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Ghods, MasoudEffect of Convection Associated with Cross-section Change during Directional Solidification of Binary Alloys on Dendritic Array Morphology and Macrosegregation
Doctor of Engineering, Cleveland State University, 2017, Washkewicz College of Engineering
This dissertation explores the role of different types of convection on macrosegregation and on dendritic array morphology of two aluminum alloys directionally solidified through cylindrical graphite molds having both cross-section decrease and increase. Al- 19 wt. % Cu and Al-7 wt. % Si alloys were directionally solidified at two growth speed of 10 and 29.1 µm s-1 and examined for longitudinal and radial macrosegregation, and for primary dendrite spacing and dendrite trunk diameter. Directional solidification of these alloys through constant cross-section showed clustering of primary dendrites and parabolic-shaped radial macrosegregation profile, indicative of “steepling convection” in the mushy-zone. The degree of radial macrosegregation increased with decreased growth speed. The Al- 19 wt. % Cu samples, grown under similar conditions as Al-7 wt. % Si, showed more radial macrosegregation because of more intense “stepling convection” caused by their one order of magnitude larger coefficient of solutal expansion. Positive macrosegregation right before, followed by negative macrosegregation right after an abrupt cross-section decrease (from 9.5 mm diameter to 3.2 mm diameter), were observed in both alloys; this is because of the combined effect of thermosolutal convection and area-change-driven shrinkage flow in the contraction region. The degree of macrosegregation was found to be higher in the Al- 19 wt. % Cu samples. Strong area-change-driven shrinkage flow changes the parabolic-shape radial macrosegregation in the larger diameter section before contraction to “S-shaped” profile. But in the smaller diameter section after the contraction very low degree of radial macrosegregation was found. The samples solidified through an abrupt cross-section increase (from 3.2 mm diameter to 9.5 mm diameter) showed negative macrosegregation right after the cross-section increase on the expansion platform. During the transition to steady-state after the expansion, radial macrosegregation profile in locations close to the expansion was found to be “S-shaped”. This is attributed to the redistribution of solute-rich liquid ahead of the mushy-zone as it transitions from the narrow portion below into the large diameter portion above. Solutal remelting and fragmentation of dendrite branches, and floating of these fragmented pieces appear to be responsible for spurious grains formation in Al- 19 wt. % Cu samples after the cross-section expansion. New grain formation was not observed in Al-7 wt. % Si in similar locations; it is believed that this is due to the sinking of the fragmented dendrite branches in this alloy. Experimentally observed radial and axial macrosegregations agree well with the results obtained from the numerical simulations carried out by Dr. Mark Lauer and Prof. David R. Poirier at the University of Arizona. Trunk Diameter (TD) of dendritic array appears to respond more readily to the changing growth conditions as compared to the Nearest Neighbor Spacing (NNS) of primary dendrites.

Committee:

Surendra Tewari, Ph.D. (Advisor); Jorge Gatica, Ph.D. (Committee Member); Orhan Talu, Ph.D. (Committee Member); Rolf Lustig, Ph.D. (Committee Member); Kiril Streletzky, Ph.D. (Committee Member)

Subjects:

Aerospace Materials; Automotive Materials; Chemical Engineering; Condensed Matter Physics; Engineering; Fluid Dynamics; High Temperature Physics; Materials Science; Metallurgy

Keywords:

Directional Solidification; Natural Convection; Fluid Flow; Binary Alloys; Macrosegregation; Dendritic Array; Dendrite Morphology; Solutal Remelting; Thermosolutal Convection; Aluminum Alloy; Cross section Change

Goparaju, KalyanFlow and Acoustic Characteristics of Complex Supersonic Jets
Doctor of Philosophy, The Ohio State University, 2017, Aero/Astro Engineering
Jet noise has been a major source of concern for commercial and military aviation sectors alike. The need to assuage the adverse impact of jet noise on human health has led to increased interest in jet noise source identification and noise level minimization/mitigation. Most previous works on common round supersonic jets have primarily explored the ideal case of simple, perfectly expanded jet configurations. In real world scenarios however, many of these simplifications do not hold. Two such considerations are examined in this work. The first is a simple configuration operating at complex operating conditions, specifically an imperfect expansion i.e. where the jets are operating at off-design conditions. The second concerns a complex configuration at simple conditions: specifically two jets (twin-jets such as those on fighter aircraft) operating in close proximity to each other. In this work, Large Eddy Simulation (LES) based high-fidelity computations are used to understand the dynamics of imperfectly expanded and twin-jets respectively, with the following objectives: 1) Identify the impact of active flow control techniques on the plume dynamics and acoustic characteristics of underexpanded jets, and 2) Investigate the interaction dynamics of the twin-jet plumes and study its associated sound field which exhibits complex radiation characteristics. To meet the first objective, a single-jet with a fully expanded Mach 1.3 jet, issuing from a converging nozzle operating at underexpanded conditions is considered. After selecting the appropriate grid based on mesh resolution studies, flow validation is conducted which indicates an excellent qualitative and quantitative agreement of the computed jet plume characteristics with the experimental observations. The analysis of underexpanded flow-field at two different Reynolds numbers indicates a relative independence of the jet flow characteristics and downstream plume evolution to the variation in Reynolds number. A detailed investigation of the near-field pressure fluctuations at different polar angles confirms the presence of three distinct noise sources i.e. downstream directed mixing noise, side-line directed broadband shock associated noise, and upstream directed screech tone noise. The frequency of the screech tone, generated as a result of a feedback process, is observed to be consistent with theoretical and previous experimental works. To investigate the effect of active flow control on the flow and acoustic features of the underexpanded jet, Localized Arc Filament Plasma Actuators (LAFPAs) are employed, which are modeled using a semi-empirical surface heating technique. For the control simulations, axisymmetric mode pulsing is considered at two different Strouhal numbers of St=0.3 and St=0.9. These simulations show that the response of the jet to flow control is a strong function of the actuation frequency. Relative to the baseline uncontrolled case, actuating at the column mode instability frequency (St=0.3) results in an increase in the rate of spreading of the shear-layer. Furthermore, analysis of the phase-averaged results reveals the formation of large-scale toroidal structures that are generated due to the excitation of jet column instabilities at this actuation frequency. As a consequence of the formation of these large-scale features, the St=0.3 case exhibits increased noise levels, relative to the uncontrolled case. On the other hand, the higher frequency actuation affects the initial shear-layer instability and interferes with the formation of the toroidal events, that are observed for the St=0.3 actuation case, and further appears to weaken even the naturally occurring turbulent structures. As a result, noise level reduction is observed, relative to the uncontrolled case. In spite of the absence of axisymmetric toroidal events, detailed integral azimuthal length scale analyses reveal the dominance of the axisymmetric (m=0) mode, even at large distances from the nozzle exit. This behavior indicates that flow control methods need not always have a visual signature of their influence on the system. To achieve the second objective of this work of computationally investigating the flow and acoustic characteristics of complex propulsion systems, a Mach 1.23 circular twin-jet configuration, at perfectly expanded conditions, with an inter-nozzle spacing of two jet diameters is considered. The validity of the simulations is established by comparing the jet flow structure and the near-field linear-array noise levels to the experimental results, which exhibit a good match. A qualitative investigation of the jet plume structure reveals the dominance of a helical (m=1) mode on both the jet plumes, which is manifested in the form of cork-screw type features encompassing the supersonic jet core of each of the plumes. Analysis of the inter-nozzle region using mean velocity profiles indicates the formation of a secondary flow, which extends a significant influence on the jet plume shear-layers in its proximity. However, this impact is confined to small azimuthal angles and as a result, many of the potential core properties are almost similar to those of a single-jet at identical flow conditions. Analysis of the near-field pressure fluctuations along the plane containing the jet centers, at various polar angles, reveals the presence of jet noise shielding, where the noise levels imposed by a twin-jet is less than the sum of two incoherent single-jets. This shielding effect is observed to diminish with increasing polar angles. On the other hand, no such shielding is observed along the plane perpendicular to that containing the jet axes and instead noise level amplification, relative to the sum of two incoherent single-jets is noticed. This is attributed to the unabated sound radiation of the twin-jet plume in this direction. To aid in the identification and understanding of the noise generation mechanisms, in addition to predicting the radiated noise levels and directivity, a decomposition of the flow-field into constituent fluid-thermal (FT) modes based on Momentum Potential Theory (MPT) is performed. The decomposed hydrodynamic mode is observed to be a true representation of the unsteady turbulence, which is evident from the identification of and m=1 helical mode patterns in the iso-levels of this variable. The noise signature associated with these turbulent features is observed to be contained within the decomposed acoustic mode, which exhibits a pronounced wavepacket structure in the jet core with an apparent m=0 axisymmetric toroidal mode dominance. Consistent with recent works on perfectly expanded supersonic jets, the acoustic wavepacket in the core of the jet exhibits significant temporal and axial modulation, which play a major role in determining acoustic radiation characteristics such as directivity and intermittency. Comparing the near-field noise level predicted by the acoustic mode with that of the overall pressure fluctuations, an excellent match is obtained both qualitatively and quantitatively. The decomposed acoustic mode correctly predicts the shielding and magnification phenomena, which signifies its ability to predict acoustic behavior of even severely complex configurations.

