Theses and Dissertations from UMD
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Item The Effects of Model Scaling on Sediment Transport in Brownout(2013) Glucksman-Glaser, Mark Samuel; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The phenomenon of “brownout” is characterized by a large cloud of sediment or dust that is formed around a rotorcraft when it takes off or lands in arid or dusty environments. To further understand the physics of brownout, a laboratory-scale rotor hovering in water was tested over a ground plane covered with a mobile sediment bed. The sensitivity of the dual-phase flow environment to changes in the values of the similarity parameters that potentially govern the fluid dynamics of the rotor flow and the transport of sediment was explored. First, dye flow visualization was performed to study the general evolution of the rotor flow and its interaction with the ground plane. Then, dual-phase flow visualization was used to expose the details of the processes that mobilize and uplift loose particles from the sediment bed. It was shown using the flow visualization that the trailed vortices from the rotor blades were a primary contributor to the mobilization and suspension of sediment. Particle image velocimetry (PIV) was also used to obtain quantitative measurements of the flow velocities found in the rotor wake and near the ground plane. It is then discussed as to why the steady flow assumptions used in the usual definitions of the classical similarity parameters governing sediment transport are not as applicable to the dual-phase flows produced by a rotor operating over a mobile sediment bed. A Buckingham-π analysis was performed to determine a set of new similarity parameters that potentially better reflect the dual-phase flow characteristics relevant to sediment mobilization and suspension by a rotor wake, including the characteristics of the tip vortices. Sixteen new similarity parameters were initially determined, five of which selected as having particular relevance. Specifically, these new similarity parameters were: 1. The mobile inertia ratio; 2. The stationary inertia ratio, 3. The terminal-swirl velocity ratio; 4. The threshold-swirl velocity ratio; 5. The terminal/threshold-swirl velocity ratio. The values of these similarity parameters were determined using the PIV measurements, and were all found to correlate to the quantity of sediment mobilized and uplifted by the rotor. The terminal/threshold-swirl velocity ratio is proposed as the potentially most important similarity parameter for further characterizing the brownout phenomenon.Item Contributions Toward Understanding the Effects of Rotor and Airframe Configurations On Brownout Dust Clouds(2014) Govindarajan, Bharath Madapusi; Leishman, J. Gordon; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Brownout dust cloud simulations were conducted for rotorcraft undergoing representative landing maneuvers, primarily to examine the effects of different rotor placement and rotor/airframe configurations. The flow field generated by a helicopter rotor in ground effect operations was modeled by using an inviscid, incompressible, time-accurate Lagrangian free-vortex method, coupled to a semi-empirical approximation for the boundary layer flow near the ground. A surface singularity method was employed to represent the aerodynamic influence of a fuselage. A rigorous coupling strategy for the free-vortex method was developed to include the effects of rotors operating at different rotational speeds, such as a tail rotor. For the dispersed phase of the flow, particle tracking was used to model the dust cloud based on solutions to a decoupled form of the Basset-Boussinesq-Oseen equations appropriate to dilute gas particle suspensions of low Reynolds number Stokes flow. Important aspects of particle mobility and uplift in such vortically driven dust flows were modeled, which included a threshold-based model for sediment mobility and bombardment effects when previously suspended particles impact the bed and eject new particles. Various techniques were employed to reduce the computational cost of the dust cloud simulations, such as particle clustering and parallel programming using graphics processing units. The predicted flow fields near the ground and resulting dust clouds during the landing maneuvers were analyzed to better understand the physics behind their development, and to examine differences produced by various rotor and airframe configurations. Metrics based on particle counts and particle velocities in the field of view were developed to help quantify the severity of the computed brownout dust clouds. The presence of both a tail rotor and the fuselage was shown to cause both local and global changes to the aerodynamic environment near the ground and also influenced the development of the resulting dust clouds. Studies were also performed to examine the accuracy of self-induced velocities of vortex filaments by augmenting the straight-line vortex segments with a curved filament correction term. It was found that while curved elements can accurately recover the self-induced velocity in the case of a vortex ring, there existed bounds of applicability when extended to three-dimensional rotor wakes. Finally, exploratory two-dimensional and three-dimensional studies were performed to examine the effects of blade/particle collisions. The loss in particle kinetic energy during the collision was adopted as a surrogate metric to quantify the extent of potential blade erosion.Item Measurement and Scaling Analysis of Rotor-Induced Sediment Mobilization(2014) Perrotta, Gino; Jones, Anya; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Flow visualization and particle imaging velocimetry (PIV) experiments were conducted in a water tank to investigate the effects of rotor wake and sediment properties on