Aerospace Engineering Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2737

Browse

Search Results

Now showing 1 - 10 of 16

EXPERIMENTAL INVESTIGATION OF BOUNDARY LAYER TRANSITION ON CONE-FLARE GEOMETRIES AT MACH 4
(2024) Norris, Gavin; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This study investigates supersonic boundary layer transition on a cone-flarewith a 5° half-angle straight cone and flared bases of +5°, +10°, and +15°. The experiments used the University of Maryland's Multiphase Flow Investigations Tunnel (MIST), a Mach 4 Ludweig tube. Experiments were performed “dry”, without aerosols or droplets, and focus on the first-mode (Tollmien-Schlichting) boundary layer instability waves and their interaction with the compression corner. Using high-speed Schlieren imaging, the boundary layer dynamics on the cone-flare's top surface were analyzed. The data were processed through Power Spectral Density (PSD) and Spectral Proper Orthogonal Decomposition (SPOD) techniques to study the behavior of the first-mode waves and the transition location changes. The findings reveal coherent wave packets within the boundary layer at frequencies characteristic of the first-mode. The wave packets power increased along the cone and peaked near the compression corner before dissipation on the flare. These findings contribute to the understanding of first-mode boundary layer transition mechanisms in hypersonic flows for the cone-flare geometry.
AEROSOL EFFECTS IN HIGH SUPERSONIC FLOWS
(2024) Schoneich, Antonio Giovanni; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The understanding of high-speed aerodynamics is becoming evermore pertinent with thegrowth of space tourism, continued interest in space exploration, and pursuit of advanced highspeed aircraft for both military and commercial use. For initial investigations, ground test facilities are preferred to flight tests as they are far cheaper and carry significantly less risk, although wind tunnels can only replicate a subset of the conditions experienced in actual flight. One of these conditions that has not been adequately captured in wind tunnels is the effect of particulates in the atmosphere. Typical wind tunnels use a pure, clean gas (air, nitrogen, etc.) for testing, but this does notcapture the aerosolized nature of the atmosphere, where humidity and condensation can produce a distribution of liquid droplet sizes ranging from the average rain drop of 2mm to sub-micron diameter particles. Similarly, volcanic eruptions and ever-present wildfires result in solid particles exhibiting a variety of species and sizes that are transported to every layer of the atmosphere. At supersonic speeds, encounters with particulates have been shown to lead to detrimental effects, such as material erosion and boundary layer transition. Previous attempts to study this problem in wind tunnels have focused mainly on sub-micronsized solid particles, since aerosol settling time is a major limiting factor. On the other hand, most high-speed experiments involving large liquid droplet impacts have been carried out in gas guns or ballistic ranges due to the difficulty of trying to accelerate a droplet to high speeds without causing it to break up. While these facilities can be used to study impacts, the moving model means that detailed aerodynamic studies are nearly impossible, leading to a large gap in knowledge. To perform high-speed wind tunnel testing with liquid aerosols representative of cloud-likeenvironments (5-20 μm), a Mach-4 facility, referred to as the Multi-phase Investigations Supersonic Tunnel (MIST) has been designed and developed at the University of Maryland (capable of producing supersonic, particle-laden flows). This range of aerosol sizes makes MIST a unique facility with significant potential for expanding the state of the art in high-speed multi-phase flows. The present work discusses the design and characterization of MIST as well as two major experimental investigations carried out using this new facility. The first investigation examines the force augmentation on a free-flying sphere exposed to supersonic, particle-laden flows. Freeflight measurements are performed with five different particle size and concentration combinations. When comparing the results for particle-free flow in the same facility, the drag coefficient of the sphere was shown to be 1.75-4.5% greater for all multi-phase cases; this is significantly higher than simple estimates based on the increased momentum flux in the freestream would indicate. In addition to force measurements, an experimental investigation into the effect of particle-ladenflows on boundary-layer transition was conducted. It is important to characterize the disturbance environment in wind tunnels since they typically do not represent the levels in atmospheric flight and can lead to earlier onset of boundary-layer transition. In performing such measurements using a single-point Focused