Changho Nam

Implementation of a Coupled Thermo-Piezoelectric-Mechanical Model in the LQG Controller Design for Smart Composite Shells

The present paper aims to develop a completely coupled thermo-piezoelectric-mechanical theory, based on an improved layerwise displacement field and higher order electrical and temperature fields, to study dynamic response and control of smart composite shells. A variational principle, addressing the interaction between thermal, piezoelectric and mechanical fields, is used to derive the governing equations of equilibrium. Finite element technique is used to ensure application to practical geometry and boundary conditions. Numerical analysis is conducted for simply supported cylindrical shells with distributed self-sensing piezoelectric actuators. Control authority is investigated using Linear Quadratic Gaussian (LQG) theory. Parametric studies are conducted to investigate the effect of two-way coupling, placement of actuators, coupling and flexibility of the primary structure.

Reduced-Order-Model Approach for Aeroelastic Analysis Involving Aerodynamic and Structural Nonlinearities

Ar educed-order model (ROM) for solving an aeroelasticity problem including structural and aerodynamic nonlinearities is introduced. The aerodynamic system is identified using a Volterra-based ROM. The appropriate procedures needed for the identification of the aerodynamic linearized kernel are presented, and a procedure to identify the first- and second-order Volterra kernels and a linearized ROM kernel from aerodynamic step response is detailed. A coupled framework for addressing the nonlinear aeroelasticity problem using a typical wing section model is introduced for the validation of the procedure. Nonlinear unsteady aerodynamic forces in both subsonic and transonic regimes are evaluated using a conventional approach using the CFL3D (version 6.1) code capable of integrating Navier‐Stokes and the structural equations. The computational-fluid-dynamics (CFD) solver is used to compute the aerodynamic response to step or impulse inputs. Aerodynamic responses to arbitrary inputs are predicted using ROM kernels. The Gaussian pulse response obtained using ROM was validated by comparison with the response obtained from CFD solver. Aeroelastic analysis was conducted using the ROM with aerodynamic and structural nonlinearities. The results show that the reduced-order model can estimate accurately flutter speed as well as the limit-cycle oscillations (LCO). Also the ROM approach is found to be computationally efficient.

Optimal Wing Planform Design for Aeroelastic Control

An integrated aeroservoelastic design synthesis for flutter suppression and gust load reduction using multiple control surfaces is presented. For this multidisciplinary optimization procedure, structural design variables, control system, and aerodynamic design variables, such as wing planform, ply orientation of the composite layer, and control surface size and location, are considered simultaneously. The analysis for a composite wing with control surfaces is conducted by the finite element method. Unsteady aerodynamic forces calculated by the doublet lattice method are approximated as transfer functions of the Laplace variable by Rogers method. The output feedback control scheme is applied to design the active control system. Using a swept wing model, the performance of the control system is investigated. The geometry of wing planform and control surface size and location are determined by using the genetic algorithm. Design objectives are to minimize the control performance index and the root mean square of the gust responses for various airspeeds. Numerical results showed substantial improvements in performance index value as well as the root-mean-square values of the gust responses compared with the baseline wing model.

Application of shape memory alloy (SMA) spars for aircraft maneuver enhancement

Modern combat aircraft are required to achieve aggressive maneuverability and high agility performance, while maintaining handling qualities over a wide range of flight conditions. Recently, a new adaptive-structural concept called variable stiffness spar is proposed in order to increase the maneuverability of the flexible aircraft. The variable stiffness spar controls wing torsional stiffness to enhance roll performance in the complete flight envelope. However, variable stiffness spar requires the mechanical actuation system in order to rotate the Variable stiffness spar during flight. The mechanical actuation system to rotate variable stiffness spar may cause an additional weight increase. In this paper, we will apply Shape Memory Alloy (SMA) spars for aeroelastic performance enhancement. In order to explore the potential of SMA spar design, roll performance of the composite smart wings will be investigated using ASTROS. Parametric study will be conducted to investigate the SMA spar effects by changing the spar locations and geometry. The results show that with activation of the SMA spar, the roll effectiveness can be increased up to 61% compared with the baseline model.

Modeling Segmented Active Constrained Layer Damping Using Hybrid Displacement Field

A finite element model for composite plates with active constrained layer (ACL) damping is developed using hybrid displacement fields. The higher order displacement field is used in the composite plate to capture the transverse shear deformations. Since viscoelestic and piezoelectric layers are made from certain isotropic material, the first and the second order displacement fields are employed in these layers to maintain computational efficiency in problem solution. The refined displacement fields defined in different material layers are derived by applying the displacement and transverse shear stress continuity conditions at interfaces of different materials and the traction-free boundary conditions at the top and the bottom surfaces of the structure. The anelastic displacement field method is used to implement the viscoelastic material model to enable time domain finite element analysis. The- finite element model is validated by comparison with NASTRAN 3D finite element modal analysis. The developed technique isused to investigate the dynamic responses of a cantilever composite plate with ACLs. Numerical results are presented with both the open and closed loop control. Significant improvements are observed with the use of active control treatment.

