A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Development and Application of Mach 10 PIV in a Large Scale Wind Tunnel(2018) Brooks, Jonathan; Gupta, Ashwani K; Marineau, Eric C; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents the development of particle image velocimetry (PIV) for use in a large-scale hypersonic wind tunnel to measure the turbulent boundary layer (TBL) and shock turbulent boundary layer interaction (STBLI) on a large hollow cylinder flare (HCF) test article. The main feature of this application of PIV is the novel local injector which injects seeding particles into the high-speed section of the flow. Development work began sub-scale in a Mach 3 wind tunnel where the seeding particle response was characterized and the local injectors were demonstrated. Once the measurement technique was refined, it was scaled up to hypersonic flow. The particle response was characterized through PIV measurements of Mach 3 TBLs under low Reynolds number conditions, $ Re_\tau=200{-}1,000 $. Effects of Reynolds number, particle response and boundary layer thickness were evaluated separately from facility specific experimental apparatus or methods. Prior to the current study, no detailed experimental study characterizing the effect of Stokes number on attenuating wall normal fluctuating velocities has been performed. Also, particle lag and spatial resolution are shown to act as low pass filters on the fluctuating velocity power spectral densities which limit the measurable energy content. High-speed local seeding particle injection has been demonstrated successfully for the first time. Prior to these measurements, PIV applications have employed global seeding or local seeding in the subsonic portion of the nozzle. The high-speed local seeding injectors accelerate the particle aerosol through a converging/diverging supersonic nozzle which exits tangentially to the wall. Two methods are used to measure the particle concentration which shows good agreement to the CFD particle tracking codes used to design the injector nozzle profiles. Based on the particle concentration distribution in the boundary layer a new phenomenon of particle biasing has been identified and characterized. PIV measurements of a Mach 10 TBL and STBLI have been performed on a large (2.4-m long, 0.23-m dia.) HCF at a freestream unit Reynolds number of 16 million per meter. These are the highest Mach number PIV measurements reported in the literature. Particles are locally injected from the leading edge of the test article and turbulent mixing dispersed the particles for a relatively uniform high concentration of particles at the measurement section 1.83-m downstream of the leading edge. The van Driest transformed mean velocity in the TBL agrees well with incompressible zero pressure gradient log law theory. Morkovin-scaled streamwise velocity fluctuations agree well with the literature for the majority of the boundary layer.Item PARTICLE CHARGING EFFECTS ON PIV MEASUREMENTS OF PLASMA ACTUATORS(2017) Masati, Arber; Sedwick, Raymond; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Plasma actuators gained popularity during the change of the millennium for their ability to induce airflow and reattach stalled flow. Since then, investigations have been conducted to characterize their performance and extend their applications. Plasma actuators can be used to increase lift, decrease drag, and increase the efficiency of wind turbines. Phase locked ensemble averaging particle image velocimetry (PIV) is used to determine the induced velocity field and characterize actuator behavior and performance. However, very few studies account for particle charging from dusty plasma theory. Particle charging theories for high pressure plasmas predict a mostly linear trend between charge and particle size, with smaller particles charging less. In this work, PIV experiments were conducted with monodisperse nanoparticles for sizes ranging between 300 nm and 1250 nm. Results showed that smaller particles follow the flow more closely. PIV uncertainty quantification was performed for ensemble averaging processing. A weighted linear fit was applied to each vector and extrapolated to the 0 nm particle speed, which is taken as the true air speed. Stokes drag force fields were calculated using the known velocity difference, and using a force balance calculation the electrostatic force acting on the particles was calculated. The electrostatic force near the actuator electrode was always acting upstream, implying that particles can attain either negative or positive charge, depending on the phase.Item PARTICLE-TURBULENCE INTERACTION OF SUSPENDED LOAD BY A FORCED JET IMPINGING ON A MOBILE SEDIMENT BED(2014) Mulinti, Rahul; Kiger, Ken; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Phase-resolved two-phase flow experiments have been conducted to predict particle suspension and sedimentation within coupled particle-laden flows relevant to rotorcraft brownout conditions. Single phase and dual-phase PIV experiments have been conducted to study the interaction of a mobile sediment bed with characteristic flow structures similar to those within a rotor wake. Even though sediment