Turbulent Diffusion Of Heat Research Articles

Gyro-Landau-fluid simulations of impurity effects on ion temperature gradient driven turbulence transport

The effects of impurities on ion temperature gradient (ITG) driven turbulence transport in tokamak core plasmas are investigated numerically via global simulations of microturbulence with carbon impurities and adiabatic electrons. The simulations use an extended fluid code (ExFC) based on a four-field gyro-Landau-fluid (GLF) model. The multispecies form of the normalized GLF equations is presented, which guarantees the self-consistent evolution of both bulk ions and impurities. With parametric profiles of the cyclone base case, well-benchmarked ExFC is employed to perform simulations focusing on different impurity density profiles. For a fixed temperature profile, it is found that the turbulent heat diffusivity of bulk ions in a quasi-steady state is usually lower than that without impurities, which is contrary to the linear and quasi-linear predictions. The evolutions of the temperature gradient and heat diffusivity exhibit a fast relaxation process, indicating that the destabilization of the outwardly peaked impurity profile is a transient state response. Furthermore, the impurity effects from different profiles can obviously influence the nonlinear critical temperature gradient, which is likely to be dominated by linear effects. These results suggest that the improvement in plasma confinement could be attributed to the impurities, most likely through adjusting both heat diffusivity and the critical temperature gradient.

The Weddell Gyre heat budget associated with the Warm Deep Water circulation derived from Argo floats

Abstract. The Weddell Gyre plays an important role in the global climate system by supplying heat to underneath the ice shelves and in the formation of deep and bottom water masses, which have been subject to widespread warming over past decades. In this study, we investigate the re-distribution of heat throughout the Weddell Gyre by diagnosing the terms of the heat conservation equation for a 1000 m thick layer of water encompassing the core of Warm Deep Water. The spatial distributions of the different advective and diffusive terms in terms of heat tendencies are estimated using gridded climatologies of temperature and velocity, obtained from Argo floats in the Weddell Gyre from 2002 to 2016. While the results are somewhat noisy on the grid scale and the representation of the effects of eddy mixing is highly uncertain due to the need to parameterise them by means of turbulent diffusion, the heat budget (i.e. the sum of all terms) closes (within the uncertainty range) when integrated over the open inflow region in the southern limb, whereas the interior circulation cell remains unbalanced. There is an overall balance in the southern limb between the mean horizontal advection and horizontal turbulent diffusion of heat, whereas the vertical terms contribute comparatively little to the heat budget throughout the Weddell Gyre. Heat convergence due to mean horizontal advection balances with divergence due to horizontal turbulent diffusion in the open southern limb of the Weddell Gyre. In contrast, heat divergence due to mean horizontal advection is much weaker than convergence due to horizontal turbulent diffusion in the interior circulation cell of the Weddell Gyre, due to large values in the latter along the northern boundary due to large meridional temperature gradients. Heat is advected into the Weddell Gyre along the southern limb, some of which is turbulently diffused northwards into the interior circulation cell, while some is likely turbulently diffused southwards towards the shelf seas. This suggests that horizontal turbulent diffusion plays a role in transporting heat both towards the gyre interior where upwelling occurs and towards the ice shelves. Horizontal turbulent diffusion is also a mechanism by which heat can be transported into the Weddell Gyre across the open northern boundary. Temporal deviations from the mean terms are not included due to study limitations. In order to appreciate the role of transient eddying processes, a continued effort to increase the spatial and temporal coverage of observations in the eastern Weddell Sea is required.

