Dissipation Region Research Articles

Model study of nutrient and phytoplankton dynamics in the Gulf of Maine: patterns and drivers for seasonal and interannual variability

Abstract Coupled physical–biological modelling experiments were made for the period of 1995–2009 to analyse the spatial and interannual variability of nutrients and phytoplankton production in the Gulf of Maine (GOM). The physical model was the Finite-Volume Community Ocean Model (FVCOM) and the biological model was a Nitrogen, Phytoplankton, Zooplankton, and Detritus (NPZD) model. The simulation was carried out with realistic meteorological surface forcing, five major tidal constituents, river discharge, and observation-based open boundary conditions. The results were robust with comparison to SeaWiFS chlorophyll data and historical data of nitrogen. An Empirical Orthogonal Function analysis clearly identified two dominant modes in nutrient and phytoplankton dynamics: (1) sustained nutrient supply and phytoplankton production from spring through autumn, and (2) a dominating phytoplankton bloom in spring, relatively low production in summer, and a noticeable bloom in autumn. Mode 1 was a dominant feature in strong tidal energy dissipation regions such as the southwestern shelf of Nova Scotia, Georges Bank, Nantucket Shoals, the Bay of Fundy, and the coastal regions of GOM, where tidal pumping and mixing were the major drivers for the sustained nutrient supply, and primary production showed certain resilience with less interannual variability. Mode 2 was a characteristic in the deep Gulf, the offshore region of the Scotian Shelf, and in the open sea area, where the timing and amplitude of the spring phytoplankton bloom is essentially controlled by the salinity regime, and its interannual variability was significantly influenced by freshening events controlled by local and remote forcing.

Transmission of planetary wave effects to the upper atmosphere through eddy diffusion modulation

Past observations have shown that the effects of planetary waves can travel beyond their dissipation region and into the thermosphere and ionosphere. One possible mechanism through which this event can occur is the modulation of turbulent mixing near the turbopause. Because turbulent mixing in the mesosphere–lower thermosphere is strongly dependent on the amount of wave dissipation, planetary wave filtering of gravity waves from below can indirectly modulate the amount of turbulent mixing. This study uses the NCAR TIE-GCM to investigate how this mechanism induces variability in upper atmospheric composition and density. It is shown that varying the amount of turbulent mixing at discrete planetary wave periods induces similar periodicities in neutral and electron density. This process is driven by species diffusive flux through isobaric (constant pressure) levels, resulting in atmospheric contraction or expansion. Results also indicate that the mechanism studied has a wave frequency dependence and is most effective for long period planetary waves.

Molecular Dynamics Study of Nanoconfined Water Flow Driven by Rotating Electric Fields under Realistic Experimental Conditions

In our recent work, J. Chem. Phys. 2013, 138, 154712, we demonstrated the feasibility of unidirectional pumping of water, exploiting translational-rotational momentum coupling using nonequilibrium molecular dynamics simulations. Flow can be sustained when the fluid is driven out of equilibrium by an external spatially uniform rotating electric field and confined between two planar surfaces exposing different degrees of hydrophobicity. The permanent dipole moment of water follows the rotating field, thus inducing the molecules to spin, and the torque exerted by the field is continuously injected into the fluid, enabling a steady conversion of spin angular momentum into linear momentum. The translational-rotational coupling is a sensitive function of the rotating electric field parameters. In this work, we have found that there exists a small energy dissipation region attainable when the frequency of the rotating electric field matches the inverse of the dielectric relaxation time of water and when its amplitude lies in a range just before dielectric saturation effects take place. In this region, that is, when the frequency lies in a small window of the microwave region around ∼20 GHz and amplitude ∼0.03 V Å(-1), the translational-rotational coupling is most effective, yielding fluid velocities of magnitudes of ∼2 ms(-1) with only moderate fluid heating. In this work, we also confine water to a realistic nanochannel made of graphene giving a hydrophobic surface on one side and β-cristobalite giving a hydrophilic surface on the other, reproducing slip-and-stick velocity boundary conditions, respectively. This enables us to demonstrate that in a realistic environment, the coupling can be effectively exploited to achieve noncontact pumping of water at the nanoscale. A quantitative comparison between nonequilibrium molecular dynamics and analytical solutions of the extended Navier-Stokes equations, including an external rotating electric field has been performed, showing excellent agreement when the electric field parameters match the aforementioned small energy dissipation region.

