Mechanical Engineering Theses and Dissertations

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A Computational Study of the Clap and Fling Aerodynamic Mechanism
(2009) PANAGAKOS, GRIGORIOS; Balaras, Elias; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Clap and fling is a particular wing kinematic pattern utilized by some insects and birds to produce enhanced aerodynamic forces. It consists of two very distinct phases: i) the leading edges of the two wings are brought together near the upper limit of the upstroke and subsequently the wings are rotated around their leading edges, `'clapping'' like a closing book; ii) at the onset of the downstroke, and while they are still close, the two wings rotate around their trailing edges `'flinging'' apart. Prior theoretical and experimental work suggested that clap-and-fling is responsible for production of unusually high lift coefficients. However, due to limitations of the theoretical models and experimental techniques, detailed quantitative results are yet to be reported. The primary objective of the present work is to provide a concrete description of the underlying physics by means of high-fidelity simulations based on the Navier-Stokes equations for incompressible flow. In particular, the effects of the kinematics and the Reynolds number are discussed in detail in the thesis. Thesis' results verify the lift enhancement trends observed in experiments and identify the particular flow patterns correlated with such increases.
STUDY OF CONDENSATION OF REFRIGERANTS IN MICRO-CHANNELS FOR DEVELOPMENT OF FUTURE COMPACT MICRO-CHANNEL CONDENSERS
(2008) Chowdhury, Sourav; Ohadi, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Mini- and micro- channel technology has gained considerable ground in the recent years in industry and is favored due to its several advantages stemming from its high surface to volume ratio and high values of proof pressure it can withstand. Micro-channel technology has paved the way to development of highly compact heat exchangers with low cost and mass penalties. In the present work, the issues related to the sizing of compact micro-channel condensers have been explored. The considered designs encompass both the conventional and MEMS fabrication techniques. In case of MEMS-fabricated micro-channel condenser, wet etching of the micro-channel structures, followed by bonding of two such wafers with silicon nitride layers at the interface was attempted. It was concluded that the silicon nitride bonding requires great care in terms of high degree of surface flatness and absence of roughness and also high degree of surface purity and thus cannot be recommended for mass fabrication. Following this investigation, a carefully prepared experimental setup and test micro-channel with hydraulic diameter 700 microns and aspect ratio 7:1 was fabricated and overall heat transfer and pressure drop aspects of two condensing refrigerants, R134a and R245fa were studied at a variety of test conditions. To the best of author's knowledge, so far no data has been reported in the literature on condensation in such high aspect ratio micro-channels. Most of the published experimental works on condensation of refrigerants are concerning conventional hydraulic diameter channels (> 3mm) and only recently some experimental data has been reported in the sub-millimeter scale channels for which the surface tension and viscosity effects play a dominant role and the effect of gravity is diminished. It is found that both experimental data and empirically-derived correlations tend to under-predict the present data by an average of 25%. The reason for this deviation could be because a high aspect ratio channel tends to collect the condensate in the corners of its cross-section leaving only a thin liquid film on the flat side surfaces for better heat transfer than in circular or low aspect ratio channels.
Effect of Swirl on the Choking Criteria, Shock Structure, and Mixing in Underexpanded Supersonic Nozzle Airflows
(2009) Abdelhafez, Ahmed; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Swirling flow in nozzles occurs in a number of important propulsion applications, including turbofans and turbojet engines, spin-stabilized rockets, and integral rocket/ramjets. This study examines the effect of imparting swirl to underexpanded supersonic nozzle airflow on the choking criteria, shock structure, and mixing. Fuel is injected coaxially along centerline at the nozzle throat. The nanosecond Schlieren and condensate-seeded Mie-scattering diagnostic techniques are utilized to visualize the shock structure and mixing within the free supersonic part of flowfield, while CFD numerical simulations are used to quantify the subsonic region inside nozzle. Thrust is measured experimentally to validate the numerical findings and assess the effect of swirl on nozzle choking criteria, primarily thrust and specific impulse. It is found that the throat velocity itself (not any of its components) is choked in a swirling flowfield. Therefore, the limiting tangential Mach number is unity. Moreover, the application of swirl always results in a reduction in axial Mach number component. The mass flow rate through nozzle is found to be primarily a function of throat static pressure and axial Mach number. The reduction in the latter with swirl explains the observed reduction in mass flow. Greater reservoir pressures, on the other hand, result in higher throat static pressures, which compensates for the reduced axial Mach number, and the mass flow rate can be kept constant at its non-swirling value. It is also found that the distribution of subsonic Mach number in a non-swirling flow is almost not affected with the application of swirl, i.e., non-swirling and swirling flows have the same subsonic Mach number profile. In terms of thrust and specific impulse, the application of swirl at matched nozzle reservoir pressure results in the expected reductions in discharge coefficient, thrust, and specific impulse. At matched mass flow, however, the application of swirl results in the enhancement of both thrust and specific impulse. This is attributed to the considerable degree of underexpansion associated with the swirling flow as a result of the higher nozzle reservoir pressure with swirl. In terms of shock strength, the application of swirl at matched reservoir pressure weakens the shock structure. Matching the mass flow, on the other hand, results in a stronger structure. Swirl is found to enhance supersonic mixing significantly, where swirl-induced vortices stir up and mix different regions of flowfield. High relative Mach numbers between air and fuel, combined with subsonic injection, are found to induce a negative-angled air/fuel shear layer, which results in mixing enhancement and a weaker shock structure.