Turbulent diffusion of scalar and heat in an off-source heated steady round jet

We study, using direct numerical simulation, the characteristics of an off-source heated jet (OSHJ), the rate of heat addition being proportional to the local concentration of a scalar. The heating disturbs the self-similar state of the unheated jet (UJ) entering the heat injection zone (HIZ). Based on the characteristics of the velocity and scalar statistics, the evolution of the OSHJ is demarcated into three axial zones. Zone I, which is dynamically the most active region, exhibits rapid variations in quantities like the mean radial velocity, the scalar-to-velocity width ratio and the second-order correlations. Zone II represents the relaxation of these quantities towards a new self-similar state which is realized in Zone III above the HIZ. We find that the temperature–velocity correlation is negative in the core of the OSHJ in Zone I, which becomes positive for all radial locations in Zones II and III. We interpret this behavior by decomposing the effect of heat addition into “heating” and “buoyancy” factors; the former describes the effect of axial velocity on heating while the latter describes the effect of added buoyancy on flow acceleration. The heating factor dominates in Zone I leading to the negative temperature–velocity correlation, which we conjecture to be the reason for the “disruption” of coherent structures in the OSHJ reported in the literature. Based on wavelet analysis of the scalar field we identify the length scale of the ring-like coherent structure that gets disrupted upon heat addition, which enhances the scalar homogeneity in the core of the OSHJ near the base of the HIZ. Where possible, we compare our results with the literature on heated jets within the context of the three demarcated zones. We find that external heating decreases the scalar-to-velocity flow widths relative to the UJ to a value in Zone III that is largely insensitive to the precise details of heat addition , such as the amount of heat added and the extent of the HIZ. We also present the effect of heat addition on turbulent Schmidt and Prandtl numbers. Our findings are relevant to the dynamics of cumulus clouds which are governed by a similar interplay of scalar, velocity and temperature fields as reported here.

A simplified model to estimate nonlinear turbulent transport by linear dynamics in plasma turbulence

A simplified model to estimate nonlinear turbulent transport only by linear calculations is proposed, where the turbulent heat diffusivity in tokamak ion temperature gradient(ITG) driven turbulence is reproduced for a wide parameter range including near- and far-marginal ITG stability. The optimal nonlinear functional relation(NFR) between the turbulent diffusivity, the turbulence intensity mathcal{T}, and the zonal-flow intensity mathcal{Z} is determined by means of mathematical optimization methods. Then, an extended modeling for mathcal{T} and mathcal{Z} to incorporate the turbulence suppression effects and the temperature gradient dependence is carried out. The simplified transport model is expressed as a modified nonlinear function composed of the linear growth rate and the linear zonal-flow decay time. Good accuracy and wide applicability of the model are demonstrated, where the regression error of sigma _{textrm{model}} = 0.157 indicates improvement by a factor of about 1/4 in comparison to that in the previous works.

Turbulent heating in a stratified medium

ABSTRACT There is considerable evidence for widespread subsonic turbulence in galaxy clusters, most notably from Hitomi. Turbulence is often invoked to offset radiative losses in cluster cores, both by direct dissipation and by enabling turbulent heat diffusion. However, in a stratified medium, buoyancy forces oppose radial motions, making turbulence anisotropic. This can be quantified via the Froude number Fr, which decreases inward in clusters as stratification increases. We exploit analogies with MHD turbulence to show that wave–turbulence interactions increase cascade times and reduce dissipation rates ϵ ∝ Fr. Equivalently, for a given energy injection/dissipation rate ϵ, turbulent velocities u must be higher compared to Kolmogorov scalings. High-resolution hydrodynamic simulations show excellent agreement with the ϵ ∝ Fr scaling, which sets in for Fr ≲ 0.1. We also compare previously predicted scalings for the turbulent diffusion coefficient D ∝ Fr2 and find excellent agreement, for Fr ≲ 1. However, we find a different normalization, corresponding to stronger diffusive suppression by more than an order of magnitude. Our results imply that turbulent diffusion is more heavily suppressed by stratification, over a much wider radial range, than turbulent dissipation. Thus, the latter potentially dominates. Furthermore, this shift implies significantly higher turbulent velocities required to offset cooling, compared to previous models. These results are potentially relevant to turbulent metal diffusion in the galaxy groups and clusters (which is likewise suppressed), and to planetary atmospheres.

