285 research outputs found
Spatial and temporal variability of turbulent mixing in an estuary
Observations with a microstructure/CTD profiler and an ADCP during five cruises in 1994/95 provide a glance at the turbulent mixing climate of a partially mixed estuary, the Hudson River. The cruises took place under widely varying conditions of river discharge and external forcing by water level fluctuations in New York Bight. Tidal and fortnightly variability patterns of stratification, shear and mixing characteristics were qualitatively repeatable, largely following the description of Peters (1997). (i) Longer-term variations of buoyancy frequency and Richardson number (Ri) during flood were weakly correlated with the river discharge. (ii) All flood tides had substantial mixing. Cruise-to-cruise variations of turbulent dissipation rate, salt flux, and stress were large and correlated with variations in Ri. (iii) During ebb tides, mixing was weak in response to stable stratification during neaps, and it increased dramatically toward spring tides parallel with the occurrence of low Ri. Cruise-to-cruise variations of mixing became small toward spring tide. (iv) The largest vertical salt flux occurred during spring ebbs, which provided about 30% of the total fortnightly salt flux; floods throughout the fortnightly cycle providing most of the remainder. (v) The observed level of small-scale mixing, i.e. stress and vertical salt flux, was consistent with the integral momentum and salt budgets in the uniform segment of the Hudson where the bulk of the measurements were taken. (vi) One topographically forced flow feature examined, lee waves behind a trench across the river, was associated with elevated mixing
Turbulentno mijeĆĄanje u srednjem Jadranu tijekom proljeÄa
U svibnju 2003. godine prikupljen je u srednjem Jadranu nevelik niz podataka kojim je dokumentirano turbulentno mijeĆĄanje u povrĆĄinskom sloju te stratifikacija i strujanje. BuduÄi da su vjetrovi bili slabi, povrĆĄinski izmijeĆĄani sloj dosizao je najviĆĄe do dubine od 10 m, Äesto i manje. Vodeni stupac ispod tog sloja bio je uglavnom jako stratificiran, uz prisutnost ostataka ranijih izmijeĆĄanih slojeva koji su dijelom ponovno stratificirani. Struje su bile malih brzina na srednjoj skali, s time da je barotropna komponenta bila znaÄajna. Slabo smicanje i dobro izraĆŸena stratifikacija doprinosili su velikim iznosima Richardsonovog broja. Ispod sloja debljine 10â20 m u kojem je mijeĆĄanje bilo pojaÄano, koeficijenti turbulentnog mijeĆĄanja uglavnom su bili malih iznosa, izmeÄu 10â6 m2 sâ1 i 5 x 10â5 m2 sâ1. MeÄutim, u nekoliko su sluÄajeva opaĆŸene znatno veÄe vrijednosti
Zur Kinematik eines stochastischen Feldes interner Wellen in einer Scherströmung
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Zur Kinematik eines stochastischen Feldes interner Wellen in einer Scherströmung.
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Peters, Hartmut (1981) Zur Kinematik eines stochastischen Feldes interner Wellen in einer Scherströmung. (Doctoral thesis/PhD), Christian-Albrechts-UniversitÀt Kiel, Kiel, Germany, 118 pp.
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Abstract
FĂŒr die im Atlantik im Bereich des Ăquatorialen Gegenstroms wĂ€hrend GATE beobachtete mittlere Schichtung und Scherströmung werden vertikale Eigenschwingungen (Moden) und Dispersionskurven hochfrequenter (0.3 bis 15 cph) interner Wellen berechnet. Ein spektrales Modell der Kinematik freier, linearer interner Wellen wird durch die stochastische Ăberlagerung so gewonnener Moden erzeugt. Modellrechnungen werden zu Vergleichs- und Testzwecken fĂŒr einfache GrundzustĂ€nde (N 2 = const.; u = const.) durchgefĂŒhrt. Die den VerhĂ€ltnissen wĂ€hrend GATE entsprechenden Rechnungen ergeben folgende Resultate: In den Energiespektren wird eine Schulter bei Frequenzen zwischen 1.5 und 4 cph beobachtet, die einen nicht unbetrĂ€chtlichen Teil der Gesamtenergie des Wellenfeldes reprĂ€sentiert.
Diese Schulter kann im wesentlichen nicht durch kinematische Effekte erklĂ€rt werden, sie muĂ dynamische Ursachen haben. Im Frequenzbereich 1.5 bis 4 cph wird das Wellenfeld praktisch ausschlieĂlich von der ersten Mode beherrscht. Nach den Beobachtungen sind die Wellen ĂŒberwiegend gegen die mittlere Strömung gerichtet. Zur ErklĂ€rung dessen mĂŒssen sowohl dynamische als auch kinematische Effekte herangezogen werden. Das beobachtete Maximum der KohĂ€renz zwischen Strömung in der Deckschicht und Temperatur in der Sprungschicht zwischen 1. 5 und 4 cph ist prinzipiell der Kinematik des Systems zuzuschreiben. Die Skalen des Wellenzahlspektrums, die sich aus der Anpassung des Modells an die Daten ergeben, sind derart, daĂ nur ein Teil der Anisotropie des Systems von Moden und Dispersionskurven wirksam wird. Die Vertikalstruktur der beobachteten Strömungsfluktuationen kann mit dem Modell nicht vollstĂ€ndig beschrieben werden. Das Modell wird kritisch diskutiert
Computer-aided Optical Plasma Postprocessing Applied on Model Spark Gaps
Spark gaps are used as surge protective devices (SPD class 1) for low voltage grids protection against surge currents and overvoltages. For practical research of the narrow gap plasma of spark gaps, high-speed camera recordings are used in modified transparent test models. In this test setup, current densities of 1010 A/m2 are generated.