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.
An Innovative Thermal Management Solution for Cooling of Chips with Various Heights and Power Densities
(2007-08-09) McMillin, Timothy Walter; Ohadi, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The challenges and benefits of using a liquid-cooled cold plate to cool a multi-processor circuit board with complex geometry were explored. Two cold plates were designed, fabricated, and tested experimentally. Thermal interface resistance was experimentally discovered and confirmed with numerical simulations. A circuit board simulator was constructed. This simulator was meant to mimic a multi-processor circuit board with heat sources of different surface areas, heights, and heat dissipations. Results and discussions are presented in this thesis.
Spray Flame and Exhaust Jet Characteristics of a Pressurized Swirl Combustor
(2006-05-17) Linck, Martin Brendan; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This work describes an investigation of swirl-stabilized flames, created in a combustor featuring co-annular swirling airflows, under unenclosed, enclosed, and submerged conditions. A centrally-located fuel nozzle, which uses air-assist atomization, creates a methanol fuel spray. This approach provides great control over fuel spray properties in a compact geometry. Factors affecting the structure of the flames, including the effect of the central atomization air jet, are investigated using three-dimensional particle image velocimetry, direct imaging, and phase-Doppler particle analysis techniques. Exhaust jet temperatures are measured. The dynamic events affecting two-phase exhaust jets from the combustor under submerged conditions are examined using high-speed cinematography and sound spectrum analysis. It is found that the structures of the flames examined, which feature low overall equivalence ratios, are closely linked to the features of the air flowfield in the combustor. Swirl numbers of flows emerging from twisted-vane swirl assemblies are characterized. The structure of the flow is affected by the swirl configuration, but does not depend heavily on the Reynolds number. The central atomization air jet (with or without fuel) reshapes the recirculation region in the swirling flow and has a significant, controllable effect on the structure of the airflow and flame. The effect is the same for nonreacting and reacting flows. In one unique case, the central atomization air interacts with the swirling flow to create two recirculation regions and a lifted flame. The lifted flame is more compact than similar non-lifted flames. The twin-fluid atomization approach is shown to provide effective atomization over a wide range of operating conditions. The two-phase interaction of the exhaust jet is found to depend on the pressure drop over the exhaust nozzle. The dynamic behavior of the exhaust jet is buoyancy-driven at low pressure drops, and is affected by complex instability mechanisms at high pressure drops. Strouhal numbers of large-scale unstable events occurring in the two-phase flow are two orders of magnitude smaller than those associated with instabilities in single-phase flows. Evidence is presented, indicating that acoustic pressure waves in the exhaust jet may be involved in the generation of bubbles surrounding exhaust jets at high pressure drops.
Adaptive Superposition of Finite Element Meshes in Linear and Nonlinear Dynamic Analysis
(2005-12-05) Yue, Zhihua; Robbins, Donald; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The numerical analysis of transient phenomena in solids, for instance, wave propagation and structural dynamics, is a very important and active area of study in engineering. Despite the current evolutionary state of modern computer hardware, practical analysis of large scale, nonlinear transient problems requires the use of adaptive methods where computational resources are locally allocated according to the interpolation requirements of the solution form. Adaptive analysis of transient problems involves obtaining solutions at many different time steps, each of which requires a sequence of adaptive meshes. Therefore, the execution speed of the adaptive algorithm is of paramount importance. In addition, transient problems require that the solution must be passed from one adaptive mesh to the next adaptive mesh with a bare minimum of solution-transfer error since this form of error compromises the initial conditions used for the next time step. A new adaptive finite element procedure (s-adaptive) is developed in this study for modeling transient phenomena in both linear elastic solids and nonlinear elastic solids caused by progressive damage. The adaptive procedure automatically updates the time step size and the spatial mesh discretization in transient analysis, achieving the accuracy and the efficiency requirements simultaneously. The novel feature of the s-adaptive procedure is the original use of finite element mesh superposition to produce spatial refinement in transient problems. The use of mesh superposition enables the s-adaptive procedure to completely avoid the need for cumbersome multipoint constraint algorithms and mesh generators, which makes the s-adaptive procedure extremely fast. Moreover, the use of mesh superposition enables the s-adaptive procedure to minimize the solution-transfer error. In a series of different solid mechanics problem types including 2-D and 3-D linear elastic quasi-static problems, 2-D material nonlinear quasi-static problems, and 2-D transient problems for linear elastic and material nonlinear materials, the s-adaptive solution is compared to a solution obtained using a non-adaptive, uniform refined mesh. These comparisons clearly demonstrate that the s-adaptive method is capable of generating a solution with the same accuracy level as a non-adaptive, uniform refined mesh; however, the s-adaptive solution uses far fewer DOF and consequently executes much faster.