Enhancement of turbulent mixing by a porous obstruction

Turbulent diffusion and mixing of heat, introduced passively by a thin ribbon in grid-generated turbulence, was enhanced drastically by a small array of thin cylinders, positioned closely downstream of the ribbon. A multi-structure flow region was formed behind the array and relaxed towards grid turbulence, albeit maintaining a turbulence intensity and an integral length scale that were twice as large as those in the absence of the array. This configuration has potential for use in applications that would benefit from intense turbulent mixing.

Transport barrier in 5D gyrokinetic flux-driven simulations

Two ways for producing a transport barrier through strong shear of the E × B poloidal flow have been investigated using GYSELA gyrokinetic simulations in a flux-driven regime. The first one uses an external poloidal momentum (i.e. vorticity) source that locally polarizes the plasma, and the second one enforces a locally steep density profile that also stabilizes the ion temperature gradient (ITG) instability modes linearly. Both cases show a very low local turbulent heat diffusivity coefficient and a slight increase in core pressure when a threshold of (respectively the E × B shear rate and average linear growth rate of ITG) is reached, validating previous numerical results. This pressure increase and quench are the signs of a transport barrier formation. This behaviour is the result of a reduced turbulence intensity which strongly correlates with the shearing of turbulent structures as evidenced by a reduction of the auto-correlation length of potential fluctuations as well as an intensity reduction of the k θ spectrum. Moreover, a small shift towards smaller poloidal wavenumber is observed in the vorticity source region which could be linked to a tilt of the turbulent structures in the poloidal direction.

Study of a lead-bismuth eutectic jet issued into a heated cavity using large eddy simulation

Low Prandtl number convection of an axisymmetric turbulent lead-bismuth eutectic (LBE) jet discharged into a heated confined geometry using large eddy simulation is investigated. The Reynolds number based on jet centerline velocity and jet diameter is Re = 44,706. The results demonstrate some features of liquid metal that deviate from those of ordinary fluids. Examples include the differences between fluctuations of temperature and velocity, the distributions of turbulent viscosity and turbulent diffusivity of heat, and the misalignment between the turbulent heat flux vector and the gradient of mean temperature, all of which imply that standard turbulence models may not be suitable for the simulation of such flows. The LBE jet at Pr = 0.03 is compared with a high-pressure water jet at Pr = 0.8 under the same Re and Fr. The centerline decay of the mean velocity for LBE demonstrates the same behavior as that of water, which is governed by a linear variation for the self-preservation region between 1.5D and 8.5D, where D is the diameter of the jet orifice. In contrast, the centerline mean temperature for LBE decreases slower compared with that of water. The difference in temperature results from the high intensity of molecular diffusion in heat transport in LBE, while the momentum transport is mainly by turbulent diffusion in both flows. Self-preserving characteristic of temperature r.m.s (Trms) is observed. The radial profiles of Trms are presented by an axisymmetric curve with a maximum at around 0.7b, where b is the half-value radius of Trms. Good agreement of mean temperature distribution between the simulation results and the experiments are obtained.

Diapycnal diffusivities in Kelvin–Helmholtz engendered turbulent mixing: the diffusive–convection regime in the Arctic Ocean

Recent progress in the direct measurement of turbulent dissipation in the Arctic Ocean has highlighted the need for an improved parametrization of the turbulent diapycnal diffusivities of heat and salt that is suitable for application in the turbulent environment characteristic of this polar region. In support of this goal we describe herein a series of direct numerical simulations of the turbulence generated in the process of growth and breaking of Kelvin–Helmholtz billows. These simulations provide the data sets needed to serve as basis for a study of the stratified turbulent mixing processes that are expected to exist in the Arctic Ocean environment. The mixing properties of the turbulence are studied using a previously formulated procedure in which the temperature and salinity fields are sorted separately in order to enable the separation of irreversible Arctic mixing from reversible stirring processes and thus the definition of turbulent diffusivities for both heat and salt that depend solely upon irreversible mixing. These analyses allow us to demonstrate that the irreversible diapycnal diffusivities for heat and salt are both solely dependent on the buoyancy Reynolds number in the Arctic Ocean environment. These are found to be in close agreement with the functional forms inferred for these turbulent diffusivities in the previous work of Bouffard & Boegman (Dyn. Atmos. Oceans, vol. 61, 2013, pp. 14–34). Based on a detailed comparison of our simulation data with this previous empirical work, we propose an algorithm that can be used for inferring the diapycnal diffusivities from turbulent dissipation measurements in the Arctic Ocean.

