NASA Astrophysics Data System (ADS)
Wong, Teresa; Solomatov, Viatcheslav S.
2016-05-01
We perform numerical simulations of lithospheric failure in the stagnant lid regime of temperature-dependent viscosity convection, using the yield stress approach. We find that the time of failure can vary significantly for the same values of the controlling parameters due to the chaotic nature of the convective system. The general trend of the dependence of the time of lithospheric failure on the yield stress can be explained by treating lithospheric failure as a type of Rayleigh-Taylor instability. This study suggests that it is important to address not only the question of whether plate tectonics can occur on a planet but also when it would occur if conditions are favorable.
NASA Astrophysics Data System (ADS)
Fernandez-Feria, Ramon; Del Pino, Carlos; Fernández-Gutiérrez, Alberto
2013-11-01
The boundary-layer flow of a cold horizontal current exiting radially from a cylindrical vertical surface with a constant velocity over a hotter horizontal wall with constant temperature is analyzed. The temperature and velocity fields are coupled by buoyancy through the pressure gradients, so that the boundary-layer equations are made dimensionless with a radial characteristic length in which natural and forced convection become of the same order of magnitude, being the Prandtl number the only nondimensional parameter governing the problem. A similarity solution valid for the leading edge boundary-layer flow is obtained, yielding as a first order correction the effect of natural convection on Blasius' thermal boundary layer. This solution is also used to start the numerical integration of the equations to find out the location where the boundary-layer flow blows up due to the termination of the solution in a singularity. The physical nature of this singularity is analyzed and its position is characterized numerically. The heat flux from the horizontal wall up to this singularity is also characterized and qualitatively compared with previous experimental results from a related experimental setup.
NASA Astrophysics Data System (ADS)
Manalo, Lawrence B.
A comprehensive, non-equilibrium, two-domain (liquid and vapor), physics based, mathematical model is developed to investigate the onset and growth of the natural circulation and thermal stratification inside cryogenic propellant storage tanks due to heat transfer from the surroundings. A two-dimensional (planar) model is incorporated for the liquid domain while a lumped, thermodynamic model is utilized for the vapor domain. The mathematical model in the liquid domain consists of the conservation of mass, momentum, and energy equations and incorporates the Boussinesq approximation (constant fluid density except in the buoyancy term of the momentum equation). In addition, the vapor is assumed to behave like an ideal gas with uniform thermodynamic properties. Furthermore, the time-dependent nature of the heat leaks from the surroundings to the propellant (due to imperfect tank insulation) is considered. Also, heterogeneous nucleation, although not significant in the temperature range of study, has been included. The transport of mass and energy between the liquid and vapor domains leads to transient ullage vapor temperatures and pressures. (The latter of which affects the saturation temperature of the liquid at the liquid-vapor interface.) This coupling between the two domains is accomplished through an energy balance (based on a micro-layer concept) at the interface. The resulting governing, non-linear, partial differential equations (which include a Poisson's equation for determining the pressure distribution) in the liquid domain are solved by an implicit, finite-differencing technique utilizing a non-uniform (stretched) mesh (in both directions) for predicting the velocity and temperature fields. (The accuracy of the numerical scheme is validated by comparing the model's results to a benchmark numerical case as well as to available experimental data.) The mass, temperature, and pressure of the vapor is determined by using a simple explicit finite
Natural convective mixing flows
NASA Astrophysics Data System (ADS)
Ramos, Eduardo; de La Cruz, Luis; del Castillo, Luis
1998-11-01
Natural convective mixing flows. Eduardo Ramos and Luis M. de La Cruz, National University of Mexico and Luis Del Castillo San Luis Potosi University. The possibility of mixing a fluid with a natural convective flow is analysed by solving numerically the mass, momentum and energy equations in a cubic container. Two opposite vertical walls of the container are assumed to have temperatures that oscillate as functions of time. The phase of the oscillations is chosen in such a way that alternating corrotating vortices are formed in the cavity. The mixing efficiency of this kind of flow is examined with a Lagrangian tracking technique. This work was partially financed by CONACyT-Mexico project number GE0044
Siu, S.; Evans, J.W.
1997-08-01
In many electrochemical cells, the flow of electrolyte has an influence on cell behavior and this investigation concerns a cell (a zinc-air cell) where that flow occurred through natural convection. The zinc was present in the form of a bed of particles, connected at its top and bottom with channels forming reservoirs of electrolyte. Dissolution of the zinc caused density differences between electrolyte in the bed interstices and that in the reservoir. In Part 1 of this two-part paper, a mathematical model for this cell is developed. The model employs the well-known Newman/Tobias description of a porous electrode and treats flow through the bed using the Blake-Kozeny equation. A fourth-order Lax-Wendroff algorithm, thought to be original, is used to solve the convective diffusion equation within the model. Sample computed results are presented.
NASA Astrophysics Data System (ADS)
Varol, Yasin; Öztop, Hakan F.; Özgen, Filiz; Koca, Ahmet
2012-01-01
Natural convection heat transfer in an inclined fin attached square enclosure is studied both experimentally and numerically. Bottom wall of enclosure has higher temperature than that of top wall while vertical walls are adiabatic. Inclined fin has also adiabatic boundary conditions. Numerical solutions have been done by writing a computer code in Fortran platform and results are compared with Fluent commercial code and experimental method. Governing parameters are Rayleigh numbers (8.105 ≤ Ra ≤ 4 × 106) and inclination angle (30° ≤ and ≤ 120°). The temperature measurements are done by using thermocouples distributed uniformly at the wall of the enclosure. Remarkably good agreement is obtained between the predicted results and experimental data. A correlation is also developed including all effective parameters on heat transfer and fluid flow. It was observed that heat transfer can be controlled by attaching an inclined fin onto wall.
Natural convection in low-g environments
NASA Technical Reports Server (NTRS)
Grodzka, P. G.; Bannister, T. C.
1974-01-01
The present state of knowledge in the area of low-g natural convection is reviewed, taking into account a number of experiments conducted during the Apollo 14, 16, and 17 space flights. Convections due to steady low-g accelerations are considered. Steady g-levels result from spacecraft rotation, gravity gradients, solar wind, and solar pressure. Varying g-levels are produced by engine burns, attitude control maneuvers, and onboard vibrations from machinery or astronaut movement. Thermoacoustic convection in a low-g environment is discussed together with g-jitter convection, surface tension-driven convection, electrohydrodynamics under low-g conditions, phase change convection, and approaches for the control and the utilization of convection in space.
Heat distribution by natural convection
Balcomb, J.D.
1985-01-01
Natural convection can provide adequate heat distribution in many situtations that arise in buildings. This is appropriate, for example, in passive solar buildings where some rooms tend to be more strongly solar heated than others or to reduce the number of heating units required in a building. Natural airflow and heat transport through doorways and other internal building apertures is predictable and can be accounted for in the design. The nature of natural convection is described, and a design chart is presented appropriate to a simple, single-doorway situation. Natural convective loops that can occur in buildings are described and a few design guidelines are presented.
Angirasa, D.; Srinivasan, J. )
1989-08-01
This paper presents a numerical study of laminar doubly diffusive free convection flows adjacent to a vertical surface in a stable thermally stratified medium. The two buoyant mechanisms are thermal diffusion and species diffusion. The species concentration is assumed to be small. Boussinesq approximations are incorporated and the governing conservation equations of mass, momentum, energy, and species are nondimensionalized. These equations are solved using a finite-difference method. The results are explained in terms of the basic physical mechanisms that govern these flows. It is observed that the ambient thermal stratification has a profound influence on the transport characteristics. The results show many interesting aspects of the complex interaction of the two buoyant mechanisms.
Self-propulsion via natural convection
NASA Astrophysics Data System (ADS)
Ardekani, Arezoo; Mercier, Matthieu; Allshouse, Michael; Peacock, Thomas
2014-11-01
Natural convection of a fluid due to a heated or cooled boundary has been studied within a myriad of different contexts due to the prevalence of the phenomenon in environmental systems such as glaciers, katabatic winds, or magmatic chambers; and in engineered problems like natural ventilation of buildings, or cooling of electronic components. It has, however, hitherto gone unrecognized that boundary-induced natural convection can propel immersed objects. We experimentally investigate the motion of a wedge-shaped object, immersed within a two-layer fluid system, due to a heated surface. The wedge resides at the interface between the two fluid layers of different density, and its concomitant motion provides the first demonstration of the phenomenon of propulsion via boundary-induced natural convection. Established theoretical and numerical models are used to rationalize the propulsion speed by virtue of balancing the propulsion force against the appropriate drag force. We successfully verified the influence of various fluid and heat parameters on the predicted speed. now at IMFT (Institut de Mécanique des Fluides de Toulouse).
Bifurcations and unfoldings in natural convection
Decker, W.J.; Dorning, J.
1996-12-31
Extensive numerical studies of bifurcations and unfoldings have been carried out for two important problems in natural convection. These are (a) the Rayleigh-Benard convection (RBC) problem-a rectangular cavity, with insulated sidewalls, heated at constant uniform temperature along the bottom and cooled at constant uniform temperature along the top; and (b) the volumetric heating convection (VHC) problem - a rectangular cavity, with insulated sidewalls and bottom, heated by a constant uniform volumetric heat source and cooled at constant uniform temperature along the top. The information available in the literature on RBC was used to evaluate and justify the approximations made in the current research, which has shed additional light on nonlinear phenomena in RBC and led to new basic information on the bifurcations and unfoldings that occur in VHC for which there were essentially no previous results available. Both problems arise in many important technological and scientific contexts, including reactor safety analysis and meteorological phenomena. In particular, VHC is relevant to the development of an understanding of the natural convective motion driven by the radioactive decay heat in the molten core mixture (corium) in the core catcher following a hypothetical reactor core meltdown accident and of that which occurs in the atmosphere due to the deposition of radiant solar energy. The calculations were done using newly developed versions of the nodal integral method (NIM) for steady-state flows in conjunction with extended system methods for numerical bifurcation analysis for the saddle-node and pitchfork bifurcation computations.
Natural convection in porous media
Prasad, V.; Hussain, N.A.
1986-01-01
This book presents the papers given at a conference on free convection in porous materials. Topics considered at the conference included heat transfer, nonlinear temperature profiles and magnetic fields, boundary conditions, concentrated heat sources in stratified porous media, free convective flow in a cavity, heat flux, laminar mixed convection flow, and the onset of convection in a porous medium with internal heat generation and downward flow.
Energy transport using natural convection boundary layers
Anderson, R
1986-04-01
Natural convection is one of the major modes of energy transport in passive solar buildings. There are two primary mechanisms for natural convection heat transport through an aperture between building zones: (1) bulk density differences created by temperature differences between zones; and (2) thermosyphon pumping created by natural convection boundary layers. The primary objective of the present study is to compare the characteristics of bulk density driven and boundary layer driven flow, and discuss some of the advantages associated with the use of natural convection boundary layers to transport energy in solar building applications.
Natural convection between concentric spheres
NASA Technical Reports Server (NTRS)
Garg, Vijay K.
1992-01-01
A finite-difference solution for steady natural convective flow in a concentric spherical annulus with isothermal walls has been obtained. The stream function-vorticity formulation of the equations of motion for the unsteady axisymmetric flow is used; interest lying in the final steady solution. Forward differences are used for the time derivatives and second-order central differences for the space derivatives. The alternating direction implicit method is used for solution of the discretization equations. Local one-dimensional grid adaptation is used to resolve the steep gradients in some regions of the flow at large Rayleigh numbers. The break-up into multi-cellular flow is found at high Rayleigh numbers for air and water, and at significantly low Rayleigh numbers for liquid metals. Excellent agreement with previous experimental and numerical data is obtained.
Suppression of Natural Convection in a Thermoacoustic Pulse Tube Refrigerator
NASA Astrophysics Data System (ADS)
Han, Jun-Qing; Liu, Qiu-Sheng
2013-05-01
The effects of gravity on the efficiency of thermoacoustic engines are investigated theoretically and experimentally, especially for thermoacoustic pulse tube refrigerators. The significant effects of gravity are found to be due to the presence of natural convection in the thermoacoustic pulse tube when the hot side of the tube is lower than the cold side. This kind of natural convection influences and reduces the efficiency of the thermoacoustic working system. Thus, how to suppress this natural convection becomes important for increasing the efficiency of thermoacoustic engines. Unlike the method of inserting a silk screen in a pulse tube, the present study uses a numerical simulation method to research the natural convection in pulse tubes, and we try to change the shape of the pulse tube to suppress this convection.
NASA Astrophysics Data System (ADS)
Javed, Tariq; Siddiqui, Muhammad Arshad; Mehmood, Ziafat; Pop, Ioan
2015-10-01
In this article, numerical simulations are carried out for fluid flow and heat transfer through natural convection in an isosceles triangular cavity under the effects of uniform magnetic field. The cavity is of cold bottom wall and uniformly/non-uniformly heated side walls and is filled with isotropic porous medium. The governing Navier Stoke's equations are subjected to Penalty finite element method to eliminate pressure term and Galerkin weighted residual method is applied to obtain the solution of the reduced equations for different ranges of the physical parameters. The results are verified as grid independent and comparison is made as a limiting case with the results available in literature, and it is shown that the developed code is highly accurate. Computations are presented in terms of streamlines, isotherms, local Nusselt number and average Nusselt number through graphs and tables. It is observed that, for the case of uniform heating side walls, strength of circulation of streamlines gets increased when Rayleigh number is increased above critical value, but increase in Hartmann number decreases strength of streamlines circulations. For non-uniform heating case, it is noticed that heat transfer rate is maximum at corners of bottom wall.
Solar Hot Water Heating by Natural Convection.
ERIC Educational Resources Information Center
Noble, Richard D.
1983-01-01
Presents an undergraduate laboratory experiment in which a solar collector is used to heat water for domestic use. The working fluid is moved by natural convection so no pumps are required. Experimental apparatus is simple in design and operation so that data can be collected quickly and easily. (Author/JN)
A Simple Classroom Demonstration of Natural Convection
ERIC Educational Resources Information Center
Wheeler, Dean R.
2005-01-01
This article explains a simple way to demonstrate natural convection, such as from a lit candle, in the classroom using an overhead projector. The demonstration is based on the principle of schlieren imaging, commonly used to visualize variations in density for gas flows.
Uncertainties in future climate predictions due to convection parameterisations
NASA Astrophysics Data System (ADS)
Rybka, H.; Tost, H.
2013-10-01
In the last decades several convection parameterisations have been developed to consider the impact of small-scale unresolved processes in Earth System Models associated with convective clouds. Global model simulations, which have been performed under current climate conditions with different convection schemes, significantly differ among each other in the simulated transport of trace gases and precipitation patterns due to the parameterisation assumptions and formulations, e.g. the simplified treatment of the cloud microphysics. Here we address sensitivity studies comparing four different convection schemes under alternative climate conditions (doubling of the CO2 concentrations) to identify uncertainties related to convective processes. The increase in surface temperature reveals regional differences up to 4 K dependent on the chosen convection parameterisation. The increase in upper tropospheric temperature affects the amount of water vapour transported to the lower stratosphere. Furthermore, the change in transporting short-lived pollutants within the atmosphere is highly ambiguous for the lower and upper troposphere. Finally, cloud radiative effects have been analysed uncovering a shift in different cloud types in the tropics.
Heterogeneous nanofluids: natural convection heat transfer enhancement
2011-01-01
Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account the thermal conductivity, viscosity versus nanofluids type and concentration and the spatial heterogeneous concentration induced by the Soret effect is presented. The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach. The resulting convective flows are found to be dependent on the local particle concentration φ and the corresponding solutal to thermal buoyancy ratio N. The induced nanofluid heterogeneity showed a significant heat transfer modification. The heat transfer in natural convection increases with nanoparticle concentration but remains less than the enhancement previously underlined in forced convection case. PMID:21711755
The Phenix ultimate natural convection test
Gauthe, P.; Pialla, D.; Tenchine, D.; Vasile, A.; Rochwerger, D.
2012-07-01
The French sodium cooled fast reactor Phenix was shut down in 2009 after 35 years of operation. Before decommissioning, a final set of tests were performed by the CEA during 9 months. Several topics were involved such as thermal hydraulics, core physics and fuel behaviour. Among these ultimate experiments, two thermal hydraulic tests were performed: an asymmetrical test consisting in a trip of one secondary pump and a natural convection test in the primary circuit. Recognizing the unique opportunity offered by these Phenix ultimate tests, IAEA decided in 2007 to launch a Coordinated Research Project (CRP) devoted to benchmarking analyses with system codes on the Phenix natural convection test. One objective of the natural convection test in Phenix reactor is the assessment of the CATHARE system code for safety studies on future and advanced sodium cooled fast reactors. The aim of this paper is to describe this test, which was performed on June 22-23, 2009, and the associated benchmark specifications for the CRP work. The paper reminds briefly the Phenix reactor with the main physical parameters and the instrumentation used during the natural convection test. After that, the test scenario is described: - initial state at a power of 120 MWth, - test beginning resulting from a manual dry out of the two steam generators, - manual scram, - manual trip on the three primary pumps without back-up by pony motors, - setting and development of natural convection in the primary circuit, in a first phase without significant heat sink in the secondary circuits and in a second phase with significant heat sink in the secondary circuits, by opening the casing of steam generators to create an efficient heat sink, by air natural circulation in the steam generators casing. The benchmark case ends after this second phase, which corresponds to the experimental test duration of nearly 7 hours. The paper presents also the benchmark specifications data supplied by the CEA to all
Laminar natural convection in right triangular enclosures
NASA Astrophysics Data System (ADS)
Fuad Kent, E.; Asmaz, E.; Ozerbay, S.
2007-12-01
In this study, natural convection in non-rectangular enclosures is analyzed numerically. Streamlines and isotherms are presented for different triangular enclosures with different boundary conditions and Rayleigh numbers. Mean Nusselt numbers on hot walls are also calculated in order to make comparisons between different cases. The solutions are obtained for different aspect ratios where boundary conditions represent the wintertime heating of an attic space. This made possible to investigate the effect of aspect ratio on natural convection. In this study, quarter circular enclosure, which is very similar to right triangles, is also examined. Consequently, we had the opportunity to analyze how shape changes affect the flow pattern. The results of the calculations are compared with the similar enclosures and boundary conditions.
Unsteady natural convection in micropolar nanofluids
NASA Astrophysics Data System (ADS)
Rup, Kazimierz; Nering, Konrad
2014-09-01
This paper presents the analysis of momentum, angular momentum and heat transfer during unsteady natural convection in micropolar nanofluids. Selected nanofluids treated as single phase fluids contain small particles with diameter size 10-38.4 nm. In particular three water-based nanofluids were analyzed. Volume fraction of these solutions was 6%. The first of the analyzed nanofluids contained TiO2 nanoparticles, the second one contained Al2O3 nanoparticles, and the third one the Cu nanoparticles.
Natural convective heat transfer from square cylinder
NASA Astrophysics Data System (ADS)
Novomestský, Marcel; Smatanová, Helena; Kapjor, Andrej
2016-06-01
This article is concerned with natural convective heat transfer from square cylinder mounted on a plane adiabatic base, the cylinders having an exposed cylinder surface according to different horizontal angle. The cylinder receives heat from a radiating heater which results in a buoyant flow. There are many industrial applications, including refrigeration, ventilation and the cooling of electrical components, for which the present study may be applicable
Natural convection in a uniformly heated pool
Tzanos, C.P.
1996-05-01
In the event of a core meltdown accident, to prevent reactor vessel failure from molten corium relocation to the reactor vessel lower head, the establishment of a coolable configuration has been proposed by flooding with water the reactor cavity. In Reference 3, it was shown that for the heavy-water new production reactor (NPW-HWR) design, this strategy, e.g., the rejection of decay heat to a containment decay heat removal system by boiling of water in the reactor cavity, could keep the reactor vessel temperature below failure limits. The analysis of Ref. 3 was performed with the computer code COMMIX-1AR/P, and showed that natural convection in the molten-corium pool was the dominant mechanism of heat transfer from the pool to the wall of the reactor vessel lower head. To determine whether COMMIX adequately predicts natural convection in a pool heated by a uniform heat source, in Ref. 4, the experiments of free convection in a semicircular cavity of Jahn and Reineke were analyzed with COMMIX. It was found that the Nusselt (Nu) number predicted by COMMIX was within the spread of the experimental measurements. In the COMMIX analysis of Ref. 4, the semicircular cavity was treated as symmetric. The objective of the work presented in this paper was to extend the COMMIX validation analysis of Ref. 4 by removing the assumption of symmetry and expanding the analysis up to the highest Rayleigh (Ra) number that leads to a steady state. In conclusion, this work shows that the numerical predictions of natural convection in an internally heated pool bounded by a curved bottom are in reasonably good agreement with experimental measurements.
Combined natural convection and radiation in a triangular enclosure
NASA Astrophysics Data System (ADS)
Hasani, Syed Muhammad Fakhir
The problem of combined natural convection and radiation heat transfer of a gray absorbing-emitting and isotropically scattering medium in a triangular enclosure is solved numerically in this dissertation. Interactions of natural convection and radiation occur in many engineering applications such as electronic cooling, solar heating, crystal growth, fire propagation etc. The radiation effect was neglected from many of the previous studies due to the complexities associated with radiation modeling. The triangular enclosure considered in the present study has been used by researchers in the past to model flows inside attic spaces, solar stills and near shore water circulation in lakes and rivers. Previous pure natural convection studies in this geometry had produced conflicting results and the effect of radiation was not considered prior to this study. The objectives of this study are: (i) to obtain a numerical solution for the combined natural convection and radiation problem in a triangular enclosure, (ii) to study the influence of radiation on thermal instabilities present in pure natural convection flow in this geometry, (iii) to perform parametric study and (iv) to verify the results of pure natural convection flow using more accurate QUICK scheme. The problem is mathematically formulated and a Fortran computer program is developed to meet the desired objectives. The two dimensional stream function equation, the time-dependent vorticity transport and energy equations, and the radiative transport equation are solved simultaneously for uniform temperature boundary conditions. Two different sets of boundary conditions are employed with inclined surfaces considered hot for one and cold for the other. The stream function equation is solved using successive overrelaxation whereas the vorticity transport and energy equations are solved using third-order upwinding QUICK scheme while the radiative transport solution is sought by means of Discrete Ordinates Method. Two
Modeling PCR in Natural Convection Systems
NASA Astrophysics Data System (ADS)
Dorfman, Kevin; Yariv, Ehud; Ben Dov, Guy
2007-03-01
Polymerase chain reaction (PCR) is a biochemical protocol for making many copies of a DNA template by thermal cycling between a hot temperature (where the strands are separated) and a cool temperature (where primers are annealed). In natural convection PCR, the requisite thermal cycling is provided by a buoyancy-driven circulating flow of the carrying buffer between a lower hot plate (at the denaturing temperature) and an upper cold plate (at the annealing temperature). We present a multi-component convection-diffusion-reaction model for natural convection-driven PCR when both primers and PCR enzyme are in excess. The evolution of the DNA population achieves a stationary state, wherein the problem is recast as an eigenvalue problem for computing the exponential amplification rate. With a realistic choice of parameters, the model predicts a doubling time on the order of two minutes, in agreement with experiments and much slower than the fluid cycling time. In contrast to what might be expected, the doubling time increases monotonically with the diffusion coefficient.
Laminar natural convection under nonuniform gravity.
NASA Technical Reports Server (NTRS)
Lienhard, J.; Eichhorn, R.; Dhir, V.
1972-01-01
Laminar natural convection is analyzed for cases in which gravity varies with the distance from the leading edge of an isothermal plate. The study includes situations in which gravity varies by virtue of the varying slope of a surface. A general integral solution method which includes certain known integral solutions as special cases is developed to account for arbitrary position-dependence of gravity. A series method of solution is also developed for the full equations. Although it is more cumbersome it provides verification of the integral method.
Natural convection in a uniformly heated pool
Tzanos, C.P.
1996-12-31
To prevent reactor vessel failure from molten corium relocation to the reactor vessel lower head in the event of a core meltdown accident, the establishment of a coolable configuration has been proposed by flooding the reactor cavity with water. In Ref. 3, it was shown that for the heavy-water new production reactor (NPW-HWR) design, this strategy (e.g., the rejection of decay heat to a containment decay heat removal system by boiling of water in the reactor cavity) could keep the reactor vessel temperature below failure limits. The analysis of Ref. 3 was performed with the COMMIX-IAR/P computer code and showed that natural convection in the molten-corium pool was the dominant mechanism of heat transfer from the pool to the wall of the reactor vessel lower head. COMMIX is a general-purpose thermal-hydraulics code based on finite differencing by the first-order upwind scheme. To determine whether COMMIX adequately predicts natural convection in a pool heated by a uniform heat source, in Ref. 4, the experiments of free convection in a semicircular cavity of Jahn and Reineke were analyzed with COMMIX in Ref. 5. It was found that the Nusselt number predicted by COMMIX was within the spread of the experimental measurements. In the COMMIX analysis of Ref. 5, the semicircular cavity was treated as symmetric. The objective of this paper was to extend the COMMIX validation analysis of Ref. 5 by removing the assumption of symmetry and expanding the analysis from the highest Rayleigh number of the experiments of Ref. 4 to the highest Rayleigh number that leads to a steady state.
Thermocapillary Convection Due to a Stationary Bubble - A Paradox
NASA Technical Reports Server (NTRS)
Balasubramaniam, R.; Subramanian, R. S.
2003-01-01
We analyze the velocity and temperature fields at steady state due to thermocapillary convection around a gas bubble that is stationary in a liquid. A linear temperature field is imposed in the undisturbed liquid. Our interest is in investigating the effect of convective transport of momentum and energy on the velocity and temperature fields. We assume the pertinent physical properties to be constant, and that buoyant convection is negligible. Suitably defined Reynolds and Marangoni numbers are assumed to be small compared with unity. When both the Reynolds and Marangoni numbers are set equal to zero, a solution can be found. In this solution, far from the bubble, the velocity field decays as the inverse of the distance from the bubble, and the disturbance temperature field decays as the inverse of the square of this distance. We now attempt to obtain a solution when the Reynolds number is zero, but the Marangoni number is small, but non-zero, by a perturbation expansion in the Marangoni number. When the temperature field is expanded in a regular perturbation series in the Marangoni number, we show that the problem for the first correction field is ill-posed. The governing equation for this perturbation field contains an inhomogeneity, and the corresponding particular solution neither decays far from the bubble, nor can be canceled by a homogeneous solution. Additional information is included in the original extended abstract.
Transport in vertical mixed convection flows and natural convection flows in cold water
NASA Astrophysics Data System (ADS)
Carey, V. P.
Computed similarity solutions are presented for thermally-driven natural convection flow adjacent to a vertical isothermal surface in cold pure or saline water. These calculations specifically explore the flow behavior at temperature conditions for which the buoyancy force reverses across the thermal transport region due to the presence of a density extremum within the region. Computed similarity solutions are given for the laminar natural convection flow adjacent to a vertical ice surface melting in saline water. The most recent transport property data and a very accurate equation of state for saline water are used to analyze the transport of momentum, salt and thermal energy in such flows. Interface motion effects are included and the interface conditions are determined from the transport. Time exposure photographs of the flow adjacent to a vertical ice surface melting in 10% saline water are presented for ambient water temperatures between 1 C and 15 C. A perturbation analysis is presented of mixed convection flow over a vertical semi infinite surface with uniform heat flux.
Natural convection in C-shaped thermosyphon
Mohamad, A.A.; Sezai, I.
1997-08-29
The present work is concerned with the numerical analysis of natural convection from a C-shaped thermosyphon. The system can be considered as a model for a Trombe wall in passive solar collectors as well as electronic cooling arrangements and it has geophysical applications. The effect of Rayleigh number (1 {times} 10{sup 3} to 1 {times} 10{sup 7}) and aspect ratios of 2.0, 4.0, 6.0, and 10.0 is investigated for a fixed Prandtl number (0.7). Local and average Nusselt numbers for heated and cold walls are discussed. Mass advected by the buoyancy force is calculated and presented for the range of investigated parameters. The flow pattern and isotherm distribution in the gap between the wall and cover plate are presented and discussed.
Conjugate natural convection between horizontal eccentric cylinders
NASA Astrophysics Data System (ADS)
Nasiri, Davood; Dehghan, Ali Akbar; Hadian, Mohammad Reza
2016-06-01
This study involved the numerical investigation of conjugate natural convection between two horizontal eccentric cylinders. Both cylinders were considered to be isothermal with only the inner cylinder having a finite wall thickness. The momentum and energy equations were discretized using finite volume method and solved by employing SIMPLER algorithm. Numerical results were presented for various solid-fluid conductivity ratios (KR) and various values of eccentricities in different thickness of inner cylinder wall and also for different angular positions of inner cylinder. From the results, it was observed that in an eccentric case, and for KR < 10, an increase in thickness of inner cylinder wall resulted in a decrease in the average equivalent conductivity coefficient (overline{{K_{eq} }} ); however, a KR > 10 value caused an increase in overline{{K_{eq} }} . It was also concluded that in any angular position of inner cylinder, the value of overline{{K_{eq} }} increased with increase in the eccentricity.
Prandtl Number Dependent Natural Convection with Internal Heat Sources
Kang Hee Lee; Seung Dong Lee; Kune Y. Suh; Joy L. Rempe; Fan-Bill Cheung; Sang B. Kim
2004-06-01
Natural convection plays an important role in determining the thermal load from debris accumulated in the reactor vessel lower head during a severe accident. Recently, attention is being paid to the feasibility of external vessel flooding as a severe accident management strategy and to the phenomena affecting the success path for retaining the molten core material inside the vessel. The heat transfer inside the molten core material can be characterized by the strong buoyancy-induced flows resulting from internal heating due to decay of fission products. The thermo-fluid dynamic characteristics of such flow depend strongly on the thermal boundary conditions. The spatial and temporal variation of heat flux on the pool wall boundaries and the pool superheat are mainly characterized by the natural convection flow inside the molten pool. In general, the natural convection heat transfer phenomena involving the internal heat generation are represented by the modified Rayleigh number (Ra’), which quantifies the internal heat source and hence the strength of the buoyancy force. In this study, tests were conducted in a rectangular section 250 mm high, 500 mm long and 160 mm wide. Twenty-four T-type thermocouples were installed in the test section to measure temperatures. Four T-type thermocouples were used to measure the boundary temperatures. The thermocouples were placed in designated locations after calibration. A direct heating method was adopted in this test to simulate the uniform heat generation. The experiments covered a range of Ra' between 1.5x106 and 7.42x1015 and the Prandtl number (Pr) between 0.7 and 6.5. Tests were conducted with water and air as simulant. The upper and lower boundary conditions were maintained uniform. The results demonstrated feasibility of the direct heating method to simulate uniform volumetric heat generation. Particular attentions were paid to the effect of Pr on natural convection heat transfer within the rectangular pool.
