103 research outputs found
Mapping the underworld multi-sensor device creation, assessment, protocols: Acoustic technologies advancement to support multi-sensor device. An assessment of the use of a scanning laser to measure ground vibration
No full textThis report concerns experimental work undertaken at ISVR under the EPSRC-funded ‘Mapping the Underworld’ programme, phase 2, EP/F065973/1. In the experimental work reported here, using a scanning laser is compared with using geophones for the measurement of ground vibration at low frequencies (typically <500Hz).The performance of the sensors was compared on a number of different ground surfaces. For all the surfaces, there was general agreement between the laser data and the geophone data; the laser performed better on some surfaces than others, but the laser data was consistently of poorer quality than the geophone data. Surface velocity was found to be the key factor in determining data quality, rather than the surface texture itself; for most of the tests, the surface velocities were close to the laser system noise floor.A number of ways to improve data quality were investigated including altering the surface texture, either by removal of surface dust/grit or by applying retroreflective tape, high pass filtering, signal averaging, both spatially and in the time/frequency domain, and using different types of input signal.Finally, effects of the laser stand-off distance were assessed
Insights into the measurement of vertibral translation in the sagittal plane
No full textIn this discussion paper, vertebral translation in the sagittal plane is considered. A number of different measurement protocols are investigated and it is shown that, in general, the protocols lack a clear notion of the ideal. This can lead to difficulties when interpreting clinical data. One possible ideal is encapsulated in the concept of George's line, the posterior vertebral body alignment line. It is shown that translation can be measured such that deviations from George's line are quantified. Furthermore, an intrinsic centre of rotation is implied, and by considering translation in conjunction with intervertebral angle, inferences can be made regarding the position of the instantaneous centre of rotation of the joint. The relationship between static values of translation viewed on a single radiograph and changes in translation viewed across a number of images is also investigated. Results from some previous clinical studies are discussed in the light of the findings
Axisymmetric wave propagation in buried, fluid-filled pipes: effects of wall discontinuities
No full textWater leakage from buried pipes is a subject of great concern in Britain and across the world because of decreasing water supplies due to changing rainfall patterns, deterioration of antiquated distribution systems, and an increasing population. Correlation techniques are widely used to locate the leaks, however, difficulties are encountered when repairs have been made to a pipe by inserting a new length of pipe to replace a damaged section. Although this practice is now discouraged, the new sections might be of a different material or possibly different cross-section or wall thickness. The wave propagation behaviour at such joints is poorly understood at present.In earlier work, simple expressions for the wavenumbers of the s=1 (fluid-dominated) and s=2 (shell dominated) axisymmetric wave types were derived for a fluid-filled elastic pipe, both in vacuo and surrounded by an elastic medium of infinite extent. In this paper, the wave transmission and reflection characteristics of these waves at an axisymmetric pipe wall discontinuity in a fluid-filled piping system are investigated theoretically.For changes in wall thickness or wall elasticity, simple expressions may be used to characterise the joint. The reason for this is that negligible energy conversion between the wavetypes occurs, so the wavetypes can be considered separately. For changes in the fluid cross-section, significant mode conversion occurs and the wavetypes must be considered together
A low-frequency anechoic lining for underwater use
No full textA novel low-frequency anechoic lining for use underwater is investigated both theoretically and experimentally. Measurements are made in an impedance tube. The reflection coefficient of any lining material placed at the end of the tube can be found from the real part of the input mobility. The results are compared with theoretical predictions. The experimental results agree qualitatively with the theoretical predictions and any quantitative discrepancy is probably due to inaccuracies in manufacturing the anechoic lining itself, rather than in the measuring technique used or in the theoretical model
Axisymmetric wave propagation in buried, fluid-filled pipes: effects of the surrounding medium
No full textThe effects of wall discontinuities on low frequency axisymmetric wave propagation in fluid-filled pipes
No full textAutomatic location of vertebrae in digitized videofluoroscopic images of the lumbar spine
No full textBack pain is a widespread problem, and the disability it engenders continues to grow, despite efforts to contain it. A major problem in the diagnosis and management of back pain is the assessment of the degree to which mechanical factors play a part. Of considerable importance in understanding these mechanical factors is being able to quantify how the human spine actually moves in vivo. Digitized videofluoroscopy is currently the only practical method available for studying spinal motion in vivo at the segmental level. Low-dose, planar motion X-rays of the spine are captured on videotape and subsequently digitized for analysis. Until now, vertebrae in the digitized images were identified and marked manually as a basis for calculating intervertebral kinematics. This paper describes a procedure for automatically identifying the vertebrae in the motion sequences. The process increases objectivity and repeatability, and significantly reduces the manual effort required in locating the vertebrae prior to calculating the kinematics. The technique has been applied to images of a calibrated model and the results are promising. In-plane rotations may be calculated to an accuracy of at least 1 degree. Repeated analysis reveals standard deviations of less than 0.5 degree for intervertebral rotations and less than 0.25 mm for translations
Leak noise propagation and attenuation in submerged plastic water pipes
No full textDetection of water leaks in buried distribution pipes using acoustic methods is common practice in many countries. Correlation techniques are widely used in leak detection, and these have been extremely effective when attempting to locate leaks in metal pipes. However, a number of difficulties have been highlighted when trying to determine the position of leaks in plastic pipes. Of particular interest here is what happens to the leak noise when the pipe passes through an expanse of water, such as across a river or lake.In this paper, the low-frequency acoustic propagation and attenuation characteristics of a submerged plastic water pipe are investigated experimentally in the laboratory, supported by predictions from a theoretical model. It is found that, whilst the signal attenuation for a submerged pipe is increased relative to that for a similar in-vacuo pipe, energy does not, in fact, radiate into the water; furthermore, the attenuation is small compared with that for a pipe buried in soil. <br/
Wavenumber prediction and measurement of axisymmetric waves in buried fluid-filled pipes: inclusion of shear coupling at a lubricated pipe/soil interface
Get PDFAcoustic methods have been widely used to detect water leaks in buried fluid-filled pipes, and these technologies also have the potential to locate buried pipes and cables. Relatively predictable for metal pipes, there is considerably more uncertainty with plastic pipes, as the wave propagation behaviour becomes highly coupled between the pipe wall, the contained fluid and surrounding medium. Based on the fully three-dimensional effect of the surrounding soil, pipe equations for n=0 axisymmetric wave motion are derived for a buried, fluid-filled pipe. The characteristics of propagation and attenuation are analysed for two n=0 waves, the s=1 wave and s=2 wave, which correspond to a predominantly fluid-borne wave and a compressional wave predominantly in the shell, respectively. At the pipe/soil interface, two extreme cases may be considered in order to investigate the effects of shear coupling: the “slip” condition representing lubricated contact; and the “no slip” condition representing compact contact. Here, the “slip” case is considered, for which, at low frequencies, analytical expressions can be derived for the two wavenumbers, corresponding to the s=1 and s=2 waves. These are both then compared with the situations in which there is no surrounding soil and in which the pipe is surrounded by fluid only, which cannot support shear. It is found that the predominant effect of shear at the pipe/soil interface is to add stiffness along with damping due to radiation. For the fluid-dominated wave, this causes the wavespeed to increase and increases the wave attenuation. For the shell-dominated wave there is little effect on the wavespeed but a marked increase in wave attenuation. Comparison with experimental measurements confirms the theoretical finding
Mathematical simulation of a fluoroscopic projection from a three-dimensional solid body model for application in spinal motion analysis
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