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Sunthornvarabhas, JackaponStudy of Methods to Create and Control Electrospun Liquid Jets
Doctor of Philosophy, University of Akron, 2009, Chemical Engineering

Electrospun fibers have so many advantages and can be used in many applications such as filtration, electronic sensor, and wound dressing. It can have diameter from tenth to hundredth nanometer in diameter which results in high mass to surface area ratio.

To create electrospun fibers, polymer solution is delivered to small nozzle which can be delivered by two methods, constant mass by using syringe pump or constant pressure by using low pressurized gas pushes polymer solution from reservoir to small nozzle. Polymer forms a small drop at the tip of the nozzle. Once electrical potential reaches a critical value, fine charged liquid jet launches form the apex of droplet. Fine charge liquid jet travels in straight line for a few centimeters before develops a bending instability. During the course of traveling, jet is stretched due to electrical force from charge that jet carried result in decreased of jet diameter or increasing of jet surface area. This promotes solvent evaporate and results with dry fibers on collector.

Disadvantage of the electrospinning process is low mass production rate. Commercial high mass production rate designs are expensive and complex. Simple and inexpensive designs for laboratory used are required. Three concepts for increasing mass production rate of electrospinning process were proposed. Innovative designs were used to implement those concepts. Electrospun fibers from 200 to 800 nanometer in diameter were created from simple designs with production rate higher than single nozzle electrospinning jet.

Committee:

George G. Chase (Advisor)

Subjects:

Chemical Engineering

Keywords:

Electrospinning; Electrospun fibers; Many jets; High production rate; Multiple nozzles launching electrospinning jets; Liquid film launching electropinning jets

Crawley, Michael BUnderstanding the Aeroacoustic Radiation Sources and Mechanisms in High-Speed Jets
Doctor of Philosophy, The Ohio State University, 2015, Mechanical Engineering
It has been well-known within the aeroacoustic community that the dominant noise sources in high-speed turbulent jets are related to the large-scale structures which are generated in the initial shear layer by instabilities and rapidly grow, interact, and disintegrate as they convect downstream. However, the exact dynamics of these large-scale structures which are relevant to the noise generation process are less clear. This work aims to study the dynamics of, and noise generated by, the large-scale structures in high-fidelity in a Mach 0.9 turbulent jet using simultaneous pressure and velocity data acquisition systems alongside plasma-based excitation to produce either individual or periodic coherent ring vortices in the shear layer. In the first phase, the irrotational near-field pressure is decomposed into its constitutive acoustic and hydrodynamic components, and two-point cross-correlations are used between the acoustic near-field and far-field in order to identify the dominant noise source region. Building upon the work of previous researchers, the decomposition is performed using a spatio-temporal wavelet transform, which was developed during the current work and found to be more robust than previous techniques. Results indicated that for both individual as well as periodic large-scale structures, the dominant noise reaching the far-field at low angles to the jet axis is being generated in the upstream region of the jet, ending just before the end of the potential core (in a time-averaged sense) in the unexcited jet. This is not to say that no noise is generated outside of this region, just that the most energetic and coherent acoustic radiation is emitted here. The large-scale structure interactions were then investigated by stochastically-estimating the time-resolved velocity fields from time-resolved near-field pressure traces and non-time-resolved planar velocity snapshots. For computational efficiency, the ensemble velocity snapshots were first decomposed into orthogonal modes, and a mapping from the near-field pressure to the expansion coefficients was then produced using a feedforward neural network using backpropagation for learning. The coherent structures generated by the excitation were then identified and tracked using standard vortex identification routines. When exciting the jet at very low frequencies, an individual structure quickly rolled up into a coherent structure within two jet diameters and then advected until roughly four jet diameters downstream, at which point it underwent a rapid disintegration. For the periodically-excited jet, multiple smaller-scale structures are initially apparent just downstream of the nozzle exit. These structures quickly undergo multiple mergings to produce a single large-scale structure with a separation distance that matches the excitation wavelength. Similar to the impulsively-excited structures, these now large-scale structures advect downstream and undergo a rapid disintegration near the end of the potential core. Finally, from Ribner's dilatation-based acoustic analogy the aeroacoustic source terms were computed using the time-resolved velocity field produced by the stochastic estimation. Interpretation of the results is challenging however, due to the number of assumptions and simplifications necessary for the computations given the limitations of the current experimental capabilities. Analysis of the computed source fields found that the coherent structures produced a convected wavepacket-like event, centered on the jet lipline though reaching into the potential core. For the individual vortex rings, a clear modulation of the spatial extent and amplitude was observed as the vortex began to break down just upstream of the end of the potential core. This behavior is also present for the periodic train of vortices observed at higher excitation frequencies, however it is obscured by an amplification of the source in the upstream region where the multiple smaller-scale structures merge. As the excitation frequency was increased, and multiple vortex mergings occurred before the end of the potential core, the aeroacoustic source associated with the merging amplified such that it was distinct from the vortex disintegration source. The results from this work indicate that the disintegration of the coherent ring vortices are the dominant aeroacoustic source mechanism for the Mach 0.9, high Reynolds number jet studied here. However, the merging of vortices in the initial shear layer was also identified as a non-trivial noise source mechanism in high-speed, turbulent jets. Future work will focus on improving the source localization by utilizing acoustic beamforming techniques to identify the source region from the acoustic near-field, in place of the two-point correlations used in this work. Additionally, the structure dynamics and noise generation process will be explored in high-order azimuthal modes.

Committee:

Mo Samimy (Advisor); Datta Gaitonde (Committee Member); James Gregory (Committee Member); Mei Zhuang (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

