Search Results (1 - 25 of 65 Results)

Sort By  
Sort Dir
 
Results per page  

Bloxham, Matthew JonA Global Approach to Turbomachinery Flow Control: Loss Reduction using Endwall Suction and Midspan Vortex Generator Jet Blowing
Doctor of Philosophy, The Ohio State University, 2010, Aeronautical and Astronautical Engineering

A flow control scheme using endwall suction and vortex generator jet (VGJ) blowing was employed to reduce the turbine passage losses associated with the endwall flow field and midspan separation. Unsteady midspan control at low Re had a significant impact on the wake total pressure losses, decreasing the area-average losses by 54%. The addition of leading edge endwall suction resulted in an area-average total pressure loss reduction of 57%. The minimal additional gains achieved with leading edge endwall suction showed that the horseshoe vortex was a secondary contributor to endwall loss production (primary contributor- passage vortex).

A similar flow control strategy was employed with an emphasis on passage vortex (PV) control. During the design, a theoretical model was used that predicted the trajectory of the passage vortex. The model required inviscid results obtained from two-dimensional CFD. It was used in the design of two flow control approaches, the removal and redirection approaches. The emphasis of the removal approach was the direct application of flow control on the endwall below the passage vortex trajectory. The redirection approach attempted to alter the trajectory of the PV by removing boundary layer fluid through judiciously placed suction holes. Suction hole positions were chosen using a potential flow model that emphasized the alignment of the endwall flow field with inviscid streamlines. Model results were validated using flow visualization and particle image velocimetry (PIV) in a linear turbine cascade comprised of the highly-loaded L1A blade profile.

Detailed wake total pressure losses were measured, while matching the suction and VGJ massflow rates, for the removal and redirection approaches at ReCx=25000 and blowing ratio, B, of 2. When compared with the no control results, the addition of steady VGJs and endwall suction reduced the wake losses by 69% (removal approach) and 68% (redirection approach). The majority of the total pressure loss reduction resulted from the spanwise VGJs, while the suction schemes provided modest additional reductions (<2%). At ReCx=50000, the endwall control effectiveness was assessed for a range of suction rates without midspan VGJs. Area-average total pressure loss reductions of up to 28% were measured in the wake at ReCx=50000, B=0, with applied endwall suction employed using the removal scheme (compared to no suction at ReCx=50000). At which point, the total pressure loss core was almost completely eliminated. PIV showed that the endwall suction changed the location of the PV eliminating its influence on the suction surface of the turbine blade. Suction with the removal approach removed the corner vortex (CV) increasing the available span by more than 10%. The redirection approach was less effective at higher suction rates due to the continual presence of the CV.

A system analysis was performed that compared the power needed to operate the flow control system to the power gained by the system. The power gains were assessed by comparing the change in lift and wake total pressure losses with and without flow control. The resultant power ratio showed that only 23% of the total power gained was needed to operate the flow control system for an L1A rotor at ReCx=50000, B=2.

Committee:

Jeffrey Bons, PhD (Advisor); James Gregory, PhD (Committee Member); Jen-Ping Chen, PhD (Committee Member); Mohammad Samimy, PhD (Committee Member)

Subjects:

Engineering

Keywords:

turbomachinery; vortex generator jet; flow control; turbine; gas turbine; horseshoe vortex; passage vortex; corner vortex; separation

Bunjevac, Joseph AntunPIV Analysis of Wake Structure of Real Elephant Seal Whiskers
Master of Science in Mechanical Engineering, Cleveland State University, 2017, Washkewicz College of Engineering
Seals are able to accurately detect minute disturbances in the ambient flow environment using their whiskers, which is attributed to the exceptional capability of their whiskers to suppress vortex-induced vibrations in the wake. To explore potential applications for designing smart flow devices, such as high-sensitivity underwater flow sensors and drag reduction components, researchers have studied how the role of some key parameters of whisker-like morphology affect the wake structure. Due to the naturally presented variation in size and curvature along the length of whiskers, it is not well understood how a real whisker changes the surrounding flow and the vortex shedding behavior. This study aims to detail the flow statistics around a real Elephant Seal whisker at low Reynolds numbers (i.e. one hundred) using particle image velocimetry in a water channel. Wake flow structures are inspected and compared between two Elephant Seal whiskers (undulating) and a California Sea Lion whisker (smooth), along with idealized whisker-like models. Undulating whiskers significantly change the mean flow properties and suppress turbulence intensities in the wake region as compared to the smooth whisker at the tested Re. The undulating whiskers are able to create a low turbulence intensity area directly behind the whiskers trailing, vproviding these whiskers with their Vortex Induced Vibration reduction properties.

Committee:

Wei Zhang, Ph.D. (Advisor); Mounir Ibrahim, Ph.D. (Committee Member); Thijs Heus, Ph.D. (Committee Member)

Subjects:

Biomechanics; Fluid Dynamics

Keywords:

Undulating whiskers, vortex dynamics, seal whiskers, elephant seal, harbor seal, PIV, water channel, flow, vortex induced vibration, VIV, turbulence, vortex supression, undulations

