2,320 research outputs found
Signal processing for experimental modal analysis
No full textThis paper is an overview of signal processing for modal analysis. The approach taken is to summarize some of the types of data that arise in modal analysis, to classify signal processing problems and to attempt to show the relevance of the latter to the former. It is intended to emphasize the diversity of approaches that may be applied and point to advanced methods that offer potential
Modelling piezoelectric excitation in waveguides using the semi-analytical finite element method
No full textIn this paper we present a wave-based technique for modelling waveguides equipped with piezoelectric actuators in which there is no need for common simplications regarding their dynamic behaviour, the interaction with the waveguide or the bonding conditions. The proposed approach is based on the semianalytical finite element (SAFE) method. We developed a new piezoelectric element and employed the analytical wave approach to model the distributed electrical excitation and scattering of the waves at discontinuities. The model was successfully verified numerically and validated against an experiment on a beam-like waveguide with emulated anechoic terminations
Identifying joints from measured reflection coefficients in beam-like structures with application to a pipe support
No full textThe properties of joints in mechanical systems are notoriously uncertain causing corresponding uncertainty in the systems’ dynamic responses. A piping system is one such example where an accurate knowledge of joint properties is useful for the purposes of structure-borne sound transmission, fatigue considerations and structural health monitoring. This paper presents an inverse technique that is applicable to joint estimation in one-dimensional structures such as a pipe. Measured wave reflection coefficients are used which have several advantages over modal information. First, they characterise just the joint and adjacent pipes and are independent of the rest of the built-up system. Second, they are potentially more sensitive to the joint parameters in question than are modal parameters.The method is illustrated by means of an experimental case study featuring a straight pipe suspended by a cantilevered hanger. The stiffness and inertia of the hanger are accurately identified from measured data at frequencies significantly higher than the fundamental modes of the structur
Experimental study on a nonlinear vibration isolator based on a post-buckled inverted L-shaped beam
No full textIn this paper, the characteristics and the effectiveness of a nonlinear passive vibration isolator based on a post-buckled beam is investigated experimentally. The intended application is specifically isolation in the vertical direction where the isolator is required to be sufficiently stiff statically to bear the weight of the isolated mass. The isolator consists of two beams joined to form an inverted L-shape and the weight of the isolated mass is taken to act at the vertex. If the weight of isolated mass is larger than the buckling load of the L-shaped beam then the beam buckles in one of two modes, one of which is unstable. In this paper, the static restoring force of the unstable mode is measured and an appropriately selected coil spring is added to counteract the negative stiffness of the beam. The resulting system presents a dramatically lower stiffness to small excursions about its equilibrium position in its buckled state but maintains its static load bearing capability. Free vibration measurements are presented which show some amplitude dependency of the natural frequency for large amplitude motion. Low amplitude harmonic base excitation measurements are also conducted from which transmissibility measurements are obtained and compared with corresponding results from a Finite Element model. The fundamental resonance is about 80% lower than that achievable by a comparable linear isolator. However the performance of the prototype design is compromised by the presence of higher frequency internal resonances of the isolator, mitigation of which is the focus of ongoing wor
Shock and vibration response of museum objects during transportation
No full textMuseum objects are frequently transported by road and air when toured internationally or when moved between storage facilities which exposes them to a range of vibration and shock inputs. Many of these vulnerable objects have incipient damage such as cracks, loose joints, partial delamination or failing repairs which can be exacerbated in transit. It is the conservator's responsibility to select which objects are safe to travel, largely based on experience and professional judgement. A joint research programme between the University of Southampton and the British Museum aims to develop a scientific framework to aid and inform such judgements. A central research question, and the focus of this paper, is which methods of transport are potentially most damaging?Vibration measurements were acquired during transit by car, train, ferry and commercial airliner. A moving average kurtosis is used to identify near-Gaussian and non-Gaussian segments in a time signal. Acceleration shock response spectra (SRS) are presented for a number of impulsive events, and corresponding vibration response spectra (VRS) shown for more normally distributed random inputs. Laboratory based vibration testing was carried out on a museum artefact packed in bespoke foam packaging inside a standard wooden transport crate. The modal response, combined with SRS and VRS, enabled the response of the object and the suitability of the packaging in reducing the response to different real-life stimuli to be assessed. Actual measurements on the artefact in transit also enabled the accuracy of SRS and VRS in predicting peak and RMS acceleration levels to be quantified
