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 sub-discipline, 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 sub-disciplines, 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 micro adaptive 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 sub disciplinary areas.

1.1 Fluid Dynamics

Research in fluid dynamics supports the development of improved or new technology for advanced helicopters, small gas turbine engines, improved airdrop (parachute) systems, maneuverable high-speed missiles, high performance gun-launched projectiles, and miniature unmanned air vehicles. While basic research studies that address the fundamental flow physics underlying these devices are solicited, innovative research in the specific topical thrust areas listed below is especially encouraged.

1.1.1 Vortex-Dominated Flows

In contrast to fixed-wing aircraft, rotorcrafts always operate under the influence of their own wakes. The prediction of rotor performance, vibratory loads, and the associated noise due to blade-vortex interaction depend strongly on the accurate prediction of the rotor wake. However, the prediction methodology of this wake remains one of the major challenges in fluid mechanics. Current computational fluid dynamics (CFD) approaches are computationally intensive, especially for Eulerian methodologies where the vorticity diffuses numerically through the grid points and makes predication inaccurate. The process by which vorticity is shed by the blade, and its eventual roll-up to form vortex filaments are not now adequately simulated. In fact, under certain flight conditions and due to negative lift, multiple vortices are observed to form over the blade tip. The application of non-intrusive optical diagnostic techniques should yield new phenomenological understanding for the study of multiple vortices, the wake structure, and its development. New numerical algorithms or different techniques to increase accuracy and reduce the computational requirements are required.

1.1.2 Unsteady Aerodynamics

The flow fields around many modern Army weapons systems are characterized by a high level of unsteady flow, which can neither be predicted adequately by steady nor quasi-steady approaches. One classical example of unsteady aerodynamics of very high Army relevance occurs on the retreating blade of a helicopter rotor blade. At high angles of attack the retreating blade of the helicopter experiences boundary-layer separation that leads to load and pitching-moment overshoots. While mild separation causes increased vibration and reduces system performance, severe dynamic stall leads to unacceptably large vibratory loads, severe limitation to forward flight speed, and diminished maneuver capabilities. The physics of this unsteady flow phenomenon are known to depend on the Mach and Reynolds numbers of the flow, and hence future research in this area needs to be performed under realistic flight conditions. In addition, improved theoretical and numerical simulation is needed for the understanding of the unsteady separation process, and for the evaluation of concepts that may lead to greater flow separation control. Flow field simulations must be capable of accounting for boundary layer transition, and under some circumstances, the transition of the separating free shear layer. To achieve a fundamental understanding of the separation process, detailed experimental measurements of velocity and pressure in the separating region and the development of new turbulence models that are valid during dynamic stall are needed. In this regard, the validation of numerical simulations must focus on quantitative flow field measurements rather than the mere quantitative measurements on the airfoil surface. In addition, the flow field measurements require the use of new non-intrusive optical methods. Combined experimental and numerical efforts towards the control of unsteady separation using passive and active flow control, including the emerging field of Micro-adaptive Flow Control, are also sought.

A second example of the importance of unsteady aerodynamics to the Army occurs during the maneuvering of missiles and projectiles. As future emphasis in flight vehicle control and "smart" systems pervades munitions design, greater understandings of unsteady aerodynamic phenomena, such as; high alpha dynamic separation, vortex shedding, control surface/vortex interaction, divert thruster/vehicle interaction, roll control stability and propulsion system integration, will be required. It is anticipated that new composite material vehicles will have stringent thermodynamic limits and enhanced nonlinear aero elastic response to maneuver forces. Smart structures and MEMS technology will redefine control strategies, control surface shape and control surface dynamics, consequently driving fluid dynamics into new areas of research. All of these developments will require the prediction and experimental verification of complex nonlinear transient flow fields. In turn, these developments will require improved CFD for turbulent flow separation prediction, large eddy simulation, vehicle vortex interactions, and the accurate computations of gross flow field response to MEMS boundary layer flow perturbations. Parallel developments in experimental techniques will also be required to measure these complex flow fields in efforts to verify and guide the predictive technology.