Some remarks on the diffusion regions in magnetic reconnection

The structure of the diffusion regions in antiparallel magnetic reconnection is investigated by means of a theory and a Vlasov simulation. The magnetic diffusion is considered as relaxation to the frozen-in state, which depends on a reference velocity field. A field-aligned component of the frozen-in condition is proposed to evaluate a diffusion-like process. Diffusion signatures with respect to ion and electron bulk flows indicate the ion and electron diffusion regions near the reconnection site. The electron diffusion region resembles the energy dissipation region. These results are favorable to a previous expectation that an electron-scale dissipation region is surrounded by an ion-scale Hall-physics region.

Electron-scale shear instabilities: magnetic field generation and particle acceleration in astrophysical jets

Strong shear flow regions found in astrophysical jets are shown to be important dissipation regions, where the shear flow kinetic energy flow is converted into electric and magnetic field energy via shear instabilities. The emergence of these self-consistent fields makes shear flows significant sites for radiation emission and particle acceleration. We focus on electron-scale instabilities, namely the collisionless, unmagnetized electron-scale Kelvin–Helmholtz instability (ESKHI) and a large-scale DC magnetic field generation mechanism on the electron scales. We show that these processes are important candidates to generate magnetic fields in the presence of strong velocity shears, which may naturally originate in energetic matter outbursts of active galactic nuclei and gamma-ray bursters. We show that the ESKHI is robust to density jumps between shearing flows, thus operating in various scenarios with different density contrasts. Multidimensional particle-in-cell (PIC) simulations of the ESKHI, performed with OSIRIS, reveal the emergence of a strong and large-scale DC magnetic field component, which is not captured by the standard linear fluid theory. This DC component arises from kinetic effects associated with the thermal expansion of electrons of one flow into the other across the shear layer, whilst ions remain unperturbed due to their inertia. The electron expansion forms DC current sheets, which induce a DC magnetic field. Our results indicate that most of the electromagnetic energy developed in the ESKHI is stored in the DC component, reaching values of equipartition on the order of in the electron time-scale, and persists longer than the proton time-scale. Particle scattering/acceleration in the self-generated fields of these shear flow instabilities is also analyzed.

Seismic demand on steel reinforcing bars in reinforced concrete frame structures

Modern design standards for reinforced concrete (r.c.) buildings allow the achievement of ductile structures, able to globally dissipate seismic energy through the development of plastic deformations located in the dissipative regions (i.e. plastic hinges). The hysteretic capacity of r.c. structures is related to the ability of reinforcing steel bars to sustain many cycles of high plastic deformations without the exhibition significant decrease of strength and stiffness; this condition, typically due to cyclic/seismic action, shall be widely investigated in order to obtain a full and detailed knowledge of the structural behaviour of modern r.c. buildings. In the present paper, elaborated inside the European research project “Rusteel”, the evaluation of the seismic ductile demand on steel reinforcing bars due to real earthquake events was carried out. Representative r.c. case study buildings were designed following the actual European and Italian prescriptions and analyzed using the Incremental Dynamic Analysis technique for the assessment of the behaviour under real seismic events. The elaboration of a simplified mechanical model for the steel reinforcing bars, calibrated on the basis of experimental monotonic and cyclic tests, allowed the evaluation of the effective level of deformation and energy dissipation required by earthquakes and the assessment of the ability of the actual European production to satisfy the effective seismic ductile requirements.