In order to optimize and automate the evaluation process of camera recordings, an image analysis tool is developed further in this contribution. After basic image improvement and segmentation, this research optimizes a detection algorithm for plasma location and distribution. As a result, the known plasma distribution gives access to significantly more information about the plasma behaviour and the spatial distribution of radiation
Mixing and entrainment in the Red Sea outflow plume. Part I : plume structure
Author Posting. © American Meteorological Society, 2005. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 35 (2005): 569â583, doi:10.1175/JPO2679.1.When the salty and heavy water of the Red Sea exits from the Strait of Bab el Mandeb, it continues downslope into the Gulf of Aden mainly along two channels. The 130-km-long âNorthern Channelâ (NC) is topographically confined and is typically only 5 km wide. In it, the Red Sea plume shows unanticipated patterns of vertical structure, turbulent mixing, and entrainment. Above the seafloor a 25â120-m-thick weakly stratified layer shows little dilution along the channel. Hence this bottom layer undergoes only weak entrainment. In contrast, a 35â285-m-thick interfacial layer shows stronger entrainment and is shown in a companion paper to undergo vigorous turbulent mixing. It is thus the interface that exhibits the bulk of entrainment of the Red Sea plume in the NC. The interfacial layer also carries most of the overall plume transport, increasingly so with downstream distance. The âSouthern Channelâ (SC) is wider than the NC and is accessed from the latter by a sill about 33 m above the floor of the NC. Entrainment into the bottom layer of the SC is diagnosed to be strong near the entry into the SC such that the near-bottom density and salinity are smaller in the SC than in the NC at the same distance from Bab el Mandeb. In comparison with winter conditions, the authors encountered weaker outflow with shallower equilibration depths during the summer cruise. Bulk Froude numbers computed for the whole plume varied within the range 0.2â1. Local maxima occurred in relatively steep channel sections and coincided with locations of significant entrainment.The Red Sea Outflow Experiment
was funded by the National Science Foundation
under Contracts OCE-9819506 and OCE-9818464. Additional
support was provided to the âClimate Process
Team Gravity Currentsâ under OCE-0336799
Mobile quantum gravity sensor with unprecedented stability
Changes of surface gravity on Earth are of great interest in geodesy, earth sciences and natural resource exploration. They are indicative of Earth system's mass redistributions and vertical surface motion, and are usually measured with falling corner-cube- and superconducting gravimeters (FCCG and SCG). Here we report on absolute gravity measurements with a mobile quantum gravimeter based on atom interferometry. The measurements were conducted in Germany and Sweden over periods of several days with simultaneous SCG and FCCG comparisons. They show the best-reported performance of mobile atomic gravimeters to date with an accuracy of 39nm/s2, long-term stability of 0.5nm/s2 and short-term noise of 96nm/s2/âHz. These measurements highlight the unique properties of atomic sensors. The achieved level of performance in a transportable instrument enables new applications in geodesy and related fields, such as continuous absolute gravity monitoring with a single instrument under rough environmental conditions.Peer Reviewe
Human securin proteolysis is controlled by the spindle checkpoint and reveals when the APC/C switches from activation by Cdc20 to Cdh1
Progress through mitosis is controlled by the sequential destruction of key regulators including the mitotic cyclins and securin, an inhibitor of anaphase whose destruction is required for sister chromatid separation. Here we have used live cell imaging to determine the exact time when human securin is degraded in mitosis. We show that the timing of securin destruction is set by the spindle checkpoint; securin destruction begins at metaphase once the checkpoint is satisfied. Furthermore, reimposing the checkpoint rapidly inactivates securin destruction. Thus, securin and cyclin B1 destruction have very similar properties. Moreover, we find that both cyclin B1 and securin have to be degraded before sister chromatids can separate. A mutant form of securin that lacks its destruction box (D-box) is still degraded in mitosis, but now this is in anaphase. This destruction requires a KEN box in the NH2 terminus of securin and may indicate the time in mitosis when ubiquitination switches from APCCdc20 to APCCdh1. Lastly, a D-box mutant of securin that cannot be degraded in metaphase inhibits sister chromatid separation, generating a cut phenotype where one cell can inherit both copies of the genome. Thus, defects in securin destruction alter chromosome segregation and may be relevant to the development of aneuploidy in cancer
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