Letteratura scientifica selezionata sul tema "Eddy-internal tide interactions"

AbstractInternal tides, generated by barotropic tides flowing over rough topography, are a primary source of energy into the internal wave field. As internal tides propagate away from generation sites, they can dephase from the equilibrium tide, becoming nonstationary. Here, we examine how low-frequency quasigeostrophic background flows scatter and dephase internal tides in the Tasman Sea. We demonstrate that a semi-idealized internal tide model [the Coupled-Mode Shallow Water model (CSW)] must include two background flow effects to replicate the in situ internal tide energy fluxes observed during the Tasmanian Internal Tide Beam Experiment (TBeam). The first effect is internal tide advection by the background flow, which strongly depends on the spatial scale of the background flow and is largest at the smaller scales resolved in the background flow model (i.e., 50–400 km). Internal tide advection is also shown to scatter internal tides from vertical mode-1 to mode-2 at a rate of about 1 mW m−2. The second effect is internal tide refraction due to background flow perturbations to the mode-1 eigenspeed. This effect primarily dephases the internal tide, attenuating stationary energy at a rate of up to 5 mW m−2. Detailed analysis of the stationary internal tide momentum and energy balances indicate that background flow effects on the stationary internal tide can be accurately parameterized using an eddy diffusivity derived from a 1D random walk model. In summary, the results identify an efficient way to model the stationary internal tide and quantify its loss of stationarity.

AbstractThe interaction of barotropic tidal currents and baroclinic geostrophic eddies is considered theoretically and numerically to determine whether energy can be transferred to an internal wave field by this process. The eddy field evolves independently of the tide, suggesting that it acts catalytically in facilitating energy transfer from the barotropic tide to the internal wave field, without exchanging energy with the other flow components. The interaction is identically zero and no waves are generated when the barotropic tidal current is horizontally uniform. Optimal internal wave generation occurs when the scales of tide and eddy fields satisfy resonant conditions. The most efficient generation is found if the tidal current horizontal scale is comparable to that of the eddies, with a weak maximum when the scales differ by a factor of two. Thus, this process is not an effective mechanism for internal wave excitation in the deep ocean, where tidal current scales are much larger than those of eddies, but it may provide an additional source of internal waves in coastal areas where horizontal modulation of the tide by topography can be significant.

Abstract This study uses a primitive equation model to estimate the time-varying M2 internal tide dissipation in the Philippine Sea in the presence of the subtidal circulation. The time-mean diapycnal diffusivity due to the M2 internal tide is estimated to be 4.0–4.8 × 10−4 m2 s−1 at the Luzon Strait and 2–9 × 10−5 m2 s−1 in the Philippine Sea basin. The variability in internal tides and their interactions with the subtidal ocean circulation results in significant spatial and temporal variability in the energy available for mixing. The subtidal circulation influences internal-tide-induced mixing in two ways: by introducing variability in internal tide generation and by increased dissipation of baroclinic energy associated with greater velocity shear. Close to the generation site, mixing is dominated by high-mode internal tide dissipation, while in the far field the influence of the mesoscale energy on internal tide dissipation is significant, resulting in increased dissipation. This study presents model-based estimates of the important and relatively unknown effect of mesoscale circulation on internal-tide-induced mixing away from internal tide generation sites in a region of high eddy kinetic energy.

AbstractA 4-month glider mission was analyzed to assess turbulent dissipation in an anticyclonic eddy at the western boundary of the subtropical North Atlantic. The eddy (radius ≈ 60 km) had a core of low potential vorticity between 100 and 450 m, with maximum radial velocities of 0.5 m s−1 and Rossby number ≈ −0.1. Turbulent dissipation was inferred from vertical water velocities derived from the glider flight model. Dissipation was suppressed in the eddy core (ε ≈ 5 × 10−10 W kg−1) and enhanced below it (>10−9 W kg−1). Elevated dissipation was coincident with quasiperiodic structures in the vertical velocity and pressure perturbations, suggesting internal waves as the drivers of dissipation. A heuristic ray-tracing approximation was used to investigate the wave–eddy interactions leading to turbulent dissipation. Ray-tracing simulations were consistent with two types of wave–eddy interactions that may induce dissipation: the trapping of near-inertial wave energy by the eddy’s relative vorticity, or the entry of an internal tide (generated at the nearby continental slope) to a critical layer in the eddy shear. The latter scenario suggests that the intense mesoscale field characterizing the western boundaries of ocean basins might act as a “leaky wall” controlling the propagation of internal tides into the basin’s interior.

