Evanescent Wave Reduction Using a Segmented Wavemaker in a Two Dimensional Wave Tank

Evanescent waves are created by the wavemaking process during tank testing. They have long been a nuisance for engineers as they contaminate the wave field in the tank and result in additional inertial force experienced by a wavemaker. Evanescent waves are created by the mismatch between the motion of the wavemaker and the motion of the uid particles in a progressive wave. To avoid contamination of test results, often a considerable distance must be left between the wavemaker and the test area. This space requirement may be costly for small research groups or companies who wish to have a facility to perform some basic proof-of-concept tests in-house, but are restricted for space. The initial aim of this project was to develop a wavemaker which minimised this space requirement over a large range of frequencies. The exploration into the behaviour of evanescent waves from the point of view of the fundamentals of hydrodynamics has been very enlightening. It became clear with the discovery of an interference pattern between the evanescent waves, that this pattern can be optimised to effectively cancel out the evanescent wave fieeld. This interference pattern arises from a phase shift of � radians experienced by some of the evanescent waves, with respect to the others. The significance of this in hydrodynamics is that it explains the existence of negative added mass. The application for this knowledge far out reaches the topic of reducing the distortion in a wave tank. The ability to minimise the added mass of a wavemaker has a great deal of potential in both active absorbing wavemakers and wave energy conversion. For active absorbing wavemakers, the minimisation of added mass may be useful in the absorption of unwanted waves which can be particularly troublesome at high frequencies. The concept of designing the geometry of a wavemaker to simply match the motion of the uid particles has long been proposed; however, the difficulty with designing such a wavemaker is that the ideal geometry is frequency dependent. Hence, a design that eliminates the evanescent waves at one particular frequency will not be able to do so for other frequencies. An investigation into the design of a segmented wavemaker is presented here, as its geometry can easily be adjusted to suit different frequencies. The wavemaker theory for the multi-body problem of the segmented wavemaker is developed, and a new aspect of wavemaker theory that predicts a phase shift of � radians in some of the evanescent waves is presented for the first time. A hypothesis is put forward, and then investigated, proposing that this phase shift can be exploited to create an interference pattern that can effectively cancel out the evanescent waves. The hydrodynamics of the segmented wavemaker were constrained using the Newton-Euler equations of motion with Eliminated Constraints (NE-EC). This approach facilitated a comparison between wavemakers with multiple degrees of freedom and traditional wavemakers with a single degree of freedom. The lengths and strokes of each segment in the wavemaker are optimised to reduce the distortion caused by the evanescent waves using two approaches. Approach one follows the traditional ideas and optimises the lengths and strokes of the segments to best approximate the motion of the uid particles in a progressive wave. Approach two optimises the lengths and strokes of the segments in order to minimise the distance between the wavemaker and the testable area in the tank. Approach two exploits the phase shift in the evanescent waves by finding the optimal interference pattern that effectively cancels out the evanescent waves. A comparison between both approaches shows that effectively eliminating the distortion caused by the evanescent waves is much more achievable by optimising the interference pattern between the evanescent waves, rather than trying to approximate a progressive wave. The results for the segmented wavemakers optimised using approach two predicted that the distortion can be effectively eliminated for a wide range of frequencies using a segment wavemaker consisting of three aps. A sensitivity analysis indicates that the performance of the wavemaker is somewhat effected by errors in the segments strokes, but the overall performance is still better than what has been developed to date

Prenatal Medication Use And Autistic Behaviors In A South Korean Cohort

Autism spectrum disorder (ASD) is thought to result from a combination of genetic and environmental factors, yet a specific cause remains unknown. In this study, exposure and outcome data from a South Korean cohort (N=3,711) were analyzed to investigate the possibility of an environmental contribution to autism etiology. No significant association was observed between use of prenatal medications and autistic behaviors, as measured by Korean versions of both the Autism Spectrum Screening Questionnaire (ASSQ; OR 1.20, 95% CI 0.95–1.51) and the Social Responsiveness Scale (SRS; OR 0.98, 95% CI 0.78–1.25). Similarly, no significant associations were observed when use of prenatal vitamins, folic acid, or iron supplements was examined. There was also no dose-response relationship observed between number of medications taken during pregnancy and increased autistic behaviors. No evidence for a role of prenatal medication use in autism etiology is suggested by the data in this study. Future investigations focusing on specific subgroups of medications in this population are warranted