Committee:

Datta Gaitonde , Professor (Advisor); Mo Samimy, Professor (Committee Member); Mei Zhuang, Professor (Committee Member); Jen-Ping Chen, Associate Professor (Committee Member)

Subjects:

Acoustics; Aerospace Engineering; Fluid Dynamics

Keywords:

jet noise; supersonic jets; LES; twin-jets; underexpanded jets; momentum potential theory; flow decomposition;

Friedrich, Brian KarlAn Experimental Study of Volumetric Quality on Fluid Flow and Heat Transfer Characteristics for Two Phase Impinging Jets
Master of Science in Engineering, Youngstown State University, 2016, Department of Mechanical and Industrial Engineering
This study further expands the current knowledge of the relationship between heat transfer and fluid mechanics. Fluid flow and heat transfer characteristics of air-assisted water jet impingement was experimentally investigated under a fixed water flow rate condition. Water and air were the test fluids. The effects of volumetric quality (ß = 0 – 0.9) on the Nusselt number, hydraulic jump diameter, and pressure were considered. The results showed that stagnation Nusselt number, hydraulic jump diameter, and stagnation pressure increased with volumetric quality to a maximum value at 0.8 of the volumetric quality, and then decreased. The stagnation Nusselt number and hydraulic jump diameter of the air assisted water jet impingement are governed by the stagnation pressure. Based on the experimental results, a new correlation for the normalized stagnation Nusselt number and hydraulic jump are developed as a function of the normalized stagnation pressure alone. This research can be applied to further enhance the cooling of industrial applications, such as, cooling of electronics and processing of materials.

Committee:

Kyosung Choo, PhD (Advisor); Guha Manogharan, PhD (Committee Member); Jae Joong Ryu, PhD (Committee Member)

Subjects:

Experiments; Fluid Dynamics; Mechanical Engineering

Keywords:

impinging jet; Hydraulic Jump; two phase; two-phase; fluid flow; Heat transfer; jet impingement; Volumetric quality;

Shaik, Muneeb Ur RahmanGas Dispersion Using an Up-Pumping Maxflo W Impeller
Master of Science (M.S.), University of Dayton, 2014, Chemical Engineering
Despite known advantages of up-pumping wide-blade axial-flow impellers for gas dispersion, little is known about the design guidelines of these impellers. In the present study, the gas dispersion capabilities of the up-pumping axial-flow Maxflo W impeller in low-viscosity liquids have been characterized to incorporate effects of scale and system geometry. Further, a comparison is made with the dispersion performance of an up-pumping pitch-blade turbine and a radial-flow CD-6. This comparison showed that the up-pumping Maxflo W impeller have dispersion speed, torque and power requirements relatively independent of the gas flow rate, providing the ability to handle varying process conditions. Testing at three scales (tank diameters of 0.29 m, 0.44 m and 0.60 m) indicates that the dispersion capabilities of the up-pumping Maxflo W can be described in terms of a scale-independent aeration number - Froude number relationship. Although a minimum impeller rotational speed is required to disperse low gas flows, the increase in dispersion speed is small with increasing gas flow rate. The effects of geometric parameters such as impeller to tank diameter ratio, sparger size and ungassed liquid height on flooded to dispersed conditions have been investigated. Unlike radial-flow gas dispersion impellers that are not strongly affected by some geometric parameters, the up-pumping Maxflo W dispersion performance is strongly dependent on system geometric parameters. Additionally, small impeller to tank diameter ratios of Maxflo W impellers have been found to perform poorly in gas dispersion operations because of their highly axial discharge flow. It was found that the up-pumping pitched-blade turbine exhibits time-dependent dispersion behavior that can permanently fail to disperse gas after appearing to be capable of dispersion for a reasonably long period of time. For this reason, collection of design data required extended and meticulous testing.