1.1.3 Micro Adaptive Flow Control

Micro Adaptive Flow Control (MAFC) technologies enable the control of large-scale aerodynamic flows using small-scale actuators. MAFC technologies combine adaptive control strategies with advanced actuator concepts; such as, micro-scale synthetic jets, microelectromechanical systems (MEMS)-based microactuators, pulsed-blowing, plasma actuators, and combustion actuators. MAFC techniques are used to cause the delay, or prevention, of fluid flow separation; to induce flow separation in previously unseparated flow; to alter supersonic flow shock structure; or to otherwise alter the large-scale flow field and provide overall system benefit. Army systems for which MAFC is currently being investigated include on-blade controls, dynamic stall control on helicopter rotor blades, separation control for drag and buffet reduction on helicopters, surge and stall control within Army gas turbines, and dispersion reduction and terminal guidance of subsonic, transonic, supersonic and future hypersonic Army projectiles.

Even though many successful demonstrations of the efficacy of MAFC technology have taken place, much of the supporting research has tended to be somewhat Edisonian in nature. To enhance and focus the adoption of MAFC technology into Army weapon systems, basic research is needed. MAFC research is needed but not limited to the following areas: the development of fundamental understandings of the methods by which MAFC actuation alters the overall flow field; the development of phenomenological models for flows and surface forces in micro and nano channels; the development of robust and efficient MAFC actuators leading to greater control authority and higher bandwidth; the development of computational analysis methodologies capable of accurately and efficiently predicting the effect of unsteady MAFC actuation on the entire flow field; and the development of methods to integrate all these technologies into Army systems.

Technical Point of Contact: Dr. Bryan Glaz, e-mail: bryan.j.glaz.civ@mail.mil,(919) 549-4322.

1.2 Solid Mechanics

Solid mechanics research plays a crucial role in the prediction of strength, damage initiation and failure progression of Army material systems under extreme loading conditions, such as blast, shock, impact, penetration, and thermal cycling. This research topic addresses the need to understand the response of Army assets to high rate impact and explosive detonation. It integrates approaches based on finite deformation, high pressure and high strain- rate, damage, and failure mechanics. Innovative use of material combinations for specific applications necessitates understanding the behavior of military systems under complex and severe constraints. The program seeks to develop an understanding of the underlying physics of solids that form the foundation of optimization tools to enhance performance, and minimize weight and volume for the future design of Army and DoD systems. Research approaches should consider the topological effects (both micro- and macro-scale), specific material geometry, layering, and interface properties on response to high rate loading. In addition, solid mechanics approaches may address non-traditional concepts including biologically inspired hierarchical structures, soft materials, human tissue and functional degradation of electronic components. Research should be conducted through a combination of physically based experiments, analysis, and computations to address these difficult multi-physics problems. Predictive models, validated by well-characterized experiments, are needed to identify dominant failure mechanisms at relevant scales. An important aspect of this research area is the deformation and fracture of materials under high strain rates (>107 s-1), strains (ranging from 2% up to 100%), high temperatures (up to the melting point), and high pressures (up to 5 GPa). An important aspect of solid mechanics research is the development of novel techniques to expand the design space of new hierarchical design principles for the purpose of creating microstructures that eliminate traditional inverse material property relationships to enable a combination of disparate properties (i.e. strength and toughness, strength and density, hardness and ductility, etc...).

1.2.1 Mechanics of Heterogeneous Systems

The mechanics of structures involves the development of integrated analytical, computational, and experimental approaches to investigate rate dependent behavior of heterogeneous materials that may include combinations of high strength and lightweight functionally graded material systems. Experimental and computational techniques are needed to optimize material microstructure as well as the topology of systems to provide the desired structural response for specific boundary and loading conditions. Of special interest is the thermo-mechanical response at strain rates encountered in high-speed impact or explosive loading. Phenomena of interest are wave propagation, scattering, dispersion, and progressive failure behavior.