Laser Differential Interferometer, it was discovered that the presence of particles in the flow could significantly attenuate the acoustic disturbances generated by the wind tunnel. This finding was further reinforced when investigating the boundary-layer transition on a 5◦ half-angle, sharp cone using high-speed schlieren visualization. For each case presented in this work, the boundary-layer disturbance amplitudes were reduced and transition Reynolds numbers increased in the particle-laden flow cases. This was contrary to expectations, given that prior numerical studies have indicated that particles can induce early transition. These findings potentially open a path to substantially reduce freestream disturbance levels in conventional hypersonic wind tunnels.
PARTICLE INDUCED TRANSITION IN HIGH-SPEED BOUNDARY-LAYER FLOWS
(2024) Abdullah Al Hasnine, Sayed Mohammad; Brehm, Christoph; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Boundary-layer transition to turbulence presents a critical challenge in aerospace engineering due to its impact on thermal load, especially for hypersonic vehicles. This transition, influenced by various disturbances such as acoustic waves, entropy waves, and particle impingement, follows complex and non-unique pathways to turbulence. It significantly affects the surface heat flux and thus will impact the design of thermal protection systems. This dissertation focuses on the transition process initiated by particle impingement, which introduces small-scale disturbances through a complex receptivity process that typically initiates a natural transition path. Using direct numerical simulations, this study explores the particle-induced transition process. The disturbance spectrum, consisting of both stable and unstable modes along with continuous acoustic contributions, is meticulously reconstructed near the particle impingement site using biorthogonal decomposition to assess the contributions of different eigenmodes to the initial disturbance spectrum. A large number of discrete and continuous eigenmodes are seeded, but the dominant eigenmodes capture only a small fraction of the disturbance energy, with the majority reflected into the freestream through the continuous modes associated with the continuous acoustic branches. The modeling fidelity is also investigated, particularly the particle-source-in-cell (PSIC) approach, commonly used due to its efficiency in capturing particle-flow interactions. Comparisons with the Immersed-Boundary-Method (IBM), however, reveal that PSIC inadequately captures particle-wall interactions and needs correction for accurate disturbance modeling. Finally, a reduced-order model is developed for the prediction of particle-induced transition. This model integrates data from high-fidelity simulations, linear stability theory, and a saturation amplitude model while also considering particle characteristics like size, density and concentration. The model’s capability is demonstrated for a wide range of transition scenarios, including data from the HIFiRE-1 flight test, offering a robust tool for rapid transition prediction in hypersonicvehicle design.
GLOBAL ANALYSIS OF TRANSITIONAL HYPERSONIC FLOW OVER CONE AND CONE-FLARE GEOMETRIES
(2024) Sousa, Cole Edward; Laurence, Stuart; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Accurately predicting the laminar-to-turbulent boundary-layer transition on hypersonic vehiclesremains one of the principal challenges in characterizing the expected heat loads and skin friction the vehicle will experience in flight. Ground facilities, while incapable of replicating the complete set of flow conditions found at hypersonic flight, play a critical role in providing physical measurements of the transition process. The experimental characterization of hypersonic boundary-layer disturbances, however, has traditionally faced limitations in its ability to provide spatiotemporally dense data sets comparable to those of computational fluid dynamics (CFD) investigations. The present work aims to provide global off-body measurements of hypersonic boundary-layer disturbances at frequencies much greater than that of the fundamental instability, enabling the exploration of nonlinear phenomena and more extensive comparisons between experimental and computational studies. The current methodology utilizes the fact that hypersonic-boundary layer disturbances havebeen observed to propagate at measurable and statistically predictable velocities. Particularly for the second-mode instability, the density gradient fields acquired by a calibrated schlieren system provide an avenue for resolving dense high-frequency spatiotemporal data. Disturbance propagation velocities extracted from the schlieren images are used to conduct a time-interpolation of the disturbances, which transforms spatially-available descriptions of the travelling waveforms into up-sampled temporal signals at specific pixel locations. When performed