Committee:

Kevin Myers, J. (Advisor); Eric Janz, E. (Committee Co-Chair); Robert Wilkens, J. (Committee Member)

Subjects:

Chemical Engineering; Fluid Dynamics; Industrial Engineering

Keywords:

Gas Dispersion; Up-pumping Axial-Flow Impeller Scale Effect and Geometrical Effect; Maxflo W; Mixing Scale-up Creteria

Bixler, GregBioinspired Surface for Low Drag, Self-Cleaning, and Antifouling: Shark Skin, Butterfly and Rice Leaf Effects
Doctor of Philosophy, The Ohio State University, 2013, Mechanical Engineering
In this thesis, first presented is an overview of inorganic-fouling and biofouling which is generally undesirable for many medical, marine, and industrial applications. A survey of nature’s flora and fauna are studied in order to discover new antifouling methods that could be mimicked for engineering applications. New antifouling methods will presumably incorporate a combination of physical and chemical controls. Presented are mechanisms and experimental results focusing on laminar and turbulent drag reducing shark skin inspired riblet surfaces. This includes new laser etched and riblet film samples for closed channel drag using water, oil, and air as well as in wind tunnel. Also presented are mechanisms and experimental results focusing on the newly discovered rice and butterfly wing effect surfaces. Morphology, drag, self-cleaning, contact angle, and contact angle hysteresis data are presented to understand the role of sample geometrical dimensions, wettability, viscosity, and velocity. Hierarchical liquid repellent coatings combining nano- and micro-sized features and particles are utilized to recreate or combine various effects. Such surfaces have been fabricated with photolithography, soft lithography, hot embossing, and coating techniques. Discussion is provided along with new conceptual models describing the role of surface structures related to low drag, self-cleaning, and antifouling properties. Modeling provides design guidance when developing novel low drag and self-cleaning surfaces for medical, marine, and industrial applications.

Committee:

Bharat Bhushan (Advisor)

Subjects:

Fluid Dynamics; Mechanical Engineering; Nanotechnology

Keywords:

Shark skin; lotus leaf; rice and butterfly wing effect; drag reduction; self-cleaning; antifouling; embossed film; fish scale; butterfly wing; rice leaf; rice leaf inspired

Stalcup, Erik JamesNumerical Modeling of Upward Flame Spread and Burning of Wavy Thin Solids
Master of Sciences, Case Western Reserve University, EMC - Aerospace Engineering
Flame spread over solid fuels with simple geometries has been extensively studied in the past, but few have investigated the effects of complex fuel geometry. This study uses numerical modeling to analyze the flame spread and burning of wavy (corrugated) thin solids and the effect of varying the wave amplitude. Sensitivity to gas phase chemical kinetics is also analyzed. Fire Dynamics Simulator is utilized for modeling. The simulations are two-dimensional Direct Numerical Simulations including finite-rate combustion, first-order pyrolysis, and gray gas radiation. Changing the fuel structure configuration has a significant effect on all stages of flame spread. Corrugated samples exhibit flame shrinkage and break-up into flamelets, behavior not seen for flat samples. Increasing the corrugation amplitude increases the flame growth rate, decreases the burnout rate, and can suppress flamelet propagation after shrinkage. Faster kinetics result in slightly faster growth and more surviving flamelets. These results qualitatively agreement with experiments.

Committee:

James T'ien (Committee Chair); Joseph Prahl (Committee Member); Yasuhiro Kamotani (Committee Member)

Subjects:

Aerospace Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

modeling;simulation;numerical modeling;combustion;computational combustion;direct numerical simulation;flame spread;burning;wavy;corrugated;fire dynamics simulator;FDS;fuel structure;fuel geometry;complex geometry;cardboard;

Bova, Anthony ScottModeling the Ventilation of Natural Animal Shelters in Wildland Fires
Master of Science, The Ohio State University, 2010, Civil Engineering

The level of protection from wildland fires that tree cavities provide to sheltered fauna is not well understood. Further, few experiments have been performed to investigate the transfer of combustion products into, and ventilation of, tree cavity shelters in wildland fires. This paucity of data is unlikely to change in the near future. However, increasingly realistic fluid and fire dynamics simulation software has made the execution of “virtual experiments” tenable. In such experiments, data from simulations are used to form empirical relationships between the investigated phenomena and simulated conditions. As an example of this approach, the National Institute of Standards and Technology’s (NIST) Fire Dynamics Simulator (FDS) was used to create formulas for estimating maximum combustion product concentrations, doses (concentration integrated over time) and maximum gas temperatures within a single-entrance cylindrical shelter at heights above 3 m.

A three-step approach was taken: First, FDS was validated for single-entrance ventilation by comparison of simulation results to data from large- and small-scale ventilation experiments. Second, data from 45 simulations of a single-entrance, cylindrical shelter subjected to frontal winds at various speeds, angles of incidence and temperatures, were used to create empirical formulas relating these variables to entrance flux and rates of temperature change. Third, these formulas were applied to data from 26 separate simulations of different surface fire scenarios. As a result, a single empirical formula was found relating gas concentrations, doses and maximum temperatures inside a shelter to fire intensity, flame depth and wind speed. The findings suggest that virtual experiments can help provide tools for forest and land managers to estimate the impact and minimize the hazards of prescribed burning, as well as evaluate the consequences of naturally occurring wildland fires.

Committee:

Gil Bohrer, PhD (Advisor); Matthew Dickinson, PhD (Committee Member); Ethan Kubatko, PhD (Committee Member); John Lenhart, PhD (Committee Member)

Subjects:

Environmental Engineering; Environmental Science; Fluid Dynamics

Keywords:

wildland fire; animal shelter; cavity ventilation; fire dynamics simulator

Hahn, Casey BernardDesign and Validation of the New Jet Facility and Anechoic Chamber
Master of Science, The Ohio State University, 2011, Mechanical Engineering

The jet facility and anechoic chamber at the Gas Dynamics and Turbulence Laboratory (GDTL) at The Ohio State University have been redesigned and rebuilt to significantly improve their capabilities. The new jet facility is capable of jets of 2-inch diameter—twice the size of the old jets. The new and much larger anechoic chamber can handle the larger jet and enables the measurements of shock noise generated by the jet of tactical aircraft. Free-field qualification requirements of ISO 3745 standard are met, and the chamber has a cutoff frequency of 160 Hz. A few improvements were incorporated into the new facility including thicker, acoustically-treated walls and an acoustically transparent grating floor above the floor anechoic wedges. Tests showed that very minor variations in the spectra are introduced by the grating floor panels.

Two additional microphones were added to the new facility with three within the upstream region of the acoustic field (a maximum polar angle of 130° compared to the maximum of 90° of the old facility). The radial distances of the microphones were increased, and far-field tests show that the microphones are safely within the far-field of 1-inch and 1.5-inch jets. For a 2-inch jet, some microphones are likely within the transition region of the acoustic field but could be moved farther outward to locate them within the far-field, as there is more room within the chamber. The stagnation chamber diameter was increased from 3.068 inches to 5.047 inches to handle the larger mass flow rate of a 2-inch jet. Initially, spectra suffered from narrowband cavity tones generated by ports upstream. The ports were modified, and a second perforated plate was added to eliminate these tones.

