Convection Field Research Articles

Adaptive Strategies for Transport Equations

Abstract This paper is concerned with a posteriori error bounds for linear transport equations and related questions of contriving corresponding adaptive solution strategies in the context of Discontinuous Petrov Galerkin schemes. After indicating our motivation for this investigation in a wider context the first major part of the paper is devoted to the derivation and analysis of a posteriori error bounds that, under mild conditions on variable convection fields, are efficient and, modulo a data-oscillation term, reliable. In particular, it is shown that these error estimators are computed at a cost that stays uniformly proportional to the problem size. The remaining part of the paper is then concerned with the question whether typical bulk criteria known from adaptive strategies for elliptic problems entail a fixed error reduction rate also in the context of transport equations. This turns out to be significantly more difficult than for elliptic problems and at this point we can give a complete affirmative answer for a single spatial dimension. For the general multidimensional case we provide partial results which we find of interest in their own right. An essential distinction from known concepts is that global arguments enter the issue of error reduction. An important ingredient of the underlying analysis, which is perhaps interesting in its own right, is to relate the derived error indicators to the residuals that naturally arise in related least squares formulations. This reveals a close interrelation between both settings regarding error reduction in the context of adaptive refinements.

Modelling of the transport phenomena for an atmospheric-pressure plasma jet in contact with liquid

The paper presents the results of a modelling study on the transport phenomena for a turbulent atmospheric-pressure plasma jet in contact with liquid. The focus is on the convective flow field in the gas and liquid phases and the transport of plasma generated long-lived hydrophilic species into the liquid. The flow field is studied by solving the Navier–Stokes equations using the finite-element method. The transport of species from the gas into the liquid phase is studied on the example of hydrogen peroxide by solving the convection-diffusion equation. Special attention is given to the implementation of the coupling boundary conditions at the gas–liquid interface. The computations are performed for the interaction of an argon jet with liquid under typical experimental conditions. The modelling results agree well with available experimental data. It is found that the three-dimensional nature of the flow in the liquid might be important in interpreting the experimental data. The model is shown to provide a reliable estimate of the hydrogen peroxide diffusion flux from the gas into the liquid phase.

Lattice Boltzmann study of pool boiling heat transfer enhancement on structured surfaces

Pool boiling heat transfer enhancement on structured surfaces with columns is investigated by a thermal two-phase lattice Boltzmann model. The effects of geometrical parameters, including column height (H), width (W) and gap spacing (D), are discussed in detail. The size of columns is comparable to the diameter of a typical detached bubble (db). It is found that the heat transfer performance mainly depends on two factors: the heated surface area A and local convective flow field. When W and D are properly chosen as about W=D≈4db, increasing H enhances the heat transfer solely due to the increase of the surface area. When W and D are small, it is observed that the top of the columns and the channels are mostly covered by a layer of vapor, respectively, which weaken the convection. Under the circumstances, although surface area A increases, heat transfer enhancement is not so significant. For the surface tension effect, the enhancement of the structured surface decreases with the increase of capillary number compared to that of the plain surface. The mechanism is that at larger capillary number, the convection close to the channel may be partially blocked, which hinders the heat transfer. An optimal enhancement could be achieved when A is as large as possible and meanwhile the convection is not hindered by the bubbles behaviors. The finding here may shed some light on mechanism of heat transfer enhancement on structured surfaces with columns.

Accelerating radiative transfer calculations for high‐resolution atmospheric models

