Showing papers by "Earl H. Dowell published in 1998"
TL;DR: In this article, the primary resonance and the 1/3 subharmonic resonance of a harmonically forced Duffing oscillator under state feedback control with a time delay were analyzed.
Abstract: The paper presents analytical and numerical studies of the primary resonance and the 1/3 subharmonic resonance of a harmonically forced Duffing oscillator under state feedback control with a time delay. By using the method of multiple scales, the first order approximations of the resonances are derived and the effect of time delay on the resonances is analyzed. The concept of an equivalent damping related to the delay feedback is proposed and the appropriate choice of the feedback gains and the time delay is discussed from the viewpoint of vibration control. In order to numerically solve the problem of history dependence prior to the start of excitation, the concepts of the Poincare section and fixed points are generalized. Then, a modified shooting scheme associated with the path following technique is proposed to locate the periodic motion of the delayed system. The numerical results show the efficacy of the first order approximations of the resonances.
180 citations
TL;DR: In this article, a three-degree-of-freedom aeroelastic model with freeplay is modeled theoretically using a small number of aerodynamic eigenmodes (i.e., a reduced order model) based upon Peters' finite-state model for two-dimensional aerodynamic flow.
151 citations
TL;DR: In this paper, an active control system is used to suppress flutter in a typical section airfoil, which is based on experimental system identifications of the transfer functions between three measured system variables - pitch, plunge, and flap position - and a single control signal that commands the flap of the air foil.
Abstract: This paper presents an experimental implementation of an active control system used to suppress flutter in a typical section airfoil. The H2 optimal control system design is based on experimental system identifications of the transfer functions between three measured system variables - pitch, plunge, and flap position - and a single control signal that commands the flap of the airfoil. Closed-loop response of the airfoil demonstrated gust alleviation below the open-loop flutter boundary. In addition, the flutter boundary was extended by 12.4% through the application of active control. Cursory robustness tests demonstrate stable control for variations in flow speed of ± 10%.
70 citations
TL;DR: In this article, the eigenmodes of a two-dimensional aerodynamic flow over an airfoil are determined using a reduced-order model, and aeroelastic model is formed by coupling them to a typical section structural model with a trailing-edge flap.
Abstract: Starting from a finite state model for a two-dimensional aerodynamic flow over an airfoil, the eigenmodes of the aerodynamic flow are determined. Using a small number of these aerodynamic eigenmodes, ie., a reduced-order model, the aeroelastic model is formed by coupling them to a typical section structural model with a trailing-edge flap. A free-play nonlinearity is modeled. Results are shown from the finite state model, the reduced-order model, and previous theoretical and experimental work. All results are in good agreement.
51 citations
TL;DR: In this article, a perturbation approach is proposed to determine the first order variation of an arbitrary eigenvalue and corresponding eigenvector of the system with feedback delay by solving a set of linear algebraic equations only.
37 citations
TL;DR: In this paper, the Ott-Grebogi-Yorke (OGY) control technique is applied to nonlinear flows, as distinct from nonlinear maps, and an algorithm for designing the optimal basis function is presented.
34 citations
29 citations
TL;DR: In this paper, an approximate expression for the local response of the concentrated mass relative to the plate and the inertial displacement of the spring end at its point of connection with the plate has been developed as the asymptotic limit of a classical component modal analysis by using Asymmptotic Modal Analysis (AMA).
17 citations
14 citations
TL;DR: In this paper, an asymptotic modal approximation of the classical modal solution is proposed to estimate the impulse response problem for high frequency regimes where typical numerical methods (e.g., finite elements) are impractical.
Abstract: The transient response ofa structure is predicted using an asymptotic modal approximation of the classical modal solution. The method is aimed at estimating the impulse response problem for high frequency regimes where typical numerical methods (e.g., finite elements) are impractical. As an example, the response of a thin elastic panel is modeled in a frequency range that includes a sufficient number of modes. Both impulsive and arbitrary. forms of excitation are considered. It is shown that the asymptotic modal analysis yields an excellent estimate of both the local displacement near the excitation location and of the spatially averaged transient response of the panel for moderate time spans after the excitation is applied. Furthermore, as this approach does not require that the mode shapes or natural frequencies of the structure to be calculated, it is an extremely efficient technique.
6 citations
TL;DR: In this article, an experimental identi cation of unsteady and stall aerodynamic coefµ cients for an external store of an aircraft has been made by a least squares procedure based on the measured time series obtained from an aeroelastic model undergoing aerodynamic excitation caused by a gust generator.
Abstract: An experimental identi cation investigation of unsteady and stall aerodynamic coef cients for an external store of an aircraft has been made. The coef cient identi cation is performed by a least-squares procedure based on the measured time series obtained from an aeroelastic model undergoing aerodynamic excitation caused by a gust generator. The results are veri ed by the good correlation between the theoretical prediction and aerodynamic measurement data. Using the identi ed unsteady aerodynamic model, numerical investigations for both the stability ( utter) and nonlinear aeroelastic response of this external store model have been made. Also, the results were compared with those obtained from a quasisteady aerodynamic model. It is seen that the effects of unsteady aerodynamics on the nonlinear aeroelastic response are signi cant when the angle of attack is large.
TL;DR: In this article, the authors address two relatively simple but fundamental questions in nonlinear oscillations: given an arbitrary initial condition where will the trajectory go, and how long will it take to get there?
Abstract: This paper addresses two relatively simple but fundamental questions in nonlinear oscillations: Given an arbitrary initial condition where will the trajectory go, and how long will it take to get there? These related questions are addressed from an experimental perspective where generating global transient behavior has received relatively little attention, despite the fact that a global view of transient behavior provides a much more complete description of the dynamics of a system than a traditional concentration on steady-state behavior. Three different physical systems are studied, each of which exhibits a specific behavior heavily influenced by transient global effects.