Research Programs from BAA - Mechanical Sciences

1.0 Overview

Research supported in the Mechanical Sciences Division of the ARO is concerned with a broad spectrum of fundamental investigations in the disciplines of fluid dynamics, solid mechanics, complex dynamics and systems, and propulsion and energetics. Though many creative and imaginative studies concentrate on a particular subdiscipline, increasingly, new contributions arise from interdisciplinary approaches such as the coupling between aerodynamics and structures, complex dynamics and systems, combustion and fluid dynamics, or solid mechanics and structures as in the structural reliability areas. Additionally, several common themes run through much of these four subdisciplines, for example, active controls and computational mechanics. Research in such areas is addressed within the context of the application rather than as a separate subject of study. Fluid dynamics research is primarily concerned with investigations in the areas of rotorcraft wakes, unsteady aerodynamics of dynamic stall and unsteady separation, and fundamental studies of microadaptive flow control. Solid mechanics include a wide array of research areas such as high-strain rate phenomena, penetration mechanics, heterogeneous material behavior, and reliability of structures. The complex dynamics and systems area is focused on investigations in vehicle structural dynamics, and simulation and air vehicle dynamics including rotor aeromechanics. Research in the propulsion and energetics area is concentrated on processes characteristic of reciprocating (diesel) and gas turbine engines and the combustion dynamics of propellants used for gun and missile propulsion. The following narratives describe the details of the scope and emphasis in each of these subdisciplinary areas.

1.1 Fluid Dynamics

Fluid dynamics plays a critical role in many Army operational capabilities. Significant challenges exist for accurate and efficient prediction of flow physics critical for improved performance and future advanced capability. Army platforms are often dominated by flows with high degrees of unsteadiness, turbulence, numerous and widely separated spatio-temporal scales, and geometrical complexity of solid or flexible boundaries. In order to gain the necessary physical insight to enable future capabilities spanning Army vehicles, munitions, medical devices, and logistics, the Fluid Dynamics program seeks to support basic research investigations of fundamental and novel flow physics. In view of the nonlinear and high-dimensional character of the governing equations, revolutionary advances in fluid dynamics research tools are also of great interest; advanced experimental methods, sophisticated computational techniques and breakthrough theoretical advances will be critical for gaining the required fundamental understanding.

1.1.1 Dynamics of Unsteady and Separated Flows

Operating conditions for many Army platforms are characterized by flows featuring unsteadiness, nonlinear interactions, turbulence, three-dimensionality and flow separation. All efforts in this thrust area will require novel and aggressive strategies for examination of the interplay between disparate spatio-temporal scales, inclusion of physically significant sources of three-dimensionality, and characterization of the role of flow instabilities and nonlinear interactions across a range of appropriate Mach and Reynolds numbers. Historical management of physical complexity has often resulted in scientific approaches that result in the elimination of potentially critical flow physics. Research efforts that are capable of gaining deep understanding of highly complicated flows are likely to allow critical physics to be exploited, leading to significant performance gains for Army systems. As an example, shortcomings in understanding the details of unsteady flow separation, reverse flow phenomena, and dynamic stall continue to limit the capabilities of Army rotorcraft vehicle platforms. While much progress has been made towards unraveling these details, it has become apparent that revolutionary advances are unlikely if the full complexity of the physics is not considered.

1.1.2 Nonlinear Flow Interactions and Turbulence.

As mentioned above, many Army relevant flows are governed by strong nonlinearities and turbulent behaviors. Historically, many analysis tools developed for linear dynamics have been applied to gain understanding of flow behaviors. While local insights can be gained through applications of these methods, the ability to provide global understanding of the evolution of flows requires new approaches that are capable of dealing directly with inherent nonlinearities. Turbulent dynamics may also benefit from new approaches based in dynamical systems theory to push modeling frameworks beyond the notions based on Reynolds averaging and stochastic dynamics and to determine if a useful underlying deterministic structure exists. Modeling flows near walls is a continuing challenge to accurate numerical prediction of complex flows that may benefit from novel non-intrusive diagnostics to inform creative numerical and theoretical constructs capable of efficiently producing a high degree of fidelity near physical boundaries.