Determining potential for pollutant impacts in dynamic coastal waters: comparing morphological settings
No full textThe coastal focus and beach culture of Australia’s population in general, and the people of New South Wales in particular, mean that coastal systems are both highly prized and subjected to great pressures. The vast majority of the wastewater generated by the 7.3 million people of New South Wales is discharged directly to the ocean. The dispersion and fate of waterborne pollutants and their potential to impact coastal ecosystems are fundamentally determined by the dynamics of the coastal boundary layer (CBL). This turbulent interface between the coastline and the deep oceans is defined and classified for the first time in this thesis. Coastal morphologies and changes in the orientation of the coastline promote turbulence and strong gradients with extreme variability and heterogeneity over a broad range of scales. Conceptual models are presented to characterise New South Wales coastal boundary layer processes.
The broad aims of this thesis are to investigate the coastal boundary layer processes that affect dispersal and advection of pollutants, and to develop conceptual models and tools to facilitate coastal management.
Remote sensed ocean colour and sea surface temperature observations define meso-scale CBL phenomena, and this study demonstrates their application to support management decisions in relation to marine algal (phytoplankton) blooms. However, considerable scope exists to improve regional algorithms to deliver better ocean colour products for the optically complex (Case 2) waters of the inner coastal boundary layer.
Past failures to consider the CBL (morphological) settings of pollutant discharges to coastal waters have led to inefficient pollutant discharge systems and potential environmental impacts. Two case studies, investigate the principal forcing mechanisms and demonstrate the importance of morphology in controlling the dispersion and retention times of pollutants.
The first case study is focused on Sydney coastal waters where pollutant loadings are greater in magnitude and different in character than elsewhere in New South Wales. Here population pressures generate large wastewater loadings but the distances to offshore discharge locations are large compared to the scale of coastal roughness (headlands and bays) and the water is deep, thus reducing the risk of local retention of pollutants and increasing the potential for rapid dilution. By considering simulations of near field effluent plume behaviour in relation to long term ambient nutrient patterns specific periods of the year and depth intervals have been identified when outfalls would have an increased opportunity to influence bloom development, especially the upper half of the water column during late summer. However, algal blooms appear to be principally driven by seasonal oceanic nutrient enrichment. The research presented in this thesis, together with companion research previously published by the author and routine ongoing monitoring, indicate the viability of disposal of the Sydney’s excess sewage effluent (after source control and re-use options have been exhausted) via existing deepwater outfalls.
In contrast, inner CBL settings with coastal irregularities (e.g. headlands and bays) have a greater propensity to trap pollutants. A new hydrodynamically relevant morphological classification of New South Wales bays, headlands and islands provides both broad context for case studies and guides preliminary assessments for other locations. This classification reveals a borderline propensity for flow separation and re-circulation in the lee of Corambirra Point which is the focus of the second case study off Coffs Harbour in northern NSW. Direct observations and 3D finite difference hydrodynamic (Eulerian) and particle tracking (Lagrangian) model simulations quantify transient re-circulation associated with local current accelerations and a persistent shear zone located in the wake to the south of Corambirra Point. The flux of ambient water across the prescribed outfall alignment increases eighteen fold, over a shear zone spanning a cross-shore distance of just 1.4km (from 1.6km to 3km offshore). In contrast, the potential for re-entrainment and trapping of effluent in transient re-circulation cells was demonstrated to be insignificant. The proposed location of the outfalls was 1.5km offshore whereas the greatest gain per unit extension of the proposed discharge point coincides with the centre of the shear zone located ~2km offshore.