Gunasekaran, SidaardRelationship Between the Free Shear Layer, the Wingtip Vortex and Aerodynamic Efficiency
Doctor of Philosophy (Ph.D.), University of Dayton, 2016, Aerospace Engineering
The overarching objective of this experimental investigation is to explore the relationship between the aerodynamic efficiency of the wing and its turbulent wake (both the free shear layer and the wingtip vortex). Recent evidence of unique turbulent signatures in the free shear layer of a turbulent generator provided the motivation behind this research. The balance of induced drag and the parasite drag was hypothesized to be mirrored in the properties of the wingtip vortex and the free shear layer respectively expanding from classical theoretical descriptions. Experimental investigations were focused on the wake of wings to understand this balance in the parasite and the induced drag and to explore the use of the properties in the turbulent wake to increase the aerodynamic efficiency of the wing. Because of the highly complex nature of the wake, the research is broken down into several individual sub-studies which explore a) the relationship between the aerodynamic efficiency and the free shear layer, b) the relationship between the aerodynamic efficiency and the wingtip vortex, and c) the relationship between the free shear layer and the wingtip vortex and their correlation to the aerodynamic efficiency. Particle Image Velocimetry (PIV) was used to measure the velocity in the wake of an SD 7003 wall-to-wall model and an AR 4 flat plate with and without a spanwise boundary layer trip in the Horizontal Free Surface Water Tunnel (HFWT) at the Air Force Research Labs (AFRL) and in the Low Speed Wind Tunnel at the University of Dayton (UD-LSWT). The results from experimental investigations were Reynolds decomposed to study the mean and fluctuating quantities in the wake of the wing. The initial prediction of these quantities in the wake of SD 7003 wall-to-wall model and AR 4 flat plate were made using the existing momentum deficit and Reynolds stress models (which are derived from simplified Navier-Stokes equations). Even though the momentum deficit model yielded a good match with the experimental data, the Reynolds stress model was not able to predict the experimental data because of the asymmetry in the distribution. The eddy viscosity parameter in the algebraic models was then identified and incorporated in the algebraic models. The variation in the surrogate eddy viscosity parameter when compared to the experimental data showed direct correlation with the variation in the aerodynamic efficiency of the wing. In order to fortify the relationship between the turbulent properties in the free shear layer and the aerodynamic efficiency, the energy loss in the wake of the SD7003 wall-to-wall model was quantified by determining the viscous dissipation (Exergy) as a function of initial conditions upstream. The changes in Exergy mirrored the aerodynamic efficiency of the SD 7003 wall-to-wall model. But in the AR 4 wing wake, there existed a net spanwise momentum due to the formation of the wingtip vortices. Using orthogonal PIV planes of interrogation in the wingtip vortex station across several distances downstream, the evolution of the wingtip vortex and its relationship with aerodynamic efficiency of the wing were investigated. The wake-like to jet-like transition in the core of the wingtip vortex was not observed at the angle of attack of maximum aerodynamic efficiency. However the maximum viscous dissipation and the Reynolds stress in the wingtip vortex shows changes in the slope at the maximum (L/D) location. In the presence of a spanwise boundary layer trip, the location of the change of slope in the viscous dissipation and Reynolds stress was changed indicating a direct correlation to the properties in the wingtip vortex and the aerodynamic efficiency of the wing. Significant changes in the boundary layer of the flat plate with the boundary layer trip were observed at lower angles of attack. The resulting changes in the turbulence character of the wingtip vortex and the free shear layer were investigated for evidence of an interaction between the free shear layer and the evolution of the wingtip vortex. The streamwise, cross-stream and spanwise oriented PIV of the wingtip vortex shows definitive evidence of the free shear layer interaction with the wingtip vortex at angles of attack lower than maximum (L/D). This interaction was reflected in the normalized azimuthal velocity profile of the wingtip vortex as well. The composite of the velocity profiles from the multiple different planes showed a transfer of momentum from the free shear layer to the wingtip vortex in the vicinity of maximum (L/D) angle of attack. This suggests that by manipulating the cross-stream flow in the wake of the wing from the wing root to the wingtip, the balance of induced drag and parasite drag can be altered given initial conditions and the aerodynamic efficiency can be improved in off-design conditions.

Committee:

Aaron Altman (Advisor); Markus Rumpfkeil (Committee Member); Jose Camberos (Committee Member); Ryan Schmit (Committee Member)

Subjects:

Aerospace Engineering; Fluid Dynamics

Keywords:

Wingtip Vortex; Free Shear Layer; Turbulence; Exergy; Wingtip Vortex Shear Layer Interaction; Aerodynamics; Fluid Dynamics; Boundary Layer Trip; Serrated Leading Edge; Streamwise Wingtip Vortex Particle Image Velocimetry; Momentum Transfer;

Smith, Todd J.Development, Design, Manufacture and Test of Flapping Wing Micro Aerial Vehicles
Doctor of Philosophy (PhD), Wright State University, 2016, Engineering PhD
The field of FlappingWing Micro Air Vehicles (FWMAV) has been of interest in recent years and as shown to have many aerodynamic principles unconventional to traditional aviation aerodynamics. In addition to traditional manufacturing techniques, MAVs have utilized techniques and machines that have gained significant interest and investment over the past decade, namely in additive manufacturing. This dissertation discusses the techniques used to manufacture and build a 30 gram-force (gf) model which approaches the lower limit allowed by current commercial off-the-shelf items. The vehicle utilizes a novel mechanism that minimizes traditional kinematic issues associated with four bar mechanisms for flapping wing vehicles. A kinematic reasoning for large amplitude flapping is demonstrated namely, by lowering the cycle averaged angular acceleration of the wings. The vehicle is tested for control authority and lift of the mechanism using three servo drives for wing manipulation. The study then discusses the wing design, manufacturing techniques and limitations involved with the wings for a FWMAV. A set of 17 different wings are tested for lift reaching lifts of 38 gf using the aforementioned vehicle design. The variation in wings spurs the investigation of the flow patterns generated by the flexible wings and its interactions for multiple flapping amplitudes. Phase-lock particle image velocimetry (PIV) is used to investigate the unsteady flows generated by the vehicle. A novel flow pattern is experimentally found, namely “trailing edge vortex capture” upon wing reversal for all three flapping amplitudes, alluding to a newly discovered addition to the lift enhancing effect of wake capture. This effect is believed to be a result of flexible wings and may provide lift enhancing characteristics to wake capture.

