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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Subject: search.filters.subject.Scaling

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Now showing 1 - 6 of 6
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    The Effects of Model Scaling on Sediment Transport in Brownout
    (2013) Glucksman-Glaser, Mark Samuel; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The phenomenon of “brownout” is characterized by a large cloud of sediment or dust that is formed around a rotorcraft when it takes off or lands in arid or dusty environments. To further understand the physics of brownout, a laboratory-scale rotor hovering in water was tested over a ground plane covered with a mobile sediment bed. The sensitivity of the dual-phase flow environment to changes in the values of the similarity parameters that potentially govern the fluid dynamics of the rotor flow and the transport of sediment was explored. First, dye flow visualization was performed to study the general evolution of the rotor flow and its interaction with the ground plane. Then, dual-phase flow visualization was used to expose the details of the processes that mobilize and uplift loose particles from the sediment bed. It was shown using the flow visualization that the trailed vortices from the rotor blades were a primary contributor to the mobilization and suspension of sediment. Particle image velocimetry (PIV) was also used to obtain quantitative measurements of the flow velocities found in the rotor wake and near the ground plane. It is then discussed as to why the steady flow assumptions used in the usual definitions of the classical similarity parameters governing sediment transport are not as applicable to the dual-phase flows produced by a rotor operating over a mobile sediment bed. A Buckingham-π analysis was performed to determine a set of new similarity parameters that potentially better reflect the dual-phase flow characteristics relevant to sediment mobilization and suspension by a rotor wake, including the characteristics of the tip vortices. Sixteen new similarity parameters were initially determined, five of which selected as having particular relevance. Specifically, these new similarity parameters were: 1. The mobile inertia ratio; 2. The stationary inertia ratio, 3. The terminal-swirl velocity ratio; 4. The threshold-swirl velocity ratio; 5. The terminal/threshold-swirl velocity ratio. The values of these similarity parameters were determined using the PIV measurements, and were all found to correlate to the quantity of sediment mobilized and uplifted by the rotor. The terminal/threshold-swirl velocity ratio is proposed as the potentially most important similarity parameter for further characterizing the brownout phenomenon.
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    Measurement and Scaling Analysis of Rotor-Induced Sediment Mobilization
    (2014) Perrotta, Gino; Jones, Anya; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Flow visualization and particle imaging velocimetry (PIV) experiments were conducted in a water tank to investigate the effects of rotor wake and sediment properties on rotor-induced sediment mobilization during hover in ground effect. The two-phase flow was separated into the carrier phase and the dispersed phase for characterization. The carrier phase was studied using PIV to acquire time-resolved planar velocity measurements for a field of view within the rotor wake. The rotor-induced flow was confirmed to be dominated by blade tip vortices and was characterized primarily in terms of the vortex characteristics. Vortices were identified using a tracking function, and were compared to the Lamb-Oseen vortex velocity profile to evaluate their size and strength. The rotor-induced flow was also characterized in terms of wall-jet velocity and turbulent kinetic energy. The dispersed phase was separated using image filtering procedures and was quantified by identifying mobilized sediment particles visible in the field of view. Characteristics of the rotor-induced flow and quantification of sediment mobilization were each averaged over time for several rotor rotations to reduce the effects of wake aperiodicity and asymmetry. New parameter groups were created by combining rotor-induced sediment mobilization system characteristics and each was inspected for correlation with sediment mobilization. Three parameter groups which correlated for all cases measure here are identified and discussed.
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    The Effects of Model Scaling on Sediment Transport in Brownout
    (2013) Glucksman-Glaser, Mark Samuel; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The phenomenon of “brownout” is characterized by a large cloud of sediment or dust that is formed around a rotorcraft when it takes off or lands in arid or dusty environments. To further understand the physics of brownout, a laboratory-scale rotor hovering in water was tested over a ground plane covered with a mobile sediment bed. The sensitivity of the dual-phase flow environment to changes in the values of the similarity parameters that potentially govern the fluid dynamics of the rotor flow and the transport of sediment was explored. First, dye flow visualization was performed to study the general evolution of the rotor flow and its interaction with the ground plane. Then, dual-phase flow visualization was used to expose the details of the processes that mobilize and uplift loose particles from the sediment bed. It was shown using the flow visualization that the trailed vortices from the rotor blades were a primary contributor to the mobilization and suspension of sediment. Particle image velocimetry (PIV) was also used to obtain quantitative measurements of the flow velocities found in the rotor wake and near the ground plane. It is then discussed as to why the steady flow assumptions used in the usual definitions of the classical similarity parameters governing sediment transport are not as applicable to the dual-phase flows produced by a rotor operating over a mobile sediment bed. A Buckingham-π analysis was performed to determine a set of new similarity parameters that potentially better reflect the dual-phase flow characteristics relevant to sediment mobilization and suspension by a rotor wake, including the characteristics of the tip vortices. Sixteen new similarity parameters were initially determined, five of which selected as having particular relevance. Specifically, these new similarity parameters were: 1. The mobile inertia ratio; 2. The stationary inertia ratio, 3. The terminal-swirl velocity ratio; 4. The threshold-swirl velocity ratio; 5. The terminal/threshold-swirl velocity ratio. The values of these similarity parameters were determined using the PIV measurements, and were all found to correlate to the quantity of sediment mobilized and uplifted by the rotor. The terminal/threshold-swirl velocity ratio is proposed as the potentially most important similarity parameter for further characterizing the brownout phenomenon.