Natural convection in a fluid layer periodically heated from above.
Hossain, M Z; Floryan, J M
2014-08-01
Natural convection in a horizontal layer subject to periodic heating from above has been studied. It is shown that the primary convection leads to the cooling of the bulk of the fluid below the mean temperature of the upper wall. The secondary convection may lead either to longitudinal rolls, transverse rolls, or oblique rolls. The global flow properties (e.g., the average Nusselt number for the primary convection and the critical conditions for the secondary convection) are identical to those of the layer heated from below. However, the flow and temperature patterns exhibit phase shifts in the horizontal directions.
Natural Convection in Enclosed Porous or Fluid Media
ERIC Educational Resources Information Center
Saatdjian, Esteban; Lesage, François; Mota, José Paulo B.
2014-01-01
In Saatdjian, E., Lesage, F., and Mota, J.P.B, "Transport Phenomena Projects: A Method to Learn and to Innovate, Natural Convection Between Porous, Horizontal Cylinders," "Chemical Engineering Education," 47(1), 59-64, (2013), the numerical solution of natural convection between two porous, concentric, impermeable cylinders was…
Onset of Convection Due to Surface Tension Variations in Multicomponent and Binary Fluid Layers
NASA Technical Reports Server (NTRS)
Skarda, J. Raymond Lee
2000-01-01
Under certain conditions, such as in thin liquid films or microgravity, surface tension variations along a free surface can induce convection. Convection onset due to surface tension variation is important to many terrestrial technological processes in addition to microgravity materials processing applications. Examples include coating, drying crystallization, solidification, liquid surface contamination, and containerless processing. In double-diffusive and multicomponent systems, the spatial variations of surface tension are associated with two or more stratifying agencies, respectively. For example, both temperature and species (concentration) gradients are associated with convection in the solidification of binary alloys or salt ponds. The direction of the two (or more) gradients has a profound effect on the nature of the flow at or slightly beyond the onset of convection. Our recent work at the NASA Lewis Research Center focused on characterizing surface-tension-induced onset of convection, often referred to as Marangoni-Benard convection. Exact solutions for the stationary neutral stability of multicomponent fluid layers with interfacial deformation were derived. These solutions also permit the computation of a boundary curve that separates the long and finite wavelength instabilities. Computing points along this boundary using the exact solution (when possible) is more efficient than the typical numerical approaches, such as finite difference or spectral methods. Above the curve, a long wavelength instability was predicted, suggesting that convection would occur principally through one large flow cell in the layer, whereas below the curve, finite wavelength instabilities occur which suggest multiple finite-sized circulation cells. For many common liquids with layer depths greater than 100 mm, finite wave instability is predicted under terrestrial conditions; however, with little exception, long wavelength instability is predicted in microgravity for the
Transient natural convection in heated inclined tubes
McEligot, D.M. . Oceanic Div.); Denbow, D.A. ); Murphy, H.D. )
1990-05-01
To simulate natural convection flow patterns in directionally drilled wellbores, experiments and analyses were conducted for a circular tube with length-to-diameter (L/D) ratio of 36 at angles of 0{degree}, 20{degree}, and 35{degree} from the vertical. The tube was heated at the bottom and cooled at the top, and the insulation was adjusted so that approximately one- to two-thirds of the power dissipated was transferred through the tube wall to the surroundings. An aqueous solution of polyvinyl alcohol was employed as the working fluid in order to obtain low Rayleigh numbers corresponding to conditions in geothermal wellbores. Results were primarily qualitative but were useful in providing insight into the phenomena occurring. Steady-state temperature distributions were measured for the three orientations and for several heating rates to demonstrate the effects of tube angle and Rayleigh number. transient measurements of the temperature distribution were obtained during cooling from a higher temperature without a heat source to calibrate the heat losses. With the electrical heat source, temporal data were taken during heating to examine the approach to steady state. Quasi-steady flow conditions were approached rapidly, but the overall time constant of the apparatus was of the order of one-third of a day. Predictions with the three-dimensional TEMPEST code were first tested by comparison with simple conduction analyses. Comparison with actual data showed good agreement of the predicted temperature levels for the maximum inclination, 35{degree}, and slightly poorer agreement for the other limit, a vertical tube. Trends of temperature level and Nusselt number with heating rate or Rayleigh number were reasonable, but the predicted variation of the end Nusselt number versus inclination was in the opposite direction from the experiment. 75 refs., 20 figs., 8 tabs.
NASA Astrophysics Data System (ADS)
Aminuddin Aftab, Syed Mohammed; Younis, Obai; Al-Atabi, Musthak
2012-09-01
Natural convection in cavities has been a field of interest to researchers over the past 50 years. One of the basic techniques used to investigate the natural convection in cavities is shadowgraph flow visualisation, used to experimentally observe the boundary layer growth, formation of double layer structures, intrusions and plume generation. This paper accounts for Various other observations also made when fins are placed along the side wall of the cavity, the effect due to change in shape and orientation of the cavity, how these changes effect the natural convection have also been discussed. The applications of various types of shadowgraph methods to understand the flow variation with density have also been included. The paper summarizes the literature of shadowgraph techniques in natural convection.
A numerical study of natural convection in a narrow annulus
NASA Astrophysics Data System (ADS)
Sahai, V.
1991-12-01
Various numerical models were used to predict the natural convection of a solidifying liquid metal in a narrow annulus. Previous work in this area does not consider the temperature variation that exists in the fluid and the resulting heat conduction in the solid mold material. The finite element fluid dynamics code FIDAP was used to solve these models. The results indicate that the natural convective effects are small.
Analysis of natural convection in a low gravity environment
NASA Technical Reports Server (NTRS)
Mattor, Ethan E.; Durgin, William W.; Bloznalis, Peter; Schoenberg, Richard
1992-01-01
Natural convection inside a spherical container was studied experimentally with two apparatuses at low buoyancy levels. The data generated by these experiments, plotted nondimensionally as the Nusselt versus Rayleigh numbers, give correlations for Rayleigh numbers between 1000 and 10 exp 8, a range previously untested. These results show that natural convection has significant effects at a Rayleigh number of 1000 and higher, although the behavior of the Nusselt number as the conduction limit is approached is still unknown for a spherical geometry.
Nature versus nurture in shallow convection
NASA Astrophysics Data System (ADS)
Romps, D. M.; Kuang, Z.
2009-12-01
We use tracers in a large-eddy simulation of shallow convection to show that stochastic entrainment, not cloud-base properties, determine the fate of convecting parcels. The tracers are used to diagnose the correlations between a parcel's state above the cloud base and both the parcel's state at the cloud base and its entrainment history. We find that the correlation with the cloud-base state goes to zero a few hundred meters above the cloud base. On the other hand, correlations between a parcel's state and its net entrainment are large. Evidence is found that the entrainment events may be described as a stochastic Poisson process. We construct a parcel model with stochastic entrainment that is able to replicate flux profiles and, more importantly, the observed variability. Turning off cloud-base variability has little effect on the results, which suggests that stochastic mass-flux models may be initialized with a single set of properties. The success of the stochastic parcel model suggests that it holds promise as the framework for a convective parameterization.
Double Diffusive Natural Convection in a Nuclear Waste Repository
Hao, Y; Nitao, J; Buscheck, T A; Sun, Y
2006-02-03
In this study, we conduct a two-dimensional numerical analysis of double diffusive natural convection in an emplacement drift for a nuclear waste repository. In-drift heat and moisture transport is driven by combined thermal- and compositional-induced buoyancy forces. Numerical results demonstrate buoyancy-driven convective flow patterns and configurations during both repository heat-up and cool-down phases. It is also shown that boundary conditions, particularly on the drip-shield surface, have strong impacts on the in-drift convective flow and transport.
Double Diffusive Natural Convection in a Nuclear Waste Repository
Hao, Y; Nitao, J J; Buscheck, T A; Sun, Y
2006-07-24
In this study, we conduct a two dimensional numerical analysis of double diffusive natural convection in an emplacement drift for a nuclear waste repository. In-drift heat and moisture transport is driven by combined thermal- and compositional-induced buoyancy forces. Numerical results demonstrate buoyancy-driven convective flow patterns and configurations during both repository heat-up and cool-down phases. It is also shown that boundary conditions, particularly on the drip-shield surface, have a strong impact on in-drift convective flow and transport.
Double Diffusive Natural Convection in a Nuclear Waste Repository
Y. Hao; J. Nitao; T.A. Buscheck; Y. Sun
2006-03-28
In this study, we conduct a two-dimensional numerical analysis of double diffusive natural convection in an emplacement drift for a nuclear waste repository. In-drift heat and moisture transport is driven by combined thermal- and compositional-induced buoyancy forces. Numerical results demonstrate buoyancy-driven convective flow patterns and configurations during both repository heat-up and cool-down phases. It is also shown that boundary conditions, particularly on the drip-shield surface, have strong impacts on the in-drift convective flow and transport.
A decoupled monolithic projection method for natural convection problems
NASA Astrophysics Data System (ADS)
Pan, Xiaomin; Kim, Kyoungyoun; Lee, Changhoon; Choi, Jung-Il
2016-06-01
We propose an efficient monolithic numerical procedure based on a projection method for solving natural convection problems. In the present monolithic method, the buoyancy, linear diffusion, and nonlinear convection terms are implicitly advanced by applying the Crank-Nicolson scheme in time. To avoid an otherwise inevitable iterative procedure in solving the monolithic discretized system, we use a linearization of the nonlinear convection terms and approximate block lower-upper (LU) decompositions along with approximate factorization. Numerical simulations demonstrate that the proposed method is more stable and computationally efficient than other semi-implicit methods, preserving temporal second-order accuracy.
Natural convection heat transfer within horizontal spent nuclear fuel assemblies
Canaan, R.E.
1995-12-01
Natural convection heat transfer is experimentally investigated in an enclosed horizontal rod bundle, which characterizes a spent nuclear fuel assembly during dry storage and/or transport conditions. The basic test section consists of a square array of sixty-four stainless steel tubular heaters enclosed within a water-cooled rectangular copper heat exchanger. The heaters are supplied with a uniform power generation per unit length while the surrounding enclosure is maintained at a uniform temperature. The test section resides within a vacuum/pressure chamber in order to subject the assembly to a range of pressure statepoints and various backfill gases. The objective of this experimental study is to obtain convection correlations which can be used in order to easily incorporate convective effects into analytical models of horizontal spent fuel systems, and also to investigate the physical nature of natural convection in enclosed horizontal rod bundles in general. The resulting data consist of: (1) measured temperatures within the assembly as a function of power, pressure, and backfill gas; (2) the relative radiative contribution for the range of observed temperatures; (3) correlations of convective Nusselt number and Rayleigh number for the rod bundle as a whole; and (4) correlations of convective Nusselt number as a function of Rayleigh number for individual rods within the array.
Convection due to an unstable density difference across a permeable membrane
NASA Astrophysics Data System (ADS)
Puthenveettil, Baburaj A.; Arakeri, Jaywant H.
We study natural convection driven by unstable concentration differences of sodium chloride (NaCl) across a horizontal permeable membrane at Rayleigh numbers (Ra) of 1010 to 1011 and Schmidt number (Sc)=600. A layer of brine lies over a layer of distilled water, separated by the membrane, in square-cross-section tanks. The membrane is permeable enough to allow a small flow across it at higher driving potentials. Based on the predominant mode of transport across the membrane, three regimes of convection, namely an advection regime, a diffusion regime and a combined regime, are identified. The near-membrane flow in all the regimes consists of sheet plumes formed from the unstable layers of fluid near the membrane. In the advection regime observed at higher concentration differences (Bb) show a common log-normal probability density function at all Ra. We propose a phenomenology which predicts /line{lambda}_b sqrt{Z_w Z_{V_i}}, where Zw and Z_{V_i} are, respectively, the near-wall length scales in Rayleighnard convection (RBC) and due to the advection velocity. In the combined regime, which occurs at intermediate values of C/2)4/3. At lower driving potentials, in the diffusion regime, the flux scaling is similar to that in turbulent RBC.
Solution of heat removal from nuclear reactors by natural convection
NASA Astrophysics Data System (ADS)
Zitek, Pavel; Valenta, Vaclav
2014-03-01
This paper summarizes the basis for the solution of heat removal by natural convection from both conventional nuclear reactors and reactors with fuel flowing coolant (such as reactors with molten fluoride salts MSR).The possibility of intensification of heat removal through gas lift is focused on. It might be used in an MSR (Molten Salt Reactor) for cleaning the salt mixture of degassed fission products and therefore eliminating problems with iodine pitting. Heat removal by natural convection and its intensification increases significantly the safety of nuclear reactors. Simultaneously the heat removal also solves problems with lifetime of pumps in the primary circuit of high-temperature reactors.
Numerical Solution of Natural Convection in Eccentric Annuli
Pepper, D.W.
2001-09-18
The governing equations for transient natural convection in eccentric annular space are solved with two high-order accurate numerical algorithms. The equation set is transformed into bipolar coordinates and split into two one-dimensional equations: finite elements are used in the direction normal to the cylinder surfaces; the pseudospectral technique is used in the azimuthal direction. This report discusses those equations.
Natural convection heat transfer in vertical triangular subchannel in Zirconia-water nanofluid
NASA Astrophysics Data System (ADS)
Tandian, N. P.; Alkharboushi, A. A. K.; Kamajaya, K.
2015-09-01
Natural convection heat transfer in vertical triangular sub-channel has important role in cooling mechanism of the APWR and the PHWR nuclear reactors. Unfortunately, natural convection correlation equations for such geometry are scarcely available. Recent studies showed that ZrO2-water nanofluid has a good prospect to be used in the nuclear reactor technology due to its low neutron absorption cross section. Although several papers have reported transport properties of ZrO2-water nanofluids, practically there is no correlation equation for predicting natural convection heat transfer in a vertical triangular sub-channel in ZrO2-water nanofluid. Therefore, a study for finding such heat transfer correlation equation has been done by utilizing Computational Fluid Dynamics software and reported in this paper. In the study, natural convection heat transfer in a vertical triangular sub-channel has been simulated at several values of heat transfer flux within 9.1 to 30.9 kW/m2 range and ZrO2 concentrations of 0 (pure water), 0.27, and 3 volume-% of ZrO2. The study shows that the ZrO2 concentration has no significant influence to the natural convection heat transfer at those concentration levels. The obtained theoretical heat transfer correlation equations were verified through experiment, and they showed very similar results. The correlation equations are reported in this paper.
On the convective-absolute nature of river bedform instabilities
NASA Astrophysics Data System (ADS)
Vesipa, Riccardo; Camporeale, Carlo; Ridolfi, Luca; Chomaz, Jean Marc
2014-12-01
River dunes and antidunes are induced by the morphological instability of stream-sediment boundary. Such bedforms raise a number of subtle theoretical questions and are crucial for many engineering and environmental problems. Despite their importance, the absolute/convective nature of the instability has never been addressed. The present work fills this gap as we demonstrate, by the cusp map method, that dune instability is convective for all values of the physical control parameters, while the antidune instability exhibits both behaviors. These theoretical predictions explain some previous experimental and numerical observations and are important to correctly plan flume experiments, numerical simulations, paleo-hydraulic reconstructions, and river works.
Polymerase chain reaction in natural convection systems: A convection-diffusion-reaction model
NASA Astrophysics Data System (ADS)
Yariv, E.; Ben-Dov, G.; Dorfman, K. D.
2005-09-01
We present a rational scheme for modeling natural convection-driven polymerase chain reaction (PCR), where many copies of a DNA template are made by cycling between hot and cold regions via a circulatory, buoyancy-driven flow. This process is described here in the framework of multiple-species formulation, using evolution equations which govern the concentrations of the various DNA species in the carrying solution. In the intermediate asymptotic limit, where a stationary amplification rate is achieved, these equations provide an eigenvalue problem for computing the exponential amplification rate of double-stranded DNA. The scheme is demonstrated using a simplified model of a Rayleigh-Bénard cell. In contrast to what may have been anticipated, diffusion tends to enhance the growth rate. The present model, intended to be used as a template for more device-specific analyses, provides a starting point for understanding the effects of the competing mechanisms (reaction, convection and diffusion) upon the amplification efficiency.
Influence of geometry on natural convection in buildings
White, M.D.; Winn, C.B.; Jones, G.F.; Balcomb, J.D.
1985-01-01
Strong free convection airflows occur within passive solar buildings resulting from elevated temperatures of surfaces irradiated by solar energy compared with the cooler surfaces not receiving radiation. The geometry of a building has a large influence on the directions and magnitudes of natural airflows, and thus heat transfer between zones. This investigation has utilized a variety of reduced-scale building configurations to study the effects of geometry on natural convection heat transfer. Similarity between the reduced-scale model and a full-scale passive solar building is achieved by having similar geometries and by replacing air with Freon-12 gas as the model's working fluid. Filling the model with Freon-12 gas results in similarity in Prandtl numbers and Rayleigh numbers based on temperature differences in the range from 10/sup 9/ to 10/sup 11/. Results from four geometries are described with an emphasis placed on the effects of heat loss on zone temperature stratification shifts.
Verification of a numerical simulation technique for natural convection
Gadgil, A.; Bauman, F.; Altmayer, E.; Kammerud, R.C.
1983-03-01
The present paper describes a verification of CONVEC2 for single-zone geometries by comparison with the results of two natural convection experiments performed in small-scale rectangular enclosures. These experiments were selected because of the high Rayleigh numbers obtained and the small heat loss through the insulated surfaces. Comparisons are presented for (1) heat transfer rates, (2) fluid temperature profiles, and (3) surface heat flux distributions.
Topological analysis of a mixing flow generated by natural convection
NASA Astrophysics Data System (ADS)
Contreras, Pablo Sebastián; de la Cruz, Luis Miguel; Ramos, Eduardo
2016-01-01
We use topological tools to describe the natural convective motion and the Lagrangian trajectories of a flow generated by stepwise, alternating heating and cooling protocol of opposite vertical walls of a cubic container. The working fluid considered is Newtonian and the system is in presence of the acceleration of gravity but the nonlinear terms are neglected, i.e., we study the piece-wise steady and linear problem. For this convective mixing flow, we identify invariant surfaces formed by the Lagrangian orbits of massless tracers that are topologically equivalent to spherical shells and period-1 lines with elliptic and hyperbolic segments that are located on symmetry planes. We describe the previous features as functions of the Rayleigh number in the range 3 × 104 ≤ Ra ≤ 5 × 105. We show that this system shares properties with other systems with non-toroidal invariant surfaces.
Upwind finite-volume method for natural and forced convection
Pan, D.; Chang, C.H. . Inst. of Aeronautics and Astronautics)
1994-03-01
A third-order upwind finite-volume method was applied to solve the incompressible Navier-Stokes equations via the use of artificial compressibility. The energy equation and the source terms representing thermal buoyancy are included in the system. The inviscid fluxes are evaluated by a MUSCL-type flux difference upwind scheme based on the inviscid eigensystem. An implicit approximate factorization (AF) scheme was used for time integration, and subiterations at each time step can be applied to obtain time accuracy. Various steady and unsteady tests are performed to validate the present method, including problems in natural convection and forced convection, and in particular the complex flow field over two circular cylinders displaced normally to free stream.
The Fractional Step Method Applied to Simulations of Natural Convective Flows
NASA Technical Reports Server (NTRS)
Westra, Douglas G.; Heinrich, Juan C.; Saxon, Jeff (Technical Monitor)
2002-01-01
This paper describes research done to apply the Fractional Step Method to finite-element simulations of natural convective flows in pure liquids, permeable media, and in a directionally solidified metal alloy casting. The Fractional Step Method has been applied commonly to high Reynold's number flow simulations, but is less common for low Reynold's number flows, such as natural convection in liquids and in permeable media. The Fractional Step Method offers increased speed and reduced memory requirements by allowing non-coupled solution of the pressure and the velocity components. The Fractional Step Method has particular benefits for predicting flows in a directionally solidified alloy, since other methods presently employed are not very efficient. Previously, the most suitable method for predicting flows in a directionally solidified binary alloy was the penalty method. The penalty method requires direct matrix solvers, due to the penalty term. The Fractional Step Method allows iterative solution of the finite element stiffness matrices, thereby allowing more efficient solution of the matrices. The Fractional Step Method also lends itself to parallel processing, since the velocity component stiffness matrices can be built and solved independently of each other. The finite-element simulations of a directionally solidified casting are used to predict macrosegregation in directionally solidified castings. In particular, the finite-element simulations predict the existence of 'channels' within the processing mushy zone and subsequently 'freckles' within the fully processed solid, which are known to result from macrosegregation, or what is often referred to as thermo-solutal convection. These freckles cause material property non-uniformities in directionally solidified castings; therefore many of these castings are scrapped. The phenomenon of natural convection in an alloy under-going directional solidification, or thermo-solutal convection, will be explained. The
Transient Convection Due to Imposed Heat Flux: Application to Liquid-Acquisition Devices
NASA Technical Reports Server (NTRS)
Duval, Walter M. B.; Chato, David J.; Doherty, Michael P.
2014-01-01
A model problem is considered that addresses the effect of heat load from an ambient laboratory environment on the temperature rise of liquid nitrogen inside an enclosure. This model has applications to liquid acquisition devices inside the cryogenic storage tanks used to transport vapor-free propellant to the main engine. We show that heat loads from Q = 0.001 to 10 W, with corresponding Rayleigh numbers from Ra = 109 to 1013, yield a range of unsteady convective states and temperature rise in the liquid. The results show that Q = 1 to 10 W (Ra = 1012 to 1013) yield temperature distributions along the enclosure height that are similar in trend to experimental measurements. Unsteady convection, which shows selfsimilarity in its planforms, is predicted for the range of heat-load conditions. The onset of convection occurs from a free-convection-dominated base flow that becomes unstable against convective instability generated at the bottom of the enclosure while the top of the enclosure is convectively stable. A number of modes are generated with small-scale thermals at the bottom of the enclosure in which the flow selforganizes into two symmetric modes prior to the onset of the propagation of the instability. These symmetric vertical modes transition to asymmetric modes that propagate as a traveling-wave-type motion of convective modes and are representative of the asymptotic convective state of the flow field. Intense vorticity production is created in the core of the flow field due to the fact that there is shear instability between the vertical and horizontal modes. For the higher Rayleigh numbers, 1012 to 1013, there is a transition from a stationary to a nonstationary response time signal of the flow and temperature fields with a mean value that increases with time over various time bands and regions of the enclosure.
An experimental investigation of a natural convection solar air loop
Mastrullo, R.; Mazzei, P.; Vanoli, R.
1983-12-01
The interest that has been shown in the use of solar energy to heat dwellings following the ''passive'' design criteria does not correspond to the development of accurate theoretical and experimental analysis. This is particularly true for natural circulation solar air heaters. A significant application of these components is wall panel to complement south-facing windows in supplying solar heat directly to buildings. This idea, formerly suggested by Trombe et al., leads to various realizations, one of which was theoretically investigated by present authors. A convective loop panel consists of a glass layer and a black absorber that is backed by insulation. In the configuration shown the air flows in the channel in front of the absorber and the deflecting panel allows cool air to settle to the bottom of the U channel, preventing reverse thermocirculation during night or very low insolation periods. Since thermocirculation is the primary mode of heat transfer for the solar air heaters, the definition of an accurate convection model for the channel is essential for performance predictions. Studies on this subject - free convection between asymmetrically heated vertical planes - deal mainly with theoretical solutions for laminar flow, with the two usual boundary conditions. As the heat transfer process in the solar air loop cannot be expected to follow this model, there is the need of extensive experimental investigation.
Instabilities of Natural Convection in a Periodically Heated Layer
NASA Astrophysics Data System (ADS)
Hossain, M. Z.; Floryan, Jerzy M.
2013-11-01
Natural convection in a horizontal layer subject to a spatially periodic heating along the lower wall has been investigated. The heating produces sinusoidal temperature variations characterized by the wave number α and the Rayleigh number Rap. The primary response has the form of stationary rolls with axis orthogonal to the heating wave vector. For large α convection is limited to a thin layer adjacent to the lower wall with a uniform conduction above it. Linear stability was used to determine conditions leading to a secondary convection. Two mechanisms of instability have been identified. For α = 0(1), the parametric resonance dominates and leads to the pattern of instability that is locked-in with the pattern of the heating according to the relation δcr = α /2, where δcr denotes the component of the critical disturbance wave vector parallel to the heating wave vector. The second mechanism, Rayleigh-Bénard (RB) mechanism, dominates for large α. Competition between these mechanisms gives rise to non-commensurable states and appearance of soliton lattices, to the formation of distorted transverse rolls, and to the appearance of the wave vector component in the direction perpendicular to the forcing direction.
Natural convection within a vertical finite-length channel in free space
Lin, S.C.; Chang, K.P.; Hung, Y.H. )
1994-04-01
Natural convection within a vertical finite length channel in free space is studied in this article to remove assumptions that need to be made on velocity and temperature profiles at the channel entrance. For small channel aspect ratios and low Rayleigh numbers, significant deviations of the Nusselt number and temperature distributions exist due to the effects of vertical thermal diffusion and free space stratification in the channel. A new correlation was proposed on induced Reynolds number for vertical finite length channel. 8 refs.
Natural convection flow of a generalized second grade fluid between two vertical walls
Massoudi, M.C.; Vaidya, Ashwin; Wulandana, Rachmadian
2008-02-01
We study the flow due to natural convection of a non-Newtonian fluid, modeled as a generalized second grade fluid, between two vertical parallel walls. The flow results from the two walls being held at different temperatures. The viscosity of the fluid is taken to be a function of temperature according to Reynolds’ exponential law. We solve for the dimensionless velocity and temperature profiles and study their dependence upon certain material parameters.
Modelling and control of natural convection in canned foods
NASA Astrophysics Data System (ADS)
Alvarez-Vazquez, L. J.; Martinez, A.
1999-12-01
In this paper we study mathematically an industrial problem related to sterilization processes involving heat transfer by natural convection. We give results of existence and regularity for the solution of this problem. We recast the whole problem as an optimal control problem with pointwise constraints on the state and the control in order to ensure the reduction of microorganism concentration and the retention of nutrients, and to save energy. Finally, we give results on existence of the optimal solution and optimality conditions for its characterization.
Numerical predictions of natural convection in a uniformly heated pool
Tzanos, C.P. Cho, D.H.
1993-05-01
In the event of a core meltdown accident, one of the accident progression paths is fuel relocation to the lower reactor plenum. In the heavy water new production reactor (NPR-HWR) design the reactor cavity is flooded with water. In such a design, decay heat removal to the water in the reactor cavity and thence to the containment may be adequate to keep the reactor vessel temperature below failure limits. If this is the case, the accident progression can be arrested by retaining a coolable corium configuration in the lower reactor plenum. The strategy of reactor cavity flooding to prevent reactor vessel failure from molten corium relocation to the reactor vessel lower head has also been considered for commercial pressurized water reactors. Previously, the computer code COMMIX-LAR/P was used to determine if the heat removal rate from the molten cerium in the lower plenum to the water in the cavity was adequate to keep the reactor vessel temperature in the NPR-HWR design below failure limits. It was found that natural convection in the molten pool resulted in heat removal rates that kept the peak reactor vessel temperature about 400{degrees}C below the steel melting point. The objective of the work presented in this paper was to determine whether COMMIX adequately predicts natural convection in a pool heated by a uniform heat source. For this purpose, the experiments of free convection in a semicircular cavity of Jahn and Reeneke were analyzed with COMMIX and code predictions were compared with experimental measurements. COMMIX is a general purpose thermalhydraulics code based on finite differencing by the first order upwind scheme.
Numerical predictions of natural convection in a uniformly heated pool
Tzanos, C.P. Cho, D.H.
1993-01-01
In the event of a core meltdown accident, one of the accident progression paths is fuel relocation to the lower reactor plenum. In the heavy water new production reactor (NPR-HWR) design the reactor cavity is flooded with water. In such a design, decay heat removal to the water in the reactor cavity and thence to the containment may be adequate to keep the reactor vessel temperature below failure limits. If this is the case, the accident progression can be arrested by retaining a coolable corium configuration in the lower reactor plenum. The strategy of reactor cavity flooding to prevent reactor vessel failure from molten corium relocation to the reactor vessel lower head has also been considered for commercial pressurized water reactors. Previously, the computer code COMMIX-LAR/P was used to determine if the heat removal rate from the molten cerium in the lower plenum to the water in the cavity was adequate to keep the reactor vessel temperature in the NPR-HWR design below failure limits. It was found that natural convection in the molten pool resulted in heat removal rates that kept the peak reactor vessel temperature about 400[degrees]C below the steel melting point. The objective of the work presented in this paper was to determine whether COMMIX adequately predicts natural convection in a pool heated by a uniform heat source. For this purpose, the experiments of free convection in a semicircular cavity of Jahn and Reeneke were analyzed with COMMIX and code predictions were compared with experimental measurements. COMMIX is a general purpose thermalhydraulics code based on finite differencing by the first order upwind scheme.
Effect of enclosure shape on natural convection velocities
NASA Technical Reports Server (NTRS)
Robertson, S. J.; Nicholson, L. A.
1982-01-01
A numerical analysis was performed to compare natural convection velocities in two dimensional enclosures of various shape. The following shapes were investigated: circle, square, horizontal and upright 2 x 1 aspect ratio rectangles, horizontal and upright half circles, diamond. In all cases, the length scale in the various dimensionless parameters, such as Rayleigh number, is defined as the diameter of the equal area circle. Natural convection velocities were calculated for Rayleigh numbers of 1000 and 5000 with the temperature difference taken to be across (1) the maximum horizontal dimension, (2) the median horizontal line (line through centroid) and (3) the horizontal distance such that the temperature gradient is the same for shapes of equal area. For the class of shapes including the square, upright half circle and upright rectangle, the computed velocities were found to agree very closely with that of the equal area circle when the temperature difference is taken to be across the maximum horizontal dimension (condition (a)). The velocities for the horizontal rectangle and half circle were found to be approximately one half that of the equal area circle for the same condition. Better overall agreement among all shapes was obtained by setting the temperature difference across a distance such that the temperature gradients were equal for shapes of equal area.
