Froude number scaling unifies impact trajectories into granular media across gravitational conditions

The interactions of solid objects with granular media is countered by a resistance force that stems from frictional forces between the grains and the media's resistance to inertia imposed by the intruder. Earlier theories of granular intrusion have suggested an additive contribution of these two families of forces and had tremendous success in predicting resistive forces on arbitrary shaped objects. However, it remains unclear how these forces are influenced by gravitational conditions. We examine the role of gravity on surface impact behavior into cohesionless granular media using hundreds of soft-sphere discrete element simulations, we demonstrate that the outcome of impacts remain qualitatively similar under varying gravitational conditions if initial velocities are scaled with the Froude number, suggesting an underlying law. Using theoretical arguments, we provide reasoning for the observed universality and show that there is a hidden dependency in resistive forces into granular media on Froude number. Following the theoretical framework, we show that Froude number scaling precisely collapses impact trajectories across gravitational conditions, setting the foundation for explorations in granular behavior beyond Earth

Research Reports: 1988 NASA/ASEE Summer Faculty Fellowship Program

The basic objectives are to further the professional knowledge of qualified engineering and science faculty members; to stimulate an exchange of ideas between participants and NASA: to enrich and refresh the research and teaching activities of the participants' institutions; and to contribute to the research objectives of the NASA centers. Topics addressed include: cryogenics; thunderstorm simulation; computer techniques; computer assisted instruction; system analysis weather forecasting; rocket engine design; crystal growth; control systems design; turbine pumps for the Space Shuttle Main engine; electron mobility; heat transfer predictions; rotor dynamics; mathematical models; computational fluid dynamics; and structural analysis

Topology optimization of compliant adaptive wing leading edge with composite materials

AbstractAn approach for designing the compliant adaptive wing leading edge with composite material is proposed based on the topology optimization. Firstly, an equivalent constitutive relationship of laminated glass fiber reinforced epoxy composite plates has been built based on the symmetric laminated plate theory. Then, an optimization objective function of compliant adaptive wing leading edge was used to minimize the least square error (LSE) between deformed curve and desired aerodynamics shape. After that, the topology structures of wing leading edge of different glass fiber ply-orientations were obtained by using the solid isotropic material with penalization (SIMP) model and sensitivity filtering technique. The desired aerodynamics shape of compliant adaptive wing leading edge was obtained based on the proposed approach. The topology structures of wing leading edge depend on the glass fiber ply-orientation. Finally, the corresponding morphing experiment of compliant wing leading edge with composite materials was implemented, which verified the morphing capability of topology structure and illustrated the feasibility for designing compliant wing leading edge. The present paper lays the basis of ply-orientation optimization for compliant adaptive wing leading edge in unmanned aerial vehicle (UAV) field

Design of a Horizontal Axis Open-Centre Tidal Stream Turbine using Computational Fluid Dynamics

Tidal energy is one of the most promising emerging renewable energy sources which remains largely untapped, due primarily to the challenges of submerged operation within sensitive marine environments. Extracting kinetic energy from dense and energetic flow streams which vary in height, reverse flow direction roughly twice a day and carry sediment as well as marine life requires a unique application of engineering knowledge. A variety of tidal turbine technologies have been developed in response, although as yet the industry is far from mature and there remains great potential for improvement. The research presented in this study introduces a new type of turbine design which has been developed specifically to address the issue of balancing marine friendly technology with efficient energy harvest. This is accomplished through the use of an open-centre concept which houses the blades between the hub and shroud, thus minimizing the risk of blade tip impact and providing free passage through the central aperture. In this study several iterations of the design are tested using the methods of computational fluid dynamics (CFD), each one featuring a different helical blade geometry of varying length and twist angle. A numerical model of the new design is presented in which the energy generation potential is assessed by measuring the amount of torque produced by a stationary blade placed in a steady flow. The torque is calculated by determining the pressure force acting on each blade surface and the resulting moment generated about the rotation axis of the turbine. This method allows for a great number of geometries to be tested under simulated turbine operating conditions, without requiring a prohibitive amount of computational resources. The initial assessment of this new type of turbine is promising, indicating that certain blade geometries produce a greater amount of torque than a model of the conventional open-centre turbine developed by OpenHydro

