ARO Mechanical Sciences

Overview

Research supported in the mechanical sciences portion of the Mechanical Sciences Division of the Army Research Office (ARO) is concerned with a broad spectrum of fundamental investigations in the disciplines of fluid dynamics, solid mechanics, structures and dynamics, 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, 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 structures and dynamics 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.

1.0 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 and 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. 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 blade-vortex interaction noise depends strongly on the accurate prediction of the rotor wake, and 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 rolls up to form vortex filaments is not now adequately simulated for rotorcraft load distributions. In fact, under certain flight conditions, multiple vortices are observed to form due to negative lift over the blade tip. The application of non-intrusive optical diagnostic techniques should yield new phenomenological understanding for the study of multiple vortices, wake structures, and wake development. New numerical algorithms or different techniques to increase accuracy and reduce the computational requirements are required.

1.2. Unsteady Aerodynamics. A high level of unsteady flow, which cannot be adequately predicted by steady or quasi-steady approaches, can characterize the flow field around many modern Army weapons systems. One classical example of very high Army relevance occurs on the retreating blade of a helicopter rotor, where the high angles of attack experienced by the retreating blade of the helicopter rotor leads to boundary-layer separation followed by load and pitching-moment overshoots. Mild separation causes increased vibration and reduces performance, while severe dynamic stall leads to unacceptably large vibratory loads and limits forward flight speeds, load, and maneuver capabilities. The physics of this 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. Improved theoretical and numerical simulation is needed for understanding the unsteady separation process and evaluating concepts for separation control. The simulations must be capable of accounting for transition of the boundary layer (and under some circumstances, the transition in the separating free shear layer). Detailed experimental measurements of velocity and pressure are needed in the separating region for the fundamental understanding the separation process, the development of new turbulence models valid during the stall process, and the validation of numerical simulations: here, the current focus is on quantitative flow field measurements rather than merely quantitative measurements on the airfoil surface. These measurements will probably require the use of new non-intrusive optical methods. Combined experimental and numerical efforts towards 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 occurs on maneuvering missiles and projectiles. As future emphasis in flight vehicle control and "smart" systems pervades munitions design, advances in aerodynamic phenomena, such as dynamic high alpha separation, vortex shedding, control surface/vortex interaction, divert thruster/vehicle interaction, roll control stability, and propulsion system integration will be required. New composite material vehicles will have stringent thermodynamic limits and enhanced nonlinear aero elastic response to maneuver forces. Smart structures and micro-electro-mechanical systems (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 require the prediction and experimental verification of complex nonlinear transient flow fields. This will require improved CFD for turbulent flow separation prediction, large eddy simulation, vehicle vortex interactions, and accurate computations of gross flow field response to MEMS boundary layer flow perturbations. Parallel developments in experimental techniques will be required to measure these complex flow fields to help verify and guide the predictive technology.

1.3. Micro Adaptive Flow Control. Micro-Adaptive Flow Control (MAFC) technologies enable control of large-scale aerodynamic flows using small-scale actuators. MAFC technologies combine adaptive control strategies with advanced actuator concepts like micro-scale synthetic jets: MEMS-based microactuators; pulsed-blowing, plasma actuators; and combustion actuators. These techniques are used to cause the delay, or prevention, of fluid flow separation; induce flow separation in previously unseparated flow; alter supersonic flow shock structure; or otherwise alter the large-scale flowfield 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; and supersonic Army projectiles.

While recent successful demonstrations of the efficacy of MAFC technology have taken place, much of this research has tended to be somewhat Edisonian in nature. Basic research needs to enhance and focus the adoption of this technology include developing fundamental understanding of the method by which MAFC actuation alters the overall flowfield; developing of robust and efficient MAFC actuators featuring greater control authority and higher bandwidth; developing of computational analysis methodologies capable of accurately and efficiently predicting the effect of unsteady MAFC actuation on the entire flowfield; and the integrating of all these technologies into Army systems.

Technical Point of Contact: Dr. Thomas Doligalski, e-mail: Thomas.Doligalski@us.army.mil , (919) 549-4251.