STARSPOTS DUE TO LARGE-SCALE VORTICES IN ROTATING TURBULENT CONVECTION
Kaepylae, Petri J.; Mantere, Maarit J.; Hackman, Thomas
2011-11-20
We study the generation of large-scale vortices in rotating turbulent convection by means of Cartesian direct numerical simulations. We find that for sufficiently rapid rotation, cyclonic structures on a scale large in comparison to that of the convective eddies emerge, provided that the fluid Reynolds number exceeds a critical value. For slower rotation, cool cyclonic vortices are preferred, whereas for rapid rotation, warm anti-cyclonic vortices are favored. In some runs in the intermediate regime both types of cyclones coexist for thousands of convective turnover times. The temperature contrast between the vortices and the surrounding atmosphere is of the order of 5%. We relate the simulation results to observations of rapidly rotating late-type stars that are known to exhibit large high-latitude spots from Doppler imaging. In many cases, cool spots are accompanied with spotted regions with temperatures higher than the average. In this paper, we investigate a scenario according to which of the spots observed in the temperature maps could have a non-magnetic origin due to large-scale vortices in the convection zones of the stars.
Adjoint optimization of natural convection problems: differentially heated cavity
NASA Astrophysics Data System (ADS)
Saglietti, Clio; Schlatter, Philipp; Monokrousos, Antonios; Henningson, Dan S.
2016-06-01
Optimization of natural convection-driven flows may provide significant improvements to the performance of cooling devices, but a theoretical investigation of such flows has been rarely done. The present paper illustrates an efficient gradient-based optimization method for analyzing such systems. We consider numerically the natural convection-driven flow in a differentially heated cavity with three Prandtl numbers (Pr=0.15{-}7 ) at super-critical conditions. All results and implementations were done with the spectral element code Nek5000. The flow is analyzed using linear direct and adjoint computations about a nonlinear base flow, extracting in particular optimal initial conditions using power iteration and the solution of the full adjoint direct eigenproblem. The cost function for both temperature and velocity is based on the kinetic energy and the concept of entransy, which yields a quadratic functional. Results are presented as a function of Prandtl number, time horizons and weights between kinetic energy and entransy. In particular, it is shown that the maximum transient growth is achieved at time horizons on the order of 5 time units for all cases, whereas for larger time horizons the adjoint mode is recovered as optimal initial condition. For smaller time horizons, the influence of the weights leads either to a concentric temperature distribution or to an initial condition pattern that opposes the mean shear and grows according to the Orr mechanism. For specific cases, it could also been shown that the computation of optimal initial conditions leads to a degenerate problem, with a potential loss of symmetry. In these situations, it turns out that any initial condition lying in a specific span of the eigenfunctions will yield exactly the same transient amplification. As a consequence, the power iteration converges very slowly and fails to extract all possible optimal initial conditions. According to the authors' knowledge, this behavior is illustrated here
Surface temperature distribution along a thin liquid layer due to thermocapillary convection
NASA Technical Reports Server (NTRS)
Lai, C. L.; Chai, A. T.
1985-01-01
The surface temperature distributions due to thermocapillary convections in a thin liquid layer with heat fluxes imposed on the free surface were investigated. The nondimensional analysis predicts that, when convection is important, the characteristics length scale in the flow direction L, and the characteristic temperature difference delta T sub o can be represented by L and delta T sub o approx. (A2Ma)/1/4 delta T sub R, respectively, where L sub R and delta sub R are the reference scales used in the conduction dominant situations with A denoting the aspect ratio and Ma the Marangoni number. Having L and delta sub o defined, the global surface temperature gradient delta sub o/L, the global thermocapillary driving force, and other interesting features can be determined. Numerical calculations involving a Gaussian heat flux distribution are presented to justify these two relations.
Natural Convection and Boiling for Cooling SRP Reactors During Loss of Circulation Conditions
Buckner, M.R.
2001-06-26
This study investigated natural convection and boiling as a means of cooling SRP reactors in the event of a loss of circulation accident. These studies show that single phase natural convection cooling of SRP reactors in shutdown conditions with the present piping geometry is probably not feasible.
Three-dimensional, transient natural convection in inclined wellbores
McEligot, D.M. . Oceanic Div.); Denbow, D.A. ); Murphy, H.D. )
1990-01-01
The occurrence of natural conduction in a wellbore can affect geothermal gradient measurements and heat flow estimates. In the Hot Dry Rock geothermal concept, the wellbores are purposely inclined in the deep regions to enhance heat production. To simulate natural convection flow patterns in directionally drilled wellbores, experiments and analyses were conducted for a circular tube with length to diameter (L/D) ratio of 36 at angles of 0{degrees}, 20{degrees}, and 35{degrees} from the vertical. The tube was heated at the bottom and cooled at the top, and the insulation was adjusted so that approximately one- to two-thirds of the power dissipated was transferred through the tube wall to the surroundings. An aqueous solution of polyvinyl alcohol was employed as the working fluid in order to obtain low Rayleigh numbers corresponding to conditions in geothermal wellbores. Temperature distributions were measured for the three orientations and for several heating rates to demonstrate the effects of tube angle and Rayleigh number. Comparison with measurements showed good agreement of the predicted temperature levels for the maximum inclination and slightly poorer agreement for the other limit, a vertical tube. 50 refs., 9 figs.
Polar vortex formation in giant-planet atmospheres due to moist convection
NASA Astrophysics Data System (ADS)
O'Neill, Morgan E.; Emanuel, Kerry A.; Flierl, Glenn R.
2015-07-01
A strong cyclonic vortex has been observed on each of Saturn’s poles, coincident with a local maximum in observed tropospheric temperature. Neptune also exhibits a relatively warm, although much more transient, region on its south pole. Whether similar features exist on Jupiter will be resolved by the 2016 Juno mission. Energetic, small-scale storm-like features that originate from the water-cloud level or lower have been observed on each of the giant planets and attributed to moist convection, suggesting that these storms play a significant role in global heat transfer from the hot interior to space. Nevertheless, the creation and maintenance of Saturn’s polar vortices, and their presence or absence on the other giant planets, are not understood. Here we use simulations with a shallow-water model to show that storm generation, driven by moist convection, can create a strong polar cyclone throughout the depth of a planet’s troposphere. We find that the type of shallow polar flow that occurs on a giant planet can be described by the size ratio of small eddies to the planetary radius and the energy density of its atmosphere due to latent heating from moist convection. We suggest that the observed difference in these parameters between Saturn and Jupiter may preclude a Jovian polar cyclone.
Torrance, K.E.; Catton, I.
1980-01-01
Natural convection in low aspect ratio rectangular enclosures is considered along with three-dimensional convection within rectangular boxes, natural convection flow visualization in irradiated water cooled by air flow over the surface, free convection in vertical slots, the stratification in natural convection in vertical enclosures, the flow structure with natural convection in inclined air-filled enclosures, and natural convection across tilted, rectangular enclosures of small aspect ratio. Attention is given to the effect of wall conduction and radiation on natural convection in a vertical slot with uniform heat generation of the heated wall, a numerical study of thermal insulation enclosure, free convection in a piston-cylinder enclosure with sinusoidal piston motion, natural convection heat transfer between bodies and their spherical enclosure, an experimental study of the steady natural convection in a horizontal annulus with irregular boundaries, three-dimensional natural convection in a porous medium between concentric inclined cylinders, a numerical solution for natural convection in concentric spherical annuli, and heat transfer by natural convection in porous media between two concentric spheres.
Natural convection in a horizontal cylinder with axial rotation
NASA Astrophysics Data System (ADS)
Sánchez, Odalys; Mercader, Isabel; Batiste, Oriol; Alonso, Arantxa
2016-06-01
We study the problem of thermal convection in a laterally heated horizontal cylinder rotating about its axis. A cylinder of aspect ratio Γ =H /2 R =2 containing a small Prandtl number fluid (σ =0.01 ) representative of molten metals and molten semiconductors at high temperature is considered. We focus on a slow rotation regime (Ω <8 ), where the effects of rotation and buoyancy forces are comparable. The Navier-Stokes and energy equations with the Boussinesq approximation are solved numerically to calculate the basic states, analyze their linear stability, and compute several secondary flows originated from the instabilities. Due to the confined cylindrical geometry—the presence of lateral walls and lids—all the flows are completely three dimensional, even the basic steady states. Results characterizing the basic states as the rotation rate increases are presented. As it occurred in the nonrotating case for higher values of the Prandtl number, two curves of steady states with the same symmetric character coexist for moderate values of the Rayleigh number. In the range of Ω considered, rotation has a stabilizing effect only for very small values. As the value of the rotation rate approaches Ω =3.5 and Ω =4.5 , the scenario of bifurcations becomes more complex due to the existence in both cases of very close bifurcations of codimension 2, which in the latter case involve both curves of symmetric solutions.
Natural convection in a horizontal cylinder with axial rotation.
Sánchez, Odalys; Mercader, Isabel; Batiste, Oriol; Alonso, Arantxa
2016-06-01
We study the problem of thermal convection in a laterally heated horizontal cylinder rotating about its axis. A cylinder of aspect ratio Γ=H/2R=2 containing a small Prandtl number fluid (σ=0.01) representative of molten metals and molten semiconductors at high temperature is considered. We focus on a slow rotation regime (Ω<8), where the effects of rotation and buoyancy forces are comparable. The Navier-Stokes and energy equations with the Boussinesq approximation are solved numerically to calculate the basic states, analyze their linear stability, and compute several secondary flows originated from the instabilities. Due to the confined cylindrical geometry-the presence of lateral walls and lids-all the flows are completely three dimensional, even the basic steady states. Results characterizing the basic states as the rotation rate increases are presented. As it occurred in the nonrotating case for higher values of the Prandtl number, two curves of steady states with the same symmetric character coexist for moderate values of the Rayleigh number. In the range of Ω considered, rotation has a stabilizing effect only for very small values. As the value of the rotation rate approaches Ω=3.5 and Ω=4.5, the scenario of bifurcations becomes more complex due to the existence in both cases of very close bifurcations of codimension 2, which in the latter case involve both curves of symmetric solutions. PMID:27415364
NASA Astrophysics Data System (ADS)
Haddad, Zoubida; Abid, Chérifa; Mohamad, A. A.; Rahli, O.; Bawazer, S.
2016-08-01
An experimental and numerical study was performed to investigate the effect of different formulas for nanofluid thermal conductivity and dynamic viscosity on natural convective heat transfer. It was found that the heat transfer across the enclosure using different models can be enhanced or deteriorated with respect to the base fluid. Also, it was found that the inconsistencies in the reported thermal conductivity and dynamic viscosity from different research groups are mainly due to the characterization of the nanofluid, including determination of colloidal stability and particle size, (i.e., aggregates size) within nanofluid.
Changes in tropical climate due to bifurcations of radiative-convective equilibrium
NASA Astrophysics Data System (ADS)
Kashinath, Karthik; O'Brien, Travis; Collins, William
2015-04-01
Radiative-convective equilibrium (RCE), a simplified model of tropical climate, has been shown to be linearly unstable to moisture perturbations above a critical sea surface temperature (Emanuel et al., JAMES 2013). The mechanism of this instability relies on the temperature dependence of changes in the emissivities of the lower and upper troposphere due to changes in their moisture content, and hence is driven by the thermodynamics of radiation. A requirement for this instability is that the lower troposphere is sufficiently moist and hence opaque in the infrared, so that variations in its radiative cooling are determined primarily by variations in the moisture content of the upper troposphere. Emanuel et al. (2013) and Raymond and Zeng (2000) have shown that the instability migrates the ordinary RCE state to either a dry state with large-scale descent, or to a moist state with mean ascent. Given that general circulation models (GCMs) describe the thermodynamics essential for this instability accurately, we examine their output to detect changes in the behavior of tropical deep convection due to global warming, as anticipated by the theoretical work described above. We do so by analyzing the dependence of convection, radiation and precipitation on SST in the warm pool region of the tropical west Pacific from the CMIP5 archive. We also examine outgoing long-wave radiation from the NASA CERES observational dataset. We find that both model output (from 6 GCMs) and observations show a consistent change in the behavior of radiation and convection, i.e. a bifurcation, across a critical SST. The critical SST is the same (to within 0.5 K) across all the models and variables examined thus far. Deep convection becomes more vigorous and cloud fraction increases across this critical SST, whilst outgoing long-wave radiation decreases. Further, we find a significant change in the behavior of precipitation across this critical SST. In particular, intense rain events and long dry
Natural convection in asymmetric triangular enclosures heated from below
NASA Astrophysics Data System (ADS)
Kamiyo, O. M.; Angeli, D.; Barozzi, G. S.; Collins, M. W.
2014-11-01
Triangular enclosures are typical configurations of attic spaces found in residential as well as industrial pitched-roof buildings. Natural convection in triangular rooftops has received considerable attention over the years, mainly on right-angled and isosceles enclosures. In this paper, a finite volume CFD package is employed to study the laminar air flow and temperature distribution in asymmetric rooftop-shaped triangular enclosures when heated isothermally from the base wall, for aspect ratios (AR) 0.2 <= AR <= 1.0, and Rayleigh number (Ra) values 8 × 105 <= Ra <= 5 × 107. The effects of Rayleigh number and pitch angle on the flow structure and temperature distributions within the enclosure are analysed. Results indicate that, at low pitch angle, the heat transfer between the cold inclined and the hot base walls is very high, resulting in a multi-cellular flow structure. As the pitch angle increases, however, the number of cells reduces, and the total heat transfer rate progressively reduces, even if the Rayleigh number, being based on the enclosure height, rapidly increases. Physical reasons for the above effect are inspected.
Experimental analysis of natural convection within a thermosyphon
Clarksean, R.
1993-09-01
The heat transfer characteristics of a thermosyphon designed to passively cool cylindrical heat sources are experimentally studied. The analysis is based on recognizing the physics of the flow within different regions of the thermosyphon to develop empirical heat transfer correlations. The basic system consists of three concentric cylinders, with an outer channel between the outer two cylinders, and an inner channel between the inner two cylinders. Tests were conducted. with two different process material container diameters, representing the inner cylinder, and several different power levels. The experimentally determined local and average Nu numbers for the inner channel are in good agreement with previous work for natural convection between vertical parallel plates, one uniformly heated and the other thermally insulated. The implication is that the heat transfer off of each surface is independent of the adjacent surface for sufficiently high Ra numbers. The heat transfer is independent because of limited interaction between the boundary layers at sufficiently high Ra numbers. As a result of the limited interaction, the maximum temperature within the system remained constant, or decreased slightly when the radii of the inner cylinders increased for the same amount of heat removal.
Analysis of Phenix natural convection test with the TRACE code
Chenu, A.; Mikityuk, K.; Chawla, R.
2012-07-01
Experimental data from the Natural Convection (NC) test performed in the Phenix reactor prior to its final shutdown have been used to further validate the single-phase sodium flow modeling in TRACE. The experimental data for the benchmark have been shared by the CEA in the frame of a Coordinated Research Project (CRP), initiated by the IAEA Technical Working Group on Fast Reactors (TWG-FR). This paper presents a complete TRACE model of the Phenix primary circuit developed for the analysis. Steady-state calculations at nominal (350 MWth) and reduced (120 MWth) power are compared to the experimental data for the validation of the model. We presents results from the 'blind' comparison, i.e. the comparison of the test results with those computed prior to the communication of the experimental data, so-called 'pre-test' results. 'Post-test' results, calculated from a model improved on the basis of the discrepancies identified from the blind comparison, are also presented. The analysis highlights the need to accurately simulate the reactor structures, since these define the thermal inertia of the system during the first phase of the transient. Furthermore, it shows the limitations of computed 1D-results when applied to the simulation of highly-stratified temperature fields. Nevertheless, the simulated reactor behavior and temperatures are found to match very well with the experimental data after the first two hours and, in general, the TRACE blind predictions may be considered as having been quite satisfactory. (authors)
Natural convection heat transfer analysis of ATR fuel elements
Langerman, M.A.
1992-05-01
Natural convection air cooling of the Advanced Test Reactor (ATR) fuel assemblies is analyzed to determine the level of decay heat that can be removed without exceeding the melting temperature of the fuel. The study was conducted to assist in the level 2 PRA analysis of a hypothetical ATR water canal draining accident. The heat transfer process is characterized by a very low Rayleigh number (Ra {approx} 10{sup {minus}5}) and a high temperature ratio. Since neither data nor analytical models were available for Ra < 0.1, an analytical approach is presented based upon the integral boundary layer equations. All assumptions and simplifications are presented and assessed and two models are developed from similar foundations. In one model, the well-known Boussinesq approximations are employed, the results from which are used to assess the modeling philosophy through comparison to existing data and published analytical results. In the other model, the Boussinesq approximations are not used, thus making the model more general and applicable to the ATR analysis.
Natural convection of ferrofluids in partially heated square enclosures
NASA Astrophysics Data System (ADS)
Selimefendigil, Fatih; Öztop, Hakan F.; Al-Salem, Khaled
2014-12-01
In this study, natural convection of ferrofluid in a partially heated square cavity is numerically investigated. The heater is located to the left vertical wall and the right vertical wall is kept at constant temperature lower than that of the heater. Other walls of the square enclosure are assumed to be adiabatic. Finite element method is utilized to solve the governing equations. The influence of the Rayleigh number (104≤Ra≤5×105), heater location (0.25H≤yh≤0.75H), strength of the magnetic dipole (0≤γ≤2), horizontal and vertical location of the magnetic dipole (-2H≤a≤-0.5H, 0.2H≤b≤0.8H) on the fluid flow and heat transfer characteristics are investigated. It is observed that different velocity components within the square cavity are sensitive to the magnetic dipole source strength and its position. The length and size of the recirculation zones adjacent to the heater can be controlled with magnetic dipole strength. Averaged heat transfer increases with decreasing values of horizontal position of the magnetic dipole source. Averaged heat transfer value increases from middle towards both ends of the vertical wall when the vertical location of the dipole source is varied. When the heater location is changed, a symmetrical behavior in the averaged heat transfer plot is observed and the minimum value of the averaged heat transfer is attained when the heater is located at the mid of vertical wall.
Conjugate natural convection flow over a vertical surface with radiation
NASA Astrophysics Data System (ADS)
Siddiqa, Sadia; Hossain, Md. Anwar; Gorla, Rama Subba Reddy
2016-06-01
Numerical study of conjugate natural convection flow over a finite vertical surface with radiation is reported in this article. Rosseland diffusion approximation is used to express the radiative heat flux term. The governing boundary-layer equations are made dimensionless by means of a suitable form of non-similarity transformation. These equations are obtained in three regimes: (1) upstream (when ξ → 0), (2) downstream (when ξ → ∞ ) and (3) entire regime and are solved numerically. The solutions in the upstream and downstream regimes are obtained via shooting method whereas two-point implicit finite difference method is used to get the solutions for the entire regime. It is seen that asymptotic solutions give accurate results when compared with the numerical solution of the entire regime. The results indicate that the flow field and the temperature distributions are greatly influenced by thermal radiation parameter , R_d, surface temperature parameter, θ _w and Prandtl number Pr. It is established from the analysis that recirculation occurs in the flow specifically for R_d=1.5.
The role of natural convection on cool flames and autoignition
NASA Astrophysics Data System (ADS)
Chapek, Richard; Neville, Donna; Wu, Ming-Shin; Hehemann, David; Sheredy, William; Pearlman, Howard
2000-01-01
Slow reaction, cool flames and autoignition are discussed in the context of unstirred, static reactors at terrestrial (1g) and microgravity (μg) environments. At 1g, conductive heat transport is important when the Rayleigh number (Ra) is less than 600, conduction and convection are important when the Ra ranges from 600 to approximately 104 and convection governs when Ra exceeds 104. Except for highly-diluted, weakly exothermic reactions, the Ra for all laboratory-scale, lg experiments ranges from 103-105. Hence, convection and conduction are important at 1g. Existing models, however, neglect convection, in lieu of conduction, to simplify mathematical formulation. While this assumption is not valid for most 1g cool flames and autoignitions, it is valid at μg where Ra numbers on the order of 10-100 are easily attained in drop towers and aircraft. Aboard the Space Shuttle and Station, typical Ra's are order 1/10-1. .
NASA Astrophysics Data System (ADS)
Yokoi, Satoru; Katsumata, Masaki; Yoneyama, Kunio
2014-03-01
This study examines the variability in surface meteorological parameters and air-sea heat fluxes due to cold pools emanating from cumulus convective systems observed over the tropical Indian Ocean in November 2011. In particular, this study focuses on convective systems that are spatially smaller than mesoscale convective systems in a southeasterly trade wind environment. Composite analyses of convectively active periods show an increase in the sensible heat flux by 15-20 W m-2 that is primarily attributed to an increase in the difference between the surface air temperature and sea surface temperature and an increase in the latent heat flux by 30-70 W m-2 due to enhanced surface wind speeds. A succession of convectively active periods leads to a greater influence than those occurring independently. The direction of the surface wind velocity anomaly due to cold pools tends to be close to that of the environmental wind velocity, resulting in an efficient enhancement of wind speed. This study also demonstrates the close relation between cold pool intensities and convective activity. In particular, two measures of cold pool intensity, a minimum surface air temperature and a maximum amount of surface wind speed enhancement, are correlated with each other and with the convective activity around the observation point measured by radar-estimated rainfall and radar echo coverage.
Joosik Yoo; Jun Young Choi; Moonuhn Kim . Dept. of Mechanical Engineering)
1994-01-01
Two-dimensional natural convection of a fluid of low Prandtl number (Pr = 0.02) in an annulus between two concentric horizontal cylinders is numerically investigated in a wide range of gap widths. For low Grashof numbers, a steady unicellular convection is obtained. Above a transition Grashof number that depends on the gap width, a steady bicellular flow occurs. With further increase of the Grashof number, steady or time-periodic multicellular convection occurs, and finally, complex unsteady convective flow appears. A plot is presented that predicts the type of flow patterns for various combination of gap widths and Grashof numbers.
Experimental study of natural convective heat transfer in a vertical hexagonal sub channel
NASA Astrophysics Data System (ADS)
Tandian, Nathanael P.; Umar, Efrizon; Hardianto, Toto; Febriyanto, Catur
2012-06-01
The development of new practices in nuclear reactor safety aspects and optimization of recent nuclear reactors, including the APWR and the PHWR reactors, needs a knowledge on natural convective heat transfer within sub-channels formed among several nuclear fuel rods or heat exchanger tubes. Unfortunately, the currently available empirical correlation equations for such heat transfer modes are limited and researches on convective heat transfer within a bundle of vertical cylinders (especially within the natural convection modes) are scarcely done. Although boundary layers around the heat exchanger cylinders or fuel rods may be dominated by their entry regions, most of available convection correlation equations are for fully developed boundary layers. Recently, an experimental study on natural convective heat transfer in a subchannel formed by several heated parallel cylinders that arranged in a hexagonal configuration has been being done. The study seeks for a new convection correlation for the natural convective heat transfer in the sub-channel formed among the hexagonal vertical cylinders. A new convective heat transfer correlation equation has been obtained from the study and compared to several similar equations in literatures.
Analysis of the heat transfer from horizontal pipes at natural convection
NASA Astrophysics Data System (ADS)
Kapjor, Andrej; Huzvar, Jozef; Ftorek, Branislav; Smatanova, Helena
2014-08-01
These article deals with heat transfer from "n" horizontal pipes one above another at natural convection. On the bases of theoretical models have been developed for calculating the thermal performance of natural convection by Churilla and Morgan, for various pipe diameters and temperatures. These models were compared with models created in CFD-Fluent Ansys the same boundary conditions. The aim of the analyze of heat and fluxional pipe fields "n" pipes one about another at natural convection is the creation of criterion equation on the basis of which the heat output of heat transfer from pipe oriented areas one above another with given spacing could be quantified.
Natural free convection in porous media: First field documentation in groundwater
NASA Astrophysics Data System (ADS)
Van Dam, Remke L.; Simmons, Craig T.; Hyndman, David W.; Wood, Warren W.
2009-06-01
Natural free convection is a process of great importance in disciplines from hydrology to meteorology, oceanography, planetary sciences, and economic geology, and for applications in carbon sequestration and nuclear waste disposal. It has been studied for over a century - but almost exclusively in theoretical and laboratory settings. Despite its importance, conclusive primary evidence of free convection in porous media does not currently exist in a natural field setting. Here, we present recent electrical resistivity measurements from a sabkha aquifer near Abu Dhabi, United Arab Emirates, where large density inversions exist. The geophysical images from this site provide, for the first time, compelling field evidence of fingering associated with natural free convection in groundwater.
Interaction of a vortex ring with a natural convective layer
NASA Astrophysics Data System (ADS)
Palacios-Morales, C.; Gelderblom, G.; Solorio, F.; Salinas-Vázquez, M.; Zenit, R.
2014-08-01
We study the dynamics of the interaction of a vortex ring with a shear flow, generated by a natural convective layer. Laminar vortex rings were generated in water with a piston-cylinder arrangement. To generate the shear flow, a vertical wall was heated by a thermal bath held at constant temperature to produce a laminar and stable thermal boundary layer with a Grashof number of O(108). Measurements of the two-dimensional velocity field were obtained with a time resolved particle image velocimetry technique. Additionally, a 3D numerical model was used to simulate the experimental conditions. We mainly conducted experiments for the piston stroke L/D0 = 1 and Re of O(1000). The velocity ratio r = Uvi/Ush (where Uvi is the initial vortex velocity and Ush is the maximum velocity of the shear layer) was in the range 2.2 ⩽ r ⩽ 3.6. The results show that as the vortex approaches the shear layer, the ring expands and stretches mainly in the vertical direction and tilts slightly forming an angle between the wall and the ring plane which increases to about 3°. The rate of reduction of circulation is slower at the lower section of the vortex ring indicating that the momentum transport is more significant in this region. Moreover, the vortex circulation at the lower section increases to about 20% compared to the isothermal case. An analysis of the different mechanisms leading to this ring-shear layer interaction is presented and comparisons with reported data are discussed.
Theoretical analysis of solar-driven natural convection energy conversion systems
Jacobs, E.W.; Lasier, D.D.
1984-01-01
This report presents a theoretical study of solar-powered natural convection tower (chimney) performance. Both heated and cooled towers are analyzed; the latter uses evaporating water as the cooling mechanism. The results, which are applicable to any open-cycle configuration, show that the ideal conversion efficiencies of both heated and cooled natural convection towers are linear functions of height. The performance of a heated tower in an adiabatic atmosphere ideally approaches the Carnot efficiency limit of approx. = 3.4%/km (1.0%/1000 ft). Including water pumping requirements, the ideal limit to cooled tower performance is approx. = 2.75%/km (0.85%/1000 ft). Ambient atmospheric conditions such as vertical temperature gradient (lapse rate) and relative humidity can have significantly adverse effects on natural convection tower performance. The combined effects of lapse rate and ambient relative humidity are especially important to cooled natural convection towers.
NASA Technical Reports Server (NTRS)
Ukanwa, A. O.; Stermole, F. J.; Golden, J. O.
1972-01-01
Natural convection effects in phase change thermal control devices were studied. A mathematical model was developed to evaluate natural convection effects in a phase change test cell undergoing solidification. Although natural convection effects are minimized in flight spacecraft, all phase change devices are ground tested. The mathematical approach to the problem was to first develop a transient two-dimensional conduction heat transfer model for the solidification of a normal paraffin of finite geometry. Next, a transient two-dimensional model was developed for the solidification of the same paraffin by a combined conduction-natural-convection heat transfer model. Throughout the study, n-hexadecane (n-C16H34) was used as the phase-change material in both the theoretical and the experimental work. The models were based on the transient two-dimensional finite difference solutions of the energy, continuity, and momentum equations.
Natural convection heat transfer simulation using energy conservative dissipative particle dynamics.
Abu-Nada, Eiyad
2010-05-01
Dissipative particle dynamics with energy conservation (eDPD) was used to study natural convection via Rayleigh-Bénard (RB) problem and a differentially heated enclosure problem (DHE). The current eDPD model implemented the Boussinesq approximation to model the buoyancy forces. The eDPD results were compared to the finite volume solutions and it was found that the eDPD method predict the temperature and flow fields throughout the natural convection domains properly. The eDPD model recovered the basic features of natural convection, such as development of plumes, development of thermal boundary layers, and development of natural convection circulation cells (rolls). The eDPD results were presented via temperature isotherms, streamlines, velocity contours, velocity vector plots, and temperature and velocity profiles. Further useful quantities, such as Nusselt number was calculated from the eDPD results and found to be in good agreement with the finite volume calculations.
Kang, S.; Ha, K. S.; Lee, S. W.; Park, S. D.; Kim, S. M.; Seo, H.; Kim, J. H.; Bang, I. C.
2012-07-01
The safety issues of the SFRs are important due to the fact that it uses sodium as a nuclear coolant, reacting vigorously with water and air. For that reason, there are efforts to seek for alternative candidates of liquid metal coolants having excellent heat transfer property and to adopt improved safety features to the SFR concepts. This study considers gallium as alternative liquid metal coolant applicable to safety features in terms of chemical activity issue of the sodium and aims to experimentally investigate the natural convection capability of gallium as a feasibility study for the development of gallium-based passive safety features in SFRs. In this paper, the design and construction of the liquid gallium natural convection loop were carried out. The experimental results of heat transfer coefficient of liquid gallium resulting in heat removal {approx}2.53 kW were compared with existing correlations and they were much lower than the correlations. To comparison of the experimental data with computer code analysis, gallium property code was developed for employing MARS-LMR (Korea version of RELAP) based on liquid gallium as working fluid. (authors)
NASA Astrophysics Data System (ADS)
Featherstone, Nicholas A.; Hindman, Bradley W.
2016-10-01
We investigate how rotationally constrained, deep convection might give rise to supergranulation, the largest distinct spatial scale of convection observed in the solar photosphere. While supergranulation is only weakly influenced by rotation, larger spatial scales of convection sample the deep convection zone and are presumably rotationally influenced. We present numerical results from a series of nonlinear, 3D simulations of rotating convection and examine the velocity power distribution realized under a range of Rossby numbers. When rotation is present, the convective power distribution possesses a pronounced peak, at characteristic wavenumber {{\\ell }}{peak}, whose value increases as the Rossby number is decreased. This distribution of power contrasts with that realized in non-rotating convection, where power increases monotonically from high to low wavenumbers. We find that spatial scales smaller than {{\\ell }}{peak} behave in analogy to non-rotating convection. Spatial scales larger than {{\\ell }}{peak} are rotationally constrained and possess substantially reduced power relative to the non-rotating system. We argue that the supergranular scale emerges due to a suppression of power on spatial scales larger than {\\ell }≈ 100 owing to the presence of deep, rotationally constrained convection. Supergranulation thus represents the largest non-rotationally constrained mode of solar convection. We conclude that the characteristic spatial scale of supergranulation bounds that of the deep convective motions from above, making supergranulation an indirect measure of the deep-seated dynamics at work in the solar dynamo. Using the spatial scale of supergranulation in conjunction with our numerical results, we estimate an upper bound of 10 m s‑1 for the Sun’s bulk rms convective velocity.
