On the need for bump event correction in vibration test profiles representing road excitations in automobiles

This paper presents an approach to the synthesis of compressed vibration test profiles representing much longer time histories obtained in road testing of ground vehicles. Vibration test profiles are defined as those related directly to operational testing on specific road surfaces and which summarise the input to the vehicle in the given conditions. The method extends classical Fourier transform technique by means of bump event correction in the background Fourier signal where the bump event term implies a high-amplitude transient event of the shock type. The orthogonal wavelet decomposition was used as a specific filtering tool facilitating bump event identification. Examples of seat guide vertical acceleration have been considered. Calculated probability density functions suggest the ability of the bump correction method to improve the statistical accuracy of the final vibration test profile with respect to the original road data. Test profiles obtained by means of Fourier transform synthesis with subsequent reinsertion of bump events from separated frequency bands were more accurate than those obtained by Fourier synthesis alone. Further developments led to advanced bump reinsertion with synchronisation of events occurring in different frequency bands at the same moment of time. Test profiles generated in this way have provided better accuracy compared to the non-synchronised algorithm

Non-Gaussian PDF modeling of turbulent boundary layer fluctuating pressure excitation

The purpose of the study is to investigate properties of the probability density function (PDF) of turbulent boundary layer fluctuating pressures measured on the exterior of a supersonic transport aircraft. It is shown that fluctuating pressure PDFs differ from the Gaussian distribution even for surface conditions having no significant discontinuities. The PDF tails are wider and longer than those of the Gaussian model. For pressure fluctuations upstream of forward-facing step discontinuities and downstream of aft-facing step discontinuities, deviations from the Gaussian model are more significant and the PDFs become asymmetrical. Various analytical PDF distributions are used and further developed to model this behavior

Variations in steepness of the probability density function of beam random vibration

Dynamic behaviour of a beam, subjected to stationary random excitation, has been investigated for the situation in which the response is different from the model of a Gaussian random process. The study was restricted to the case of symmetric non-Gaussian probability density functions of beam vibrations. There are two possible causes of deviations of the system response from the Gaussian model: the first, nonlinear behaviour, concerns the system itself and the second is external when the excitation is not Gaussian. Both cases have been considered in the paper. To clarity the conclusions for each case and to avoid interference of these different types of system behaviour, two beam structures, clamped-clamped and cantilevered, have been studied. A numerical procedure for prediction of the nonlinear random response of a clamped-clamped beam under the Gaussian excitations was based on a linear modal expansion. Monte Carlo simulation was undertaken using Runge–Kutta integration of the generalised coordinate equations. Probability density functions of the beam response were analysed and approximated making use of different theoretical models. An experimental study has been carried out for a linear system of a cantilevered beam with a point mass at the free end. A pseudo-random driving signal was generated digitally in the form of a Fourier expansion and fed to a shaker input. To generate a non-Gaussian excitation a special procedure of harmonic phase adjustment was implemented instead of the random choice. In so doing, the non-Gaussian kurtosis parameter of the beam response was controlle