Committee:

George Huang, Ph.D. (Advisor); James Menart, Ph.D. (Committee Member); Zifeng Yang, Ph.D. (Committee Member); Richard Cobb, Ph.D. (Committee Member); Michael Oppenheimer, Ph.D. (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

FWMAV; MAV; flapping wings; vortex capture; Phase-Lock; PIV; trailing edge vortex capture; vortex capture; flexible wings

Barnes, Caleb J.Unsteady Physics and Aeroelastic Response of Streamwise Vortex-Surface Interactions
Doctor of Philosophy (PhD), Wright State University, 2015, Engineering PhD
Streamwise vortex-surface interactions can occur in aviation intentionally in the context of formation flight as an energy saving mechanism, unintentionally in wake crossings when aircraft fly in close proximity, and as a consequence of aircraft design through the interaction of fluid dynamics between different aerodynamic surfaces. The bulk of past work on streamwise vortex-surface interactions has focused on steady inviscid analysis for optimizing aerodynamic loads in the context of formation flight or experimental analysis on fin buffeting problems. A fundamental understanding of the viscous and unsteady effects that may occur is both important and currently lacking in the literature. This dissertation seeks to fill this need by using a high-fidelity implicit large-eddy simulation approach coupled with geometrically non-linear finite elements to identify and analyze important physics that may occur. Simple, canonical configurations are employed in order help disentangle the many interrelated factors of a very complex problem. Analysis of a tandem wing configuration elucidated mutual induction between the incident vortex from the leader wing and tip vortex of the follower wing that resulted in a broad taxonomy of flow structure, wake evolution, and unsteady behaviors for several lateral impingement locations. Interaction of an isolated streamwise vortex with a wing revealed a robust helical instability develops when a strong vortex impinges directly with the leading-edge. This spiraling behavior was found to occur as a result of the upstream influence of adverse pressure gradients provided by the wing that drive the vortex into its linearly unstable regime allowing for the growth of shortwave perturbations. Stability can be augmented through vertical positioning of the vortex. A negative offset can enhance stability by providing a stronger adverse pressure gradient while a positive offset exploits a favorable gradient and removes the upstream instability altogether. The effects of wing compliance were revealed through full aeroleastic simulations. Essentially static, vortex-induced bending deformations reposition the vortex and drive it further into its unstable regime. Static and dynamic components of the aeroelastic response were systematically isolated where the static deformations were shown to provide the greatest influence. Dynamic effects provide some influence to the incident vortex behavior but these are secondary to the static behavior.

Committee:

George Huang, Ph.D. (Advisor); Joseph Shang, Ph.D. (Committee Member); Zifeng Yang, Ph.D. (Committee Member); Miguel Visbal, Ph.D. (Committee Member); Aaron Altman, Ph.D. (Committee Member)

Subjects:

Aerospace Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

formation flight; streamwise vortex interaction; fluid structure interaction; aeroelasticity; unsteady fluid dynamics; vortex dynamics; vortex surface interaction

Golubeva, Natalia YurievnaSingularities in the spatial complex plane for vortex sheets and thin vortex layers
Doctor of Philosophy, The Ohio State University, 2003, Mathematics
An important special case of shear layers is the narrowly confined vortex layer whose mathematical model is a vortex sheet with the motion described by the Birkhoff equation. Consistent discretizations of the Birkhoff equation fail to yield reliable results; in particular, a curvature singularity forms in finite time. This failure motivated the development of alternative approaches. One of them is to replace the vortex sheet by a layer of finite thickness and uniform vorticity. That prevents singularity formation. The limiting behaviour of a thin layer may be addressed by understanding the formation and subsequent motion of singularities formed in the spatial complex plane. Next, the motion of these singularities can be compared to that of the equivalent vortex sheet. The difficulty is that the parametrizations for vortex sheet and the boundaries of the thin layer do not coincide. Therefore, a new and easy way is found to compare the motion of a thin layer with the motion of a vortex sheet. The motion of the fluid particles on one side of the sheet is used, and this choice is referred to as a one-sided vortex sheet. As a result, the fluid motion of the boundary of the thin layer becomes the motion of the fluid particles of a one-sided vortex sheet in the limit of vanishing thickness. The equations of motion for a one-sided vortex sheet are derived and analytically continued into the complex plane of the new parametrization. Asymptotic methods applied to these equations reveal the presence of a 3/2-power singularity moving towards the real axis. This singularity is related to the one for the classical vortex sheet by studying the mapping between the parametrizations. The equations of motion for the thin layer are also analytically continued into the complex plane of the parametrization variable. By studying the limit of vanishing thickness, confirmation is made that the motion of the boundary of the thin layer does become the motion of the one-sided vortex sheet. Finally, asymptotic methods are applied to the analytically continued equations to understand the initial formation of singularities. Numerical calculations of the boundary motion confirm the presence of 3/2-power singularities. The trajectory of these singularities approach the one for the vortex sheet in the limit of vanishing thickness before the singularity time.

Committee:

Gregory Baker (Advisor)

Subjects:

Mathematics

Keywords:

vortex sheet; vortex layer; thin shear layer; asymptotic methods; singularity

Garmann, Daniel JCharacterization of the vortex formation and evolution about a revolving wing using high-fidelity simulation
PhD, University of Cincinnati, 2013, Engineering and Applied Science: Aerospace Engineering
A numerical study is conducted to examine the vortex structure and aerodynamic loading about a revolving wing in quiescent flow. The high-fidelity, implicit large eddy simulation (ILES) technique is employed to simulate a revolving wing configuration consisting of a rectangular plate extended out half a chord from the rotational axis at a fixed geometric angle relative to this axis. Shortly after the onset of the motion, the rotating wing generates a coherent vortex system along the leading-edge. This vortex system remains attached throughout the motion for the range of Reynolds numbers explored despite the unsteadiness and vortex breakdown observed at higher Reynolds numbers. The average and instantaneous wing loading also increases with Reynolds number. At a fixed Reynolds number, the attachment of the leading-edge vortex (LEV) is shown to be insensitive to geometric orientation of the wing. Additionally, the flow structure and forcing generated by a purely translating wing is investigated and compared with that of the revolving wing. Similar features are present at the inception of the motion; however, the two flows evolve very differently for the remainder of the maneuver. The role of aspect ratio is also examined, and the span-wise evolution of the leading-edge vortex is analyzed. The mean lift and drag both increase with aspect ratio until the chord-wise growth of the leading-edge vortex becomes constrained by the trailing edge causing a saturation of the aerodynamic loads. Additional LEV substructures form with increased aspect ratio from the increase in local span-based Reynolds number. The genesis of these substructures has been traced to the eruption of secondary, wall-induced vorticity under the LEV that penetrates the LEV feeding sheet. The disrupted shear layer then rolls-up under self-induction to form discrete substructures. Vortex breakdown of the vortex core occurs around mid-span despite aspect ratio of the wing indicating that it does not correlate with the local span-based Reynolds number, but instead, is a result of the pressure gradient established between the root and tip of the wing. Very favorable comparisons of the computational solution with experimental measurements are provided through application of a new data-reduction technique used to accurately compare simulations and experiments of differing spatial resolutions.