Stream Channel Stability and Channel Evolution in a Rapidly Urbanizing, Ridge-and-Valley Watershed, Beaver Creek, Knox County, Tennessee

In Tennessee, sedimentation is among the leading causes of stream impairment. Excessive loads of alluvium are detrimental to the ecological health and human use of these resources. Sediments in streams have many sources, but there is evidence that stream bank erosion is a major contributing factor. Development and urbanization in a stream’s watershed will have impacts on the concentration of stream sediment because the increase in the area covered by impervious surfaces, which reduces initial abstraction and retention times. This, in turn will increase the peak storm water discharge and sediment carrying capacity. If the stream channel cannot accommodate these flows, the form of its bed and banks will begin to adjust. These adjustments are described by the Channel Evolution Model developed by the USDA National Sedimentation Laboratory. The channel response will proceed through 6 stages, moving from a premodified condition through periods of degradation and periods of aggradation until a new, stable channel form is attained. Theoretically, it would be possible to use an evaluation of the stage of channel evolution at several sites along a disturbed stream to predict the response of the entire stream network. However, this can only happen in streams in which there are no controls on the ability of a channel to adjust freely. If this pattern were to hold true in the case of a rapidly developing watershed and could be detected by a relatively fast and easy assessment scheme, it would ease the difficulty of determining where to focus stream bank stabilization projects. In an effort to determine whether or not this was the case, a semi-quantitative Rapid Geomorphic Assessment, introduced by Andrew Simon, was used to evaluat

Dialogue Education, Parts 1 & 2

Blog post to AEA365, a blog sponsored by the American Evaluation Association (AEA) dedicated to highlighting Hot Tips, Cool Tricks, Rad Resources, and Lessons Learned for evaluators. The American Evaluation Association is an international professional association of evaluators devoted to the application and exploration of program evaluation, personnel evaluation, technology, and many other forms of evaluation. Evaluation involves assessing the strengths and weaknesses of programs, policies, personnel, products, and organizations to improve their effectiveness

A pattern for a health team-centered classroom based on a survey of literature on the modern philosophy and goals of a medical center, general education, medical and nursing education, the medical social worker's education and needs affecting the teaching of nursing

Thesis (M.S.)--Boston Universit

P2Y12 inhibition in patients with NSTEMI--can later be better

Acute coronary syndrome is an umbrella term that is used to describe the abrupt reduction of blood flow to myocardial tissue, typically associated with the rupture of a coronary atherosclerotic plaque. Rupture exposes the blood to plaque contents, resulting in the deposition and activation of platelets and the formation of thrombi. Complete thrombotic occlusion produces ST-segment elevation myocardial infarction, whereas incomplete impairment of coronary blood flow results in unstable angina or, when biomarkers for myocardial injury are present, non–ST-segment elevation myocardial infarction (NSTEMI). Because the rupture of a plaque incites platelet activation and thrombosis, treatments for unstable angina and NSTEMI have focused on inhibiting platelet function and the coagulation cascade. In patients at high risk for future events (i.e., reinfarction or recurrent ischemia), an early invasive strategy of cardiac catheterization and revascularization is recommended, and in most of these patients intracoronary stents are implanted to treat the plaque rupture. Since stents can produce further plaque trauma, platelet-dependent thrombosis, and embolization into the coronary microcirculation, it is best practice to treat patients with agents that inhibit platelet activation to prevent recurrent ischemia after percutaneous coronary intervention (PCI). As a consequence, current guidelines recommend dual antiplatelet therapy with aspirin plus another agent in patients with NSTEMI who are undergoing PCI