200 research outputs found
Launch Vehicle Performance Enhancement Using Aerodynamic Assist
No full textA complete preliminary design model of a three-stage solid-fuel launch vehicle
has been combined with a genetic algorithm to perform an optimization study with the
goal to enhance performance using aerodynamic assist. Three studies have been
completed which include improving the suborbital and orbital flights of a modern
intercontinental ballistic missile and improving on the orbital flight of a generic three-stage
launch vehicle. The performance is enhanced by varying the geometric definition of
the attached wing structure and the internal propellant geometry while the external
geometry remains constant. Initial system weights and propellant mass fractions were
found to decrease with the addition of the wing structure. Further enhancement involves a
payload increase by 10% with a negligible increase in system weight, and a propellant
mass decrease by 5.2% without impairing the flight performance
Tomographic Plenoptic Background Oriented Schlieren Imaging
No full textWithin the last decade, tomographic background oriented schlieren has become an advanced flow diagnostic used to reconstruct three-dimensional density and/or refractive index fields. These three-dimensional measurements have been used to characterize a wide range of applications that contain flow features varying in both shape and length scale. This dissertation presents the development of the tomographic plenoptic BOS implementation, which was used to: (1) perform a systematic study assessing the reconstruction of multiple flow features and how their interaction affects the final solution and (2) highlight the implications of using plenoptic cameras in a tomographic BOS setting. The experiments outlined in this work used four plenoptic cameras surrounding an octagonal tank facility. Solid, transparent objects were submerged in a nearly refractive index matched solution in order to create a small refractive index difference. These semi-rigid objects provided the ability to systematically change an array of desired variables.
The tomographic plenoptic BOS implementation was successfully developed and tested using both ad hoc phantoms and experimental data sets. The use of such phantoms in implementation testing not only provided validation of implementation performance, but it also provided insight on achievable resolution of the experimental measurement system. The experimental data sets showed that: (1) two features will be individually resolved as long as measurements from a single camera are able to observe feature separation, (2) the error in the solution increases as the size of the feature decreases as a result of spatial resolution, and (3) the use of volumetric masking in the implementation is critical in order to achieve an accurate solution. It was also determined that the limited angular information collected by a single plenoptic camera does not replace the need to acquire measurements across a large angular range. The benefit to using plenoptic cameras stems from the ability to generate multiple views from a single camera, where there is potential for hardware reduction in future tomographic experiments
Three Dimensional Flow Visualization of the Adverse Pressure Gradient Turbulent Boundary Layer
No full textAn adverse pressure gradient (APG) turbulent boundary layer is investigated using a three dimensional scanning flow visualization technique. This dissertation includes an introduction to the structures and past research of the zero pressure gradient (ZPG) and APG turbulent boundary layer, as well as a description of three experiments to investigate the turbulent boundary layer.
The first experiment of this work uses simultaneous 2-D particle image velocimetry (PIV) to complement 3-D flow visualization measurements in a turbulent boundary layer to compare the boundary layer edge. Building off of the first experiment, a separate experiment was performed to compare 2-D velocity with 2-D flow visualization. The edge of the boundary layer identified from flow visualization generally matches the edge determined from velocity and vorticity. The correlation between velocity deficit and smoke intensity indicated a moderate to strong relationship between the two. In many cases, velocity fields estimated from smoke intensity were similar to the actual velocity fields. While not suitable for estimating the velocity given an intensity field, the correlations validate the use of flow visualization techniques for determining the edge and large-scale shape of a turbulent boundary layer, specifically when quantitative velocity measurements are not possible.
The third experiment described in this dissertation consists of using a scanning 3-D flow visualization technique to compare the large scale features in a ZPG and APG boundary layer. In general, structures in a ZPG boundary layer are larger and spaced farther apart compared to the APG cases. Flow features identified through conditional averaging extend higher into the freestream flow for the APG cases and are more distinct and separated from the wall than in the ZPG case. The difference in the three cases is more pronounced with increasing wall-normal height
Embedded Imaging of Internal Shock Boundary Layer Interactions
No full textRamjets and scramjets continue to be a central developmental focus spurred by both offensive and defensive needs on the battlefield. The development of field-able versions of these supersonic and hypersonic air-breathing propulsion systems will rely on improving the collective physical understanding of inlet flow physics enabled by the continued development of improved non-intrusive measurement systems. Present experimental methods of characterizing these internal, shock-dominated flows largely rely on the use of pressure taps in practical, optically-inaccessible geometries. The present work offers a solution in the form of a miniature embedded camera system which is designed to perform non-intrusive measurements which is then applied to the measurement of wall-bounded, incident/reflecting shock boundary layer interactions.