across the entire schlieren field of view, the resulting time-resolved signals have a new sampling frequency much greater than the original camera frame rate and a spatial density equal to the camera resolution. This enables the spectral analysis of high-frequency disturbances, including superharmonics of the fundamental instability, which are not originally resolvable from raw time series of the video data. The methodology is employed here in three different experimental data sets, comprising a7° half-angle sharp cone at zero incidence in Mach 6 flow, a 7° half-angle sharp cone at variable incidence in Mach 14 flow, and a cone-flare geometry composed of a 5° frustum with compression angles of +5°, +10°, and +15° at zero incidence in Mach 14 flow. A comprehensive global analysis is conducted on the linear and nonlinear development of the second-mode instability waves in each case. Pointwise measures of the autobicoherence are used to identify specific triadic interactions and the locations of their highest levels of quadratic phase coupling. Significant resonance interactions between the second-mode fundamental and harmonic instabilities are found along with interactions between these and the mean flow. Bispectral mode decomposition is employed to educe the flow structures associated with these interactions. A similar analysis is performed for the power spectrum, with power spectral densities computed for each pixel’s timeseries and spectral proper orthogonal decomposition employed to derive the modal structure and energy of the flow at specific frequencies. The instability measurements taken on the cone-flare geometry are the first of their kind atMach 14. The analysis reveals that incoming second-mode waves undergo extended interactions with the shock waves present at the corner, consistently leading to amplification of the waves and accelerating their nonlinear activity. The disturbance energy is also found to strongly radiate along the shock waves, a behavior that appears to be intensified at high Mach numbers. In the case of separated flow at the corner, additional low-frequency disturbances arise along the shear layer. Self-resonance of these disturbances leads to the radiation of elongated structures upstream of reattachment, which extend outward from the shear layer and terminate at the separation shock. This shear-layer disturbance is determined to be dominantly unstable between separation and reattachment but is significantly damped after reattachment.
Physics and Modelling of Compressible Turbulent Boundary Layer
(2023) Lee, Hanju; Martin, Pino; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Key findings from a research study that focuses on understanding the effect of Mach number, Reynolds number and wall temperature on compressible turbulent boundary layers (CTBL) in the hypersonic regime are presented in this dissertation. The study utilizes a comprehensive CTBL database developed using an in-house direct numerical simulation (DNS) code at the CRoCCo laboratory. The database encompasses a range of semi-local Reynolds numbers (800 to 34,000) and Mach numbers up to 12, incorporating wall-cooling. The effects of density and viscosity fluctuations on the total stress balance are identified and used to create a new mean velocity transformation for compressible boundary layers. The role, significance and physical mechanisms connecting density and viscosity fluctuations to the momentum balance and to the viscous, turbulent and total stresses are presented, allowing the creation of generalized formulations. We identify the significant properties that thus-far have been neglected in the derivation of velocity transformations: (1) the Mach-invariance of the near-wall momentum balance for the generalized total stress, and (2) the Mach-invariance of the relative contributions from the generalized viscous and Reynolds stresses to the total stress. The proposed velocity transformation integrates both properties into a single transformation equation and successfully demonstrates a collapsing of all currently considered compressible cases onto the incompressible law of the wall, within the bounds of reported slope and intercept for incompressible data. Based on the physics embedded in the two scaling properties, the success of the newly proposed transformation is attributed to considering the effects of the viscous stress and turbulent stresses as well as mean and fluctuating density viscosity in a single transformation form. The Reynolds number trends of large turbulent structures in compressible turbulent boundary layers are investigated using the pre-multiplied energy spectra based on the density corrected fluctuating streamwise velocity signal. Results demonstrate the existence of friction as well as semi-local Reynolds number trend associated with large-scale structures, similar to trends observable in incompressible turbulent boundary layers (ITBL). In particular, the