AbstractThis study presents a method for calculating all‐sky, broadband radiative fluxes for high‐resolution atmospheric model domains, such as limited‐area numerical weather prediction models and Cloud System‐Resolving Models, that have many columns (i.e. ≫ 105). The aim is to replace the Independent Column Approximation (ICA) which applies 1D radiative transfer models (RTMs) to all columns; but usually after skipping many dynamical time steps due to the ICA's computational burden. The proposed method, referred to as Partitioned Gauss–Legendre Quadrature (PGLQ), begins by partitioning a domain into sub‐domains. This study is concerned just with cloudy columns so partitioning was done according to cloud‐top altitudes. Then, for each sub‐domain, sort columns from smallest to largest cloud water path , assign each column a sequence number, identify nG GLQ points, and apply 1D RTMs to the nG columns with resulting flux profiles assigned to nearby columns in the sorted sequence. Last, reposition columns back to the full domain. Marine boundary‐layer and deep convective cloud fields were used to assess PGLQ. For ratios of RTM calculations between ICA and PGLQ of fRT = /(nG · )−1 ∈ [103, 104], MBEs and RMSEs for net short‐wave (SW) surface fluxes scale close to (nG · )−1, RMSEs being typically ∼200× smaller than domain‐average . Structure function analyses of indicate that PGLQ errors resemble white noise. Long‐wave (LW) fluxes perform less well due to sorting on but seem likely to be acceptable, RMSEs being ∼5–10× smaller than domain‐average . All MBEs are typically <±0.5 W/m2. Estimates of radiative heating rate profiles are unbiased, and while noisy at small scales, they are portrayed well at scales larger than typical individual clouds. Overall speed‐up of PGLQ relative to ICA, for serial computation as consider here, is expected to be >300×; likely to be comparable to parallel computation. This should allow application of RTMs at every dynamical time step.

Correlations between the leading Lyapunov vector and pattern defects for chaotic Rayleigh-Bénard convection.

We probe the effectiveness of using topological defects to characterize the leading Lyapunov vector for a high-dimensional chaotic convective flow field. This is accomplished using large-scale parallel numerical simulations of Rayleigh-Bénard convection for experimentally accessible conditions. We quantify the statistical correlations between the spatiotemporal dynamics of the leading Lyapunov vector and different measures of the flow field pattern's topology and dynamics. We use a range of pattern diagnostics to describe the flow field structures which includes many of the traditional diagnostics used to describe convection as well as some diagnostics tailored to capture the dynamics of the patterns. We use the ideas of precision and recall to build a statistical description of each pattern diagnostic's ability to describe the spatial variation of the leading Lyapunov vector. The precision of a diagnostic indicates the probability that it will locate a region where the Lyapunov vector is larger than a threshold value. The recall of a diagnostic indicates its ability to locate all of the possible spatial regions where the Lyapunov vector is above threshold. By varying the threshold used for the Lyapunov vector magnitude, we generate precision-recall curves which we use to quantify the complex relationship between the pattern diagnostics and the spatiotemporally varying magnitude of the leading Lyapunov vector. We find that pattern diagnostics which include information regarding the flow history outperform pattern diagnostics that do not. In particular, an emerging target defect has the highest precision of all of the pattern diagnostics we have explored.

SuperDARN Evidence for Convection‐Driven Lagrangian Coherent Structures in the Polar Ionosphere

AbstractPolar cap patches are large sporadic enhancements of plasma density on the scale of hundreds of kilometers, which can impact the performance of Global Navigation Satellite Systems. Lagrangian Coherent Structures (LCSs) are ridges that show areas of maximal separation in a time‐evolving flow. Previous work based on modeled ionospheric flow showed that LCSs exist in the ionosphere and are barriers governing patch formation. In this work, we identify the first data‐driven LCSs in the high‐latitude ionosphere using Super Dual Auroral Radar Network (SuperDARN) ion convection fields. The LCSs found using the Ionosphere‐Thermosphere Algorithm for LCSs are compared during geomagnetically quiet and active periods. The shape of the LCS is found to be dependent on the electric potential pattern. A consistent two‐cell pattern results in a W‐shaped LCS, but when the two‐cell pattern breaks down, the LCS loses this characteristic shape. The changes in the electric potential, and thus the LCS, are likely due to changes in the interplanetary magnetic field. A comparison between LCSs obtained from empirical models and data reveal that the data‐driven LCSs are poleward of and have a shorter longitudinal span than the model‐based LCSs. A comparison of the LCS location and the formation of a polar cap patch on 17 March 2015 showed that the center of the patch developed from plasma on the main LCS ridge, and this is confirmed with a separate polar cap patch event from 26 September 2011.