Abstract. We investigate the interaction between an anticyclonic eddy (AE) and semidiurnal internal tide (SIT) on the continental slope of the northeastern South China Sea (SCS), using a high spatiotemporal resolution numerical model. Two key findings are as follows: first, the AE promotes energy conversion from low-mode to higher-mode SIT. Additionally, production terms indicate that energy is also transferred from the SIT field to the eddy field at an average rate of 3.0 mW m−2 (accounting for 7 % of the incoming energy flux of SIT when integrated over the eddy diameter). Second, the AE can modify the spatial distribution of tidal-induced dissipation by refracting, scattering, and reflecting low-mode SIT. The phase and group velocities of the SIT are significantly influenced by the eddy field, resulting in a northward or southward shift in the internal tidal rays. These findings deepen our understanding of the complex interactions between AE and SIT, as well as their impacts on energy conversion, wave propagation, and coastal processes.

Direct microstructure observations and fine-scale measurements of an anticyclonic mesoscale eddy were conducted in the northern South China Sea in July 2020. An important finding was that suppressed turbulent mixing in the thermocline existed at the center of the eddy, with an averaged diapycnal diffusivity at least threefold smaller than the peripheral diffusivity. Despite the strong background shear and significant wave–mean flow interactions, the results indicated that the lack of internal wave energy in the corresponding neap tide period during measurement of the eddy’s center was the main reason for the suppressed turbulent mixing in the thermocline. The applicability of the fine-scale parameterization method in the presence of significant wave–mean flow interactions in a mesoscale eddy was evaluated. Overprediction via fine-scale parameterization occurred in the center of the eddy, where the internal waves were inactive; however, the parameterization results were consistent with microstructure observations along the eddy’s periphery, where active internal waves existed. This indicates that the strong background shear and wave–mean flow interactions affected by the mesoscale eddy were not the main contributing factors that affected the applicability of fine-scale parameterization in the northern South China Sea. Instead, our results showed that the activity of internal waves is the most important consideration.

AbstractThe role of mesoscale eddies in modulating the semidiurnal internal tide (SIT) in the northern South China Sea (SCS) is examined using the data from a cross-shaped mooring array. From November 2013 to January 2014, an anticyclonic eddy (AE) and cyclonic eddy (CE) pair crossed the westward SIT beam originating in Luzon Strait. Observations showed that, because of the current and stratification modulations by the eddy pair, the propagation speed of the mode-1 SIT sped up (slowed down) by up to 0.7 m s−1 (0.4 m s−1) within the AE’s (CE’s) southern portion. As a result of the spatially varying phase speed, the mode-1 SIT wave crest was clockwise rotated (counterclockwise rotated) within the AE (CE) core, while it exhibited convex and concave (concave and convex) patterns on the southern and northern peripheries of the AE (CE), respectively. In mid-to-late November, most of the mode-1 SIT energy was refracted by the AE away from Dongsha Island toward the north part of the northern SCS, which resulted in enhanced internal solitary waves (ISWs) there. Corresponding to the energy refraction, responses of the depth-integrated mode-1 SIT energy to the eddies were generally in phase at the along-beam-direction moorings but out of phase in the south and north parts of the northern SCS at the cross-beam-direction moorings. From late December to early January, intensified mode-2 SIT was observed, whose energy was likely transferred from the mode-1 SIT through eddy–wave interactions. The observation results reported here are helpful to improve the capability to predict internal tides and ISWs in the northern SCS.