aeroacoustics; high-speed jets; flow control

Speth, Rachelle LeaControlling The Development of Coherent Structures in High Speed Jets and The Resultant Near Field
Doctor of Philosophy, The Ohio State University, 2015, Aero/Astro Engineering
This work uses Large-Eddy Simulations to examine the effect of actuator parameters and jet exit properties on the evolution of coherent structures and their impact on the near-acoustic field without and with control. For the controlled cases, Localized Arc Filament Plasma Actuators (LAFPAs) are considered, and modeled with a simple heating approach that successfully reproduces the main observations and trends of experiments. A parametric study is first conducted, using the flapping mode (m=±1), to investigate the sensitivity of the results to various actuator parameters including: actuator model temperature, actuator duty cycle, and excitation frequency. It is shown by considering a Mach 1.3 jet at Reynolds number of 1 ×106 that the response of the jet is relatively insensitive to actuator model temperature within the limits of the experimentally measured temperature values. Furthermore, duty cycles in the range of 20%−90% were observed to be effective in reproducing the characteristic coherent structures of the flapping mode. The 90% duty cycle exhibits strengthened coherent structures and slightly higher jet growth along the flapping plane, but the overall dynamics remain almost identical to the lower duty cycle cases. However, a 100% duty cycle had no perceptible effect on the jet. Therefore, increasing the energy inserted into the flow via actuator temperature or duty cycle does not significantly alter the flow dynamics. The largest sensitivity was associated with excitation frequency, with the most significant effect associated with the column mode instability frequency (St ≅0.3), which results in alternating vortex rings for this mode. Higher and lower frequencies reduced the rate of decay of the centerline velocity. Although the higher frequencies increased the number of features observed in the phase-averaged data, their prominence is reduced due to their breakdown into smaller structures. These actuator parameter results confirm that the flow response and control authority is associated with manipulation of flow instability, rather than heat deposition. Next, jet flow parameters were explored to determine the control authority under different operating conditions. To begin, the effect of the laminar nozzle exit boundary layer thickness was examined by varying its value from essentially uniform flow to 25% of the diameter. In the absence of control, the distance between the nozzle lip and the initial appearance of breakdown is proportional to the boundary-layer thickness, which is consistent with theory and previous results obtained by other researchers at Mach 0.9. However, the subsequent growth towards the centerline is faster for the thicker boundary layers. For flapping mode control, increasing the thickness of the boundary layer has different effects on the flapping and non-flapping planes. The rapid spreading of the jet observed on the flapping plane with thin boundary layers is greatly diminished as the nozzle exit layer is thickened. Conversely, the rate of spreading on the non-flapping plane is increased. The characteristic vortical rings observed with thin layers in experiment and simulations become less prominent with increasing nozzle exit boundary layer thickness, indicating reduced control authority. Thus, increasing the boundary-layer thickness reduces the differences between controlled and uncontrolled cases. The second flow parameter studied was the effect of Reynolds number on a Mach 1.3 jet controlled by the flapping mode at an excitation Strouhal number of 0.3. The higher Reynolds number (Re=1,100,000) jet exhibited reduced control authority compared to the Re=100,000 jet. Like the effect of increasing the nozzle exit boundary layer thickness, increasing the Reynolds number cause a reduction in spreading on the flapping plane and an increase on the non-flapping plane. Therefore, these thicker layers and higher Reynolds number jets may require actuators with a higher energy input (i.e. higher duty cycle, higher actuator temperature, more actuators) to ensure the excitation of the flow instability. The final parameter studied is the effect of Mach number on the development and decay of large scale structures for no-control and control cases for Mach 0.9 and Mach 1.3 jets. For this exercise, the axisymmetric mode (m=0) was considered at excitation frequencies of St=0.05, 0.15, and 0.25, with emphasis on the evolution of coherent structures and their effects on the resultant near field pressure map. Without control, the two jets have similar shear layer growth until the end of the potential core length of the subsonic case, at which point the subsonic jet spreads at a higher rate. For the controlled cases, relatively larger streamwise hairpin vortices have been noted for the subsonic cases than the supersonic cases resulting in stronger entrainment of the ambient fluid. This increased entrainment in the subsonic cases causes a reduction in the normalized convective velocity resulting in similar normalized values to that of the supersonic cases. As the excitation frequency is increased, more hairpin vortices are present and the normalized convective velocity is reduced for both subsonic and supersonic cases. A detailed study of the connection between the coherent structures and the pressure in the near-acoustic field and lipline supports the theory that the successive interacting pulses produce a quasi-linear superposition of the impulse response of the jet to excitation. The velocity of waves just outside of the shear layer is very near the acoustic value due to the exponential decay of the hydrodynamic waves. In the nearfield, the acoustic influence on the convective velocity is greater on the sideline angles for the subsonic cases due to the directivity of the large scale structures and the lower acoustic signature of the subsonic jet compared to the supersonic jet. The supersonic jet region maintains higher correlations to the near acoustic field than the subsonic jet. The higher excitation frequencies cause a more directed propagation path to the downstream angles due to the consistent location of the large scale structure decay from the excitation.

Committee:

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

Subjects:

Acoustics; Aerospace Engineering

Keywords:

acoustics; large eddy simulation; jets; plasma control

Mora Sánchez, Pablo AInvestigation of the Noise Radiation from Heated Supersonic Jets
PhD, University of Cincinnati, 2016, Engineering and Applied Science: Aerospace Engineering
This work focuses in the investigation of crackle and Mach wave radiation in heated supersonic jets. The skewness and kurtosis of the acoustic pressure signal and its time derivative were adopted as metrics for identifying crackling jets and quantifying levels of crackle. Cold and heated jets from supersonic nozzles with different geometric parameters and scales are analyzed to draw conclusions on noise sources and propagation. In order to complement the investigation, results are also presented for the mixing noise, broadband shock-associated noise and screech. Chapter 4 focuses on the impact of jet operating condition on the skewness and kurtosis levels of a jet issuing from a converging-diverging conical nozzle, with a 1.5 design Mach number. An increase in convective Mach number, achieved by increasing jet temperature, proved to be related to elevated values of OASPL, skewness, and kurtosis, in both the near and far fields. Intense levels of the dP/dt high-order statistics appear to be generated at different locations in the shear layer of the jet and strengthen away from the jet by non-linear propagation effects. Chapter 5 studies how adding chevrons to a converging-diverging nozzle impacts Mach wave radiation and crackle. The chevrons decreased OASPL in the downstream angles but increased the broadband shock-associated noise. Pressure skewness, dP/dt skewness and kurtosis were all reduced by the chevrons in the near field and far field, and thus they effectively mitigated crackle and Mach wave radiation; however, chevrons showed no evidence of changing the convective Mach number. The evolution of noise signals was analyzed in the near-field to the far-field to identify the strengthening of skewness through nonlinear propagation effects. Chapter 6 investigates a jet exhausting over a plate at different stand-off distances, to simulate jets exhausting over airframe surfaces and jet-ground interaction during take-off and landing operations. Far-field acoustics were measured at the reflected direction, sideline, and shielded azimuthal directions. At the sideline, the plate attached to the nozzle lip diminished broadband shock-associated noise, and mitigated screech for the cold case. When the plate was moved away from the nozzle, screech tones were intensified at the under-expanded condition. Crackle levels were significantly intensified in the sideline, within a range of stand-off positions. Chapter 7 analyzes the impact of nozzle scale and nozzle internal contours on the levels of crackle. Three scaled converging-diverging nozzles, with jet exit diameters of 0.542 in, 0.813 in, and 1.085 in were investigated. Far-field arrays were setup at a constant non-dimensionalized radial distance of 40 nozzle exit diameters. The pressure skewness and kurtosis plots collapsed for all three scaled nozzles when the pressure signals were not filtered. The dP/dt statistics collapsed when the signals were downsampled proportional to the nozzle exit diameters. Baseline nozzle results were also compared to a smooth contoured nozzle designed by the Method of Characteristics. This nozzle almost had no broadband shock-associated noise, but contained the same skewness and kurtosis levels, concluding that crackle is not linked to the shock-cell structures in the jet.

Committee:

Ephraim Gutmark, Ph.D. D.Sc. (Committee Chair); Kailas Kailasanath, Ph.D. (Committee Member); Jeffrey Kastner, Ph.D. (Committee Member); Mark Turner, Sc.D. (Committee Member)

Subjects:

Aerospace Materials

Keywords:

Supersonic Jet Noise;Crackle;Mach Wave Radiation;Supersonic Heated Jets

Kaveh, FarrokhAn experimental and theoretical study of the interaction of an electrostatic field with a two-dimensional jet flow.
Doctor of Philosophy, The Ohio State University, 1981, Graduate School

Committee:

Not Provided (Other)

Subjects:

Engineering

Keywords:

Electrostatics;Jets--Fluid dynamics

Gupta, SwatiTime Dependent Radiation Spectra From Jets of Microquasars
Doctor of Philosophy (PhD), Ohio University, 2007, Physics (Arts and Sciences)

X-ray binary systems in our galaxy exhibiting relativistic jets (microquasars) present one of the most recent additions to the field of high energy astrophysics. Jet models of high energy emission from these sources have created significant interest lately with detailed spectral and timing studies of the X-ray emission from microquasars, and their recent establishment as a new distinctive class of γ-ray emitting sources after the detection of very-high-energy (VHE) γ-rays from the microquasars LS 5039 and LS I +61° 303.

