Sheremet
Vitalii A.
Sheremet
Vitalii A.
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ArticleNear-bottom circulation and dispersion of sediment containing Alexandrium fundyense cysts in the Gulf of Maine during 2010–2011(Elsevier, 2013-12-13) Aretxabaleta, Alfredo L. ; Butman, Bradford ; Signell, Richard P. ; Dalyander, P. Soupy ; Sherwood, Christopher R. ; Sheremet, Vitalii A. ; McGillicuddy, Dennis J.The life cycle of Alexandrium fundyense in the Gulf of Maine includes a dormant cyst stage that spends the winter predominantly in the bottom sediment. Wave-current bottom stress caused by storms and tides induces resuspension of cyst-containing sediment during winter and spring. Resuspended sediment could be transported by water flow to different locations in the Gulf and the redistribution of sediment containing A. fundyense cysts could alter the spatial and temporal manifestation of its spring bloom. The present study evaluates model near-bottom flow during storms, when sediment resuspension and redistribution are most likely to occur, between October and May when A. fundyense cells are predominantly in cyst form. Simulated water column sediment (mud) concentrations from representative locations of the Gulf are used to initialize particle tracking simulations for the period October 2010–May 2011. Particles are tracked in full three-dimensional model solutions including a sinking velocity characteristic of cyst and aggregated mud settling (0.1 mm s−1). Although most of the material was redeposited near the source areas, small percentages of total resuspended sediment from some locations in the western (~4%) and eastern (2%) Maine shelf and the Bay of Fundy (1%) traveled distances longer than 100 km before resettling. The redistribution changed seasonally and was sensitive to the prescribed sinking rate. Estimates of the amount of cysts redistributed with the sediment were small compared to the inventory of cysts in the upper few centimeters of sediment but could potentially have more relevance immediately after deposition.
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ArticleLaboratory experiments with tilted convective plumes on a centrifuge: a finite angle between the buoyancy force and the axis of rotation(Cambridge University Press, 2004-04-28) Sheremet, Vitalii A.The effect of both vertical and horizontal components of the Earth's rotation on plumes during deep convection in the ocean is studied. In the laboratory, the misalignment, characterized by the angle $\alpha$, between the buoyancy force (‘effective’ free-fall acceleration ${\bm g}_e$) and the rotation axis ${\bm \Omega}$ is produced by using the centrifugal force: an experimental tank was placed at a large distance from the centre of the turntable. The mathematical analogy between the laboratory model and the oceanic environment is presented. For $\alpha\,{=}\,30^\circ$, a number of laboratory experiments spanning a wide range of the buoyancy flux parameter, and correspondingly Reynolds number, is used to illustrate the development of the convective plume from a point source in regimes ranging from weakly to highly turbulent. New features of the flow, as compared to $\alpha\,{=}\,0$, are documented and explained. The incoming heavier dyed fluid jet disintegrates into fast-sinking coherent blobs (in a low-Reynolds-number regime) or turbulent billows (in a high-Reynolds-number regime) and a more diffuse cloud of highly diluted dyed water. An analysis of the forces acting on an ellipsoid moving in a rotating fluid with the main balance including the buoyancy, Coriolis forces, and the hydrodynamic reaction due to generation of inertial waves correctly predicts the trajectory of a descending blob. It also explains the tendency of the plume to develop in the direction intermediate between ${\bm g}_e$ and ${\bm \Omega}$ and to shift ‘eastward’ (lagging the rotation of the centrifuge) if the plume is envisaged as an ensemble of blobs. The stretching of the highly diluted dyed water along the absolute vorticity tubes with simultaneous shearing by horizontal quasi-two-dimensional flow produces conspicuous tilted structures or tilted Taylor ‘ink walls’. The misalignment between ${\bm g}_e$ and ${\bm \Omega}$ enhances the turbulent mixing and development of tilted structures by breaking the symmetry and producing motions directed away from the rotation axis. We argue that the conditions at the sites of ocean deep convection are favourable for the development of tilted structures because of the smallness of the Rossby number and an extreme homogenization of the mixed layer. We hypothesize that the homogenized sublayers observed within actively convecting regions in the ocean may not be horizontal, but in fact analogous to the tilted ‘ink walls’ observed in the laboratory experiments and that they represent the internal structure of a plume on horizontal scales smaller than its depth.