Hydrodynamics and Thermodynamics of Ice Particle Accretion

Icing in warm environments, e.g. in aircraft engines or heated measurement probes, occurs if airplanes fly through areas with high amounts of atmospheric ice crystals. Ingested into the warm engine, they start to melt, resulting in an airflow laden with mixed-phase particles consisting of water and ice. Liquid water deposits on component surfaces, which enables ice particles to adhere to them, forming ice accretion of considerable thickness. Such an accretion reduces reliability, power and efficiency of the engine and impedes the function of probes. While light icing reduces the aircraft’s economic viability and environmental-friendliness by increasing fuel consumption, it may lead to engine failure and damage as well as probe malfunction in severe cases, which threatens aircraft safety significantly. The aviation industry is highly interested in eliminating this problem and in developing accurate ice accretion models. As the comprehension of the underlying physics is still rudimentary, the accuracy of current prediction tools is rather limited. The goal of this work is to investigate the physical mechanisms leading to ice accretion by developing theoretical models and the implementation of them in numerical codes. Within the scope of this work, three main phenomena related to the process of ice crystal accretion are studied: the melting of non-spherical particles, the impact of small particles on a liquid surface and the accretion and shedding of ice layers. In order to investigate the particle melting, a theoretical model is developed based on an approximation of the particle shape as a spheroid. Due to capillary forces, the arising meltwater is presumed to accumulate in the particle mid-section, where the curvature is minimal. Numerically realized with a Level-Set approach, the model is able to predict the evolution of the shape of the melting particle and the time of its melting with high accuracy. It yields results which confirm the model’s superiority over currently employed melting models. The particle impact onto a liquid surface is studied numerically. In addition to pressure and viscous forces acting on the particle, capillary forces arising in the three phase contact line are taken into account by the Finite-Volume algorithm. An appropriate mesh motion allows for the movement of the particle which constitutes a boundary on the domain while the liquid-gaseous interface is accounted for by a Volume-of-Fluid method. The code accurately predicts the impact behavior of high Weber number processes as well as of low Weber impacts in which surface tension and the contact line force resulting from it prevails. By means of data obtained with the algorithm and a dimensional analysis, a simple correlation is found which is able to predict whether particles stick or rebound. Investigation of the behavior of accreted ice layers is carried out using two approaches. In the first approach, a detailed three-dimensional thermal model which resolves ice particles and liquid droplets is developed. It demonstrates that a porous ice/water layer behaves differently than solid ice. Theoretical modeling of the effective thermal properties and accounting for the transport of heat and mass in the ice layer is the basis of the second approach. It yields a numerical algorithm which efficiently predicts the composition of the accretion, which is then utilized to anticipate the instant of ice plate shedding. The obtained results agree very well with experimental data

A small perturbation based optimization approach for the frequency placement of high aspect ratio wings

Design denotes the transformation of an identified need to its physical embodiment in a traditionally iterative approach of trial and error. Conceptual design plays a prominent role but an almost infinite number of possible solutions at the outset of design necessitates fast evaluations. The traditional practice of empirical databases loses adequacy for novel concepts and an ever increasing system complexity and resource scarsity mandate new approaches to adequately capture system characteristics. Contemporary concerns in atmospheric science and homeland security created an operational need for unconventional configurations. Unmanned long endurance flight at high altitudes offers a unique showcase for the exploration of new design spaces and the incidental deficit of conceptual modeling and simulation capabilities. The present research effort evolves around the development of an efficient and accurate optimization algorithm for high aspect ratio wings subject to natural frequency constraints. Foundational corner stones are beam dimensional reduction and modal perturbation redesign. Local and global analyses inherent to the former suggest corresponding levels of local and global optimization. The present approach departs from this suggestion. It introduces local level surrogate models to capacitate a methodology that consists of multi level analyses feeding into a single level optimization. The innovative heart of the new algorithm originates in small perturbation theory. A sequence of small perturbation solutions allows the optimizer to make incremental movements within the design space. It enables a directed search that is free of costly gradients. System matrices are decomposed based on a Timoshenko stiffness effect separation. The formulation of respective linear changes falls back on surrogate models that approximate cross sectional properties. Corresponding functional responses are readily available. Their direct use by the small perturbation based optimizer ensures constitutive laws and eliminates a previously necessary optimization at the local level. The great economy of the developed algorithm makes it suitable for the conceptual phase of aircraft design.Ph.D.Committee Chair: Mavris, Dimitri; Committee Member: Bauchau, Olivier; Committee Member: Schrage, Daniel; Committee Member: Volovoi, Vitali; Committee Member: Yu, Wenbi