This dissertation presents a study of radiation signatures from a leptonic jet model, based on time-dependent electron injection and acceleration, followed by their subsequent adiabatic and radiative cooling. The radiation mechanisms included are synchrotron, synchrotron self Compton and external Compton with soft photons provided by the companion star and the accretion disk. Compton scattering is treated both in the Thomson and the Klein-Nishina regimes, thus making the model applicable to microquasars that are candidates for VHE γ-ray emission as well. An analytical solution to the electron kinetic equation is introduced for the Thomson regime treatment, while a numerical approach is adopted for the Klein-Nishina regime. Predictions regarding rapid flux and spectral variability signatures in the form of spectral hysteresis in the X-ray hardness intensity diagrams are made, which should be testable with monitoring observations using Chandra and/or XMM-Newton. Detections of such variability would help in distinguishing between various competing models for the high energy emission from these sources. Our results show that the shape and orientation of the hysteresis loops would allow identification of the dominant emission components as well as quantify physical parameters like the magnetic field, spectral index, Doppler boosting factor, etc.

The model is applied to available broadband observations of the two microquasars that have been very recently detected in VHE γ-rays, namely LS I +61° 303 and LS 5039. In the case of LS I +61° 303, we explain the observed orbital modulation of the VHE γ-ray emission solely by the geometrical effect of changes in the relative orientation of the stellar companion with respect to the compact object affecting the position and depth of the γ-γ absorption trough. For LS 5039, our results imply that an orbital modulation of the velocity of the stellar wind in addition to γ-γ absorption effects may be necessary to explain the orbital variability of the VHE γ-ray emission.

Committee:

Markus Boettcher (Advisor)

Subjects:

Physics, Astronomy and Astrophysics

Keywords:

X-ray binaries; Microquasars; Jets; Black Holes; Leptonic Models

Gompertz, Kyle AdlerSeparation Flow Control with Vortex Generator Jets Employed in an Aft-Loaded Low-Pressure Turbine Cascade with Simulated Upstream Wakes
Master of Science, The Ohio State University, 2009, Aeronautical and Astronautical Engineering
Detailed pressure and velocity measurements were acquired at Rec = 20,000 with 3% inlet free stream turbulence intensity to study the effects of position, phase and forcing frequency of vortex generator jets employed on an aft-loaded low-pressure turbine blade in the presence of impinging wakes. The L1A blade has a design Zweifel coefficient of 1.34 and a suction peak at 58% axial chord, making it an aft-loaded pressure distribution. At this Reynolds number, the blade exhibits a non-reattaching separation region beginning at 60% axial chord under steady flow conditions without upstream wakes. Wakes shed by an upstream vane row are simulated with a moving row of cylindrical bars at a flow coefficient of 0.91. Impinging wakes thin the separation zone and delay separation by triggering transition in the separated shear layer, although the flow does not reattach. Instead, at sufficiently high forcing frequencies, a new time-mean separated shear layer position is established which begins at approximately 72%Cx. Reductions in area-averaged wake total pressure loss of more than 75% were documented. One objective of this study was to compare pulsed flow control using two rows of discrete vortex generator jets (VGJs). The VGJs are located at 59%Cx, approximately the peak Cp location, and at 72%Cx. Effective separation control was achieved at both locations. In both cases, wake total pressure loss decreased 35% from the wake only level and the shape of the Cp distribution indicates that the cascade recovers its high Reynolds number (attached flow) performance. The most effective separation control was achieved when actuating at 59%Cx where the VGJ disturbance dominates the dynamics of the separated shear layer, with the wake disturbance assuming a secondary role only. On the other hand, when actuating at 72%Cx, the efficacy of VGJ actuation is derived from the relative mean shear layer position and jet penetration. When the pulsed jet actuation (25% duty cycle) was initiated at the 72%Cx location, synchronization with the wake passing frequency (8.7Hz) was critical to produce the most effective separation control. A 20% improvement in effectiveness over the wake-only level was obtained by aligning the jet actuation between wake events. A range of blowing ratios was investigated at both locations to maximize separation reduction with minimal mass flow. The optimal control parameter set for VGJ actuation at 72%Cx does not represent a reduction in required mass flow compared to the optimal parameter set for actuation at 59%Cx. Differences in the fundamental physics of the jet interaction with the separated shear layer are discussed and implications for the application of flow control in a full engine demonstrator are reviewed. Evidence suggests that flow control using VGJs will be effective in the highly unsteady LPT environment of an operating gas turbine, provided the VGJ location and amplitude are adapted for the specific blade profile.

Committee:

Jeffrey Bons, PhD (Advisor); James Gregory, PhD (Committee Member)

Subjects:

Aerospace Materials; Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

flow control; vortex generator jets; negative jet; low pressure turbine aerodynamics; simulating wakes

KISHORE, ARAVINDNumerical Modeling of Pollutant Dispersal from Watercraft Exhaust Systems
MS, University of Cincinnati, 2008, Engineering : Mechanical Engineering
The goal of this thesis is to analyze Carbon Monoxide (CO) dispersal in water.Watercraft exhaust systems issue gaseous products of combustion, of which Carbon Monoxide is a toxic component, from under the hull of the watercraft into the water. The aim of the present study is to simulate CO dispersal using Computational Fluid Dynamics (CFD). Due to the similarity in physical configuration to the watercraft exhaust system, the canonical problem of a single Jet In Cross Flow (JICF) is chosen as a validation case. Simulation of the JICF affords insight into the adequacy of the computational methodology before the methodology can be extended to simulate the present problem. Watercraft exhaust systems include dual exhaust ports, and hence, the methodology used to simulate the single jet is extended to a dual-jet system. Results show a pronounced interaction of the flow structures developed in the flow field. Further, the jets in the dual-jet configuration do not penetrate the cross flow to the extent that the single jet does. This phenomenon will prove important in the design and control of systems where multiple jets are used for optimum mixing. The watercraft exhaust system is simulated and the results presented show that the prevailing large-scale flow structures affect the dispersal of CO. Based on these results, three ways of mitigating CO dispersal are proposed.

