Spectrograms of ship wakes: identifying linear and nonlinear wave signals

A spectrogram is a useful way of using short-time discrete Fourier transforms to visualise surface height measurements taken of ship wakes in real world conditions. For a steadily moving ship that leaves behind small-amplitude waves, the spectrogram is known to have two clear linear components, a sliding-frequency mode caused by the divergent waves and a constant-frequency mode for the transverse waves. However, recent observations of high speed ferry data have identified additional components of the spectrograms that are not yet explained. We use computer simulations of linear and nonlinear ship wave patterns and apply time-frequency analysis to generate spectrograms for an idealised ship. We clarify the role of the linear dispersion relation and ship speed on the two linear components. We use a simple weakly nonlinear theory to identify higher order effects in a spectrogram and, while the high speed ferry data is very noisy, we propose that certain additional features in the experimental data are caused by nonlinearity. Finally, we provide a possible explanation for a further discrepancy between the high speed ferry spectrograms and linear theory by accounting for ship acceleration.Comment: 21 pages, 10 figures, submitte

Efficient computation of two-dimensional steady free-surface flows

We consider a family of steady free-surface flow problems in two dimensions, concentrating on the effect of nonlinearity on the train of gravity waves that appear downstream of a disturbance. By exploiting standard complex variable techniques, these problems are formulated in terms of a coupled system of Bernoulli's equation and an integral equation. When applying a numerical collocation scheme, the Jacobian for the system is dense, as the integral equation forces each of the algebraic equations to depend on each of the unknowns. We present here a strategy for overcoming this challenge, which leads to a numerical scheme that is much more efficient than what is normally employed for these types of problems, allowing for many more grid points over the free surface. In particular, we provide a simple recipe for constructing a sparse approximation to the Jacobian that is used as a preconditioner in a Jacobian-free Newton-Krylov method for solving the nonlinear system. We use this approach to compute numerical results for a variety of prototype problems including flows past pressure distributions, a surface-piercing object and bottom topographies.Comment: 20 pages, 13 figures, under revie

Explaining fish consumption in Sri-Lanka: The role of consideration set size, attitude, knowledge, convenience orientation, price consciousness, and variety seeking tendency

The primary purpose of this study to understand how the consideration set size affect for consumption frequency of fish in Sri-Lanka. Consideration set size of fish is considered to be affected by consumer attitude, convenience orientation, and consumer knowledge in Sri-Lankan context. Thus, the second objective was to investigate how consumer attitude, knowledge, convenience orientation, variety seeking tendency and price consciousness affect the formation of consideration set size. Based on that, it aimed to suggest a marketing strategy implication for food marketers, in addition with strategy implication for increasing fish consumption. The proposed conceptual frame work was used to achieve the objectives in this study. Thus, the proposed model consisted with the constructs of attitude, knowledge, convenience orientation, variety seeking tendency and price consciousness. This thesis was an extension of Rortveit and Olsen (2007: 2009) study with the inclusion of additional antecedents of variety seeking tendency and price consciousness. The questionnaire survey was carried out in Galle district in Sri-Lanka with the convenience sample of 250 respondents. The measurement scales used here were adopted from previous studies found in literature. The confirmatory factor analysis and structural equation modeling in Amos 16.0 were used as statistical analysis tool. It was found that significant positive relationship between consideration set size and fish consumption frequency as a main hypothesis. Further, this study has found significant positive relationships between knowledge and consideration set size, between variety seeking related to personality and variety seeking related to food and between convenience orientation and consideration set size, while having insignificant relationships between knowledge and fish consumption frequency, and between variety seeking related to food and consideration set size. Surprisingly, attitude has an insignificant effect on both consideration set size and the consumption frequency of food. The main reason for this problem is skewneess of the data set. The study found that that belief of sensory aspects, health and convenience were significant predictors of attitude toward fish. Further this study revealed that price consciousness related to food has a negative significant impact on consideration set size. These findings indicated that from a marketing point of view, being chosen is effected by the size of the consideration set size, food marketers should advocate that consumers consider many kind of fish products in choice occasion. As shown by the study, set size can be increased through the consumer knowledge variable. In such a condition, manufacturer must tend to put in to consumer education which will lead to have a positive impact on consideration set size. From the theoretical point of view, this research contributed to fill the gap in the consumer consideration literature in general, but also to the literature trying to explain fish consumption in Sri-Lanka. A limitation of this study is that the relationship between the consideration set size and the consumption was only tested for the size dimension of the consideration set. Further more, sample is relatively small and they are not statistically representative to the total population in Galle district or Sri-Lanka. Therefore, the results could not be generalized to Sri-Lanka. Keywords: Consideration set size, attitude, convenience orientation, knowledge, and variety seeking tendency and price consciousness, fish consumption, Sri-Lanka