Quantitative prediction and measurement of parameters related to dominant heterogeneities and mechanisms are needed at appropriate length and time scales for specific material systems in order to relate atomistic and micro effects to the macro scale. Deterministic and statistical scaling methodologies are needed for toughness, strength, and geometrical effects which account for heterogeneities such as interfaces, interphases, reinforcement distribution and their combined effects on failure. Constitutive relations for multi-scale mechanisms should include mechanism- based and experimentally verifiable failure and damage criteria to investigate hierarchical structures for blast- and ballistic-impact mitigation. The integration of sensor technology and the concept of adaptive microstructures may be explored for enhanced control of energy dissipation and load transfer under multiple loading conditions.

1.2.2 Mechanics of Soft Materials & Biologic Systems

The mechanics of biologic structures involves the development and validation of a hierarchical approach to accurately predict stresses, strains, and cavitation in biologic tissue resulting from blast shock waves and high rate blunt trauma. The development of improved protection and injury prevention for military specific injuries requires experimental techniques capable of characterizing the response of soft materials in compression, tension, or shear for strain rates that range from 102 to105 s-1. High rate loading of different durations and amplitudes may lead to cascading events that cause functional loss and impairment of human tissues. Identification of resultant fundamental processes ranging from molecular changes to physical injury is in their infancy for high rate loading conditions. Such identification will lead to the quantification of high fidelity injury thresholds and novel protective solutions. Several aspects of tissue damage are of interest: (a) high strain rate characterization of constitutive behavior of damaged tissues, (b) damage initiation and evolution at the cellular level, (c) force transduction through temporal and spatial scales from organs to cellular substructures, and (d) effect of blast load (pressure and electromagnetic loads, toxicity, etc.) on short and long term functionality of lower extremities. The aforementioned issues must be placed in perspective of high rate loads and their transmission through protective equipment, human tissue and bone. Advances require interdisciplinary teams combining solid mechanics and biological sciences.

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

1.3 Complex Dynamics and Systems

Modern research concerning the dynamics of complex and innovative engineering systems to enable unprecedented Army operational capabilities requires a highly interdisciplinary approach spanning many aspects of engineering, mathematics, complexity science, and physics as well as the dynamics of biological structures and systems. Numerous research needs of Army interest are driven by interactions amongst interconnected dynamics and field phenomena ranging from thermal fluctuations in nanosystems to nonlinear aeroacoustic induced vibrations. Consequently, the Complex Dynamics and Systems program encompasses a broad research spectrum and seeks to cultivate a cadre of avant-garde engineers, mathematicians, and scientists capable of skillfully transcending traditional disciplinary bounds to generate leap-ahead fundamental understanding and pivotal innovation across the full range of length scales concerning the underlying physics, information flow, mathematical modeling, and control of isolated or interdependent dynamical systems. Further information on the programmatic thrust areas of greatest interest are detailed in the paragraphs that follow.