1.1.3 Flow Stability and Control

Many of the previously described flows are susceptible to initially small amplitude, but dynamically significant, instabilities that can ultimately lead to fundamental changes in global flow behaviors. Thorough understanding and prediction of these instabilities and their growth is crucial not only to maintaining robustness in the face of disturbances, but also to gain advanced control over the evolution of flows through their exploitation. Research breakthroughs in global and local stability characteristics and their subsequent manipulation are of interest. Theoretical, computational, and experimental examinations of canonical problems to enable focused studies of interactions between instability mechanisms and global flow characteristics are highly encouraged, especially those that seek nonlinear descriptions. Flow control efforts should seek to exploit understanding of these mechanisms to permit the flow evolution to be prescribed. Flow control investigations should also seek to understand not only the impact of control actuation on the flow, but should also consider strategies for closed-loop feedback control and appropriate scaling laws that lead to the potential for such strategies to transition into new capabilities in real-world flows.

Technical Point of Contact: Dr. Matthew Munson, e-mail: matthew.j.munson6.civ@mail.mil,(919) 549-4284.

1.2 Solid Mechanics

The goal of the Solid Mechanics Program is to investigate and understand behavior of complex material systems under a broad range of loading regimes in various environments, to develop analytical and computational methods to characterize material models and to serve as physically-based tools for the quantitative prediction, control, and optimization of Army relevant material systems subjected to extreme battlefield environments. Army systems are frequently limited by material strength and failure resistance. Solid mechanics research plays a crucial role in the development of new materials, prediction of strength, toughness and potential damage development and failure of Army relevant material systems, structures and individual protective equipment under extreme loading conditions such as impact or blast, and prolonged normal operating conditions. Research in analytical, computational and experimental solid mechanics forms the foundation of optimization tools to enhance performance while minimizing equipment weight, and its theories provide a strong link between the underlying mechanical behavior of solids and the resulting design and functionality of actual systems resulting in reduced development cost by minimizing the need for expensive field testing, and it leads to novel ideas and concepts for revolutionary capabilities.

The below outlined research thrusts are broad and serve as a guideline of potential research project development. The goal of the program is to support basic research in solid mechanics addressing fundamental issues the subject is facing today and developing new concepts and methods with high potential for future applications for the Army.

1.2.1 Mechanical Behavior of Complex Material Systems

Modern Army structural service conditions and design requirements continually demand development of new materials and extensions of applicability limits and reliability of classical materials. Detailed understanding of material behavior and its capability during service is crucial for future applications. To utilize materials' capabilities close to their limiting capacity helps to reduce structural weight, increase service cycle, and reduce costs of the equipment. The understanding of the mechanics of material behavior in its intrinsic details during materials' transitions into states of damage and fracture initiation due to extreme loading, high rate loading, high temperature variations, repetitive loading, is important for setting appropriate limitations for material applications and for design and development of new materials with specific capabilities for particular applications. The specific component the subject of solid mechanics brings to that task is development of models of material behavior that are based on essential physical processes taking place on different scales and establishing essential quantitative relationships characterizing various processes taking place.

The common goal of the model development is to establish the critical relationships in the form useful for quantitative evaluation of the described processes during material service as a structural component. A material model should give a capability to evaluate and identify the specific internal material processes playing most significant role in a particular service environment and loading conditions. While current modeling techniques based on computational simulations of multiple micro- or nano- events are capable to include in the consideration a large number of these events and internal material components interactions, it is important for a model to have capability of quantitative determination of the significance of these events on the final material characteristics and to characterize as they may depend on the loading regimes, temperature, and other environmental factors.

The range of material groups of special interest is broad. They include various heterogeneous material systems like composite materials, various compositions of reinforced classical materials, materials undergoing special treatments and phase transformations.

1.2.2 Methods for Material and Structural Analysis

Development of analytical, computational and experimental methods capable to utilize material models in the analysis of extreme situations leading to damage and failure development.

Experimental methods aimed to investigate and validate material models play critical role in understanding material behavior on all scales. Development of new experimental techniques is essential for solid mechanics development. In particular, when the experiment is set to enhance the understanding of the physical processes taking place during deformation and failure, leading to development of new understanding and observation of a new physical processes, validating the new material models, helping to visualize the stress fields in complex situations where current analytical or computational methods cannot be successfully used or need validation.