Acoustic data of the new and old jets are compared, and some minor differences in the high frequency content of the spectra are found. Early guesses point to internal rig noise created by flow through the second perforated plate. Work will continue to remove these differences. Finally, PIV results of the old and new jets are compared. The Mach number decay and spreading rates of a new Mach 0.9 jet compare well to an old Mach 0.9 jet. The old Mach 0.9 jets had slightly lower levels of turbulent kinetic energy. A new Mach 1.3 jet compares well with an old Mach 1.3 jet all these statistics.

Committee:

Mo Samimy, PhD (Advisor); Datta Gaitonde, PhD (Committee Member)

Subjects:

Acoustics; Aerospace Engineering; Engineering; Experiments; Fluid Dynamics; Mechanical Engineering

Keywords:

jet facility; anechoic chamber; aeroacoustics

Mikhail, Salam R.The Effect of Inclination on the Rayleigh-Benard Convection of Mercury in a Small Chamber
Doctor of Philosophy, The Ohio State University, 2011, Physics

The effect of inclination on the convection of mercury in a small chamber heated from below was studied. Mercury's low viscosity and high thermal conductivity makes it a low Prandtl number fluid. Because it is opaque, traditional optical techniques for visualizing flow states were not possible. Instead, we made use of the variation of the speed of sound with temperature. The time between sending an ultrasound pulse and receiving its echo was converted to a temperature measurement. Flow states were deduced from the temperature distributions. A phase diagram of flow states for different angles of inclination and different Rayleigh numbers was produced.

In the horizontal chamber with low temperature differences across the mercury layer, cellular convection rolls parallel to shorter side of the chamber are observed. With higher temperature differences, they are not observed. As the angle of inclination is increased, the range in which cellular convection rolls are observed decreases. In the ranges where rolls are not observed, it is likely that the rolls have changed their orientation by 90 degrees, changing from transverse to longitudinal. Our measurement technique does not allow visualization of longitudinal rolls. Linear stability theory indicates that longitudinal rolls occur when thermal effects are important, while transverse rolls occur when hydrodynamic effects are important. The flow state in which there are no visible transverse rolls is found to be more efficient at heat transfer than the transverse roll state. This supports the idea that at higher temperature differences, when no transverse rolls are observed, longitudinal rolls are present.

Committee:

C. David Andereck, PhD (Advisor); Thomas Gramila, PhD (Committee Member); Bruce Patton, PhD (Committee Member); Robert Perry, PhD (Committee Member)

Subjects:

Fluid Dynamics; Physics

Keywords:

Convection; Rayleigh-Benard Convection, Low Prandtl Number; Mercury; Liquid Metal; Inclination

Moses, Kenneth C.A Durable Terrestrial Drive Train for a Small Air Vehicle
Master of Sciences (Engineering), Case Western Reserve University, 2010, EMC - Mechanical Engineering
Weight, aerodynamic profile, and strength are considered in the design of a terrestrial drive train for a small air vehicle. Several drive trains were developed and their performance characteristics compared in order to show a progression in their designs. Each iteration contained minor improvements, approaching the goal of a durable terrestrial drive train for a small air vehicle. These drive trains were analyzed in the Case Western Reserve University low-speed wind tunnel for their influence on the performance of the aircraft. The change in lift produced by the aircraft’s airfoil ranges from -1.0% to -4.5%. The drive trains were also tested for their ability to withstand the shock and reduce the impact of landing. Spring steel wire wheel-legs are found to reduce the peak deceleration by 15.8%. The results identify one drive train that meets the performance goals and is suitable for general use in this scale application.

Committee:

Roger Quinn, PhD (Committee Chair); Yasuhiro Kamotani, PhD (Committee Member); Joseph Prahl, PhD (Committee Member)

Subjects:

Engineering; Fluid Dynamics; Mechanical Engineering; Robots; Technology

Keywords:

Micro Air Vehicle; MAV; hybrid vehicle; drive train; terrestrial locomotion; aerial locomotion; wind tunnel; impact test; aircraft design; aerodynamics;

Allen, Jeremy L.The Effect of Baffle Arrangements on Flow Uniformity in a Manifold for a Unique Solid Oxide Fuel Cell Stack Design
Master of Science (MS), Ohio University, 2011, Mechanical Engineering (Engineering and Technology)
Flow uniformity through channels of a complex fuel cell stack is studied for several baffle arrangements using ANSYS Fluent, a computational fluid dynamics (CFD) package. Flow mal-distribution occurs from pressure differentials throughout the flow structure and causes a drop in stack performance. Three baffle arrangements were introduced into the flow structure and compared to a control case with no baffle in an attempt to improve the flow regime. A flow uniformity coefficient Γ was introduced to compare results from case to case. It was found that all three arrangements significantly increased flow uniformity, with the slotted baffle arrangement providing the most uniform flow. By increasing flow uniformity, the efficiency of the stack is also increased.

Committee:

David Bayless (Advisor); Gregory Kremer (Committee Member); John Cotton (Committee Member); Greg Van Patten (Committee Member)

Subjects:

Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

SOFC; fuel cell; Simulation; CFD; ANSYS Fluent; manifold; baffle; uniform flow

Packard, Nathan OwenActive Flow Separation Control of a Laminar Airfoil at Low Reynolds Number
Doctor of Philosophy, The Ohio State University, 2012, Aero/Astro Engineering
Detailed investigation of the NACA 643-618 is obtained at a Reynolds number of 6.4x104 and angle of attack sweep of -5° < α < 25°. The baseline flow is characterized by four distinct regimes depending on angle of attack, each exhibiting unique flow behavior. Active flow control is exploited from a row of discrete holes located at five percent chord on the upper surface of the airfoil. Steady normal blowing is employed at four representative angles; blowing ratio is optimized by maximizing the lift coefficient with minimal power requirement. The range of effectiveness of pulsed actuation with varying frequency, duty cycle and blowing ratio is explored. Pulsed blowing successfully reduces separation over a wide range of reduced frequency (0.1-1), blowing ratio (0.5–2), and duty cycle (0.6–50%). A phase-locked investigation, by way of particle image velocimetry, at ten degrees angle of attack illuminates physical mechanisms responsible for separation control of pulsed actuation at a low frequency and duty cycle. Temporal resolution of large structure formation and wake shedding is obtained, revealing a key mechanism for separation control. The Kelvin-Helmholtz instability is identified as responsible for the formation of smaller structures in the separation region which produce favorable momentum transfer, assisting in further thinning the separation region and then fully attaching the boundary layer. Closed-loop separation control of an oscillating NACA 643-618 airfoil at Re = 6.4x104 is investigated in an effort to autonomously minimize control effort while maximizing aerodynamic performance. High response sensing of unsteady flow with on-surface hot-film sensors placed at zero, twenty, and forty percent chord monitors the airfoil performance and determines the necessity of active flow control. Open-loop characterization identified the use of the forty percent sensor as the actuation trigger. Further, the sensor at twenty percent chord is used to distinguish between pre- and post- leading edge stall; this demarcation enables the utilization of optimal blowing parameters for each circumstance. The range of effectiveness of the employed control algorithm is explored, charting the practicality of the closed-loop control algorithm. To further understand the physical mechanisms inherent in the control process, the transients of the aerodynamic response to flow control are investigated. The on-surface hot-film sensor placed at the leading edge is monitored to understand the time delays and response times associated with the initialization of pulsed normal blowing. The effects of angle of attack and pitch rate on these models are investigated. Black-box models are developed to quantify this response. The sensors at twenty and forty percent chord are also monitored for a further understanding of the transient phenomena.