Constraint-based navigation for safe, shared control of ground vehicles

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 138-147).Human error in machine operation is common and costly. This thesis introduces, develops, and experimentally demonstrates a new paradigm for shared-adaptive control of human-machine systems that mitigates the effects of human error without removing humans from the control loop. Motivated by observed human proclivity toward navigation in fields of safe travel rather than along specific trajectories, the planning and control framework developed in this thesis is rooted in the design and enforcement of constraints rather than the more traditional use of reference paths. Two constraint-planning methods are introduced. The first uses a constrained Delaunay triangulation of the environment to identify, cumulatively evaluate, and succinctly circumscribe the paths belonging to a particular homotopy with a set of semi autonomously enforceable constraints on the vehicle's position. The second identifies a desired homotopy by planning - and then laterally expanding - the optimal path that traverses it. Simulated results show both of these constraint-planning methods capable of improving the performance of one or multiple agents traversing an environment with obstacles. A method for predicting the threat posed to the vehicle given the current driver action, present state of the environment, and modeled vehicle dynamics is also presented. This threat assessment method, and the shared control approach it facilitates, are shown in simulation to prevent constraint violation or vehicular loss of control with minimal control intervention. Visual and haptic driver feedback mechanisms facilitated by this constraint-based control and threat-based intervention are also introduced. Finally, a large-scale, repeated measures study is presented to evaluate this control framework's effect on the performance, confidence, and cognitive workload of 20 drivers teleoperating an unmanned ground vehicle through an outdoor obstacle course. In 1,200 trials, the constraint-based framework developed in this thesis is shown to increase vehicle velocity by 26% while reducing the occurrence of collisions by 78%, improving driver reaction time to a secondary task by 8.7%, and increasing overall user confidence and sense of control by 44% and 12%, respectively. These performance improvements were realized with the autonomous controller usurping less than 43% of available vehicle control authority, on average.by Sterling J. Anderson.Ph.D

Pressure Systems Energy Release Protection (Gas Pressurized Systems)

A survey of studies into hazards associated with closed or pressurized system rupture and preliminary guidelines for the performance design of primary, secondary, and protective receptors of these hazards are provided. The hazards discussed in the survey are: blast, fragments, ground motion, heat radiation, biological, and chemical. Performance guidelines for receptors are limited to pressurized systems that contain inert gas. The performance guidelines for protection against the remaining unaddressed degenerative hazards are to be covered in another study

Comet Hitchhiker: NIAC Phase 1 Final Report

Summary of Activities-Developed the Comet Hitchhiker concept, which is to hitch rides on small bodies (asteroids and comets) using a tethered spacecraft. (Section 2)-Identified five scientifically important missions that would be enabled or significantly benefited by the Comet Hitchhiker concept.The five mission concepts are: KBO rendezvous, Centaur rendezvous, Trojan rendezvous, Damocloid rendezvous, and Main asteroid belt tour to rendezvous with multiple (10) objects. (Section 3)-Derived the Space Hitchhike Equation, or "the rocket equation for hitchhiker", which relates the specific strength of tether, mass ratio, and V. (Section 4.1)-Performed in-depth feasibility analysis of the critical components of the concept through: Finite-element simulations of tether and spacecraft dynamics, as shown in Figure 1 (Section 4.4); Supercomputer simulations of the hypervelocity impact of harpoon on a small body, as shown in Figure 2. (Section 6)-Performed public outreach activities including the collaboration with a concept artist of the Museum of Science Fiction, exposure to media, and public presentations. (Section 8

The Development of Computational Models for Melting-Solidification Applications Using the Volume-Of-Fluid Method

Fluids-related issues in the Aerospace industry are often multiphase in scope. Numerical modeling, such as computational fluid dynamics, is used to describe these problems, as they are difficult or impossible to describe analytically. This research uses computational fluid dynamics to describe multiphase problems related to melting-solidification and particle impingement. Firstly, a numerical model was established that uses the Volume-of-Fluid method to resolve a melting/solidifying particle. This model was verified against experiments and simplified analytical models, and a mesh independence study was done to ensure the results were independent of the mesh sizing. Next, the model was applied to two separate but related problems. The Artemis program has renewed interest in lunar dust mitigation. It is proposed that lunar regolith partially melts and becomes sticky when coming into contact with a jet flame, like a landing rocket. The method above was applied to a lunar regolith particle to show how these sticky particles can adhere to surfaces. The direct resolution methodology was also applied to a melted sand particle impinging and infiltrating a yttria-stabilized zirconia thermal barrier coating, as seen in engine turbines. Sand can infiltrate the thermal barrier coating and decrease its effectiveness. The infiltration from a single particle was compared to the infiltration from a stream of melted sand. These three efforts showcase the usefulness of directly resolving small particles using the Volume-of-Fluid method