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    Experimental Studies to Investigate Pressure Loading on Target Plates
    (2012) Leiste, Hans Ulrich; Fourney, William L; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The pressure distribution on the bottom of an armored vehicle caused by a detonating explosive buried in soil has been long of interest in the community of the Armed Services. This experimental study discusses results from small-scale tests conducted to determine the distribution of pressure on target plates when subjected to loading due to the detonation of buried mines. A new methodology has been developed in order to measure the pressure - time profiles on the bottom of a vehicle. The tests are conducted by using water, saturated sand, and dry sand as the loading media. Different stand off distances are investigated using saturated sand. Kolsky bars and pressure sensors, on the target plate, are used to determine the free-field pressure - time profile of the output of the charge at any location for the vehicle bottom. For verification of the pressure results at certain distances, the specific impulse measured by utilizing different size round plates are employed. In order to investigate the loading mechanisms, high speed cameras are applied. The small-scale test results can be scaled to full scale by using verified scaling laws. The results of this research are of interest to the U.S NAVY and U.S ARMY in order to develop and evaluate computer codes which are mainly used in soil models. These computer codes are to be used to design mine resistant personnel carriers and in the field to predict the threat assessments.
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    The Scaling of Performance and Losses in Miniature Internal Combustion Engines
    (2010) Menon, Shyam Kumar; Cadou, Christopher P; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Miniature glow ignition internal combustion (IC) piston engines are an off-the-shelf technology that could dramatically increase the endurance of miniature electric power supplies and the range and endurance of small unmanned air vehicles provided their overall thermodynamic efficiencies can be increased to 15% or better. This thesis presents the first comprehensive analysis of small (<500 g) piston engine performance. A unique dynamometer system is developed that is capable of making reliable measurements of engine performance and losses in these small engines. Methodologies are also developed for measuring volumetric, heat transfer, exhaust, mechanical, and combustion losses. These instruments and techniques are used to investigate the performance of seven single-cylinder, two-stroke, glow fueled engines ranging in size from 15 to 450 g (0.16 to 7.5 cm3 displacement). Scaling rules for power output, overall efficiency, and normalized power are developed from the data. These will be useful to developers of micro-air vehicles and miniature power systems. The data show that the minimum length scale of a thermodynamically viable piston engine based on present technology is approximately 3 mm. Incomplete combustion is the most important challenge as it accounts for 60-70% of total energy losses. Combustion losses are followed in order of importance by heat transfer, sensible enthalpy, and friction. A net heat release analysis based on in-cylinder pressure measurements suggest that a two-stage combustion process occurs at low engine speeds and equivalence ratios close to 1. Different theories based on burning mode and reaction kinetics are proposed to explain the observed results. High speed imaging of the combustion chamber suggests that a turbulent premixed flame with its origin in the vicinity of the glow plug is the primary driver of combustion. Placing miniature IC engines on a turbulent combustion regime diagram shows that they operate in the 'flamelet in eddy' regime whereas conventional-scale engines operate mostly in the 'wrinkled laminar flame sheet' regime. Taken together, the results show that the combustion process is the key obstacle to realizing the potential of small IC engines. Overcoming this obstacle will require new diagnostic techniques, measurements, combustion models, and high temperature materials.
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    A Study of Nanometer Semiconductor Scaling Effects on Microelectronics Reliability
    (2009) White, Mark; Bernstein, Joseph B.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The desire to assess the reliability of emerging scaled microelectronics technologies through faster reliability trials and more accurate acceleration models is the precursor for further research and experimentation in this relevant field. The effect of semiconductor scaling on microelectronics product reliability is an important aspect to the high reliability application user. From the perspective of a customer or user, who in many cases must deal with very limited, if any, manufacturer's reliability data to assess the product for a highly-reliable application, product-level testing is critical in the characterization and reliability assessment of advanced nanometer semiconductor scaling effects on microelectronics reliability. This dissertation provides a methodology on how to accomplish this and provides techniques for deriving the expected product-level reliability on commercial memory products. Competing mechanism theory and the multiple failure mechanism model are applied to two separate experiments; scaled SRAM and SDRAM products. Accelerated stress testing at multiple conditions is applied at the product level of several scaled memory products to assess the performance degradation and product reliability. Acceleration models are derived for each case. For several scaled SDRAM products, retention time degradation is studied and two distinct soft error populations are observed with each technology generation: early breakdown, characterized by randomly distributed weak bits with Weibull slope Beta=1, and a main population breakdown with an increasing failure rate. Retention time soft error rates are calculated and a multiple failure mechanism acceleration model with parameters is derived for each technology. Defect densities are calculated and reflect a decreasing trend in the percentage of random defective bits for each successive product generation. A normalized soft error failure rate of the memory data retention time in FIT/Gb and FIT/cm2 for several scaled SDRAM generations is presented revealing a power relationship. General models describing the soft error rates across scaled product generations are presented. The analysis methodology may be applied to other scaled microelectronic products and key parameters.