Barthold, W.P.
1984-08-01
The scope of work is to summarize inherent safety advantages that are unique to the use of a carbide based fuel system and to summarize the technical issues regarding natural convection flow in LMFBR cores. As discussed in this report, carbide fuel provides the designer with far greater flexibility than oxide fuel. Carbide fuel systems can be designed to eliminate major accident initiators. They turn quantitative advantages into a qualitative advantage. The author proposed to LANL a series of core design and component concepts that would greatly enhance the safety of carbide over oxide systems. This report cites a series of safety advantages which potentially exist for a carbide fuel system. Natural convection issues have not been given much attention in the past. Only during the last few years has this issue been addressed in some detail. Despite claims to the contrary by some of the LMR contractors, the author does not think that the natural convection phenomena is fully understood. Some of the approximations made in natural convection transient analyses have probably a greater impact on calculated transient temperatures than the effects under investigation. Only integral in-pile experimental data and single assembly out-of-pile detailed data are available for comparisons with analytical models and correlations. Especially for derated cores, the natural convection capability of a LMR should be far superior to that of a LWR. The author ranks the natural convection capability of the LMR as the most important inherent safety feature.
Study of plasma natural convection induced by electron beam in atmosphere [
Deng, Yongfeng Han, Xianwei; Tan, Yonghua
2014-06-15
Using high-energy electron beams to ionize air is an effective way to produce a large-size plasma in the atmosphere. In particular, with a steady-state high power generator, some unique phenomena can be achieved, including natural convection of the plasma. The characteristics of this convection are studied both experimentally and numerically. The results show that an asymmetrical temperature field develops with magnitudes that vary from 295 K to 389 K at a pressure of 100 Torr. Natural convection is greatly enhanced under 760 Torr. Nevertheless, plasma transport is negligible in this convection flow field and only the plasma core tends to move upward. Parameter analysis is performed to discern influencing factors on this phenomenon. The beam current, reflecting the Rayleigh number Ra effect, correlates with convection intensity, which indicates that energy deposition is the underlying key factor in determining such convections. Finally, natural convection is concluded to be an intrinsic property of the electron beam when focused into dense air, and can be achieved by carefully adjusting equipment operations parameters.
Nature, theory and modelling of geophysical convective planetary boundary layers
NASA Astrophysics Data System (ADS)
Zilitinkevich, Sergej
2015-04-01
Geophysical convective planetary boundary layers (CPBLs) are still poorly reproduced in oceanographic, hydrological and meteorological models. Besides the mean flow and usual shear-generated turbulence, CPBLs involve two types of motion disregarded in conventional theories: 'anarchy turbulence' comprised of the buoyancy-driven plumes, merging to form larger plumes instead of breaking down, as postulated in conventional theory (Zilitinkevich, 1973), large-scale organised structures fed by the potential energy of unstable stratification through inverse energy transfer in convective turbulence (and performing non-local transports irrespective of mean gradients of transporting properties). C-PBLs are strongly mixed and go on growing as long as the boundary layer remains unstable. Penetration of the mixed layer into the weakly turbulent, stably stratified free flow causes turbulent transports through the CPBL outer boundary. The proposed theory, taking into account the above listed features of CPBL, is based on the following recent developments: prognostic CPBL-depth equation in combination with diagnostic algorithm for turbulence fluxes at the CPBL inner and outer boundaries (Zilitinkevich, 1991, 2012, 2013; Zilitinkevich et al., 2006, 2012), deterministic model of self-organised convective structures combined with statistical turbulence-closure model of turbulence in the CPBL core (Zilitinkevich, 2013). It is demonstrated that the overall vertical transports are performed mostly by turbulence in the surface layer and entrainment layer (at the CPBL inner and outer boundaries) and mostly by organised structures in the CPBL core (Hellsten and Zilitinkevich, 2013). Principal difference between structural and turbulent mixing plays an important role in a number of practical problems: transport and dispersion of admixtures, microphysics of fogs and clouds, etc. The surface-layer turbulence in atmospheric and marine CPBLs is strongly enhanced by the velocity shears in
Numerical simulation of natural convection in a sessile liquid droplet
NASA Astrophysics Data System (ADS)
Bartashevich, M. V.; Marchuk, I. V.; Kabov, O. A.
2012-06-01
Heat transfer in a sessile liquid droplet was studied with numerical methods. A computer code was developed for solving the problem of convection in an axisymmetric hemispherical droplet and in a spherical layer as well. The problem of establishing an equilibrium state in a droplet was solved using several variables: temperature, stream function, and vorticity. Simulation was performed for droplets of water, ethyl alcohol, and model liquids. Variable parameters: intensity of heat transfer from droplet surface, Rayleigh and Marangoni dimensionless criteria, and the characteristic temperature difference. It was revealed that the curve of convective flow intensity versus heat transfer intensity at droplet surface has a maximum. A dual-vortex structure was obtained in a stationary hemispherical profile of liquid droplet for the case of close values for thermocapillary and thermogravitational forces. Either thermocapillary or thermogravitational vortex might be dominating phenomena in the flow structure.
NASA Astrophysics Data System (ADS)
Weaver, J. A.; Viskanta, Raymond
1992-01-01
An investigation of natural convection is presented to examine the influence of a horizontal temperature gradient and a concentration gradient occurring from the bottom to the cold wall in a cavity. As the solutal buoyancy force changes from augmenting to opposing the thermal buoyancy force, the fluid motion switches from unicellular to multicellular flow (fluid motion is up the cold wall and down the hot wall for the bottom counterrotating flow cell). Qualitatively, the agreement between predicted streamlines and smoke flow patterns is generally good. In contrast, agreement between measured and predicted temperature and concentration distributions ranges from fair to poor. Part of the discrepancy can be attributed to experimental error. However, there remains considerable discrepancy between data and predictions due to the idealizations of the mathematical model, which examines only first-order physical effects. An unsteady flow, variable thermophysical properties, conjugate effects, species interdiffusion, and radiation were not accounted for in the model.
Weaver, J.A.; Viskanta, R. )
1992-01-01
An investigation of natural convection is presented to examine the influence of a horizontal temperature gradient and a concentration gradient occurring from the bottom to the cold wall in a cavity. As the solutal buoyancy force changes from augmenting to opposing the thermal buoyancy force, the fluid motion switches from unicellular to multicellular flow (fluid motion is up the cold wall and down the hot wall for the bottom counterrotating flow cell). Qualitatively, the agreement between predicted streamlines and smoke flow patterns is generally good. In contrast, agreement between measured and predicted temperature and concentration distributions ranges from fair to poor. Part of the discrepancy can be attributed to experimental error. However, there remains considerable discrepancy between data and predictions due to the idealizations of the mathematical model, which examines only first-order physical effects. An unsteady flow, variable thermophysical properties, conjugate effects, species interdiffusion, and radiation were not accounted for in the model. 31 refs.
Experimental investigation of turbulent natural convection flow in a converging channel
Ayinde, T.F.
2008-05-15
This paper reports the results of fluid flow measurements for natural convection in a converging plates channel using the particle image velocimetry (PIV) system. The channel walls were symmetrically subjected to uniform temperature conditions. Velocity characteristics were obtained for two inclination angles, {theta} = 15 and 45 , and two heating conditions corresponding to Ra{sub L} = 2.7 x 10{sup 8} and 4.4 x 10{sup 8}, where Ra{sub L} is the Raleigh number based on the length of the channel wall. Results are presented as vector plots as well as profiles of mean velocities and turbulence quantities. They show that the main flow is aligned with the orientation of the channel walls, due to the effect of buoyancy force, which is no longer exclusively in the vertical direction. They also reveal the presence of reverse flow, which leads to the formation of two symmetric vortices in the core. (author)
NASA Technical Reports Server (NTRS)
Seybert, C. D.; Evans, J. W.; Leslie, F.; Jones, W. K., Jr.
2000-01-01
Natural convection, driven by temperature-or concentration gradients or both, is an inherent phenomenon during solidification of materials on Earth. This convection has practical consequences (e.g effecting macrosegregation) but also renders difficult the scientific examination of diffusive/conductive phenomena during solidification. It is possible to halt, or even reverse, natural convection by exploiting the variation (with temperature, for example) of the susceptibility of a material. If the material is placed in a vertical magnetic field gradient, a buoyancy force of magnetic origin arises and, at a critical field gradient, can balance the normal buoyancy forces to halt convection. At higher field gradients the convection can be reversed. The effect has been demonstrated in experiments at Marshall Space Flight Center where flow was measured by PIV in MnCl2 solution in a superconducting magnet. In auxiliary experiments the field in the magnet and the properties of the solution were measured. Computations of the natural convection, its halting and reversal, using the commercial software FLUENT were in good agreement with the measurements.
Onset of convection in a finite two-dimensional container due to unipolar injection of ions.
Wu, Jian; Traoré, Philippe; Vázquez, Pedro A; Pérez, Alberto T
2013-11-01
This work addresses the stability of a two-dimensional plane layer of a dielectric liquid enclosed in wall bounded cavities of different aspect ratios and subjected to unipolar injection of ions. Numerical simulations have been conducted to investigate the effect of lateral walls, especially in the development of the electroconvective instability. It is found that an unexpected change of the bifurcation nature occurs for certain cavity aspect ratios. We show that above the linear stability threshold for the rest state a supercritical bifurcation arises. This bifurcation takes place at a given value T(c1) of the parameter T (the electric Rayleigh number). Then, a second subcritical bifurcation occurs at a second threshold T(c2), featuring a typical hysteresis loop with an associated nonlinear criterion T(f), which is very characteristic of the Coulomb-driven convection. This behavior has been confirmed by different numerical codes based on different numerical methods. The physical mechanism which leads to this situation is analyzed and discussed. The evolution of the bifurcation diagrams with the aspect ratio of the cavity is also provided and analyzed. PMID:24329362
Onset of convection in a finite two-dimensional container due to unipolar injection of ions.
Wu, Jian; Traoré, Philippe; Vázquez, Pedro A; Pérez, Alberto T
2013-11-01
This work addresses the stability of a two-dimensional plane layer of a dielectric liquid enclosed in wall bounded cavities of different aspect ratios and subjected to unipolar injection of ions. Numerical simulations have been conducted to investigate the effect of lateral walls, especially in the development of the electroconvective instability. It is found that an unexpected change of the bifurcation nature occurs for certain cavity aspect ratios. We show that above the linear stability threshold for the rest state a supercritical bifurcation arises. This bifurcation takes place at a given value T(c1) of the parameter T (the electric Rayleigh number). Then, a second subcritical bifurcation occurs at a second threshold T(c2), featuring a typical hysteresis loop with an associated nonlinear criterion T(f), which is very characteristic of the Coulomb-driven convection. This behavior has been confirmed by different numerical codes based on different numerical methods. The physical mechanism which leads to this situation is analyzed and discussed. The evolution of the bifurcation diagrams with the aspect ratio of the cavity is also provided and analyzed.
NASA Astrophysics Data System (ADS)
Eshagh, Mehdi; Romeshkani, Mohsen
2015-11-01
Sub-lithospheric stress due to mantle convection can be determined from gravimetric data based on Runcorn's theory. In this paper, the satellite gradiometric data of the recent European satellite mission, the Gravity field and steady-state Ocean Circulation Explorer (GOCE) is used to determine the sub-lithospheric stress locally in Iran. The method of S function (SF) with numerical differentiation is developed further and an integral equation connecting satellite gradiometric data to SF is presented. The integral equation will be used to invert the real gradiometric data of GOCE to recover the SF. Later on, the sub-lithospheric shear stresses, which are the northward and eastward derivatives of the SF, are computed numerically. Our numerical results show that the mean squares error of the recovered SF is smaller than the values of the SF meaning that the recovery process is successful. Also, the recovered stress has a good agreement with the tectonic boundaries and active seismic points of the world stress map (WSM) database. This stress reaches amplitude of 100 MPa in the territory.
Environmental exposures due to natural disasters
Knap, Anthony H.; Rusyn, Ivan
2016-01-01
The environmental mobilization of contaminants by “natural disasters” is a subject of much interest; however, little has been done to address these concerns, especially in the developing world. Frequencies and predictability of events, both globally and regionally as well as the intensity, vary widely. It is clear that there are greater probabilities for mobilization of modern contaminants in sediments. Over the past 100 years of industrialization many chemicals are buried in riverine, estuarine and coastal sediments. There are a few studies, which have investigated this potential risk especially to human health. Studies that focus on extreme events need to determine the pre-existing baseline, determine the medium to long term fate and transport of contaminants and investigate aquatic and terrestrial pathways. Comprehensive studies are required to investigate the disease pathways and susceptibility for human health concerns. PMID:26982607
Environmental exposures due to natural disasters.
Knap, Anthony H; Rusyn, Ivan
2016-03-01
The environmental mobilization of contaminants by "natural disasters" is a subject of much interest, however, little has been done to address these concerns, especially in the developing world. Frequencies and predictability of events, both globally and regionally as well as the intensity, vary widely. It is clear that there are greater probabilities for mobilization of modern contaminants in sediments. Over the past 100 years of industrialization many chemicals are buried in riverine, estuarine and coastal sediments. There are a few studies, which have investigated this potential risk especially to human health. Studies that focus on extreme events need to determine the pre-existing baseline, determine the medium to long term fate and transport of contaminants and investigate aquatic and terrestrial pathways. Comprehensive studies are required to investigate the disease pathways and susceptibility for human health concerns. PMID:26982607
Natural convection heat transfer on two horizontal cylinders in liquid sodium
Hata, K.; Shiotsu, M.; Takeuchi, Y.
1995-09-01
Natural convection heat transfer on two horizontal 7.6 mm diameter test cylinders assembled with the ratio of the distance between each cylinder axis to the cylinder diameter, S/D, of 2 in liquid sodium was studied experimentally and theoretically. The heat transfer coefficients on the cylinder surface due to the same heat inputs ranging from 1.0 X 10{sup 7} to 1.0 x 10{sup 9} W/m{sup 3} were obtained experimentally for various setting angeles, {gamma}, between vertical direction and the plane including both of these cylinder axis over the range of zero to 90{degrees}. Theoretical equations for laminar natural convection heat transfer from the two horizontal cylinders were numerically solved for the same conditions as the experimental ones considering the temperature dependence of thermophysical properties concerned. The average Nusselt numbers, Nu, values on the Nu versus modified Rayleigh number, R{sub f}, graph. The experimental values of Nu for the upper cylinder are about 20% lower than those for the lower cylinder at {gamma} = 0{degrees} for the range of R{sub f} tested here. The value of Nu for the upper cylinder becomes higher and approaches that for the lower cylinder with the increase in {gamma} over range of 0 to 90{degrees}. The values of Nu for the lower cylinder at each {gamma} are almost in agreement with those for a single cylinder. The theoretical values of Nu on two cylinders except those for R{sub f}<4 at {gamma} = 0{degrees} are in agreement with the experimental data at each {gamma} with the deviations less than 15%. Correlations for Nu on the upper and lower cylinders were obtained as functions of S/D and {gamma} based n the theoretical solutions for the S/D ranged over 1.5 to 4.0.
Tagawa, Toshio; Ozoe, Hiroyuki
1996-08-23
Numerical calculations were carried out for natural convection of low-Prandtl-number fluid. These calculations include the inertial terms that were approximated by six kinds of schemes, i.e., upwind scheme, hybrid scheme, second-order central difference method, Kawamura-Kuwahara scheme, Utopia scheme, and fourth-order central difference method. The average Nusselt number depended significantly on the schemes. The occurrence of oscillatory flow also depended on the schemes for inertial terms. Higher order up-winding approximations for inertial terms appear to be required to calculate natural convection of low-Prandtl-number fluids like liquid metal, even if the Rayleigh number is not large enough.
Natural Convection Cooling of the Advanced Stirling Radioisotope Generator Engineering Unit
NASA Technical Reports Server (NTRS)
Lewandowski, Edward J.; Hill, Dennis
2011-01-01
After fueling and prior to launch, the Advanced Stirling Radioisotope Generator (ASRG) will be stored for a period of time then moved to the launch pad for integration with the space probe and mounting on the launch vehicle. During this time, which could be as long as 3 years, the ASRG will operate continuously with heat rejected from the housing and fins. Typically, the generator will be cooled by forced convection using fans. During some of the ground operations, maintaining forced convection may add significant complexity, so allowing natural convection may simplify operations. A test was conducted on the ASRG Engineering Unit (EU) to quantify temperatures and operating parameters with natural convection only and determine if the EU could be safely operated in such an environment. The results show that with natural convection cooling the ASRG EU Stirling convertor pressure vessel temperatures and other parameters had significant margins while the EU was operated for several days in this configuration. Additionally, an update is provided on ASRG EU testing at NASA Glenn Research Center, where the ASRG EU has operated for over 16,000 hr and underwent extensive testing.
NASA Astrophysics Data System (ADS)
Bayani Cardenas, M.; Lagmay, Alfredo Mahar F.; Andrews, Benjamin J.; Rodolfo, Raymond S.; Cabria, Hillel B.; Zamora, Peter B.; Lapus, Mark R.
2012-01-01
Thermal springs are ubiquitous features whose underground kinematic structure is mostly unknown but are typically thought to originate from deep sources. We documented a type of thermal springs at the banks of a volcanic lake that are discharge zones of hydrothermal convection cells circulating groundwater within the near shore environment. The convection captures lake water through the lakebed, mixes it with deeper groundwater at velocities of 100s of m d-1, then returns the water to the lake via the spring. The convection cell is flushed in a few hours and turns over the lake's volume in a few days. Most volcanic lakes and other relatively cool surface water bodies in areas of elevated geothermal heat fluxes meet the conditions for the occurrence of local hydrothermal circulation of groundwater. The type of spring we studied, the terrestrial version of black smokers, is likely present but perhaps unrecognized at many areas.
Pebay, Cécile; Sella, Catherine; Thouin, Laurent; Amatore, Christian
2013-12-17
Mass transport at infinite regular arrays of microband electrodes was investigated theoretically and experimentally in unstirred solutions. Even in the absence of forced hydrodynamics, natural convection limits the convection-free domain up to which diffusion layers may expand. Hence, several regimes of mass transport may take place according to the electrode size, gap between electrodes, time scale of the experiment, and amplitude of natural convection. They were identified through simulation by establishing zone diagrams that allowed all relative contributions to mass transport to be delineated. Dynamic and steady-state regimes were compared to those achieved at single microband electrodes. These results were validated experimentally by monitoring the chronoamperometric responses of arrays with different ratios of electrode width to gap distance and by mapping steady-state concentration profiles above their surface through scanning electrochemical microscopy. PMID:24283775
Pressure transfer function of a JT15D nozzle due to acoustic and convected entropy fluctuations
NASA Astrophysics Data System (ADS)
Miles, J. H.
An acoustic transmission matrix analysis of sound propagation in a variable area duct with and without flow is extended to include convected entropy fluctuations. The boundary conditions used in the analysis are a transfer function relating entropy and pressure at the nozzle inlet and the nozzle exit impedance. The nozzle pressure transfer function calculated is compared with JT15D turbofan engine nozzle data. The one dimensional theory for sound propagation in a variable area nozzle with flow but without convected entropy is good at the low engine speeds where the nozzle exit Mach number is low (M=0.2) and the duct exit impedance model is good. The effect of convected entropy appears to be so negligible that it is obscured by the inaccuracy of the nozzle exit impedance model, the lack of information on the magnitude of the convected entropy and its phase relationship with the pressure, and the scatter in the data. An improved duct exit impedance model is required at the higher engine speeds where the nozzle exit Mach number is high (M=0.56) and at low frequencies (below 120 Hz).
Pressure transfer function of a JT15D nozzle due to acoustic and convected entropy fluctuations
NASA Technical Reports Server (NTRS)
Miles, J. H.
1982-01-01
An acoustic transmission matrix analysis of sound propagation in a variable area duct with and without flow is extended to include convected entropy fluctuations. The boundary conditions used in the analysis are a transfer function relating entropy and pressure at the nozzle inlet and the nozzle exit impedance. The nozzle pressure transfer function calculated is compared with JT15D turbofan engine nozzle data. The one dimensional theory for sound propagation in a variable area nozzle with flow but without convected entropy is good at the low engine speeds where the nozzle exit Mach number is low (M=0.2) and the duct exit impedance model is good. The effect of convected entropy appears to be so negligible that it is obscured by the inaccuracy of the nozzle exit impedance model, the lack of information on the magnitude of the convected entropy and its phase relationship with the pressure, and the scatter in the data. An improved duct exit impedance model is required at the higher engine speeds where the nozzle exit Mach number is high (M=0.56) and at low frequencies (below 120 Hz).
Effects of electrode location on EHD-enhanced natural convection in an enclosure
Liu, K.S.; Lai, F.C.
1997-07-01
Numerical results are presented for natural convection in an enclosure under the influence of electric field. The geometry considered is a two-dimensional cavity with an aspect ratio of 5. The electrical field is generated by positive corona from an electrode wire charged with a high dc voltage. Three wire locations have been considered, which result in symmetric and non-symmetric electric fields. Numerical calculations have covered a wide range of parameters (i.e., V{sub o} = 12, 15 and 18 kV, 10{sup 3} {le} Ra {le} 10{sup 6}). In the presence of electric field, the flow and temperature fields may reach a steady, steady-periodic or non-periodic state. For low Rayleigh numbers, it is observed that the flow and temperature fields are basically oscillatory in nature. When the Rayleigh number is sufficiently increased, a steady state may be reached. Due to the oscillatory flows, there is a significant increase in heat transfer. It is found that heat transfer enhancement increases with the applied voltage but decreases with the Rayleigh number. In addition, it is found that heat transfer enhancement can be maximized by placing the electrode toward the leading edge of the heat transfer surface, that is, to perturb the thermal boundary layer as early as it begins to develop.
A three-dimensional analysis of natural convection in a toroidal loop
NASA Astrophysics Data System (ADS)
Lavine, A. G.
1984-06-01
Flow through a toroidal loop oriented in a vertical plane is studied. This system is a simple example of a class of devices known as thermosyphons or natural circulation loops, which have many applications. When the toroidal loop is heated from below and cooled from above, an unstable density gradient is created in the fluid. Under the influence of gravity, the lighter fluid rises and the heavier fluid fails. Thus, the fluid flows due only to natural convection. Experiments on the toroidal thermosyphon have shown that under steady state flow conditions, the axial velocity and the temperature are nonaxisymmetric, the cross stream velocities are nonzero and regions of streamwise flow reversal exist. The simplified one and two dimensional analyses performed to date have not been able to predict these phenomena. The development of a finite difference computer program for performing a three dimensional analysis of the steady state fluid flow and heat transfer in the toroidal thermosyphon is described. Regions of streamwise flow reversal are predicted in some cases. Simplified analyses which do not take the flow reversals into account are shown to be substantially in error. Hence, the current analysis yields insight into the three dimensional aspects of the flow, which could not be predicted by earlier more simplified analyses.
Meng, Xiangyin; Li, Yan
2015-01-01
Natural heat convection of water-based alumina (Al2O3/water) nanofluids (with volume fraction 1% and 4%) in a horizontal cylinder is numerically investigated. The whole three-dimensional computational fluid dynamics (CFD) procedure is performed in a completely open-source way. Blender, enGrid, OpenFOAM and ParaView are employed for geometry creation, mesh generation, case simulation and post process, respectively. Original solver 'buoyantBoussinesqSimpleFoam' is selected for the present study, and a temperature-dependent solver 'buoyantBoussinesqSimpleTDFoam' is developed to ensure the simulation is more realistic. The two solvers are used for same cases and compared to corresponding experimental results. The flow regime in these cases is laminar (Reynolds number is 150) and the Rayleigh number range is 0.7 × 10(7) ~ 5 × 10(7). By comparison, the average natural Nusselt numbers of water and Al2O3/water nanofluids are found to increase with the Rayleigh number. At the same Rayleigh number, the Nusselt number is found to decrease with nanofluid volume fraction. The temperature-dependent solver is found better for water and 1% Al2O3/water nanofluid cases, while the original solver is better for 4% Al2O3/water nanofluid cases. Furthermore, due to strong three-dimensional flow features in the horizontal cylinder, three-dimensional CFD simulation is recommended instead of two-dimensional simplifications.
NASA Astrophysics Data System (ADS)
Meng, Xiangyin; Li, Yan
2015-03-01
Natural heat convection of water-based alumina (Al2O3/water) nanofluids (with volume fraction 1% and 4%) in a horizontal cylinder is numerically investigated. The whole three-dimensional computational fluid dynamics (CFD) procedure is performed in a completely open-source way. Blender, enGrid, OpenFOAM and ParaView are employed for geometry creation, mesh generation, case simulation and post process, respectively. Original solver `buoyantBoussinesqSimpleFoam' is selected for the present study, and a temperature-dependent solver `buoyantBoussinesqSimpleTDFoam' is developed to ensure the simulation is more realistic. The two solvers are used for same cases and compared to corresponding experimental results. The flow regime in these cases is laminar (Reynolds number is 150) and the Rayleigh number range is 0.7 × 107 ~ 5 × 107. By comparison, the average natural Nusselt numbers of water and Al2O3/water nanofluids are found to increase with the Rayleigh number. At the same Rayleigh number, the Nusselt number is found to decrease with nanofluid volume fraction. The temperature-dependent solver is found better for water and 1% Al2O3/water nanofluid cases, while the original solver is better for 4% Al2O3/water nanofluid cases. Furthermore, due to strong three-dimensional flow features in the horizontal cylinder, three-dimensional CFD simulation is recommended instead of two-dimensional simplifications.
Asfia, F.; Dhir, V.
1998-03-01
One strategy for preventing the failure of lower head of a nuclear reactor vessel is to flood the concrete cavity with subcooled water in accidents in which relocation of core material into the vessel lower head occurs. After the core material relocates into the vessel, a crust of solid material forms on the inner wall of the vessel, however, most of the pool remains molten and natural convection exists in the pool. At present, uncertainty exists with respect to natural convection heat transfer coefficients between the pool of molten core material and the reactor vessel wall. In the present work, experiments were conducted to examine natural convection heat transfer in internally heated partially filled spherical pools with external cooling. In the experiments, Freon-113 contained in a Pyrex bell jar was used as a test liquid. The pool was bounded with a spherical segment at the bottom, and was heated with magnetrons taken from a conventional microwave oven. The vessel was cooled from the outside with natural convection of water or with nucleate boiling of liquid nitrogen.
A numerical study of natural convection during an in-package pasteurisation process
Engelman, M.E.; Sani, R.L.
1983-07-01
Results of a numerical study of transient axisymmetric natural convection during an in-package pasteurisation process are reported. The numerical technique employed was a penalty Galerkin finite element procedure using a variable step implicit time integration technique coupled with a quasi-Newton nonlinear equation solver. The numerical results display excellent agreement with experimental results.
FREQUENCY SHIFTS OF RESONANT MODES OF THE SUN DUE TO NEAR-SURFACE CONVECTIVE SCATTERING
Bhattacharya, J.; Hanasoge, S.; Antia, H. M.
2015-06-20
Measurements of oscillation frequencies of the Sun and stars can provide important independent constraints on their internal structure and dynamics. Seismic models of these oscillations are used to connect structure and rotation of the star to its resonant frequencies, which are then compared with observations, the goal being that of minimizing the difference between the two. Even in the case of the Sun, for which structure models are highly tuned, observed frequencies show systematic deviations from modeled frequencies, a phenomenon referred to as the “surface term.” The dominant source of this systematic effect is thought to be vigorous near-surface convection, which is not well accounted for in both stellar modeling and mode-oscillation physics. Here we bring to bear the method of homogenization, applicable in the asymptotic limit of large wavelengths (in comparison to the correlation scale of convection), to characterize the effect of small-scale surface convection on resonant-mode frequencies in the Sun. We show that the full oscillation equations, in the presence of temporally stationary three-dimensional (3D) flows, can be reduced to an effective “quiet-Sun” wave equation with altered sound speed, Brünt–Väisäla frequency, and Lamb frequency. We derive the modified equation and relations for the appropriate averaging of 3D flows and thermal quantities to obtain the properties of this effective medium. Using flows obtained from 3D numerical simulations of near-surface convection, we quantify their effect on solar oscillation frequencies and find that they are shifted systematically and substantially. We argue therefore that consistent interpretations of resonant frequencies must include modifications to the wave equation that effectively capture the impact of vigorous hydrodynamic convection.
NASA Astrophysics Data System (ADS)
Danaila, Ionut; Moglan, Raluca; Hecht, Frédéric; Le Masson, Stéphane
2014-10-01
We present a new numerical system using finite elements with mesh adaptivity for the simulation of solid-liquid phase change systems. In the liquid phase, the natural convection flow is simulated by solving the incompressible Navier-Stokes equations with Boussinesq approximation. A variable viscosity model allows the velocity to progressively vanish in the solid phase, through an intermediate mushy region. The phase change is modeled by introducing an implicit enthalpy source term in the heat equation. The final system of equations describing the liquid-solid system by a single domain approach is solved using a Newton iterative algorithm. The space discretization is based on a P2-P1 Taylor-Hood finite elements and mesh adaptivity by metric control is used to accurately track the solid-liquid interface or the density inversion interface for water flows. The numerical method is validated against classical benchmarks that progressively add strong non-linearities in the system of equations: natural convection of air, natural convection of water, melting of a phase-change material and water freezing. Very good agreement with experimental data is obtained for each test case, proving the capability of the method to deal with both melting and solidification problems with convection. The presented numerical method is easy to implement using FreeFem++ software using a syntax close to the mathematical formulation.
Campbell, A N
2015-07-14
When any exothermic reaction proceeds in an unstirred vessel, natural convection may develop. This flow can significantly alter the heat transfer from the reacting fluid to the environment and hence alter the balance between heat generation and heat loss, which determines whether or not the system will explode. Previous studies of the effects of natural convection on thermal explosion have considered reactors where the temperature of the wall of the reactor is held constant. This implies that there is infinitely fast heat transfer between the wall of the vessel and the surrounding environment. In reality, there will be heat transfer resistances associated with conduction through the wall of the reactor and from the wall to the environment. The existence of these additional heat transfer resistances may alter the rate of heat transfer from the hot region of the reactor to the environment and hence the stability of the reaction. This work presents an initial numerical study of thermal explosion in a spherical reactor under the influence of natural convection and external heat transfer, which neglects the effects of consumption of reactant. Simulations were performed to examine the changing behaviour of the system as the intensity of convection and the importance of external heat transfer were varied. It was shown that the temporal development of the maximum temperature in the reactor was qualitatively similar as the Rayleigh and Biot numbers were varied. Importantly, the maximum temperature in a stable system was shown to vary with Biot number. This has important consequences for the definitions used for thermal explosion in systems with significant reactant consumption. Additionally, regions of parameter space where explosions occurred were identified. It was shown that reducing the Biot number increases the likelihood of explosion and reduces the stabilising effect of natural convection. Finally, the results of the simulations were shown to compare favourably with
Controlling natural convection in a closed thermosyphon using neural networks
NASA Astrophysics Data System (ADS)
Cammarata, L.; Fichera, A.; Pagano, A.