Committee:

Urmila Ghia (Committee Chair); Karman Ghia (Committee Member); Milind Jog (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

Jet In Cross Flow; Multiple Jets; Watercraft Exhaust System; CFD

Woods, Nathan MichaelPHASE-LOCKED PIV INVESTIGATION OF THE EFFECTS OF THE BLOWING RATIO OF A PULSED VORTEX GENERATOR JET IN A LOW-PRESSURE TURBINE
Master of Science in Engineering (MSEgr), Wright State University, 2007, Mechanical Engineering
At very high altitudes the Reynolds number flow through the low pressure turbine section of the gas turbine engine can drop below 25,000. At these low Reynolds numbers the flow is laminar and extremely susceptible to separation which can lead to increased losses and reduced lift. Small jets of air injected through the suction surface of the airfoil, called Vortex Generator Jets (VGJs), have been shown successful in suppressing separation and maintaining attached flow. Pulsing of these jets has been shown to be as effective as steady jets while reducing the amount of mass flow needed. An experiment using Particle Image Velocimetry (PIV) was set up to study the interaction of the VGJ flow with the main flow. A cascade of Pratt and Whitney Pack-B turbine blades were mounted in the test section of a low speed wind tunnel at Wright Patterson Air Force Base. On the middle six blades were rows of 1mm VGJ holes. The VGJ holes were oriented with a 30o pitch angle and 90o skew angle. The pitch angle is the angle the jet makes with the surface of the turbine blade while the skew angle is the angle the jet makes with the cross-flow. Blowing ratios, a ratio of the jet velocity to the cross-flow velocity, of 0.5, 1, and 2 were examined. These three blowing ratios were selected because they represent when the cross-flow momentum dominates the fluid interaction (B=0.5); when the momentums of the jet flow and cross-flow are equal (B=1); and when the momentum of the jet flow dominates the interaction. Blowing ratios of 0.5 and 1 were studied for pulsing frequencies of 10Hz and 0.4Hz while the blow ratio of 2 was studied only with 10Hz pulsing. A duty cycle of 50% was used for both pulsing frequencies. The two pulsing frequencies allowed data to be taken to show how the pulsed VGJ maintains attached flow (10Hz) and how the pulsed VGJ suppresses the separation bubble (0.4Hz). Results show that jets interacting with separated flow are able to suppress the separation bubble almost immediately for a blowing ratio of 1 and 0.5. The results for suppression and separation growth show the response of the crossflow is very similar in magnitude and timing between the two blowing ratios. The results for the 10Hz pulsing frequency show blowing ratios of 0.5, 1, and 2 are effective. A blowing ratio of 2 is undesirable because it carries more momentum than is needed and would therefore use more massflow than the B=1 or 0.5 case. Results from the B=0.5 case suggest that a blowing ratio of 0.5 is near the minimum effective blowing ratio.

Committee:

Mitch Wolff (Advisor)

Keywords:

Turbine; Pack-B; Pak-B; separation; Low Pressure Turbine; Vortex Generator Jets; PIV

Smith, Geoffrey NA Search For the Standard Model Higgs Boson Produced in Association with Top Quarks in the Lepton + Jets Channel at CMS
Doctor of Philosophy, The Ohio State University, 2014, Physics
A search for the Standard Model Higgs boson in the ttH production mode is presented. The search uses 19.3 fb-1 of data collected at a center-of-mass energy of 8 TeV by the Compact Muon Solenoid detector at the Large Hadron Collider. The focus of the analysis is the semi-leptonic decay of the top quark pair, accompanied by the decay of the Higgs boson to bottom quarks. The search sets a 95% confidence upper limit of μ < 5.1, where μ is the ratio of the ttH cross-section to the ttH cross-section predicted by the Standard Model.

Committee:

Richard Hughes (Advisor); Brian Winer (Committee Member); Michael Lisa (Committee Member); Eric Braaten (Committee Member)

Subjects:

Particle Physics; Physics

Keywords:

Higgs; boson; top; quark; CMS; LHC; limit; boosted decision trees; BDT; multivariate; ttH; lepton; jets

Mustafa, MansoorInvestigation into Offset Streams for Jet Noise Reduction
Master of Science, The Ohio State University, 2015, Aero/Astro Engineering
This effort investigates the near field behavior of two ideally-expanded subsonic dual-stream jets. One case implements a traditional symmetric, concentric dual-stream nozzle configuration while the other imposes an asymmetric, eccentric layout to model the behavior of an offset stream. The essence of an offset stream is to force an uneven azimuthal distribution of the secondary coflow and create an outside stream that varies in thickness. Past studies have shown a benefit in acoustic propagation in the direction of the thickest coflow and the present work further analyzes this phenomenon. A LES (Large Eddy Simulation) approach is implemented to run the simulations for both cases and a number of qualitative and quantitative analyses tools are used for post-processing. A reduction in the noise levels for the lower, thicker side of the eccentric nozzle is observed in comparison to the baseline concentric case. Examination of the mean flow behavior shows a shorter, thinner primary potential core for the offset case and a faster axial velocity decay rate. The asymmetric distribution of the coflow causes varying velocity profiles in the radial direction for the top and bottom regions and consequently produces unique flow features on either side. Lower levels of shear stress and slower decay rates lead to less turbulence production on the lower side of the eccentric nozzle. An investigation into the flow structures reveals lower vorticity and weaker convective structures on the bottom which influences propagation in that direction. Two-point correlation analysis reveals the presence of smaller turbulence scales in the lower, thicker portion of the eccentric case. This is further confirmed by an Empirical Mode Decomposition (EMD) study that shows lower frequency ranges dominate the concentric near field in comparison to the eccentric. The combination of these unique features demonstrate the principles behind the acoustic benefit of implementing offset stream flows in dual-stream nozzle configurations.

Committee:

Datta Gaitonde (Advisor); Mei Zhuang (Committee Member)

Subjects:

Aerospace Engineering

Keywords:

offset; noise reduction; jets; aerospace; aerodynamics; LES; computational simulation; jet noise; eccentric; concentric; dual-stream; nozzle

Knezek, Robert AloisAn investigation of the influence of sound on flow stability, flame noise, and nitrogen oxide levels in natural gas flames /
Doctor of Philosophy, The Ohio State University, 1974, Graduate School

Committee:

Not Provided (Other)

Subjects:

Engineering

Keywords:

Flame;Sound;Jets;Nitric oxide;Nitrogen dioxide

Ortwerth, Paul James,1939-Mechanism of mixing of two non-reacting gases /
Doctor of Philosophy, The Ohio State University, 1971, Graduate School

Committee:

Not Provided (Other)

Subjects:

Engineering

Keywords:

Jets;Gases

Moore, Kenneth JayLarge Scale Visualization of Pulsed Vortex Generator Jets
Master of Science in Engineering (MSEgr), Wright State University, 2005, Mechanical Engineering
Moore, Kenneth Jay. M.S., Department of Mechanical and Materials Engineering, Wright State University, 2005. Large Scale Visualization of Pulsed Vortex Generator Jets. The use of small jets of air has proven to be an effective means of flow control on low Reynolds number turbine blades. Pulsing of these jets has also shown benefits in reducing the amount of air needed to achieve the same level of flow control. An experiment using Hot Wire Anemometry and Particle Image Velocimetry (PIV) has been used to investigate how these pulsed jets interact with the boundary layer to help keep the flow attached. A 25x scaled jet in a flat plate has been utilized. The 25.4 mm diameter jet has a pitch angle of 30° and a skew angle of 90°. Pitch angle is defined as the angle the jet makes with the surface of the plate, and the skew angle is the angle that the projection of the jet on the surface makes with the crossflow. The jet was pulsed at both 0.5 Hz and 4 Hz with varying pulse durations (duty cycles), as well as various blowing ratios (ratio of the jet velocity to the freestream velocity). Duty cycles of 10, 25, 50, and100 percent were implemented at a blowing ratio of unity. Blowing ratios of 0.5, 1, 2, and 4 were implemented at a 50% duty cycle and at 0.5 Hz. Velocity and vorticity planes were obtained at various spanwise locations and used in the characterization of the jetflow. Both the free jet as well as the jet in crossflow were studied. A calibration experiment was also performed using PIV on a rotating disk. The calibration experiment was successful and the PIV results averaged a 1.56% error. The hot wire experiment with the free jet showed that the starting vortex is a key event at the beginning of each cycle, and the end of each cycle included a “kick-back” and a suction effect that could also have an influence on the boundary layer. The PIV experiment was performed first on the free jet, and results were comparable to the hot wire results. When the PIV experiment was performed on the jet in crossflow, it was clear that both the beginning and ending events of the jet cycle were keys to eliminating or delaying flow separation.The effect of the beginning and ending events can be used to keep the flow attached for longer periods of time by increasing the frequency of the jet pulse. Due to limitations of the setup, higher frequency cases could not be studied. However, the experiment was successful in controlling a separated crossflow for blowing ratios greater than unity. The larger blowing ratios resulted in larger attachment size, and were able to sustain attachment for longer time periods.

Committee:

Mitch Wolff (Advisor)

Subjects:

Engineering, Mechanical

Keywords:

PIV; Flow Control; Separation Control; VGJ; Pulsed Jets

Xia, Bingπ0 - h± Jet Correlations in d + Au Collisions at √SNN = 200 GeV
Doctor of Philosophy (PhD), Ohio University, 2014, Physics and Astronomy (Arts and Sciences)
Various quark gluon plasma signatures have been experimentally established in Au+Au and Pb+Pb collisions at RHIC and LHC, such as elliptical flow and jet quenching. For decades, the quark gluon plasma was believed not to be created in p+A collisions. However, recent experimental discoveries of collective flow in p+Pb and d+Au collisions indicate a hot nuclear medium and a thermal equilibrium in such small systems. An energy loss analysis would be a good alternative measurement for such pictures. Di-hadron correlation measurements are widely used in jet analysis. In this study, π0 - h± azimuthal correlations and the per trigger yields in d+Au collisions at √SNN = 200 GeV are measured and compared with p+p collisions at the same energy. In order to cancel out part of the systematic uncertainties and measure the subtle jet modifications, a new quantity RI is proposed and measured from data. In central d+Au collisions, RI shows a clear suppression in large zT regions and a delicate enhancement of about 2s in low zT regions. Such jet modifications are qualitatively similar, but in a much smaller scale, to the ones observed in central Au+Au collisions, which are attributed to the jet quenching and energy loss in a quark gluon plasma created in central Au+Au collisions. No theory is available to explain this new experimental phenomenon. To constrain the cold nuclear matter effects, we perform a series of simulations to investigate the possible physics origin of these modifications. Various kT setting in Pythia and Hijing simulations are not able to reproduce the features observed in data. Also, the gluon jet mixing and nuclear modification of the parton distribution unctions are studied. None of them are sufficient to explain the RI modification observed in central d+Au collisions. No significant away-side IdA modifications is observed in peripheral d+Au collisions. This result suggests that previous unexpected RdA enhancement in peripheral d+Au collisions from π0 and jet reconstruction analysis might come from an issue with the determination of Ncoll scale factor instead of a real enhancement in physics data. Regardless of the theoretical origin, the data provide constraints that should be able to set concrete limits on the contributions of various cold nuclear or possibly hot QGP-like effects.

Committee:

Justin Frantz (Advisor); Brune Carl (Committee Member); Hicks Kenneth (Committee Member); Masson Eric (Committee Member)

Subjects:

Nuclear Physics; Physics

Keywords:

Quark Gluon Plasma; d Au collisions; jets; pi0-h azimuthal correlations

DIltz, Christopher S.Time Dependent Leptonic and Lepto-Hadronic Modeling of Blazar Emission
Doctor of Philosophy (PhD), Ohio University, 2016, Physics and Astronomy (Arts and Sciences)
Active galactic nuclei (AGN) are known to exhibit multi-wavelength variability across the whole electromagnetic spectrum. In the context of blazars, the variability timescale can be as short as a few minutes. Correlated variability has been seen in different bands of the electromagnetic spectrum: from radio wavelengths to high energy γ-rays. This correlated variability in different wavelength bands can put constraints on the particle content, acceleration mechanisms and radiative properties of the relativistic jets that produce blazar emission. Two models are typically invoked to explain the origin of the broadband emission across the electromagnetic spectrum: Leptonic and Hadronic Modeling. Both models have had success in reproducing the broadband spectral energy distributions (SEDs) of blazar emission with different input parameters, making the origin of the emission difficult to determine. However, flaring events cause the spectral components that produce the SED to evolve on different timescales, producing different light curve behavior for both models. My Ph.D. research involves developing one-zone time dependent leptonic and lepto-hadronic codes to reproduce the broadband SEDs of blazars and then model flaring scenarios in order to find distinct differences between the two models. My lepto-hadronic code also considers the time dependent evolution of the radiation emitted by secondary particles (pions and muons) generated from photo-hadronic interactions between the photons and protons in the emission region. I present fits to the broadband SEDs of the flat spectrum radio quasars (FSRQs) 3C 273 and 3C 279 using my one-zone leptonic and lepto-hadronic model, respectively. I showed that by considering perturbations of any one of the selected input parameters for both models: magnetic field, particle injection luminosity, particle spectral index, and stochastic acceleration time scale, distinct differences arise in the light curves for the optical, X-ray and γ-ray bandpasses that can separate leptonic and lepto-hadronic models. I find that decreasing the stochastic acceleration timescale for a one-zone leptonic model will result in a decrease in flux in the X-ray band as opposed to the lepto-hadronic model, in which an increase is seen. I also find that increasing the magnetic field produces a drop in the X-ray and ¿-ray bands while for the lepto-hadronic model, an increase is observed in both bands. In the final part of my Ph.D. research, I use my leptonic and lepto-hadronic codes to reproduce the SED of the FSRQ 3C 454.3. The SED fits are then used to model a large multi-wavelength flare that 3C 454.3 exhibited in November 2010. I find that a combination of parameter changes to the magnetic field, particle injection luminosity, stochastic acceleration timescale and particle spectral index is needed to model the November flare. Using both codes to model the ¿are, I found that the lepto-hadronic model can reproduce both the broadband spectral energy distribution of 3C 454.3 in its quiescent and flaring states and can reproduce the integrated light curves in three bandpasses; optical R, Swift XRT and Fermi γ-rays. I also found that the fits to the SED of 3C 454.3 in its flaring state could not be reproduced by our one-zone leptonic model.