An Investigation of the Effect of interrupted fin arrangements on the Thermal Performance of a Heat Sink under a Free Convection Condition

The heat sink with vertically rectangular interrupted fins was investigated numerically in a natural convection field, with steady-state heat transfer. A numerical study has been conducted using ANSYS Fluent software (R16.1) in order to develop a 3-D numerical model. The dimensions of the fins are (305 mm length, 100 mm width, 17 mm height, and 9.5 mm space between fins. The number of fins used on the surface is eight. In this study, the heat input was used as follows: 20, 40, 60, 80, 100, and 120 watts. This study focused on interrupted rectangular fins with a different arrangement and angle of the fins. Results show that the addition of interruption in fins in various arrangements will improve the thermal performance of the heat sink, and through the results, a better interruption rate as an equation can be obtained.

Morphological Development of Sub-Grain Cellular/Bands Microstructures in Selective Laser Melting.

In this paper, single-layer and bulk 316 L selective laser melting (SLM) experiments were conducted, fine submicron-scale geometric symmetrical cellular (hexagonal, pentagonal and square), elongated cellular and bands solidification morphologies were found in the laser-melt top surface. Meanwhile, morphological developed sub-grain patterns with quasi-hexagonal cellular, elongated cellular and bands structures (size ~1 μm) coexisting inside one single macro-solidified grain were also identified. This demonstrated the transitions from quasi-hexagonal-cells to elongated cells/bands, and transitions reverse, occurred in the whole bulk under some circumstances during SLM. Based on the experimental realities, these morphologies are formed by the local convection and Bénard instabilities in front of the solid/liquid interface (so-called mushy zones) affected by intricate temperature and surface tension gradients. Quasi-hexagonal cellular convective fields are then superimposed on macro-grain solidification to form the sub-grain patterns and micro-segregations. This explanation seems reasonable and is unifying as it can be expanded to other eutectic alloys with face center cubic (FCC) prevenient phase prepared by SLM, e.g., the Al-Si and Co-Cr-Mo systems.

Automated Mapping of Convective Clouds (AMCC) Thermodynamical, Microphysical, and CCN Properties from SNPP/VIIRS Satellite Data

AbstractThe advent of the Visible Infrared Imager Radiometer Suite (VIIRS) on board the Suomi NPP (SNPP) satellite made it possible to retrieve a new class of convective cloud properties and the aerosols that they ingest. An automated mapping system of retrieval of some properties of convective cloud fields over large areas at the scale of satellite coverage was developed and is presented here. The system is named Automated Mapping of Convective Clouds (AMCC). The input is level-1 VIIRS data and meteorological gridded data. AMCC identifies the cloudy pixels of convective elements; retrieves for each pixel its temperature T and cloud drop effective radius re; calculates cloud-base temperature Tb based on the warmest cloudy pixels; calculates cloud-base height Hb and pressure Pb based on Tb and meteorological data; calculates cloud-base updraft Wb based on Hb; calculates cloud-base adiabatic cloud drop concentrations Nd,a based on the T–re relationship, Tb, and Pb; calculates cloud-base maximum vapor supersaturation S based on Nd,a and Wb; and defines Nd,a/1.3 as the cloud condensation nuclei (CCN) concentration NCCN at that S. The results are gridded 36 km × 36 km data points at nadir, which are sufficiently large to capture the properties of a field of convective clouds and also sufficiently small to capture aerosol and dynamic perturbations at this scale, such as urban and land-use features. The results of AMCC are instrumental in observing spatial covariability in clouds and CCN properties and for obtaining insights from such observations for natural and man-made causes. AMCC-generated maps are also useful for applications from numerical weather forecasting to climate models.

