Active vibration control (AVC) of a satellite boom structure using optimally positioned stacked piezoelectric actuators

In this paper, results for active vibration control predicted from experimental measurements on a lightweight structure are compared with purely computational predictions. The structure studied is a 4.5m long satellite boom consisting of 10 identical bays with equilateral triangular cross sections. First, the results from a Fortran code that is based on a receptance analysis are validated against the experimental forced response of the boom structure. Exhaustive searches are then carried out to find the optimum positions for one and two actuators. Finally, a genetic algorithm is employed to find high-quality positions for three actuators on the structure that will achieve the greatest reductions in vibration transmission. Having found these actuator positions, experiments are then carried out to verify the quality of the theoretical predictions. It was found that the attenuation achievable in practice for one, two and three actuators were, respectively, 15.1, 26.1 and 33.5 dB

Active vibration control (AVC) of a satellite boom structure using optimally positioned stacked piezoelectric actuators

In this paper, results for active vibration control predicted from experimental measurements on a lightweight structure are compared with purely computational predictions. The structure studied is a 4.5m long satellite boom consisting of 10 identical bays with equilateral triangular cross sections. First, the results from a Fortran code that is based on a receptance analysis are validated against the experimental forced response of the boom structure. Exhaustive searches are then carried out to find the optimum positions for one and two actuators. Finally, a genetic algorithm is employed to find high-quality positions for three actuators on the structure that will achieve the greatest reductions in vibration transmission. Having found these actuator positions, experiments are then carried out to verify the quality of the theoretical predictions. It was found that the attenuation achievable in practice for one, two and three actuators were, respectively, 15.1, 26.1 and 33.5 dB

Point vibration measurements for the detection of shallow-buried objects

AbstractA major UK initiative, entitled ‘Mapping the Underworld’, is seeking to address the serious social, environmental and economic consequences arising from an inability to locate accurately and comprehensively the buried utility service infrastructure without resorting to extensive excavations. Mapping the Underworld aims to develop and prove the efficacy of a multi-sensor device for accurate remote buried utility service detection, location and, where possible, identification. One of the technologies to be incorporated in the device is low-frequency vibro-acoustics, and application of this technique for detecting buried infrastructure is currently being investigated. Here, the potential for making a number of simple point vibration measurements in order to detect shallow-buried objects, in particular plastic pipes, is explored. Point measurements can be made relatively quickly without the need for arrays of surface sensors, which can be expensive, time-consuming to deploy, and sometimes impractical in congested areas.At low frequencies, the ground behaves as a simple single-degree-of-freedom (mass–spring) system with a well-defined resonance, the frequency of which will depend on the density and elastic properties of the soil locally. This resonance will be altered by the presence of a buried object whose properties differ from the surrounding soil. It is this behavior which can be exploited in order to detect the presence of a buried object, provided it is buried at a sufficiently shallow depth. The theoretical background is described and preliminary measurements are made both on a dedicated buried pipe rig and on the ground over a domestic waste pipe. Preliminary findings suggest that, for shallow-buried pipes, a measurement of this kind could be a quick and useful adjunct to more conventional methods of buried pipe detection

Private and censorship-resistant communication over public networks

Society’s increasing reliance on digital communication networks is creating unprecedented opportunities for wholesale surveillance and censorship. This thesis investigates the use of public networks such as the Internet to build robust, private communication systems that can resist monitoring and attacks by powerful adversaries such as national governments. We sketch the design of a censorship-resistant communication system based on peer-to-peer Internet overlays in which the participants only communicate directly with people they know and trust. This ‘friend-to-friend’ approach protects the participants’ privacy, but it also presents two significant challenges. The first is that, as with any peer-to-peer overlay, the users of the system must collectively provide the resources necessary for its operation; some users might prefer to use the system without contributing resources equal to those they consume, and if many users do so, the system may not be able to survive. To address this challenge we present a new game theoretic model of the problem of encouraging cooperation between selfish actors under conditions of scarcity, and develop a strategy for the game that provides rational incentives for cooperation under a wide range of conditions. The second challenge is that the structure of a friend-to-friend overlay may reveal the users’ social relationships to an adversary monitoring the underlying network. To conceal their sensitive relationships from the adversary, the users must be able to communicate indirectly across the overlay in a way that resists monitoring and attacks by other participants. We address this second challenge by developing two new routing protocols that robustly deliver messages across networks with unknown topologies, without revealing the identities of the communication endpoints to intermediate nodes or vice versa. The protocols make use of a novel unforgeable acknowledgement mechanism that proves that a message has been delivered without identifying the source or destination of the message or the path by which it was delivered. One of the routing protocols is shown to be robust to attacks by malicious participants, while the other provides rational incentives for selfish participants to cooperate in forwarding messages