Committee:

Jeffrey Bons, Dr. (Advisor); Mohammad Samimy, Dr. (Committee Member); Jen-Ping Chen, Dr. (Committee Member); Andrea Seranni, Dr. (Committee Member)

Subjects:

Aerospace Engineering; Fluid Dynamics

Keywords:

active flow control; experimental fluid dynamics; closed-loop control

Kearney-Fischer, Martin A.The Noise Signature and Production Mechanisms of Excited High Speed Jets
Doctor of Philosophy, The Ohio State University, 2011, Mechanical Engineering

Following on previous works showing that jet noise has significant intermittent aspects, the present work assumes that these intermittent events are the dominant feature of jet noise. A definition and method of detection for intermittent noise events are devised and implemented. Using a large experimental database of acoustically subsonic jets with different acoustic Mach numbers (Ma = 0.5 – 0.9), nozzle exit diameters (D = 2.54, 5.08, & 7.62 cm), and jet exit temperature to ambient temperature ratios (ETR = 0.84 – 2.70), these events are extracted from the noise signals measured in the anechoic chamber of the NASA Glenn AeroAcoustic Propulsion Laboratory. It is shown that a signal containing only these events retains all of the important aspects of the acoustic spectrum for jet noise radiating to shallow angles relative to the jet axis, validating the assumption that intermittent events are the essential feature of the peak noise radiation direction. The characteristics of these noise events are analyzed showing that these events can be statistically described in terms of three parameters (the variance of the original signal, the mean width of the events, and the mean time between events) and two universal statistical distribution curves. The variation of these parameters with radiation direction, nozzle diameter, exit velocity, and temperature are discussed.

A second experimental database from the Ohio State University Gas Dynamics and Turbulence Laboratory of far-field acoustic data from an excited subsonic jet with hydrodynamic Mach number of 0.9 (Mj = 0.9) at various total temperature ratios (TTR = 1.0 - 2.5) is analyzed using the same process. In addition to the experimental acoustic database, conclusions and observations from previous works using Localized Arc Filament Plasma Actuators (LAFPAs) are leveraged to inform discussion of the statistical results and their relationship to the jet flow dynamics. Analysis of the excited jet reveals the existence of a resonance condition. When excited at the resonance condition, large amounts of noise amplification can occur – this is associated with each large-scale structure producing a noise event. Conversely, noise reduction occurs when only one noise event occurs per several large-scale structures. One of the important conclusions from these results is that there seems to be a competition for flow energy among neighboring structures that dictates if and how their dynamics will produce noise that radiates to the far-field.

Utilizing the results from both databases, several models for noise sources addressing different aspects of the results are discussed. A simple model for this kind of noise signal is used to derive a relationship between the characteristics of the noise events and the fluctuations in the integrated noise source volume. Based on the known flow-field dynamics and the acoustic results from the excited jet, a hypothetical model of the competition process is described. These various models speculate on the dynamics relating the noise sources to the signal in the far-field and, as such, the present work cannot provide a definitive description of jet noise sources, but can serve as a guide to future exploration.

Committee:

Mo Samimy, PhD (Advisor); Igor Adamovich, PhD (Committee Member); James Bridges, PhD (Committee Member); Michael Dunn, PhD (Committee Member); Walter Lempert, PhD (Committee Member)

Subjects:

Acoustics; Aerospace Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

Jet Noise; Aeroacoustics; Active Flow Control; Turbulence

Zhang, HuaijianBoundary Integral Techniques in Three Dimensions for Deep Water Waves
Doctor of Philosophy, The Ohio State University, 2011, Mathematics

The motion of deep water waves is a long-existing problem in fluid dynamics. Because it has many applications in academics and industries, numerous scientists have done much research to tackle this problem. The evolution of deep water wave is governed by Euler equations. Because of its nonlinearity, there are few available analytical expressions to fully describe the evolution of the deep water waves. Thus efficient numerical approximation becomes very critical to understand the properties of water wave motion.

An improved boundary integral technique is presented in this thesis to simulate the motion of deep water waves. One difficulty is the singularity in the Green’s function. Two methods to treat this singularity are discussed. One is blob regularization with third-order accuracy, and the other is based on polar coordinates with spectral accuracy. In the blob regularization, we replace the Green’s function by a regularized smooth Green’s function, which provides a good approximation to the original integral. For the other approach, integral identities are applied to reduce the strength of the singularity and then a polar coordinate transformation is applied to obtain a nonsingular integrand. The results from these two methods will be examined.

Another challenge is that the integrands are integrated over an infinite surface. For a doubly periodic water wave, we have to sum the images of Green’s function over the free-surface of the water. Ewald summation technique is used to expedite the calculation. Three-dimensional interpolation technique is suggested to reduce the time spent even further.

The numerical method is tested with several examples and then applied to the motion of a perturbed Stokes wave. The perturbation grows until the resolution fails.