Energy dissipation in viscous‐plastic sea‐ice models

AbstractIn viscous‐plastic (VP) sea‐ice models, small deformations are approximated by irreversible viscous deformations, introducing a nonphysical energy sink. As the spatial resolution and the degree of numerical convergence of the models increase, linear kinematic features (LKFs) are better resolved and more states of stress lie in the viscous regime. Energy dissipation in this nonphysical viscous regime therefore increases. We derive a complete kinetic energy (KE) balance for sea ice, including plastic and viscous energy sinks to study energy dissipation. The main KE balance is between the energy input by the wind and the dissipation by the water drag and the internal stresses (dissipating 87% and 13% of the energy input on an annual average). The internal stress term is mostly important in winter when ice‐ice interactions are dominant. The energy input that is not dissipated locally is redistributed laterally by the internal stresses into regions of dissipation by small‐scale deformations (LKFs). Of the 13% dissipated annually by the internal stress term, 93% is dissipated in plastic friction along LKFs (14% in ridging, 79% in shearing) and 7% is stored as potential energy in ridges. For all time and spatial scales tested, the frictional viscous dissipation is negligible in the KE balance. This conclusion remains valid regardless of the degree of numerical convergence of the simulations. Overall, the results confirm the applicability, from an energetical point of view, of the VP approximation.

A New Accurate Closed-Form Analytical Solution for Junction Temperature of High-Powered Devices

The peak junction temperature has a profound effect on the operational lifetime and performance of high powered microwave devices. Although numerical analysis can help to estimate the peak junction temperature, it can be computationally expensive and time consuming when investigating the effect of the device geometry and material properties on the performance of the device. On the other hand, a closed-form analytical method will allow similar studies to be done easily and quickly. Although some previous analytical solutions have been proposed, the solutions either require over-long computational times or are not so accurate. In this paper, an accurate closed-form analytical solution for the junction temperature of power amplifier field effect transistors (FETs) or monolithic microwave integrated circuits (MMICs) is presented. Its derivation is based on the Green's function integral method on a point heat source developed through the method of images. Unlike most previous works, the location of the heat dissipation region is assumed to be embedded under the gate. Since it is a closed-form solution, the junction temperature as well as the temperature distribution around the gate can be easily calculated. Consequently, the effect of various design parameters and material properties affecting the junction temperature of the device can be easily investigated. This work is also applicable to multifinger devices by employing superposition techniques and has been shown to agree well with both numerical and experimental results.

Near-surface structure and energy characteristics of the Antarctic Circumpolar Current

Historical surface drifter observations collected from the Southern Ocean are used to study the near-surface structure, variability, and energy characteristics of the Antarctic Circumpolar Current (ACC). A strong, nearly zonal ACC combined with complex fronts dominates the circulation system in the Southern Ocean. Standard variance ellipses indicate that both the Agulhas Return Current and the East Australian Warm Current are stable supplements of the near-surface ACC, and that the anticyclonic gyre formed by the Brazil warm current and the Malvinas cold current is stable throughout the year. During austral winter, the current velocity increases because of the enhanced westerly wind. Aroused by the meridional motion of the ACC, the meridional velocity shows greater instability characteristics than the zonal velocity does over the core current. Additionally, the ACC exhibits an eastward declining trend in the core current velocity from southern Africa. The characteristics of the ACC are also argued from the perspective of energy. The energy distribution suggests that the mean kinetic energy (MKE), eddy kinetic energy (EKE), and $\sqrt{\text{EKE}}$ are strong over the core currents of the ACC. However, in contrast, EKE/MKE suggests there is much less (more) eddy dissipation in regions with strong (weak) energy distribution. Both meridional and zonal energy variations are studied to illustrate additional details of the ACC energy characteristics. Generally, all the energy forms except EKE/MKE present west-east reducing trends, which coincide with the velocity statistics. Eddy dissipation has a much greater effect on MKE in the northern part of the Southern Ocean.

Material driven workability simulation by FEM including 3D processing maps for magnesium alloy

The three-dimensional (3D) processing maps considering strain based on the two-dimensional (2D) processing maps proposed by PRASAD can describe the distribution of the efficiency of power dissipation and flow instability regions at various temperatures, strain rates and strains, which exhibit intrinsic workability related to material itself. Finite element (FE) simulation can obtain the distribution of strain, strain rate, temperature and die filling status, which indicates state-of-stress (SOS) workability decided by die shape and different processing conditions. On the basis of this, a new material driven analysis method for hot deformation was put forward by the combination of FE simulation with 3D processing maps, which can demonstrate material workability of the entire hot deformation process including SOS workability and intrinsic workability. The hot forging process for hard-to-work metal magnesium alloy was studied, and the 3D thermomechanical FE simulation including 3D processing maps of complex hot forging spur bevel gear was first conducted. The hot forging experiments were carried out. The results show that the new method is reasonable and suitable to determine the appropriate process parameters.