1.3.1 Nonlinear Dynamics, Force Generation, and Field Interactions

Nonlinear dynamics is a rich research endeavor of direct impact on a multitude of Army systems from nano/microsystems to conventional rotorcraft aeromechanics. Research in this thrust focuses on novel mathematical, numerical, and reduced order modeling methods, fundamental physical understanding, and exploitation of structural, multi-body and multi-physics dynamical systems whose state transition and control parameter behavior are necessarily characterized by either strong, smooth, or non- smooth nonlinearities; holonomic and nonholonomic constraints; stochastic dynamics; or high- dimensional interactions with granular media and field phenomena. Novel methods for reduced order modeling and exploiting these complex behaviors and interactions for superior performance are especially sought. Research topics of interest are diverse and some examples include (but are not limited to): non-Lyapunov based stability theory of nonlinear systems; nonlinear inverse problems and system identification; geometric mechanics; chaotic and stochastic transitions; symmetry; integrability and stability of nonholonomic systems; nonlinear aeroelasticity and wake/vortex/acoustic-field induced structural oscillations; de-stabilizing time-delays; bio- dynamics; wave phenomena, multi-body, and structural interactions within granular media of varying viscosity; nano- to macro-scale continuum field interactions (thermal, fluid, electromagnetic, photomechanical, viscoelastic); chaos, instabilities, bifurcations, chimera states, and synchronization of coupled nonlinear oscillators; solitons; high-frequency oscillations; nonlinear, impact, blast, and stochastic excitations of nonlinear systems; opportunistic exploitation of nonlinearities and unique physical interactions in NEMS/MEMS; nonlinear dynamics of multi- body hybrid translational/rotary systems with nonlinear/non-smooth connections; intelligent force perception, interpretation, and action in compliant environments; nonlinear physics of dissipation, slipping, and damping; or tightly coupled unsteady aeroelastic-rigid body dynamics. Inspiration from nature's solutions to problems relevant to micro-vehicle dynamics and robotics is of value, however, understanding how competing factors in biology lead to sub-optimal solutions that modern engineering science can overcome may lead to truly innovative solutions.

1.3.2 Multi-Dimensional and Dissipative Dynamical Systems

This thrust focuses on the mathematically rigorous methods and physics of continuous and discrete high-dimensional dynamical systems; the engineering science of complexity; renormalization, projection-operator, and uncertainty propagation methods in spatiotemporally heterogeneous dynamical systems; and development of a formalism for systems synthesis. Novel methods for the engineering analysis of high-dimensional linear, nonlinear, and stochastic dynamical systems is sought to include model order reduction, stability, bifurcations, attractors, uncertainty propagation, statistical fields, etc. In large-scale networks of interdependent dynamical systems, temporal anisotropy, spatial non- locality, and multiscale processes precipitate local dynamics to global responses in a non- straightforward manner such that ergodicity may only apply at certain spatiotemporal scales and hierarchies. Fundamental investigations are required to develop the abstract formalisms and frameworks guiding the scaling analysis, causality, energy and multidimensional information flow, hierarchal, adaptive dynamics, multi-objective synthesis, and new mathematical performance metrics of increasingly complex and dissipative (non-equilibrium) systems comprised of distributed, interdependent, and heterogeneous subsystems. The engineering science of complexity constitutes an effort to leverage the unprecedented insight from complex systems science regarding a wide variety of biological, economic, and social systems. While discovery and understanding of complex systems in nature, economics, and society are of profound value and impact, our ability to exploit this knowledge to engineer the controllability, fragility, propensity for self-organization, and/or robustness of interdependent dynamical systems will demonstrate true mastery. Of particular interest are methods and techniques from complexity science, dynamical systems theory, non-equilibrium physics, differential topology, graph theory, stochastic calculus, percolation theory, and associated stability and control principles. Multiscale projection-operator methods and renormalization groups for novel reduced order modeling strategies for complex dynamical systems is of major interest. Within the context of the aforementioned theoretical approaches, characterization of the uncertainty propagation and non- Gaussian stochastic dynamics is of paramount significance. Determining, understanding, and manipulating the emergent behaviors, persistent or transient mathematical structures, self- similarity, chaotic and fractional dynamics, etc, will lead to unprecedented capabilities of complex engineered systems and high dimensional multi-scale simulation. Furthermore, research developing the formalism for systems synthesis and systems-level stochastic nonlinear inverse problem analysis is sought after, where deep understanding of the nonlinear and stochastic dynamics of complex systems drives optimal system modeling. The theoretical framework for determining the efficient allocation of computational resources or component characteristics for the optimum desired systems-level dynamics remains an open challenge and the intricate dynamics of multi-dimensional and complex systems bring about a wealth of challenges in this endeavor.