rotor-induced sediment mobilization during hover in ground effect. The two-phase flow was separated into the carrier phase and the dispersed phase for characterization. The carrier phase was studied using PIV to acquire time-resolved planar velocity measurements for a field of view within the rotor wake. The rotor-induced flow was confirmed to be dominated by blade tip vortices and was characterized primarily in terms of the vortex characteristics. Vortices were identified using a tracking function, and were compared to the Lamb-Oseen vortex velocity profile to evaluate their size and strength. The rotor-induced flow was also characterized in terms of wall-jet velocity and turbulent kinetic energy. The dispersed phase was separated using image filtering procedures and was quantified by identifying mobilized sediment particles visible in the field of view. Characteristics of the rotor-induced flow and quantification of sediment mobilization were each averaged over time for several rotor rotations to reduce the effects of wake aperiodicity and asymmetry. New parameter groups were created by combining rotor-induced sediment mobilization system characteristics and each was inspected for correlation with sediment mobilization. Three parameter groups which correlated for all cases measure here are identified and discussed.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 Contributions Towards the Detailed Understanding of Rotor Flow Fields in Ground Effect Operations(2014) Milluzzo, Joseph; Leishman, John G; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)High-speed flow visualization and high-resolution particle image velocimetry experiments were conducted on a two-bladed rotor that was operated in a hovering state, both out of ground effect (well away from the ground) and in ground effect at several rotor heights. Recent advances in flow diagnostic instrumentation allowed measurements of the rotor wake to be performed with unprecedented levels of resolution. In particular, the goal of the present work was to gain a better understanding of the fluid dynamics of the wake sheets (and the blade tip vortices) that were trailed from the blades. The present work examined the effects produced by two blade sets: 1. A baseline untwisted blade, 2. A twisted blade with $-17^{\circ}$ of linear twist, and has revealed fluid dynamic details of the wake sheet that were hitherto unknown. For the measurements made with the rotor operating out of ground effect, the blade sets were tested at two blade loading coefficients of 0.053 and 0.08, although only the higher loading condition was tested with the rotors operating in ground effect. For the rotor operating out of ground effect, a helicoidal tip vortex was shown to form at the blade tip and the associated wake sheets were initially laid down as small-scale counterrotating vortical pairs. However, this initial vorticity quickly diffused and the sheet was then convected as a concentrated bands of turbulence, including several dominant eddies. Several types of sheet dynamics were documented in the rotor wake, including sheet interactions with the tip vortices, the detailed behavior of this interaction depending on both the blade twist and the rotor thrust. At earlier wake ages, a sinusoidal wave-like perturbation was seen to be formed on the wake sheets, although the growth in wave amplitude was limited as the sheets were convected and stretched in the velocity gradients in the downstream wake. When the rotor was operated in ground effect, the vorticity in the wake sheets persisted to much older wake ages. Wave-like perturbations did not form on the wake sheets when the rotors were operating in ground effect because the outward radial stretching of the rotor wake in the presence of the ground suppressed their development. The wake sheets were found to convect to the ground and introduce significant vorticity into the near-wall flow field closer to the rotor, contributing to fluctuations in the local flow velocities. The flow field near the ground was also observed to be significantly affected by the use of twist on the blade, with the wake impinging on the ground further inboard and closer to the rotor, which also resulted in higher flow velocities being produced further downstream.Item EVALUATION OF PARTICLE CLUSTERING ALGORITHMS IN THE PREDICTION OF BROWNOUT DUST CLOUDS(2011) Govindarajan, Bharath Madapusi; Leishman, Gordon; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A study of three Lagrangian particle clustering methods has been conducted with application to the problem of predicting brownout dust clouds that develop when rotor- craft land over surfaces covered with loose sediment. A significant impediment in per- forming such particle modeling simulations is the extremely large number of particles needed to obtain dust clouds of acceptable fidelity. Computing the motion of each and every individual sediment particle in a dust cloud (which can reach into tens of billions per cubic meter) is computationally prohibitive. The reported work involved the development of computationally efficient clustering algorithms that can be applied to the simulation of dilute gas-particle suspensions at low Reynolds numbers of the relative particle motion. The Gaussian distribution, k-means and Osiptsov's clustering methods were studied in detail to highlight the nuances of each method for a prototypical flow field that mimics the highly unsteady, two-phase vortical particle flow obtained when rotorcraft encounter brownout conditions. It is shown that although clustering algorithms can be problem dependent and have bounds of applicability, they offer the potential to significantly re- duce computational costs while retaining the overall accuracy of a brownout dust cloud solution.Item The Effects of Model Scaling on Sediment Transport in