behavior of turbulence in the inner layer is seen to exhibit dependence based on both definitions of Reynolds numbers. On the contrary, the strength of large turbulent structures is seen to be only dependent on friction Reynolds number. This result directly contrasts with the observation of the near-wall turbulent intensity peak increasing with semi-local Reynolds number. The discrepancy is mended with a suggestion that the large turbulent scales in the log layer of which the strength increases with friction Reynolds number, are modified through the changes in local fluid properties such that the scale interaction near the wall increases as semi-local Reynolds number. In another words, closer to the wall, the CTBL flow behaves like a semi-local Reynolds number flow, while closer to the freestream, it behaves like a friction Reynolds number flow. Furthermore, the present study examines the Reynolds number dependence of the length scale between small and large turbulent scales. The analysis highlights the inadequacy of using a univariable wavelength based on viscous, semi-local or outer length scales to differentiate small and large scales. Based on this, the use of Reynolds number-dependent length scales is recommended. Overall, the study provides valuable insights into the Reynolds number trends of large turbulent structures in CTBL, emphasizing the influence of both semi-local Reynolds number and friction Reynolds number on turbulence characteristics.
Mitigation of transverse gusts via open- and closed-loop pitching maneuvers
(2022) Sedky, Girguis; Jones, Anya R.; Lagor, Francis D.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Unsteady flow conditions present significant challenges to stable flight, and gust rejectionremains a concern for flight control in many modern flight environments. Examples of gustdominated flight conditions include flight in stormy conditions, aircraft takeoff and landing in strong crosswinds or ship air wakes, and micro air vehicles in strong shear flow engendered by urban settings and complex terrain. Improving flight stability during gust encounters relies on an improved understanding of the flow physics and the development of effective mitigation control strategies. To this end, the present work seeks to (1) improve our understanding of the unsteady flow physics of a pitching wing encountering a transverse gust and (2) develop and characterize successful open- and closed-loop control strategies to mitigate aerodynamic lift transients induced by the gust using wing pitching input. Classic unsteady aerodynamic theory was used to construct the open-loop pitch maneuvers and tune the closed-loop controller for closed-loop control. The dynamical systems treatment of the problem during control design revealed several important physical features important to vehicle control. Two sets of wing-gust encounter experiments were conducted using a flat-plate wing model in a water towing tank. The transverse gust was generated in the center of the towing tank using a recirculating water jet. Data was acquired using a combination of Particle Image Velocimetry (PIV), force, and torque measurements. In the first set of experiments, the constructed openloop pitch maneuvers were implemented as open-loop kinematics in the water towing tank. This study revealed several findings regarding the change in the flow topology due to pitch actuation, the necessity of modeling added mass for open-loop pitch maneuver construction, and the increase in the pitching moment transients due pitch control. This study also demonstrated how lift-mitigating pitching maneuvers minimized the disturbance to the gust’s flow field, thereby reducing the momentum exchange between the gust and the wing. The second set of experiments implemented a proportional control strategy based on classic unsteady aerodynamic theory using a pitch acceleration input and real-time force measurements. The closed-loop control experiments spanned upwards and downwards gusts of various strengths and lift tracking at pre- and post-stall angles of attack. The controller yielded an average rejection performance of 80% without a priori knowledge of gust strength or onset time and for various aerodynamic conditions. Reasons for the controller’s success include using lift measurements directly in control feedback, aerodynamic models that capture the salient physics in the control design process, and wing pitching as input. Simultaneous time-resolved PIV and force measurements were used to discover and understand the flow physics underlying the lift transients and how applying closed-loop control mitigated those transients.
Experimental Investigation of Force Transients during Gust Encounters