1.3.3. Strategic Program Challenges.

Strategic program challenges focus on deep questions concerning scientific focus areas deemed beyond the scope of single investigator awards. The challenges emphasize high-risk, high-reward exploratory research to create breakthroughs, push science in truly novel directions, or to support mathematical abstractions and precise physical foundations for emerging technologies deemed likely to be of significant future Army and DoD impact. Potential investigator teams are encouraged to contact the program manager for further information concerning any Strategic Program Challenge topic.

For each topic, proposed efforts should be limited to 48 months and $1.2 million. There is no formal limit on the number of investigators, but white papers should clearly indicate significant, complementary, and meaningful intellectual commitment by each proposed team member. Potential individual investigators interested in a particular aspect of any topic are welcome to discuss single investigator proposals with the program manager. Related single investigator efforts will be considered, with support at a commensurate level.

Topic 1: Energetic Versatility of Muscles: Principles and Emulation

Discern, through both theory and experiment, the fundamental physical principles underlying the energetic versatility of muscles in dynamic operating conditions (e.g. force generation, shock absorption, energy dissipation, power conversion and distribution etc.). Of complementary interest are novel experimental approaches with sustained in-vitro electromechanical coupling as well as compelling new ideas to leverage biophysical principles to potentially engineer new classes of soft active matter exhibiting muscle-like energetic versatility.

Topic 2: Controlling Hyperelastic Matter

Bring together soft condensed matter physics, continuum mechanics, neuromechanics, and under-actuated systems theory to generate novel frameworks for directing complex spatial motions and wave propagation capabilities within soft continua with nonlinear material properties. Theoretical principles are expected to be integrated with novel experimental platforms, possibly leveraging advances in printed programmable matter.

Topic 3: Theory of Morphological Energetics

Develop a formal abstraction of the principles underlying the efficiency of information processing, control, and energetic transformation on the intricate coupling with, and exploitation of, a physical system's shape (either articulated or amorphous), topological structure (functional morphology), and nonlinear dynamics (including transient and non-smooth dynamics). Emphasize the less-considered dynamics of energy and entropy transformations in the context of the challenge objective and present generality of the results across a broad range of data sets and experimental test beds.

Topic 4: Control and Creation of Critical Dynamics

Steer statistical physics towards a science of the artificial. The goal is to develop 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. Questions of interest for this challenge include: Can we develop the concept of an active universality class? Can we dynamically stabilize, through feedback, nonequilibrium states of matter that have no equilibrium counterparts? Can renormalization group theory be unified with control theory through geometry? What is the role of information dynamics in the control of critical dynamics? Proposed efforts should have a strong experimental component to test theoretical abstractions.

Technical Point of Contact: Dr. Samuel Stanton, e-mail: 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) 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, high temperature combustion processes and on the peculiarities of combustion 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 a strong emphasis on fuel injection processes, jet break-up, atomization and spray dynamics, ignition, and subsequent heterogeneous flame propagation. 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. Of particular interest are investigations of fundamental characteristics related to highly stressed engines, such as elevated temperature and pressure combustion, accelerated mixing, and transient heat transfer. Engine performance degradation under low temperature conditions due to reduced fuel volatility, high oil viscosity, poor atomization and vaporization, etc., is a major concern. Fundamental research is needed in many areas, including low temperature physical and chemical rate processes, instantaneous friction and wear mechanisms, and combustion instability effects at low temperatures. New characterization methods to investigate kinetics and flame phenomena at high pressure are needed. New computational 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 major goal of the program.

1.4.2 Propellant Combustion Processes

Research on propellant combustion processes is focused 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: thermal pyrolysis of basic ingredients and solid propellants; 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, 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, e-mail: ralph.a.anthenien2.civ@mail.mil, (919) 549-4317.

 

Last Update / Reviewed: February 25, 2014