Numerical Investigation for the Influence of Turbulent Heat Transfer of Mushy Zone on Shell Growth in the Slab Mold

The turbulent heat transfer in the mushy zone caused by the impinging of molten steel jets in the slab mold has a significant effect on the growth of the solidified shell and the formation of impinging zone, as well as the quality of steel products. In the present work, a Peclet number of the mushy zone (Pem) is proposed to analyze the influence of turbulent heat transfer on the shell growth in the mushy zone and define the boundary of the impinging zone. A three-dimensional mathematical model based on the enthalpy-porosity method has been established to predict the flow, heat transfer and solidification processes in the slab mold. The influences of casting speed and secondary dendrite arm spacing (SDAS) on the turbulent heat transfer behavior are also investigated. The results indicate that the turbulent heat diffusion has a great effect on the formation of the impinging zone and is the only source of energy to maintain the boundary of the impinging zone. The Pem can evaluate the turbulent heat transfer in the mushy zone and is a more reasonable method to obtain a clearer boundary of the impinging zone. The impinging zone is formed when the Pem reaches a critical value, been dependent on the casting speed and SDAS.

Impact of modified turbulent diffusion of PM<sub>2.5</sub> aerosol in WRF-Chem simulations in eastern China

Abstract. Correct description of the boundary layer mixing process of particle is an important prerequisite for understanding the formation mechanism of pollutants, especially during heavy pollution episodes. Turbulent vertical mixing determines the distribution of momentum, heat, water vapor and pollutants within the planetary boundary layer (PBL). However, what is questionable is that the turbulent mixing process of particles is usually denoted by turbulent diffusion of heat in the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). With mixing-length theory, the turbulent diffusion relationship of particle is established, embedded into the WRF-Chem and verified based on long-term simulations from 2013 to 2017. The new turbulent diffusion coefficient is used to represent the turbulent mixing process of pollutants separately, without deteriorating the simulation results of meteorological parameters. The new turbulent diffusion improves the simulation of pollutant concentration to varying degrees, and the simulated results of PM2.5 concentration are improved by 8.3 % (2013), 17 % (2014), 11 % (2015) and 11.7 % (2017) in eastern China, respectively. Furthermore, the pollutant concentration is expected to increase due to the reduction of turbulent diffusion in mountainous areas, but the pollutant concentration did not change as expected. Therefore, under the influence of complex topography, the turbulent diffusion process is insensitive to the simulation of the pollutant concentration. For mountainous areas, the evolution of pollutants is more susceptible to advection transport because of the simulation of obvious wind speed gradient and pollutant concentration gradient. In addition to the PM2.5 concentration, the concentration of CO as a primary pollutant has also been improved, which shows that the turbulent diffusion process is extremely critical for variation of the various aerosol pollutants. Additional joint research on other processes (e.g., dry deposition, chemical and emission processes) may be necessary to promote the development of the model in the future.

Turbulent cylinder-stirred flow heat and momentum transfer research in batch operated single-phase square reactor