The camera system developed in this work leverages recent developments in the field of board-mounted camera systems to produce a packaged camera with integrated illumination which is capable of performing oil flow visualization (OFV) and pressure-sensitive paint (PSP) experiments. The system was fielded in the University of Tennessee Space Institute Mach 4.0 Ludwieg Tube, obtaining direct optical measurements at the throat of a custom-built Busemann streamline-traced inlet model. The camera system obtained pressure and shear fields while surviving the harsh tunnel environment. Following this initial success, the Auburn University variable Mach number tunnel was modified by adding an extension to enable the main experimental effort. The data from these tests suggest that corner flow/shock interaction is primarily driven by the swept shock interaction on the sidewall rather than Free-Interaction Theory (FIT). PSP and OFV results indicate that the corner region interaction length increases with Mach number, and decreases with wedge angle and shock strength. This trend is explained by comparison to the swept shock upstream interaction measurements of Settles and Lu. An interaction length scaling is proposed based on this work which acts synergistically with FIT to produce the scaled interaction
Development of an Actively-Cooled Imaging System for Embedded Optical Measurements in High-Speed Flows
No full textHigh speed airbreathing propulsion systems have become increasingly complex with the development of hypersonic scramjet engines. Analysis of scramjet flow structures is difficult due to extreme thermal environments and limited optical access to internal flow paths. This study introduces an innovative actively-cooled miniature imaging system designed to broaden the applications of low-cost, small form factor image sensors for integration into extreme thermal environments, facilitating the observation of high-enthalpy flows.
The developed system enables non-intrusive optical flow field measurements in confined flows where traditional imaging systems have limited optical access. The design incorporates a cooled image sensor housing that actively cools the image sensor and lens assembly by immersing them in a dielectric cooling liquid, Fluorinert FC-72. The internal geometry of the housing facilitates the flow of the dielectric coolant, effectively regulating the thermal environment around the entire sensor package. A thermal management system regulates the thermal environment inside of the enclosed housing to protect the image sensor. A closed-loop cooling system continuously supplies fresh coolant to regulate hardware temperatures.
Experimental analysis demonstrated the actively-cooled miniature imaging system's capability to maintain the thermal environment of an image sensor at 100◦F during extended exposure to 1,450◦F, simulating a high-temperature flow path. This validates the system's ability to capture images in high-temperature supersonic flows, laying the groundwork for potential applications in high-enthalpy supersonic test facilities. Implementation of optical bandpass filters in the system design allows the system to capture chemiluminescence emissions produced by radicals in a methane flame, verifying the system's ability to generate spectrally-resolved images consistent with traditional image-based diagnostics of reacting flows.
This actively-cooled miniature imaging system offers a novel solution for internal flow measurements in high enthalpy flows, employing dielectric immersion cooling and cost-effective hardware components to observe regions that have been traditionally challenging to observe and measure. With further development, this system has the potential to be adapted as an inexpensive, lightweight, economical tool to obtain images during flight experiments in demanding thermal environments
Three-Dimensional Velocity Measurements in the Wake of a Hemispherical Roughness Element Using Plenoptic Particle Image Velocimetry
No full textPlenoptic particle image velocimetry (PIV) was used to perform instantaneous three-dimensional (3D) velocity measurements in the near-wake of a wall-mounted hemispherical roughness element at a Reynolds number (based on roughness height) of 4.57 X 10^3 and boundary layer to roughness height-ratio of 4.67. The experiment was performed in a refractive index matched flow facility to mitigate laser reflections from the hemispherical surface. Data gathered from this experiment represents one of the first applications of plenoptic PIV. The time-average flow is characterized by a separated shear layer off of the hemisphere, a symmetric recirculation region, and an arch-shaped vortex. In the instantaneous 3D velocity fields, a separated boundary layer and recirculation region with asymmetric characteristics are present. Additionally, arch vortices are found that are both attached and detached to the hemispherical surface, similar to previously studied recirculation arch (RA) vortices. The proper orthogonal decomposition (POD) was applied to both the 3D velocity and 3D vorticity fields. Velocity modes produced features associated with the overall flow whereas vorticity modes produced features associated with the wake. The most energetic POD modes confirm the fluctuations in the boundary layer and recirculation region, as well as suggest the existence of shed arch-shaped vortices
Vortex Topology of a Pitching and Rolling Wing in Forward Flight
No full textVortex topology is analyzed from measurements of flow over a flat, rectangular plate with an aspect ratio of 2 which was articulated in pitch and roll, individually and simultaneously. The plate was immersed into a Re = 10,000 flow (based on chord length) to provide forward flight component of the study. Measurements were made using a 3D-3C plenoptic PIV system to allow for the study of complete vortex topology of the entire wing. The prominent focus is the early development of the leading-edge vortex (LEV) and resulting topology. The effect of the wing kinematics on the topology was explored through a parameter space involving multiple values of pitch rate and roll rate at pitch and roll angles up to 50°. Characterization and comparisons across the expansive data set are made possible through the use of a newly defined dimensionless parameter, k_rg. Termed the effective reduced pitch rate, k_rg is a measure of the pitch rate that considers the relative rolling motion of the wing in addition to the pitching motion and freestream velocity. The study has found the addition of a rolling motion to a pitching wing removes the symmetries in the vortical structures, delays vortex evolution, and inhibits the extent of detachment of the LEV. Additionally, it was found that increasing the k_rg parameter accelerates the evolution of the LEV, from formation to detachment, as well as advances the evolution of the LEV in nondimensionalized time
On the Development of a Volumetric Velocimetry Technique using Multiple Plenoptic Cameras
No full textPlenoptic PIV was recently introduced as a viable three-dimensional, three-component velocimetry technique based on light field cameras. One of the main benefits of this technique is its single camera configuration allowing the technique to be applied in facilities with limited optical access. The main drawback of this configuration is decreased accuracy in the out-of-plane dimension. This dissertation presents a solution with the addition of a second plenoptic camera in a stereo-like configuration. A framework for reconstructing volumes with multiple plenoptic cameras including the volumetric calibration and reconstruction algorithms are presented. It is shown that the addition of a second camera doubles the reconstruction quality and removes the `cigar'-like elongation associated with the single camera system. In addition, it was found that adding a third camera provided minimal benefit for the reconstruction quality of sparse particle fields. Further metrics of the reconstruction quality are quantified in terms of particle density, number of cameras, camera separation angle, voxel size, and the effect of common image noise sources. In addition, a synthetic Gaussian ring vortex is used to compare the accuracy of the single and two camera configurations. It was determined that the addition of a second camera reduces the RMSE velocity error from 0.85 to 0.23 voxels. Finally, the technique is applied experimentally on a ring vortex and comparisons are drawn from the four presented reconstruction algorithms.