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ArticlePredictability( 2005-12) Hacker, Joshua ; Hansen, James ; Berner, Judith ; Chen, YangQuan ; Eshel, Gidon ; Hakim, Gregory ; Lazarus, Steven ; Majumdar, Sharanya ; Morss, Rebecca ; Poje, Andrew ; Sheremet, Vitalii A. ; Tang, Youmin ; Webb, ColleenA group of junior faculty members and UCAR junior scientists convened in Boulder, CO on June 16-18, 2003 for discussion on future scientific directions. This report summarizes the goals and products of one of the three foci selected for special consideration: predictability. About 15 people, representing physical, mathematical, and biological sciences, were present for round-table discussions. The discussion sought common interpretation of the predictability problem, points of generalization, identification of major hurdles, and potential approaches to their solution. The diverse background of the participants generated a wide-ranging discussion. The participants addressed predictability generally, while supplying specific examples from their own areas of expertise. Recurring themes included the relationship between models and initial conditions, the importance of definitions and the choice of a norm for evaluation, and generalization across systems and disciplines. The group explored potential avenues for generalization through interdisciplinary networking. Short- and long-range challenges were identified related to probabilistic state estimation, verifying predictions and understanding error, and dealing with nonlinearity. In this essay we expand on these themes and challenges, and describe possible future research objectives.
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ArticleBall release experiments on a centrifuge : misalignment between the buoyancy force and the axis of rotation(Cambridge University Press, 2006-09-15) Riemenschneider, Ulrike ; Sheremet, Vitalii A.Motivated by work on tilted convection (Sheremet, J. Fluid Mech., vol. 506, 2004, p. 217), a set of experiments is presented here using the same set-up of a tilted tank attached to a rotating centrifuge with a 2.5 m arm. Within the tank small, almost neutrally buoyant, spheres are released, and their trajectories are recorded. Thus the forces acting on a sphere can be analysed in the case of misalignment between the buoyancy force and the axis of rotation. The angles of descent characterizing the trajectory are compared with inviscid linear theory developed by Stewartson (Q. J. Math. Appl. Mech., vol. 6, 1953, p. 141), and the agreement is found to be good. The angles should be independent of the density anomaly of the spheres compared to their environment. Using the descent velocity from non-rotating experiments, the density of the spheres is estimated and used to determine the drag acting on them in the rotating experiments. It is found that the drag is up to 50% larger than expected from Stewartson's theory. The agreement is best, not for infinitesimal, but for small Rossby numbers. The results are consistent with observations recorded by Maxworthy (J. Fluid Mech., vol. 40, 1970, p. 453).
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ArticleExperimental and numerical investigation of shelf flow crossing over a strait(Springer, 2024-05-20) Kuehl, Joseph ; Sheremet, Vitalii A.Motivated by the phenomenon of Scotian Shelf Crossover events, the problem of a shelf flow that is interrupted by a strait is considered. Laboratory experiments in a rotating tank with barotropic and baroclinic flow over flat and sloping shelves confirm that the flow is steered by the bathymetric contours and mainly circumnavigates the gulf. In order to jump across the strait, as suggested by earlier theories, the flow must have unrealistically high Rossby numbers. However, the near bottom friction relaxes the bathymetric constraint and causes the formation of a peculiar jet crossing the strait diagonally. For the dissipation values such that a half of the transport goes around the gulf and half crosses the strait diagonally, the diagonal crossover jet becomes most evident. Numerical solutions for realistic values of the frictional parameter reproduce the results of the laboratory experiments and consideration of the actual Gulf of Maine bathymetry reproduces patterns similar to those observed by drift trajectories and in the satellite derived sea surface temperature fields.