Delamination modeling and detection in smart composite plates

A refined model for smart composite plates with delamination is developed. The modeling is based on a third order refined displacement field, which satisfies the stress free boundary conditions at all free surfaces including delamination interfaces. The displacement field also accounts for the presence of distributed piezoelectric actuators. The mathematical model is implemented using the finite element method. The dynamic strain is analyzed for smart composites with and without delamination. Next, a new approach is investigated for the detection of delamination in smart composites based on the Root-Mean Square (RMS) values of the plate response subject to disturbances. Numerical results presented show accurate location of the delamination and its size.

A volterra kernel reduced-order model approach for nonlinear aeroelastic analysis

A procedure to develop CFD-based reduced order models (ROMs) which capture the essence of an aerodynamic system while reducing the complexity of the computational model is introduced. The Volterra-based ROM is obtained using the derivative of unsteady aerodynamic step-response. Transient responses and Gaussian responses were calculated using ROM and compared with the CFD solver for validation of ROM approach. An Eigensystem Realization Algorithm is used to convert ROM unsteady aerodynamics into the LTI state space model. A reduction in the cost of the realization of the ROM kernel is obtained by the identification of the state-space model. Aeroelastic analysis is conducted using state space model. The weakened wall-mounted AGARD wing 445.6 has been used for validation. An aeroelastic analysis of a NACA 65A004 composite wing model is also conducted at M=0.90 including structural nonlinearities. While highly optimized, the state-space model remains a decoupled system and has enough meaningful information about the nature of the aeroelastic system. The proposed approach is computationally efficient without losing structural/aerodynamic nonlinearities.

Aeroelastic Control of Smart Composite Plate with Delaminations

In this paper, aeroelastic performance of smart composite wing in the presence of delaminations is investigated. A control system is designed to enhance the dynamic stability of the delaminated composite wing. The refined higher order theory for analyzing an adaptive composite plate in the presence of delaminations is used. The theory accurately captures the transverse shear deformation through the thickness, which is important in anisotropic composites, particularly in the presence of discrete actuators and sensors and delaminations. The effects of delamination on the aeroelastic characteristics of composite plates are investigated. An active control system is designed to redistribute the loads and to minimize the effect of delamination. The pole placement technique is applied to design the closed loop system by utilizing piezoelectric actuators. Due to delamination, the significant changes in the natural frequencies of the lower modes are observed. This causes the reduction on the flutter speed of the delaminated plate model. The aeroelastic control results show that controller makes the delaminated plate model behave like a normal plate. The controller also reduces a significant amount of the root mean square values of the gust response due to gust.

Neural net-based controller for flutter suppression using ASTROS with smart structures

Recent development of a smart structures module and its successful integration with a multidisciplinary design optimization software ASTROS* and an Aeroservoelasticity (ASE) module is presented. A modeled F-16 wing using piezoelectric (PZT) actuators was used as an example to demonstrate the integrated software capability to design a flutter suppression system. For an active control design, neural network based robust controller will be used for this study. A smart structures module is developed by modifying the existing thermal loads module in ASTROS* in order to include the effects of the induced strain due to piezoelectric (PZT) actuation. The thermal-PZT equivalence model enables the modifications of the thermal stress module to accommodate the smart structures module in ASTROS*. ZONA developed the control surface (CS)/PZT equivalence model principle, which ensures the interchangeability between the CS force input and the PZT force input to the ASE modules in ASTROS*. The results show that the neural net based controller can increase the flutter speed.

Active control of composite box beams using in-plane piezoelectric actuation and structural coupling with optimization

An integrated structures/controls optimization is developed for vibration suppression of a composite box beam with surface bonded piezoelectric actuators. The penalty approach is used to perform the multi-objective hybrid optimization to enhance damping of the first lag, flap, and torsion modes while minimizing control input. The objective functions and constraints include damping ratios, and natural frequencies. The design variables include ply orientations of the box beam walls, and the location and size of the actuators. Two box beam configurations are investigated and the results are compared. In the first, piezoelectric actuators are bonded to the top and bottom surfaces and in the second, actuators are bonded to all four walls for additional in-plane actuation. Optimization results show that significant reductions in control input and tip displacement can be achieved in both cases, however, improved response trends are obtained with in- plane actuation.