Electron nongyrotropy in the context of collisionless magnetic reconnection

Collisionless magnetized plasmas have the tendency to isotropize their velocity distribution function around the local magnetic field direction, i.e., to be gyrotropic, unless some spatial and/or temporal fluctuations develop at the particle gyroscales. Electron gyroscale inhomogeneities are well known to develop during the magnetic reconnection process. Nongyrotropic electron velocity distribution functions have been observed to play a key role in the dissipative process breaking the field line connectivity. In this paper, we present a new method to quantify the deviation of a particle population from gyrotropy. The method accounts for the full 3D shape of the distribution and its analytical formulation allows fast numerical computation. Regions associated with a significant degree of nongyrotropy are shown, as well as the kinetic origin of the nongyrotropy and the fluid signature it is associated with. Using the result of 2.5D Particle-In-Cell simulations of magnetic reconnection in symmetric and asymmetric configurations, it is found that neither the reconnection site nor the topological boundaries are generally associated with a maximized degree of nongyrotropy. Nongyrotropic regions do not correspond to a specific fluid behavior as equivalent nongyrotropy is found to extend over the electron dissipation region as well as in non-dissipative diamagnetic drift layers. The localization of highly nongyrotropic regions in numerical models and their correlation with other observable quantities can, however, improve the characterization of spatial structures explored by spacecraft missions.

On the Resonance and Shelf/Open-Ocean Coupling of the Global Diurnal Tides

AbstractThe resonance of diurnal tidal elevations is investigated with a forward ocean tide model run in a realistic near-global domain and a synthesis of free oscillations (normal modes) computed for realistic global ocean geometries and ocean physics. As a prelude to performing the forward ocean tide simulations, the topographic wave drag, which is now commonly employed in forward ocean tide models, is tuned specifically for diurnal tides. The synthesis of global free oscillations predicts reasonably well the forward ocean diurnal tide model sensitivity to changes in the frequency, zonal structure, and meridional structure of the astronomical diurnal tidal forcing. Three global free oscillations that are important for understanding diurnal tides as a superposition of forced-damped, resonant, free oscillations are identified. An admittance analysis of the frequency sweep experiments demonstrates that some coastal locations such as the Sea of Okhotsk are resonant to diurnal tidal forcing. As in earlier work done with semidiurnal tides, a series of simulations are performed in which regions possessing significant coastal diurnal tides are blocked out. The largest perturbations to the open-ocean diurnal tides take place in Blocked Sea of Okhotsk experiments. Lesser but still significant perturbations also arise from the blocking out of other regions of large diurnal tidal elevations or dissipation. Interpretation of the results is made more complex, however, by the fact that substantial perturbations also arise from blocking out regions where neither tidal elevations nor dissipation are large. The “blocking” experiments are relevant to understanding tides of the ice age, during which lower sea levels entail a reduced area of continental shelves.

Analysis of Friction and Stabilized Wear in the Area of Mechanical Energy Dissipation

Friction, its concomitant thermal processes and wear are analyzed in a tribological system which is formed by a separate fragment of a friction pair element where mechanical energy is dissipated. A phenomenological (macroscopic) interpretation of stationary processes in a thermodynamic perspective is proposed. The tribological system is assumed to form an open thermodynamic system. An original model of a frictional source of heat is formulated. Balancing of mass and energy, especially the first law of thermodynamics, is employed in the discussion. Analytical dependences are found between work of friction, wear, heat of friction, heat carried away to the environment, and physical properties of the system material are determined. Variation of the system temperature and dimensions of the energy dissipation region are taken into consideration. The proposed model is illustrated by means of selected tests. Reference is made to an earlier, energetic interpretation of friction and its associated processes (Maciąg, M., 2010, “Thermodynamic Model of the Metallic Friction Process,” ASME J. Tribol., 132(3), pp. 1–7). A method of defining a tribological system and a new mechanism of frictional heating are some of the original elements. Introduction of these elements to equations of mass and energy balances resulted in new analytical dependences characterizing properties of a tribological system.

