Accurate Computing of Higher Free Vibration Modes for Thin Flexible Structures

Motivated by the need for high-fidelity modeling and accurate control of future hypersonic vehicles subjected to complex aerothermomechanical loads and many other such applications, a novel scheme is proposed to accurately compute higher modes of vibration for one-dimensional structures by coupling the classic Ritz method as a predictor and the linear two-point boundary value problem solver SUPORE as a corrector. The Ritz method is used to compute a preliminary estimate of the natural frequencies for the desired modes. These estimates are then used as initial guesses by SUPORE, which employs superposition and reorthonormalization to accurately solve the governing boundary value problem. Compared to a high-fidelity Ritz approximation or a highly refined finite element modeling procedure, this is a simple, computationally less expensive, and yet highly accurate method to compute mode shapes for applications that require higher modes. This scheme is used to compute mode shapes within a certain  absolute and relative error tolerance for the idealized case of a free–free Euler–Bernoulli beam and a typical air-breathing hypersonic vehicle modeled as a thin-walled structure and taking into account deformations due to bending, shear, and torsion.

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Flowchart of the Proposed Approach

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Comparison between SUPORE and Ritz solution for n=22, using MAC

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Comparison between SUPORE and Ritz solution for n=52, using MAC