Tropical intraseasonal oscillation at monthly time scale during boreal summer and winter

This study describes the space–time structure of an intraseasonal oscillation with the time scale of a month over the South Asian monsoon region and the tropical Pacific for the boreal summer and winter seasons. These nonlinear oscillations were extracted by applying multichannel singular spectrum analysis on daily anomalies of three-dimensional diabatic heating separately during the summer and winter. The monthly oscillations (MOs) are distinct from the leading monsoon intraseasonal oscillation with a period of 45 days during the summer and the Madden–Julian Oscillation during the winter. The summer MO exhibits horizontal quadrupole structure predominantly in the Northern Hemisphere and propagates eastward over the Indian region and westward over the Pacific along with northward movement. The winter MO is confined to the equatorial region in the Southern Hemisphere and propagates eastward and southward. Both the summer and winter MOs consist of propagating deep vertical columns of heating and cooling anomalies. The associated fields of convection, precipitation and three-dimensional circulation show similar patterns and propagation. The corresponding specific humidity has revealed moisture preconditioning for horizontal propagation and co-located vertical advection. However, there is no clear pattern of ocean–atmosphere interaction associated with the MO as seen in the examination of sea surface temperature and latent heat flux.

Studies on the mechanism of large cavity to improve the growth rate of large diamond under HPHT conditions

In this work, we investigate the effect of cavity size on the change of the physical fields of diamond synthetic cavity and the mechanism of large cavity to improve the growth rate of the large single crystal diamond under high pressure and high temperature (HPHT) conditions. Finite element method (FEM) is used to simulate the temperature and convection field of the Φ15 and Φ10 cavity assemblies. Temperature simulation results indicate that the increase of cavity size leads to the increase of axial and radial temperature difference of catalyst. Convection simulation results show that the convection velocity and strength of the catalyst increases significantly with the increase of the cavity size. The increase of temperature difference and convection velocity and strength lead to the acceleration of crystal growth. The simulation results were perfectly validated by the synthetic experiments. This work not only elucidates the mechanism of large cavity to improve the growth rate, but also provides a theoretical reference for the application of the large-size cavity in the diamond synthetic industry.

Implementation Euler implicit scheme of unsteady magnetohydrodinamic micropolar fluid pass a sphere affected by mixed convection

This paper considers the effect of mixed convection and magnetic field sphere in the unsteady magnetohydrodynamic micropolar fluid. For solving this problem, we develop dimensional governing equations from mass conservation law, energy conservation law, and Second Newton Law for momentum and angular momentum. We further convert them into non dimensional equations by using non dimensional variables. These non dimensional equations are further transformed into similarity equations using stream functions. Similarity equations are further solved numerically by implementing numerical method with euler implicit scheme. The result shows that the velocity and temperature will increase when parameter magnetic and mixed convection decrease. The higher magnetic parameter leads to higher microrotation of micropolar fluid particle. Whereas microrotation profile decrease with increasing mixed convection parameter.

NUMERICAL STUDY FOR INTERRUPTED RECTANGULAR FINS IN A NATURAL CONVECTION FIELD

The heat sink with vertically rectangular interrupted fins investigated numerically in a natural convection field, and with steady-state heat transfer. Numerical study has been conducted using ANSYS Fluent software (R16.1) in order to develop a 3-D numerical model. The dimensions of fins are (305 mm length, 100 mm width, 17 mm height, and 9.5 mm space between fins). The number of fins used on the surface are eight. In this study, the heat input that is used as follow (20, 40, 60, 80, 100, and 120 watts). The study is focused on interrupted rectangular fins with different arrangement of fins. The results show that the addition of interruption fins in various arrangements will improve the thermal performance of the heat sink, and through the results, a better interruption rate obtained as an equation.

Use of dielectric padding to eliminate low convective field artifact in cr-MREPT conductivity images.