Committee:

Gregory Baker, Dr. (Advisor); Chiu-Yen Kao, Dr. (Committee Member); Ed Overman, Dr. (Committee Member)

Subjects:

Applied Mathematics; Fluid Dynamics; Mathematics

Keywords:

Three Dimensions Deep Water Wave; Singularity; Blob Regularization; Exponetially Accurate Method; Polar Coordinate Transformation; Third-Order Accurate Method; Ewald Summation; Green's function; Simulation of Perturbed Stokes Wave

Saracoglu, Bayindir HuseyinTurbine Base Pressure Active Control Through Trailing Edge Blowing
Doctor of Philosophy (PhD), Wright State University, 2012, Engineering PhD
The desire for high performance and low fuel consumption aero-engines has been pushing the limits of the turbomachinery and leading cutting-edge engine designs to fulfill the demand. The number of stages is reduced to achieve the same pressure ratios over lighter turbines. The extreme expansion requirements result in transonic-supersonic flow fields. Transonic and supersonic turbines are exposed to the shock waves that appear at the trailing edge of the airfoils, generating substantial efficiency deduction due to the interaction with the boundary layer. Furthermore, pressure fluctuations created by the shocks result in unsteady forcing on downstream components and eventually cause high cycle fatigue. Component failure may lead reduced service life and further damage on the engine. A novel proposal to control the resulting fish tail shock waves consists on, pulsating coolant blowing through the trailing edge of the airfoils. The changes in the base region topology and fish tail shock wave were numerically investigated for a wide range of purge flow at simplified blunt and circular trailing edge geometries. An optimum purge rate which increases the base pressure and significantly reduces the trailing edge shock wave intensity was found. The effects of pulsating base pressure on the shock properties and the base region was investigated in detail to understand the mechanisms driving the flow field under unsteady bleed. A linear cascade representative of modern turbine bladings was specifically designed and constructed. The test matrix comprised four Mach numbers, from subsonic to supersonic regimes (0.8, 0.95, 1.1 and 1.2) together with two engine representative Reynolds numbers (4 and 6 million) at various blowing rates. The blade loading, the downstream pressure distributions and the unsteady wall temperature measurements allowed understanding the effects on each leg of the shock structure. Minimum shock intensities were achieved using pulsating cooling. A substantial increase in base pressure and significant reduction in trailing edge loss were observed for low coolant blowing rate. Analysis of the high frequency Schlieren pictures revealed the modulation of the shock waves with the coolant pulsation. The Strouhal number of the vortex shedding was analyzed for all of the conditions. Finally, the statistical analyses of the results showed that the effects of the state of cooling and free stream conditions were statistically significant on the flow properties.

Committee:

George Huang, PhD (Advisor); Guillermo Paniagua, PhD (Advisor); Joseph Shang, PhD (Committee Member); Mitch Wolff, PhD (Committee Member); Paul King, PhD (Committee Member)

Subjects:

Aerospace Engineering; Engineering; Experiments; Fluid Dynamics; Mechanical Engineering

Keywords:

Shock wave; turbine; base pressure; trailing edge cooling; flow control; vortex shedding; pulsating cooling;

Sampath, Aravind RohanEffect of Rib Turbulators on Heat Transfer Performance in Stationary Ribbed Channels
Master of Science in Mechanical Engineering, Cleveland State University, 2009, Fenn College of Engineering
The thermal performance was examined computationally for the stationary channels with rib turbulators oriented at 90 degrees. Ribs were placed on opposite walls and the heat transfer coefficients and frictional loss were calculated. Three stationary channels with aspect ratios (W/H) 1, 2 and 4 were considered for the analysis. The thermal performance was measured by calculating the Nusselt number and frictional losses. Square ribs (w/e = 1) were considered as the baseline configuration. The rib width and rib spacing varies while the rib height is maintained constant. Rib spacing (P/e) of 10 and 20 and rib width to rib height ratios (w/e) ranging from 1/8 to 14 were considered. The heat transfer performance for all the channels were calculated for Reynolds numbers 10,000, 30,000 and 60,000. The code was validated by comparing the results for channels with square ribs (w/e =1) with the experimental results. The results obtained for all the channels with different rib configuration proved that the increase in rib width reduced the thermal performance of the channels. By combined effect of rib width, rib spacing and flow parameters, the optimal cooling configuration was obtained.

Committee:

Mounir Ibrahim, Ph.D. (Committee Chair); George Chatzimavroudis, Ph.D. (Committee Member); Miron Kaufman, Ph.D. (Committee Member)

Subjects:

Chemical Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

Ribbed channels; heat transfer; frictional loss; cooling configuration; turbine blades

Jeffrey, BargielCommercialization of Lateral Displacement Array for the Dewatering of Microalgae
Master of Sciences, Case Western Reserve University, 2009, Physics
It is shown that a lateral displacement array is technically and commercially feasible for the dewatering of microalgae for the production of algal oil for energy. Process economics of algal oil are examined for dewatering process target cost. Device requirements and the underlying physics of the technology are investigated. Technical feasibility is evaluated for real world operation, scale-up, and manufacturability. Designs are proposed for cost reduction based on underlying theory and manufacturability. Multiple manufacturing methods are evaluated, ranked for technology readiness level and manufacturing readiness level, and cost of goods sold. A gravity-fed unit manufactured with the hot embossing method is recommended for full scale production and is shown to have a total cost reduction of 67% to the economic threshold of $5 per barrel of oil.

Committee:

Brown Robert, PhD (Committee Chair); Taylor Cyrus, PhD (Committee Member); Berner J. Kevin, PhD (Committee Member); Lane Christopher, PhD (Committee Member); Caner Edward (Committee Member)

Subjects:

Energy; Engineering; Fluid Dynamics; Physics

Keywords:

algae; dewatering; biofuels

Zhao, QiuyingTowards Improvement of Numerical Accuracy for Unstructured Grid Flow Solver
Doctor of Philosophy in Engineering, University of Toledo, 2012, College of Engineering

An effort to improve the numerical accuracy of a three dimensional unstructured grid finite volume scheme is pursued in the present work. Unstructured grid methods have been widely used in computational fluid dynamics for the convenience of modeling complex geometries in realistic applications. In the present work, improvements towards high order unstructured grid schemes are proposed using high order flux formula for spatial discretizations. The Riemann variables on the left and right sides of the interface are reconstructed using quadratic and quartic polynomials composed of both flow variables and their gradients. The high order flux is then calculated using the concept of MUSCL (Monotone Upstream-centered Schemes for Conservation Laws) approach. In order to maintain the accuracy for the finite volume scheme, an innovative method based on Radial Basis Function interpolation is introduced as a substitution to Gaussian quadrature to achieve the higher order surface integration on mixed element unstructured grids.

The proposed high order improvements for unstructured grid schemes have been tested for a wide range of flows from very low Mach number to supersonic speeds. The observed accuracy for the improved schemes is verified using a benchmark case about an inviscid vortex transporting in a free stream flow. In addition, the ability to capture the tip trailing vortex, which is a major challenge in computational fluid dynamics today, is extensively verified on two vortex dominated viscous flows, a fixed NACA0015 wing at a subsonic Mach number and a rotating NACA0012 hovering rotor at a transonic tip speed. The numerical validations are also performed on two realistic industrial applications including a marine propeller P5168 and a Bell Helicopter aircraft 427 main rotor. Computational results indicate that the methods proposed in the present work can significantly improve numerical accuracy in predicting the strong vortical flows in smooth regions, while maintaining the stability of the schemes in discontinuous regions such as shockwaves.