The trade-off between spatial and angular resolution is the main consideration when designing a plenoptic camera. This dissertation provides guidelines for the selection of the microlens array using theoretical analysis as well as synthetic and experimental data for validation. It was determined that the optimal selection of the microlens size depends heavily on the desired volume depth and a good rule-of-thumb is the span of the volume should be ~1.1 DoFp (single pixel, or perspective, depth-of-field). It was also determined that while this is the optimal selection, the robustness of the cross-correlation algorithm mitigates the effect of sub-optimal microlens selection allowing for a single configuration to be used in a wide variety of situations
Development of a High Dynamic Velocity Range Processing Scheme for Time-Resolved Particle Image Velocimetry Measurements
No full textThe development, validation, and demonstration of a novel computational procedure to supplementally process time-resolved particle image velocimetry (TR PIV) measurements are described. The motivation for such work stems from an experimental investigation to characterize the near-nozzle velocity field in a high-temperature, shock-containing jet. The computational procedure, termed dynamic evaluation via ordinary least squares (DEVOLS), offers substantial improvements over conventional processing methods for its ability to increase the dynamic velocity range. Unique to DEVOLS is an iterative validation scheme that enables a variable number of displacement results to be utilized in the determination of a single velocity vector. This approach is significant since it provides enhanced robustness in the presence of significant image noise. Validation of the procedure is provided through the use of temporally resolved, synthetically generated particle images simulating the fluid dynamics of a Hamel-Oseen vortex. Following such validation, the experimental investigation is described wherein TR PIV measurements were acquired for a flow field centered axially at the end of the jet potential core and radially along the lower half of the shear layer. For all cases the nozzle was operated at over-expanded conditions, and images were acquired through the combined use of a pulse burst laser and a high-speed, gated intensified CCD framing camera. Results achieved by the DEVOLS processing scheme are presented for both the experimental jet as well as a synthetic jet derived from computational fluid dynamics. Estimations of the measurement errors associated with these results are also given. Finally, steps for improving the quality of the experimental data as well as the analysis procedure are offered as suggestions for future investigations
Study on the aerodynamic interactions of a coaxial rotor hovering in-ground effect
No full textCounter-rotating coaxial rotors (CCR) offer notable advantages over conventional single-rotor systems in flight speed, payload capacity, and maneuverability. As a result, CCR configurations are being considered for future vertical flight applications ranging from urban air mobility to space exploration. However, operating two rotors in proximity on a shared axis results in complex aerodynamic interactions between the rotors that significantly influence individual rotor performance. Understanding these interactions is critical to developing safer and more efficient aircraft. This thesis aims to quantify the aerodynamic rotor-rotor interactions in a CCR operating in-ground effect (IGE), where an additional effect, the rotor-ground interaction, competes with the rotor-rotor interactions in driving the performance characteristics. An experimental facility consisting of two mechanically decoupled rotors with a height-adjustable ground plane was developed. Load measurements were collected to assess the rotor performance. They were coupled with a momentum theory-based theoretical framework to develop empirical correction factors that quantify how these interactions influence the induced power required to hover. The results indicated that in IGE conditions, the rotor-ground interaction dominates rotor performance, leading to significant performance improvements in each rotor. However, rotor-rotor interactions were modified by the presence of the ground, varying by up to approximately 8% in certain conditions, suggesting a complex coupling between rotor-rotor and rotor-ground interactions. A secondary study investigating the effects of Reynolds number on these interactions revealed that the lower rotor interactions were altered. In contrast, the upper rotor interactions were invariant to the Reynolds number, suggesting a discrepancy in the wake characteristics of the upper rotor as it impinges on the lower rotor. This thesis introduces a formulation that isolates the rotor-rotor and rotor-ground interactions of a CCR operating IGE. It provides a fundamental understanding of how ground proximity impacts the power requirements for a hovering CCR and establishes a framework for decomposing the complex aerodynamic interactions between the rotors and the ground
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