2.0 Solid Mechanics

The light, lethal, survivable, continental United States (CONUS)-based modern Army with quick power projection capabilities around the globe has abiding interest in building the most effective fixed and mobile assets with advanced materials and systems. Weapons, platforms, ammunition, and ground structures are designed with severe weight and volume restrictions, and are frequently limited by material strength and failure. Innovative use of material combinations for specific applications necessitates understanding the behavior of materials and structures under complex and severe constraints. Solid mechanics research plays a crucial role in the prediction of strength, damage initiation and progression, and failure of Army material systems under extreme loading conditions, such as blast and impact. Two major research aims in solid mechanics are to reduce development cost by minimizing the need for expensive testing and to optimize performance. The program seeks approaches based on the underlying physics of solids that form the foundation of optimization tools to enhance performance, while minimizing weight and volume for the design of actual systems. The program focuses on the material and system response to blast, shock, impact, and penetration, as well as the mechanics of heterogeneous systems including computational modeling-based Solid Mechanics Research. Thrust areas related to the development of better protective systems against blast and impact loading are directly related to the challenges facing the current and future Army. Under these conditions, the Army faces research problems with the unique constraints of very high strain rates, large deformations, high pressures, and rapid changes in temperature. Hence, a fundamental understanding of the behaviors of a variety of complex materials and systems, including engineered materials and soft human tissue, is needed. Research approaches should consider the topological effects (both micro- and macro-scale), specific material geometry, layering, and interface properties on the response to blast and impact loading. In addition, it may also address non-traditional concepts including nanotechnology, biologically inspired hierarchical structures, active protection, and the functional degradation of electronic components. Interrelated analytical, experimental, and computational formulations are needed to address these difficult multi-physics problems. Predictive models, validated by well-characterized experiments, are needed to identify dominant mechanisms at relevant scales.

2.1. Blast, Shock, Impact, and Penetration. This research topic addresses the need to understand the response of Army assets to impact and explosive detonation. It integrates approaches based on finite deformation, high pressure and high strain-rate, damage, and failure mechanics. Research should be conducted through a combination of physically based experiments, analysis, and computations. No single material will fulfill all of the Army’s functional requirements. Therefore, combinations of materials such as polymer-, metal-, and ceramic-matrix composites, ceramics, metals, active materials, and functionally graded materials will be needed to achieve the desired thermo-mechanical response. The complexity of Army systems comprised of these materials is compounded by their highly anisotropic and heterogeneous nature, and the severe gradients and complex stress states resulting from blast and impact loading environments. Paramount to this effort is a better understanding of the mechanics of interfaces and impact mechanisms, such as high velocity behaviors that might occur at the penetrator/target interface or within developing cracks at macroscopic and microscopic scales. An important aspect of this research area is the deformation and fracture of materials under high strain rates (up to 107 s-1), large strains (up to 500%), high temperatures (up to the melting point), and high pressures (up to 5 GPa). Innovative research on processes in materials and structures that absorb energy, deflect penetrators, and/or laterally disperse momentum is strongly encouraged.

Blast and impact into brittle materials presents special challenges due to cracking and comminution of material ahead of the penetrator, high-speed granular flow of comminuted material, and the mixing of eroded penetrator material and comminuted target material. Ceramics and geological materials exhibit extreme sensitivity to defects and loading histories, which may result in highly rate dependent failure strengths and the propagation of failure waves. Metals are susceptible to inelastic deformations leading to ductile failure under low to medium loading rates, but transition to more brittle behavior under higher loading rates. In addition, heat generation, and thermal softening occur in metals, further complicating the behavior of these materials under extreme loads and loading rates. Blast and impact of composite systems present still another set of challenges that result from their complex microstructure, material boundaries, and the number of different and coupled failure modes that may occur. Since personnel protection is a key objective for Army platforms, an understanding of the response of biological tissue to blast and impact is essential.

All of this requires greatly improved understanding, effective modeling, and efficient computational schemes. Because blast and impact events often involve erosion and sliding of both the projectile and the target, explicit modeling of these processes with friction-based theories and computational techniques are essential. Computational methods for treating discontinuities in a three dimensional context are required. These methods must concentrate not only on the techniques required to track a moving boundary, but also on the relevant physics and mechanics associated with those surfaces. Examples might include boundaries between dissimilar materials, shock fronts, elastic/plastic boundaries, phase boundaries, shear bands and cracks, as well as penetrator/target interfaces. Constitutive models should be three-dimensional and should allow for system nonlinearities.

In blast and impact environments, complex interactions between the shock and release waves usually initiate damage mechanisms in the target; accurate modeling of the target behavior will require controlled, high fidelity experiments. Therefore, innovative experimental techniques that incorporate high-speed data acquisition and imaging are necessary to capture the deformation processes and relative motion between surfaces. Accurate experimental techniques are required to delineate the nature, timing, and evolution of damage and failure in heterogeneous materials. An important aspect of this area of research is the development of novel experimental techniques that can be used to generate data for the wide ranges and combinations of strain rates, strains, temperatures, and pressures of interest.