Committee:

Chunhua Sheng, PhD (Committee Chair); Abdollah Afjeh, PhD (Committee Member); Glenn Lipscomb, PhD (Committee Member); Ray Hixon, PhD (Committee Member); Terry Ng, PhD (Committee Chair)

Subjects:

Aerospace Engineering; Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

CFD; Higher Order; Unstructured Grid; NS Equations

Hegedus, PhilEvaluation and Prediction of Elastohydrodynamic Lubrication Film Thickness Under Conditions of Severe Sliding and Zero Entrainment
Doctor of Philosophy, Case Western Reserve University, 2018, EMC - Mechanical Engineering
The Hamrock-Dowson expressions for theoretical minimum and central film thickness of non-conformal contacts in fully flooded elastohydrodynamic lubrication (EHL) predict film collapse in the case of zero entrainment velocity (ZEV); however, past experiments show that films exist and support appreciable loads under this condition. To explore this phenomenon further, as well as the intermediate sliding speeds between pure sliding (ZEV) and pure rolling, a previously established experimental tribometer was resurrected and returned to useful service. Through independent control of two servo motors and a variable pressure loading mechanism, the tribometer is able to generate sliding velocities up to 8.0 m/s, entrainment velocities up to 3.0 m/s, and contact pressures up to 1.5 GPa or 364 N between two spherical test specimens. Using polyalphaolefin (PAO) NYE 182 as the working lubricant, the fluid film generated under these varying conditions was measured by a capacitance method integral to the design of the tribometer. While the lubricant was subjected to zero entrainment speeds, separating films of 0.52 ± 0.22 µm and 0.46 ± 0.15 µm were recorded at contact pressures of 0.5 GPa and 0.6 GPa (13.5 N and 23.3 N), respectively. The experimental results acquired during the present study are used to empirically extend the theoretical film equations to predict film thickness over the entire range of sliding speeds, from pure rolling to pure sliding, including zero entrainment velocity.

Committee:

Joseph Prahl, Dr (Committee Chair); James T'ien, Dr (Committee Member); Paul Barnhart, Dr (Committee Member); Kenneth Loparo, Dr (Committee Member)

Subjects:

Aerospace Engineering; Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

tribology; zero entrainment; ZEV; Hamrock Dowson; elastohydrodynamic; EHL; film thickness; sliding; rolling; lubrication; prediction

Madan, AshishIn vitro assessment of the effects of valvular stenosis on aorta hemodynamics and left ventricular function
Master of Science in Mechanical Engineering (MSME), Wright State University, 2018, Mechanical Engineering
Calcific aortic stenosis (CAS) is the most common valvular heart disease and is associated with aortopathy and ventricular dysfunction. Hemodynamic alterations due CAS could affect the aorta lining (endothelium), that is in direct contact with the blood, triggering adverse biological responses that may possibly cause aortic dilation and dissection. Also, CAS could impose excessive ventricular load leading to ventricular wall thickening, thus putting an individual at a higher risk of heart attack or stroke. These pathophysiological effects of CAS are highly dependent on the degree of calcification. However, the impact of CAS development on aorta flow and left ventricular workload remains largely unknown. Hence the objective of this study is to measure experimentally the effect of CAS on aorta hemodynamics using particle image velocimetry; and left ventricular function in terms of left ventricular work, at different stages of calcification. This study will provide insights on aorta flow abnormalities and left ventricular overload, due CAS, which can be linked to aortopathy and heart failure.

Committee:

Philippe Sucosky, Ph.D., F.A.H.A. (Advisor); George Huang, Ph.D. (Committee Member); Zifeng Yang, Ph.D. (Committee Member)

Subjects:

Biomechanics; Fluid Dynamics; Mechanical Engineering; Physiology

Keywords:

Calcific aortic stenosis; aortic valve; ascending aorta; aortic dilation; left ventricle; particle image velocimetry

Kang, Ming-FangINVESTIGATION OF PASSIVE CYCLONIC GAS-LIQUID SEPARATOR PERFORMANCE FOR MICROGRAVITY APPLICATIONS
Doctor of Philosophy, Case Western Reserve University, 2017, EMC - Mechanical Engineering
Gas-liquid separation is a key task for various systems that are utilized onboard the International Space Station, such as active thermal control and waste management. In most of these system designs, performance is either significantly degraded or the equipment undergoes increased wear if two phase flow is not first separated. Unlike being under the influence of Earth gravity, where gas and liquid separate spontaneously due to buoyancy forces, gas bubbles remain suspended within the liquid under microgravity conditions. In the passive cyclonic separator studied in this research, the gas-liquid mixture is separated by the centrifugal force created by tangentially injecting the two phase fluid into a cylindrical housing. The inertia of the flow itself creates a swirling motion which generates the centrifugal acceleration. Due to the density differences between the two phases, the lower density gas phase moves towards the center of rotation creating a gas core surrounded by an annular liquid film. In such an operation, the separation efficiency is strongly influenced by the gas core behavior and the transport of bubbles within the turbulent liquid annulus. The main focus of the present study is to obtain physical insight into the gas core behavior as well as the bubble dynamics within the liquid film through experimentation and computational modeling. The experimental portion of this work entails the construction and operation of a specific cyclonic two phase separator operated across its useful parameter range under the influence of Earth gravity. The useful operating ranges and separation efficiencies are mapped. The numerical modeling work includes two hybrid-multiphase computational fluid dynamics techniques coupled with large eddy simulation turbulence modeling and are implemented by using the OpenFOAM library. Data analysis and comparison reveal that the injection nozzle design, swirl number, and volumetric gas quality all have a major influence on the gas core size. The control volume analysis reveals the importance of the skin friction coefficient. The trajectories of single gas bubbles are simulated numerically and graphed. It is found that fluid turbulence tends to disperse small gas bubbles in the liquid film resulting in a longer residence time.

Committee:

Yasuhiro Kamotani (Advisor); Jaikrishnan Kadambi (Advisor)

Subjects:

Fluid Dynamics; Mechanical Engineering

Keywords:

passive cyclonic gas-liquid separator; gas core; bubble dynamics; OpenFOAM; microgravity

Miranda, Gregorio do CoutoThe Detection of Journal Bearing Cavitation with Use of Ultrasound Technology
Master of Sciences (Engineering), Case Western Reserve University, 2016, EMC - Mechanical Engineering
A method utilizing the implementation of ultrasound technology is developed to detect cavitation in an operating journal bearing. A modified Reynolds equation is derived and applied to a journal bearing design in order to determine the pressure distribution of the fluid film present in the bearing during operation. This model is used to predict the location of the low pressure region, or gaseous cavitation region, of the bearing to determine the target location for the ultrasound technology instrumentation. A journal bearing test bench apparatus was constructed under the guidance of the pressure distribution model. Ultrasound technology in the form of a small 7mm diameter 10MHz sensor is applied to the back side of the fabricated bearing, opposite of the predicted low fluid film pressure region of bearing operation. An experiment is conducted in order to compare the ultrasound signal attenuation when the measurement region is exposed to both high and low fluid film pressures. The results of the experimental testing indicate a correlation of the ultrasound signal response with whether the measurement region is exposed to high or low fluid film pressures from journal operation. Correlation of changes in signal attenuation with cavitation located in the low fluid film pressure region present itself as an increase in signal attenuation variability when compared to the signal measured in high fluid film pressure regions. The presence of gaseous cavities in the low fluid film pressure region is confirmed with the fabrication and operation of a translucent bearing of identical geometries. Cavities in the fluid film are observed in the low pressure region during operation of the translucent bearing, validating the presence of cavitation in the target region, further confirming the ability of the technique to detect cavitation in journal bearing operation.

