Showing papers by "Earl H. Dowell published in 2002"
TL;DR: In this article, the authors investigated the effect of nonlinear aerodynamic effects on the divergence, utter, and limit-cycle oscillation (LCO) characteristics of a transonic airfoil cone guration.
Abstract: By the use of a state-of-the-art computational e uid dynamic (CFD) method to model nonlinear steady and unsteady transonice owsin conjunction with a linearstructural model,an investigationismadeinto how nonlinear aerodynamics can effect the divergence, e utter, and limit-cycle oscillation (LCO) characteristics of a transonic airfoil cone guration. A single-degree-of-freedom (DOF) model is studied for divergence, and one- and two-DOF models are studied for e utter and LCO. A harmonicbalancemethod in conjunction with the CFD solver is used to determine the aerodynamics for e nite amplitude unsteady excitations of a prescribed frequency. A procedure for determining the LCO solution is also presented. For the cone guration investigated, nonlinear aerodynamic effects are found to produce a favorable transonic divergence trend and unstable and stable LCO solutions, respectively, for the one- and two-DOF e utter models. Nomenclature a = nondimensional location of airfoil elastic axis, e=b b, c = semichord and chord, respectively cl, cm = coefe cients of lift and moment about elastic axis, respectively e = location of airfoil elastic axis, measured positive aft of airfoil midchord h, ® = airfoil plunge and pitch degrees of freedom I® = second moment of inertia of airfoil about elastic axis
282 citations
TL;DR: The von Karman lecture as mentioned in this paper discusses the state of the art in nonlinear aeroelasticity and unsteady aerodynamics and identifies several of the most significant nonlinearities arising in a structure or in an aerodynamic flow field.
Abstract: In this von Karman lecture, a subject is addressed whose foundations were significantly influenced by the work of Theodore von Karman. A classic paper by von Karman and Sears first considered the determination of aerodynamic forces on an airfoil undergoing general time-dependent motion. Also, early in his career, von Karman investigated fundamental issues in structural mechanics and derived the celebrated von Karman plate equations for determining the large (nonlinear) deflections of an elastic plate under a distributed force. Finally, he authored a widely cited paper on the importance of nonlinearities for engineers and engineering. In this lecture, these themes are recalled and the current state of the art in nonlinear aeroelasticity and unsteady aerodynamics is discussed. Several of the most significant nonlinearities arising in a structure or in an aerodynamic flow field are identified. Recent and relevant theoretical and experimental studies are reviewed and future developments are projected that are expected to have a significant impact on our ability to understand and beneficially use nonlinear dynamic aeroelastic behavior
188 citations
TL;DR: In this paper, a nonlinear response analysis of a high-aspect-ratio wing aeroelastic model excited by gust loads is presented along with a companion wind-tunnel test program.
Abstract: A nonlinear response analysis of a high-aspect-ratio wing aeroelastic model excited by gust loads is presented along with a companion wind-tunnel test program. For the wind-tunnel tests, a high-aspect-ratio wing aeroelastic experimental model with a slender body at the tip has been constructed, and a rotating slotted cylinder gust generator has been used to generate a gust excitation field. A LabVIEW 5.1 measurement and analysis system is used to measure the gust response, flutter boundary, and limit-cycle oscillation behavior. Structural equations of motion based on a nonlinear beam theory are combined with the ONERA aerodynamic stall model to study the effects of geometric structural nonlinearity and steady angle of attack on nonlinear gust response of high-aspect-ratio wings. Also a dynamic perturbation analysis about a nonlinear static equilibrium is used to determine the small perturbation flutter boundary. The fair to good quantitative agreement between theory and experiment demonstrates that the present analysis method has reasonable accuracy.
107 citations
TL;DR: In this paper, the effects of a freeplay structural nonlinearity on an aeroelastic system were studied experimentally, and particular attention was paid to the stability of a periodic nonlinear aero-elastic response.
Abstract: The effects of a freeplay structural nonlinearity on an aeroelastic system are studied experimentally. Particular attention is paid to the stability of a periodic nonlinear aeroelastic response, kn...
64 citations
TL;DR: In this article, an experimental high-aspect-ratio wing aeroelastic model with a slender body at the tip has been constructed, and the response due to flutter and limit-cycle oscillations (LCOs) has been measured in a wind-tunnel test.
Abstract: An experimental high-aspect-ratio wing aeroelastic model with a slender body at the tip has been constructed, and the response due to flutter and limit-cycle oscillations (LCOs) has been measured in a wind-tunnel test. A theoretical model has been developed and calculations made to correlate with the experimental data. Structural equations of motion based on nonlinear beam theory are combined with the ONERA aerodynamic stall model to study the LCO hysteresis phenomenon of a high-aspect-ratio wing model. Time simulation and a harmonic balance approach are each used to compute the LCO hysteretic response.The results between the theory and experiment are in good agreement.
54 citations
TL;DR: The limit cycle oscillations of a two-dimensional panel in low subsonic flow have been studied theoretically and experimentally as mentioned in this paper, where the panel is clamped at its leading edge and free at its trailing edge.