2.2. Mechanics of Heterogeneous Systems. The mechanics of heterogeneous structures involves the development of integrated analytical, computational, and experimental approaches to investigate the response of hybrid structures that may include combinations of high strength and lightweight engineered composites, ceramics, and functionally graded materials. Heterogeneity at all scales should be considered, from nanomaterials to systems created through combinations of different materials at larger scales. 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. Physically based structural design guidelines for energy absorbing structural systems comprised of tailored combinations of materials and heterogeneities at different length and time scales are sought. There are continuing technology barriers that need to be overcome if reliable Army structures such as helicopters, ground vehicles, bridges, and weapons systems are to be designed, manufactured, and maintained over a long period of time. Of special interest to the Army is the thermo-mechanical response at strain rates encountered in high-speed impact or explosive loading. Probabilistic as well as deterministic approaches are encouraged. Phenomena of interest are wave propagation, scattering, dispersion, damage evolution, and failure.

At appropriate length and time scales, the quantitative prediction and measurement of parameters related to dominant heterogeneities and mechanisms are needed for specific material systems in order to relate nano and micro effects to the macro scale. Deterministic and statistical scaling methodologies for toughness, strength, and geometrical effects that account for the multitude and variability of heterogeneities such as interfaces, interphases, particulate dispersion, fiber volume fraction and distribution, constituent shape, and their combined effects on failure are needed. Innovative methods and models to control material properties and damage by graded interfaces, coatings, and mechanical impedance mismatches are required. Constitutive relations for multi-scale mechanisms should include failure and damage criteria, which are mechanism-based and experimentally verifiable. The determination of universal scaling laws that can be used to bridge physical scales would greatly enhance our understanding and prediction of phenomena such as inelastic deformations, localization, distributed damage and failure, and fragmentation.

2.3 Computational Modeling Based Solid Mechanics Research. Over the past decades, the finite element methods that were developed in the mechanics community have had a tremendous impact on engineering practice and Army designs. Although these methods have been highly effective for linear and some nonlinear analyses, substantial breakthroughs are needed for failure modeling and representations of blast and impact live-fire tests. Commercially developed algorithms that rely on the deletion of elements to model the nucleation and propagation of cracks are inadequate. These algorithms are highly sensitive to the size and mesh arrangement and distribution. Furthermore, no convergence theories or even empirical evidence of convergence has been obtained so far for these methods. New theories are needed that can overcome the limitations of traditional continuum fracture models that are based on crack tip singularities. New principles need to be devised for the construction of failure surfaces and cohesion strength. New methodologies are needed to account for energy dissipation due to crack progression, large-scale yielding, and interfacial separation. It has become important to be able to develop material properties in terms of subscale models because, with the rapid advent of new material systems, testing is becoming prohibitively time consuming and expensive. New theoretical approaches are needed to develop hierarchical methods for failure that account for the inherent statistical aspects of failure. Methodologies that explore fracture processes at the micro-scale and relate them to the meso- and macroscopic levels should be investigated. Computational models for the creation of free surfaces that are mesh independent and that incorporate evolving time-dependent boundary conditions and physically based failure initiation criteria need to be developed. These computational models should capture the complex interactions of failure processes and defects in three dimensions. To achieve the requisite accuracy in three-dimensional problems, effective adaptive methods that can treat crack propagation, shear band formation, and other failure modes are needed. The capability to model fragmentation, contact, and penetration is also of strong interest.

Technical Point of Contact: Dr. Bruce LaMattina, e-mail: Bruce.LaMattina@us.army.mil, (919) 549-4379.

3.0 Structures and Dynamics

A significant challenge facing Army laboratory engineers is the determination of the influence of inertial, thermal, electrical, magnetic, impact, damping, and aerodynamic forces on the dynamic response of adaptive armament systems, ground vehicles, rotorcraft, missiles, projectiles, gears, parachutes, and shelters. Such challenges are of fundamental importance to the design and construction of affordable, reliable, durable, and maintainable Army equipment with acceptable levels of personnel safety and comfort. Consequently, the ARO is supporting basic research in these areas, with emphasis on air vehicle dynamics, including missile, unmanned air vehicles, and rotorcraft dynamics; the dynamics, non-linear vibrations, structural control, and simulation of land vehicles and weapon systems; and the dynamic response of structural components and systems fabricated from advanced composite materials, with or without embedded actuators and sensors. Submittal of fundamental research proposals on the general topics described above is encouraged, keeping in view the paramount importance of Army relevance. More specific details of the program’s predominant thrust areas are described in the following paragraphs.