transport has been extensively studied in the past, the rapidly evolving transient nature of brownout calls many of the simplifying assumptions that have been made to understand sediment transport mechanisms into question. Image intensity based phase-separation and a hybrid PIV/PTV techniques have been implemented to identify the gas and solid phases as well as to the resolve multi-valued velocity displacements within a given interrogation region. A calibration technique to identify the measurement volume using size-brightness as well as PIV correlation based criteria has been outlined. Simultaneous velocity measurements of the fluid and dispersed phase in two vertical co-planar planes are analyzed to examine the role of vortex interaction and its subsequent breakdown on sediment transport process. The mobilization conditions and wall-normal flux of particulates by the vortex-wall interaction are reported and are correlated to the local vortex conditions such as proximity to the wall and subsequent decay. The effect of the changing sediment bed profile on sediment transport rates is also studied. Modulation of mean and stochastic fluid flow properties due to the presence of particles and the effect of turbulent coupling between the particle and fluid momentum, as based on a modified drag law with dependence on particle Reynolds number as well as local volume fraction has been examined. A mesoscopic Eulerian formalism has been implemented to study the effect of particle inertia on the suspension process.Item CONTRIBUTIONS TOWARDS THE UNDERSTANDING OF ROTOR-INDUCED DUST PARTICLE MOBILIZATION AND TRANSPORT(2014) Sydney, Anish Joshua; Leishman, John G; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)To better understand the problem of rotor-induced particle motion and rotorcraft brownout, time-resolved, dual-phase particle image velocimetry and particle tracking velocimetry measurements were made in the flow produced by a small laboratory rotor that was hovering over a ground plane covered with a mobile sediment bed. To investigate the three-dimensionality of the wake and resultant particle field, flow measurements were made in vertical and horizontal planes around the rotor and near the ground. The primary goals of the work were to: 1. Characterize the fundamental flow physics of a rotor wake interacting with a sediment bed; 2. Investigate how rotor operating parameters, such as the disk loading, blade loading coefficient, and wake shedding frequency affected the mobilization, uplift and overall development of the particle field; 3. Examine the effects of placing a body between the rotor and the ground to understand how the interactions of the rotor wake with the body affected the transport of particles from the bed. The results showed that the rotor wake was very three-dimensional, with highly non-uniform velocities near the ground that resulted in the radially asymmetric mobilization, uplift and suspension of particles. The tip vortices were found to be the primary contributor to the uplift of particles, with the aperiodic variations in their trajectories near the ground causing intermittent particle mobilization events. These effects were caused, in part, by wave-like displacements that developed along the lengths of the tip vortices, which caused some parts of the filaments to convect closer to the ground than other parts and so uplift discrete bursts or plumes of particles. The quantity and distribution of uplifted particles were shown to be affected by the operating condition of the rotor, with the overall complexity of the rotor wake generally resulting in the formation of a highly three-dimensional and time-varying particle field. The rotor operating parameters were shown to interdependently alter the characteristics of the groundwash flow and the tip vortices produced by the rotor. Stronger wake vortices that impinged on the bed generally uplifted more particles, however, higher near-wall flow velocities over the bed also convected particles further downstream before they could be suspended. The near-wall flow developments were further complicated by the interaction of the rotor wake with a body, which significantly distorted the development of the rotor wake at the ground, the resulting near-wall flow velocities generally being lower in magnitude. The degree of wake distortion, however, was found to be sensitive to the cross-sectional shape of the body. In cases where there was direct impingement of the tip vortices on the body surfaces, the distortions to the wake caused lower near-wall flow velocities but still contained vortices that were able to suspend sediment particles radially closer to the rotor compared to the isolated rotor case.Item EFFECTS OF BLADE TIP SHAPE ON ROTOR IN-GROUND-EFFECT AERODYNAMICS(2011) Milluzzo, Joseph; Leishman, John G; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)High-speed videographic flow visualization and detailed two-component particle image velocimetry (PIV) measurements were conducted to examine the wake produced by a hovering single-bladed rotor as it interacted with a horizontal ground plane. The mo- tivation of the work was to better understand the nature