Crystal L. Pasiliao

Simulation of Aeroelastic Limit-Cycle Oscillations of Aircraft Wings with Stores

Aircraft wings carrying stores are susceptible to nonlinear aeroelastic limit-cycle oscillations, which can lead to reduced flight and mission performance. Limit-cycle-oscillation dynamics and control simulation studies, although of great importance, are often based on simplified typical-section airfoil models. In this work, the more accurate beam–rod representation is used to capture the spanwise varying displacement of a wing with store. The spanwise variation of wing geometry and structural properties, as well as the presence of multiple stores on rigid or flexible mounts, is efficiently modeled by the primitive-modes approach. Aeroelastic limit-cycle oscillation due to structural nonlinearity is demonstrated via time-marching simulations, as well as the computationally more efficient harmonic-balance method. Some novel forms of limit-cycle-oscillation behavior are observed as the model parameters are varied, and these are explained in terms of the flutter/divergence properties of the base linear aeroe...

Lyapunov-Based Tracking of Store-Induced Limit Cycle Oscillations in an Aeroelastic System

Store-induced limit cycle oscillations (LCOs) are a common issue on current fighter aircraft and are expected to be present on next generation fighter aircraft. Current efforts in control systems designed to suppress LCO behavior have been either linear and restricted to specific flight regimes or nonlinear but require uncertainties in the system dynamics to be linear-in-the-parameters and only present in the torsional stiffness. Furthermore, the aerodynamic model used in prior research efforts neglects any nonlinear effects. This paper presents the development of a controller consisting of a continuous RISE feedback term with a neural network feedforward term to achieve semiglobal asymptotic tracking of the wing angle of attack in the presence of structural and aerodynamic uncertainties that do not satisfy the linear-in-the-parameter assumption.© 2012 ASME

Store-Induced Limit-Cycle Oscillations Due to Nonlinear Wing-Store Attachment

Several high-performance fighter aircraft exhibit store-induced limit-cycle oscillations, leading to pilot discomfort, potential structural fatigue, and flight envelope restrictions. The roles of various aerodynamic and structural factors causing the limit-cycle oscillation are not sufficiently understood, and their numerical exploration via time marching is computationally expensive. In this paper, the effects of nonlinear stiffness and damping in the wing-store attachments of the F-16 aircraft are examined, in the presence of steady flow aerodynamic nonlinearity, using the computationally efficient harmonic balance method. Structural mechanisms including cubic restoring force of both softening and hardening types, freeplay, and Coulomb friction are systematically evaluated, and the most likely among these are identified by comparing the computed limit-cycle oscillation results to flight data. An extension of the harmonic balance method to handle nonlinear unsteady aerodynamics along with structural nonl...

Prediction of Wing Flutter Boundary Using High Fidelity Delayed Detached Eddy Simulation

This paper conducts Delayed Detached Eddy Simulation(DDES) of a 3D wing flutter with free stream Mach number varied from subsonic to supersonic using a fully coupled fluid/structure interaction (FSI). Unsteady 3D compressible Navier-Stokes equations are solved with a system of 5 decoupled structure modal equations in a fully coupled manner. The low diffusion E-CUSP scheme with a 5th order WENO reconstruction for the inviscid flux and a set of 2nd order central differencing for the viscous terms are used to accurately capture the shock wave/turbulent boundary layer interaction of the vibrating wing. The predicted flutter boundaries at different free stream Mach numbers achieve very good agreement with experiment. It appears that the transonic dip phenomenon is due to the anticlimax contribution of the second mode, which is caused by the complicated shock oscillation on the wing.At the flutter boundary including at the sonic dip, no flow separation due to shock/boundary layer interaction is observed.

Micromechanical modeling of metal-ceramic composites for high temperature applications

This paper is concerned with the evaluation of the overall temperature-dependent elastic, thermal, and thermo-elastic material properties of metal-ceramic composites for high temperature applications. Effective properties of an aluminum/zirconia composite are obtained using micromechanics models and finite element analysis of representative volume elements (RVEs). RVE microstructures consisting of mono-sized spherical reinforcement particles embedded in a unit cube matrix were generated using a random sequential adsorption algorithm and event driven molecular dynamics simulation. The adopted algorithm allowed generating microstructures with high volume fractions of reinforcement particles up to 61%. Finite element analysis was performed to determine effective properties of the composite over a wide temperature range. The obtained computational results for effective elastic, thermal, and thermal expansion properties are consistent with the known analytical bounds.