Committee:

Paul Orkwis, Ph.D. (Committee Chair); Miguel Visbal, Ph.D. (Committee Member); Shaaban Abdallah, Ph.D. (Committee Member); Mark Turner, Sc.D. (Committee Member)

Subjects:

Aerospace Materials

Keywords:

vortex dynamics;bio-inspired aerodynamics;large eddy simulation;revolving wings;vortex transition and breakdown;high-fidelity simulation;

Luo, GuoSingularities in the Complex Spatial Plane of a Vortex Sheet with Blob Regularization
Doctor of Philosophy, The Ohio State University, 2009, Mathematics
I study in this thesis a periodic version of Krasny's vortex blob equation (1987), whose solutions are known to converge to a classical weak solution of the Euler equation upon vanishing blob size (Liu and Xin 1995). The equation is analytically continued into the complex spatial plane of its parametrization variable, and for a specific initial condition used by Meiron et al. (1982), a simplified far-field equation is derived which is then studied using Taylor expansions in small time and small blob size. Like the unregularized vortex sheet studied by Cowley et al. (1999), the solutions of the far-field equation develop singularities immediately after the sheet starts to evolve. These singularities propagate in from infinity and local analysis suggests they are square-root branch points, possibly infinitely many in quantity. All analytic results are confirmed in a subsequent numerical study, and certain properties of the complex singularities, like their locations and strengths, are studied numerically in some detail. It appears that all singularities of the blob-regularized vortex sheet collapse to the 3/2-power singularity of the unregularized vortex sheet as the blob size vanishes, but the precise way in which this collapse happens is yet to be determined.

Committee:

Gregory Baker, PhD (Advisor); Saleh Tanveer, PhD (Committee Member); Ed Overman, PhD (Committee Member)

Subjects:

Mathematics

Keywords:

vortex sheet; vortex blob; complex singularity

Memory, Curtis L.Turbulent Transition Behavior in a Low Pressure Turbine Subjected to Separated and Attached-Flow Conditions
Doctor of Philosophy, The Ohio State University, 2010, Mechanical Engineering
Various time accurate numerical simulations were conducted on the aft-loaded L1A low pressure turbine airfoil operating at Reynolds numbers presenting with fully-stalled, non-reattaching laminar separation. The numerical solver TURBO was modified from its annular gas turbine simulation configuration to conduct simulations based on a linear cascade wind tunnel facility. Simulation results for the fully separated flow fields revealed various turbulent decay mechanisms. Separated shear layer decay, in the form of vortices forming between the shear layer and the blade wall, was shown to agree with experimental particle image velocimetry (PIV) data in terms of decay vortex size and core vorticity levels. These vortical structures eventually mix into a large recirculation zone which dominates the blade wake. Turbulent wake extent and time-averaged velocity distributions agreed with PIV data. Steady-blowing vortex generating jet (VGJ) flow control was then applied to the flow fields. VGJ-induced streamwise vorticity was only present at blowing ratios above 1.5. VGJs actuated at the point of flow separation on the blade wall were more effective than those actuated downstream, within the separation zone. Pulsed-blowing VGJs at the upstream blade wall position were then actuated at various pulsing frequencies, duty cycles, and blowing ratios. These condition variations yielded differing levels of separation zone mitigation. Pulsed VGJs were shown to be more effective than steady blowing VGJs at conditions of high blowing ratio, high frequency, or high duty cycle, where blowing ratio had the highest level of influence on pulsed jet efficacy. The characteristic "calm zone" following the end of a given VGJ pulse was observed in simulations exhibiting high levels of separation zone mitigation. Numerical velocity fields near the blade wall during this calm zone was shown to be similar to velocity fields observed in PIV data. Instantaneous numerical vorticity fields indicated that the elimination of the separation zone directly downstream of the VGJ hole is a primary indicator of pulsed VGJ efficacy. This indicator was confirmed by numerical time-averaged velocity magnitude rms data in the same region.

Committee:

JenPing Chen, PhD (Advisor); Jeffrey P. Bons, PhD (Committee Member); James W. Gregory, PhD (Committee Member); Mei Zhuang, PhD (Committee Member)

Subjects:

Aerospace Engineering; Mechanical Engineering

Keywords:

vortex generating jet; low pressure turbine; direct numeric simulation; steady blowing; pulsed blowing; particle image velocimetry; vortex

Hammer, Patrick RichardA Discrete Vortex Method Application to Low Reynolds Number Aerodynamic Flows
Master of Science (M.S.), University of Dayton, 2011, Aerospace Engineering
Although experiments and CFD are very powerful tools in analyzing a niche of fluid dynamics problems relevant to developing Micro Aerial Vehicles (MAVs), reduced order methods have shown to be very capable in helping researchers achieve a basic understanding of flow physics with application to highly iterative design processes due to the less computationally expensive nature of the low order models. The current study used one low order method, the Discrete Vortex Method, to model the aerodynamic flow fields and forces around a thin airfoil undergoing a variety of flows, as well as parametric studies to determine the important factors that had to be adjusted to make the results more representative of the physical phenomenon being modeled. Initial investigations validated the code’s use in steady flow and low amplitude unsteady flow cases by comparing it with circulation distributions of various airfoil shapes, the Wagner function, and Theodorsen’s function. The results showed a strong dependency on bound vortex number and time step size. The code was then used to capture the flow behavior around the airfoil for various AIAA Fluid Dynamics Technical Committee Low Reynolds iv Number Working Group (FDTC-LRWG) canonical cases. Implementing the Uhlman method in the Discrete Vortex Method allowed for the calculation of the pressure at the airfoil surface and in the flow field during high angle attack maneuvers. This method proved very capable in calculating the pressures, forces, and force coefficients around the airfoil post-flow separation in the canonical cases where other methods (such as the Unsteady Bernoulli Method) fall short. The code was also tuned with respect to the results with respect to vortex size, leading edge separation strength factor, desingularization function, wake radius size factor, and in the Uhlman method itself to yield an optimal comparison with experimental and CFD results. The study found a bound vortex number of 30, a leading edge separation strength factor of 1.0, the planetary desingularization function, a wake radius size factor of 1.0, and using just the volume integral term on the RHS of the Uhlman method gave the best results for the geometry analyzed. An investigation then determined the dependency of reduced frequency on the lift and drag coefficients for the canonical cases. Finally, the code was used to model a “true perch” by implementing a curve fit function which caused the horizontal free stream velocity to decrease to zero. In this context, the forces were of more interest than the force coefficients since the coefficients experienced anomalous behavior as the free stream velocity approached zero. It was also interesting to find that the code modeled behavior very similar to shear layer instabilities in the LE and TE shear layers, caused by a rippling effect as the bound circulation changed in strength and sign as the LEV and TEV interacting with it. Recommendations were then made to apply the code to airfoils with either fixed or variable camber since camber acts as a high lift device and could prove very beneficial in the design and development of MAVs