(2021) Biler, Hulya; Jones, Anya; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The small size and low speed of Micro Air Vehicles make them vulnerable to wind gusts such that sustaining controlled flight becomes a challenge in the unsteady environments. A better understanding of the gust flow is crucial to develop models capable of predicting unsteady forcing. Therefore, this study aims to improve understanding of fundamental flow physics behind the unsteady force production during gust encounters. The bulk of the results presented here were obtained experimentally for a flat plate passing through a transverse gust created in a water towing tank and found to have a sine-squared velocity profile. The effects of 4 different parameters, namely angle of attack, gust ratio, effective angle of attack, and aspect ratio, were explored. A wide investigation range was used for each parameter. The forces increased significantly from their steady-state values during the encounter. 2D flowfields showed the formation and shedding of vortices from the leading and trailing edges of the wing. The flow was found to stay nominally two-dimensional until the forces peak. Only thereafter, spanwise variations were observed in the 3D flowfields. The accuracy and limitations of Kussner's model were evaluated by comparing the sine-squared and top-hat transverse gusts (the latter experiments performed by collaborators). The gradients in the transverse velocity profile were found to significantly affect the force response such that sharper gradients cause higher nonlinearities. Kussner's model was found to provide accurate predictions for the sine-squared gust even when the flow is highly nonlinear, whereas it failed to do so for the top-hat gust. A momentum flux-based normalization was proposed and found to scale the responses of different velocity profiles as long as the response can be predicted by Kussner's model. The effect of gust type on the unsteady forcing was examined by comparing the sine-squared transverse and vortex gusts (the latter experiments performed by collaborators). The results showed that both gust encounters result in large transients in the lift. The increase in the lift force and the leading-edge vortex strength for the transverse gust was found to be steeper than the vortex gust. A flowfield-based force prediction method was proposed and found to be effective for low-to-moderate effective angles of attack.
AERODYNAMIC SEPARATION OF FRAGMENTED BODIES IN HIGH-SPEED FLOW
(2021) Whalen, Thomas James; Laurence, Stuart; Brehm, Christoph; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Atmospheric entry of meteoroids poses danger to humans in the form of blast-wave overpressure, impact craters, tsunamis, and other assorted threats. The relative risks of each are highly dependent on the details of the unavoidable structural disruption that occurs and the subsequent aerodynamic separation sequence, so accurate prediction of fragment trajectories is required for threat mitigation. However, the physics of aerodynamic separation immediately following meteor fragmentation are virtually uncharacterized, allowing for only low confidence in threat assessment projections. The present work endeavors to constrain the separation behavior of fragmenting bodies by examining the model problem of close-packed sphere clusters and, to a lesser extent, clouds of dusty debris. Free-flight experimentation in UMD HyperTERP, a Mach-6 shock tunnel, is conducted to provide a foundation for both statistical and aerodynamic analyses, while coupled inviscid CFD/FEA provides complementary insight into the mechanisms driving fragment separation. First, computations of equal-sized sphere pairs reveal a previously unidentified phenomenon wherein two bodies in continual mechanical contact oscillate about a stable angle-of-attack equilibrium and achieve anomalously high lateral velocities. Proceeding to higher cluster populations, separation procedure can be divided into two stages: mutual repulsion from a common center and subsequent subcluster interactions dictated by the influence of an upstream body. The degree of repulsion induced by the former demonstrates close correlation with the initial angular position of a fragment, whereas the lateral velocities resulting from the latter appear to be normally distributed about a slightly positive value. The transverse separation characteristics of equal-sphere clusters numbering from 2 to 115 bodies are used to constrain a power-law fit between the lateral extent of a disrupted swarm and its population, providing a significant improvement to existing models of aerodynamic separation following fragmentation. Furthermore, experiments of unequal-sphere clusters, whose compositions are governed by realizations of truncated power laws, reveal a systematic underestimate in the equal-sphere correlation, resulting largely from massive subclusters suppressing high expulsion. A unified model of fragment separation, based on both the aforementioned power-law fit and a combined Rayleigh—exponential distribution, is then proposed. Finally, the dynamics of dusty debris clouds are discussed, with implications for mass depletion and energy deposition of rubble-pile-type impactors highlighted.