Committee:

Markus Boettcher (Advisor); Joseph Shields (Committee Member); Douglas Clowe (Committee Member)

Subjects:

Astronomy; Astrophysics; Physics

Keywords:

Galaxies; Active Galaxies; Blazars; Jets; Radiation Mechanisms; Non-Thermal; Relativistic Processes

Landers, Brian DMixing Characteristics of Turbulent Twin Impinging Axisymmetric Jets at Various Impingement Angles
MS, University of Cincinnati, 2016, Engineering and Applied Science: Aerospace Engineering
An experimental study is first presented on the comparison between two commonly used velocity measurement techniques applied in experimental fluid dynamics: Constant Temperature Anemometry (CTA) and Particle Image Velocimetry (PIV). The comparison is performed in the near-field region of an axisymmetric circular turbulent jet where the flow field contains large scale turbulent structures. The comparison was performed for five Reynolds numbers, based on diameter, between 5,000 and 25,000. The Reynolds numbers selected cover the critical Reynolds number range, 10,000 to 20,000 where the characteristics of the flow transition to a fully developed turbulent mixing layer. A comparison between these two measurement techniques was performed in order to determine the differences between an intrusive (CTA) and non-intrusive (PIV) method when applied to a practical application. The results and observations obtained from the comparison between the two techniques were applied to better characterize the time-averaged characteristics of a single axisymmetric turbulent jet with a Reynolds number of 7,500. The mean and fluctuating velocities, turbulent kinetic energy (TKE), and vorticity were measured as a baseline case. Additionally, smoke visualization was utilized to determine the mixing characteristics of the transient start of an axisymmetric turbulent jet. The shedding frequencies, also known as, the `preferred mode’ were investigated for a single jet. Particle Image Velocimetry (PIV) was also utilized to characterize the pre-and post-regions of the interaction region of two axisymmetric, incompressible turbulent jets at included angles: 30, 45, and 60 degrees. The Reynolds number selected (7,500) was within the range of critical Reynolds numbers and the geometrical distance to twin jet impingement, X0, remained constant at 10.33D for each impingement angle. The mean and fluctuating velocities, vorticity, and turbulent kinetic energy (TKE) were measured. Smoke Visualization was utilized to measure the mixing characteristics of impinging jets during the transient start as well as when the jets had reached a steady state condition.

Committee:

Peter Disimile, Ph.D. (Committee Chair); Shaaban Abdallah, Ph.D. (Committee Member); Milind Jog, Ph.D. (Committee Member)

Subjects:

Aerospace Materials

Keywords:

Impinging Jets;Axisymmetric Turbulent Jet;Particle Image Velocimetry;Constant Temperature Anemometry;Hotwire Anemometry;Turbulent Flow

KASLIWAL, AMITFLOW SEPARATION CONTROL FOR CYLINDER FLOW AND CASCADE FLOW USING GENERATOR JETS
MS, University of Cincinnati, 2006, Engineering : Aerospace Engineering
Many attempts have been made by researchers, worldwide, to comprehend the physics of separated flows. Study of flow separation is vital as it is encountered in many engineering applications, and is generally detrimental. One such example is flow through a low pressure turbine (LPT) cascade, at relatively low-Re values, where flow separates on the suction surface of the LPT blade, and adversely affects the efficiency of the aircraft engine. Contemporary research is focused on understanding the physics of the separated flow, and identifying control strategies to delay or, if possible at all, prevent the flow separation phenomenon. The main objective of the present research is to study a model separated flow, and identify a control strategy, which can subsequently be applied to manage the flow in the LPT cascade. To achieve this, a model problem of flow past a circular cylinder is considered, as the geometry for this flow is simple and facilitates a focus on the flow itself. Despite of its simple geometry, the flow past a circular cylinder exhibits a variety of complex flow features which make this a challenging problem to solve. As a validation study, the flow for Re = 3,900 is simulated, and the results obtained are compared with the numerical and experimental data available in the literature. For the flow control study, a baseline solution for flow past a circular cylinder at Re = 13,400 is obtained as a first step towards implementation of flow separation control for preventing or delaying the flow separation. The Re value of 13,400 ensures laminar separation and serves to approximate the flow conditions prevailing in a LPT cascade. Later, flow control is incorporated by employing vortex generator jets (VGJs) on the upper surface of the cylinder at about 750 from the stagnation point. The jets are issued into the flow with a blowing ratio of 2.0 and are pitched and skewed by 300 and 700, respectively. A non-dimensional pulsation frequency F+ of 1.0 is used, along with 50% duty cycle. With this understanding, VGJs are then incorporated for the LPT cascade flow. VGJs are placed in a range of 63.5% to 67% Cax. All the jet parameters, i.e., blowing ratio, pitch angle, skew angle and duty cycle ratio, are kept the same as for the cylinder case, while the F+ value of 2.33 is employed for the LPT cascade problem. The three-dimensional, unsteady, full Navier-Stokes equations are solved to obtain accurate prediction of unsteady separated flows governed by the Navier-Stokes (N-S) equations. A fourth-order accurate, compact-difference scheme is used for spatial discretization, with sixth-order filtering to minimize the oscillations in the flow solution. For the cylinder, a multi-block structured grid generated using the grid generation software, GRIDGEN, is used for the present numerical analysis. The grid contains approximately 3.9M grid points, and approximately 70% of the total grid points are concentrated in the wake region to capture the small scales that are expected to exist in this region. A MPI-based higher-order, Chimera version of the FDL3DI flow solver developed by the Air Force Research Laboratory at Wright Patterson Air Force base is used for the numerical computations. PEGSUS a NASA Ames research code is used for storing the connectivity data at the block interfaces. The baseline case for the cylinder flow at Re = 13,400 displays a wide range of vortical structures in the wake region. The separating shear layers are subject to spanwise instability which leads to the formation of an unsteady and three-dimensional wake, with the characteristic features of typical turbulent flow. It is observed that after the jets are being turned on, the pressure on the surface of the cylinder redistributes in a way so as to reduce the pressure drag significantly. The total pressure loss coefficient and momentum thickness are calculated in the wake at x/D = 3.0 and x/D = 5.0, and are found to reduce by 10% and 30%, respectively. The flow control simulation for the LPT cascade flow reveals 27% reduction in total pressure loss coefficient, along with the total elimination of separation upon application of VGJs.

Committee:

Dr. Kirti Ghia (Advisor)

Keywords:

Flow Separation; Low Pressure Turbine Cascade; LPT Cascade; Flow over Cylinder; Vortex Generator Jets; VGJ; Multiblock Grid

Sai Ramesh, PrassannaInstability and Breakup of Non-Newtonian Viscoelastic Liquid Jets
MS, University of Cincinnati, 2012, Engineering and Applied Science: Mechanical Engineering
A mathematical model for predicting the breakup length and resultant drop size for a liquid jet emanating from an orifice into a gaseous atmosphere is presented in this thesis. The non-Newtonian viscoelastic liquids are considered owing to their importance in a variety of engineering applications. The viscoelastic liquid rheology is characterized by the Jeffrey's equation. Results for a Newtonian jet are obtained by making the viscoelastic time constants zero in the model. Furthermore, when the liquid viscosity is neglected in the model, the behavior of an inviscid liquid jet can be predicted. The perturbation expansion technique is used to analyze the stability of the liquid jet by imposing a small disturbance on the jet surface and tracking the growth and development of the disturbance. In this temporal analysis, the jet is considered to disintegrate into droplets when the amplitude of the surface disturbance reaches the jet radius. The time taken for the surface deformation to evolve and break the jet, or the breakup time, and the breakup length are calculated. The model is validated by comparing numerical predictions with several theoretical analyses and experimental measurements available in the literature. A parametric study is carried out to investigate the effects of fluid properties (liquid viscosity, liquid elasticity, and time constant ratios), flow conditions (liquid and gas velocities), and disturbance characteristics (initial amplitude and wave number) on the breakup length and resultant drop size. Significant findings are that increase in gas velocity decreases the breakup length and increases the size of the secondary drops formed. Also, jets of higher viscosity liquids have longer breakup lengths. Increasing the liquid elasticity decreases the breakup length and aids in the breakup process. Higher initial disturbance amplitude results in shorter breakup length but the initial amplitude does not significantly affect the resulting drop size.