Heat transfer and momentum transfer due to water flow in a cylinder-stirred reactor of square cross section were investigated. Important industrial processes like mixing depend on combined effects of heat transfer and fluid flow which can be understood by analyzing the turbulence structure. A reactor of aspect ratio η = 0.21 (diameter of cylinder to tank side wall) was considered. It uses a small-diameter cylinder to stir the water in batch condition with rotating Reynolds number in a range 1.68 × 104 <Re < 10.1 × 104. The cylinder released a constant heat flux rate of 158 W/m2 through its surface. The steady state mean flow was numerically simulated using two models for turbulence, isothermally with a k-ε and thermally with the RSM model. The results revealed a fluid motion like Taylor-Couette vortex which was validated with PIV streamlines for Re = 6.1 × 104 and 10.1 × 104. Turbulent angular momentum and shear rate revealed differences for the corner direction as the Re number increased compared with the wall direction. Temperature fluctuations and thermal gradient in the gap allowed to analyze the turbulent heat diffusivity coefficient and the ratio to predicted diffusivity for momentum. The results revealed that heat and momentum diffusivity increase for higher Re number and show that heat diffusivity is faster than momentum in the gap. The main frequencies of fluid motion showed large-scale structures and secondary cells of fluid motion. The averaged heat transfer as a function of Re number indicate that this reactor is an enhanced mean for mixing process compared to concentric cylinders reactors.

Numerical Investigation on Turbulent Flow and Heat Transfer of Helium-Xenon Gas Mixture in a Circular Tube

Gas-cooled space nuclear reactor system usually utilizes the helium-xenon gas mixture as the working fluid. Since the typical helium-xenon mixture has the Prandtl number of about 0.2, which is lower than that of water and air, the turbulent flow and heat transfer features need to be further investigated among the helium-xenon mixture and other fluids. In the current paper, numerical investigations by ANSYS Fluent are performed on helium-xenon mixture flow (HeXe40, M = 40.0 g/mol, Pr = 0.21), airflow (Pr = 0.71), and water flow (Pr = 6.99) in the circular tube. Direct numerical simulation results of liquid metal flow (Pr = 0.01) are also adopted for comparison. Results show that the dimensionless velocity profile and shear stress in the boundary layer of HeXe40 are close to those of other fluids. The empirical correlations from other fluids can also predict well the friction factor of helium-xenon mixtures. Due to the discrepancy in turbulent heat diffusivity ratio, the dimensionless radial temperature profile and turbulent heat conduction of HeXe40 significantly differ from those of other fluids. The molecular conduction region of HeXe40 develops up to y+ ≈ 30 and extends to the logarithmic region of the flow boundary layer. Moreover, the available experimental Nusselt numbers of helium-xenon mixtures are compared with several convective heat transfer correlations, in which Kays correlation is better.

Remote Sensing of Global Daily Evapotranspiration based on a Surface Energy Balance Method and Reanalysis Data

AbstractCurrently available evapotranspiration (ET) products have not provided us with an accurate estimation for global irrigated land area. Thermal energy balance (EB) model has the potential to solve this problem. The accurate estimation of aerodynamic resistances is a major complication in most remote sensing ET models. An EB model using a column canopy‐air turbulent heat diffusion method was developed to more realistically depict dynamic changes in aerodynamic resistance. In order to estimate global ET and land surface fluxes for all weather conditions, Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua and Terra land surface temperature fields were combined and a nearest‐evaporative‐fraction gap‐filling method was merged into the EB model. A global ET product covering the period 2003–2017 was produced using the EB model. By combining thermal and optional information from MODIS satellites and surface meteorological forcing data from ERA‐Interim reanalysis data, the EB model provides a 5 × 5 km resolution estimate of daily ET without spatio‐temporal gaps at the global scale. Assessment of the daily EB ET at 238 flux sites showed a mean bias of 0.05 mm/day and an RMSE of 1.56 mm/day. Analysis of global precipitation minus ET demonstrated that EB ET has a relatively higher potential for agriculture water resource management than currently available global ET products, such as Landflux, GLEAM, MOD16, GLDAS, and ERA‐Interim ET. In addition, the EB model developed in this study can be applied to both polar and geostationary satellite thermal sensors.

