, in flows that are steady in the mean, WMLES with stochastic forcing gave more accurate results than the logarithmic law or RANS. For the oscillating boundary layer, in which stochastic forcing could not be applied, the logarithmic law yielded the best results.Item Interaction of Intense Short Laser Pulses with Gases of Nanoscale Atomic and Molecular Clusters(2006-08-09) Gupta, Ayush; Antonsen, Thomas M.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We study the interaction of intense laser pulses with gases of van der Waals bound atomic aggregates called clusters in the range of laser-cluster parameters such that kinetic as well as hydrodynamic effects are active. The clustered gas absorbs the laser pulse energy efficiently producing x-rays, extreme ultraviolet radiation, energetic particles and fusion neutrons. First, we investigate the effect of pulse duration on the heating of a single cluster in a strong laser field using a 2-D electrostatic particle-in-cell (PIC) code. Heating is dominated by a collision-less resonant absorption process that involves energetic electrons transiting through the cluster. A size-dependent intensity threshold defines the onset of this resonance [Taguchi et al., Phys. Rev. Lett., v90(20), (2004)]. It is seen that increasing the laser pulse width lowers this intensity threshold and the energetic electrons take multiple laser periods to transit the cluster instead of one laser period as previously recorded [Taguchi et al., Phys. Rev. Lett.,v90(20), (2004)]. Results of our numerical simulations showing the effect of pulse duration on the heating rate and the evolution of the electron phase space are presented in this dissertation. Our simulations show that strong electron heating is accompanied by the generation of a quasi-monoenergetic high-energy peak in the ion kinetic energy distribution function. The energy at which the peak occurs is pulse duration dependent. Calculations of fusion neutron yield from exploding deuterium clusters using the PIC model with periodic boundary conditions are also presented. We also investigate the propagation of the laser pulse through a gas of clusters that is described by an effective dielectric constant determined by the single cluster polarizability. For computational advantage, we adopt a uniform density description of the exploding clusters, modified to yield experimentally consistent single cluster polarizability, and couple it to a Gaussian description of the laser pulse. This model is then used to study self-focusing, absorption, and spectral broadening of the laser pulse. The model is further extended to allow for a fraction of the gas to be present as unclustered monomers and to include the effect of unbound electrons produced in the laser-cluster interaction.Item INTERACTION OF INTENSE LASER PULSES WITH DROPLET AND CLUSTER SOURCES: APPLICATION TO EXTREME ULTRAVIOLET LITHOGRAPHY AND PLASMA WAVEGUIDE GENERATION(2006-08-08) Sheng, Hua; Milchberg, Howard M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Several topics were studied in the interaction of intense laser pulses with droplet and cluster sources. Laser pulse-formatting that can enhance laser-to-EUV conversion efficiency in the 13.5nm band for next geration lithography was first explored, using droplet sources as the laser target. Xenon droplets size distribution was measured, and the droplet plasma spectrum irradiated by various laser energies was scanned. A 2-pulse heater setup and a 4-pulse stacker scheme were built and studied. Results suggest that, unlike droplets of argon (Ar) and krypton (Kr), the ionizationstate distribution in xenon may be much more transient. The decay timescales for 13.5nm emissions are approximately 0.5-1.5ns, a timescale intermediate to the results from excitation and recombination emission in Ar and Kr droplet targets. Comparing xenons results with argons, the argon droplets generated a more robust ionizationstage distribution, while the xenon droplets generated a more transient ionizationstage distribution. The second topic studied was an investigation of plasma waveguides generated in clustered gases, using 100ps long pump pulses axicon-generated Bessel beam. The plasma waveguide space and time evolution was measured, using picosecond interferometry. The resulting waveguides have both central densities as low as ~1018cm-3 and small diameters, a desirable but hard to achieve combination for either hydrodynamic shock waveguides using conventional gases or for other techniques, such as discharge capillaries. Extremely efficient absorption of laser pulses by cluster targets was shown to extend to pulses significantly longer than the timescale for cluster explosive disassembly. These long pulse absorption efficiencies can be more than a factor of 10 greater than those in unclustered gas targets of the same volume average atomic density. The maximum long pulse absorption observed in cluster jets under our range of conditions was 35%. A third topic explored was resonant pulse-shortening of Bessel beams in the under-dense (Ne is ~10^(-2) Ncr) plasma channels formed by 100ps Nd:YAG laser pulses. Pulse shortening was seen at two pressures under our range of conditions: 340torr and 460torr.Item LINEAR AND NONLINEAR ANALYSIS OF A GYRO-PENIOTRON OSCILLATOR AND STUDY OF START-UP SCENARIO IN A HIGH ORDER MODE GYROTRON(2006-01-31) Yeddulla, Muralidhar; Antonsen, Thomas; Nusinovich, Gregory; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Cyclotron Resonant Maser (CRM) is a device in which electrons gyrating in an external magnetic field produce coherent EM radiation. A DC electron beam current must be converted to an AC beam current to create RF energy. There are two possible approaches: phase bunching (O-type) and spatial segregation (Mtype). In phase bunching, electrons are either accelerated or decelerated depending on when the electrons enter the interaction region, causing phase bunching. The electron bunches are then slowed down by the RF field for energy extraction. Not all electrons lose energy; some even gain energy. In spatial segregation, electrons entering the interaction region at different times are deflected in different directions. With an appropriate spatially varying RF field, all electrons can lose energy leading to very high conversion efficiency. A CRM with a smooth walled cylindrical waveguide interaction cavity and an annular electron beam passing through it can generate very large amount of RF energy. Depending on the electron beam position a gyrotron (O-type device) and a gyro-peniotron (M-type device) are possible. In this work, first, a nonlinear theory to study CRMs with a smooth walled cylindrical waveguide interaction cavity is presented. The nonlinear set of differential equations are linearized to study the starting conditions of the device. A gyropeniotron operating in the TE0,2 - mode is studied using the theory presented. It is found that a gyro-peniotron operating in a low order mode can be self excited without mode competition from gyrotron modes, leading to the possibility of a very efficient high power RF source. A higher order mode gyro-peniotron experiences severe mode competition from gyrotron modes. The cavity Q required for gyropeniotron operation is very high, which can lead to excessive heat in the cavity walls due to ohmic losses. Hence, a gyro-peniotron operation seems practical only in low order modes and in short pulses. Second, an existing linear theory of gyrotrons is extended to include effects of magnetic field tapering, cavity wall profile, finite beam thickness, velocity spread and axially dependent beam coupling to the fields of competing modes. Starting currents are calculated for the operating and the most dangerous competing mode in a 140 GHz gyrotron, which was developed at Communications and Power Industries (CPI). Start-up scenario of this device is also studied using the non-stationary code MAGY, which is a tool for modeling slow and fast microwave sources.