. The aim of this paper is to present a neural network-based approach to identification and control of a rectangular natural circulation loop. The first part of the paper defines a NARMAX model for the prediction of the experimental oscillating behavior characterizing the fluid temperature. The model has been generalized and implemented by means of a Multilayer Perceptron Neural Network that has been trained to simulate the system experimental dynamics. In the second part of the paper, the NARMAX model has been used to simulate the plant during the training of another neural network aiming to suppress the undesired oscillating behavior of the system. In order to define the neural controller, a cascade of several couples of neural networks representing both the system and the controller has been used, the number of couples coinciding with the number of steps in which the control action is exerted.
Natural convection in tunnels at Yucca Mountain and impact on drift seepage
Halecky, N.; Birkholzer, J.T.; Peterson, P.
2010-04-15
The decay heat from radioactive waste that is to be disposed in the once proposed geologic repository at Yucca Mountain (YM) will significantly influence the moisture conditions in the fractured rock near emplacement tunnels (drifts). Additionally, large-scale convective cells will form in the open-air drifts and will serve as an important mechanism for the transport of vaporized pore water from the fractured rock in the drift center to the drift end. Such convective processes would also impact drift seepage, as evaporation could reduce the build up of liquid water at the tunnel wall. Characterizing and understanding these liquid water and vapor transport processes is critical for evaluating the performance of the repository, in terms of water-induced canister corrosion and subsequent radionuclide containment. To study such processes, we previously developed and applied an enhanced version of TOUGH2 that solves for natural convection in the drift. We then used the results from this previous study as a time-dependent boundary condition in a high-resolution seepage model, allowing for a computationally efficient means for simulating these processes. The results from the seepage model show that cases with strong natural convection effects are expected to improve the performance of the repository, since smaller relative humidity values, with reduced local seepage, form a more desirable waste package environment.
Luo, Kang; Yi, Hong-Liang Tan, He-Ping
2014-05-15
Transitions and bifurcations of transient natural convection in a horizontal annulus with radiatively participating medium are numerically investigated using the coupled lattice Boltzmann and direct collocation meshless (LB-DCM) method. As a hybrid approach based on a common multi-scale Boltzmann-type model, the LB-DCM scheme is easy to implement and has an excellent flexibility in dealing with the irregular geometries. Separate particle distribution functions in the LBM are used to calculate the density field, the velocity field and the thermal field. In the radiatively participating medium, the contribution of thermal radiation to natural convection must be taken into account, and it is considered as a radiative term in the energy equation that is solved by the meshless method with moving least-squares (MLS) approximation. The occurrence of various instabilities and bifurcative phenomena is analyzed for different Rayleigh number Ra and Prandtl number Pr with and without radiation. Then, bifurcation diagrams and dual solutions are presented for relevant radiative parameters, such as convection-radiation parameter Rc and optical thickness τ. Numerical results show that the presence of volumetric radiation changes the static temperature gradient of the fluid, and generally results in an increase in the flow critical value. Besides, the existence and development of dual solutions of transient convection in the presence of radiation are greatly affected by radiative parameters. Finally, the advantage of LB-DCM combination is discussed, and the potential benefits of applying the LB-DCM method to multi-field coupling problems are demonstrated.
Critical heat flux in natural convection cooled TRIGA reactors with hexagonal bundle
Yang, J.; Avery, M.; De Angelis, M.; Anderson, M.; Corradini, M.; Feldman, E. E.; Dunn, F. E.; Matos, J. E.
2012-07-01
A three-rod bundle Critical Heat Flux (CHF) study at low flow, low pressure, and natural convection condition has been conducted, simulating TRIGA reactors with the hexagonally configured core. The test section is a custom-made trefoil shape tube with three identical fuel pin heater rods located symmetrically inside. The full scale fuel rod is electrically heated with a chopped-cosine axial power profile. CHF experiments were carried out with the following conditions: inlet water subcooling from 30 K to 95 K; pressure from 110 kPa to 230 kPa; mass flux up to 150 kg/m{sup 2}s. About 50 CHF data points were collected and compared with a few existing CHF correlations whose application ranges are close to the testing conditions. Some tests were performed with the forced convection to identify the potential difference between the CHF under the natural convection and forced convection. The relevance of the CHF to test parameters is investigated. (authors)
NASA Astrophysics Data System (ADS)
Pérez Grande, Isabel; Rodriguez Sevillano, Angel; Meseguer, Jos
In June, 8th, 2009 the balloon-borne solar telescope SUNRISE was launched from the Swedish Space Corporation balloon facility Esrange. A telescope with a mirror of 1 m in diameter ob-served the Sun during six days until the mission was terminated in Canada. The design process of SUNRISE and of any optical telescope requires the analysis of the effect of surrounding air on the quality of images. The turbulence encountered in the local telescope environment de-grades its optical performance. This phenomenon called `seeing' consists of optical aberrations produced by density non-homogeneities in the air along the optical path. The refraction index of air changes due to thermal non-uniformities so that the wavefront incident on the mirror is randomly distorted, and therefore, images are altered. When telescope mirrors are heated, as it happens in solar telescopes, and therefore they are at a temperature different from the environment's, natural convection occurs. It is then crucial to know whether the flow in front of the mirror is laminar or turbulent. After reviewing the literature, it was found that the scattering of results about the onset of the transition gives only rough orders of magnitude of the values of the critical Grashof numbers. Aiming to obtain more information about it, the problem of determination of the turbulence onset in natural convection on heated inclined plates in air environment was experimentally revisited. The transition has been determined from hot wire velocity measurements. The onset of turbulence has been considered to take place where velocity perturbations start to grow. Experiments have shown that the onset depends not only on the Grashof number, but also on other parameters as the temperature difference between the heated plate and the surrounding air. A correlation between dimensionless Grashof and Reynolds numbers has been obtained, fitting extraordinarily well the experimental data. The results are obtained in terms of non
Effects of finiteness on the thermo-fluid-dynamics of natural convection above horizontal plates
NASA Astrophysics Data System (ADS)
Guha, Abhijit; Sengupta, Sayantan
2016-06-01
A rigorous and systematic computational and theoretical study, the first of its kind, for the laminar natural convective flow above rectangular horizontal surfaces of various aspect ratios ϕ (from 1 to ∞) is presented. Two-dimensional computational fluid dynamic (CFD) simulations (for ϕ → ∞) and three-dimensional CFD simulations (for 1 ≤ ϕ < ∞) are performed to establish and elucidate the role of finiteness of the horizontal planform on the thermo-fluid-dynamics of natural convection. Great care is taken here to ensure grid independence and domain independence of the presented solutions. The results of the CFD simulations are compared with experimental data and similarity theory to understand how the existing simplified results fit, in the appropriate limiting cases, with the complex three-dimensional solutions revealed here. The present computational study establishes the region of a high-aspect-ratio planform over which the results of the similarity theory are approximately valid, the extent of this region depending on the Grashof number. There is, however, a region near the edge of the plate and another region near the centre of the plate (where a plume forms) in which the similarity theory results do not apply. The sizes of these non-compliance zones decrease as the Grashof number is increased. The present study also shows that the similarity velocity profile is not strictly obtained at any location over the plate because of the entrainment effect of the central plume. The 3-D CFD simulations of the present paper are coordinated to clearly reveal the separate and combined effects of three important aspects of finiteness: the presence of leading edges, the presence of planform centre, and the presence of physical corners in the planform. It is realised that the finiteness due to the presence of physical corners in the planform arises only for a finite value of ϕ in the case of 3-D CFD simulations (and not in 2-D CFD simulations or similarity theory
NASA Astrophysics Data System (ADS)
Zhang, Wencan; Chen, Jiqing; Lan, Fengchong
2013-02-01
A numerical simulation of combined natural convection and radiation in a square enclosure heated by a centric circular cylinder and filled with absorbing-emitting medium is presented. The ideal gas law and the discrete ordinates method are used to model the density changes due to temperature differences and the radiation heat transfer correspondingly. The influence of Rayleigh number, optical thickness and temperature difference on flow and temperature fields along with the natural convection, radiation and total Nusselt number at the source surfaces is studied. The results reveal that the radiation heat transfer as well as the optical thickness of the fluid has a distinct effect on the fluid flow phenomena, especially at high Rayleigh number. The heat transfer and so the Nusselt number decreases with increase in optical thickness, while increases greatly with increase in temperature difference. The variation in radiation heat transfer with optical thickness and temperature difference is much more obvious as comparison with convection heat transfer.
Generation of coronal electric currents due to convective motions on the photosphere
NASA Technical Reports Server (NTRS)
Sakurai, T.; Levine, R. H.
1981-01-01
Generation of electric currents in a magnetized plasma overlying a dense convective layer is studied, assuming that the magnetic field perturbation is small and satisfies the force-free equation. Currents are produced by rotational motions on the boundary in the case of a uniform equilibrium field. In a simple two-dimensional bipolar configuration, however, both irrotational and incompressible motions give rise to currents, and the current density has a peak at the magnetic neutral line. Scaling laws for the current density as well as for the stored magnetic energy are derived, and the possibility of heating the solar corona through the dissipation of coronal currents generated in this way is discussed.
Uvarov, A. V.; Sakharova, N. A.; Vinnichenko, N. A.
2011-12-15
The parameters of the positive column of a glow discharge in neon are calculated with allowance for the induced hydrodynamic motion. It is shown that natural convection in the pressure range of {approx}0.1 atm significantly affects the profiles of the parameters of the positive column and its current-voltage characteristic. The convection arising at large deposited energies improves heat removal, due to which the temperature in the central region of the discharge becomes lower than that calculated from the heat conduction equation. As a result, the current-voltage characteristic is shifted. With allowance for convection, the current-voltage characteristic changes at currents much lower than the critical current at which a transition into the constricted state is observed. This change is uniquely related to the Rayleigh number in the discharge. Thus, a simplified analysis of thermal conduction and diffusion, even with detailed account of kinetic processes occurring in the positive column, does not allow one to accurately calculate the current-voltage characteristic and other discharge parameters at intermediate gas pressures.
Extreme dissipation event due to plume collision in a turbulent convection cell
NASA Astrophysics Data System (ADS)
Schumacher, Jörg; Scheel, Janet D.
2016-10-01
An extreme dissipation event in the bulk of a closed three-dimensional turbulent convection cell is found to be correlated with a strong reduction of the large-scale circulation flow in the system that happens at the same time as a plume emission event from the bottom plate. The reduction in the large-scale circulation opens the possibility for a nearly frontal collision of down- and upwelling plumes and the generation of a high-amplitude thermal dissipation layer in the bulk. This collision is locally connected to a subsequent high-amplitude energy dissipation event in the form of a strong shear layer. Our analysis illustrates the impact of transitions in the large-scale structures on extreme events at the smallest scales of the turbulence, a direct link that is observed in a flow with boundary layers. We also show that detection of extreme dissipation events which determine the far-tail statistics of the dissipation fields in the bulk requires long-time integrations of the equations of motion over at least a hundred convective time units.
Delmas, A.A.; Wilkes, K.E.
1992-04-01
A two-dimensional code for solving equations of convective heat transfer in porous media is used to analyze heat transfer by conduction and convection in the attic insulation configuration. The particular cases treated correspond to loose-fill fiberglass insulation, which is characterized by high porosity and air permeability. The effects of natural convection on the thermal performance of the insulation are analyzed for various densities, permeabilities, and thicknesses of insulation. With convection increasing the total heat transfer through the insulation, the thermal resistance was found to decrease as the temperature difference across the insulating material increases. The predicted results for the thermal resistance are compared with data obtained in the large-scale climate simulator at the Roof Research Center using the attic test module, where the same phenomenon has already been observed. The way the wood joists within the insulation influence the start of convection is studied for differing thermophysical and dynamic properties of the insulating material. The presence of wood joists induces convection at a lower temperature difference.
Potential of enhancing a natural convection loop with a thermomagnetically pumped ferrofluid
NASA Astrophysics Data System (ADS)
Aursand, Eskil; Gjennestad, Magnus Aa.; Lervåg, Karl Yngve; Lund, Halvor
2016-11-01
The feasibility of using a thermomagnetically pumped ferrofluid to enhance the performance of a natural convection cooling loop is investigated. First, a simplified analytical estimate for the thermomagnetic pumping action is derived, and then design rules for optimal solenoid and ferrofluid are presented. The design rules are used to set up a medium-scale (1 m, 10-1000 W) case study, which is modeled using a previously published and validated model (Aursand et al. [1]). The results show that the thermomagnetic driving force is significant compared to the natural convection driving force, and may in some cases greatly surpass it. The results also indicate that cooling performance can be increased by factors up to 4 and 2 in the single-phase and two-phase regimes, respectively, even when taking into the account the added heat from the solenoid. The performance increases can alternatively be used to obtain a reduction in heat-sink size by up to 75%.
Scaling of the turbulent natural convection flow in a heated square cavity
NASA Astrophysics Data System (ADS)
Henkes, R. A. W. M.; Hoogendoorn, C. J.
1994-05-01
By numerically solving the Reynolds equations for air and water in a square cavity, with differentially heated vertical walls, at Rayleigh numbers up to 10(exp 20) the scalings of the turbulent natural convection flow are derived. Turbulence is modeled by the standard k-epsilon model and by the low-Reynolds-number k-epsilon models of Chien and of Jones and Launder. Both the scalings with respect to the Rayleigh number (based on the cavity size H) and with respect to the local height (y/H) are considered. The scalings are derived for the inner layer, outer layer, and core region. The Rayleigh number scalings are almost the same as the scalings for the natural convection boundary layer along a hot vertical plate. The scalings found are almost independent of the k-epsilon model used.
Meng, Xiangyin; Li, Yan
2015-01-01
Natural heat convection of water-based alumina (Al2O3/water) nanofluids (with volume fraction 1% and 4%) in a horizontal cylinder is numerically investigated. The whole three-dimensional computational fluid dynamics (CFD) procedure is performed in a completely open-source way. Blender, enGrid, OpenFOAM and ParaView are employed for geometry creation, mesh generation, case simulation and post process, respectively. Original solver 'buoyantBoussinesqSimpleFoam' is selected for the present study, and a temperature-dependent solver 'buoyantBoussinesqSimpleTDFoam' is developed to ensure the simulation is more realistic. The two solvers are used for same cases and compared to corresponding experimental results. The flow regime in these cases is laminar (Reynolds number is 150) and the Rayleigh number range is 0.7 × 10(7) ~ 5 × 10(7). By comparison, the average natural Nusselt numbers of water and Al2O3/water nanofluids are found to increase with the Rayleigh number. At the same Rayleigh number, the Nusselt number is found to decrease with nanofluid volume fraction. The temperature-dependent solver is found better for water and 1% Al2O3/water nanofluid cases, while the original solver is better for 4% Al2O3/water nanofluid cases. Furthermore, due to strong three-dimensional flow features in the horizontal cylinder, three-dimensional CFD simulation is recommended instead of two-dimensional simplifications. PMID:25852431
Simulation of natural convection in a rectangular loop using finite elements
Pepper, D W; Hamm, L L; Kehoe, A B
1984-01-01
A two-dimensional finite-element analysis of natural convection in a rectangular loop is presented. A psi-omega formulation of the Boussinesque approximation to the Navier-Stokes equation is solved by the false transient technique. Streamlines and isotherms at Ra = 10/sup 4/ are shown for three different modes of heating. The results indicate that corner effects should be considered when modeling flow patterns in thermosyphons.
NASA Astrophysics Data System (ADS)
Shen, Chunyun; Yang, Mo; Zhang, Yuwen; Li, Zheng
2016-09-01
Natural convection in a cylinder with an internally slotted annulus was solved by SIMPLE algorithm, and the effects of different slotted structures on nonlinear characteristics of natural convection were investigated. The results show that the equivalent thermal conductivity Keq increases with Rayleigh number, and reaches the maximum in the vertical orientation. Nonlinear results were obtained by simulating the fluid flow at different conditions. With increasing Rayleigh number, heat transfer is intensified and the state of heat transfer changes from the steady to unsteady. We investigated different slotted structures effects on natural convection, and analyze the corresponding nonlinear characteristics.
The Prediction of Noise Due to Jet Turbulence Convecting Past Flight Vehicle Trailing Edges
NASA Technical Reports Server (NTRS)
Miller, Steven A. E.
2014-01-01
High intensity acoustic radiation occurs when turbulence convects past airframe trailing edges. A mathematical model is developed to predict this acoustic radiation. The model is dependent on the local flow and turbulent statistics above the trailing edge of the flight vehicle airframe. These quantities are dependent on the jet and flight vehicle Mach numbers and jet temperature. A term in the model approximates the turbulent statistics of single-stream heated jet flows and is developed based upon measurement. The developed model is valid for a wide range of jet Mach numbers, jet temperature ratios, and flight vehicle Mach numbers. The model predicts traditional trailing edge noise if the jet is not interacting with the airframe. Predictions of mean-flow quantities and the cross-spectrum of static pressure near the airframe trailing edge are compared with measurement. Finally, predictions of acoustic intensity are compared with measurement and the model is shown to accurately capture the phenomenon.
CYCLIC MAGNETIC ACTIVITY DUE TO TURBULENT CONVECTION IN SPHERICAL WEDGE GEOMETRY
Kaepylae, Petri J.; Mantere, Maarit J.; Brandenburg, Axel
2012-08-10
We report on simulations of turbulent, rotating, stratified, magnetohydrodynamic convection in spherical wedge geometry. An initially small-scale, random, weak-amplitude magnetic field is amplified by several orders of magnitude in the course of the simulation to form oscillatory large-scale fields in the saturated state of the dynamo. The differential rotation is solar-like (fast equator), but neither coherent meridional poleward circulation nor near-surface shear layer develop in these runs. In addition to a poleward branch of magnetic activity beyond 50 Degree-Sign latitude, we find for the first time a pronounced equatorward branch at around 20 Degree-Sign latitude, reminiscent of the solar cycle.
Emergency cooling down of fast-neutron reactors by natural convection (a review)
NASA Astrophysics Data System (ADS)
Zhukov, A. V.; Sorokin, A. P.; Kuzina, Yu. A.
2013-05-01
Various methods for emergency cooling down of fast-neutron reactors by natural convection are discussed. The effectiveness of using natural convection for these purposes is demonstrated. The operating principles of different passive decay heat removal systems intended for cooling down a reactor are explained. Experimental investigations carried out in Russia for substantiating the removal of heat in cooling down fast-neutron reactors are described. These investigations include experimental works on studying thermal hydraulics in small-scale simulation facilities containing the characteristic components of a reactor (reactor core elements, above-core structure, immersed and intermediate heat exchangers, pumps, etc.). It is pointed out that a system that uses leaks of coolant between fuel assemblies holds promise for fast-neutron reactor cooldown purposes. Foreign investigations on this problem area are considered with making special emphasis on the RAMONA and NEPTUN water models. A conclusion is drawn about the possibility of using natural convection as the main method for passively removing heat in cooling down fast-neutron reactors, which is confirmed experimentally both in Russia and abroad.
Shitzer, Avraham
2006-03-01
The wind-chill index (WCI), developed in Antarctica in the 1940s and recently updated by the weather services in the USA and Canada, expresses the enhancement of heat loss in cold climates from exposed body parts, e.g., face, due to wind. The index provides a simple and practical means for assessing the thermal effects of wind on humans outdoors. It is also used for indicating weather conditions that may pose adverse risks of freezing at subfreezing environmental temperatures. Values of the WCI depend on a number of parameters, i.e, temperatures, physical properties of the air, wind speed, etc., and on insolation and evaporation. This paper focuses on the effects of various empirical correlations used in the literature for calculating the convective heat transfer coefficients between humans and their environment. Insolation and evaporation are not included in the presentation. Large differences in calculated values among these correlations are demonstrated and quantified. Steady-state wind-chill-equivalent temperatures (WCETs) are estimated by a simple, one-dimensional heat-conducting hollow-cylindrical model using these empirical correlations. Partial comparison of these values with the published "new" WCETs is presented. The variability of the estimated WCETs, due to different correlations employed to calculate them, is clearly demonstrated. The results of this study clearly suggest the need for establishing a "gold standard" for estimating convective heat exchange between exposed body elements and the cold and windy environment. This should be done prior to the introduction and adoption of further modifications to WCETs and indices. Correlations to estimate the convective heat transfer coefficients between exposed body parts of humans in windy and cold environments influence the WCETs and need to be standardized. PMID:16397760
Shitzer, Avraham
2006-03-01
The wind-chill index (WCI), developed in Antarctica in the 1940s and recently updated by the weather services in the USA and Canada, expresses the enhancement of heat loss in cold climates from exposed body parts, e.g., face, due to wind. The index provides a simple and practical means for assessing the thermal effects of wind on humans outdoors. It is also used for indicating weather conditions that may pose adverse risks of freezing at subfreezing environmental temperatures. Values of the WCI depend on a number of parameters, i.e, temperatures, physical properties of the air, wind speed, etc., and on insolation and evaporation. This paper focuses on the effects of various empirical correlations used in the literature for calculating the convective heat transfer coefficients between humans and their environment. Insolation and evaporation are not included in the presentation. Large differences in calculated values among these correlations are demonstrated and quantified. Steady-state wind-chill-equivalent temperatures (WCETs) are estimated by a simple, one-dimensional heat-conducting hollow-cylindrical model using these empirical correlations. Partial comparison of these values with the published "new" WCETs is presented. The variability of the estimated WCETs, due to different correlations employed to calculate them, is clearly demonstrated. The results of this study clearly suggest the need for establishing a "gold standard" for estimating convective heat exchange between exposed body elements and the cold and windy environment. This should be done prior to the introduction and adoption of further modifications to WCETs and indices. Correlations to estimate the convective heat transfer coefficients between exposed body parts of humans in windy and cold environments influence the WCETs and need to be standardized.
NASA Astrophysics Data System (ADS)
Shitzer, Avraham
2006-03-01
The wind-chill index (WCI), developed in Antarctica in the 1940s and recently updated by the weather services in the USA and Canada, expresses the enhancement of heat loss in cold climates from exposed body parts, e.g., face, due to wind. The index provides a simple and practical means for assessing the thermal effects of wind on humans outdoors. It is also used for indicating weather conditions that may pose adverse risks of freezing at subfreezing environmental temperatures. Values of the WCI depend on a number of parameters, i.e, temperatures, physical properties of the air, wind speed, etc., and on insolation and evaporation. This paper focuses on the effects of various empirical correlations used in the literature for calculating the convective heat transfer coefficients between humans and their environment. Insolation and evaporation are not included in the presentation. Large differences in calculated values among these correlations are demonstrated and quantified. Steady-state wind-chill-equivalent temperatures (WCETs) are estimated by a simple, one-dimensional heat-conducting hollow-cylindrical model using these empirical correlations. Partial comparison of these values with the published “new” WCETs is presented. The variability of the estimated WCETs, due to different correlations employed to calculate them, is clearly demonstrated. The results of this study clearly suggest the need for establishing a “gold standard” for estimating convective heat exchange between exposed body elements and the cold and windy environment. This should be done prior to the introduction and adoption of further modifications to WCETs and indices. Correlations to estimate the convective heat transfer coefficients between exposed body parts of humans in windy and cold environments influence the WCETs and need to be standardized.
Aksenova, A.E.; Chudanov, V.V.; Strizhov, V.F.; Vabishchevich, P.N.
1995-09-01
Unsteady natural convection of a heat-generating fluid with phase transitions in the enclosures of a square section with isothermal rigid walls is investigated numerically for a wide range of dimensionless parameters. The quasisteady state solutions of conjugate heat and mass transfer problem are compared with available experimental results. Correlation relations for heat flux distributions at the domain boundaries depending on Rayleigh and Ostrogradskii numbers are obtained. It is shown that generally heat transfer is governed both by natural circulation and crust formation phenomena. Results of this paper may be used for analysis of experiments with prototypic core materials.
Natural convection in a square cavity with thin porous layers on its vertical walls
Le Breton, P.; Caltagirone, J.P.; Arquis, E. )
1991-11-01
Natural convection in a square cavity in which differentially heated vertical walls are covered with thin porous layers is studied by using a control volume formulation and a SIMPLER algorithm for pressure-velocity coupling. Comparisons with benchmark solutions for natural convection in fluid-filled cavities are first presented for Rayleigh numbers up to 10{sup 8}. The problem of the square cavity with thin porous layers on its vertical walls is then studied by using a modified form of the Navier-Stokes equations by addition of a Darcy term. It is shown that the main effect of the introduction of porous layers is to produce a large decrease of the overall Nusselt number when the permeability is reduced. The higher the Rayleigh number is, the stronger is the decrease, and obviously the decrease also increases with the layer thickness. Moreover, porous layers having a thickness of the order of the boundary layer thickness are sufficient, and taking thicker ones only induces a small decrease of the heat transfer. The main effect of porous layers is to reduce the upwind flow and then to decrease the convective heat transfer.
NASA Astrophysics Data System (ADS)
Chen, Yan-Jun; Wang, Ping-Yang; Liu, Zhen-Hua
2016-11-01
The natural convective heat transfer and flow characteristics of nanofluids in an enclosure are numerically simulated using the multiphase-flow model and single phase model respectively. The simulated results are compared with the experimental results from the published papers to investigate the applicability of these models for nanofluids from a macro standpoint. The effects of Rayleigh number, Grashof number and volume concentration of nanoparticles on the heat transfer and flow characteristics are investigated and discussed. Comparisons of the horizontal and vertical central dimensionless velocity profiles between nanofluid and water for various Grashof numbers are studied. In addition, both streamline contours and isotherms lines for different volume concentrations of nanofluids are analyzed as well. The study results show that a great deviation exists between the simulated result of the single phase model and the experimental data on the relation of Nusselt number and Rayleigh number, which indicates that the single phase model cannot reflect the heat transfer characteristic of nanofluid. While the simulated results using the multiphase-flow model show a good agreement with the experimental data of nanofluid, which means that the multiphase-flow model is more suitable for the numerical study of nanofluid. For the natural convection, the present study holds the point that using Grashof numbers as the benchmark would be more appropriate to describe the heat transfer characteristics of nanofluid. Moreover, the simulated results demonstrate that adding nanoparticles into the base fluid can enhance both the motion of fluid and convection in the enclosure significantly.
Finite element, stream function-vorticity solution of steady laminar natural convection
NASA Astrophysics Data System (ADS)
Stevens, W. N. R.
1982-12-01
Stream function-vorticity finite element solution of two-dimensional incompressible viscous flow and natural convection is considered. Steady state solutions of the natural convection problem have been obtained for a wide range of the two independent parameters. Use of boundary vorticity formulae or iterative satisfaction of the no-slip boundary condition is avoided by application of the finite element discretization and a displacement of the appropriate discrete equations. Solution is obtained by Newton-Raphson iteration of all equations simultaneously. The method then appears to give a steady solution whenever the flow is physically steady, but it does not give a steady solution when the flow is physically unsteady. In particular, no form of asymmetric differencing is required. The method offers a degree of economy over primitive variable formulations. Physical results are given for the square cavity convection problem. The paper also reports on earlier work in which the most commonly used boundary vorticity formula was found not to satisfy the no-slip condition, and in which segregated solution procedures were attempted with very minimal success.
Solar drying of whole mint plant under natural and forced convection.
Sallam, Y I; Aly, M H; Nassar, A F; Mohamed, E A
2015-03-01
Two identical prototype solar dryers (direct and indirect) having the same dimensions were used to dry whole mint. Both prototypes were operated under natural and forced convection modes. In the case of the later one the ambient air was entered the dryer with the velocity of 4.2 m s(-1). The effect of flow mode and the type of solar dryers on the drying kinetics of whole mint were investigated. Ten empirical models were used to fit the drying curves; nine of them represented well the solar drying behavior of mint. The results indicated that drying of mint under different operating conditions occurred in the falling rate period, where no constant rate period of drying was observed. Also, the obtained data revealed that the drying rate of mint under forced convection was higher than that of mint under natural convection, especially during first hours of drying (first day). The values of the effective diffusivity coefficient for the mint drying ranged between 1.2 × 10(-11) and 1.33 × 10(-11) m(2) s(-1). PMID:25750751
Triplett, C.E.
1996-12-01
This thesis presents the results of an experimental investigation of natural convection heat transfer in a staggered array of heated cylinders, oriented horizontally within a rectangular enclosure. The main purpose of this research was to extend the knowledge of heat transfer within enclosed bundles of spent nuclear fuel rods sealed within a shipping or storage container. This research extends Canaan`s investigation of an aligned array of heated cylinders that thermally simulated a boiling water reactor (BWR) spent fuel assembly sealed within a shipping or storage cask. The results are presented in terms of piecewise Nusselt-Rayleigh number correlations of the form Nu = C(Ra){sup n}, where C and n are constants. Correlations are presented both for individual rods within the array and for the array as a whole. The correlations are based only on the convective component of the heat transfer. The radiative component was calculated with a finite-element code that used measured surface temperatures, rod array geometry, and measured surface emissivities as inputs. The correlation results are compared to Canaan`s aligned array results and to other studies of natural convection in horizontal tube arrays.
Numerical modeling of crystal growth on a centrifuge for unstable natural convection configurations
NASA Technical Reports Server (NTRS)
Ramachandran, N.; Downey, J. P.; Curreri, P. A.; Jones, J. C.
1993-01-01
The fluid mechanics associated with crystal growth processes on centrifuges is modeled using 2D and 3D models. Two-dimensional calculations show that flow bifurcations exist in such crystal growth configurations where the ampoule is oriented in the same direction as the resultant gravity vector and a temperature gradient is imposed on the melt. A scaling analysis is formulated to predict the flow transition point from the natural convection dominated regime to the Coriolis force dominated regime. Results of 3D calculations are presented for two thermal configurations of the crystal growth cell: top heated and bottom heated with respect to the centrifugal acceleration. In the top heated configuration, a substantial reduction in the convection intensity within the melt can be attained by centrifuge operations, and close to steady diffusion-limited thermal conditions can be achieved over a narrow range of the imposed microgravity level. In the bottom heated configuration the Coriolis force has a stabilizing effect on fluid motion by delaying the onset of unsteady convection.