of the flow field at the ground and the possible aerodynamic mechanisms that create brownout dust clouds when rotorcraft take off and land over surfaces covered with loose sediment. Rotors with four different blade tips were tested: 1. A baseline rectangular tip, 2. A simple 20◦ swept tip, 3. A BERP-like tip, and 4. A slotted tip. Flow visualization was performed using a high- repetition rate Nd:YLF laser that illuminated appropriately seeded flows in radial planes, with imaging performed using a high-speed CMOS camera. PIV measurements were performed in regions near the blades and at the ground plane by using a Nd:YAG laser with a CCD camera. Measurements as functions of wake age were obtained to examine the morphology of the vortical rotor wake during its interaction with the ground. The results showed that the wake was subjected to powerful curvature and straining effects as it interacted with the ground plane and was deflected into a radially outward direction along the plane. Reintensification of the tip vortices during the interaction caused them to remain very distinct features in the flow near the ground to as old as six or more ro- tor revolutions. The unsteady outward flow over the ground plane was shown to have similarities to a classical turbulent wall jet, especially further away from the rotor. Flow measurements were obtained deep into the boundary layer region at the ground, and in some cases into the laminar sublayer. The results showed certain common flow features between the four blade tips, but also differences in the flows that may ultimately affect the problem of brownout. The slotted-tip was shown to be particularly effective in diffusing the tip vortices and reducing the overall intensity of the fluctuating aspects of the flow at the ground.Item Flow Field and Performance Measurements of a Flapping-Wing Device Using Particle Image Velocimetry(2011) Ramsey, Joseph Patrick; Leishman, John G; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A flexible flapping wing was tested using various flow interrogation techniques including particle image velocimetry (PIV) to further the understanding of its complex, unsteady, three-dimensional flow field. The flow field was characterized using high-speed flow visualization (FV) in chordwise and spanwise planes to observe and characterize the evolution of flow structures produced. The formation, growth, convection, and shedding of a leading-edge vortex (LEV) was observed on the upper surface of the wing, which was found to mimic the classical process of dynamic stall. A motion-tracking system was used to characterize the complex wing kinematics and aeroelastic deformations of the flexible wing. These measurements were then used to estimate the noncirculatory forces and moments acting on the wing. Two-dimensional velocity fields around the wing contour and in its wake were obtained using PIV. These velocity fields were used to calculate the circulatory lift as well as the drag produced on the wing. It was found that the process of LEV formation, growth, and convection significantly increased the lift production on the flapping wing. The noncirculatory and circulatory lift measurements were then combined in amplitude and phase to calculate the total lift on the wing. It was shown that the noncirculatory contributions to the airloads were small except near pronation and supination. The flow field results were also used to calculate the lift-to-drag ratio during the wing stroke, where surprisingly it was found that the lift-to-drag ratio increased during the process of LEV formation and shedding. This observation perhaps suggests a reason why flapping-wing flyers intentionally produce LEVs during their wing stroke.Item Mechanisms of Sediment Entrainment and Transport in Rotorcraft Brownout(2009) Johnson, Bradley Stephen Curtis; Leishman, J. Gordon; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)To advance the understanding of the phenomenon of rotorcraft brownout, a dual-phase flow environment induced by a small-scale rotor hovering above a sediment bed was studied using high-speed flow visualization and particle image velocimetry (PIV). The high frame rate of the camera, combined with advanced particle recognition and tracking software, permitted an understanding of the temporal evolution of the rotor wake in ground effect simultaneously with the processes of sediment entrainment and transport by the rotor flow. High-resolution near-wall PIV measurements showed that large excursions in the surface boundary layer were produced by the convecting rotor wake vortices. These excursions acted to suppress an equilibrium state in the boundary layer within the zone of vortex impingement on the ground. The highest sediment entrainment levels were observed to occur within this impingement zone, which can be attributed to the increase in groundwash and wall shear produced beneath the vortices. Once entrained, significant quantities of sediment were then trapped and locally suspended by the vortex-induced upwash field. This effect resulted in a noticeable level of intermittency in the initial vertical transport of sediment from the ground. The ground and upwash flow