Abstract Internal wave–wave interaction theories and observations support a parameterization for the turbulent dissipation rate ɛ and eddy diffusivity K that depends on internal wave shear 〈Vz2〉 and strain 〈ξz2〉 variances. Its latest incarnation is applied to about 3500 lowered ADCP/CTD profiles from the Indian, Pacific, North Atlantic, and Southern Oceans. Inferred diffusivities K are functions of latitude and depth, ranging from 0.03 × 10−4 m2 s−1 within 2° of the equator to (0.4–0.5) × 10−4 m2 s−1 at 50°–70°. Diffusivities K also increase with depth in tropical and subtropical waters. Diffusivities below 4500-m depth exhibit a peak of 0.7 × 10−4 m2 s−1 between 20° and 30°, latitudes where semidiurnal parametric subharmonic instability is expected to be active. Turbulence is highly heterogeneous. Though the bulk of the vertically integrated dissipation ∫ɛ is contributed from the main pycnocline, hotspots in ∫ɛ show some correlation with small-scale bottom roughness and near-bottom flow at sites where strong surface tidal dissipation resulting from tide–topography interactions has been implicated. Average vertically integrated dissipation rates are 1.0 mW m−2, lying closer to the 0.8 mW m−2 expected for a canonical (Garrett and Munk) internal wave spectrum than the global-averaged deep-ocean surface tide loss of 3.3 mW m−2.

AbstractDirect and large eddy simulations are performed to study the internal waves generated by the oscillation of a barotropic tide over a model ridge of triangular shape. The objective is to go beyond linear theory and assess the role of nonlinear interactions including turbulence in situations with low tidal excursion number. The criticality parameter, defined as the ratio of the topographic slope to the characteristic slope of the tidal rays, is varied from subcritical to supercritical values. The barotropic tidal forcing is also systematically increased. Numerical results of the energy conversion are compared with linear theory and, in laminar flow at low forcing, they agree well in subcritical and supercritical cases but not at critical slope angle. In critical and supercritical cases with higher forcing, there are convective overturns, turbulence and significant reduction (as much as 25 %) of the radiated wave flux with respect to laminar flow results. Analysis of the baroclinic energy budget and spatial modal analysis are performed to understand the reduction. The near-bottom velocity is intensified at critical angle slope leading to a radiated internal wave beam as well as an upslope bore of cold water with a thermal front. In the critical case, the entire slope has turbulence while, in the supercritical case, turbulence originates near the top of the topography where the slope angle transitions through the critical value. The phase dependence of turbulence within a tidal cycle is examined and found to differ substantially between the ridge slope and the ridge top where the beams from the two sides cross.

AbstractUnderstanding and predicting how internal tides distort and lose coherence as they propagate through the ocean has been identified as a key issue for interpreting data from the upcoming wide-swath altimeter mission Surface Water and Ocean Topography (SWOT). This study addresses the issue through the analysis of numerical experiments where a low-mode internal tide propagates through a quasigeostrophic turbulent jet. Equations of motion linearized around the slower turbulent field are projected onto vertical modes and assumed to describe the dynamics of the low-mode internal tide propagation. Diagnostics of the terms responsible for the interaction between the wave and the slow circulation are computed from the numerical outputs. The large-scale change of stratification, on top of eddies and jet meanders, contributes significantly to these interaction terms, which is shown to be consistent with an independent scaling analysis. The sensitivity of interaction terms to a degradation of the slow field spatial and temporal resolution indicates that present-day observing systems (Argo network, altimetry) may lack the spatial resolution necessary to correctly predict internal tide evolution. The upcoming SWOT satellite mission may improve upon this situation. The number of vertical modes required to properly estimate interaction terms is discussed. These results advocate development of a simplified model based on solving a modest number of the linearized equations subject to a prescribed mesoscale field and internal tide sources.

