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Unformatted text preview: AA242B: MECHANICAL VIBRATIONS AA242B: MECHANICAL VIBRATIONS Direct TimeIntegration Methods These slides are based on the recommended textbook: M. G´ eradin and D. Rixen, “Mechanical Vibrations: Theory and Applications to Structural Dynamics,” Second Edition, Wiley, John & Sons, Incorporated, ISBN13:9780471975465 AA242B: MECHANICAL VIBRATIONS AA242B: MECHANICAL VIBRATIONS Outline 1 Stability and Accuracy of TimeIntegration Operators 2 Newmark’s Family of Methods AA242B: MECHANICAL VIBRATIONS AA242B: MECHANICAL VIBRATIONS Stability and Accuracy of TimeIntegration Operators Multistep TimeIntegration Methods General multistep timeintegration method for firstorder systems of the form ˙ u = Au u n +1 = m X j =1 α j u n +1 j h m X j =0 β j ˙ u n +1 j where h = t n +1 t n is the computational timestep, u n = u ( t n ), and u n +1 = q n +1 ˙ q n +1 is the statevector calculated at t n +1 from the m preceding state vectors and their derivatives as well as the derivative of the statevector at t n +1 β 6 = 0 leads to an implicit scheme — that is, a scheme where the evaluation of u n +1 requires the solution of a system of equations β = 0 corresponds to an explicit scheme — that is, a scheme where the evaluation of u n +1 does not require the solution of any system of equations and instead can be deduced directly from the results at the previous timesteps AA242B: MECHANICAL VIBRATIONS AA242B: MECHANICAL VIBRATIONS Stability and Accuracy of TimeIntegration Operators Multistep TimeIntegration Methods General multistep integration method for firstorder systems (continue) u n +1 = m X j =1 α j u n +1 j h m X j =0 β j ˙ u n +1 j trapezoidal rule u n +1 = u n + h 2 ( ˙ u n + ˙ u n +1 ) backward Euler formula (implicit) u n +1 = u n + h ˙ u n +1 forward Euler formula (explicit) u n +1 = u n + h ˙ u n AA242B: MECHANICAL VIBRATIONS AA242B: MECHANICAL VIBRATIONS Stability and Accuracy of TimeIntegration Operators Numerical Example: the OneDegreeofFreedom Oscillator Consider an undamped onedegreeoffreedom oscillator ¨ q + ω 2 q = 0 with ω = π rad/s, submitted to an initial displacement q (0) = 1 , ˙ q (0) = 0 exact solution q ( t ) = cos ω t associated firstorder system ˙ u = Au where A = » ω 2 1 – u = [ ˙ q , q ] T , and initial condition u (0) = » 1 – AA242B: MECHANICAL VIBRATIONS AA242B: MECHANICAL VIBRATIONS Stability and Accuracy of TimeIntegration Operators Numerical Example: the OneDegreeofFreedom Oscillator Numerical solution T = 3 s , h = T 32 0.5 1 1.5 2 2.5 34321 1 2 3 t q Exact solution Trapezoidal rule Euler backward Euler forward AA242B: MECHANICAL VIBRATIONS AA242B: MECHANICAL VIBRATIONS Stability and Accuracy of TimeIntegration Operators Stability Behavior of Numerical Solutions Analysis of the characteristic equation of a timeintegration method consider the firstorder system ˙ u = Au for this problem, the general multistep method can be written as u n +1 = m X j =1 α j u n...
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 Spring '10
 CHARBELFARHAT
 Acceleration, Trigraph, Complex number, Numerical differential equations, Numerical ordinary differential equations

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