Ahmed, Mahmoud; Eslamian, Morteza
2015-12-01
Laminar natural convection in differentially heated (β = 0°, where β is the inclination angle), inclined (β = 30° and 60°), and bottom-heated (β = 90°) square enclosures filled with a nanofluid is investigated, using a two-phase lattice Boltzmann simulation approach. The effects of the inclination angle on Nu number and convection heat transfer coefficient are studied. The effects of thermophoresis and Brownian forces which create a relative drift or slip velocity between the particles and the base fluid are included in the simulation. The effect of thermophoresis is considered using an accurate and quantitative formula proposed by the authors. Some of the existing results on natural convection are erroneous due to using wrong thermophoresis models or simply ignoring the effect. Here we show that thermophoresis has a considerable effect on heat transfer augmentation in laminar natural convection. Our non-homogenous modeling approach shows that heat transfer in nanofluids is a function of the inclination angle and Ra number. It also reveals some details of flow behavior which cannot be captured by single-phase models. The minimum heat transfer rate is associated with β = 90° (bottom-heated) and the maximum heat transfer rate occurs in an inclination angle which varies with the Ra number.
Ahmed, Mahmoud; Eslamian, Morteza
2015-12-01
Laminar natural convection in differentially heated (β = 0°, where β is the inclination angle), inclined (β = 30° and 60°), and bottom-heated (β = 90°) square enclosures filled with a nanofluid is investigated, using a two-phase lattice Boltzmann simulation approach. The effects of the inclination angle on Nu number and convection heat transfer coefficient are studied. The effects of thermophoresis and Brownian forces which create a relative drift or slip velocity between the particles and the base fluid are included in the simulation. The effect of thermophoresis is considered using an accurate and quantitative formula proposed by the authors. Some of the existing results on natural convection are erroneous due to using wrong thermophoresis models or simply ignoring the effect. Here we show that thermophoresis has a considerable effect on heat transfer augmentation in laminar natural convection. Our non-homogenous modeling approach shows that heat transfer in nanofluids is a function of the inclination angle and Ra number. It also reveals some details of flow behavior which cannot be captured by single-phase models. The minimum heat transfer rate is associated with β = 90° (bottom-heated) and the maximum heat transfer rate occurs in an inclination angle which varies with the Ra number. PMID:26183389
The effect of natural and forced melt convection on dendritic solidification in Ga-In alloys
NASA Astrophysics Data System (ADS)
Shevchenko, N.; Roshchupkina, O.; Sokolova, O.; Eckert, S.
2015-05-01
The directional solidification of Ga-25 wt%In alloys within a Hele-Shaw cell was visualized by means of X-ray radioscopy. The experimental investigations are especially focused on the impact of melt convection on the dendritic growth. Natural convection occurs during a bottom up solidification because lighter solute is rejected at the solid-liquid interface leading to an unstable density stratification. Forced convection was produced by a rotating wheel with two parallel disks containing at their inner sides a set of permanent NdFeB magnets with alternating polarization. The direction of forced melt flow is almost horizontal at the solidification front whereas local flow velocities in the range between 0.1 and 1.0 mm/s were achieved by controlling the rotation speed of the magnetic wheel. Melt flow induces various effects on the grain morphology primarily caused by the convective transport of solute. Our observations show a facilitation of the growth of primary trunks or lateral branches, suppression of side branching, dendrite remelting and fragmentation. The manifestation of all phenomena depends on the dendrite orientation, local direction and intensity of the flow. The forced flow eliminates the solutal plumes and damps the local fluctuations of solute concentration. It provokes a preferential growth of the secondary arms at the upstream side of the primary dendrite arms, whereas the high solute concentration at the downstream side of the dendrites can inhibit the formation of secondary branches completely. Moreover, the flow changes the inclination angle of the dendrites and the angle between primary trunks and secondary arms.
Passive decay heat removal by natural air convection after severe accidents
Erbacher, F.J.; Neitzel, H.J.; Cheng, X.
1995-09-01
The composite containment proposed by the Research Center Karlsruhe and the Technical University Karlsruhe is to cope with severe accidents. It pursues the goal to restrict the consequences of core meltdown accidents to the reactor plant. One essential of this new containment concept is its potential to remove the decay heat by natural air convection and thermal radiation in a passive way. To investigate the coolability of such a passive cooling system and the physical phenomena involved, experimental investigations are carried out at the PASCO test facility. Additionally, numerical calculations are performed by using different codes. A satisfying agreement between experimental data and numerical results is obtained.
Drying characteristic of barley under natural convection in a mixed-mode type solar grain dryer
Basunia, M.A.; Abe, T.
1999-07-01
Thin-layer solar drying characteristics of barley were determined at average natural air flow temperature ranging from 43.4 to 51.7 C and for relative humidities ranging from 16.5% to 37.5%. A mixed-mode type natural convection solar dryer was used for this experiment. The data of sample weight, and dry and wet bulb temperatures of the drying air were recorded continuously throughout the drying period for each test. The drying data were then fitted to the Page model. The model gave a good fit for the moisture content with an average standard error of 0.305% dry basis. The parameter N in Page's equation was assumed as a product-dependent constant which made it easy to compare the effects of independent variables on the natural convection solar drying rate without causing considerable error in predicting the drying rate for barley. A linear relationship was found between the parameter K, temperature T, and relative humidity R{sub H}.
NASA Astrophysics Data System (ADS)
Šprlák, Michal; Eshagh, Mehdi
2016-08-01
Two integral transformations between the stress function, differentiation of which gives the meridian and prime vertical components of the sub-crustal stress due to mantle convection, and the satellite-to-satellite tracking (SST) data are presented in this article. In the first one, the SST data are the disturbing potential differences between twin-satellites and in the second one the line-of-sight (LOS) gravity disturbances. It is shown that the corresponding integral kernels are well-behaving and therefore suitable for inversion and recovery of the stress function from the SST data. Recovery of the stress function and the stress components is also tested in numerical experiments using simulated SST data. Numerical studies over the Himalayas show that inverting the disturbing potential differences leads to a smoother stress function than from inverting LOS gravity disturbances. Application of the presented integral formulae allows for recovery of the stress from the satellite mission GRACE and its planned successor.
Imtiaz, Maria; Hayat, Tasawar; Alsaedi, Ahmed
2016-01-01
This paper looks at the flow of Jeffrey fluid due to a curved stretching sheet. Effect of homogeneous-heterogeneous reactions is considered. An electrically conducting fluid in the presence of applied magnetic field is considered. Convective boundary conditions model the heat transfer analysis. Transformation method reduces the governing nonlinear partial differential equations into the ordinary differential equations. Convergence of the obtained series solutions is explicitly discussed. Characteristics of sundry parameters on the velocity, temperature and concentration profiles are analyzed by plotting graphs. Computations for pressure, skin friction coefficient and surface heat transfer rate are presented and examined. It is noted that fluid velocity and temperature through curvature parameter are enhanced. Increasing values of Biot number correspond to the enhancement in temperature and Nusselt number. PMID:27583457
Imtiaz, Maria; Hayat, Tasawar; Alsaedi, Ahmed
2016-01-01
This paper looks at the flow of Jeffrey fluid due to a curved stretching sheet. Effect of homogeneous-heterogeneous reactions is considered. An electrically conducting fluid in the presence of applied magnetic field is considered. Convective boundary conditions model the heat transfer analysis. Transformation method reduces the governing nonlinear partial differential equations into the ordinary differential equations. Convergence of the obtained series solutions is explicitly discussed. Characteristics of sundry parameters on the velocity, temperature and concentration profiles are analyzed by plotting graphs. Computations for pressure, skin friction coefficient and surface heat transfer rate are presented and examined. It is noted that fluid velocity and temperature through curvature parameter are enhanced. Increasing values of Biot number correspond to the enhancement in temperature and Nusselt number. PMID:27583457
Convection due to surface-tension gradients. [in reduced gravity spacecraft environments
NASA Technical Reports Server (NTRS)
Ostrach, S.
1978-01-01
The use of dimensionless parameters to study fluid motions that could occur in a reduced-gravity environment is discussed. The significance of the Marangoni instability is considered, and the use of dimensionless parameters to investigate problems such as thermo and diffusocapillary flows is described. Characteristics of fluid flow in space are described, and the relation and interaction of motions due to capillarity and buoyancy is examined.
NASA Astrophysics Data System (ADS)
Sheikhzadeh, G. A.; Dastmalchi, M.; Khorasanizadeh, H.
2013-12-01
The effect of wall temperature variations on double diffusive natural convection of Al2O3-water nanofluid in a differentially heated square enclosure with constant temperature hot and cold vertical walls is studied numerically. Transport mechanisms of nanoparticles including Brownian diffusion and thermophoresis that cause heterogeneity are considered in non-homogeneous model. The hot and cold wall temperatures are varied, but the temperature difference between them is always maintained 5 °C. The thermophysical properties such as thermal conductivity, viscosity and density and thermophoresis diffusion and Brownian motion coefficients are considered variable with temperature and volume fraction of nanoparticles. The governing equations are discretized using the control volume method. The results show that nanoparticle transport mechanisms affect buoyancy force and cause formation of small vortexes near the top and bottom walls of the cavity and reduce the heat transfer. By increasing the temperature of the walls the effect of transport mechanisms decreases and due to enhanced convection the heat transfer rate increases.
NASA Astrophysics Data System (ADS)
Selimefendigil, Fatih; Öztop, Hakan F.
2016-11-01
In this numerical study, magnetohydrodynamics natural convection in a flexible sided triangular cavity with internal heat generation is investigated. The inclined wall of the cavity is cooled and flexible while the left vertical wall is partially heated. Galerkin weighted residual finite element method is used to solve the governing equations. The effects of pertinent parameters such as external Rayleigh number (between 104 and 106), internal Rayleigh number (between 104 and 107), elastic modulus of flexible wall (between 500 and 105), Hartmann number (between 0 and 40) and inclination angle of the magnetic field (between 0° and 90°) on the fluid flow and heat transfer characteristics were numerically investigated. It was observed local and averaged Nusselt number enhance with external Rayleigh number but in the vicinity of the upper location of the heater local heat transfer deteriorates due to the inclined wall deformation with increasing external Rayleigh number. Heat transfer reduces with internal Rayleigh number and Hartmann number. Averaged heat transfer decreases 13.25% when internal Rayleigh number is increased from 104 to 107 and decreases 40.56% when Hartmann number is increased from 0 to 10. The reduction in the convection with magnetic field is effective for higher values of external Rayleigh numbers and averaged heat transfer increases with magnetic field inclination angle.
NASA Astrophysics Data System (ADS)
Jha, B. K.; Sani, I.
2015-02-01
This paper investigates the role of induced magnetic field on a transient natural convection flow of an electrically conducting, incompressible and viscous fluid in a vertical channel formed by two infinite vertical parallel plates. The transient flow formation inside the channel is due to sudden asymmetric heating of channel walls. The time dependent momentum, energy and magnetic induction equations are solved semi-analytically using the Laplace transform technique along with the Riemann-sum approximation method. The solutions obtained are validated by comparisons with the closed form solutions obtained for the steady states which have been derived separately and also by the implicit finite difference method. Graphical results for the temperature, velocity, induced magnetic field, current density, and skin-friction based on the semi-analytical solutions are presented and discussed.
NASA Astrophysics Data System (ADS)
Mayor, T. S.; Couto, S.; Psikuta, A.; Rossi, R. M.
2015-12-01
The ability of clothing to provide protection against external environments is critical for wearer's safety and thermal comfort. It is a function of several factors, such as external environmental conditions, clothing properties and activity level. These factors determine the characteristics of the different microclimates existing inside the clothing which, ultimately, have a key role in the transport processes occurring across clothing. As an effort to understand the effect of transport phenomena in clothing microclimates on the overall heat transport across clothing structures, a numerical approach was used to study the buoyancy-driven heat transfer across horizontal air layers trapped inside air impermeable clothing. The study included both the internal flow occurring inside the microclimate and the external flow occurring outside the clothing layer, in order to analyze the interdependency of these flows in the way heat is transported to/from the body. Two-dimensional simulations were conducted considering different values of microclimate thickness (8, 25 and 52 mm), external air temperature (10, 20 and 30 °C), external air velocity (0.5, 1 and 3 m s-1) and emissivity of the clothing inner surface (0.05 and 0.95), which implied Rayleigh numbers in the microclimate spanning 4 orders of magnitude (9 × 102-3 × 105). The convective heat transfer coefficients obtained along the clothing were found to strongly depend on the transport phenomena in the microclimate, in particular when natural convection is the most important transport mechanism. In such scenario, convective coefficients were found to vary in wavy-like manner, depending on the position of the flow vortices in the microclimate. These observations clearly differ from data in the literature for the case of air flow over flat-heated surfaces with constant temperature (which shows monotonic variations of the convective heat transfer coefficients, along the length of the surface). The flow patterns and
Experimental study of natural convection enhancement using a Fe3O4-water based magnetic nanofluid.
Stoian, Floriana D; Holotescu, Sorin
2012-10-01
The effect of nanoparticles dispersed in a carrier fluid on the natural convection heat transfer is still raising controversies. While the reported experimental results show no improvement or even worsening of the heat transfer performance of nanofluids, the numerical simulations show an increase of the heat transfer coefficient, at least for certain ranges of Ra number. We report an experimental investigation regarding the natural convection heat transfer performance of a Fe3O4-water based nanofluid, in a cylindrical enclosure. The fluid was heated linearly from the bottom wall using an electric heater and cooled from the upper wall by a constant flow of water, such that a constant temperature difference between the upper and bottom walls was obtained at steady-state. The experiment was also carried out using water, in order to observe the effect of the addition of Fe3O4 nanoparticles on the heat transfer coefficient. Several regimes were tested, both for water and nanofluid. The experimental results showed that values obtained for the heat transfer coefficient for Fe3O4-water nanofluid were higher than those for water, at the same temperature difference. The present experimental results are also compared with our previous work and the reference literature. PMID:23421199
A new look at natural convection from isothermal vertical parallel plates
Li, H.H.; Chung, B.T.F.
1996-12-31
Natural convection between isothermal plates is solved numerically by applying the full Navier-Stokes equations. The elliptic formulation allows separating the effect of the Rayleigh number, Ra, and the aspect ratio, L/B. Calculations are made on a wide range of the Rayleigh number and the aspect ratio, and the Nusselt number is provided as a function of both Ra and B/L. The conventional correlations in the literature presenting the Nusselt number in terms of a single parameter, RaB/L, have been found inaccurate. At a small value of RaB/L, multiple values of Nusselt number are obtained for different combinations of Ra and B/L. Previous results are found to be the special cases of the present study. A minimum Rayleigh number is also obtained above which a fully-developed flow is possible. To simulate the natural convective flow, the ambient pressure is given at the exit while the pressure at the entrance is related to the ambient pressure by the Bernoulli equation. Velocities at the entrance and exit are also solved from the Navier-Stokes equations.
Natural convection of a high Prandlt number fluid in a cavity
Poujol, F.T.; Rojas, J.; Ramos, E.
2000-01-01
The dynamic and thermal properties of transient natural convection in cavities have been studied in the context of limnology, geophysics, solar energy and mixing applications as an effort to build realistic models of various physical phenomena. It is of interest to understand the heat transfer process to be able to enhance or reduce it, depending on the requirements. Here, transient natural convection in a square cavity heated with a time-dependent heat flux on one vertical wall and cooled by maintaining the opposite wall at a constant temperature was studied experimentally and numerically. The working fluid was silicon oil (Dow Corning fluid 20--200) with a prandtl number of 230. All experiments were carried out in a cubic cavity of 0.13 m in each side. The heating rate used was 460 W/m{sup 2}, which corresponds to a Rayleigh number of 2 x 10{sup 9}. Experimental data included temperature records at particular points and velocity measurements obtained from video images of tracers. The dynamics of the transition to the steady state is characterized by a vortex structure that forms near the heated wall. This structure is generated by shear at the heated wall boundary layer. The results were compared with a numerical simulation and qualitative agreement was obtained.
Natural and mixed convection in the cylindrical pool of TRIGA reactor
NASA Astrophysics Data System (ADS)
Henry, R.; Tiselj, I.; Matkovič, M.
2016-05-01
Temperature fields within the pool of the JSI TRIGA MARK II nuclear research reactor were measured to collect data for validation of the thermal hydraulics computational model of the reactor tank. In this context temperature of the coolant was measured simultaneously at sixty different positions within the pool during steady state operation and two transients. The obtained data revealed local peculiarities of the cooling water dynamics inside the pool and were used to estimate the coolant bulk velocity above the reactor core. Mixed natural and forced convection in the pool were simulated with a Computational Fluid Dynamics code. A relatively simple CFD model based on Unsteady RANS turbulence model was found to be sufficient for accurate prediction of the temperature fields in the pool during the reactor operation. Our results show that the simple geometry of the TRIGA pool reactor makes it a suitable candidate for a simple natural circulation benchmark in cylindrical geometry.
Natural convection in a cylindrical porous enclosure with internal heat generation
Prasad, V.; Chui, A. )
1989-11-01
A numerical study is performed on natural convection inside a cylindrical enclosure filled with a volumetrically heated, saturated porous medium for the case when the vertical wall is isothermal and the horizontal walls are either adiabatic or isothermally cooled. When the horizontal walls are insulated, the flow in the cavity is unicellular and the temperature field in upper layers is highly stratified. However, if the top wall is cooled, there may exist a multicellular flow and an unstable thermal stratification in the upper region of the cylinder. Under the influence of weak convection, the maximum temperature in the cavity can be considerably higher than that predicted for pure conduction. The local heat flux on the bounding walls is generally a strong function of the Rayleigh number, the aspect ratio, and the wall boundary conditions. The heat removal on the cold upper surface decreases with the aspect ratio, thereby increasing the Nusselt number on the vertical wall. The effect of Rayleigh number is, however, not straightforward. Several correlations are presented for the maximum cavity temperature and the overall Nusselt number.
Developing natural convection in a fluid layer with localized heating and large viscosity variation
Hickox, C.E.; Chu, Tze Yao.
1991-01-01
Numerical simulations and laboratory experiments are used to elucidate aspects of transient natural convection in a magma chamber. The magma chamber is modeled as a horizontal fluid layer confined within an enclosure of square planform and heated from below by a strip heater centered on the lower boundary of the enclosure. The width of the strip heater and the depth of the fluid layer are one-fourth of the layer width. Corn syrup is used as the working fluid in order to approximate the large viscosity variation with temperature and the large Prandtl number typical of magma. The quiescent, uniform, fluid layer is subjected to instantaneous heating from the strip heater producing a transient flow which is dominated by two counter-rotating convective cells. Experimentally determined characteristics of the developing flow are compared with numerical simulations carried out with a finite element computer program. The results of numerical simulations are in essential agreement with experimental data. Differences between the numerical simulations and experimental measurements are conjectured to result from non-ideal effects present in the experiment which are difficult to represent accurately in a numerical simulation.
NASA Astrophysics Data System (ADS)
Osipov, Aleksei I.; Uvarov, A. V.
2005-02-01
The general type of boundary conditions in heat exchange problems, including heat exchange in laser systems, is discussed. The appearance of convection in a plane layer of a nonequilibrium gas with the volume energy release and its temperature dependence is briefly considered. Convection in a cylindrical system and in a system of coaxial cylinders with a cooled central part is considered in detail. Such systems simulate real laser devices. It is shown that the maximum temperature in the cylindrical system decreases due to convection, whereas the maximum temperature in the system of coaxial cylinders increases, i.e., the analysis of heat removal in a laser system reveals a very important role of convection.
NASA Technical Reports Server (NTRS)
Greenwald, R. A.; Baker, K. B.; Ruohoniemi, J. M.; Dudeney, J. R.; Pinnock, M.; Mattin, N.; Leonard, J. M.; Lepping, R. P.
1990-01-01
Data from two conjugate HF radars currently operating at Goose Bay (Labrador) and the Halley Station (Antarctica), obtained for a single 45-min period about local noon on April 22, 1988, were used to study the near-instantaneous conjugate two-dimensional patterns of plasma convection in the vicinity of the cusp. In particular, the response of these plasma convection patterns to changes in the By component of the IMF was examined. Results indicate that, under quasi-stationary IMF conditions, the conjugate convection patterns are quite similar to the synthesized patterns of Heppner and Maynard (1987) and that the patterns respond rapidly to changes in the IMF By component. Results also show that transitions between convection states begin to occur within minutes of the time that an IMF state change is incident on the magnetospheric boundary, and that the convection reconfigurations expand poleward, completely filling the field of view of an HF radar within 6 min of the time of onset.
NASA Astrophysics Data System (ADS)
Oztop, Hakan F.; Abu-Nada, Eiyad; Varol, Yasin; Al-Salem, Khaled
2011-04-01
In this study, the problem of steady state natural convection in an enclosure filled with a nanofluid has been analyzed numerically by using heating and cooling by sinusoidal temperature profiles on one side. The governing partial differential equations, in terms of the dimensionless stream function-vorticity and temperature, are solved numerically using the finite volume method for various inclination angles 0∘≤ϕ≤90∘, different types of nanoparticles (TiO 2 and Al 2O 3) and fractions of nanoparticles 0≤φ≤0.1, whereas the range of the Rayleigh number Ra is 10 3-10 5. It is found that the addition of nanoparticles into water affects the fluid flow and temperature distribution especially for higher Rayleigh numbers. An enhancement in heat transfer rate was registered for the whole range of Rayleigh numbers. However, low Rayleigh numbers show more enhancement compared to high Rayleigh numbers.
MHD natural convection flow along a vertical wavy surface with heat generation and pressure work
NASA Astrophysics Data System (ADS)
Alim, M. A.; Kabir, K. H.; Andallah, L. S.
2016-07-01
In this paper, the influence of pressure work on MHD natural convection flow of viscous incompressible fluid along a uniformly heated vertical wavy surface with heat generation has been investigated. The governing boundary layer equations are first transformed into a non-dimensional form using suitable set of dimensionless variables. The resulting nonlinear system of partial differential equations are mapped into the domain of a vertical flat plate and then solved numerically employing the implicit finite difference method, known as Keller-box scheme. The numerical results for the velocity profiles, temperature profiles, skin friction coefficient, the rate of heat transfers, the streamlines and the isotherms are shown graphically and skin friction coefficient and rate of heat transfer have been shown in tabular form for different values of the selective set of parameters consisting of pressure work parameter Ge, the magnetic parameter M, Prandtl number Pr, heat generation parameter Q and the amplitude of the wavy surface.
Enhancement of natural-convection heat transfer from a horizontal heated plate using grid fins
Kitamura, Kenzo; Nagae, Naoyuki; Kimura, Fumiyoshi
1996-01-01
An enhancement technique was developed for natural-convection heat transfer from a horizontal heated plate. In order to enhance the heat transfer, grid fins made of copper plates were soldered to the copper base plate. These grid fins function not only as an extended surface but also as a heat-transfer promoter. The apparent heat-transfer coefficient of the above enhanced plate were measured and compared with those of a nontreated, smooth plate and a conventional plate with vertical straight fins. It was found that the highest performance is achieved by the present plate. By adopting grid fins with appropriate size and height, the heat-transfer coefficient at the central portion of the present plate is increased by 35% compared to that of the conventional finned plate with the same fin area of fin height.
Design and Scaling of the Natural Convection Shutdown Heat Removal Test Facility
Lisowski, Darius D.; Gerardi, Craig D.; Bremer, Nathan C.; Farmer, Mitchell T.
2014-01-01
The Natural convection Shutdown heat removal Test Facility (NSTF) at Argonne National Laboratory (ANL) reflects a 1/2 scale model of one conceptual design for passive safety in advanced reactors. The project was initiated in 2010 primarily to conduct ex-vessel, passive decay heat removal experiments in support of the Advanced Reactor Concepts (ARC), Small Modular Reactor (SMR), and Next Generation Nuclear Plant (NGNP) programs while also generating data for code validation purposes. The facility successfully demonstrated scoping objectives in late 2013, and is expected to begin testing by early 2014. The following paper summarizes some of the key design and scaling considerations used in construction of the experimental facility, along with an overview of the current instrumentation and data acquisition methods. Details of the distributed fiber optic temperature system will be presented, which introduces a level of data density suitable for CFD validation and is a first-of-its-kind for largescale thermal hydraulics facilities.
Flow patterns of natural convection in an air-filled vertical cavity
NASA Astrophysics Data System (ADS)
Wakitani, Shunichi
1998-08-01
Flow patterns of two-dimensional natural convection in a vertical air-filled tall cavity with differentially heated sidewalls are investigated. Numerical simulations based on a finite difference method are carried out for a wide range of Rayleigh numbers and aspect ratios from the onset of the steady multicellular flow, through the reverse transition to the unicellular pattern, to the unsteady multicellular flow. For aspect ratios (height/width) from 10 to 24, the various cellular structures characterized by the number of secondary cells are clarified from the simulations by means of gradually increasing Rayleigh number to 106. Unsteady multicellular solutions are found in some region of Rayleigh numbers less than those at which the reverse transition has occurred.
Natural convection flow in porous enclosure with localized heating from below with heat flux
NASA Astrophysics Data System (ADS)
Siddiki, Md. Noor-A.-Alam; Molla, Md. Mamun; Saha, Suvash C.
2016-07-01
Unsteady natural convection flow in a two dimensional fluid saturated porous enclosure with localized heating from below with heat flux, symmetrical cooling from the sides and the insulated top wall has been investigated numerically. The governing equations are the Darcy's law for the porous media and the energy equation for the temperature field has been considered. The non-dimensional Darcy's law in terms of the stream function is solved by finite difference method using the successive over-relaxation (SOR) scheme and the energy equation is solved by Alternative Direction Alternative (ADI) scheme. The uniform heat flux source is located centrally at the bottom wall. The numerical results are presented in terms of the streamlines and isotherms, as well as the local and average rate of heat transfer for the wide range of the Darcy's Rayleigh number and the length of the heat flux source at the bottom wall.
Dunn, T.A.; McCallen, R.C.
2000-10-17
The Galerkin Finite Element Method was used to predict a natural convection flow in an enclosed cavity. The problem considered was a differentially heated, tall (8:1), rectangular cavity with a Rayleigh number of 3.4 x 10{sup 5} and Prandtl number of 0.71. The incompressible Navier-Stokes equations were solved using a Boussinesq approximation for the buoyancy force. The algorithm was developed for efficient use on massively parallel computer systems. Emphasis was on time-accurate simulations. It was found that the average temperature and velocity values can be captured with a relatively coarse grid, while the oscillation amplitude and period appear to be grid sensitive and require a refined computation.
Fossa, M.; Menezo, C.; Leonardi, E.
2008-02-15
An experimental study on natural convection in an open channel is carried out in order to investigate the effect of the geometrical configuration of heat sources on the heat transfer behaviour. To this aim, a series of vertical heaters are cooled by natural convection of air flowing between two parallel walls. The objective of the work is to investigate the physical mechanisms which influence the thermal behaviour of a double-skin photovoltaic (PV) facade. This results in a better understanding of the related phenomena and infers useful engineering information for controlling the energy transfers from the environment to the PV surfaces and from the PV surfaces to the building. Furthermore increasing the heat transfer rate from the PV surfaces increases the conversion efficiency of the PV modules since they operate better as their temperature is lower. The test section consists in a double vertical wall, 2 m high, and each wall is constituted by 10 different heating modules 0.2 m high. The heater arrangement simulates, at a reduced scale, the presence of a series of vertical PV modules. The heat flux at the wall ranges from 75 to 200 W/m{sup 2}. In this study, the heated section is 1.6 m in height, preceded by an adiabatic of 0.4 m in height. Different heating configurations are analyzed, including the uniform heating mode and two different configurations of non uniform, alternate heating. The experimental procedure allows the wall surface temperature, local heat transfer coefficient and local and average Nusselt numbers to be inferred. The experimental evidences show that the proper selection of the separating distance and heating configuration can noticeably decrease the surface temperatures and hence enhance the conversion efficiency of PV modules. (author)
NASA Technical Reports Server (NTRS)
Skarda, J. Raymond Lee; McCaughan, Frances E.
1998-01-01
Stationary onset of convection due to surface tension variation in an unbounded multicomponent fluid layer is considered. Surface deformation is included and general flux boundary conditions are imposed on the stratifying agencies (temperature/composition) disturbance equations. Exact solutions are obtained to the general N-component problem for both finite and infinitesimal wavenumbers. Long wavelength instability may coexist with a finite wavelength instability for certain sets of parameter values, often referred to as frontier points. For an impermeable/insulated upper boundary and a permeable/conductive lower boundary, frontier boundaries are computed in the space of Bond number, Bo, versus Crispation number, Cr, over the range 5 x 10(exp -7) less than or equal to Bo less than or equal to 1. The loci of frontier points in (Bo, Cr) space for different values of N, diffusivity ratios, and, Marangoni numbers, collapsed to a single curve in (Bo, D(dimensional variable)Cr) space, where D(dimensional variable) is a Marangoni number weighted diffusivity ratio.
Risk Due to Radiological Terror Attacks With Natural Radionuclides
Friedrich, Steinhaeusler; Lyudmila, Zaitseva; Stan, Rydell
2008-08-07
The naturally occurring radionuclides radium (Ra-226) and polonium (Po-210) have the potential to be used for criminal acts. Analysis of international incident data contained in the Database on Nuclear Smuggling, Theft and Orphan Radiation Sources (CSTO), operated at the University of Salzburg, shows that several acts of murder and terrorism with natural radionuclides have already been carried out in Europe and Russia. Five different modes of attack (T) are possible: (1) Covert irradiation of an individual in order to deliver a high individual dose; (2) Covert irradiation of a group of persons delivering a large collective dose; (3) Contamination of food or drink; (4) Generation of radioactive aerosols or solutions; (5) Combination of Ra-226 with conventional explosives (Dirty Bomb).This paper assesses the risk (R) of such criminal acts in terms of: (a) Probability of terrorist motivation deploying a certain attack mode T; (b) Probability of success by the terrorists for the selected attack mode T; (c) Primary damage consequence (C) to the attacked target (activity, dose); (d) Secondary damage consequence (C') to the attacked target (psychological and socio-economic effects); (e) Probability that the consequences (C, C') cannot be brought under control, resulting in a failure to manage successfully the emergency situation due to logistical and/or technical deficits in implementing adequate countermeasures. Extensive computer modelling is used to determine the potential impact of such a criminal attack on directly affected victims and on the environment.