velocities were shown to strengthen significantly during the viscous merging of adjacent wake vortices. This mechanism proved fundamental in defining the concentration of suspended sediment, as well as the maximum height to which sediment could be transported. Sediment particles reaching sufficient heights were observed to recirculate into the rotor wake, and convect back towards the ground at a high speed. This process caused sediment ejection by means of bombardment or "splash." The classical process of saltation bombardment was also visualized for larger particles whose inertia prevented them from being suspended in the vortical flow. While providing new insight into the time- and length-scales associated with sediment transport by a rotor wake, the observations made here also bring into question the validity of equilibrium particle flux models currently being used for brownout simulations.Item SEDIMENT SUSPENSION EVENTS FROM RIPPLE BEDS IN OSCILLATORY FLOW: EXPERIMENTS(2009) Knowles, Philip Leland; Kiger, Kenneth T; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An experimental sediment flume is used to investigate sediment transport mechanics within an oscillatory turbulent boundary layer over a mobile sediment bed in the ripple bed regime. Two-phase PIV is utilized to simultaneously capture data from each phase, allowing examination of suspension mechanisms, carrier phase stresses, and to obtain statistics to describe the momentum exchange between the phases. The technique employs median filtering, as well as size and brightness criteria to separate and accurately identify each phase. Independent well-conditioned tests have been conducted to improve the algorithm to account for the imaging conditions encountered in the vicinity of a mobile bed in order to minimize cross-talk between the phases and allow quantification of the dispersed phase concentration. Results show that large-scale vortical structures are responsible for the ejection of sediment from the bed into the outer flow. These structures are a significant source of turbulent transport, but their overall contribution to the bed stress is small compared to the mean flow. Triple decomposition of the equations of motion show that long time averaged sediment flux is of similar magnitude to cyclic fluctuations and the time averaged flow consists of two counter rotating cells. Turbulent kinetic energy created at flow reversal advects over the sediment bed and keeps particles suspended in the flow. Calculation of the vertical particle drag, body force, and convection terms revels that at flow reversal the body force terms are larger than the drag causing the particles on average to settle. The particle convection terms are small compared to particle drag and body force terms. Some of the terms most significant in the particle drag are the fluctuating components indicating that the turbulence is keeping the particles suspended in the outer flow.Item Experimental and Numerical Characterization of Turbulent Slot FIlm Cooling(2008-05-08) CRUZ, Carlos Alberto; MARSHALL, André W.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study presents an experimental and numerical characterization of the turbulent mixing in two-dimensional slot film cooling flows. Three different flows are considered by varying the coolant to mainstream velocity ratio (VR): a wall jet case (VR ≈ 2.0), a boundary layer case (VR ≈ 1.0) and a wall-wake case (VR ≈ 0.5). For each flow, detailed measurements of the film cooling effectiveness, the heat flux, and the heat transfer coefficient are obtained for adiabatic and backside cooled wall conditions. Additionally, detailed flow velocity and temperature are measured under hot conditions using Particle Image Velocimetry (PIV) and a micro-thermocouple probe, respectively. These comprehensive measurements provide a unique data set for characterizing the momentum and thermal mixing of the turbulent flows, and for validating turbulence models in Reynolds averaged Navier-Stokes (RANS) simulations and large-eddy simulations (LES). The three flow families display different performances. The mixing of the film is strongly influenced by the mean shear between the coolant and the hot mainstream, thus explaining that the boundary layer case provides the best effectiveness. Initially governed by the film kinematics at the injection point, the convective heat transfer is influence by the mainstream when the film mixes. Additionally, measurements indicate that semi-empirical correlations largely overpredict the mixing of the film. The results obtained with the Spalart-Allmaras RANS model compare favorably with the measurements, thereby proving that this model is a viable alternative to using correlations for the film cooling effectiveness. A Large-Eddy Simulation (LES) with the dynamic models is performed for the wall jet case under adiabatic wall conditions with inflow conditions prescribed from precursor simulations. The LES results show good agreement with measured adiabatic wall temperatures and provide unique insight into the turbulent transport mechanism and interaction between the near wall and outer shear regions responsible for the mixing of the film.