Browsing by Subject "Aeroelasticity"
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Item A Conformal Mapping Grid Generation Method for Modeling High-Fidelity Aeroelastic Simulations(2010-07-14) Worley, GregoryThis work presents a method for building a three-dimensional mesh from two- dimensional topologically identical layers, for use in aeroelastic simulations. The method allows modeling of large deformations of the wing in both the span direction and deformations in the cord of the wing. In addition, the method allows for the modeling of wings attached to fuselages. The mesh created is a hybrid mesh, which allows cell clustering in the viscous region. The generated mesh is high quality and allows capturing of nonlinear uid structure interactions in the form of limit cycle oscillation.Item Experimental investigation of stochastic vibration of nonlinear structures(Texas Tech University, 1986-12) Sullivan, Douglas GrantThe purpose of this experimental investigation is to measure the dynamic response of an aeroelastic structure involving nonlinear coupling to random parametric vibration. Two main series of tests are conducted under two different bandwidths. The first test corresponds to isolation of the first normal mode natural frequency. The second test corresponds to a bandwidth which covers the second normal mode frequency. The tests are conducted when the structure is tuned internally such that the second normal mode frequency is twice the first normal mode frequency. Experimental measurements are processed to estimate the response mean squares. The influence of excitation spectral density level and internal detuning on the response mean squares is examined. The results confirm regions of instability as predicted in harmonic excitation but there is no evidence of the well known saturation phenomenon. The results differ from the theoretical results obtained for the same model under white band random excitation. This disagreement is mainly due to the fact that the excitation is represented by a physical white noise process in the analytical model while it is a band— limited process in the actual experiment.Item Gust Load Alleviation for an Aeroelastic System Using Nonlinear Control(2010-10-12) Lucas, Amy MarieThe author develops a nonlinear longitudinal model of an aircraft modeled by rigid fuselage, tail, and wing, where the wing is attached to the fuselage with a torsional spring. The main focus of this research is to retain the full nonlinearities associated with the system and to perform gust load alleviation for the model by comparing the impact of a proportional-integral- lter nonzero setpoint linear controller with control rate weighting and a nonlinear Lyapunov-based controller. The four degree of freedom longitudinal system under consideration includes the traditional longitudinal three degree of freedom aircraft model and one additional degree of freedom due to the torsion from the wing attachment. Computational simulations are performed to show the aeroelastic response of the aircraft due to a gust load disturbance with and without control. Results presented in this thesis show that the linear model fails to capture the true nonlinear response of the system and the linear controller based on the linear model does not stabilize the nonlinear system. The results from the Lyapunov-based control demonstrate the ability to stabilize the nonlinear response, including the presence of an LCO, and emphasize the importance of examining the fully nonlinear system with a nonlinear controller.Item Linear and autoparametric modal analysis of aeroelastic structural systems(Texas Tech University, 1984-05) Woodall, Tommy DaleThis investigation deals with the linear modal analysis and autoparametric interaction of aeroelastic systems such as an airplane fuselage and wing with fuel storage. The mathematical modeling is derived by applying Lagrange's equations taking into consideration the Christoffel symbol of the first kind to account for the nonlinear coupling of the system coordinates, velocities, and accelerations. The linear modal analysis will be obtained by considering the linear, conservative portion of the equations of motion. The normal mode frequencies and the associated mode shapes are obtained in terms of the system parameters. The main objective of the linear analysis is to explore the critical regions of autoparametric (or internal) resonance conditions, £kiwi=0 (where ki are integers and wi are the normal mode frequencies). The results show that for certain system parameters the condition of internal resonance is satisfied. The dynamic behavior of the structure in the neighborhood of internal resonance conditions is obtained by considering the nonlinear coupling of the normal modes. The asymptotic approximation technique due to Struble is employed. Three groups of internal and normal resonance conditions are obtained from the secular terms of the first-order perturbational equations. The transient and steady-state responses cure obtained numerically by using the IBM Continuous System Modeling Program (CSMP) with double precision Milne integration. The transient response shows a build up in the interacted modes to a level which exceeds the steady-state response. In addition, the excited mode is suppressed by virtue of the nonlinear feedback of other modes. Under certain conditions, the steady-state response is derived analytically. It is concluded that the nonlinear modal analysis reveals certain types of response characteristics which cannot be interpreted within the framework of the linear theory of small oscillations.Item Non-linear stochastic flutter of aeroelastic structural systems(Texas Tech University, 1985-12) Heo, HunThe main objective of t h i s investigation is to examine the linear and non-linear modal interactions of a two-degree-of-freedom aero-elastic structure subjected to a wide band random excitation. The linear analysis