Committee:

Aaron Altman (Committee Chair); Frank Eastep (Committee Member); Greg Reich (Committee Member)

Subjects:

Aerospace Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

Unsteady Aerodynamics; High Angle of Attack; Discrete Vortex Method; Vortex Particle Method; Perching

Cheng, WenPropagation of Vortex Beams through a turbulent atmosphere
Master of Science (M.S.), University of Dayton, 2009, Electro-Optics

This thesis study and compare the propagation properties of both scalar and vector vortex beams through turbulent atmosphere. The irradiance pattern, degree ofpolarization, and scintillation index of radially polarized beam are computed for different propagation distance into an atmosphere with weak and strong turbulence. Corresponding properties of a fundamental Gaussian beam, a scalar vortex beam with topological charge of +1 propagating through an atmosphere under the same turbulence condition are calculated for comparison.

The results demonstrate that the existence of the vectorial vortex can be identified with longer propagation distance than the scalar vortex even with disappearing characteristic vortex structure in the irradiance images. This indicates the potential advantages of using vector vortex to mitigate atmospheric effects and enable a more robust free space communication channel with longer link distance.

Committee:

Qiwen Zhan, PhD (Advisor); Joseph Haus, PhD (Committee Member); Peter Powers, PhD (Committee Member)

Subjects:

Optics

Keywords:

Vortex Beam Propagation; Turbulence; Scintillation Index

Ramamoorthy, ThiagarajanMechanical Behavior of Membranes in Electrostatic Precipitators
Master of Science (MS), Ohio University, 2005, Mechanical Engineering (Engineering)

Air pollution is one of the world’s major concerns from home to industry. We have come a long way in controlling air pollution using Electrostatic Precipitators (ESP). Recent advancements in ESP has lead to the use of membranes as collecting electrodes, instead of huge sheet metal electrodes. Membrane electrodes are better than conventional metal electrodes in many ways. They are light in weight, inexpensive, and can withstand corrosive environments. Woven meshes are materials which have similar mechanical properties like membranes [13].

The latest development in ESP is the so called Sieving Electrostatic Precipitator (SEP). SEP is a novel technology in the field of Electrostatic Precipitators (ESP) developed by Dr. H. Pasic, Ohio University. The placement of mesh electrodes perpendicular to the gas flow ensures maximum particle charging and collection efficiency of the fly-ash particles. This thesis concentrates on the mechanical behavior of membranes and meshes in both conventional ESP and SEP.

Committee:

Hajrudin Pasic (Advisor)

Subjects:

Engineering, Mechanical

Keywords:

Electrostatic Precipitators; Membrane Based ESP; Sieving Electrostatic Precipitator; Vortex Shedding; Dust Removal

Bhattacharya, SamikInvestigation of Three Dimensional Forcing of Cylinder Wake with Segmented Plasma Actuators and the Determination of the Optimum Wavelength of Forcing
Doctor of Philosophy, The Ohio State University, 2013, Aero/Astro Engineering
The wake of a circular cylinder was forced in a three dimensional manner with the help of spanwise non-uniformly located dielectric barrier discharge plasma actuators. The segmented actuators created a square wave forcing profile due to alternate existence of plasma and no plasma region on the cylinder at ±80 degree from the forward stagnation point. The wavelength (lambda) of the actuators was varied from 1d to 6d (d = diameter of the cylinder) and the induced velocities from all the actuators were matched. Below a threshold power level, vortex shedding was not significantly attenuated for any wavelengths, although distinct patterns of streamwise vortices formed for lambda > 2d. Due to this, the near wake developed a spanwise wavy structure: in the no-plasma region the spanwise vortex came closer to the centerline, while it was shifted away from the centerline in the plasma region. In this low-power forcing regime, the drag in the wake was not significantly affected as compared to the case when the forcing power level was above the threshold. Forcing with the lambda = 4d actuator at this high power level resulted in substantial drag reduction and an increase in three-dimensionality of the near wake accompanied by a drastic attenuation of vortex shedding. The energy of the shedding was distributed over a wide range of frequencies, with the most prominent frequency in the velocity spectra being lower than the natural shedding frequency indicating reduced formation length. For lambda > 4d, the wake behind the no-plasma region was much wider compared to that of the plasma region along with a clear difference in formation length, which resulted in higher drag than the lambda = 4d case. This finding lead to the recognition of lambda = 4d as the optimum wavelength of forcing. In the high power-forcing regime, counter-rotating vortices were formed in the horizontal plane at the edge of each buried electrode, which created an alternate zone of backflow in the no-plasma region and downstream flow in the plasma region. Appearance of a saddle point marked the boundary of the backflow region indicating increased level of strain in the no-plasma region in the high power forcing case. The transition from a lower power level below the threshold to that above it was marked by a change in the sense of rotation of the streamwise vortices for lambda > 3d. This change was the result of the dominance of vortices created by high power forcing over a secondary vorticity whose sign matched that of the low power case. It is concluded that this change in the sense of rotation of streamwise vortices with power level of actuation is an inherent feature of the segmented forcing for any wavelength; however, it is the optimum wavelength for which this transition is achieved with minimum induced velocity. The streamwise vortices in the low power case could not disrupt the shedding process, whereas in the high power regime, existence of strong counter-rotating vortices created backflow in front of no plasma region, which diverted flow from the spanwise vortex and thus stymied its growth.