L'équilibre énergétique de l'océan traduit des échanges d'énergie entre les termes sources aux échelles planétaires et la dissipation aux micro-échelles. Cette cascade d'énergie, cruciale dans la compréhension du système océanique, demande à être mieux comprise et quantifiée. La plus grande part de l'énergie dans l'océan est associée à la dynamique mésoéchelle. Les échelles spatiales correspondantes sont aussi celles des marées internes et dans les régimes océaniques où l'énergie des ondes internes est suffisamment forte celles-ci représentent un transfert d'énergie majeur vers les échelles dissipatives. La nouvelle mission satellite SWOT (Surface Water Ocean Topography) permettra d'observer globalement pour la première fois ces processus de fines échelles. Une motivation de cette thèse est l'observabilité du niveau de la mer SWOT avec le challenge de comprendre la part respective de la dynamique méso et sous-mésoéchelle et des ondes internes. La région d'étude se situe autour de la Nouvelle Calédonie dans le Pacifique sud-ouest et plus particulièrement dans la région sud survolée par une fauchée SWOT lors de la phase de cal-val du satellite caractérisée par une orbite à 1 jour. La thèse s'appuie sur une simulation régionale dédiée à haute résolution (1/60°) forcée ou pas par la marée barotrope en plus des forçages "classiques". Pour la première fois la dynamique des marées internes autour de la Nouvelle-Calédonie est décrite. Cette région s'avère être un hot spot de génération de marées internes associé aux principales structures bathymétriques,. La marée interne est principalement semi diurne. Elle se caractérise par un premier mode barocline très important et une forte signature dans la SSH (>6cm). Cette énergie de marée interne se propage dans l'océan ouvert à partir de deux zones localisées au nord et au sud de la Nouvelle Calédonie malgré des taux de dissipation d'énergie élevés à proximité des zones de génération. Cette propagation est majoritairement associée à de la marée interne cohérente mais l'activité méso-échelle s'avère être une source potentielle de perte de cohérence de la marée interne (marée incohérente). Cette marée interne incohérente est associée aux interactions avec les tourbillons océaniques soit par la réfraction de la propagation de l'énergie du faisceau de marée par les courants à méso-échelle lors de la propagation de l'énergie de marée, soit par les variations de la conversion de l'énergie barotrope en énergie barocline dues aux changements de stratification induits par les tourbillons à méso-échelle. Des observations in situ obtenues par des planeurs sous-marins autonomes révèlent le réalisme du modèle numérique quant à la simulation des marées internes tout en s'avérant être une plateforme in-situ appropriée pour documenter les marées internes, y compris leur signature de la SSH. Dans les régions de forte marée interne, celle-ci domine la variance de la SSH pour des longueurs d'onde jusqu'à 200 km correspondant. Une attention particulière est accordée à la marée incohérente, qui se manifeste dans la SSH à des échelles inférieures à 100 km. Cette thèse initie également l'étude de l'impact des marées internes sur la circulation à méso-échelle et sous-méso-échelle, avec des voies prometteuses pour les travaux futurs sur les échanges d'énergie entre les échelles et la fermeture du bilan énergétique océanique. Ces travaux participeront à la valorisation des données SWOT dans le cadre de SWOT-AdAC avec la campagne SWOTALIS. Enfin, ils sont une première initiative dans l'implication des marées internes en lien avec l'écosystème marin de la Nouvelle-Calédonie associée à un objectif de mise en place d'aires marines protégées au sein du parc naturel de la mer de Corail
The oceanic energy cascade and the associated redistribution of energy from planetary scales to microscales are crucial to achieve climate equilibrium, yet they remain to be fully understood and quantified. Among the submesoscale flow regime which is characterized by equal contributions from rotational (balanced) and non-rotational (unbalanced) effects, it is internal tides (internal gravity waves at tidal frequency) which have been shown to represent a major energy transfer toward dissipative scales. The Surface Water Ocean Topography (SWOT) satellite mission will push forward global sea surface height (SSH) observations of fine-scale physics of combined balanced and unbalanced motions, and their interactions. Our understanding of these processes will ultimately depend on our ability to disentangle these two different dynamical flow regimes. This thesis aims to tackle SWOT SSH observability of balanced and unbalanced motions around New Caledonia, an area with pronounced internal tide activity alongside elevated level of mesoscale to submescale eddy variability located beneath two swaths of SWOT's fast-sampling phase during which SWOT orbited on a 1-day repeat cycle to collect high-frequency measurements. As an initial step, this thesis provides the first comprehensive description of internal-tide dynamics around New Caledonia, an internal generation hot spot in the southwestern tropical Pacific that has not yet been explored in the literature, based on a tailored regional high-resolution (1/60°) numerical modeling effort. Internal tide generation around New Caledonia is associated with the main bathymetric structures, i.e. continental slope, shelf breaks, small- and large-scale ridges, and seamounts, strongly dominated by the semidiurnal tide and low-vertical modes, with a strong signature in SSH. It is found to be a major source of tidal energy propagation toward the open ocean despite enhanced energy dissipation rates close to the generation sites. Mesoscale eddy variability is shown to be a potential source for the loss of tidal coherence (or tidal incoherence) due to eddy-internal tide interactions, either through the refraction of tidal beam energy propagation by mesoscale currents toward the open ocean or by mesoscale-eddy induced variations of barotropic-to-baroclinic energy conversion. Important insight is provided by in-situ observations of autonomous underwater gliders. They reveal the numerical model's realism of internal-tide dynamics while proving to be a suitable in-situ platform to infer internal tides, including SSH signature. SWOT SSH observability of balanced and unbalanced motions represent a challenge around New Caledonia as the internal tide dominates SSH variance at wavelengths similar to those of balanced motion at scales less than 200~km wavelength. Particular emphasis is given to the incoherent tide, which manifests in SSH at scales less than 100~km, while restricting the observability of mesoscale and submesoscale motions. An outlook is given on the impact of internal tides on the mesoscale to submesoscale circulation with promising routes for future work on cross-scale energy exchanges and the closure of the oceanic energy budget. Finally, the comprehensive description of internal-tide dynamics conducted in this thesis has important implications for the New Caledonia marine ecosystem, with the hope of paving the way for the island's efforts in the conservation of marine protected areas

