▪ Abstract The interaction of a flexible structure with a flowing fluid in which it is submersed or by which it is surrounded gives rise to a rich variety of physical phenomena with applications in many fields of engineering, for example, the stability and response of aircraft wings, the flow of blood through arteries, the response of bridges and tall buildings to winds, the vibration of turbine and compressor blades, and the oscillation of heat exchangers. To understand these phenomena we need to model both the structure and the fluid. However, in keeping with the overall theme of this volume, the emphasis here is on the fluid models. Also, the applications are largely drawn from aerospace engineering although the methods and fundamental physical phenomena have much wider applications. In the present article, we emphasize recent developments and future challenges. To provide a context for these, the article begins with a description of the various physical models for a fluid undergoing time-dependent mot...

Modeling of Fluid-Structure Interaction

An analytical design technique for an active flutter-suppression and gust-alleviation control system is presented. It is based on a rational approximation of the unsteady aerodynamic loads in the entire Laplace domain, which yields matrix equations of motion with constant coefficients. Some existing rational approximation schemes are reviewed, and a new technique which yields a minimal number of augmented states for a desired accuracy is presented. The state-space aeroelastic model is used to design a constant gain, partial-feedback control system, which simultaneously assures stability and optimizes any desired combination of gust response parameters throughout the entire flight envelope.

Design for Active Flutter Suppression and Gust Alleviation Using State-Space Aeroelastic Modeling

A new method for constructing reduced-order models (ROM) of unsteady small-disturbance e ows is presented. The reduced-order models are constructed using basis vectors determined from the proper orthogonal decomposition (POD) of an ensemble of small-disturbance frequency-domain solutions. Each of the individual frequencydomain solutions is computed using an efe cient time-linearized e ow solver. We show that reduced-order models can be constructed using just a handful of POD basis vectors, producing low-order but highly accurate models of the unsteady e ow over a wide range of frequencies. We apply the POD/ROM technique to compute the unsteady aerodynamic and aeroelastic behavior of an isolated transonic airfoil and to a two-dimensional cascade of airfoils.

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Proper Orthogonal Decomposition Technique for Transonic Unsteady Aerodynamic Flows

Linear and nonlinear aeroelasticresponseisexamined using a uniquetest apparatusthat allowsforexperiments of plunge and pitch motion of a wing with prescribed stiffness characteristics. The addition of a control surface, combined with an active control system, extends the stable e ight region. Unsteady aerodynamics are modeled with an approximation to Theodorsen’ s theory appropriate for the low reduced frequencies associated with the experiment. Incorporated with a full-state feedback control law, an optimal observer is utilized to stabilize the system above the open-loop e utter velocity. Coulomb damping and hardening of the pitch stiffness are included to examine nonlinear control behavior. The nonlinear model is tested using the control laws developed from an extensionoflineartheory.EachmodelissimulatedusingMATLAB ® andcomparedwithexperimentalresultsofthe active control system. Excellent correlation between theory and experiment isachieved. Using an optimal observer and full-state feedback, the linear and nonlinear systems are stabilized at velocities that exceed the open-loop e utter velocity. Limited control is achieved when the system is undergoing limit cycle oscillations.

Applied Active Control for a Nonlinear Aeroelastic Structure

Applications of the unsteady vortex-lattice method in aircraft aeroelasticity and flight dynamics

Analytical design techniques for active and passive control of aeroelastic systems are based on a rational approximation of the unsteady aerodynamic loads in the entire Laplace domain, which yields matrix equations of motion with constant coefficients. Some existing schemes are reviewed, the matrix Pade approximant is modified, and a technique which yields a minimal number of augmented states for a desired accuracy is presented. The state-space aeroelastic model is used to design an active control system for simultaneous flutter suppression and gust alleviation. The design target is for a continuous controller which transfers some measurements taken on the vehicle to a control command applied to a control surface. Structural modifications are formulated in a way which enables the treatment of passive flutter suppression system with the same procedures by which active control systems are designed.

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Mordechay Karpel

Papers

Design for Active Flutter Suppression and Gust Alleviation Using State-Space Aeroelastic Modeling

Design for active and passive flutter suppression and gust alleviation

Design for active flutter suppression and gust alleviation using state-space aeroelastic modeling