Brownout(2013) Glucksman-Glaser, Mark Samuel; Jones, Anya R; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The phenomenon of “brownout” is characterized by a large cloud of sediment or dust that is formed around a rotorcraft when it takes off or lands in arid or dusty environments. To further understand the physics of brownout, a laboratory-scale rotor hovering in water was tested over a ground plane covered with a mobile sediment bed. The sensitivity of the dual-phase flow environment to changes in the values of the similarity parameters that potentially govern the fluid dynamics of the rotor flow and the transport of sediment was explored. First, dye flow visualization was performed to study the general evolution of the rotor flow and its interaction with the ground plane. Then, dual-phase flow visualization was used to expose the details of the processes that mobilize and uplift loose particles from the sediment bed. It was shown using the flow visualization that the trailed vortices from the rotor blades were a primary contributor to the mobilization and suspension of sediment. Particle image velocimetry (PIV) was also used to obtain quantitative measurements of the flow velocities found in the rotor wake and near the ground plane. It is then discussed as to why the steady flow assumptions used in the usual definitions of the classical similarity parameters governing sediment transport are not as applicable to the dual-phase flows produced by a rotor operating over a mobile sediment bed. A Buckingham-π analysis was performed to determine a set of new similarity parameters that potentially better reflect the dual-phase flow characteristics relevant to sediment mobilization and suspension by a rotor wake, including the characteristics of the tip vortices. Sixteen new similarity parameters were initially determined, five of which selected as having particular relevance. Specifically, these new similarity parameters were: 1. The mobile inertia ratio; 2. The stationary inertia ratio, 3. The terminal-swirl velocity ratio; 4. The threshold-swirl velocity ratio; 5. The terminal/threshold-swirl velocity ratio. The values of these similarity parameters were determined using the PIV measurements, and were all found to correlate to the quantity of sediment mobilized and uplifted by the rotor. The terminal/threshold-swirl velocity ratio is proposed as the potentially most important similarity parameter for further characterizing the brownout phenomenon.Item DEVELOPMENT OF A LAGRANGIAN-LAGRANGIAN METHODOLOGY TO PREDICT BROWNOUT DUST CLOUDS(2012) Syal, Monica; Leishman, J. Gordon; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A Lagrangian-Lagrangian dust cloud simulation methodology has been developed to help better understand the complicated two-phase nature of the rotorcraft brownout problem. Brownout conditions occur when rotorcraft land or take off from ground surfaces covered with loose sediment such as sand and dust, which decreases the pilot's visibility of the ground and poses a serious safety of flight risk. The present work involved the development of a comprehensive, computationally efficient three-dimensional sediment tracking method for dilute, low Reynolds number Stokes-type flows. The flow field generated by a helicopter rotor in ground effect operations over a mobile sediment bed was modeled by using an inviscid, incompressible, Lagrangian free-vortex method, coupled to a viscous semi-empirical approximation for the boundary layer flow near the ground. A new threshold model for the onset of sediment mobility was developed by including the effects of unsteady pressure forces that are induced in vortically dominated rotor flows, which can significantly alter the threshold conditions for particle motion. Other important aspects of particle mobility and uplift in such vortically driven dust flows were also modeled, including bombardment effects when previously suspended particles impact the bed and eject new particles. Bombardment effects were shown to be a particularly significant contributor to the mobilization and eventual suspension of large quantities of smaller-sized dust particles, which tend to remain suspended. A numerically efficient Lagrangian particle tracking methodology was developed where individual particle or clusters of particles were tracked in the flow. To this end, a multi-step, second-order accurate time-marching scheme was developed to solve the numerically stiff equations that govern the dynamics of particle motion. The stability and accuracy of this scheme was examined and matched to the characteristics of free-vortex method. One-way coupling of the flow and the particle motion was assumed. Particle collisions were not considered. To help reduce numerical costs, the methodology was implemented on graphic processing units, which gave over an order of magnitude reduction in simulation time without any loss in accuracy. Validation of the methodology was performed against available measurements, including flow field measurements that have been made with laboratory-scale and full-scale rotors in ground effect operations. The predicted dust clouds were also compared against measurements of developing dust clouds produced by a helicopter during taxi-pass and approach-to-touchdown flight maneuvers. The results showed that the problem of brownout is mostly driven by the local action of the rotor wake vortices and the grouping or bundling of vortex filaments near the sediment bed. The possibilities of mitigating the intensity of brownout conditions by diffusing the blade tip vortices was also explored. While other means of brownout mitigation may be possible, enhancing the diffusion of the tip vortices was shown to drastically reduce the quantity of mobilized particles and the overall severity of the brownout dust cloud.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.