The influence of intense electric fields on three-dimensional asymmetric magnetic reconnection

A three-dimensional particle-in-cell simulation of magnetic reconnection in an asymmetric configuration without a guide field and with temperature ratio Ti/Te>1 demonstrates that intense perpendicular electric fields are produced on the low-density side of the current layer where there is a strong gradient in the plasma density. The simulation shows that the 3-D reconnection rate is unaffected by these intense electric fields, that the electron current layer near the X line remains coherent and does not break up, but that localized regions of strong energy dissipation exist along the low-density separatrices. Near the X line the dominant term in the generalized Ohm's law for the reconnection electric field remains the off-diagonal electron pressure gradient ∂Pexy/∂x. On the low-beta separatrix, however, the anomalous drag −⟨δnδEy⟩/⟨n⟩ makes an equally important contribution to that of the pressure gradient to the average Ey field.

A Statistical Study on the Reconnection in Boundary Layers of Small-scale Magnetic Flux Tubes in Solar Winds

With the data from WIND satellite in 1995—2005, the small-scale magnetic flux tubes determined with two methods, i.e., the artificial recognition and program selection respectively, are compared. It is found that there are magnetic reconnections in the boundary layers of 41% small-scale magnetic flux tubes determined with the program selection method, which is similar to the re- sult of small-scale magnetic flux tubes determined with the method of artificial recognition. The features of magnetic reconnections, such as the magnetic shear angle, magnetic field strength, and duration of the dissipation region of recon- nection at the small-scale flux tubes’ boundaries determined from both methods have the same statistical tendency. This shows that there is no essential differ- ence in the properties of reconnections in the boundary layers of the small-scale magnetic flux tubes determined with the two methods. Hence the data yielded by both methods can be used as the samples for statistically studying the events of reconnection in the front and back boundary layers of small-scale flux ropes. There are totally 71 magnetic reconnection events selected in this paper. Our statistical result shows that in 50 events (70%) the decrease of magnetic field strength in the dissipation region of reconnection is larger than 20%, and in 47 events (66%) the magnetic shear angle is larger than 90 degrees. These indicate that the magnetic reconnections in boundaries of the small-scale magnetic flux tubes are more likely to be anti-parallel. The statistics has been performed sep- arately in the reconnections of the front and back boundary layers of small-scale magnetic flux tubes. The results show that the features of reconnections in the front and back boundaries are similar to each other, which is different from the various properties in the front and back boundaries of the magnetic clouds, and this means that the expansion in the large-scale magnetic flux tubes, such as magnetic clouds, does not happen in the small-scale magnetic flux tubes.

Uniform decay rate estimates for Schrödinger and plate equations with nonlinear locally distributed damping

On a compact n-dimensional Riemannian manifold (M,g), we establish uniform decay rate estimates for the linear Schrödinger and plate equations subject to an internal nonlinear damping locally distributed on the manifold. Our approach can be also employed for other equations provided that inverse inequality for the linear model occurs. In the particular case of the wave equation, where the well-known geometric control condition (GCC) is equivalent to the observability inequality, our method generalizes the results due to Cavalcanti et al. (2010, 2009) [9,10] regarding the optimal choice of dissipative regions.

Fermi-LAT Gamma-Ray Variability Study of Misaligned AGN

We review the gamma-ray variability properties of AGN with a jet not directly pointing at the observer, i.e. Misaligned AGN. This class is mainly populated by nearby low power radio galaxies (i.e. FRIs). The high power radio sources (i.e. FRIIs) are indeed rare in the GeV sky. Our Fermi-LAT inspection of 4 years of data reveals different temporal behaviors, supporting the idea that beaming/jet structural differences could characterize the FRI-FRII jets. The case of 3C111 (FRII) and NGC6251 (FRI) are treated in detail. New insights into the location and dimension of the gamma-ray dissipation regions are provided for galaxies with different radio morphologies.