3.1.Structural Dynamics and Simulation. This topic consists of five thrusts: smart structures, structural dynamics, structural damping, active structural control, and structural health monitoring. Advances in these areas are required to improve capabilities of modeling, computing the dynamic response, reducing noise levels, suppressing vibrations, detecting the presence of damage, and assuring the integrity and performance of structural components used in military systems.

Adaptive structures are currently being considered for application in helicopter rotor systems, missiles, projectiles, electromagnetic antenna structures, unmanned air vehicles, and land vehicles and weapon systems. They offer opportunities, for example, to realize structural vibration suppression or isolation in rotorcraft and weapon systems, unsteady load control on rotor blades, reduction of blade/vortex interaction noise, airfoil shape change, gust load alleviation, aeromechanical stability augmentation, beam shaping and steering in antennas, and structural health monitoring. Research areas include sensors and actuators, formulation of suitable constitutive relations, improved structural damping concepts, modeling and optimal design of smart composite structures, finite element formulations, and control algorithms. Concepts for novel sensing and actuation techniques, based on MEMS, nanotechnology or other innovative concepts, are encouraged. New active damping techniques, based, for example, on combinations of viscoelastic, carbon nano-tubes, and active materials, combined with shunted electric circuits and non-linear adaptive control strategies, have emerged as candidates for improving structural performance and reliability. Topics of interest include the role of viscoelastic materials, nano-technology, constitutive equations, elastomeric dampers for missiles and rotorcraft, magnetorheological fluid dampers, modeling and design, actuation of missile flight control surfaces, non-linear control techniques, and techniques for including damping effects in mathematical and computational models.

The trend toward the increasing use of composite materials in the fabrication of military vehicles to reduce their weight and augment fuel efficiency requires that Army engineers have the tools necessary to predict the static and dynamic response of composite structures. During the course of service, virtually all-composite structures should be monitored to assure their condition of health and integrity to prolong their life span or prevent catastrophic failure. Recent developments in sensor and actuator technologies have opened the way to develop new diagnostic technologies particularly suitable for composite materials. Such enhancements might involve approaches such as wavelet transforms, neural networks, fuzzy logic, probabilistic estimations, system identification, electro-mechanical impedance methods, electric impedance tomography, etc. The development of the associated software will have to include the presence of distributed sensors, actuators, and controllers based on fiber optics, piezoelectric materials, (MEMS) devices, or other concepts. The development of new active materials, such as relaxor ferroelectrics and alkaline-based piezoelectric materials, has recently been reported by the materials science research community. These materials appear to offer significant opportunities to create improved actuation devices that will deliver greater authority (force, stroke) than do the conventional piezoelectric materials. The potential of new actuators in Army applications should be energetically pursued.

The assurance of structural reliability of military air and land vehicles and weapon systems will greatly enhance confidence in their safety, reduce the probability of mission failures, and diminish the costs of operation and maintenance. An important element in achieving reliable systems is a strong capability of inspecting and assessing the physical condition of critical structural components. Significantly improved techniques for inspection, analysis, and interpretation are urgently needed to facilitate the assessment of the health of a structure and to promote the design, fabrication, and reliable operation of future and current military systems. Inability to detect damage in heterogeneous structures that may comprise combinations of composites, ceramics, and metals is a limiting factor to their use in practice. The application of active materials to the development of novel sensing techniques, such as MEMS, and the ability to interpret sensor signals effectively and accurately in nearly real time are fundamental for improving the reliability of physical systems. Miniaturized sensory devices could be incorporated into heterogeneous structures to signal the presence, location, and extent of local and global failure modes, such as fiber breakage, fiber pull-out, delamination, and large matrix structural cracking. Accordingly, new design and maintenance technologies are critically needed for military systems. An idea that shows considerable promise in reducing operating costs while enhancing system safety is the concept of condition-based operation. This is a concept that encompasses maintenance, system characteristics, scheduling, and operations. Condition-based operation attempts to enhance the reliability and survivability of the system under adverse conditions, such as battle damage and critical system failures, using online system identification, health monitoring and failure detection, and adaptive fault-tolerant reconfigurable operational control. With advances in micro-sensors (including MEMS devices), piezoelectric actuator technology, system identification, information technology, adaptive control theory for sensor nets and wireless telemetry, condition-based operation of military systems will lead to enormous gains.