Committee:

James W. Gregory (Advisor)

Subjects:

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

Keywords:

cylinder wake, flow control, plasma actuator, stream wise vortex, three dimensional forcing, active forcing

Peng, DiVortex Dynamics and Induced Pressure/Load Fluctuations During Blade-Vortex Interactions
Doctor of Philosophy, The Ohio State University, 2014, Aero/Astro Engineering
A comprehensive study on vortex dynamics during parallel blade-vortex interaction (BVI) was conducted in a subsonic wind tunnel. A vortex was generated by applying a rapid pitch-up motion to an airfoil through a pneumatic system, and then interacted with an unloaded target airfoil placed downstream. Both Particle Image Velocimetry (PIV) and unsteady pressure measurements were performed to investigate the vortex dynamics during BVI, as well as its impact on the induced pressure and load fluctuations. The Reynolds number ranges of PIV and pressure measurements are 80,000 to 115,000, and 170,000 to 250,000, respectively. Based on the PIV data, the interaction was classified into three categories in terms of vortex behavior: close interaction, very close interaction and direct interaction. For each type of interaction, the vortex trajectory and strength variation were obtained from phase-averaged PIV data. The PIV results revealed the mechanisms of vortex decay and the effects of several key parameters on vortex dynamics, including separation distance (h/c), Reynolds number, vortex strength (G) and vortex sense. Generally, BVI has two main stages: interaction between vortex and leading edge (vortex-LE interaction), and interaction between vortex and boundary layer (vortex-BL interaction). Vortex-LE interaction with small separation distance is usually dominated by inviscid effects, in which the decay in vortex strength is due to pressure gradients near the leading edge. Therefore, the decay rate is determined by separation distance and vortex strength, but it is relatively insensitive to Reynolds number. Vortex-LE interaction will become a viscous-type interaction if there is enough separation distance. Vortex-BL interaction is inherently dominated by viscous effects, so the decay rate is dependent on Reynolds number. Vortex sense also has great impact on vortex-BL interaction because it changes velocity field and shear stress near the surface. Unsteady pressure data on the target airfoil were acquired for each combination of Reynolds number, vortex strength and separation distance, yielding the magnitudes of pressure and load fluctuations in each case. Both pressure and load fluctuations show asymmetric distribution about h/c = 0, with generally larger fluctuations for h/c < 0 (clockwise vortex below the target airfoil). The maximum fluctuation occurs when a clockwise vortex is slightly below the target airfoil due to the high velocity on the lower side of the leading edge induced by the vortex. This peak in fluctuation level biases more toward h/c < 0 with a stronger vortex, due to its larger size. The load fluctuation level is dependent on both the velocity fields near the leading edge and the decay in vortex strength. However, the role of vortex decay becomes insignificant at higher Reynolds number due to less decay in the viscous-type interaction. The above findings suggest that the local velocity field around the leading edge of the target airfoil is critical in both the vortex dynamics and the induced pressure and load fluctuations. The magnitudes of fluctuations can be reduced by passing the airfoil below a clockwise vortex or above a counter-clockwise vortex.

Committee:

James Gregory (Advisor); Jeffrey Bons (Committee Member); Mei Zhuang (Committee Member); Jen-Ping Chen (Committee Member)

Subjects:

Aerospace Engineering

Keywords:

BVI, vortex dynamics, pressure fluctuation, load fluctuation

Smith, Erik TThe Characteristics of Cold Air Outbreaks in the eastern United States and the influence of Atmospheric Circulation Patterns
MA, Kent State University, 2017, College of Arts and Sciences / Department of Geography
Periods of extreme cold impact the mid-latitudes every winter. Depending on the magnitude and duration of the occurrence, extremely cold periods may be deemed cold air outbreaks (CAOs). Atmospheric teleconnections impact the displacement of polar air, but the relationship between the primary teleconnections and the manifestation of CAOs is not fully understood. A systematic CAO index was developed from 20 surface weather stations based on a set of criteria concerning magnitude, duration, and spatial extent. Statistical analyses of the data were used to determine the overall trends in CAOs. Clusters of sea level pressure (SLP), 100mb, and 10mb geopotential height anomalies were mapped utilizing self-organizing maps (SOMs) to understand the surface, tropospheric Polar Vortex (PV), and stratospheric PV patterns preceding CAOs. The Arctic Oscillation (AO), North Atlantic Oscillation (NAO), and Pacific-North American (PNA) teleconnections were used as variables to explain the magnitude and location of mid-latitude Arctic air displacement. Persistently negative SLP anomalies across the Arctic and North Atlantic were evident 1 – 2 weeks prior to the CAOs throughout the winter. The tropospheric and stratospheric PV were found to be persistently weak/weakening prior to mid-winter CAOs and predominantly strong and off-centered prior to early and late season CAOs. Negative phases of the AO and NAO were favored prior to CAOs, while the PNA was found to be less applicable. This method of CAO and synoptic pattern characterization benefits from a continuous pattern representation and provides insight as to how specific teleconnections impact the atmospheric flow in a way that leads to CAOs in the eastern U.S.

Committee:

Scott Sheridan, Dr. (Advisor); Thomas Schmidlin, Dr. (Committee Member); Cameron Lee, Dr. (Committee Member)

Subjects:

Climate Change; Environmental Science; Geography; Meteorology

Keywords:

Climate; Synoptic Climatology; Cold Air Outbreaks; Atmospheric Circulation Patterns; United States; Self-Organizing Maps; Polar Vortex; Extreme Weather; Teleconnections

