Simplified contact filters in wheel/rail noise prediction

When predicting rolling noise due to wheel and rail roughness a “contact filter” is generally applied to account for the effect of the finite size of the wheel/rail contact. For time-domain analysis these calculations must be fast enough to get results in a reasonable time. Remington and Webb have devised a versatile three-dimensional ‘distributed point reacting spring’ (DPRS) contact model that is relatively quick, but if only one line of data is available along the contact it is unnecessarily complex, so a simpler two-dimensional version has been developed here. When this new model was checked against a Boussinesq analysis of the contact, the results in one-third octave bands were found to agree to within 3 dB. These results further suggest that the two-dimensional DPRS model might have an unexpectedly wide range of applicability, including large amplitude sinusoidal roughness and discrete features such as a rail joint. When implemented at each step in a time-domain wheel/rail interaction analysis, this model gave similar results to quasi-static roughness filtering with a constant load for moderate roughness, but dynamic effects became significant when the roughness amplitudes were large, particularly with dipped rail joints

The role of anti-resonance frequencies from operational modal analysis in finite element modelling

Finite element model updating traditionally makes use of both resonance and modeshape information. The mode shape information can also be obtained from anti-resonance frequencies, as has been suggested by a number of researchers in recent years. Anti-resonance frequencies have the advantage over mode shapes that they can be much more accurately identified from measured frequency response functions. Moreover, anti-resonance frequencies can, in principle, be estimated from output-only measurements on operating machinery. The motivation behind this paper is to explore whether the availability of anti-resonances from such output-only techniques would add genuinely new information to the model updating process, which is not already available from using only resonance frequencies.This investigation employs two-degree-of-freedom models of a rigid beam supported on two springs. It includes an assessment of the contribution made to the overall anti-resonance sensitivity by the mode shape components, and also considers model updating through Monte Carlo simulations, experimental verification of the simulation results, and application to a practical mechanical system, in this case a petrol generator set.Analytical expressions are derived for the sensitivity of anti-resonance frequencies to updating parameters such as the ratio of spring stiffnesses, the position of the centre of gravity, and the beam's radius of gyration. These anti-resonance sensitivities are written in terms of natural frequency and mode shape sensitivities so their relative contributions can be assessed. It is found that the contribution made by the mode shape sensitivity varies considerably depending on the value of the parameters, contributing no new information for significant combinations of parameter values.The Monte Carlo simulations compare the performance of the update achieved when using information from: the resonances only; the resonances and either anti-resonance; and the resonances and both anti-resonances. It is found that the addition of anti-resonance information improves the updating performance for some combinations of parameter values, but does not improve the update in significant other regions.The simulated results are verified using resonance and anti-resonance frequencies measured on a steel beam test rig. The investigation is extended to include the updating of parameters of a petrol generator set. It is found that the contribution of the anti-resonances to the model update is heavily dependent on the geometry of the model and the choice of variables to be updated, suggesting that, for some models, the pursuit of anti-resonance information through expensive operational modal analysis may be inappropriate

Cyclostationarity and the cepstrum for operational modal analysis of mimo systems - part 1: modal parameter identification

This paper presents a new technique for operational modal analysis (OMA) of multiple input multiple output (MIMO) systems excited by at least one cyclostationary input with a unique cyclic frequency. The technique is based on two signal separation steps; the cyclostationary properties of the input are exploited to estimate the cyclic spectral density, effectively reducing the system from a MIMO to a single input multiple output (SIMO) situation, and curve-fitted in the cepstrum domain, which allows for the separation of the input and transfer function.This technique is demonstrated using measurements taken on a steel beam test rig and a passenger rail vehicle. The performance of this technique is discussed and compared to traditional input/output modal analysis and an existing cepstrum-based OMA technique. It is shown that the technique is able to correctly identify modal parameters, but like other spectrum-based OMA techniques, long time records are required in order to obtain both smooth cyclic spectrum estimates and sufficient resolution for accurate damping estimates. The nature of the input may also inhibit its performance in the very low-frequency region

Blind system identification using cyclostationarity and the complex cepstrum

This paper presents a new technique for blind identification of MIMO systems excited by cyclostationary inputs with unique cyclic frequencies. The cyclic spectral density is determined, effectively reducing the system from a MIMO to a SIMO scenario, and curve fit in the cepstrum domain, which allows for the separation of the input and path effects. This technique is suitable for determining the in-service properties of passenger vehicles with internal combustion engines, and the resulting properties can be used to update finite element models. The effectiveness of the new technique is demonstrated on a mechanical system

Cyclostationarity and the cepstrum for operational modal analysis of MIMO systems—Part II: Obtaining scaled mode shapes through finite element model updating

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An in-service dynamic model of a diesel railcar from operational modal analysis and finite-element model updating

Builders of passenger rail vehicles need methods for predicting vibrational behaviour so that they can meet ride quality specifications. Conventional finite-element (FE) models used for stress analysis may not be accurate enough for this because of imponderables such as the stiffness of spot-welded joints. One solution is to adjust FE models based on vibration measurements. This model updating is now possible using commercially available software. Usually, the updating is based on vibration tests carried out under laboratory conditions, which may produce different results from normal operation. In this paper, two simple FE models of a railcar are updated using vibration data obtained from a newly designed railcar under in-service conditions. Measurements were made on a railcar loaded to represent crush-loaded conditions and running down track, with the only excitation that naturally occurring due to track roughness, engine excitation, aerodynamic loading etc. Unlike standard laboratory vibration testing, this input excitation could not be measured, so advanced forms of output-only analysis were required. By using these data as input to a commercial updating package, the simple FE models have been updated and the effectiveness of the technique assessed