Abstract The process of fuel-air mixing in the Direct Injection Spark Ignition (DISI) engine is highly unsteady and three-dimensional with wide cycle-to-cycle variations involving vaporization of droplets and its interaction with large-scale turbulent flow field. Although the majority of the past numerical studies of mixing in an Internal Combustion (IC) engines have employed Reynolds-Averaged Navier-Stokes (RANS) equations with empirical turbulence model, here we have implemented a Large-Eddy Simulations (LES) with the Linear-Eddy Model (LEM) for subgrid scalar mixing into a commercial IC engine simulation code (KIVA-3V). This study shows that when time-accurate effects are included significantly different results are obtained. These differences between the original KTVA-3V and the new KIVALES in predicting the in-cylinder turbulent fuel-air mixing are discussed. LES shows highly unsteady, anisotropic in-cylinder fuel-air mixing process compared to the original KIVA-3V. The implications for combustion is also discussed.

A large eddy simulation (LES) of a rocket turbopump inducer in non-cavitating and cavitating flows is presented. The computation takes full account of the interaction between the rotating inducer and the stationary casing by using a multi-frame-of-reference dynamic overset grid approach. A streamline-upwind finite element formulation with second-order accuracy both in time and space is used to discretize the governing equation. It is implemented in parallel by a domain-decomposition-programming model. The evolution of cavitation is represented by the source/sink of vapor phase in the incompressible liquid flow. The pressure-velocity coupling is based on the fractional-step method for incompressible fluid flows, in which the compressibility is taken into account through the low Mach number assumption. The internal flow of an inducer is simulated and compared with water tunnel experiments at the design (φ = 0.078) and off-design conditions (φ = 0.05 and 0.09) in non-cavitating flows. The overall head-flow characteristics of computed results show good agreement with experiments. Such results show that the applied LES code can be used as a design tool for rocket turbopump inducers.

Computational fluid dynamics (CFD) has become a critical tool in the design of aeroengines. Increasing demand for higher efficiency, performance and reduced emissions of noise and pollutants has focused attention on secondary flows, small scale internal flows and flow interactions. In conjunction with low order correlations and experimental data, RANS (Reynolds-Averaged Navier-Stokes) modelling has been used effectively for some time, particularly at high Reynolds numbers and at design conditions. However, the range of flows throughout an engine is vast, with most, in reality being inherently unsteady. There are many cases where RANS can perform poorly, particularly in zones characterised by strong streamline curvature, separation, transition, relaminarisation and heat transfer. The reliable use of RANS has also been limited by its strong dependence on turbulence model choice and related ad-hoc corrections. For complex flows, Large-Eddy Simulation (LES) methods provide reliable solutions, largely independent of turbulence model choice and at a relatively low cost for particular flows. LES can now be used to provide in depth knowledge of flow physics, for example in areas such as transition and real wall roughness effects. This can be used to inform RANS and lower order modelling. For some flows, LES can now even be used for design. Existing literature is used to show the potential of LES for a range of flows in different zones of the engine. Based on flow taxonomy, best practices including meshing requirements and turbulent inflow conditions are introduced, leading to the proposal of a tentative expert system for industrial use. In this way, LES becomes a well controlled tool, suitable for design use and reduces the burden on the end user. Further attention is also given to how LES can be used currently and in the future.