Wendt, Bruce JamesThe structure and development of streamwise vortex arrays embedded in a turbulent boundary layer
Doctor of Philosophy, Case Western Reserve University, 1991, Aerospace Engineering
An investigation of the structure and development of streamwise vortices embedded in a turbulent boundary layer was conducted in the test facility CW-22 at NASA Lewis Research Center. The vortices were generated by a single spanwise row of rectangular vortex generator blades. A single embedded vortex was examined, as well as arrays of embedded counter-rotating vortices produced by equally spaced vortex generators. Measurements of the secondary velocity field in the crossplane provided the basis for characterization of vortex structure. Vortex structure was characterized by four descriptors. The center of each vortex core was located at the spanwise and normal position of peak streamwise vorticity. Vortex concentration was characterized by the magnitude of the peak streamwise vorticity, and the vortex strength by its circulation. Measurements of the secondary velocity field were conducted at two crossplane locations to examine the streamwise development of the vortex arrays. Large initial spacings of the vortex generators produced pairs of strong vortices which tended to move away from the wall region while smaller spacings produced tight arrays of weak vortices close to the wall. The crossplane structure of embedded vortices is observed to be very s imilar to that exhibited by the two dimensional Oseen vortex with matching descriptors. Quantitative comparisons are established. A model of vortex interaction and development is constructed using the experimental results. The model is based on the structure of the Oseen vortex. Vortex trajectories are successfully modelled by including the convective effects of neighbors, and images to represent the wall. The streamwise decay of circulation is successfully modelled for the single vortex, and for large initial spacings, by accounting for the effects of wall friction. An additional mechanism associated with the turbulent stress field in the near vicinity of the vortex cores is postulated to explain the large losses in circulation obtained for the smaller initial spacings. The streamwise decay of vortex circulation at the smaller spacings is successfully modelled by summing wall friction losses and "proximity" losses. These proximity losses are found to be proportional to the gradient in streamwise vorticity occurring between an embedded vortex and its adjacent counter-rotating neighbors.

Committee:

Isaac Greber (Advisor)

Keywords:

streamwise vortex arrays turbulent boundary layer

Tornes, Ivan EdwardTopics in the physics of underdamped Josephson systems
Doctor of Philosophy, The Ohio State University, 2006, Physics
We describe a variety of numerical models to characterize the behavior of several types of underdamped Josephson systems. We first discuss a stack of small junctions in a single mode resonant cavity. Our results show self-induced-resonant-steps in the current voltage (IV) curve. These steps represent a coherent phase locking between the junctions in the stack and the resonant cavity. Next we generalize these results to treat a single long junction coupled to a single mode resonant cavity. Our numerical results show that if the electromagnetic fields of the cavity mode are spatially periodic, we again see steps in the IV curve and furthermore, for sufficiently large coupling, the cavity frequency is shifted by coupling to the long junction. We then turn to some properties of single junctions in the quantum regime where the phase/number non-commutation is important. This quantum regime is crucial to all proposals to use such junctions as solid state quantum bits (qubits). We show that such qubits, when subjected to external a.c. perturbations, exhibit not only the expected Rabi oscillations but also multiphoton transitions between the quasi-bound states of the junction. We also consider the behavior of Josephson junction ladders in an external magnetic field. For magnetic fields equal to 1/n flux quanta per plaquette, we observe a periodic vortex lattice; here n is an integer evenly divisible into the total number of plaquettes, N, in the ladder. When a single fluxon is added to such a ladder, we show that it does not enter that ladder as a single unit of flux. Instead, it produces n fractional vortices. Our final chapter is motivated by some dramatic recent experiments on Josephson junctions formed between high temperature cuprate superconductors and low temperature s-wave superconductors. These experiments are intended to test phase sensitivity, i.e., the dependence of the measured critical current on an external magnetic field. We consider a ladder array of alternating 0 and π junctions with open boundary conditions in the presence of an external magnetic field to model this zigzag junction. Our calculations show many of the salient features seen in the experiments.

Committee:

David Stroud (Advisor)

Subjects:

Physics, Condensed Matter

Keywords:

Josephson junctions; Multiphoton transitions; Rabi Oscillations; Qubits; Self induced resonant steps; Soliton; Vortex Fractionalization; Zigzag ladder

Cheng, WenOptical Vortex Beams: Generation, Propagation and Applications
Doctor of Philosophy (Ph.D.), University of Dayton, 2013, Electro-Optics
An optical vortex (also known as a screw dislocation or phase singularity) is one type of optical singularity that has a spiral phase wave front around a singularity point where the phase is undefined. Optical vortex beams have a lot of applications in areas such as optical communications, LADAR (laser detection and ranging) system, optical tweezers, optical trapping and laser beam shaping. The concepts of optical vortex beams and methods of generation are briefly discussed. The properties of optical vortex beams propagating through atmospheric turbulence have been studied. A numerical modeling is developed and validated which has been applied to study the high order properties of optical vortex beams propagating though a turbulent atmosphere. The simulation results demonstrate the advantage that vectorial vortex beams may be more stable and maintain beam integrity better when they propagate through turbulent atmosphere. As one important application of optical vortex beams, the laser beam shaping is introduced and studied. We propose and demonstrate a method to generate a 2D flat-top beam profile using the second order full Poincare beams. Its applications in two-dimensional flat-top beam shaping with spatially variant polarization under low numerical aperture focusing have been studied both theoretically and experimentally. A novel compact flat-top beam shaper based on the proposed method has been designed, fabricated and tested. Experimental results show that high quality flat-top profile can be obtained with steep edge roll-off. The tolerance to different input beam sizes of the beam shaper is also verified in the experimental demonstration. The proposed and experimentally verified LC beam shaper has the potential to become a promising candidate for compact and low-cost flat-top beam shaping in areas such as laser processing/machining, lithography and medical treatment.

Committee:

Zhan Qiwen, Dr. (Advisor)

Subjects:

Electromagnetics; Optics

Keywords:

Flat-top, Polarization, Vortex Beams, Beam Shaping

Cho, Soo-YongThree dimensional compressible turbulent flow computations for a diffusing S-duct with/without vortex generators
Doctor of Philosophy, Case Western Reserve University, 1993, Mechanical Engineering
Numerical investigations on a diffusing S-duct with/without vortex generators and a straight duct with vortex generators are presented. The investigation consists of solving the full three-dimensional unsteady compressible mass averaged Navier-Stokes equations. An implicit finite volume lower-upper time marching code (RPLUS3D) has been employed and modified. A three-dimensional Baldwin-Lomax turbulence model has been modified in conjunction with the flow physics. A model for the analysis of vortex generators in a fully viscous subsonic internal flow is evaluated. A vortical structure for modelling the shed vortex is used as a source term in the computation domain. The injected vortex paths in the straight duct are compared with the analysis by two kinds of prediction models. The flow structure by the vortex generators are investigated along the duct. Computed results of the flow in a circular diffusing S-duct provide an understanding of the flow structure within a typical engine inlet system. These are compared with the experimental wall static-pressure, static- and total-pressure field, and secondary velocity profiles. Additionally, boundary layer thickness, skin friction values, and velocity profiles in wall coordinates are presented. In order to investigate the e ffect of vortex generators, various vortex strengths are examined in this study. The total-pressure recovery and distortion coefficients are obtained at the exit of the S-duct. The numerical results clearly depict the interaction between the low velocity flow by the flow separation and the injected vortices.