involves linear dynamic coupling and parametric random coupling. In terms of normal coordinates the response mean squares are obtained as functions of the system frequency ratio. The analysis shows that for modest values of mass ratio the first mode is suppressed when the natural frequencies of the two beams are identical. Furthermore, the system mean square responses are governed mainly by the external forced excitation, while the influence of the random parametric component is almost negligible. The non-linear modal analysis involves quadratic non-linearity referred to as autoparametric coupling. This type of coupling gives rise to a new type of instability when the relationship between normal mode frequencies is linear. In the neighborhood of the internal resonance condition w2/w1=0.5 (where w1 and w2 are the normal mode frequencies of the system), a general differential equation of the response moments is derived and found to constitute an infinite hierarchy set. Two different closure schemes, based on a cumulant-neglect concept, are used to truncate the moment differential equations. The first is the Gaussian closure, which leads to fourteen coupled differential equations, while the second, known as the non- Gaussian closure, gives 69 coupled differential equations. These two sets of equations are solved numerically for the response moments. The Gaussian closure solution results in a quasi-stationary response, while the non-Gaussian closure solution gives a strict stationary response. The two solutions exhibit an exchange of energy between the two modes in such a manner that one mode acts as a vibration absorber of the second mode in the neighborhood of internal resonance condition w2/w1=0.5±0(e)f where e is a small parameter. The influence of ranc3cam variation of the system parameters such as damping and stiffness is investigated. It is found that the damping variation has less effect on the random response of the structure than the stiffness variation. Numerical solutions for different initial conditions are obtained to find out if the system possesses more than one limit cycle. It is found that the initial conditions affect only the transition response, while the steady state response does not change by changing the initial conditions.Item Nonlinear aeroelastic analysis of high aspect-ratio wings using the method of numerical continuation(Texas A&M University, 2006-08-16) Nichkawde, ChetanThis research explores the impact of kinematic structural nonlinearities on the dynamics of a highly deformable cantilevered wing. Two different theoretical formulations are presented and analysed for nonlinear behavior. The first formulation, which is more conventional, assumes zero equilibrias and structural nonlinearities occur as terms up to third order in the Taylor series expansion of structural nonlinearities. In the second approach, no prior assumption about equilibria states of the wing is made. Kinematic nonlinearities due to curvature and inertia were retained in their exact form. Thus, the former becomes a special case of the latter. This nonlinear formulation permits the analysis of dynamics about nonzero trims. Nonzero trim states are computed as a system parameter is varied using a continuation software tool. The stability characteristics of these trim states are also ascertained. Various bifurcation points of the system are determined. Limit-cycle oscillations are also investigated for and are characterized in terms of amplitude of vibration. The research in particular examines the impact of in-plane degree of freedom on the stability of nonzero trim states. The effect of variation of system parameters such as stiffness ratio, aspect ratio and root angle of attack is also studied. The method of direct eigenanalysis of nonzero equilibria is novel and new for an aeroelastic system.Item Nonlinear Analysis of a Two DOF Piecewise Linear Aeroelastic System(2011-10-21) Elgohary, Tarek Adel AbdelsalamThe nonlinear dynamic analysis of aeroelastic systems is a topic that has been covered extensively in the literature. The two main sources of nonlinearities in such systems, structural and aerodynamic nonlinearities, have analyzed numerically, analytically and experimentally. In this research project, the aerodynamic nonlinearity arising from the stall behavior of an airfoil is analyzed. Experimental data was used to fit a piecewise linear curve to describe the lift versus angle of attack behavior for a NACA 0012 2 DOF airfoil. The piecewise linear system equilibrium points are found and their stability analyzed. Bifurcations of the equilibrium points are analyzed and applying continuation software the bifurcation diagrams of the system are shown. Border collision and rapid/Hopf bifurcations are the two main bifurcations of the system equilibrium points. Chaotic behavior represented in the intermittent route to chaos was also observed and shown as part of the system dynamic analysis. Finally, sets of initial conditions associated with the system behavior are defined. Numerical simulations are used to show those sets, their subsets and their behavior with respect to the system dynamics. Poincar? sections are produced for both the periodic and the chaotic solutions of the system. The proposed piecewise linear model introduced some interesting dynamics for such systems. The introduction of the border collision bifurcation and the existence of periodic and chaotic solutions for the system are some examples. The model also enables the understanding of the mapping of initial conditions as it defines clear boundaries with different dynamics that can be used as Poincar? sections to understand further the global system dynamics. One of the constraints of the system is its validity as it is dependent on the range of the experimental data used to generate the model. This can be addressed by adding more linear pieces to the system to cover a wider range of the dynamics.