Committee:

Milind Jog, PhD (Committee Chair); Shaaban Abdallah, PhD (Committee Member); Raj Manglik, PhD (Committee Member)

Subjects:

Engineering

Keywords:

Instability;Breakup;Liquid Jets;;;;

Speth, Rachelle LeaParametric Study of the Effects of the Flapping Mode Excitation on the Near Field Structures of a Mach 1.3 Cold Jet
Master of Science, The Ohio State University, 2012, Aero/Astro Engineering
This effort investigates numerical and physical parameters influencing an ideally-expanded Mach 1.3 jet excited by the m=+/-1 flapping mode. The excitation is imposed by eight Localized Arc Filament Plasma Actuators (LAFPA) placed around the periphery of the circular nozzle exit. The devices are modeled with a proven surface heating approach. The reference case considers the most amplified (jet column mode) frequency corresponding to a Strouhal number of 0.3, based on the diameter of the nozzle and the jet velocity, with an actuator-imposed temperature of 1500K and a duty cycle of 20%. Relative to this reference, the effects of changing frequency, duty cycle and actuator model temperature are explored. In some cases, e.g., actuator temperature, experimental data is not available, but for frequency, there is. The results are analyzed with several different quantitative and qualitative metrics, including time-averaged centerline decay and jet half width as well as phase-averaged coherent structures. Raising the frequency affects the dynamics in several ways. The number of vortical features observed in the phase-averaged data increases and the rate of decay of the centerline velocity is reduced. Furthermore, the alternating vortex ring interactions observed in the reference case are not distinct but are rather replaced by smaller structures, trends which are also observed in experiment. The flow mixes the fastest around the jet column mode (St~0.22). The higher duty cycles exhibits strengthened coherent structures and slightly higher jet growth along the flapping plane, but the overall dynamics remain the same. The response of the jet is relatively insensitive to actuator temperature model within reasonable limits. The latter two studies, with different duty cycles and actuator temperatures, are consistent with previous analyses demonstrating that instability manipulation, rather than heat deposition is the primary mechanism of control.

Committee:

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

Subjects:

Aerospace Engineering; Engineering; Mechanical Engineering

Keywords:

supersonic jets; Large Eddy Simulation; plasma actuators

Naigle, Shawn ChristopherFlow Control of Compressible Dynamic Stall using Vortex Generator Jets
Master of Science, The Ohio State University, 2016, Aeronautical and Astronautical Engineering
Dynamic stall is an airspeed and maneuver limiting event which occurs on helicopter retreating blades at high advance ratios and is associated with aerodynamic flutter and large negative pitch moments. The potentially violent dynamic stall sequence amplifies pitch link stress and can lead to loss of aircraft control. Steady Vortex Generator Jet (VGJ) blowing has proven to delay the onset of dynamic stall. This work presents results of an experimental investigation into active flow control of a Sikorsky SSC-A09 airfoil undergoing periodic pitching motion in a variety of flow conditions including, steady incompressible, steady compressible, and time-varying compressible freestream, representative of a helicopter rotor system in flight. The airfoil was evaluated at reduced pitching frequencies of 0.025 and 0.050 with a nominal angle of attack schedule, a=9.5°-10.5°cos(wt). Flow conditions were at steady Mach numbers of M=0.2 and M=0.4 and time-varying phase-locked freestream oscillations at Mach number M=0.4+0.07cos(wt), at Reynolds numbers Re=1.5 M and Re=3.0 M. Flow control was achieved through a spanwise row of jets located at 10% chord, oriented normal to the surface, with an effective activated control width of 75% airfoil span. Blowing flow control was evaluated at a jet mass flux ratios from Cq=0.002 to Cq=0.005. Flow control enhancements evaluated include stall penetration, lift and moment improvements, reduction in negative damping, and flow reattachment angle. Quantitative measurements of lift and moment coefficients were calculated through the integration of airfoil surface pressure taps. Qualitative, time-resolved Background Oriented Schlieren (BOS) supplemented surface pressure measurements to assess spanwise averaged dynamic stall vortex progression as well as shock interaction. No optimal mass flux ratio completely controls dynamic stall, but VGJs delayed boundary layer separation, consistently improved cycle average moment, and increased cycle average lift. VGJs triggered an earlier flow reattachment which reduced hysteresis and circuits of clockwise rotation on the CM curve related to negative damping. BOS imagery confirmed the presence of leading edge shock formation and showed VGJ capability to delay shock-induced flow separation. The effectiveness of VGJ flow control is primarily a function of maximum angle of attack, pitching frequency, and freestream compressibility. A comparison of VGJ flow control evaluated on a pitching airfoil in a steady compressible freestream at M=0.4 versus a pitching airfoil in a time-varying compressible freestream at M=0.04+0.07cos(wt) at matched mean reduced frequency and Reynolds number, experienced similar quantitative improvements. Comparison of BOS imagery reveal the same physical VGJ to shear layer interaction between the steady and time-varying freestream cases. Thus, performance measurements based on active VGJs in a steady compressible freestream provide a good prediction of the expected performance measurements when blowing is applied to an airfoil in a low amplitude, time-varying compressible freestream. At low freestream oscillations, airfoil pitching frequency is the dominant factor influencing VGJ effectiveness.

Committee:

Jeffrey Bons (Advisor); James Gregory (Committee Member)

Subjects:

Aerospace Engineering

Keywords:

dynamic stall; flow control; vortex generator jets

Kirschenbaum, MarcThe degree of C⁰-sufficiency of Weierstrauss jets via the tree model /
Doctor of Philosophy, The Ohio State University, 1982, Graduate School

Committee:

Not Provided (Other)

Subjects:

Mathematics

Keywords:

Jets ;Geometry

Abd El-Nabi, BassamSingle Annular Combustor: Experimental investigations of Aerodynamics, Dynamics and Emissions
PhD, University of Cincinnati, 2010, Engineering : Aerospace Engineering
The present work investigates the aerodynamics, dynamics and emissions of a Single Cup Combustor Sector. The Combustor resembles a real Gas Turbine Combustor with primary, secondary and dilution zones (also known as fuel rich dome combustor). The primary jets considerably contribute to the heat release process at high power conditions. Also, the primary jets drastically impact the flow field structure. Therefore, the parameters influencing the primary jets are studied using PIV (pressure drop, jets size, off-centering, interaction with convective cooling air, jet blockage and fuel injection). This study is referred to as a jet sensitivity study. The results indicate that the primary jets can be used effectively in controlling the flow field structure. A pressure drop of 4.3% and 7.6% result in similar flows with no noticeable effect on the size of the CRZ and the four jets wake regions. On the other hand, the results show that the primary jets are very sensitive to perturbations. The cooling air interacts with the primary jet and influences the flow field although the momentum ratio has an order of magnitude of 100:1. The results also show that the big primary jets dictate the flow field in the primary zone as well as the secondary zone. However, relatively smaller jets mainly impact the primary zone. Also, the results point to the presence of a critical jet diameter beyond which the dilution jets have minimum impact on the secondary region. The jet off-centering shows significant effect on the flow field though it is on the order of 1.0 mm. The jet sensitivity study provides the combustion engineers with useful methods to control the flow field structure, an explanation for observed flow structure under different conditions and predictable flow field behavior with engine aging. All results obtained from the jet sensitivity study could be explained in terms of jet opposition. Hence, similar results are expected under reacting conditions. The combustion instabilities are studied using a microphone, high speed camera and regular cameras. The frequency spectrum for the sector is established at different pressure drops (2, 4 and 6%) as well as different pre-heat temperatures (200, 400 and 600F). The acoustic spectrum suggests that there are three frequencies of concern (280, 400 and 600 HZ). The high frequency appears to be related to the combustor ¼ longitudinal wave. The 280 Hz is due to a rotating instability while the 400 Hz is related to the primary jets. The emissions emanating from the combustor are studied using FTIR at pressure drop of 4% and different power conditions. The sector emissions characteristics are determined. Water injection is also used to control the pollutant emissions. Water fuel ratio of 100% and 50% results in a corresponding reduction in the NOx concentration with 50% and 22%. No noticeable effects are observed on the NOx and CO at low power conditions. A high degree of homogeneity in the emissions contours is observed at the combustor exit at low power conditions. However, this homogeneity is noticeably reduced at high power conditions.

Committee:

San-Mou Jeng, PhD (Committee Chair); Milind Jog, PhD (Committee Member); Mustafa Andac, PhD (Committee Member); Shaaban Abdallah, PhD (Committee Member)

Subjects:

Aerospace Materials

Keywords:

Primary jets senstivity;Swirling flow;Combustion Aerodynamics;Combustion Dynamics;Emissions control;Engine Aging effects

Munday, DavidFlow and Acoustics of Jets from Practical Nozzles for High-Performance Military Aircraft
PhD, University of Cincinnati, 2010, Engineering and Applied Science: Aerospace Engineering

This research project examines supersonic jets from nozzles representative of the practical variable-geometry convergent-divergent nozzles used on high-performance military aircraft. The nozzles employed have conical convergent sections, sharp throats and conical divergent sections. Nozzles with design Mach numbers of 1.3, 1.5, 1.56 and 1.65 are tested and the flow and acoustics examined. Such nozzles are found to produce a double-diamond shock structure consisting of two overlapping sets of shock cells, one cast from the nozzle lip and one cast from the nozzle throat. These nozzles are found to produce no shock-free condition at or near the design condition. As a result they produce shock-associated noise at all supersonic conditions. The shock cell spacing, broad-band shock-associated noise peak frequency and screech frequency all match those of more traditional nearly isentropic convergent-divergent nozzles.

A correlation is proposed which improves upon the Prandtl-Pack relation for shock cell spacing in that it accounts for differences in nozzle design Mach number which the Prandtl-Pack relation does not. This proposed relation reverts to the Prandtl-Pack equation for the case of a design Mach number of 1.0.

Chevrons are applied to the nozzles with design Mach numbers of 1.5 and 1.56. The effective penetration of the chevrons is found to be a function of the jet Mach number. Increasing jet Mach number increases effective penetration of the chevrons and increases the magnitude of all chevron effects. Chevrons on supersonic jets are found to reduce shock cell length, increase mixing and spreading, decrease turbulent kinetic energy at the end of the potential core and increase it near the nozzle. Chevrons corrugate the shear layer but not the shock structures inside the jet which remain axisymmetric. Chevrons thicken the shear layer, reducing the sonic diameter and reducing the diameter of the shock cells. By reducing their diameter they also reduce the shock cell spacing. Chevrons reduce low-frequency mixing noise near the end of the potential core, increase high-frequency noise near the nozzle exit. They eliminate screech and reduce broad-band shock-associated noise and shift it to higher frequencies.

Fluidic injection is applied to the nozzle with design Mach number of 1.5. Fluidic injection corrugates the shear layer, increases mixing and spreading, reduces low frequency mixing noise, increases high frequency noise, reduces broad-band shock-associated noise and shifts its peak to higher frequency.

Committee:

Ephraim Gutmark, PhD, DSc (Committee Chair); Shaaban Abdallah, PhD (Committee Member); Paul Orkwis, PhD (Committee Member); James Bridges, PhD (Committee Member); Kailas Kailasanmath, PhD (Committee Member)

Subjects:

Aerospace Materials

Keywords:

jet noise;chevrons;microjets;fluidics;Prantdl-Pack;supersonic jets

Kartuzova, Olga ValeryevnaA computational study for the utilization of jet pulsations in gas turbine film cooling and flow control
Doctor of Engineering, Cleveland State University, 2010, Fenn College of Engineering

Jets have been utilized in various turbomachinery applications in order to improve gas turbines performance. Jet pulsation is a promising technique because of the reduction in the amount of air removed from compressor, which helps to increase turbine efficiency. In this work two areas of pulsed jets applications were investigated, first one is film cooling of High Pressure Turbine (HPT) blades and second one is flow separation control over Low Pressure Turbine (LPT) airfoil using Vortex Generator Jets (VGJ)

The inlet temperature to the HPT significantly affects the performance of the gas turbine. Film cooling is one of the most efficient methods for cooling turbine blades. This technique is simply employing cool air discharged from rows of holes into the hot stream. Using pulsed jets for film cooling purposes can help to improve the effectiveness and thus allow higher turbine inlet temperature without affecting the blade's life. Engine cost will thus be reduced by providing the same capacity from smaller, lighter engines. Fuel consumption will be lowered, resulting in lower fuel cost. Effects of the film hole geometry, blowing ratio and density ratio of the jet, pulsation frequency and duty cycle of blowing on the film cooling effectiveness were investigated in the present work.

As for the low-pressure turbine (LPT) stages, the boundary layer separation on the suction side of airfoils can occur due to strong adverse pressure gradients. The problem is exacerbated as airfoil loading is increased. If the boundary layer separates, the lift from the airfoil decreases and the aerodynamic loss increases, resulting in a drop in an overall engine efficiency. A significant increase in efficiency could be achieved if separation could be prevented, or minimized. Active flow control could provide a means for minimizing separation under conditions where it is most severe (low Re), without causing additional losses under other conditions (high Re). Minimizing separation will allow improved designs with fewer stages and fewer airfoils per stage to generate the same power. The effects of the jet geometry, blowing ratio, density ratio, pulsation frequency and duty cycle on the size of the separated region were examined in this work. The results from Reynolds Averaged Navier-Stokes and Large Eddy Simulation computational approaches were compared with the experimental data.

Committee:

Dr. Mounir Ibrahim (Committee Chair); Dr. Asuquo Ebiana (Committee Member); Dr. Hanz Richter (Committee Member); Dr. Miron Kaufman (Committee Member); Dr. Petru Fodor (Committee Member); Dr. Ralph Volino (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

CFD; flow control; film cooling; LES; airfoil; separation; Vortex Generator Jets

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