Evaluation of the Acoustic Doppler Velocimeter (ADV) for Turbulence Measurements

Accuracy of the acoustic Doppler velocimeter (ADV) is evaluated in this paper. Simultaneous measurements of open-channel flow were undertaken in a 17-m flume using an ADV and a laser Doppler velocimeter. Flow velocity records obtained by both instruments are used for estimating the true (‘‘ground truth’’) flow characteristics and the noise variances encountered during the experimental runs. The measured values are compared with estimates of the true flow characteristics and values of variance (^u92&, ^w92&) and covariance (^u9w9&) predicted by semiempirical models for open-channel flow. The analysis showed that the ADV sensor can measure mean velocity and Reynolds stress within 1% of the estimated true value. Mean velocities can be obtained at distances less than 1 cm from the boundary, whereas Reynolds stress values obtained at elevations greater than 3 cm above the bottom exhibit a variation that is in agreement with the predictions of the semiempirical models. Closer to the boundary, the measured Reynolds stresses deviate from those predicted by the model, probably due to the size of the ADV sample volume. Turbulence spectra computed using the ADV records agree with theoretical spectra after corrections are applied for the spatial averaging due to the size of the sample volume and a noise floor. The noise variance in ADV velocity records consists of two terms. One is related to the electronic circuitry of the sensor and its ability to resolve phase differences, whereas the second is flow related. The latter noise component dominates at rapid flows. The error in flow measurements due to the former noise term depends on sensor velocity range setting and ranges from 60.95 to 63.0 mm s21. Noise due to shear within the sample volume and to Doppler broadening is primarily a function of the turbulence dissipation parameter. Noise variances calculated using spectral analysis and the results of the ground truthing technique are compared with theoretical estimates of noise

Turbulence in the shallow nearshore environment during SANDYDUCK '97

An array of five acoustic Doppler velocimeters (ADV), which produce high quality measurements of the three-dimensional velocity vector in a sample volume with a scale of one centimeter, was deployed from late August through late November of 1997 at a water depth of approximately 4.5 m off Duck, North Carolina. The sensors were deployed near the sea floor but above the centimeters-thick wave boundary layer, and the sampling scheme was designed to resolve turbulence statistics averaged over tens of minutes, much longer than typical wave periods but shorter than time scales associated with variablity of energetic wind-driven and wave-driven alongshore flows.Funding was provided by the National Science Foundation under Grant No. OCE-9810609, the Mellon Foundation and Rinehart Coastal Research Center

Controls on Floc Size in a Continental Shelf Bottom Boundary Layer

Simultaneous in situ observations of floc size, waves, and currents in a continental shelf bottom boundary layer do not support generally accepted functional relationships between turbulence and floc size in the sea. In September and October 1996 and January 1997, two tripods were deployed in 70 m of water on the continental shelf south of Woods Hole, Massachusetts. On one a camera photographed particles in suspension 1.2 m above the bottom that had equivalent circular diameters larger than 250 um, and on the other, three horizontally displaced acoustic current meters measured flow velocity 0.35 m above the bottom. The tripods were separated by ~ 150 m. Typically, maximal floc diameter stayed relatively constant, around 1 mm, and it showed a dependence on turbulence parameters that was significantly weaker than that predicted by any model that assumes that turbulence-induced stresses limit floc size. Occasionally, when waves and currents generated intense near-bed turbulence, flocs were destroyed. These precipitous decreases in maximal floc size also were not predicted by conventional models. The correlation in time between episodes of floc destruction and elevated combined wave current stresses provides the first quantitative support for the hypothesis that floc size throughout bottom boundary layers can be controlled by breakup in the intensely sheared near-bed region. These observations demand a reassessment of the forces limiting floc size in the sea, and they indicate the potential for significant simplifying assumptions in models of floc dynamics

Spatial variability of bottom turbulence over a linear sand ridge mooring deployment and AUTOSUB AUV survey cruise report R/V RRS Challenger, cruise number 146 Broken Bank, North Sea, U.K., 17 – 28 August 1999 cruise report

Two successful AUTOSUB deployments were carried out during August 1999 as part of the AUTOSUB Thematic Program project titled “Spatial Variability of Bottom Turbulence over a Linear Sand Ridge,” funded by the Natural Environment Research Council (NERC), U.K. The AUTOSUB Autonomous Underwater Vehicle (AUV) was deployed and used to survey flow patterns at a location near the Broken Bank, southern North Sea, U.K. The AUV was equipped with acoustic flow and turbulence sensors and its surveys aimed at mapping the spatial variation of flow and turbulence near the bed and over topographic features. Three instrumented bottom mounted frames were also deployed, around the AUV survey area, for a period of approximately 5 days. The purpose of this array was to gather information on the temporal variability of the flow and turbulence near the seabed and to identify the important terms that drive circulation around the bank. Additional data were gathered including CTD casts, seabed samples and acoustic images of the seabed (side-scan sonar). The purpose of this data collection was to help identify the flow patterns around ridges and to understand the mechanisms controlling the maintenance and evolution of such features. This report describes the operations carried out by researchers from the University of South Carolina, Woods Hole Oceanographic Institution, Southampton Oceanography Centre and the AUTOSUB Team on the R.V. RRS Challenger during the period 17th –28th August 1999.Funding was provided by the Office of Naval Research under Contract No. N00014-01-10255 and the Natural Environment Research Council, UK Award GST/02/2155 to the University of Southampton