Risk Due to Radiological Terror Attacks With Natural Radionuclides
NASA Astrophysics Data System (ADS)
Friedrich, Steinhäusler; Stan, Rydell; Lyudmila, Zaitseva
2008-08-01
The naturally occurring radionuclides radium (Ra-226) and polonium (Po-210) have the potential to be used for criminal acts. Analysis of international incident data contained in the Database on Nuclear Smuggling, Theft and Orphan Radiation Sources (CSTO), operated at the University of Salzburg, shows that several acts of murder and terrorism with natural radionuclides have already been carried out in Europe and Russia. Five different modes of attack (T) are possible: (1) Covert irradiation of an individual in order to deliver a high individual dose; (2) Covert irradiation of a group of persons delivering a large collective dose; (3) Contamination of food or drink; (4) Generation of radioactive aerosols or solutions; (5) Combination of Ra-226 with conventional explosives (Dirty Bomb). This paper assesses the risk (R) of such criminal acts in terms of: (a) Probability of terrorist motivation deploying a certain attack mode T; (b) Probability of success by the terrorists for the selected attack mode T; (c) Primary damage consequence (C) to the attacked target (activity, dose); (d) Secondary damage consequence (C') to the attacked target (psychological and socio-economic effects); (e) Probability that the consequences (C, C') cannot be brought under control, resulting in a failure to manage successfully the emergency situation due to logistical and/or technical deficits in implementing adequate countermeasures. Extensive computer modelling is used to determine the potential impact of such a criminal attack on directly affected victims and on the environment.
ERIC Educational Resources Information Center
Saatadjian, Esteban; Lesage, Francois; Mota, Jose Paulo B.
2013-01-01
A project that involves the numerical simulation of transport phenomena is an excellent method to teach this subject to senior/graduate chemical engineering students. The subject presented here has been used in our senior/graduate course, it concerns the study of natural convection heat transfer between two concentric, horizontal, saturated porous…
Natural remobilization of multicomponent DNAPL pools due to dissolution.
Roy, J W; Smith, J E; Gillham, R W
2002-12-01
Mixtures of dense nonaqueous phase liquids (DNAPLs) trapped in the subsurface can act as long-term sources of contamination by dissolving into flowing groundwater. If the components have different solubilities then dissolution will alter the composition of the remaining DNAPL. We theorized that a multicomponent DNAPL pool may become mobile due to the natural dissolution process. In this study, we focused on two scenarios: (1) a DNAPL losing light component(s), with the potential for downward migration; and (2) a DNAPL losing dense component(s), with the potential for upward migration following transformation into a less dense than water nonaqueous phase liquid (LNAPL). We considered three binary mixtures of common groundwater contaminants: benzene and tetrachloroethylene (PCE), PCE and dichloromethane (DCM), and DCM and toluene. A number of physical properties that control the retention and transport of DNAPL in porous media were measured for the mixtures, namely: density, interfacial tension, effective solubility, and viscosity. All properties except density exhibited nonlinear relationships with changing molar ratio of the DNAPL. To illustrate the potential for natural remobilization, we modelled the following two primary mechanisms: the reduction in pool height as mass is lost by dissolution, and the changes in fluid properties with changing molar ratio of the DNAPL. The first mechanism always reduces the capillary pressure in the pool, while the second mechanism may increase the capillary pressure or alter the direction of the driving force. The difference between the rate of change of each determines whether the potential for remobilization increases or decreases. Static conditions and horizontal layering were assumed along with a one-dimensional, compositional modelling approach. Our results indicated that for initial benzene/PCE ratios greater than 25:75, the change in density was sufficiently faster than the decline in pool height to promote DNAPL
T. Hadgu; S. Webb; M. Itamura
2004-02-12
Yucca Mountain, Nevada has been designated as the nation's high-level radioactive waste repository and the U.S. Department of Energy has been approved to apply to the U.S. Nuclear Regulatory Commission for a license to construct a repository. Heat transfer in the Yucca Mountain Project (YMP) drift enclosures is an important aspect of repository waste emplacement. Canisters containing radioactive waste are to be emplaced in tunnels drilled 500 m below the ground surface. After repository closure, decaying heat is transferred from waste packages to the host rock by a combination of thermal radiation, natural convection and conduction heat transfer mechanism?. Current YMP mountain-scale and drift-scale numerical models often use a simplified porous medium code to model fluid and heat flow in the drift openings. To account for natural convection heat transfer, the thermal conductivity of the air was increased in the porous medium model. The equivalent thermal conductivity, defined as the ratio of total heat flow to conductive heat flow, used in the porous media models was based on horizontal concentric cylinders. Such modeling does not effectively capture turbulent natural convection in the open spaces as discussed by Webb et al. (2003) yet the approach is still widely used on the YMP project. In order to mechanistically model natural convection conditions in YMP drifts, the computational fluid dynamics (CFD) code FLUENT (Fluent, Incorporated, 2001) has been used to model natural convection heat transfer in the YMP emplacement drifts. A two-dimensional (2D) model representative of YMP geometry (e.g., includes waste package, drip shield, invert and drift wall) has been developed and numerical simulations made (Francis et al., 2003). Using CFD simulation results for both natural convection and conduction-only heat transfer in a single phase, single component fluid, equivalent thermal conductivities have been calculated for different Rayleigh numbers. Correlation
NASA Astrophysics Data System (ADS)
Sheremet, M. A.; Pop, I.; Shenoy, A.
2016-03-01
Natural convective heat transfer and fluid flow in an open porous cavity filled with a nanofluid is studied numerically using the Tiwari and Das nanofluid model. The transport equations for mass, momentum and energy formulated in dimensionless stream function and temperature are solved numerically using a second-order accurate finite difference method. Particular efforts are focused on the effects of the governing parameters on the heat and fluid flow. It is found that an increase in undulation number of the wavy vertical wall leads to an attenuation of convective flow and a decrease in the heat transfer rate.
Laminar Natural Convection in Vertical Tubes with One End Open to a Large Reservoir
NASA Astrophysics Data System (ADS)
Wu, Yissu
1995-01-01
The problem of laminar natural convection in vertical tubes with one end open to a large reservoir, designated open thermosyphons, is numerically and experimentally examined to predict flow behavior and the heat transfer rates. In the numerical study, a semi-implicit, time-marching, finite -volume solution procedure was adopted to solve the three governing equations--mass, momentum, and energy--sequentially. Experimental work involved the use of a Mach-Zehnder interferometer to examine the temperature field for a modified rectangular open thermosyphon through the interpretation of fringe patterns. These experimental fringe patterns were used for the qualitative comparison with those obtained from the numerical analyses. Nusselt numbers were determined from the interferometer results and compared with numerical results. Heat transfer rates through the tube wall were found to be strong functions of the tube radius, and approached an asymptotic limit as the tube radius was increased. Both experimental and numerical results exhibited an oscillatory nature for large height-to-width (aspect ratio) open cavities. comparisons between experimental and numerically-generated fringe patterns indicated good agreement. Based on experimental results, a correlation between Nusselt number, Nu, and Rayleigh number, Ra_{rm w}, for different aspect ratios, L/W, was determined to be Nu = 0.036cdotrm Ra_sp{w }{2/5}cdot(L/W)^{-1/5} .
NASA Astrophysics Data System (ADS)
McGee, B. W.
2006-12-01
A synthesis of terrestrial and Martian data suggests that a convective vortex, or "dust devil," is a significant, non-random terrestrial eolian sediment transport phenomenon, which has implications for sediment-based migration of radionuclides on Frenchman Flat playa, a 20 square-mile mountain-bounded dry lake bed approximately centered in Frenchman Flat on the Nevada Test Site (NTS). Planetary scientists are often forced to rely on terrestrial analogues to begin characterizing extraterrestrial processes. However, as the planetary database matures, an increasing number of well-characterized extraterrestrial analogues for terrestrial processes will become available. Such analogues may provide a convenient means to investigate poorly understood or otherwise inaccessible terrestrial phenomena. Historical atmospheric nuclear experiments conducted from 1951 to 1962 deposited radionuclides into surface sediments across parts of Frenchman Flat playa, where dust devils are known to commonly occur, especially during the summer months. Recent information from both terrestrial and Martian studies yields that dust devils can be significant contributors to both the local eolian sediment transport regime and the regional climate system. Additionally, the use of terrestrial desert environments as Martian analogues, as well as the recent, unique discovery of Mars-like dust devil tracks in Africa, has established a working correlation between Earth, Mars, and the dust devil phenomenon. However, while the difficulty in tracking dust devil paths on Earth has hindered the determination of any net sediment transport due to dust devils, the dramatic albedo contrast in disturbed sediment on Mars lends to the formation of persistent, curvilinear dust devil tracks. These tracks illustrate that in zones of preferential formation, dust devils possess non-random orientations over seasonal timescales with respect to prevailing wind. By calibrating these Martian orientations with meteorological
Natural convection heat transfer of nanofluids along a vertical plate embedded in porous medium.
Uddin, Ziya; Harmand, Souad
2013-02-07
The unsteady natural convection heat transfer of nanofluid along a vertical plate embedded in porous medium is investigated. The Darcy-Forchheimer model is used to formulate the problem. Thermal conductivity and viscosity models based on a wide range of experimental data of nanofluids and incorporating the velocity-slip effect of the nanoparticle with respect to the base fluid, i.e., Brownian diffusion is used. The effective thermal conductivity of nanofluid in porous media is calculated using copper powder as porous media. The nonlinear governing equations are solved using an unconditionally stable implicit finite difference scheme. In this study, six different types of nanofluids have been compared with respect to the heat transfer enhancement, and the effects of particle concentration, particle size, temperature of the plate, and porosity of the medium on the heat transfer enhancement and skin friction coefficient have been studied in detail. It is found that heat transfer rate increases with the increase in particle concentration up to an optimal level, but on the further increase in particle concentration, the heat transfer rate decreases. For a particular value of particle concentration, small-sized particles enhance the heat transfer rates. On the other hand, skin friction coefficients always increase with the increase in particle concentration and decrease in nanoparticle size.
Numerical modeling of a lead melting front under the influence of natural convection
NASA Astrophysics Data System (ADS)
Coulson, Ryan
This work presents a study of the Effective Heat Capacity (EHC) method applied to the numerical simulation of the interface between a solid and a naturally convecting pool of liquid lead under pseudo-steady-state and transient conditions using COMSOL Multiphysics. The EHC method is implemented as a temperature dependent pseudo-material with discontinuities in the heat capacity, dynamic viscosity, and thermal conductivity to simulate the melting front. The approach is validated with experimental data for a vertical melting front between two walls. The hot wall heat flux and the cold wall temperature are adjusted until the numerical model that best matches the experimental data is found. The best case boundary conditions then serve as the control in subsequent studies of key modeling parameters, including the mesh refinement, the discontinuity width and location, the maximum allowable time step, and the jump in dynamic viscosity. An extra fine mesh with a maximum element size of 1.24 * 10--3 m2 results in the most accurate model. For pseudo-steady-state models the width and location of the discontinuity does not affect the results substantially but it does affect the settling times and transient behavior of the models. The maximum allowable time step is dependent on the mesh resolution. The behavior of the pseudo-solid transitions from solid to liquid when the dynamic viscosity is less then 1.0 * 104 Pa · s.
Pulsating and traveling wave modes of natural convection in spherical shells
NASA Astrophysics Data System (ADS)
Scurtu, N.; Futterer, B.; Egbers, C.
2010-11-01
A numerical study is made of the natural convective fluid motion in the spherical shell geometry, i.e., the gap between two concentric spheres. The case of homogeneously heated inner sphere and cooled outer sphere is considered for the radius ratio η =0.714 and Prandtl number Pr=0.7. Patterns of fluid flow are established by the variation of the Rayleigh number Ra and its heat transfer is characterized by the Nusselt number Nu. For small values of the Rayleigh number, a crescent shaped axisymmetric vortex is formed and is regarded as the basic flow. By increasing the Rayleigh number, two transitions occur to a fully developed three-dimensional irregular flow. On the first bifurcation branch, a pulsating wave flow was found with petal-like formations pulsating in meridional direction. On the second branch, a traveling wave flow exists with an azimuthal rotation of the spirally distributed petal patterns. Various characteristics of the flow patterns are investigated as well as their transition to chaos. Both branches conjoin in the very supercritical domain, where the traveling wave dominates.
Characterization of Fuego for laminar and turbulent natural convection heat transfer.
Francis, Nicholas Donald, Jr.
2005-08-01
A computational fluid dynamics (CFD) analysis is conducted for internal natural convection heat transfer using the low Mach number code Fuego. The flow conditions under investigation are primarily laminar, transitional, or low-intensity level turbulent flows. In the case of turbulent boundary layers at low-level turbulence or transitional Reynolds numbers, the use of standard wall functions no longer applies, in general, for wall-bounded flows. One must integrate all the way to the wall in order to account for gradients in the dependent variables in the viscous sublayer. Fuego provides two turbulence models in which resolution of the near-wall region is appropriate. These models are the v2-f turbulence model and a Launder-Sharma, low-Reynolds number turbulence model. Two standard geometries are considered: the annulus formed between horizontal concentric cylinders and a square enclosure. Each geometry emphasizes wall shear flow and complexities associated with turbulent or near turbulent boundary layers in contact with a motionless core fluid. Overall, the Fuego simulations for both laminar and turbulent flows compared well to measured data, for both geometries under investigation, and to a widely accepted commercial CFD code (FLUENT).
Natural convection in a differentially heated square enclosure with a solid polygon.
Roslan, R; Saleh, H; Hashim, I
2014-01-01
The aim of the present numerical study is to analyze the conjugate natural convection heat transfer in a differentially heated square enclosure containing a conductive polygon object. The left wall is heated and the right wall is cooled, while the horizontal walls are kept adiabatic. The COMSOL Multiphysics software is applied to solve the dimensionless governing equations. The governing parameters considered are the polygon type, 3 ≤ N ≤ ∞, the horizontal position, 0.25 ≤ X 0 ≤ 0.75, the polygon size, 0 ≤ A ≤ π/16, the thermal conductivity ratio, 0.1 ≤ K r ≤ 10.0, and the Rayleigh number, 10(3) ≤ Ra ≤ 10(6). The critical size of the solid polygon was found exists at low conductivities. The heat transfer rate increases with the increase of the size of the solid polygon, until it reaches its maximum value. Here, the size of the solid polygon is reaches its critical value. Further, beyond this critical size of the solid polygon, will decrease the heat transfer rate.
NASA Technical Reports Server (NTRS)
Chang, C. J.; Brown, R. A.
1983-01-01
The roles of natural convection in the melt and the shape of the melt/solid interface on radial dopant segregation are analyzed for a prototype of vertical Bridgman crystal growth system by finite element methods that solve simultaneously for the velocity field in the melt, the shape of the solidification isotherm, and the temperature distribution in both phases. Results are presented for crystal and melt with thermophysical properties similar to those of gallium-doped germanium in Bridgman configurations with melt below (thermally destabilizing) and above (stabilizing) the crystal. Steady axisymmetric flow are classified according to Rayleigh number as either being nearly the growth velocity, having a weak cellular structure or having large amplitude cellular convention. The flows in the two Bridgman configurations are driven by different temperature gradients and are in opposite directions. Finite element calculations for the transport of a dilute dopant by these flow fields reveal radial segregation levels as large as sixty percent of the mean concentration. Segregation is found most severe at an intermediate value of Rayleigh number above which the dopant distribution along the interface levels as the intensity of the flow increases.
Study of natural convection cooling of a nanofluid subjected to a magnetic field
NASA Astrophysics Data System (ADS)
Mahmoudi, Ahmed; Mejri, Imen; Omri, Ahmed
2016-06-01
This paper presents a numerical study of natural convection cooling of water-Al2O3 nanofluid by two heat sinks vertically attached to the horizontal walls of a cavity subjected to a magnetic field. The left wall is hot, the right wall is cold, while the horizontal walls are insulated. Lattice Boltzmann method (LBM) is applied to solve the coupled equations of flow and temperature fields. This study has been carried out for the pertinent parameters in the following ranges: Rayleigh number of the base fluid, Ra =103 to 105, Hartmann number varied from Ha = 0 to 60 and the solid volume fraction of nanoparticles between ϕ = 0 and 6%. In order to investigate the effect of heat sinks location, three different configurations of heat sinks are considered. The effects of Rayleigh numbers, Hartmann number and heat sinks location on the streamlines, isotherms, Nusselt number are investigated. Results show that the heat transfer rate decreases with the increase of Hartmann number and increases with the rise of Rayleigh number. In addition it is observed that the average Nusselt number increases linearly with the increase of the nanoparticles solid volume fraction. Also, results show that the heat sinks positions greatly influence the heat transfer rate depending on the Hartmann number, Rayleigh number and nanoparticle solid volume fraction.
Natural convection inside a porous trapezoidal enclosure with wavy top surface
NASA Astrophysics Data System (ADS)
Eshon, Sehrina Muzahid; Mustafa, Rakib; Hasan, Mohammad Nasim
2016-07-01
The aim of the present work is analysis of heat flow during natural convection inside a trapezoidal porous cavity having wavy top surface. The bottom wall of the cavity is sinusoidally heated whereas the top wall is kept at constant low temperature and the side walls are maintained adiabatic. The physical problem has been represented mathematically by various governing equations along with the corresponding boundary conditions and hence solved by using Galerkin Finite Element scheme. Numerical simulations were carried out and the flow and thermal fields inside the cavity were analyzed in terms of distribution of isothermal lines (θ), streamlines (ψ) and heatlines (Π). To compare heat transfer characteristics local Nusselt number (Nu), and average Nusselt number (Nuavg) along the hot bottom wall are studied for various system parameters, such as, Rayleigh number (Ra) and Darcy number (Da). The range of Ra, Da considered in the present study are as follows; 104 ≤ Ra ≤ 106, 10-5 ≤ Da ≤ 10-3. The present study has been conducted for the trapezoidal cavity being filled with two different types of fluids; water (Pr = 7.2), and molten gallium (Pr = 0.026). It has been found that an increase in flow intensity and heat transfer occurs at higher Rayleigh number (Ra) and Darcy number (Da) whereas the effect of Prandtl number (Pr) is somewhat negligible.
Natural convection heat transfer from a horizontal wavy surface in a porous enclosure
Murthy, P.V.S.N.; Kumar, B.V.R.; Singh, P.
1997-02-07
The effect of surface undulations on the natural convection heat transfer from an isothermal surface in a Darcian fluid-saturated porous enclosure has been numerically analyzed using the finite element method on a graded nonuniform mesh system. The flow-driving Rayleigh number Ra together with the geometrical parameters of wave amplitude a, wave phase {phi}, and the number of waves N considered in the horizontal dimension of the cavity are found to influence the flow and heat transfer process in the enclosure. For Ra around 50 and above, the phenomenon of flow separation and reattachment is noticed on the walls of the enclosure. A periodic shift in the reattachment point from the bottom wall to the adjacent walls in the clockwise direction, leading to the manifestation of cycles of unicellular and bicellular clockwise and counterclockwise flows, is observed, with the phase varying between 0{degree} and 350{degree}. The counterflow in the secondary circulation zone is intensified with the increase in the value of Ra. The counterflow on the wavy wall hinders the heat transfer into the system. An increase in either wave amplitude or the number of waves considered per unit length decreases the global heat flux into the system. Only marginal changes in global heat flux are noticed with increasing Ra. On the whole, the comparison of global heat flux results in the wavy wall case with those of the horizontal flat wall case shows that, in a porous enclosure, the wavy wall reduces the heat transfer into the system.
Boyd, R.D.
1980-01-01
The natural convective heat transfer across an annulus with irregular boundaries was studied using a Mach-Zehnder interferometer. The annulus was formed by an inner hexagonal cylinder and an outer concentric circular cylinder. This configuration models, in two dimensions, a liquid metal fast breeder reactor spent fuel subassembly inside a shipping container. During the test, the annulus was filled with a single gas, either neon, air, argon, krypton, or xenon, at a pressure of about 0.5 MPa. From temperature measurements, both local and mean Nusselt numbers (Nu/sub ..delta../) at the surface of the inner cylinder were evaluated, with the mean Rayleigh number (anti Ra/sub ..delta../) varying from 4.54 x 10/sup 4/ to 0.915 x 10/sup 6/ (..delta.. is the local gas width). The data correlation for the mean Nusselt and Rayleigh numbers is given by anti Nu/sub ..delta../ = 0.183 anti Ra/sub ..delta..//sup 0/ /sup 310/.
Natural Convection in a Differentially Heated Square Enclosure with a Solid Polygon
Roslan, R.; Saleh, H.; Hashim, I.
2014-01-01
The aim of the present numerical study is to analyze the conjugate natural convection heat transfer in a differentially heated square enclosure containing a conductive polygon object. The left wall is heated and the right wall is cooled, while the horizontal walls are kept adiabatic. The COMSOL Multiphysics software is applied to solve the dimensionless governing equations. The governing parameters considered are the polygon type, 3 ≤ N ≤ ∞, the horizontal position, 0.25 ≤ X 0 ≤ 0.75, the polygon size, 0 ≤ A ≤ π/16, the thermal conductivity ratio, 0.1 ≤ K r ≤ 10.0, and the Rayleigh number, 103 ≤ Ra ≤ 106. The critical size of the solid polygon was found exists at low conductivities. The heat transfer rate increases with the increase of the size of the solid polygon, until it reaches its maximum value. Here, the size of the solid polygon is reaches its critical value. Further, beyond this critical size of the solid polygon, will decrease the heat transfer rate. PMID:24991643
Natural convection in an enclosure with discrete roughness elements on a vertical heated wall
Shakerin, S; Bohn, M S; Loehrke, R I
1986-02-01
Natural convection flow next to a heated wall with single and repeated, two-dimensional, rectangular roughness elements is studied numerically and experimentally. The objective is to determine how these roughness elements influence heat transfer rates from the wall. Each roughness element consists of a thermally conducting, horizontal cylinder of rectangular cross section attached to the heated, isothermal wall of an enclosure. The height of roughness is on the order of the boundary layer thickness. Dye flow visualization in water confirms the numerical prediction that the steady flow over these elements does not separate. Only at high Rayleigh numbers, when the boundary layer below the roughness is unsteady, is local instantaneous flow reversal observed. Although steady flow reversals near the wall are not predicted or observed, nearly stagnant regions are formed, particularly between closely spaced cylinders. The surface heat flux in these stagnant regions is relatively low, so the total heat transfer rate may be nearly the same as for a smooth wall in spite of the increased surface area.
OXYGEN TRANSFER ACROSS THE AIR-WATER INTERFACE DUE TO NATURAL CONVECTION IN LAKES. (R825428)
The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Concl...
Natural solutal convection in magnetic fluids: First-order phase transition aspect
NASA Astrophysics Data System (ADS)
Ivanov, Aleksey S.
2016-10-01
Concentration stratification of magnetic fluids under the action of external magnetic field can disturb mechanical equilibrium in the system and cause intensive solutal convection. The current paper is devoted to the study of free solutal convection in magnetic fluids undergoing first-order phase transition. Simulation of solutal convection in OpenFOAM package makes it possible to compare numeric results with physical experiment observations. The numeric simulation of convective hydrodynamic flows was carried out in the framework of several theories of first-order phase transition in ferrocolloids. The numerical results are compared with experimental observations in order to choose the theory which predicts most accurately the concentration stratification in magnetic fluids undergoing magneto-controllable first-order phase transition.
NASA Astrophysics Data System (ADS)
Somavilla Cabrillo, Raquel; Schauer, Ursula; Budeus, Gedeon; Latarius, Katrin
2015-04-01
There are only a few sites where the deep ocean is ventilated from the surface. The responsible process known as deep convection is recognized to be a key process on the Earth's climate system, but still it is scarcely observed, and its good representation by global oceanographic and climate models remains unclear. In the Arctic Ocean, the halt of deep convection in the Greenland Sea during the last three decades serves as a natural experiment to study: (1) the conditions that drive the occurrence or not of deep convection and (2) the effects of the halt of deep convection on the thermohaline properties of the deep water masses and circulation both locally and in adjacent ocean basins. Combining oceanic and atmospheric in-situ data together with reanalysis data, we observe that not only on average the winter net heat losses from the ocean to the atmosphere (Qo) have decreased during the last three decades in the Greenland Sea (ΔQo (before the 1980s- after the 1980s) = 25 Wm-2) but the intensity and number of strong cooling events (Qo ≥ 800Wm-2). This last value for convection reaching 2000 m in the Greenland Sea seems critical to make the mixed layer deepening from being a non-penetrative process to one arrested by baroclinic instabilities. Besides, changes in the wind stress curl and preconditioning for deep convection have occurred, hindering also the occurrence of deep convection. Concerning the effects of the halt of deep convection, hydrographic data reveal that the temperature between 2000 meters depth and the sea floor has risen by 0.3 °C in the last 30 years, which is ten times higher than the temperature increase in the global ocean on average, and salinity rose by 0.02 because import of relatively warm and salty Arctic Ocean deep waters continued. The necessary transports to explain the observed changes suggest an increase of Arctic Ocean deep water transport that would have compensated the decrease in deep water formation rate after the 1980s. The
NASA Astrophysics Data System (ADS)
Hyland, P.; Biggerstaff, M. I.; Uman, M. A.; Hill, J. D.; Krehbiel, P. R.; Rison, W.
2012-12-01
During the summers of 2011-2012, a C-band polarimetric Shared Mobile Atmospheric Research and Teaching (SMART) radar from the University of Oklahoma was deployed to Keystone Heights, FL to study the relationship between cloud structure and the propagation of triggered and natural lightning channels. The radar was operated in Range-Height-Indicator (RHI) volume scanning mode over a narrow azimuthal sector that provided high spatial vertical resolution every 90 seconds over the rocket launch facility at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, FL. In this presentation, we will focus on observations collected in 2011. Seven successful triggers (with return strokes) out of 20 attempts were sampled by the SMART-R from June to August. Most of the trigger attempts occurred during the dissipating stages of convection with steady ground electric field values. Specific differential phase (KDP) showed evidence of ice crystal alignment due to strong electric fields within the upper portions of the convection over ICLRT around the time of launch attempts. Consecutive RHI sweeps over ICLRT revealed changes in KDP that suggested the building of electric fields and subsequent relaxation after a triggered flash. KDP signatures relative to other radar variables will also be investigated to determine the microphysical and convective nature of the storms in which natural and triggered lightning strikes occurred. Lightning Mapping Array (LMA) sources of the triggered flash channels showed a preference for horizontal propagation just above the radar bright band associated with the melting layer. This finding agrees with several past studies that used balloon soundings and found intense layers of charge near the 0°C isotherm. The propagation path also seemed to be related to the vertical distribution of KDP in some of the triggered flashes. A preferred path through areas of generally positive values of KDP suggests that triggered lightning
Marcus, F. A.; Beyer, P.; Fuhr, G.; Monnier, A.; Benkadda, S.
2014-08-15
With the resonant magnetic perturbations (RMPs) consolidating as an important tool to control the transport barrier relaxation, the mechanism on how they work is still a subject to be clearly understood. In this work, we investigate the equilibrium states in the presence of RMPs for a reduced MHD model using 3D electromagnetic fluid numerical code with a single harmonic RMP (single magnetic island chain) and multiple harmonics RMPs in cylindrical and toroidal geometry. Two different equilibrium states were found in the presence of the RMPs with different characteristics for each of the geometries used. For the cylindrical geometry in the presence of a single RMP, the equilibrium state is characterized by a strong convective radial thermal flux and the generation of a mean poloidal velocity shear. In contrast, for toroidal geometry, the thermal flux is dominated by the magnetic flutter. For multiple RMPs, the high amplitude of the convective flux and poloidal rotation are basically the same in cylindrical geometry, but in toroidal geometry the convective thermal flux and the poloidal rotation appear only with the islands overlapping of the linear coupling between neighbouring poloidal wavenumbers m, m – 1, and m + 1.
NASA Astrophysics Data System (ADS)
Chen, Wen Ruey
2016-10-01
This paper studies the steady laminar natural convection of micropolar fluids in the complex annuli between the inner sphere and outer vertical cylinder to present a numerical analysis of the flow and heat transfer characteristics with buoyancy effects. Computations were carried out systematically by the several different parameters of geometric ratio, micropolar material parameter and Rayleigh number to determine the average Nusselt number and the skin friction coefficient on the flow and the thermal fields.
Soucasse, L.; Rivière, Ph.; Soufiani, A.; Xin, S.
2014-02-15
The transition to unsteadiness and the dynamics of weakly turbulent natural convection, coupled to wall or gas radiation in a differentially heated cubical cavity with adiabatic lateral walls, are studied numerically. The working fluid is air with small contents of water vapor and carbon dioxide whose infrared spectral radiative properties are modelled by the absorption distribution function model. A pseudo spectral Chebyshev collocation method is used to solve the flow field equations and is coupled to a direct ray tracing method for radiation transport. Flow structures are identified by means of either the proper orthogonal decomposition or the dynamic mode decomposition methods. We first retrieve the classical mechanism of transition to unsteadiness without radiation, characterized by counter-rotating streamwise-oriented vortices generated at the exit of the vertical boundary layers. Wall radiation through a transparent medium leads to a homogenization of lateral wall temperatures and the resulting transition mechanism is similar to that obtained with perfectly conducting lateral walls. The transition is due to an unstable stratification upstream the vertical boundary layers and is characterized by periodically oscillating transverse rolls of axis perpendicular to the main flow. When molecular gas radiation is accounted for, no periodic solution is found and the transition to unsteadiness displays complex structures with chimneys-like rolls whose axes are again parallel to the main flow. The origin of this instability is probably due to centrifugal forces, as suggested previously for the case without radiation. Above the transition to unsteadiness, at Ra = 3 × 10{sup 8}, it is shown that both wall and gas radiation significantly intensify turbulent fluctuations, decrease the thermal stratification in the core of the cavity, and increase the global circulation.
NASA Astrophysics Data System (ADS)
Kamajaya, Ketut; Umar, Efrizon; Sudjatmi, K. S.
2012-06-01
This study focused on natural convection heat transfer using a vertical rectangular sub-channel and water as the coolant fluid. To conduct this study has been made pipe heaters are equipped with thermocouples. Each heater is equipped with five thermocouples along the heating pipes. The diameter of each heater is 2.54 cm and 45 cm in length. The distance between the central heating and the pitch is 29.5 cm. Test equipment is equipped with a primary cooling system, a secondary cooling system and a heat exchanger. The purpose of this study is to obtain new empirical correlations equations of the vertical rectangular sub-channel, especially for the natural convection heat transfer within a bundle of vertical cylinders rectangular arrangement sub-channels. The empirical correlation equation can support the thermo-hydraulic analysis of research nuclear reactors that utilize cylindrical fuel rods, and also can be used in designing of baffle-free vertical shell and tube heat exchangers. The results of this study that the empirical correlation equations of natural convection heat transfer coefficients with rectangular arrangement is Nu = 6.3357 (Ra.Dh/x)0.0740.
An evaluation of gas transfer velocity parameterizations during natural convection using DNS
NASA Astrophysics Data System (ADS)
Fredriksson, Sam T.; Arneborg, Lars; Nilsson, Hâkan; Zhang, Qi; Handler, Robert A.