Convection-reaction equation-based magnetic resonance electrical properties tomography (cr-MREPT) provides conductivity images that are boundary artifact-free and robust against noise. However, these images suffer from the low convective field (LCF) artifact. We propose to use dielectric pads to alter the transmit magnetic field (B1 + ), shift the LCF region, and eliminate the LCF artifact. Computer simulations were conducted to analyze the effects of pad electrical properties, pad thickness, pad height, arc angle, and thickness of the pad-object gap. In 3T MR experiments, water pads and BaTiO3 pads were used with agar-saline phantoms. Two data sets (e.g., with the pad located on the left or on the right of the object [phantom]) were acquired, and the corresponding linear systems were simultaneously solved to get LCF artifact-free conductivity images. A pad needed to have 180° arc angle and the same height with the phantom for maximum benefit. Increasing the pad thickness and/or the relative permittivity of the pad increased the LCF shift, whereas excessive amounts of these parameters caused errors in conductivity reconstructions because the effect of neglected Bz terms became noticeable. Conductivity of the pad, on the other hand, had minimal effect on elimination of the LCF artifact. Combining 2 data sets (i.e., with 2 different dielectric pad positions) resulted in more accurate conductivity maps (low L2 -errors) as opposed to no pad or single pad cases in experiments and simulations. Using the proposed technique, LCF artifact is significantly removed, and the reconstructed conductivity values are improved.

Study of flow field, heat transfer, and entropy generation of nanofluid turbulent natural convection in an enclosure utilizing the computational fluid dynamics‐artificial neural network hybrid method

AbstractIn this study, the turbulent natural convection of Ag‐water nanofluid in a tall, inclined enclosure has been investigated. The main objective of this study is finding the optimized angle of the enclosure with operational boundary condition in cooling from ceiling utilizing the computational fluid dynamics‐artificial neural network (CFD‐ANN) hybrid method, which has not been noticed in previous studies. To achieve this, we proposed two approaches. First, the simulations have been done with a deviation angle of 0 to 90° by using water and Ag‐water nanofluid. And second, a new prediction approach is proposed based on radial basis function artificial neural networks (RBF‐ANN) to predict the mean Nusselt number and entropy generation with the variation of Rayleigh numbers, deviation angles, and volume fractions as inputs. The results from the first approach indicate that the Rayleigh number has a considerable function in the determination of optimized angle. The results from the second approach, which used the first approach simulation results as training data set, could predict the mean Nusselt number and entropy generation with 1.4577e−022 and 1.552e−015 mean square error, respectively. Moreover, a new set of data for Rayleigh numbers, deviation angles, and volume fractions were used to test the performance of the prediction model, which shows promising and superior prospects for RBF‐ANN.

Velocity and geometry of propagating fronts in complex convective flow fields.

We numerically study the propagation of reacting fronts through three-dimensional flow fields composed of convection rolls that include time-independent cellular flow, spatiotemporally chaotic flow, and weakly turbulent flow. We quantify the asymptotic front velocity and determine its scaling with system parameters including the local angle of the convection rolls relative to the direction of front propagation. For cellular flow fields, the orientation of the convection rolls has a significant effect upon the front velocity and the front geometry remains relatively smooth. However, for chaotic and weakly turbulent flow fields, the front velocity depends upon the geometric complexity of the wrinkled front interface and does not depend significantly upon the local orientation of the convection rolls. Using the box counting dimension we find that the front interface is fractal for chaotic and weakly turbulent flows with a dimension that increases with flow complexity.

Effects of solute and flow field on 3D dendritic growth of superalloys in melt convection

Solute field, convection field and temperature field are important factors in determining the formation mechanism of solidification microstructure defects in superalloys. In the behavior of melt convective, different orientations of dendrites hinder the convective flow of the melt. And a special segregation channel may be formed between the dendrites to cause solidification defects. In this study, a numerical model for accurately calculating three-dimensional dendrite growth in superalloy melt convection is proposed. And a 27-point solute field discretization scheme is used to discretize the dendrite growth model in multi-physical field. Then, the model is solved by using the in-situ grid technique and SIMPLE algorithm. The results of numerical simulation are similar to those of LGK analysis and Oseen-Ivantsov solution, which verifies the practicability and effectiveness of the model algorithm to a certain extent. With the increase of undercooling, the effect of asymmetric growth of dendrite morphology decreases gradually, but with the increase of inlet velocity, the effect of asymmetric growth of dendrite morphology increases gradually.