Effects of Variable Phase Volume Fractions on the Effective Thermo-Mechanical Properties of Metal-Ceramic Composites With Graded Microstructures

The present paper is specifically concerned with the evaluation of the effective temperature-dependent elastic, thermal and thermo-elastic material properties of artificially graded Ti-TiB2 microstructures (through thickness only). Effective properties of Ti-TiB2 composite are obtained using micromechanics models and finite element analysis of representative volume elements (RVEs). Two approaches have been adopted and compared to determine the proper RVE. In a fashion similar to previous studies [1], RVEs are generated by considering regions that have a uniform to slow variation in material composition (i.e., constant volume fraction), resulting in statistically homogenous piece-wise RVEs of the graded microstructure neglecting interaction from neighboring cells. In the second approach, continuous RVEs are generated by considering the entire FGM. As pointed out by Anthoine [2], modeling of the complete variation in a microstructure may influence the surrounding layers due to the interactions of varying material composition, particularly when there is a steep variation in material composition along the grading direction. To determine these effects of interlayer interactions, FGM microstructures were generated using three different types of material grading functions, linear, quadratic and square root, providing uniform, gradual and steep variations, respectively. Finite element analysis was performed to determine effective properties of the composite over a wide temperature range.Copyright

Tracking Control of Limit Cycle Oscillations in an Aero-Elastic System

Limit cycle oscillations (LCOs) affect current fighter aircraft and are expected to be present on next generation fighter aircraft. Current efforts in control systems designed to suppress LCO behavior have either used a linear model, restricting the flight regime, require exact knowledge of the system dynamics, or require uncertainties in the system dynamics to be linear-in-the-parameters and only present in the torsional stiffness. Furthermore, the aerodynamic model used in prior research efforts neglects nonlinear effects. This paper presents the development of a controller consisting of a continuous robust integral of the sign of the error (RISE) feedback term with a neural network (NN) feedforward term to achieve asymptotic tracking of uncertainties that do not satisfy the linear-in-the-parameters assumption. Simulation results are presented to validate the performance of the developed controller.

New Yield Criterion for Description of Plastic Deformation of Face-Centered Cubic Single Crystals

In this paper an analytical yield criterion for description of the plastic behavior of face-centered cubic single crystals is presented. The new criterion is written in terms of the generalized invariants of the stress deviator proposed by Cazacu and Barlat (Int J Eng Sci 41:1367–1385, 2003 [1]), specialized to cubic symmetry. The octahedral projections of the yield surfaces for different crystal orientations according to the new model are presented, and compared with the yield surfaces according to the regularized Schmid law (Bishop and Hill, in Lond Edinb Dublin Philos Mag J Sci 42:1298–1307 (1951) [2], Darrieulat and Piot, in Int J Plas 12:575–612 (1996) [3]).

On Modeling the Mechanical Behavior and Texture Evolution of Rolled AZ31 Mg for Complex Loadings Involving Strain Path Changes

An accurate description of the deformation response of AZ31 Mg under changing strain paths requires consideration of its strong anisotropy and its evolution with accumulated plastic deformation. The general held belief is that without modeling de-twining it is impossible to describe the effect of the pre-strain on hardening behavior in low cycle compression-tension-compression and tension-compression-tension tests, respectively. In this paper, it is shown that using the viscoplastic self-consistent crystal plasticity model in conjunction with the pre-dominant twinning reorientation (PTR) scheme it is possible to accurately model the reorientation of the microstructure for such complex loadings. Comparison with recent data reported by Hama et al. [1, 2] shows that the stress-strain response is also very well predicted.

Constitutive modeling and simulation at room-temperature deformation and failure of polycrystalline Molybdenum

In this paper is presented a systematic experimental investigation of the mechanical response of polycrystalline commercially pure molybdenum (Mo). It was established that the material has ductility in tension at 10-5/s and that the failure strain is strongly dependent on the orientation. A specimen taken along the rolling direction sustains large axial strains (20%), while a specimen cut at an angle of 45o to the rolling direction could only sustain 5% strain. Irrespective of the loading orientation the yield stress in uniaxial compression is larger than in uniaxial tension. While in tension the material has a strong anisotropy in Lankford coefficients, in uniaxial compression it displays weak strain-anisotropy. An elastic- plastic orthotropic model that accounts for all the specificities of the plastic deformation of the material was developed. Validation of the model was done through comparison with data on notched specimens. Quantitative agreement with both global and local strain fields was obtained.