Upper‐level trajectories in the prototype problem for tropical cyclone intensification

AbstractData from an idealized three‐dimensional numerical simulation of tropical cyclone intensification are used to calculate Lagrangian air parcel trajectories emanating from the inflow layer that develops beneath the upper‐tropospheric outflow layer. It is found that about half of these trajectories end up in the outflow layer itself. The other half slowly subside to the mid‐ to upper troposphere, below the outflow layer, and drift slowly outwards as a result of a relatively weak overturning circulation in that region. Calculations show that pseudo‐equivalent potential temperature is not approximately conserved along the air parcel trajectories, indicating that the turbulent diffusion of heat and moisture and/or the latent heat changes by freezing or melting along the trajectories is appreciable in the mid‐ and upper troposphere.

Positive and negative turbulent heat diffusivity observed on a 325-m meteorological tower in Beijing 基于北京325米气象塔观测的正, 负湍流热扩散系数

Turbulent diffusion efficiently transports momentum, heat, and matter and affects their transfers between the atmosphere and the surface. As a key parameter in describing turbulent diffusion, the turbulent heat diffusivity KH has rarely been studied in the context of frequent urban pollution in recent years. In this study, KH under urban pollution conditions was directly calculated based on K-theory. The authors found an obvious diurnal variation in KH, with variations also in the vertical distributions between each case and over time. Interestingly, the height corresponding to the high occurrence frequency of negative KH rises gradually after sunrise, peaks at noon, falls near sunset, and concentrates around 140 m during most of the night. The KH magnitude and fluctuation are smaller in the pollutant accumulation stage (CS) at all levels than in the pollutant transport stage and pollutant removal stage. Turbulent diffusion may greatly affect PM2.5 concentrations at the CS because of the negative correlation between PM2.5 concentrations and the absolute value of KH at the CS accompanied by weak wind speeds. The applicability of K-theory is not very good during either day or at night. These problems are inherent in K-theory when characterizing complex systems, such as turbulent diffusion, and require new frameworks or parameterization schemes. These findings may provide valuable insight for establishing a new turbulence diffusion parameterization scheme for KH and promote the study of turbulent diffusion, air quality forecasting, and weather and climate modeling.摘要本文基于K理论和北京325m铁塔湍流观测资料直接计算了城市污染条件下的湍流热扩散系数KH.KH具有明显的日变化特征, 其垂直分布随个例和时间而变化.负KH较高发生频率对应高度在日出后逐渐升高, 在中午达到峰值, 在日落附近下降, 而在夜间大部分时间集中在140 m左右.和传输和清除阶段相比, 污染物累积阶段的KH大小和波动程度均较小.此阶段PM2.5浓度与KH绝对值之间的负相关关系表明湍流扩散可能会极大地影响PM2.5浓度.这些发现可能为建立新湍流扩散参数化方案提供有价值的启示.

Quantification of modeling uncertainties in turbulent flames through successive dimension reduction

For turbulent flames involving intense turbulence-chemistry interaction, quantifying the uncertainty originating from the parameters of chemical kinetics and physical models leads to a more rigorous assessment of the predictability of simulations. In the present work, a successive dimension reduction framework based on the active subspace (AS) method is formulated to efficiently quantify modeling uncertainties associated with chemical kinetics, and turbulent combustion model parameters in turbulent flame simulations. The approach is demonstrated in simulating a turbulent H 2 /O 2 lifted wall-jet flame. The reduction of the high-dimensional kinetic uncertainty space is first achieved through cheap surrogate autoignition tests, and a single active uncertain kinetic variable is identified. Then a one-dimensional active subspace of the uncertainty space consisting of such an active kinetic variable and four turbulent combustion model parameters are further identified using 25 runs of turbulent flame simulations. Finally, the probability distribution function (PDF) of the flame lift-off length is characterized through Monte Carlo simulations within a cheap response surface that is constructed within the active subspace. The components of the active subspace reveal that both chemical kinetics and turbulent mixing are critical for the flame stabilization. Further analysis shows that the uncertainty in the turbulent heat diffusion could change the dominant reactions between R1 (H+O 2 ⇋ O+OH) and R9 (H+O 2 (+M) ⇋ HO 2 (+M)) through varying the local temperature in the flame stabilization zone. In addition, comparisons of the PDFs of the flame lift-off length show that the uncertainty induced by chemical kinetics is comparable with that induced by turbulent combustion model parameters. The successive dimension reduction of uncertain physicochemical parameter space via AS enables efficient uncertainty quantification for turbulent flames, meanwhile providing insights into the controlling physiochemical processes.