Computational methods play a critical role in solid mechanics and its applications. Development of new computational methods and techniques addressing limitations of the currently widely used methods is of great importance in the field of solid mechanics. As material compositions and structures become more complex, and material models trend to be based on more detailed analysis of the material components interactions, and microstructural changes on multiple scales, demand for a reliable computational capabilities increases. Despite the rapid growth of the new methods and computational initiatives, and extensions of the established methods, a high demand remains for computational methods which are precise, high fidelity, efficient, capable to function in error controlled environment, validated by comparison with the benchmark analytical solutions, and capable to address and resolve computational challenges raised due to high stress/strain gradients or geometrical irregularities.

The advancement of analytical methods and development of analytical solutions to key characteristic problems in solid mechanics is extremely important as a fundamental development of the subject. Usually, analytical solutions serve as benchmark solutions for future development of material theories, understanding of the mechanical events and for verification of the validity and accuracy of the numerical methods used for similar problems before it could be reliably used for more complicated problems.

Technical Point of Contact: Dr. Denise C. Ford, e-mail: denise.c.ford2.civ@mail.mil, (919) 549-4244

1.3 Complex Dynamics and Systems

The Complex Dynamics and Systems Program emphasizes fundamental understanding of the dynamics, both physical and information theoretic, of nonlinear and nonconservative systems as well as innovative scientific approaches for engineering and exploiting nonlinear and nonequilibrium physical and information theoretic dynamics for a broad range of future capabilities (e.g. novel energetic and entropic transduction, agile motion, and force generation). The program seeks to understand how information, momentum, energy, and entropy is directed, flows, and transforms in nonlinear systems due to interactions with the system's surroundings or within the system itself. Central to the mission of the program is emphasis on pushing beyond descriptive understanding toward engineering and exploiting time-varying interactions, fluctuations, inertial dynamics, phase space structures, modal interplay and other dynamic phenomena in novel ways to enable the generation of useful work, agile motion, and engineered energetic and entropic transformations. Further information on the current scientific thrust areas and opportunities are detailed in the paragraphs that follow.

1.3.1 High-Dimensional Nonlinear Dynamics Classical dynamics has produced limited fundamental insight and theoretical methods concerning strongly nonlinear, high-dimensional, dissipative, and time-varying systems. For over a century, qualitative geometric approaches in low-dimensions have dominated research in dynamics and reduced-order-modeling of high-dimensional dynamics are often premised on empirical and statistical model fitting and are incapable of capturing the effects of slowly growing instabilities and memory. The program is interested in developing novel theoretical and experimental methods for understanding the physical and information dynamics of driven dissipative continuous systems and novel reduced-order-modeling methodologies capable of retaining time-dependent and global nonlinearities. Novel research pertaining to the analysis and fundamental physics of time-varying nonlinear systems and transient dynamics is a high-priority as well as integrating new insights from data sciences with distributed dynamical systems.

1.3.2 Active and Nonlinear Mechanical Metastructures Metamaterials research has demonstrated profound potential for micro-architected materials to surpass the intrinsic properties and functionality of natural and conventional materials. In this thrust area, interest concerns the intersection of structural dynamics, distributed sensing and actuation, and mechanical metamaterials. Behaviors due to non-infinite boundary conditions, vibrational modes, buckling modes, or external loading that are not typically considered in metamaterial research are of particular interest. Emphasis is on exploiting nonlinear behavior within nonlinear mechanical lattices and lattices of nonlinear mechanical modules from the millimeter to meter scale. Proposals exploring interactions between nonlinear meta-structures with fluids, especially if such interactions augment desired dynamic behavior, is strongly encouraged.

1.3.3 Embodied and Distributed Control, Sensing, and Actuation This thrust develops deeper understanding, through supporting theory and experiment, of the role of embodiment and dynamics on a physical system's capability to process information and transform energy. Proposals emphasizing the mechanics and control of soft, continuous bodies is encouraged along with novel experimental paradigms leveraging programmable printed matter. Generally, this thrust strongly leverages advances in, and approaches from, sensory biomechanics, neuromechanics, underactuated systems theory, and mechanical locomotion dynamics to understand the motion of both articulated and continuum dynamical systems operating in highly-dynamic environments. The scientific principles sought, however, are not limited to biological movement and manipulation. Proposals are strongly encouraged that view morphology in an abstract sense. For example, understanding morphology as a system's symmetry, its confinement (e.g. chemical reactions), or its coupling topology.