Abstract: Limit cycle oscillations of a two-dimensional panel in low subsonic flow have been studied theoretically and experimentally. The panel is clamped at its leading edge and free at its trailing edge. A structural non-linearity arises in both the bending stiffness and the mass inertia. Two-dimensional incompressible (linear) vortex lattice aerodynamic theory and a corresponding reduced order aerodynamic model were used to calculate the linear flutter boundary and also the limit cycle oscillations (that occur beyond the linear flutter boundary).
43 citations
01 Jan 2002
TL;DR: In this paper, a frequency domain harmonic balance (HB) technique was used to model nonlinear unsteady aerodynamic forces due to finite amplitude motions of a prescribed unstrainedy oscillation frequency.
Abstract: Presented is a frequency domain harmonic balance (HB) technique for modeling nonlinear unsteady aerodynamics of three-dimensional transonic inviscid flows about wing configurations. The method can be used to model efficiently nonlinear unsteady aerodynamic forces due to finite amplitude motions of a prescribed unsteady oscillation frequency. When combined with a suitable structural model, aeroelastic (fluid-structure), analyses may be performed at a greatly reduced cost relative to time marching methods to determine the limit cycle oscillations (LCO) that may arise. As a demonstration of the method, nonlinear unsteady aerodynamic response and limit cycle oscillation trends are presented for the AGARD 445.6 wing configuration. Computational results based on the inviscid flow model indicate that the AGARD 445.6 wing configuration exhibits only mildly nonlinear unsteady aerodynamic effects for relatively large amplitude motions. Furthermore, and most likely a consequence of the observed mild nonlinear aerodynamic behavior, the aeroelastic limit cycle oscillation amplitude is predicted to increase rapidly for reduced velocities beyond the flutter boundary. This is consistent with results from other time-domain calculations. Although not a configuration that exhibits strong LCO characteristics, the AGARD 445.6 wing nonetheless serves as an excellent example for demonstrating the HB/LCO solution procedure.
32 citations
TL;DR: In this article, an inviscid flow through a cascade of oscillating airfoils is investigated, and the steady flow is linearized about the nonlinear steady response based on the observation that in many practical cases the unsteadiness in the flow has a substantially smaller magnitude than the steady component.
Abstract: An unsteady inviscid flow through a cascade of oscillating airfoils is investigated. An inviscid nonlinear subsonic and transonic model is used to compute the steady flow solution. Then a small amplitude motion of the airfoils about their steady flow configuration is considered. The unsteady flow is linearized about the nonlinear steady response based on the observation that in many practical cases the unsteadiness in the flow has a substantially smaller magnitude than the steady component. Several reduced-order modal models are constructed in the frequency domain using the proper orthogonal decomposition technique. The dependency of the required number of aerodynamic modes in a reduced-order model on the far-field upstream Mach number is investigated
24 citations
22 Apr 2002
TL;DR: In this paper, a harmonic-balance computational fluid dynamic approach for modeling limit-cycle oscillation behavior of aeroelastic airfoil configurations in a viscous transonic flow is presented.
Abstract: Presented is a harmonic-balance computational fluid dynamic approach for modeling limit-cycle oscillation behavior of aeroelastic airfoil configurations in a viscous transonic flow. For the NLR 7301 airfoil configuration studied, accounting for viscous effects is shown to significantly influence computed limit-cycle oscillation trends when compared to an inviscid analysis. A methodology for accounting for changes in mean angle of attack during limit-cycle oscillation is also developed.
23 citations
TL;DR: In this paper, the state-space presentation of an aerodynamic vortex model from a classical and system identification perspective is considered, and modal controllability and observability are also considered.
Abstract: The state-space presentation of an aerodynamic vortex model is considered from a classical and system identification perspective. Using an aerodynamic vortex model as a numerical simulator of a wing tunnel experiment, both full state and limited state data or measurements are considered. Two possible approaches for system identification are presented and modal controllability and observability are also considered. The theory then is applied to the system identification of a flow over an aerodynamic delta wing and typical results are presented.
17 citations
TL;DR: There are several time-tested survival skills to consider when plotting a course to tenure in a "publish or perish" environment, such as choosing an institution that matches your expectations, abilities, and goals, knowing what to expect (and avoid) once a position is accepted, and using best practices when preparing grant proposals and publishing research results as discussed by the authors.
Abstract: There are several time-tested survival skills to consider when plotting a course to tenure in a “publish or perish” environment. These are: choosing an institution that matches your expectations, abilities, and goals; knowing what to expect (and avoid) once a position is accepted; and using best practices when preparing grant proposals and publishing research results.
22 Apr 2002
TL;DR: The effects of a steady angle of attack on the nonlinear aeroelastic response of a delta wing model to a periodic gust have been studied in this article, where a three-dimensional time-domain vortex lattice aerodynamic model and a reduced order aerodynamic technique were used and the structure was modelled using von Karman plate theory that allows for geometric strain-displacement nonlinearities in the delta wing structure.