Direct Numerical Simulation of a buoyant triple jet at low-Prandtl number

Mixing of buoyant streams is a phenomenon of relevance in many practical cases like pollutant emission in the atmosphere, discharges from marine outfalls and cooling of fuel rods in nuclear reactors to name a few. A canonical configuration for this class of flows consists in three buoyant jets at different temperatures vertically entering a pool from the bottom. This work reports a Direct Numerical Simulation study performed on the triple jet configuration. The Reynolds number based on the average jet centerline velocity and jet width is set to Re=5000 and mixed convection regime is established at a Richardson number, Ri=0.25. In order to represent flows occurring inside liquid metal fast reactors, the selected Prandtl number is Pr=0.031.Statistics computed show that in the first stages of development, the three jets undergo a strong interaction. In that same region the shedding of large-scale vortices is originated accompanied by low-frequency undulations. Further from the inlet, the three jets are observed to coalesce in a single, isothermal stream. The analysis of momentum fluxes clarifies the mutual entrainment mechanism behind coalescence, which is commonly known as Coandă effect. At distances larger than ten times the jet width the self-similar characteristics of single and isothermal planar jets are recovered. The flow configuration presented includes several peculiar features, namely buoyancy effects at low Prandtl number, interaction between jets and the presence of multiple shear layers. This leads to an irregular behaviour of the turbulent diffusivity of momentum and heat as well as the misalignment between the temperature gradient and turbulent heat flux. Therefore the flow can be considered very complex and might constitute a demanding test bench for the development and validation of turbulence models.

Seasonality and Buoyancy Suppression of Turbulence in the Bay of Bengal

AbstractA yearlong record from moored current, temperature, conductivity, and four mixing meters (χpods) in the northernmost international waters of the Bay of Bengal quantifies upper‐ocean turbulent diffusivity of heat (Kt) and its response to the Indian monsoon. Data indicate (1) pronounced intermittency in turbulence at semidiurnal, diurnal, and near‐inertial timescales, (2) strong turbulence above 25‐m depth during the SW (summer) and NE (winter) monsoon relative to the transition periods (compare Kt &gt; 10−4 m2/s to Kt ∼ 10−5 m2/s, and (3) persistent suppression of turbulence (Kt &lt; 10−5 m2/s) for 3 to 5 months in the latter half of the SW monsoon coincident with enhanced near‐surface stratification postarrival of low‐salinity water from the Brahmaputra‐Ganga‐Meghna delta and monsoonal precipitation. This suppression promotes maintenance of the low‐salinity surface waters within the interior of the bay preconditioning the upper northern Indian Ocean for the next year's monsoon.

Modeling of turbulent particle and heat transport in helical plasmas based on gyrokinetic analysis

The particle and heat transport driven by the ion temperature gradient instability in helical plasmas is investigated by the gyrokinetic analysis taking into account the kinetic electron response. High and low ion temperature plasma cases for the discharge in the Large Helical Device (LHD) are studied. Two types of transport models with a lower computational cost to reproduce the nonlinear gyrokinetic simulation results within allowable errors are presented for application in quick transport analyses. The turbulent electron and ion heat diffusivity models are given in terms of the linear growth rate and the characteristic quantity for the linear response of zonal flows, while the model of the effective particle diffusivity is not obtained for the flattened density profile observed in the LHD. The quasilinear flux model is also shown for the heat transport. The quasilinear flux models for the energy fluxes are found to reproduce the nonlinear simulation results at the accuracy similar to that of the heat diffusivity models. In addition, the quasilinear particle flux model, which is applicable to the transport analysis for LHD plasmas, is constructed. These turbulent reduced models enable coupling to the other simulation in the integrated codes for the LHD.