1.3.4 Criticality and Statistical Physics of Control and Learning The goal of this focus area is to lay the foundations for an algorithmic theory of control and learning that considers energetics. Additional interests include an experimentally tested theoretical framework for controlling and creating new types of critical dynamics, phase transitions, and universality classes by bringing together theory and physical principles in statistical dynamics with control and dynamical systems theory (controlling statistical dynamics). Topics of interest relating to this include: nonlinear control of distributions with non-Gaussian uncertainty; understanding relationships between work absorption and dynamics in the presence of fluctuations leading to emergent prediction and emergent centralization; steering multi-critical interacting dynamical systems toward desired universal scaling behaviors; externally controlling the strength of stochastic fluctuations and intrinsic noise in systems that are driven far from thermal equilibrium; and selectively targeting and stabilizing self-generated spatiotemporal patterns. Stochastic control at the microscale to enable novel manipulation of the dynamics of synthetic and natural biomolecular machines is also of interest.

1.3.5 Other Inspired, Curiosity-Driven, and Emerging Concepts The program has additional interests that span multiple thrust and focus areas listed above. Some of these areas include, but are not limited to: thermodynamics of cooperative/non-cooperative games and algorithms, the emergence of allometric scaling relationships during dynamic accretion and growth in active matter, scatter of active particles, highly non-intuitive and peculiar physics of programmable granular matter subject to mechanical loads (e.g., programmable dilatency), understanding distributed power vice power distribution in multi-degree-of-freedom and soft systems, etc. Prospective researchers are highly encouraged to contact the program manager with curiosity-driven and inspired ideas. Support can range from high-risk "seedling" efforts to full grants. Early-career researchers are especially highly encouraged to discuss potential "seedlings" to explore completely new areas.

1.3.6 Transdisciplinary Sabbatical Experiences Researchers are encouraged to contact the program manager who have interest in transdisciplinary growth enabled by collaboration with a group or laboratory in a different discipline. Domains should be broadly relevant to the CD&S program.

1.3.7 ARO-VTD Basic Research Challenges The purpose of these challenges is to develop novel collaborative research programs with scientists and engineers in the Army Research Laboratory's Vehicle Technology Directorate (VTD). It is anticipated that the research scope, objectives, and goals will involve substantial coordination and possible collaboration with VTD scientists and engineers. Interested researchers should contact the program manager to discuss the challenge topics further.

Challenge Topic 1: Criticality and Self-Organization in Structural Active Matter. A thermodynamic system can be changed from a solid to a liquid phase by appropriate tuning of external control parameters, e.g. increasing temperature of the environment. Self-adaptation of a material, e.g. changing from a solid to a liquid-like phase even when external control parameters are held constant, requires the presence of internal control parameters that the system can adjust as necessary to modify its macroscopic properties. This implies an extension of standard notions of self-organized criticality to include dynamics near to and passing through different critical regimes, and of critical phenomena to include the development of new basic mechanisms for microscopic dynamics that are capable of generating rich phase diagrams and achieving such internal control. Active matter provides a route from conventional condensed matter physics to materials capable of adapting their structure and self-organizing their phase.

However, research in active matter to date has focused on controlling particle velocity and emergent collective motion phenomena more than emergent structural properties or interactions between active and passive matter. Therefore new ideas are sought for how to experimentally realize structural active matter in the laboratory in order to explore hypotheses related to critical phenomena and dynamics of active condensed matter systems along with thermodynamically efficient adaptation and self-organization of their macroscopic properties.

Challenge Topic 2: Foundations for Robotic Play and Situated Experimentation.

As many advancements as have been made in AI and related fields, solutions to problems remain brittle, confined to highly structured tasks, and/or require too much effort/supervision (e.g. data labeling). Humans are able to intelligently handle new situations and improvise solutions using our "general intelligence." The foundation of that intelligence seems to come largely from "play" as children, where we learn about how to interact with the world by trying and failing at things on our own. Is it possible for robots to develop deeper capabilities and problem solving abilities through a more general embodied intelligence using self-directed exploration? See also, "Situated Experimental Agents for Scientific Discovery", Science Robotics, Vol 3, Issue 24, 2018.