3.2. Air Vehicle Dynamics. Rotorcraft aeromechanics analytical prediction capability must be improved to increase military effectiveness of rotorcraft through better mission performance, improved availability and dependability, and reduced lifecycle costs. Advanced comprehensive analyses must address rotor blade control surface devices that use aerodynamic forces to excite structural response to minimize blade and fixed system vibratory loads and/or to improve the vehicle's aeroelastic stability characteristics. Of great importance in helicopter dynamics is the development of numerical analysis tools that are applicable to the special challenges associated with moderate to very large systems of equations (typically finite element based) that are needed to determine solutions for rotorcraft trim, periodic response, and transient behavior. The types of numerical analyses that are needed include 1) the determination of the periodic solutions to the equations (both stable and unstable orbits) and of the unknown parameters that are associated with a specified flight condition, 2) traditional constant and periodic coefficient eigenanalysis of these system orbits and limit cycle or chaotic behavior of unstable orbits, and 3) determination of optimal design, optimal trim, and optimal control of such systems. The dynamics and control of micro-aerial vehicles is also of interest to the program.

Smart structures concepts offer the Army the potential to address critical problems in helicopter systems including vehicle vibration suppression, control of rotor blade vibratory loads and fatigue stress; reduction of interior and exterior noise; gust load alleviation; enhancement of rotor aerodynamic efficiency and performance; and augmentation of aeroelastic/aeromechanical stability. These advances may be achieved by using smart structures approaches, for example, to twisting the rotor blade along its length, actuating flap or elevon control surface at the blade trailing edge, or changing the airfoil camber or leading edge shape. The development of control algorithms is needed to tailor the inputs to multiple actuation sites, integrate information from multiple sensors, and optimize overall controller architecture including the development of appropriate data processing and software techniques.

The Army's requirement to deploy Soldiers and equipment rapidly and safely dictates the use of parachute insertion, usually at high speed and low altitude, to minimize detection and exposure to enemy fire and maximize the drop accuracy. Parachute deployment and inflation is a challenging problem in aeroelasticity requiring multidisciplinary modeling for coupling the structural deformations of the parachute material with the three‑dimensional and highly unsteady aerodynamic environment. Prediction of a parachute system's response to user control and environmental factors once deployed also requires a coupled approach. For instance, an airdrop problem for which no three‑dimensional coupled simulation capability currently exists is that of predicting the aerodynamic performance of fully deployed airdrop systems such as a steerable parafoil or a steerable round or cross canopy. Issues include determination of 1) the lift to drag ratio of such systems, 2) the outcome of a control input, and 3) the system response to environmental inputs such as winds.

3.3. Weapon System and Land Vehicle Dynamics. The overarching goal of weapon system research is to improve firing accuracy. Improved weapon system accuracy reduces the number of rounds required to complete a mission; thus, the ammunition logistics requirements of a unit are reduced. Vehicle generated disturbances (environmental or internal) and firing disturbances excite the structural dynamics between the sighting system and the weapon mount and the dynamics of the weapon itself. Innovative, unique, and far-reaching research is required to explore fundamental issues in simultaneous control and structure design; ultra-high performance hybrid weapon drive systems; smart structures for vibration suppression and micro-positioning of gun barrels, high speed emplacement mechanisms and non-traditional barrel structures. Specific areas include mechanism theory and optimization, vibration, multi-body dynamics, smart materials, distributed servo control, software development tools for mechanical design, and optimization.

Numerous large, complex mechanical systems used by the Army consist of interconnected multi-body structures, e.g., heavy machinery, wheeled/tracked military land vehicles, machine tools, rotorcraft, weapon systems, etc. These complicated systems often consist of numerous combinations of rigid and flexible elements. New and innovative approaches are needed for the efficient analysis, design, and control of large vehicles that consist of interconnected flexible bodies. Recent advances in computer and graphics hardware and software capabilities are stimulating recent advances in motion based simulators with computer generated imagery that interfaces vehicle dynamic models and their physical environments. Innovative approaches for modeling the deformation of vehicle system components based on the finite element method and experimental identification techniques are needed to develop more detailed models of complex vehicles. Examples of potential research areas are automatic formulation of the constrained equations of motion; symbolic equation processing: generation of computational methods and associated computer codes; algorithm optimization for computer architectures; model reduction and error quantification techniques; fluid payload dynamics; suspension systems and control; weapons positioning control; optimization techniques; and nonlinear control algorithms.

Technical Point of Contact: Dr. Bruce LaMattina, e-mail: Bruce.LaMattina@us.army.mil, (919) 549-4379.