These case studies illustrate specific coastal boundary layer effects and indicate how an understanding of the spatial and temporal scales of these effects can be used to target more specific assessments of potential pollutant impacts. Simple morphological risk assessment tools are also presented to identify factors and processes which limit the exposure of sensitive environments to high pollutant concentrations and loads. Eddy retention effects are generally not incorporated in existing near field models but potential re-entrainment effects in wake zones can be assessed through the eddy retention value, which is introduced in this thesis. Although the approach presented here is focused on New South Wales coastal waters, the framework serves as a basis for general application elsewhere, and as a foundation for further refinement for application to NSW coastal waters.
Existing scientific literature indicates that coastal boundary layer processes also shape the distributions of the biological species and communities. This further motivates the development of a process based understanding of coastal boundary layer dynamics as a fundamental platform to support environmental protection and biodiversity conservation initiatives
Semi-analytical modelling of piezoelectric excitation of guided waves
No full textPiezoelectric elements are a key component of modern non-destructive testing (NDT) and structural health monitoring (SHM) systems and play a significant role in many other areas involving dynamic interaction with the structure such as energy harvesting, active control, power ultrasonics or removal of surface accretions using structural waves. In this paper we present a wave-based technique for modelling waveguides equipped with piezoelectric actuators in which there is no need for common simplifications regarding their dynamic behaviour or mutual interaction with the structure. The proposed approach is based on the semi-analytical finite element (SAFE) method. We developed a new piezoelectric semi-analytical element and employed the analytical wave approach to model the distributed electric excitation and scattering of the waves at discontinuities. The model is successfully validated against an experiment on a beam-like waveguide with emulated anechoic termination
Removing surface accretions with piezo-excited high-frequency structural waves
No full textUnwanted accretions on structures are a common machinery maintenance problem, which can pose a serious safety threat if not treated effectively and punctually. In this paper we investigate the capability of piezo-excited structural waves for invoking delamination of accreted material from waveguides. We apply a wave-based technique for modelling piezoelectric excitation based on semi-analytical finite elements to model the interface shear stress associated with piezo-actuated structural waves. As a proof of concept, we present a demonstration experiment in which patches of material are removed from a beam-like waveguide with emulated anechoic terminations using ultrasonic excitation.<br/
Active damping control for vibration isolation of high-static-low-dynamic-stiffness isolators
No full textSurveillance of adenoviruses and noroviruses in European recreational waters
No full textExposure to human pathogenic viruses in recreational waters has been shown to cause disease outbreaks. In the context of Article 14 of the revised European Bathing Waters Directive 2006/7/EC (rBWD, CEU, 2006) a Europe-wide surveillance study was carried out to determine the frequency of occurrence of two human enteric viruses in recreational waters. Adenoviruses were selected based on their near-universal shedding and environmental survival, and noroviruses (NoV) selected as being the most prevalent gastroenteritis agent worldwide. Concentration of marine and freshwater samples was done by adsorption/elution followed by molecular detection by (RT)-PCR. Out of 1410 samples, 553 (39.2%) were positive for one or more of the target viruses. Adenoviruses, detected in 36.4% of samples, were more prevalent than noroviruses (9.4%), with 3.5% GI and 6.2% GII, some samples being positive for both GI and GII. Of 513 human adenovirus-positive samples, 63 (12.3%) were also norovirus-positive, whereas 69 (7.7%) norovirus-positive samples were adenovirus-negative. More freshwater samples than marine water samples were virus-positive. Out of a small selection of samples tested for adenovirus infectivity, approximately one-quarter were positive. Sixty percent of 132 nested-PCR adenovirus-positive samples analysed by quantitative PCR gave a mean value of over 3000 genome copies per L of water. The simultaneous detection of infectious adenovirus and of adenovirus and NoV by (RT)PCR suggests that the presence of infectious viruses in recreational waters may constitute a public health risk upon exposure. These studies support the case for considering adenoviruses as an indicator of bathing water quality
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