Committee:

Isaac Greber (Advisor)

Subjects:

Engineering, Aerospace

Keywords:

dimensional compressible turbulent flow computations diffusing S-duct vortex generators

Godavarty, DineshEffects of Perturbations on the Onset of Vortex Asymmetry
MS, University of Cincinnati, 2001, Engineering : Aerospace Engineering
An implicit upwind symmetric factorization finite volume solver was used to compute asymmetric vortices about a 5° half-angle cone at a 20° angle of attack. Perturbations were placed at a variety of spatial locations and various analysis methods were developed to track these perturbations in space and time. Although the saddle points seem to play an important role in the development of asymmetry, the separation point/region seems to be most influential for asymmetry.

Committee:

Dr. Paul Orkwis (Advisor)

Subjects:

Engineering, Aerospace

Keywords:

Computational Fluid Dynamics; Vortex Asymmetry; Conical Flow

Beer, Christopher PatrickAnalysis of Velocity Data to Determine the Structure of Tornado-like Vortices
Master of Science, Miami University, 2006, Physics
The goal of this thesis is to provide a detailed examination of the vertical and tangential velocity profiles within the supercritical core of a vortex for a wide range of swirl ratios and surface roughness. This is possible due to the development of an innovative analytical procedure and its application to high quality experimental data that was previously obtained in the Miami University Tornado Vortex Chamber.

Committee:

Christopher Church (Advisor)

Subjects:

Physics, Atmospheric Science

Keywords:

Tornado; Vortex; Laboratory Modeling

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

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

Huang, XinGeneration of Versatile Vortex Linear Light Bullets
Master of Science (M.S.), University of Dayton, 2015, Electro-Optics
We demonstrate a versatile vortex linear light bullet as a three-dimensional vortex Airy-Bessel wave packet for the first time. It combines a temporal Airy pulse with a higher order vortex Bessel beam in the spatial domain. Its non-varying feature in linear propagation is verified by three-dimensional (3D) measurements. Advanced from previously reported linear light bullets which are limited to specific materials, the vortex Airy-Bessel wave packet works as a light bullet for any material while carrying orbital angular momentum. It is believed that the versatile vortex linear light bullet is useful in many applications such as nano-lithography, nano-surgery, etc. The study of controlled collisions between optical vortices in the time domain is also reported in this thesis.

Committee:

Andy Chong (Advisor); Joseph Haus (Committee Member); Imad Agha (Committee Member)

Subjects:

Optics; Physics

Keywords:

Ultrafast optics; Airy pulses; Bessel beams; Vortex

Cornwell, MichaelCauses of Combustion Instabilities with Passive and Active Methods of Control for practical application to Gas Turbine Engines
PhD, University of Cincinnati, 2011, Engineering and Applied Science: Aerospace Engineering
Combustion at high pressure in applications such as rocket engines and gas turbine engines commonly experience destructive combustion instabilities. These instabilities results from interactions between combustion heat release, fluid mechanics and acoustics. This research explores the significant affect of unstable fluid mechanics processes in augmenting unstable periodic combustion heat release. The frequency of the unstable heat release may shift to match one of the combustors natural acoustic frequencies which then can result in significant energy exchange from chemical to acoustic energy resulting in thermoacoustic instability. The mechanisms of the fluid mechanics in coupling combustion to acoustics are very broad with many varying mechanisms explained in detail in the first chapter. Significant effort is made in understanding these mechanisms in this research in order to find commonalities, useful for mitigating multiple instability mechanisms. The complexity of combustion instabilities makes mitigation of combustion instabilities very difficult as few mitigation methods have historically proven to be very effective for broad ranges of combustion instabilities. This research identifies turbulence intensity near the forward stagnation point and movement of the forward stagnation point as a common link in what would otherwise appear to be very different instabilities. The most common method of stabilization of both premixed and diffusion flame combustion is through the introduction of swirl. Reverse flow along the centerline is introduced to transport heat and chemically active combustion products back upstream to sustain combustion. This research develops methods to suppress the movement of the forward stagnation point without suppressing the development of the vortex breakdown process which is critical to the transport of heat and reactive species necessary for flame stabilization. These methods are useful in suppressing the local turbulence at the forward stagnation point, limiting dissipation of heat and reactive species significantly improving stability. Combustion hardware is developed and tested to demonstrate the stability principles developed as part of this research. In order to more completely understand combustion instability a very unique method of combustion was researched where there are no discrete points of combustion initiation such as the forward stagnation point typical in many combustion systems including swirl and jet wake stabilized combustion. This class of combustion which has empirical evidence of great stability and efficient combustion with low CO, NOx and UHC emissions is described as high oxidization temperature distributed combustion. This mechanism of combustion is shown to be stable largely because there are no stagnations points susceptible to fluid mechanic perturbations. The final topic of research is active combustion control by fuel modulation. This may be the only practical method of controlling most instabilities with a single technique. As there are many papers reporting active combustion control algorithms this research focused on the complexities of the physics of fuel modulation at frequencies up to 1000 Hz with proportionally controlled flow amplitude. This research into the physics of high speed fluid movement, oscillation mechanical mechanisms and electromagnetics are demonstrated by development and testing of a High Speed Latching Oscillator Valve.

Committee:

Ephraim Gutmark, PhDDSc (Committee Chair); Shaaban Abdallah, PhD (Committee Member); Jeffrey Kastner, PhD (Committee Member); Prem Khosla, PhD (Committee Member)

Subjects:

Aerospace Materials

Keywords:

Combustion Instability;Thermoacoustic Instability;Active Combustion Control;Flameless Combustion;Swirl Stabilized Combustion;Vortex Breakdown

Next Page