Study on the effect of samplers on the disturbance of dredger-filled soil structure by numerical simulation layering method

Through the use of the particle flow software simulation, the influences on the microscopic dynamic properties of high viscosity soft soil caused by the inner diameter, cut angle and wall thickness of the samplers were studied with the example of the dredged mud. The motion behavior of the inflection point of particle characteristics was compared in seven soil samples by dividing the soil layers. The results show that a convective displacement field of the soil particles is formed at the bottom, and the particles above the convection are mainly subject to tensile expansion while the particles below the convection are mainly compressed and contracted, which result in the regular changes in porosity and bending deformation of the soil layers. There is a synergistic relationship between the intersection of the porosity curve and the initial porosity curve and the convective position of the particle displacement field. There is a characteristic inflection point in the interlayer particle displacement with the soil layers behave as a bending deformation. The particle disturbance between the inflection points is not obvious and the particle disturbance outside the inflection points is larger. The deformation feature can be fitted to a rotating paraboloid with the lower opening and the disturbance of the seven groups soil samples can be assessed initially by the a value of the surface equation. The line connecting the inflection points of the interlayer features can be refitted to a paraboloid of rotation. The ratio of the volume of the paraboloid to the volume of the soil sample can be used to evaluate the originality of the soil sample.

Analysis of critical fluid velocity and heat transfer in temperature-dependent nanocomposite pipes conveying nanofluid subjected to heat generation, conduction, convection and magnetic field

In this paper, analysis of critical fluid velocity and heat transfer in the nanocomposite pipes conveying nanofluid is presented. The pipe is reinforced by carbon nanotubes (CNTs) and the fluid is mixed by AL2O3 nanoparticles. The material properties of the nanocomposite pipe and nanofluid are considered temperature-dependent and the structure is subjected to magnetic field. The forces of fluid viscosity and turbulent pressure are obtained using momentum equations of fluid. Based on energy balance, the convection of inner and outer fluids, conduction of pipe and heat generation are considered. For mathematical modeling of the nanocomposite pipes, the first order shear deformation theory (FSDT) and energy method are used. Utilizing the Lagrange method, the coupled pipe-nanofluid motion equations are derived. Applying a semi-analytical method, the motion equations are solved for obtaining the critical fluid velocity and critical Reynolds and Nusselt numbers. The effects of CNTs volume percent, AL2O3 nanoparticles volume percent, length to radius ratio of the pipe and shell surface roughness were shown on the critical fluid velocity, critical Reynolds and Nusselt numbers. The results are validated with other published work which shows the accuracy of obtained results of this work. Numerical results indicate that for heat generation of Q = 10 MW/m3, adding 6% AL2O3 nanoparticles to the fluid increases 20% the critical fluid velocity and 15% the Nusselt number which can be useful for heat exchangers.

A Statistical Model for Isolated Convective Precipitation Events

AbstractWe present a highly simplified model to describe the diurnal evolution of a convective cloud field in idealized large eddy simulations. The life cycles of individual precipitation events are detected by a storm tracking algorithm which records the autonomous appearance and decay, as well as the merging and fragmentation of convective precipitation cells. Conditioned on the area covered by each cell, the tracking method records the time evolution of the precipitation intensity, the anomalies of near‐surface temperature and moisture, convective available potential energy, and convective inhibition. For tracks that do not merge or split (termedsolitary), many of these quantities show generic, often nearly linear relations that hardly depend on the forcing conditions of the simulations, such as surface temperature. This finding allows us to propose a simple idealized model of precipitation events, where the surface precipitation area is circular and a cell's precipitation intensity falls off linearly with the distance from the respective cell center. The drop‐off gradient is nearly independent of track duration and cell size. Multiple track properties, that is, track duration, peak, and mean intensity, as well as the associated cell area can hence be specified by knowing only one remaining parameter. In contrast to the simple and robust behavior of solitary tracks, tracks that result from merging of two or more cells show a much more complicated behavior. The most intense, long lasting, and largest tracks stem from tracks involved in repeated merging.