514 research outputs found

    Experimental measurement of focused wave group and solitary wave overtopping

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    Prediction of individual wave overtopping events is important in assessing danger to life and property, but data are sparse and hydrodynamic understanding is lacking. Laboratory-scale waves of three distinct types were generated at the Coastal Research Facility to model extreme waves overtopping a trapezoidal embankment. These comprised wave groups of compact form, wave groups embedded in a background wave field, and a solitary wave. The inshore wave propagation was measured and the time variation of overtopping rate estimated. The total volume overtopped was measured directly. The experiments provide well-defined data without uncertainty due to the effect of reflection on the incident wave train. The dependence of overtopping on a range of wave shapes is thus determined and the influence of wave-wave interactions on overtopping assessed. It was found that extreme overtopping may arise from focused waves with deep troughs rather than large crests. Furthermore, overtopping waves can be generated from small wave packets without affecting the applicability of results to cases in which there are surrounding waves. Finally, overtopping from a solitary wave is comparable with overtopping from focused wave groups of the same amplitude. © 2011 Copyright International Association for Hydro-Environment Engineering and Research

    Homocysteine in Myointimal Hyperplasia

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    AbstractIntroduction: homocysteine, a sulphur-containing non-essential amino acid, appears to play a role in the pathophysiology of atherosclerosis. However, its role in myointimal hyperplasia, the cause of almost 30% of failures of interventional therapeutic procedures, is much less clear. Methods: a review of the published scientific data concerning the role of homocysteine in myointimal hyperplasia was performed using MEDLINE and other on-line databases. Evidence was sought from cell culture experiments, animal models and clinical studies. Results: several clinical studies have recently been published linking plasma homocysteine levels to restenosis in coronary and peripheral arterial disease. However, several contradictory studies also exist making the role of homocysteine unclear. There are currently no published randomised trials. Cell culture and animal model experiments have elucidated several potential mechanisms by which may stimulate myointimal hyperplasia. Possible mechanisms include endothelial cell activation with the enhanced release of inflammatory cytokines and growth factors and a direct effect on vascular smooth muscle cell migration and proliferation. Conclusion: further studies are required before the true role of homocysteine in the pathogenesis of myointimal hyperplasia can be clearly evaluated. If evidence does confirm a role, the ease with which homocysteine levels can be normalised makes it an attractive alternative therapeutic target for intervention

    Non-causal Linear Optimal Control with Adaptive Sliding Mode Observer for Multi-Body Wave Energy Converters

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    As a non-causal optimal control problem, the performance of wave energy converter (WEC) control relies on the accuracy of the future incoming wave prediction. However, the inevitable prediction errors can degrade WEC performance dramatically especially when a long prediction horizon is needed by a WEC non-causal optimal controller. This paper proposes a novel non-causal linear optimal control with adaptive sliding mode observer (NLOC+ASMO) scheme, which can effectively mitigate the control performance degradation caused by wave prediction errors. This advantage is achieved by embedding the following enabling techniques into the scheme: (i) an adaptive sliding mode observer (ASMO) to estimate current excitation force in real-time with explicitly formulated boundary of estimation error, (ii) an auto-regressive (AR) model to predict the incoming excitation force with explicitly formulated boundary of prediction error using a set of latest historical data of ASMO estimations from (i), and (iii) a compensator to compensate for both the estimation error and the prediction error of excitation force. Moreover, the proposed NLOC+ASMO scheme does not cause heavy computational load enabling its real-time implementation on standard computational hardware, which is especially critical for the control of WECs with complicated dynamics. The proposed NLOC+ASMO framework is generic and can be applied to a wide range of WECs, and in this paper we demonstrate the efficacy by using a multi-float and multi-motion WEC called M4 as a case study, whose control problem is more challenging than the widely studied point absorbers. Simulation results show the effectiveness of the proposed control scheme in a wide range of sea states, and it is also found that the controller is not sensitive to change of ASMO parameters

    High-capacity wave energy conversion by multi-floats, multi-PTO, control and prediction: generalised state-space modelling with linear optimal control and arbitrary headings

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    Wave energy converters with capacity similar to, or greater than, wind turbines are desirable for the supply of electricity to the grid. It is shown that this may be provided by multiple floats in a hinged raft-type configuration with multimode forcing. The case analysed has 8 floats and 4 power take off (PTO) units. Analysis is based on linear diffraction-radiation modelling, validated in wave basin experiments with a smaller number of floats. Control is desirable to improve energy capture, mainly demonstrated for point absorbers, but this has not previously been applied to such a complex problem with many freedoms. The linear hydrodynamic model in a state-space form makes it possible to implement advanced control algorithms in real time. Linear non-causal optimal control (LNOC) is applied with wave force prediction from auto-regression. For the design case with zero heading, as the configuration heads naturally into the wave direction, energy capture is improved by between 21% and 83%. The energy capture is about 62% the maximum possible from idealised analyses. Off-design, non-zero headings are also analysed to indicate how energy capture can be reduced; this is again improved by control, by several times at 90 degrees heading