Several lines of inquiry are important to achieving this:

  • Self-direction. How is the goal established, whether observational or achievement-based? How can emotions such as boredom, curiosity, and fun/amusement be encoded as algorithms driving the system to attempt novel actions? How can fear be encoded to assure reasonable safety, but in such a way that it is not overly restrictive (e.g. guaranteeing death will not result, and limiting risk of breaking bones, but not entirely preventing that possibility)?
  • Knowledge. How can observations during an action sequence be captured as generalizable knowledge? How does the agent know what are the relevant parameters to learn (e.g. coefficient of friction, restitution, etc.)? To what extent can a priori knowledge of physical principles bend the learning curve?
  • Efficiency. How are goals/actions chosen so as to maximize learning with as few repetitions as possible? Multiple principles may be extracted from a single experiment or action sequence. Humans are capable of "zero-shot" and "one-shot" learning, where we are able to either visualize or predict what would happen, or learn it with one example. What kinds of tasks could be amenable to learning with very few examples?
  • Application. How can the agent recognize non-obvious similarities between two disparate situations to act on prior knowledge, akin to our "gut feeling"?

Technical Point of Contact: Dr. Samuel C. Stanton, samuel.c.stanton2.civ@mail.mil, (919) 549-4225

1.4 Propulsion and Energetics

Propulsion and Energetics Research supports the Army's need for higher performance propulsion systems. Future systems must provide reduced logistics burden (lower fuel/propellant usage), enhanced insensitivity to inadvertent ignition, and longer life than today's systems. Fundamental to this area are the extraction of stored chemical energy and the conversion of that energy into useful work for vehicle and projectile propulsion. In view of the high temperature and pressure environments encountered in these combustion systems, it is important to advance current understanding of fundamental processes as well as to advance the ability to make accurate, detailed measurements for the understanding of the dominant physical processes and the validation of predictive models. Thus, research in this area is characterized by a focus on high pressure, ignition and combustion processes and on the peculiarities of behavior in systems of Army interest.

1.4.1 Engines

Research on combustion phenomena relevant to engines is focused on intermittent reacting flows encountered in diesel combustion chambers and on continuous combustion characteristics of small, gas turbine combustors. Optimizing engine performance, through understanding and control of in-cylinder combustion dynamics, while retaining high power density, is a major objective. This focus leads to emphasis on fuel injection processes, jet break-up, atomization and spray dynamics, ignition, and subsequent heterogeneous flame propagation that will elucidate the underlying physics of these processes. Research on heterogeneous flames requires supporting study into kinetic and fluid dynamic models, turbulent flame structure, soot formation and destruction, flame extinction, surface reactions, multiphase heat transfer, and other factors that are critical to an understanding of engine performance and efficiency. An additional consideration is the high pressure/temperature environment encountered in advanced engines, which influences liquid behavior and combustion processes at near-critical and super-critical conditions. Fundamental research is needed in many areas, including low temperature physical and chemical rate processes, and combustion instability effects at low temperatures. New characterization methods to investigate kinetics and flame phenomena at high pressure are needed. New computational and theoretical methods to be able to model complex reacting systems are also needed. With advances in sensing, modeling, and control architectures, it is becoming possible to further optimize the performance of combustion systems. Providing the foundations for such active control is also a goal of the program.

1.4.2 Propellant Combustion Processes

Research on propellant combustion processes isfocused on understanding the dynamics of the planned and inadvertent ignition and subsequent combustion of energetic materials used for propulsion in gun and missile systems and in ordinance. The program is also addressing the characterization of advanced energetic materials, e.g., those based on nanoscale structures and/or ingredients. Basic research is needed in several areas, including: condesnsed phase reactions, flame spreading over unburned surfaces (particularly in narrow channels); surface reaction zone structure of burning propellants; chemical kinetics and burning mechanisms; propellant flame structures; characterization of physical and chemical properties of propellants and their pyrolysis products; and coupling effects among the ignition, combustion, and mechanical deformation/fracture processes. The use of advanced combustion diagnostic techniques for reaction front measurements, flame structure characterization, and determination of reaction mechanisms is highly encouraged. Especially of interest are novel methods which can well characterize the ignition and burning behavior of a material utilizing only minute quantities of that material. Complementary model development and numerical solution of these same ignition and combustion processes are also essential. There is also need to understand the unplanned or accidental ignition of energetic materials due to stimuli such as electrostatic discharge, impact, friction, rapid straining, etc. This requires, for example, research on the processes of energy absorption and energy partitioning in the materials, the effect of mechanical damage on the ignition events, and other topics relating to the safety of energetic materials.

Technical Point of Contact: Dr. Ralph A. Anthenien, ralph.a.anthenien2.civ@mail.mil, (919) 549-4317

 

Last Update / Reviewed: January 20, 2015