Vacuum-UV negative photoion spectroscopy of CH4

Using synchrotron radiation in the range 12-35 eV, negative ions are detected by mass spectrometry following vacuum-UV photoexcitation of methane. Ion yields for H$^-$, CH$^-$ and CH$_2^-$ are recorded, the spectra of CH$^-$ and CH$_2^-$ for the first time. All ions display a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation. Cross sections for ion-pair formation are put onto an absolute scale by calibrating the signal strengths with those of F$^-$ from SF$_6$ and CF$_4$. Following normalisation to total vacuum-UV absorption cross sections, quantum yields for anion production are reported. There is a major discrepancy in the H$^-$ cross section with an earlier measurement, which remains unresolved. The anions arise from both direct and indirect ion-pair mechanisms. For a generic polyatomic molecule AB, the former is defined as AB $\rightarrow$ A$^-$ + B$^+$ (+ neutrals), the latter as the predissociative crossing of an initially-excited Rydberg state of AB by an ion-pair state. In a separate experiment, the threshold photoelectron spectrum of the second valence band of CH$_4$, ionisation to CH$_4^+$ A $^2$A$_1$ at 22.4 eV, is recorded with an instrumental resolution of 0.004 eV; many of the Rydberg states observed in indirect ion-pair formation converge to this state. The widths of the peaks are lifetime limited, increasing with increasing $v$ in the $v_1$ (a$_1$) vibrational ladder. They are the first direct measurement of an upper value to the dissociation rate of these levels into fragment ions

The hospital costs associated with acute paediatric burn injuries

Background. Ongoing rationing of healthcare threatens services that are well established, and cripples others that desperately require investment. Burn, for one, remains a neglected epidemic in South Africa (SA), despite the magnitude of the problem.Objective. To identify the prominent components contributing to the cost of hospital admission with paediatric burn injury. Determining the true costs of specialist services is important, so that resources can be allocated appropriately to achieve the greatest possible impact.Methods. A retrospective study was undertaken over 1 year to determine patient demographics and injury details of 987 patients admitted with burn injuries to Red Cross War Memorial Children’s Hospital, Cape Town, SA. The in-hospital financial records of 80 randomly selected patients were examined. This was followed by a prospective study to determine the financial implications of four cost drivers, i.e. bed cost per day, costs of medications received, costs of dressings for wound care, and costs of surgical intervention. A random selection of 37 dressing changes (in 31 paediatric patients) and 19 surgical interventions was observed, during which all costs were recorded.Results. As expected, severe flame burns are responsible for more prolonged hospital stays and usually require surgical intervention. Scald burns comprise the greatest proportion of burn injuries, and therefore account for a considerable part of the hospital’s expenditure towards burn care.Conclusion. While community programmes aiming to prevent burn injuries are important, this study motivates for the implementation of accessible ambulatory services in low-income areas. This strategy would enable the burn unit to reduce its costs by limiting unnecessary admissions, and prioritising its resources for those with more severe burn injuries

The hospital costs associated with acute paediatric burn injuries

__Background__ Ongoing rationing of healthcare threatens services that are well established, and cripples others that desperately require investment. Burn, for one, remains a neglected epidemic in South Africa (SA), despite the magnitude of the problem. __Objective__ To identify the prominent components contributing to the cost of hospital admission with paediatric burn injury. Determining the true costs of specialist services is important, so that resources can be allocated appropriately to achieve the greatest possible impact. __Methods__ A retrospective study was undertaken over 1 year to determine patient demographics and injury details of 987 patients admitted with burn injuries to Red Cross War Memorial Children’s Hospital, Cape Town, SA. The in-hospital financial records of 80 randomly selected patients were examined. This was followed by a prospective study to determine the financial implications of four cost drivers, i.e. bed cost per day, costs of medications received, costs of dressings for wound care, and costs of surgical intervention. A random selection of 37 dressing changes (in 31 p

Application of the group-theoretical method to physical problems

The concept of the theory of continuous groups of transformations has attracted the attention of applied mathematicians and engineers to solve many physical problems in the engineering sciences. Three applications are presented in this paper. The first one is the problem of time-dependent vertical temperature distribution in a stagnant lake. Two cases have been considered for the forms of the water parameters, namely water density and thermal conductivity. The second application is the unsteady free-convective boundary-layer flow on a non-isothermal vertical flat plate. The third application is the study of the dispersion of gaseous pollutants in the presence of a temperature inversion. The results are found in closed form and the effect of parameters are discussed

‘Choicest unguents’: molecular evidence for the use of resinous plant exudates in late Roman mortuary rites in Britain

YesResinous substances were highly prized in the ancient world for use in ritual contexts. Details gleaned from classical literature indicate that they played a significant role in Roman mortuary rites, in treatment of the body and as offerings at the tomb. Outside of Egypt, however, where research has shown that a range of plant exudates were applied as part of the mummification process, resins have rarely been identified in the burial record. This is despite considerable speculation regarding their use across the Roman Empire. Focusing on one region, we investigated organic residues from forty-nine late Roman inhumations from Britain. Using gas chromatographyemass spectrometry and the well-attested biomarker approach, terpenic compounds were characterized in fourteen of the burials analysed. These results provided direct chemical evidence for the presence of exudates from three different plant families: coniferous Pinaceae resins, Mediterranean Pistacia spp. resins (mastic/terebinth) and exotic Boswellia spp. gum-resins (frankincense/olibanum) from southern Arabia or beyond. The individuals accorded this rite had all been interred with a package of procedures more elaborate than the norm. These findings illuminate the multiplicity of roles played by resinous substances in Roman mortuary practices in acting to disguise the odour of decomposition, aiding temporary soft-tissue preservation and signifying the social status of the deceased. Nevertheless, it was their ritual function in facilitating the transition to the next world that necessitated transportation to the most remote outpost of the late Roman Empire, Britain.R.C.B is supported by a PhD studentship from the Art and Humanities Research Council (43019R00209)