Alongshore momentum balance analysis on a cuspate foreland

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 118 (2013): 5280–5295, doi:10.1002/jgrc.20358.Nearshore measurements of waves and currents off Cape Hatteras, North Carolina, U.S.A, are used to investigate depth-averaged subtidal circulation and alongshore momentum balances in the surf and inner shelf region around a cuspate foreland. Data were collected on both sides of the cape representing shorefaces with contrasting shoreline orientation (north-south vs. northwest-southeast) subjected to the same wind forcing. In the nearshore, the subtidal flow is aligned with the local coastline orientation while at the cape point the flow is along the existing submerged shoal, suggesting that cape associated shoals may act as an extension of the coastline. Alongshore momentum balance analysis incorporating wave-current interaction by including vortex and Stokes-Coriolis forces reveals that in deep waters surface and bottom stress are almost in balance. In shallower waters, the balance is complex as nonlinear advection and vortex force become important. Furthermore, linearized momentum balance analysis suggests that the vortex force can be of the same order as wind and wave forcing. Farther southwest of Cape Hatteras point, wind and wave forcing alone fail to fully explain subtidal flow variability and it is shown that alongshore pressure gradient as a response to the wind forcing can close the momentum balance. Adjacent tide gauge data suggest that the magnitude of pressure gradient depends on the relative orientation of local coastline to the wind vector, and in a depth-averaged sense the pressure gradient generation due to change in coastline orientation even at km length scale is analogous to the effect of alongshore variable winds on a straight coastline.The experimental work was funded by the Carolinas Coastal Processes Project, a cooperative study supported by the US Geological Survey. Additional support during data analysis and preparation of this manuscript was provided by the National Science Foundation (award: OCE-1132130).2014-04-1

Analyzing the effect of socio-political context on tourism: perceptions of young tourists in Greece, Portugal and Israel

International crises that affect tourism, such as terror attacks, political unrest, and economic crises have become more frequent, and their influence has become broader. The influence of such extreme events depends on their salience in the tourists' awareness. Hence, it is important to understand the mechanisms underlying tourists' selection of travel destinations, especially their perceptions of crisis-related events and the impact of the sociopolitical and economic context in their countries of origin. The current study examined how the socio-political and economic context in the home countries of potential young tourists affected their selection of travel destinations. The objective was to elucidate how the salience of various crises (economic and political) in the tourists' perceptions, due to their experiences at home, color their construal of destinations affected by similar hazards and influence their travel intentions. The study focused on student tourists from Israel, Greece, and Portugal. Today about a fifth of international tourism is based on young people, especially students. These countries were chosen since Greece and Portugal are in the midst of economic crises. In addition, Greece and Portugal have experienced political instability, while Israel has security-related problems (including terrorist incidents). In 2013, a total of 648 students, responded to a questionnaire that included questions concerning attitudes and risk perceptions regarding travel to destinations with various risk hazards as well as socio-demographic details. The results indicate that over half of the Israelis intend to visit Greece or Portugal. The majority of the Portuguese intend to visit Greece, while less than a third of them intend to visit Israel. About half of the Greeks intend to visit Portugal, and most of them do not intend to visit Israel. The results indicate that greater perceived importance of economic crises mitigates the intention to travel to destinations with economic crises for tourists from origin countries that are also marked by economic crises, such as Greece and Portugal. However, for tourists from Israel, a country with a relatively stable economy, issues related to the economy barely affect their intention to travel to the other two countries. The findings also suggest that Greeks and Portuguese who are highly concerned about political unrest are unlikely to select Israel as a tourist destination. In addition, strong apprehension regarding terrorism impedes the intention to travel to destinations marked by terrorist incidents, such as Israel. The current research contributes to the existing literature by highlighting the impact of travelers' personal previous experience with crisis on their risk perceptions and in turn on their intentions to travel to countries with similar risks. Therefore, in a world where such incidents are on the rise, understanding tourists' risk perceptions and behavior and the factors influencing their destination-related decisions are crucial for countries that wish to increase the numbers of incoming tourists.info:eu-repo/semantics/publishedVersio

Factors shaping young tourists' intentions to travel to Greece, Israel and Portugal: Universal or idisyncratic perceptions of young Greeks, Israelis and Portugese?

Tourism constitutes an important branch of economy, especially in countries experiencing economic crisis and harsh socio-political events. The current study compares the intention of young people from three different countries – Greece, Israel and Portugal - to travel to two selected destinations. For each country of origin the other two countries were the destination of interest. This research examined the factors affecting young tourists' intention to travel to different destinations assuming that those determinants depend on the country of origin as well as on the destination country. The sample comprised of 648 students: Israelis (277), Portuguese (200), Greeks (171). The research questionnaire was distributed in Israeli, Greek and Portuguese academic institutions in 2014. The results indicate that over half of the Israelis intend to visit Greece or Portugal. The majority of the Portuguese anticipate visiting Greece, while only 32.5% expect to visit Israel. Among the Greeks nearly 50% intend to visit Portugal and do not intend to visit Israel. The study showed that the main factors affecting the decisions of young Portuguese to travel to Israel were: the image of Israel, the perceived risk of traveling to a country with political unrest and the number of trips abroad. The main factors affecting the decisions of young Greeks were the perceived risk of traveling to a country with political unrest and Israel's image. Israelis' decision to travel to Greece was affected by their image of Greece; the Portuguese also took into account the perceived risk regarding economic situation. For Israelis, considering Portugal as a traveling destination was related to the destination's image. The Greeks were affected by the number of Facebook friends and the Innovation, Entrepreneurship and Digital Ecosystems perceived economic risk. This research suggests that the socio-economic, political context of the country of origin and of the tourist destination plays an important role in young tourists' travelling-related decision-making.info:eu-repo/semantics/publishedVersio