Committee:

Joseph Prahl (Committee Chair); Paul Barnhart (Committee Member); Roger Quinn (Committee Member)

Subjects:

Fluid Dynamics; Mechanical Engineering

Keywords:

Cavitation Detection, Ultrasound, Journal Bearing, Hydrodynamic Bearing, Lubrication, Tribology, Vaporous Cavitation, Gaseous Cavitation

Gerber, MatthewThe Effect of Anode Geometry on Power Output in Microbial Fuel Cells
Master of Science, The Ohio State University, 2014, Mechanical Engineering
Microbial fuel cells (MFCs) are bio-electrochemical devices that use micro-organisms, predominately bacteria, to directly convert the chemical energy of substrates contained within wastewater to electricity. The suspended and dissolved organic matter contained within typical wastewater streams such as those from domestic, municipal, and agricultural sources are directly oxidized by an MFC’s bacteria to produce electrons, which can be collected and drawn through an external, electrical circuit and used to power a load. In doing so, an MFC can simultaneously produce electrical power while purifying wastewater though chemical conversion of the substrates. However, despite their potential to integrate wastewater purification with electrical power production, MFCs currently suffer from low output power: typical MFCs have been reported to produce only 10-3–10-2 W. Compared to an output power of 1–107 W for other fuel cell types such as solid oxide or hydrogen, this gap represents significant room for improvement, especially considering that the theoretical maximum for an MFC is on the order of 106 W. This thesis studies the effect on power output of adding geometrical structures to the surface of the fuel cell anode. Through a broad parametric numerical study, two forms were selected for physical fabrication: a right pyramid and a cone. A working MFC was constructed and run in continuous-flow mode with an acetate-based substrate feed. Power outputs were recorded and compared between different anode designs to show that surface shear rate and not surface area is the determining factor in dictating power output in a MFC.

Committee:

Shaurya Prakash (Advisor); A. Terrence Conlisk (Committee Member)

Subjects:

Engineering; Fluid Dynamics; Microbiology

Keywords:

microbial fuel cell; microstructure; surface shear rate; geometric form

Ozcakir, OzgeVortex-Wave Solutions of Navier-Stokes Equations in a Cylindrical Pipe
Doctor of Philosophy, The Ohio State University, 2014, Mathematics
We find a systematic approach to compute two-fold symmetric VW solutions through alternate azimuthal suction and injection on the boundary. For computational efficiency, number of unknown variables are reduced and GMRES iterative method is employed to solve linear equations in a Newton iteration. Then, we investigate asymptotic properties of the solutions and compare our results with findings of Hall & Sherwin for Couette flows in a channel.

Committee:

Saleh Tanveer (Advisor)

Subjects:

Fluid Dynamics; Mathematics

Chatziefstratiou, EfthaliaSIMULATION OF TREE STEM INJURY, AIR FLOW AND HEAT DISPERSION IN FORESTS FOR PREDICTION OF FIRE EFFECTS
Doctor of Philosophy, The Ohio State University, 2015, Environmental Science
This work presents two computational tools, Firestem2D and the fire module of Regional Atmospheric Modelling System (RAMS)-based Forest Large Eddy Simulation (RAFLES), which will help to make predictions of fire effects on trees and the atmosphere. FireStem2D is a software tool for predicting tree stem heating and injury in forest fires. It is a physically-based, two-dimensional model of stem thermodynamics that results from heating at the bark surface. It builds on an earlier one-dimensional model (FireStem) and provides improved capabilities for predicting fire-induced mortality and injury before a fire occurs by resolving stem moisture loss, temperatures through the stem, degree of bark charring, and necrotic depth around the stem. The results of numerical parameterization and model evaluation experiments for FireStem2D that simulate laboratory stem-heating experiments of 52 tree sections from 25 trees are presented. A set of virtual sensitivity analysis experiments were also conducted to test the effects of unevenness of heating around the stem and with above ground height using data from two studies: a low-intensity surface fire and a more intense crown fire. The model allows for improved understanding and prediction of the effects of wildland fire on injury and mortality of trees of different species and sizes. Further, a study of the effects of particular properties of a high-resolution canopy resolving large eddy simulation (RAFLES) was conducted. RAFLES was later used to simulate the dispersion of heat and smoke inside and above forest canopies during low-intensity prescribed surface fires. RAFLES is the only large eddy simulation model that can resolve the effects of the volume of the trees in the canopy. All other models neglect the volume effects and only allow the flow to interact with the forest through a prescribed drag term. As a preliminary study for the heat dispersion simulations, the effects of resolving the tree volumes on air flow inside and around semi porous barriers, such as forests and cities were evaluated. The effects of the numerical representation of volume restriction, independent of the effects of the leaf drag were explicitly tested by comparing drag-only simulations, where neither volume nor aperture restrictions to the flow were prescribed, restriction-only simulations, where no drag was prescribed, and control simulations, where both drag and volume plus aperture restrictions were included. Finally, RAFLES was used to investigate how different canopy structures interact with augmented surface heat flux, simulating a low-intensity surface fire and how these interactions influence turbulence and heat exchange between canopy and atmosphere. A simplified, low-intensity fire event was simulated by directly prescribing heat flux to the bottom three grid layers of the simulation, below the canopy top. The effect of canopy structure on heat accumulation in the canopy and heat dispersion in and above canopy was tested. Furthermore, for each canopy structure, homogeneous and heterogeneous fire patterns were prescribed to contrast the effect of heterogeneity of the fire pattern with that of the canopy structure.

Committee:

Gil Bohrer, Prof. (Advisor)

Subjects:

Civil Engineering; Environmental Engineering; Environmental Science; Fluid Dynamics

Keywords:

heat transfer, atmospheric modeling, fire, turbulence, heat flux, air flow, fire effects, tree stem injury, smoke dispersion, large eddy simulations, computational fluid dynamics

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