Cylinder-Airfoil Interactions and the Effect on Airfoil Performance
(2021) Lefebvre, Jonathan; Jones, Anya; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
From micro air vehicles flying in the wake of buildings to aircraft operating in ship airwakes, turbulent flows generate unsteady aerodynamic loads on airfoils that may promote structural failure, loss of flight control, and produce noise radiation. In order to develop engineering solutions capable of mitigating these effects, accurate force prediction of airfoils encountering turbulent wakes is necessary. A barrier to such force prediction techniques is the lack of a fundamental understanding of the aerodynamics of wake-airfoil interactions. The goal of this work is to investigate the cylinder-airfoil configuration by quantifying the effect of cylinder wake turbulence on airfoil force production and identifying the underlying flow physics. Results were obtained from both wind tunnel experiments and numerical simulations using a NASA OVERFLOW solver. Four cylinder-airfoil configuration parameters were evaluated: the gap G/D and offset z/D distances between the cylinder and airfoil, the cylinder-diameter-to-airfoil-chord ratio D/c, and the cylinder cross-sectional geometry. During the investigation of each parameter, the airfoil angle of attack varied from α= -5 to 40 while the Reynolds number based on the airfoil chord c was fixed at Rec =1×10^5. Flow characterization of the region between the cylinder and airfoil revealed that the airfoil encounters a highly unsteady inflow. Turbulence intensity reaches 55% of the freestream velocity upstream of the airfoil's leading edge while the flow oscillates at the cylinder vortex shedding frequency. The influence of the upstream cylinder wake on airfoil performance was quantified by time-averaged force measurements and showed three modifications compared to a clean inflow: (1) lift augmentation, (2) negative drag or thrust, and (3) delay in stall. The unsteady airfoil behavior was also investigated, showing that the amplitude of unsteady airloads increases for small gap and offset distances, while the airfoil frequency response matches the cylinder vortex shedding frequency. Flowfield measurements show that the cylinder-airfoil interaction induces flow separation at the leading edge of the airfoil, generating a leading edge vortex (LEV). The LEV is identified as the main flow structure responsible for modifying airfoil performance as it provides lift enhancement and delays stall at large angles of attack, while at low angles of attack the LEV promotes reverse flow at the surface, contributing to negative drag. The results and analysis from this work advance the fundamental flow physics of the cylinder-airfoil interactions by revealing key flow structures responsible for the unsteady force production on an airfoil in the wake of a cylinder.
DIRECT AND LARGE-EDDY SIMULATION AND ANALYSIS OF SHOCK-SEPARATED FLOWS
(2021) Helm, Clara Marie; Martin, Pino; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The in-house CRoCCo code is used to generate a database of high-fidelity direct numerical simulation (DNS) and large eddy simulation (LES) data of shock wave and turbulent boundary layer interactions (STBLI) at supersonic to hypersonic conditions. The DNS data is employed in the validation of the LES method and the assessment of the sub-grid-scale (SGS) models in application to the STBLI flow problem. It is determined that, under hypersonic conditions, a scale similar model term in both the shear stress and heat transfers SGS terms is necessary to produce the correct STBLI separation flow. The use of the dynamic eddy viscosity term alone produced as much as 30% error in separation length. The high grid-resolving efficiency (equivalently the practicality over the DNS) of the CRoCCo code LES method for the simulation of STBLI flows is also demonstrated with a typical reduction of 95% grid size and 67% in number of time steps as compared to the DNS, a feature that makes spectral convergence of the STBLI low-frequency cycle feasible. The thorough documentation of DNS-validated, high-fidelity LES solutions of hypersonic STBLI flows is a unique contribution of this work. Thanks to the detail in the turbulence data afforded by the LES, an extensive and novel characterization of the separation shear layer in the STBLI flows is possible and the results are related to compressible mixing layer theory. In addition, visualizations of the numerical data show the form of the inviscid instability in hypersonic shock-separated flows. These visualizations combined with the extended CRoCCo Lab numerical database provide significant insight into the nature of the separation length scaling in STBLI at hypersonic Mach numbers.

Aerospace Engineering Theses and Dissertations

Browse

Filters

Settings

Sort By

Results per page

Search Results