Preservation of Fine Structures in PDE-Based Image Denoising

Image denoising processes often lead to significant loss of fine structures such as edges and textures. This paper studies various innovative mathematical and numerical methods applicable for conventional PDE-based denoising models. The method of diffusion modulation is considered to effectively minimize regions of undesired excessive dissipation. Then we introduce a novel numerical technique for residual-driven constraint parameterization, in order for the resulting algorithm to produce clear images whose corresponding residual is as free of image textures as possible. A linearized Crank-Nicolson alternating direction implicit time-stepping procedure is adopted to simulate the resulting model efficiently. Various examples are presented to show efficiency and reliability of the suggested methods in image denoising.

GRB 110721A: PHOTOSPHERE “DEATH LINE” AND THE PHYSICAL ORIGIN OF THE GRB BAND FUNCTION

The prompt emission spectra of gamma-ray bursts (GRBs) usually have a dominant component that is well described by a phenomenological "Band" function. The physical origin of this spectral component is debated. Although the traditional interpretation is synchrotron radiation of non-thermal electrons accelerated in internal shocks or magnetic dissipation regions, a growing trend in the community is to interpret this component as modified thermal emission from a dissipative photosphere of a GRB fireball. We analyze the time dependent spectrum of GRB 110721A detected by {\em Fermi} GBM and LAT, and pay special attention to the rapid evolution of the peak energy $E_p$. We define a "death line" of thermally-dominated dissipative photospheric emission in the $E_p - L$ plane, and show that $E_p$ of GRB 110721A at the earliest epoch has a very high $E_p \sim 15$ MeV that is beyond the "death line". Together with the finding that an additional "shoulder" component exists in this burst that is consistent with a photospheric origin, we suggest that at least for some bursts, the "Band" component is not from a dissipative photosphere, but must invoke a non-thermal origin (e.g. synchrotron or inverse Compton) in the optically thin region of a GRB outflow. We also suggest that the rapid "hard-to-soft" spectral evolution is consistent with the quick discharge of magnetic energy in a magnetically-dominated outflow in the optically thin region.

Topographic patterns in the suspension structure in standing waves

The study of suspension dynamics and structure isa traditional problem in fluid mechanics [1]; it is ofcurrent interest due to its fundamentality [2] and thelarge variety of applications where engineering andenvironmental problems have an important place.Within the theory, models of matter transport by nonsteady flows are developed [3], and the conditions ofdifferentiation and accumulation of disperse phases incertain parts of a flow are determined [4]. In practice,wave suspension flows in the hydrosphere cause transportand accumulation of deposits that change the bottomrelief [5], which is shown in satellite images of the deepocean surface. Remote methods for monitoring bottomtopography have become common recently in practicalbathymetry despite the absence of mathematical modelsof the formation of such images [6].A series of laboratory studies of matter transport bywaves and bottom topography formation was openedby the pioneering work [7]. In recent years, the correlations between the statistical characteristics of a flowwith processes of wave ripple formation and ablation,as well as transition of bottom sediments to suspension, have mainly been discussed [8].In modern dynamic wave models in inhomogeneous media, largescale energycarrying componentsare supplemented with a rich family of finestructurecomponents influencing dissipation and localizationof highvorticity regions and matter transport [9]. Theeffect of the soluble admixture concentration on thininterlayers in the field of inner waves was noted insome works [10]. However, changes in the spatialstructure of a suspension in the body of liquid involvedin the wave flow had not been studied before. In thiswork, we first trace structuring of an initially homogeneous suspension to the standingwave field, graduallycovering the whole volume of a rectangular laboratorycontainer.The container is transparent, 50 × 4 × 40 cm in size,and it fluctuates in the vertical direction in the mainFaraday resonance mode [11]. The working media iswater suspension, the disperse phase of which consistsof platelike particles of pigmented aluminum powderwith an equivalent diameter of 10 μm and a fall diameter of about 10