EFFECTS OF PREHEATED COMBUSTION AIR ON LAMINAR COFLOW DIFFUSION FLAMES UNDER NORMAL AND MICROGRAVITY CONDITIONS
(2005-08-30) Ghaderi Yeganeh, Mohammad; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Global energy consumption has been increasing around the world, owing to the rapid growth of industrialization and improvements in the standard of living. As a result, more carbon dioxide and nitrogen oxide are being released into the environment. Therefore, techniques for achieving combustion at reduced carbon dioxide and nitric oxide emission levels have drawn increased attention. Combustion with a highly preheated air and low-oxygen concentration has been shown to provide significant energy savings, reduce pollution and equipment size, and uniform thermal characteristics within the combustion chamber. However, the fundamental understanding of this technique is limited. The motivation of the present study is to identify the effects of preheated combustion air on laminar coflow diffusion flames. Combustion characteristics of laminar coflow diffusion flames are evaluated for the effects of preheated combustion air temperature under normal and low-gravity conditions. Experimental measurements are conducted using direct flame photography, particle image velocimetry (PIV) and optical emission spectroscopy diagnostics. Laminar coflow diffusion flames are examined under four experimental conditions: normal-temperature/normal-gravity (case I), preheated-temperature/normal gravity (case II), normal-temperature/low-gravity (case III), and preheated-temperature/low-gravity (case IV). Comparisons between these four cases yield significant insights. In our studies, increasing the combustion air temperature by 400 K (from 300 K to 700 K), causes a 37.1% reduction in the flame length and about a 25% increase in peak flame temperature. The results also show that a 400 K increase in the preheated air temperature increases CH concentration of the flame by about 83.3% (CH is a marker for the rate of chemical reaction), and also increases the C2 concentration by about 60% (C2 is a marker for the soot precursor). It can therefore be concluded that preheating the combustion air increases the energy release intensity, flame temperature, C2 concentration, and, presumably, NOx production. Our work is the first to consider preheated temperature/low-gravity combustion. The results of our experiments reveal new insights. Where as increasing the temperature of the combustion air reduces the laminar flame width under normal-gravity, we find that, in a low-gravity environment, increasing the combustion air temperature causes a significant increase in the flame width.
IN SITU INFRARED DIAGNOSTICS FOR A MICRO-SCALE COMBUSTION REACTOR
(2004-08-19) Heatwole, Scott; Buckley, Steve G; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The development of centimeter to millimeter scale engines and power supplies have created a need for micro-scale combustion diagnostics. Fuel concentrations, product concentrations, and temperature are useful measurements in determining combustion behavior, chemical efficiency, and flame structures. However, to the present there have been few efforts to develop non-intrusive diagnostic techniques appropriate for application in such small engines. Non-intrusive measurements in these engines are complicated by short path length and lack of optical access. In this thesis in situ FTIR spectroscopy is used to measure temperature and concentrations of fuel, and carbon dioxide in a micro-combustor. The measurements are made through silicon walls spaced a few millimeters apart. This is possible because silicon is transmissive in the infrared. Experimental issues, including the optical setup, limitations associated with etaloning, calibration, and interpretation of the resulting spectra using wide-band models are discussed in detail.
WAVE PROPAGATION IN RODS, SHELLS, AND ROTATING SHAFTS WITH NON-UNIFORM GEOMETRY
(2004-03-16) Toso, Mario; Baz, Amr; Mechanical Engineering
The propagation of waves in rods, shells and rotating shafts with variable thickness is studied through numerical models and experimental measurements. All numerical models are formulated using the Transfer Matrix approach, which accurately reproduces the dynamic behavior and wave propagation characteristics of the considered structures at each frequency. The numerical predictions show that exponential and linear thickness profiles generate a cut-off frequency, below which waves do not propagate along the structure. Hence, the considered rods and shells are capable of filtering out low frequency and they behave as high-pass mechanical filters. The filtering capabilities of the considered class of rods and shells are investigated for different types of profiles. Furthermore, the effect introduced by using periodicity and changing the material properties of the structure in a functionally graded manner is investigated. The effect of linear profiles is practically evaluated by determining both the frequency and time response for excitations applied at one side of the structure. These results are compared to uniform profiles through the Wavelet Transform (WT), which visualizes the structure vibrational energy simultaneously in both the time and frequency domain. The agreement between the theoretical and experimental results validates the numerical models and demonstrates the effectiveness of the proposed design configurations in attenuating the propagation of waves especially in the low-frequency range. The filtering characteristics are also investigated for rotating shafts with tapered and stepped geometry. It is found out that rotation at a constant speed does not significantly modify the flexural wave propagation characteristics of the system. Also, the interest is extended to studying the Campbell diagrams of tapered and periodically stepped profiles. Experiments on the propagation of vibration from a gearbox through rotating shafts prove that tapered and periodic profiles can effectively redistribute the energy spectrum by confining the propagation to specific frequency bands. Such characteristics become more evident when the shaft is provided with active periodic piezoelectric inserts. The effectiveness of the constant axial loads and feedback control on the shaft performance is determined and compared to the alternative passive periodic treatments.

Mechanical Engineering Theses and Dissertations

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