    Effect of Intermittent Pneumatic Foot Compression on Popliteal Artery Haemodynamics

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    AbstractPurpose: the aim was to investigate the effect of intermittent pneumatic foot compression (IPCfoot) on popliteal artery haemodynamics in normal individuals and in patients with intermittent claudication due to peripheral vascular disease (PVD) (Fontaine stage II). Material and methods: popliteal artery volume flow [vFl], pulsatility index [PI], mean velocity [mV], peak systolic [PSV] and end diastolic velocity [EDV], in 25 limbs of 20 normal subjects and 40 limbs of 32 stable claudicants were obtained in the sitting position before, during and within 30 seconds after the application of IPCfoot(applied pressure: 120 mmHg; inflation time: 3 seconds; deflation time: 17 seconds) using colour-flow duplex imaging (CFDI). The reproducibility of flow velocity estimations using CFDI in the horizontal [hor] (recovery) and sitting [sit] positions was evaluated in 20 limbs of normal controls and 20 limbs of claudicants. Results: popliteal artery vFl, mV, PSV and PI measurements were performed with a coefficient of variation (CV) of less than 14.6% among claudicants and of less than 13.3% in normal subjects. EDV is the least reproducible parameter with an overall CV range of 10.2–21.5% in normal controls and 9.1–18.6% in arteriopaths. On application of IPCfootpopliteal artery vFl increased by 111% in the control group (p<0.001) and by 51% in the claudicants (p<0.001). Within 30 seconds of the cessation of pump action flow decreased significantly in both groups (p<0.001), but maintained a significantly higher level than that at baseline (p<0.001, in both groups). The mV, PSV and EDV showed a similar pattern of significant changes. Both in normals and claudicants, the PI decreased with IPCfoot(p<0.001) and increased post-compression; however, it was significantly lower than baseline (p<0.005) within 30 seconds of impulse delivery. Conclusions: current CFDI technology enables a reproducible estimation of popliteal artery flow velocities. IPCfootcan significantly augment arterial calf inflow on an acute basis both in normals and claudicants. The increase of EDV and decrease of PI indicate that attenuation of peripheral resistance to flow is the main mechanism underlying the popliteal artery vFl enhancement on application of IPCfoot. Prospective trials on the long-term effect of IPCfootin the management of patients with PVD are indicated from the results of this study

    Characteristics and Treatments of Patients with Peripheral Arterial Disease Referred to UK Vascular Clinics: Results of a Prospective Registry

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    BackgroundPeripheral arterial disease (PAD) is often associated with risk factors including cigarette smoking, hypertension and hypercholesterolaemia, and patients have a high risk of future vascular events. Good medical management results in improved outcomes and quality of life, but previous studies have documented sub-optimal treatment of risk factors. We assessed the management of cardiovascular risk factors in patients with PAD referred to specialist vascular clinics.MethodsThis was a prospective, protocol driven registry carried out in UK vascular clinics. Patients who were first-time referrals for evaluation of PAD were eligible if they had claudication plus ankle-brachial pressure index (ABPI) ≤0.9. Statistical associations between key demographic and treatment variables were explored using a chi-squared test.ResultsWe enrolled 473 patients from 23 sites. Mean age was 68 years (SD 10) and 66% were male. Mean estimated claudication distance was 100m, and ABPI was 0.74. Mean systolic blood pressure (SBP) was 155mmHg, and 42% had a SBP >160mmHg. Forty percent were current smokers and half had tried to give up in the prior 6 months, but there was no evidence of a systematic method of smoking cessation. Mean total cholesterol was 5.4 (SD1.2) mmol/l and 30% had levels >6mmol/l. Antiplatelet therapy had been given to 70% and statins to 44%. Prior CHD was present in 29% and these patients had significantly higher use of antiplatelet therapy, statins and ACE-inhibitors.ConclusionsIn spite of attempts to raise awareness about PAD as an important marker of cardiovascular risk, patients are still poorly treated prior to referral to a vascular clinic. In particular, the use of evidence-based treatments is sub-optimal, while hypertension and cigarette smoking are poorly managed. More work needs to be done to educate health professionals about the detection and optimal medical management of PAD
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