Mothers’ Knowledge Regarding Neonatal Baby Care

Neonatal mortality is a major problem, all over the world especially, in the developing countries. The aim of this study was to determine the knowledge of mothers, who were admitted to the postnatal ward of Teaching Hospital Kurunegala regarding the neonatal baby care. This descriptive cross sectional study was carried out in the post-natal ward, at the Teaching Hospital in Kurunegala (North Western Province), Sri Lanka. The data was collected by interviewing 246 post-natal mothers, aged 15 – 45 years, who were in the post-natal ward, during the period of 18th April to 2nd May 2011. The mean age of the sample was 26 years. Nearly half of mothers (48.4%) completed education up to O/L. The percentage of mothers who completed education up to secondary and higher education level was 41.3%. A considerable proportion of mothers were unemployed (86%). Half of them had one child. The percentage of postnatal mothers, who had two children, is 30%. Others had more than two children.Education level was found to be positively and significantly associated with knowledge regarding neonatal care. Additionally, occupation and age too were found to be significantly associated with knowledge level. Parity did not show a significant association with maternal knowledge regarding neonatal care as expected. The results showed that mothers’ knowledge and practices regarding breast feeding was at a satisfactory level in mothers selected to the sample population. But knowledge of mothers was not at a satisfactory level regarding umbilical cord care. More than half of them were in less than average knowledge level. It was found that mothers’ knowledge of prevention of neonatal hypothermia was poor. A large proportion of them (45.52%) were in the poor knowledge group and 42.27% of them were in very poor knowledge group. The results showed clearly that mothers’ knowledge of prevention of neonatal hypothermia was the most deficient knowledge field of mothers who lived in Kurunegala area. Maternal knowledge of breastfeeding was the only satisfactory knowledge field from these three areas for mother who lived in Kurunegala area. The study showed in majority of mothers, adequate knowledge and practices regarding neonatal care were lacking. Especially, in the field of prevention of neonatal hypothermia and care for the neonatal umbilical cord

Generosity during COVID-19: investigating socioeconomic shocks and game framing

This paper investigates two research questions. First, we examine the impact of the negative socioeconomic shock of the COVID-19 pandemic on generosity towards different types of recipients, and changes in generosity as the shock worsens over time. This is carried out by analysing the responses of 1255 US citizens to four dictator games played over 8 weeks of the early stages of the COVID-19 pandemic, corresponding to four different recipients: relatives, neighbours, strangers, and the state. Second, we investigate whether the order in which the games are played create a framing effect that influences social preferences in terms of generosity towards the different recipients. Results capture an increase in generosity despite the worsening pandemic, indicating strong perceived increasing marginal benefits of generosity in times of hardship. There is significant heterogeneity in the effects of additional regressors, such as perceived contagion risk, on the likelihood and amount donated to strangers, family members, or the government. At the same time, some significant effects of framing bring new evidence regarding the stability of social preferences

Time-frequency analysis of ship wave patterns in shallow water: modelling and experiments

A spectrogram of a ship wake is a heat map that visualises the time-dependent frequency spectrum of surface height measurements taken at a single point as the ship travels by. Spectrograms are easy to compute and, if properly interpreted, have the potential to provide crucial information about various properties of the ship in question. Here we use geometrical arguments and analysis of an idealised mathematical model to identify features of spectrograms, concentrating on the effects of a finite-depth channel. Our results depend heavily on whether the flow regime is subcritical or supercritical. To support our theoretical predictions, we compare with data taken from experiments we conducted in a model test basin using a variety of realistic ship hulls. Finally, we note that vessels with a high aspect ratio appear to produce spectrogram data that contains periodic patterns. We can reproduce this behaviour in our mathematical model by using a so-called two-point wavemaker. These results highlight the role of wave interference effects in spectrograms of ship wakes.Comment: 14 pages, 7 figure

Diversification of shrub frogs (Rhacophoridae, Pseudophilautus) in Sri Lanka - Timing and geographic context

US National Science Foundation (DEB 0345885) to CJS and JH; National Geographic Society (7612-04) to CJS; and Society of Systematic Biologists Graduate Student Award for Research to MM. MM was also supported by a Harvard University Center for the Environment (HUCE) Ziff Environmental Postdoctoral Fellowship. (DEB 0345885 - US National Science Foundation; 7612-04 - National Geographic Society; Society of Systematic Biologists Graduate Student Award for Research; Harvard University Center for the Environment (HUCE) Ziff Environmental Postdoctoral Fellowship)Accepted manuscrip