In an attempt to validate a Large Eddy Simulation (LES) approach, computations of a transonic centrifugal compressor with a backswept, unshrouded impeller followed by radial and axial vaned diffusers are performed. A sector composed of one main blade and one splitter blade, two radial diffuser vanes and six axial diffuser vanes is simulated including all the technological effects of the experimental rig. The LES methodology to simulate the rotor/stator configuration is introduced. Emphasis is put on the best trade-off between accuracy of the simulation and affordable CPU cost. A law-of-the-wall boundary condition is used to reduce the mesh size, with a target of y+ around a hundred for all walls except in the tip leakage with y+ around five. Computation of one entire characteristic line is obtained continuously in time: the transient from the flow at rest to the converged points at blockage, peak efficiency, near surge and path to deep surge is computed increasing progressively the outlet pressure as in the experiments. First, LES results are compared to experiments and show excellent agreement both in terms of overall performance and time-averaged internal flow fields previously obtained by Laser Doppler Anemometry. Then, a focus is proposed on the complementary information LES provide in the rotor. The key findings are that contrary to previous URANS studies in this centrifugal compressor, LES capture influential details of the flow structures in the rotor: secondary structures, shock/boundary layer interaction and boundary layer separation at the tip of the impeller. Moreover, it is clearly shown that the tip leakage vortex increases in size and intensity from peak efficiency to surge and becomes much more erratic. Emphasis is put on the causes and consequences of the tip leakage spillage in the neighbouring rotor channels. Pressure fluctuations were also found to increase from peak efficiency to surge downstream the splitter blade leading edge. The whole results finally show that LES with a law-of-the-wall provides excellent results in such a complex case.

Abstract The present work shows an innovative design framework for fluid Topology Optimization (TO) able to fully exploit the flexibility offered by Additive Manufacturing (AM) in the production of fluid-structure interaction systems. We present a geometry optimization method able to automatically design complex and efficient heat exchangers, adapted to maximizing fluid-structure heat transfer while minimizing turbulent flow pressure drop. The core of the method is the in-house Fluid Topology Optimization solver extended to include conjugate heat transfer problems. The TO method consists in emulating a sedimentation process inside an empty cavity in which a fluid dynamics problem is numerically solved. A design variable, in this case impermeability, is iteratively updated across the fluid dynamics domain. This mechanism leads to the formation of internal solid structures accordingly to a Lagrangian multi-objective optimization approach, adopted to include a multi-objective function. The solution of the optimization routine is the set of solidified structures, shaping the final optimized geometry. In order to match engineering applications, real conditions are implemented: an impermeability dependent thermal conductivity is included and a smoother operator is adopted to bound numerical thermal conductivity gradients across solid and fluid regions. The optimization is performed on a 3-dimensional straight duct: on the walls the temperature is constant and a coolant turbulent flow is simulated (Re 10000) inside the duct. The solver builds structures enhancing the heat transfer level between the walls of the domain and a coolant flow, by generating counter rotating vortices and complex fluid patterns. This is consistent to solution proposed in the open literature, such as v-shaped ribs, even if the geometry generated is more complex and efficient. The solution is validated with a high fidelity numerical simulation on StarCCM+, using a Detached Eddy Simulation (DES). Validation results shows higher heat transfer efficiency compared to the results present in the literature: the average Nusselt number computed on the domain walls is about 20% higher than the value obtained through experimental investigations on v-shape ribbed ducts. It is the first time that this method is applied and validated on real working conditions.