Mechanical Engineering
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Item Characterization and Modeling of Two-Phase Heat Transfer in Chip-Scale Non-Uniformly Heated Microgap Channels(2010) Ali, Ihab A.; Bar-Cohen, Avram; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A chip-scale, non-uniformly heated microgap channel, 100 micron to 500 micron in height with dielectric fluid HFE-7100 providing direct single- and two-phase liquid cooling for a thermal test chip with localized heat flux reaching 100 W/cm2, is experimentally characterized and numerically modeled. Single-phase heat transfer and hydraulic characterization is performed to establish the single-phase baseline performance of the microgap channel and to validate the mesh-intensive CFD numerical model developed for the test channel. Convective heat transfer coefficients for HFE-7100 flowing in a 100-micron microgap channel reached 9 kW/m2K at 6.5 m/s fluid velocity. Despite the highly non-uniform boundary conditions imposed on the microgap channel, CFD model simulation gave excellent agreement with the experimental data (to within 5%), while the discrepancy with the predictions of the classical, "ideal" channel correlations in the literature reached 20%. A detailed investigation of two-phase heat transfer in non-ideal micro gap channels, with developing flow and significant non-uniformities in heat generation, was performed. Significant temperature non-uniformities were observed with non-uniform heating, where the wall temperature gradient exceeded 30°C with a heat flux gradient of 3-30 W/cm2, for the quadrant-die heating pattern compared to a 20°C gradient and 7-14 W/cm2 heat flux gradient for the uniform heating pattern, at 25W heat and 1500 kg/m2s mass flux. Using an inverse computation technique for determining the heat flow into the wetted microgap channel, average wall heat transfer coefficients were found to vary in a complex fashion with channel height, flow rate, heat flux, and heating pattern and to typically display an inverse parabolic segment of a previously observed M-shaped variation with quality, for two-phase thermal transport. Examination of heat transfer coefficients sorted by flow regimes yielded an overall agreement of 31% between predictions of the Chen correlation and the 24 data points classified as being in Annular flow, using a recently proposed Intermittent/Annular transition criterion. A semi-numerical first-order technique, using the Chen correlation, was found to yield acceptable prediction accuracy (17%) for the wall temperature distribution and hot spots in non-uniformly heated "real world" microgap channels cooled by two-phase flow. Heat transfer coefficients in the 100-micron channel were found to reach an Annular flow peak of ~8 kW/m2K at G=1500 kg/m2s and vapor quality of x=10%. In a 500-micron channel, the Annular heat transfer coefficient was found to reach 9 kW/m2K at 270 kg/m2s mass flux and 14% vapor quality level. The peak two-phase HFE-7100 heat transfer coefficient values were nearly 2.5-4 times higher (at similar mass fluxes) than the single-phase HFE-7100 values and sometimes exceeded the cooling capability associated with water under forced convection. An alternative classification of heat transfer coefficients, based on the variable slope of the observed heat transfer coefficient curve), was found to yield good agreement with the Chen correlation predictions in the pseudo-annular flow regime (22%) but to fall to 38% when compared to the Shah correlation for data in the pseudo-intermittent flow regime.Item Physics of Breaking Bow Waves: A Parametric Investigation using a 2D+T Wave Maker(2009) Maxeiner, Eric; Duncan, James H; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A mechanical 2-dimensional wave maker with a flexible surface was used to create waves similar to those formed at the bow of a moving ship. Utilizing the 2D+T approximation, the wave maker was programmed so that its deformable wave board creates a time sequence of shapes that simulate the line of intersection between one side of the hull of a slender ship model moving at constant speed and an imaginary vertical plane oriented normal to the ship model track. Instead of trying to simulate a particular ship hull, however, the wave maker simulates a parametric set of flat plate motions that contain components of typical bow shapes. The resulting surface waves were measured using a cinematic laser-induced fluorescence technique and the resulting wave profiles were analyzed. A tremendous variation of wave shapes was observed. A variety of wave characteristics including the peak contact point height, peak wave height, wave crest speed and plunging jet thickness distribution were measured and related to the corresponding wave maker motion parameters. Despite the complexity of the wave maker motions, it was observed that wave maker velocity and acceleration along the water line were the wave maker parameters with the strongest influence on many of the measured wave characteristics. Additional analysis reveals that the initial acceleration of the wave maker affects some wave characteristics, especially those related to plunging jet behavior, but does not significantly affect the overall size and shape of the wave. It was also observed that the behavior of wave formation and breaking ranged between two distinct modes. The first mode consists of an overdriven wave that contains a pronounced vertical jet along the face of the wave maker. The overdriven wave breaks close to the wave maker, before a wave crest has fully formed. The second mode is a more slowly developing wave that breaks further away from the wave maker. The developing waves do not contain the pronounced vertical jet observed in overdriven waves. The two modes appear to be related to the initial wave maker acceleration and amount of water displaced by the wave maker.Item Critical Asset and Portfolio Risk Analysis for Homeland Security(2008-07-21) McGill, William L; Ayyub, Bilal M; Reliability Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Providing a defensible basis for allocating resources for critical infrastructure and key resource protection is an important and challenging problem. Investments can be made in countermeasures that improve the security and hardness of a potential target exposed to a security hazard, deterrence measures to decrease the likeliness of a security event, and capabilities to mitigate human, economic, and other types of losses following an incident. Multiple threat types must be considered, spanning everything from natural hazards, industrial accidents, and human-caused security threats. In addition, investment decisions can be made at multiple levels of abstraction and leadership, from tactical decisions for real-time protection of assets to operational and strategic decisions affecting individual assets and assets comprising a regions or sector. The objective of this research is to develop a probabilistic risk analysis methodology for critical asset protection, called Critical Asset and Portfolio Risk Analysis, or CAPRA, that supports operational and strategic resource allocation decisions at any level of leadership or system abstraction. The CAPRA methodology consists of six analysis phases: scenario identification, consequence and severity assessment, overall vulnerability assessment, threat probability assessment, actionable risk assessment, and benefit-cost analysis. The results from the first four phases of CAPRA combine in the fifth phase to produce actionable risk information that informs decision makers on where to focus attention for cost-effective risk reduction. If the risk is determined to be unacceptable and potentially mitigable, the sixth phase offers methods for conducting a probabilistic benefit-cost analysis of alternative risk mitigation strategies. Several case studies are provided to demonstrate the methodology, including an asset-level analysis that leverages systems reliability analysis techniques and a regional-level portfolio analysis that leverages techniques from approximate reasoning. The main achievements of this research are three-fold. First, this research develops methods for security risk analysis that specifically accommodates the dynamic behavior of intelligent adversaries, to include their tendency to shift attention toward attractive targets and to seek opportunities to exploit defender ignorance of plausible targets and attack modes to achieve surprise. Second, this research develops and employs an expanded definition of vulnerability that takes into account all system weaknesses from initiating event to consequence. That is, this research formally extends the meaning of vulnerability beyond security weaknesses to include target fragility, the intrinsic resistance to loss of the systems comprising the asset, and weaknesses in response and recovery capabilities. Third, this research demonstrates that useful actionable risk information can be produced even with limited information supporting precise estimates of model parameters.Item 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.