2016-02-01
Direct numerical simulations (DNS) of free surface flows driven by natural convection are used to evaluate different methods of estimating air-water gas exchange at no-wind conditions. These methods estimate the transfer velocity as a function of either the horizontal flow divergence at the surface, the turbulent kinetic energy dissipation beneath the surface, the heat flux through the surface, or the wind speed above the surface. The gas transfer is modeled via a passive scalar. The Schmidt number dependence is studied for Schmidt numbers of 7, 150 and 600. The methods using divergence, dissipation and heat flux estimate the transfer velocity well for a range of varying surface heat flux values, and domain depths. The two evaluated empirical methods using wind (in the limit of no wind) give reasonable estimates of the transfer velocity, depending however on the surface heat flux and surfactant saturation. The transfer velocity is shown to be well represented by the expression, ks=A |Bν|1/4 Sc-n, where A is a constant, B is the buoyancy flux, ν is the kinematic viscosity, Sc is the Schmidt number, and the exponent n depends on the water surface characteristics. The results suggest that A=0.39 and n≈1/2 and n≈2/3 for slip and no-slip boundary conditions at the surface, respectively. It is further shown that slip and no-slip boundary conditions predict the heat transfer velocity corresponding to the limits of clean and highly surfactant contaminated surfaces, respectively. This article was corrected on 22 MAR 2016. See the end of the full text for details.
Spatial Durbin model analysis macroeconomic loss due to natural disasters
NASA Astrophysics Data System (ADS)
Kusrini, D. E.; Mukhtasor
2015-03-01
Magnitude of the damage and losses caused by natural disasters is huge for Indonesia, therefore this study aimed to analyze the effects of natural disasters for macroeconomic losses that occurred in 115 cities/districts across Java during 2012. Based on the results of previous studies it is suspected that it contains effects of spatial dependencies in this case, so that the completion of this case is performed using a regression approach to the area, namely Analysis of Spatial Durbin Model (SDM). The obtained significant predictor variable is population, and predictor variable with a significant weighting is the number of occurrences of disasters, i.e., disasters in the region which have an impact on other neighboring regions. Moran's I index value using the weighted Queen Contiguity also showed significant results, meaning that the incidence of disasters in the region will decrease the value of GDP in other.
Geohazards due to technologically enhanced natural radioactive wastes
NASA Astrophysics Data System (ADS)
Steinhäusler, Friedrich
2010-10-01
Human activities can modify naturally occurring radioactive material (NORM) into technologically enhanced naturally occurring radioactive material (TENORM) as a result of industrial activities. Most of these industries do not intend to work with radioactive material a priori. However, whenever a uranium- or thorium-bearing mineral is exploited, NORM-containing by-products and TENORM-contaminated wastes are created. The industrial use of NORM can result in non-negligible radiation exposure of workers and members of the public, exceeding by far the radiation exposure from nuclear technologies. For decades, millions of tons of NORM have been released into the environment without adequate control or even with the lack of any control. Various technologies have been developed for the control of NORM wastes. The paper discusses the merits and limitations of different NORM-waste management techniques, such as Containment, Immobilization, Dilution/Dispersion, Natural Attenuation, Separation, and - as an alternative - Cleaner Technologies. Each of these methods requires a comprehensive risk-benefit-cost analysis.
Halecky, N.; Birkholzer, J.T.; Webb, S.W.; Peterson, P.F.; Bodvarsson, G.S.
2006-04-14
In heated tunnels such as those designated for emplacementof radioactive waste at Yucca Mountain, axial temperature gradients maycause natural convection processes that can significantly influence themoisture conditions in the tunnels and in the surrounding fractured rock.Large-scale convection cells would provide an effective mechanism foraxial vapor transport, driving moisture out of the formation away fromthe heated tunnel section into cool end sections (where no waste isemplaced). To study such processes, we have developed and applied anenhanced version of TOUGH2 (Pruess et al., 1999) adding a new module thatsolves for natural convection in open cavities. The new TOUGH2 simulatorsimultaneously handles (1) the flow and energy transport processes in thefractured rock; (2) the flow and energy transport processes in thecavity; and (3) the heat and mass exchange at the rock-cavity interface.The new module is applied to simulate the future thermal-hydrological(TH) conditions within and near a representative waste emplacement tunnelat Yucca Mountain. Particular focus is on the potential for condensationalong the emplacement section, a possible result of heat outputdifferences between individual waste packages.
The nature and geochemical role of density convection in the East European evaporite basin
NASA Astrophysics Data System (ADS)
Popov, V. G.; Abdrakhmanov, R. F.; Puchkov, V. N.
2015-09-01
The role of the gravitation factor in the formation of the hydrostratisphere in the East European evaporate basin is considered. The features of Paleozoic sedimentation are characterized, as are the mechanism and litho-hydrogeochemical effects of the density concentration convection of mother brines of the Low-Permian salt-bearing basin to the underlying terrigenous-carbonate Paleozoic and Proterozoic layers. It is shown that the convection processes resulted in the formation of multicomponent calcium chloride brines prevailing in the sedimentary layer of the basis; they also caused the metasomatic dolomitization of limestones with growth of their filtration capacity.
Porous media flow problems: natural convection and one-dimensional flow of a non-Newtonian fluid
Walker, K.L.
1980-01-01
Two fluid problems in porous media are studied: natural convection of a Newtonian fluid and one-dimensional flow of a non-Newtonian fluid. Convection in a rectangular porous cavity driven by heating in the horizontal is analyzed by a number of different techniques which yield a fairly complete description of the 2-dimensional solutions. The solutions are governed by 2 dimensionless parameters: the Darcy-Rayleigh number R and cavity aspect ratio A. The flow behavior of a dilute solution of polyacrylamide in corn syrup flowing through porous media also is studied. Measurements of the pressure drop and flow rate are made for the solution flowing through a packed bed of glass beads. At low velocities the pressure drop as a function of velocity is the same as that for a Newtonian fluid of equal viscosity. At higher flow rates the non-Newtonian fluid exhibited significantly higher pressure drops than a Newtonian fluid.
NASA Astrophysics Data System (ADS)
Dhote, Yogesh; Thombre, Shashikant
2016-05-01
This paper presents the thermal performance of the proposed double flow natural convection solar air heater with in-built liquid (oil) sensible heat storage. Unused engine oil was used as thermal energy storage medium due to its good heat retaining capacity even at high temperatures without evaporation. The performance evaluation was carried out for a day of the month March for the climatic conditions of Nagpur (India). A self reliant computational model was developed using computational tool as C++. The program developed was self reliant and compute the performance parameters for any day of the year and would be used for major cities in India. The effect of change in storage oil quantity and the inclination (tilt angle) on the overall efficiency of the solar air heater was studied. The performance was tested initially at different storage oil quantities as 25, 50, 75 and 100 l for a plate spacing of 0.04 m with an inclination of 36o. It has been found that the solar air heater gives the best performance at a storage oil quantity of 50 l. The performance of the proposed solar air heater is further tested for various combinations of storage oil quantity (50, 75 and 100 l) and the inclination (0o, 15o, 30o, 45o, 60o, 75o, 90o). It has been found that the proposed solar air heater with in-built oil storage shows its best performance for the combination of 50 l storage oil quantity and 60o inclination. Finally the results of the parametric study was also presented in the form of graphs carried out for a fixed storage oil quantity of 25 l, plate spacing of 0.03 m and at an inclination of 36o to study the behaviour of various heat transfer and fluid flow parameters of the solar air heater.
Evolutionary stasis in pollen morphogenesis due to natural selection.
Matamoro-Vidal, Alexis; Prieu, Charlotte; Furness, Carol A; Albert, Béatrice; Gouyon, Pierre-Henri
2016-01-01
The contribution of developmental constraints and selective forces to the determination of evolutionary patterns is an important and unsolved question. We test whether the long-term evolutionary stasis observed for pollen morphogenesis (microsporogenesis) in eudicots is due to developmental constraints or to selection on a morphological trait shaped by microsporogenesis: the equatorial aperture pattern. Most eudicots have three equatorial apertures but several taxa have independently lost the equatorial pattern and have microsporogenesis decoupled from aperture pattern determination. If selection on the equatorial pattern limits variation, we expect to see increased variation in microsporogenesis in the nonequatorial clades. Variation of microsporogenesis was studied using phylogenetic comparative analyses in 83 species dispersed throughout eudicots including species with and without equatorial apertures. The species that have lost the equatorial pattern have highly variable microsporogenesis at the intra-individual and inter-specific levels regardless of their pollen morphology, whereas microsporogenesis remains stable in species with the equatorial pattern. The observed burst of variation upon loss of equatorial apertures shows that there are no strong developmental constraints precluding variation in microsporogenesis, and that the stasis is likely to be due principally to selective pressure acting on pollen morphogenesis because of its implication in the determination of the equatorial aperture pattern.
Evolutionary stasis in pollen morphogenesis due to natural selection.
Matamoro-Vidal, Alexis; Prieu, Charlotte; Furness, Carol A; Albert, Béatrice; Gouyon, Pierre-Henri
2016-01-01
The contribution of developmental constraints and selective forces to the determination of evolutionary patterns is an important and unsolved question. We test whether the long-term evolutionary stasis observed for pollen morphogenesis (microsporogenesis) in eudicots is due to developmental constraints or to selection on a morphological trait shaped by microsporogenesis: the equatorial aperture pattern. Most eudicots have three equatorial apertures but several taxa have independently lost the equatorial pattern and have microsporogenesis decoupled from aperture pattern determination. If selection on the equatorial pattern limits variation, we expect to see increased variation in microsporogenesis in the nonequatorial clades. Variation of microsporogenesis was studied using phylogenetic comparative analyses in 83 species dispersed throughout eudicots including species with and without equatorial apertures. The species that have lost the equatorial pattern have highly variable microsporogenesis at the intra-individual and inter-specific levels regardless of their pollen morphology, whereas microsporogenesis remains stable in species with the equatorial pattern. The observed burst of variation upon loss of equatorial apertures shows that there are no strong developmental constraints precluding variation in microsporogenesis, and that the stasis is likely to be due principally to selective pressure acting on pollen morphogenesis because of its implication in the determination of the equatorial aperture pattern. PMID:26248868
Nature of Convective Instabilities in Explosive Volcanic Clouds Inferred by Analog Experiments
NASA Astrophysics Data System (ADS)
Carazzo, G.; Jellinek, M.
2009-12-01
Understanding the mechanisms controlling the dynamics of a volcanic cloud generated by the rise and spread of an explosive eruption is a central issue in volcanology for the assessment of associated hazards. The last decades have seen the development of sophisticated numerical simulations and particle-tracking models with the aim of better understanding and forecasting the transport and sedimentation of the solid fraction in the cloud. In these models, the lateral spreading of an umbrella cloud is strongly influenced by stratospheric winds and its loss of mass with time is assumed be controlled by the opposing effects of particles settling and turbulent diffusion. However, recent observations suggest that additional spatially complex and time-dependent phenomena may govern the dynamics in a volcanic cloud. Here we investigate the mechanisms governing the lateral transport and residence time of ash in the atmosphere using analog experiments. In these experiments, a mixture of small particles and fresh water is injected upwards at a fixed rate into a chamber containing a salt water layer beneath a fresh water layer. Our results show that the formation of a thin particle-rich layer at the base of the cloud (a particle boundary layer) can dramatically modify its dynamics and lead to a variety of behaviors not detected previously. Depending on the conditions imposed at the source and on the magnitude of the density gradient in the environment, the cloud may either break up into discrete layers or release material as dense batches of particle-laden fluid. In natural eruptions the formation of this dense layer is found to be mainly controlled by the grain size distribution and to a lesser extent the altitude reached by the plume. An exhaustive review of field data available in the literature suggests that several past eruptions meet the required conditions to form a particle boundary layer. This study shows that large convective instabilities induced by the presence of a
Non-Darcy double-diffusive natural convection in axisymmetric fluid saturated porous cavities
NASA Astrophysics Data System (ADS)
Nithiarasu, P.; Seetharamu, K. N.; Sundararajan, T.
Double-diffusive natural convection in a fluid saturated porous medium has been investigated using the finite element method. A generalised porous medium model is used to study both Darcy and non-Darcy flow regimes in an axisymmetric cavity. Results indicate that the Darcy number should be a separate parameter to understand flow characteristics in non-Darcy regime. The influence of porosity on heat and mass transfer is significant and the transport rates may differ by 25% or more, at higher Darcy and Rayleigh numbers. When compared with the Darcy and other specialised models of Brinkman and Forchheimer, the present generalised model predicts the least heat and mass transfer rates. It is also observed that an increase in radius ratio leads to higher Nusselt and Sherwood numbers along the inner wall. Zusammenfassung Mit Hilfe der Finitelement-Methode wurde die Doppeldiffusion bei natürlicher Konvektion in einem fluidgetränktem porösen Medium untersucht, wobei ein verallgemeinertes Modell für poröse Medien Verwendung fand, das sich sowohl für Darcysches, wie für nicht-Darcysches Fluidverhalten in einem achsialsymmetrischen Ringraum eignet. Aus den Ergebnissen geht hervor, daß die Darcy-Zahl als zusätzlicher Parameter eingeführt werden muß, um das Strömungsverhalten im nicht-Darcyschen Regime verstehen zu können. Die Porosität hat großen Einfluß auf den Wärme- und Stoffaustausch, so daß bei höheren Darcy- und Rayleigh-Zahlen diesbezüglich Unterschiede bis über 25% auftreten können. Im Vergleich mit den speziellen Modellen nach Darcy, Brinkman und Forchheimer liefert das hier untersuchte verallgemeinerte Modell die geringsten Wärme- und Stoffflüsse. Es zeigt sich ferner, daß die Vergrößerung des Radienverhältnisses höhere Nusselt- und Sherwood- Zahlen entlang der Innenwand zur Folge hat.
Eulerian-Lagrangian solution of the convection-dispersion equation in natural co-ordinates.
Cheng, R.T.; Casulli, V.; Milford, S.N.
1984-01-01
The vast majority of numerical investigations of transport phenomena use an Eulerian formulation for the convenience that the computational grids are fixed in space. An Eulerian-Lagrangian method (ELM) of solution for the convection-dispersion equation is discussed and analyzed. The ELM uses the Lagrangian concept in an Eulerian computational grid system.-from Authors
Chen, Bingyan; Fowler, Alex; Bhowmick, Sankha
2006-06-01
Trehalose is believed to offer desiccation protection to mammalian cells by forming stable glassy matrices. The goal of the current study was to explore the desiccation kinetics of thin films of trehalose-water solution under forced and natural convective conditions and to investigate the thermophysical state of mammalian cells at the bottom of the thin film. We developed a finite difference model based on the mass and energy conservation equations coupled to the water transport model from the cells. The boundary conditions were obtained from correlations or experimental measurements and the Gordon-Taylor equation was used to predict the glass transition temperature at every location. Results indicated that there are three distinct regimes for drying for both forced and natural convection, characterized by the slope of the moisture content plot as a function of time. Our results also indicate that the surface of the solution reached the glassy state in less than 10 min for the Reynolds (forced) numbers explored and approximately 30 min for some Rayleigh (natural convective) numbers; however, significant water was trapped at this instant. Larger drying force hastened quicker glass formation but trapped more water. The numerical model was capable of predicting the drying kinetics for the dilute region accurately, but deviated while predicting the other regimes. Based on these experimental validations of the model, the osmotic response of different cells located at the bottom of the solution with orders of magnitude difference in their membrane permeability (Lp) was predicted. The results suggested that extracellular glass formed around cells at the bottom of a trehalose-water solution by the propagation of glass into the solution; however it takes more than an order of magnitude time (approximately 7 min to >100 min for forced convective drying) to remove sufficient water to form glass around cells from the time when the first surface glass is formed. This is
Rasmussen, Kristen L.; Zuluaga, Manuel D.; Brodzik, Stella R.
2015-01-01
Abstract For over 16 years, the Precipitation Radar of the Tropical Rainfall Measuring Mission (TRMM) satellite detected the three‐dimensional structure of significantly precipitating clouds in the tropics and subtropics. This paper reviews and synthesizes studies using the TRMM radar data to present a global picture of the variation of convection throughout low latitudes. The multiyear data set shows convection varying not only in amount but also in its very nature across the oceans, continents, islands, and mountain ranges of the tropics and subtropics. Shallow isolated raining clouds are overwhelmingly an oceanic phenomenon. Extremely deep and intense convective elements occur almost exclusively over land. Upscale growth of convection into mesoscale systems takes a variety of forms. Oceanic cloud systems generally have less intense embedded convection but can form very wide stratiform regions. Continental mesoscale systems often have more intense embedded convection. Some of the most intense convective cells and mesoscale systems occur near the great mountain ranges of low latitudes. The Maritime Continent and Amazonia exhibit convective clouds with maritime characteristics although they are partially or wholly land. Convective systems containing broad stratiform areas manifest most strongly over oceans. The stratiform precipitation occurs in various forms. Often it occurs as quasi‐uniform precipitation with strong melting layers connected with intense convection. In monsoons and the Intertropical Convergence Zone, it takes the form of closely packed weak convective elements. Where fronts extend into the subtropics, broad stratiform regions are larger and have lower and sloping melting layers related to the baroclinic origin of the precipitation. PMID:27668295
Experimental study of laminar natural convection in cells with various convex and concave bottoms
Lewandowski, W.M.; Khubeiz, M.J. )
1992-02-01
The subject of this work is connected with experimental explanation of influence of bottom shape on free convective heat transfer in cylindrical closed space. Heat transfer and free convective motion in limited space from the bottoms of different hemispherical convex or concave shapes have been studied experimentally. The ratio of the diameter of the hemisphere (d) to the diameter of the bottom (D) (0 < d/D < 1) has been tested for a range of Rayleigh numbers (10{sup 5} < Ra < 10{sup 7}). In comparison with a flat bottom (d/D = O), about 40 percent inhibition or about 50 percent intensification depending on the bottom configuration (d/D) have been observed. The mechanism of the phenomenon based on dead space, local overheating, and shape influence effects has been proposed.
Studies of heat-source driven natural convection: A numerical investigation
NASA Technical Reports Server (NTRS)
Emara, A. A.; Kulacki, F. A.
1977-01-01
Thermal convection driven by uniform volumetric energy sources was studied in a horizontal fluid layer bounded from above by a rigid, isothermal surface and from below by a rigid, zero heat-flux surface. The side walls of the fluid domain were assumed to be rigid and perfectly insulating. The computations were formally restricted to two-dimensional laminar convection but were carried out for a range of Rayleigh numbers which spans the regimes of laminar and turbulent flow. The results of the computations consists of streamline and isotherm patterns, horizontally averaged temperature distributions, and horizontally averaged Nusselt numbers at the upper surface. Flow and temperature fields do not exhibit a steady state, but horizontally averaged Nusselt numbers reach limiting, quasi-steady values for all Rayleigh numbers considered. Correlations of the Nusselt number in terms of the Rayleigh and Prandtl numbers were determined.
On the episodic nature of derecho-producing convective systems in the United States
NASA Astrophysics Data System (ADS)
Ashley, Walker S.; Mote, Thomas L.; Bentley, Mace L.
2005-11-01
Convectively generated windstorms occur over broad temporal and spatial scales; however, one of the larger-scale and most intense of these windstorms has been given the name derecho. This study illustrates the tendency for derecho-producing mesoscale convective systems to group together across the United States - forming a derecho series. The derecho series is recognized as any succession of derechos that develop within a similar synoptic environment with no more than 72 h separating individual events. A derecho dataset for the period 1994-2003 was assembled to investigate the groupings of these extremely damaging convective wind events. Results indicate that over 62% of the derechos in the dataset were members of a derecho series. On average, nearly six series affected the United States annually. Most derecho series consisted of two or three events; though, 14 series during the period of record contained four or more events. Two separate series involved nine derechos within a period of nine days. Analyses reveal that derecho series largely frequent regions of the Midwest, Ohio Valley, and the south-central Great Plains during May, June, and July. Results suggest that once a derecho occurred during May, June, or July, there was a 58% chance that this event was the first of a series of two or more, and about a 46% chance that this was the first of a derecho series consisting of three or more events. The derecho series climatology reveals that forecasters in regions frequented by derechos should be prepared for the probable regeneration of a derecho-producing convective system after an initial event occurs. Copyright
The nature of symmetric instability and its similarity to convective and inertial instability
NASA Technical Reports Server (NTRS)
Xu, Q.; Clark, J. H. E.
1985-01-01
It is shown that there exists a local similarity among SI (Symmetric Instability), BI (Buoyancy or Convective Instability), and II (Inertial Instability) even for fully nonlinear viscous motion. The most unstable slope angles for SI and Moist SI motions are analyzed based on parcel energetics. These considerations also suggest qualitatively that CSI (Conditional SI) circulations will be slantwise and lie between the moist most unstable slope and dry least stable slope of the basic state.
NASA Astrophysics Data System (ADS)
Torczynski, J. R.; Henderson, J. A.; Ohern, T. J.; Chu, T. Y.; Blanchat, T. K.
Three-dimensional natural convection of a fluid in an enclosure is examined. The geometry is motivated by a possible magmaenergy extraction system, and the fluid is a magma simulant and has a highly temperature-dependent viscosity. Flow simulations are performed for enclosures with and without a cylinder, which represents the extractor, using the finite-element code FIDAP (Fluid Dynamics International). The presence of the cylinder completely alters the flow pattern. Flow-visualization and PIV experiments are in qualitative agreement with the simulations.
Transitions and chaos in natural convection of a fluid with Pr = 0.1 in a horizontal annulus
NASA Astrophysics Data System (ADS)
Yoo, Joo-Sik; Han, Seung-Moo
2000-10-01
Bifurcation sequence to chaos in natural convection in a horizontal annulus is numerically investigated for the fluid of Pr=0.1. As Ra is increased, a transition from a steady unicellular flow to an oscillatory multicellular flow with a counter-rotating eddy on the top of the annulus occurs. After the first Hopf bifurcation from steady to monoperiodic flow, a period-3 solution first appears. Quasi-periodic flow and two period-doubling bifurcations are recorded before chaos, and a window of period 4 is observed between chaotic states.
Torczynski, J.R.; Henderson, J.A.; O`Hern, T.J.; Chu, T.Y.; Blanchat, T.K.
1994-01-01
Three-dimensional natural convection of a fluid in an enclosure is examined. The geometry is motivated by a possible magmaenergy extraction system, and the fluid is a magma simulant and has a highly temperature-dependent viscosity. Flow simulations are performed for enclosures with and without a cylinder, which represents the extractor, using the finite-element code FIDAP (Fluid Dynamics International). The presence of the cylinder completely alters the flow pattern. Flow-visualization and PIV experiments are in qualitative agreement wit the simulations.
Film boiling heat transfer from a sphere in natural and forced convection of freon-113
Dix, D.; Orozco, J. )
1990-01-01
Boiling heat transfer fluxes were measured on a 3.84-cm hollow copper sphere, in both forced convection and pool boiling, as a function of angular position in Freon 113. This paper reports on forced-convection tests run at speeds of 0.5 to 1.9 m/s. These tests were conducted in the stable film boiling region of the boiling curve. Significant heat transfer rates were measured in the vapor wake region of the sphere for flow film boiling. Video observations of the boiling process revealed that the flow film boiling vapor removal mechanism consisted of periodic formation and detachment of a vapor wake in the rear of the sphere. For pool boiling it was found that the heated surface had a uniform rate of energy dissipation in the stable film boiling regime, whereas in forced convection the film boiling rate was dependent on angular position. Pool film boiling tests also showed multiple humps (more than one maximum heat flux) in the boiling curve when the liquid was subcooled.
NASA Astrophysics Data System (ADS)
Torres, Juan F.; Henry, Daniel; Komiya, Atsuki; Maruyama, Shigenao
2015-08-01
The transition from the complex Rayleigh-Bénard convection to the simple heated-from-the-sides configuration in a cubical cavity filled with a Newtonian fluid is numerically studied. The cavity is tilted by an angle θ around its lower horizontal edge and is heated and cooled from two opposite tilted sides. We first analyze the effect of a marginal inclination angle on quasi-Rayleigh-Bénard convection (θ ≈0∘ ), which is a realistic physical approximation to the ideal Rayleigh-Bénard convection. We then yield the critical angles where multiple solutions that were initially found for θ ≈0∘ disappear, eventually resulting in the single steady roll solution found in the heated-from-the-sides configuration (θ =90∘ ). We confirm the existence of critical angles during the transition θ :0∘→90∘ , and we demonstrate that such angles are a consequence of either singularities or collisions of bifurcation points in the Rayleigh-number-θ parameter space. We finally derive the most important critical angles corresponding to any Newtonian fluid of Prandtl number greater than that of air.
THE IMPACT OF NATURAL CONVECTION ON NEAR-FIELD TH PROCESSES IN THE FRACTURED ROCK AT YUCCA MOUNTAIN
Yvonne Tsang
2006-04-16
The heat output of the radioactive waste proposed to be emplaced at Yucca Mountain will strongly affect the thermal-hydrological (TH) conditions in and near the geologic repository for thousands of years. Recent computational fluid dynamics (CFD) analysis has demonstrated that the emplacement tunnels (drifts) will act as important conduits for gas flows driven by natural convection. As a result, vapor generated from boiling/evaporation of formation water near elevated-temperature sections of the drifts may effectively be transported to cooler end sections (where no waste is emplaced), would condense there, and subsequently drain into underlying rock units. To study these processes, we have developed a new simulation method that couples existing tools for simulating TH conditions in the fractured formation with modules that approximate natural convection in heated emplacement drifts. The new method is applied to evaluate the future TH conditions at Yucca Mountain in a three-dimensional model domain comprising a representative emplacement drift and the surrounding fractured rock.
Lee, S.R.; Irvine, T.F. Jr.; Greene, G.A.
1998-04-01
An implicit finite difference method was applied to analyze laminar natural convection in a vertical channel with a modified power law fluid. This fluid model was chosen because it describes the viscous properties of a pseudoplastic fluid over the entire shear rate range likely to be found in natural convection flows since it covers the shear rate range from Newtonian through transition to simple power law behavior. In addition, a dimensionless similarity parameter is identified which specifies in which of the three regions a particular system is operating. The results for the average channel velocity and average Nusselt number in the asymptotic Newtonian and power law regions are compared with numerical data in the literature. Also, graphical results are presented for the velocity and temperature fields and entrance lengths. The results of average channel velocity and Nusselt number are given in the three regions including developing and fully developed flows. As an example, a pseudoplastic fluid (carboxymethyl cellulose) was chosen to compare the different results of average channel velocity and Nusselt number between a modified power law fluid and the conventional power law model. The results show, depending upon the operating conditions, that if the correct model is not used, gross errors can result.
O`Brien, J.E.
1991-12-01
Experimental measurements of surface emissivities of three metallic samples have been obtained in support of an experiment aimed at determining natural convection and total heat transfer for a heated vertical cylinder surrounded by an array of cooled vertical tubes. In some cases, the heated stainless steel cylinder was shrouded by a perforated aluminum outer cylinder. The surrounding cooled tubes were also aluminum. In this experiment, heat transfer from the heated tube and the surrounding outer cylinder will occur by a combination of natural convection and radiation. At temperatures near the melting point of aluminum, the radiant contribution is particularly important, accounting for 50% or more of the total heat transfer. Consequently, accurate knowledge of surface emissivities of the heated rods, outer cylinders and surrounding structures is needed in order to predict the system thermal response during the transient. Direct measurements of surface emissivities have been obtained for one stainless steel and two aluminum samples. The measurements were obtained using an infrared pyrometer sensitive to the 8--14 {mu}m wavelength range. A procedure for estimating total hemispherical emissivities based on the measured spectral, normal results is also provided.
NASA Astrophysics Data System (ADS)
Adesanya, S. O.; Oluwadare, E. O.; Falade, J. A.; Makinde, O. D.
2015-12-01
In this paper, the free convective flow of magnetohydrodynamic fluid through a channel with time periodic boundary condition is investigated by taking the effects of Joule dissipation into consideration. Based on simplifying assumptions, the coupled governing equations are reduced to a set of nonlinear boundary valued problem. Approximate solutions are obtained by using semi-analytical Adomian decomposition method. The effect of pertinent parameters on the fluid velocity, temperature distribution, Nusselt number and skin friction are presented graphically and discussed. The result of the computation shows that an increase in the magnetic field intensity has significant influence on the fluid flow.
Influence of Natural Convection and Thermal Radiation Multi-Component Transport in MOCVD Reactors
NASA Technical Reports Server (NTRS)
Lowry, S.; Krishnan, A.; Clark, I.
1999-01-01
The influence of Grashof and Reynolds number in Metal Organic Chemical Vapor (MOCVD) reactors is being investigated under a combined empirical/numerical study. As part of that research, the deposition of Indium Phosphide in an MOCVD reactor is modeled using the computational code CFD-ACE. The model includes the effects of convection, conduction, and radiation as well as multi-component diffusion and multi-step surface/gas phase chemistry. The results of the prediction are compared with experimental data for a commercial reactor and analyzed with respect to the model accuracy.
Wu, Y.; Welty, J.R. . Dept. of Mechanical Engineering)
1994-08-01
A two-dimensional finite difference computer program in cylindrical coordinates has been developed to solve the case of laminar natural convection in a vertical tube open to large reservoir. Such a device, the open thermosyphon, is used in a number of applications, including the cooling of gas turbines, geothermal energy extraction, and thermosyphon solar water heaters. The objective of this work were to study the nature of fluid flow and the heat transfer rate along the tube wall. A semi-implicit, time marching, finite difference solution procedure was used, satisfying continuity, momentum, and energy equations for incompressible flow. Results show three well-defined flow regimes appearing as functions of the tube length-to-radius (aspect) ratio. Fluid motion in the tube and heat transfer rates became oscillatory at long time intervals. Plots of streamlines and isotherms at selected times for different aspect ratio tubes are also presented to show the transition behavior of fluid motion.
Natural convection in horizontal porous layers with localized heating from below
Prasad, V. ); Kulacki, F.A. )
1987-08-01
Convective flow of fluid through saturated porous media heated from below is of considerable interest, and has been extensively studied. Most of these studies are concerned with either infinite horizontal porous layers or rectangular (or cylindrical) porous cavities with adiabatic vertical walls. A related problem of practical importance occurs when only a portion of the bottom surface is heated and the rest of it is either adiabatic or isothermally cooled. This situation is encountered in several geothermal areas which consists of troughs of volcanic debris contained by walls of nonfragmented ignimbrite. Thus, the model region considered is a locally heated long trough of isotropic porous medium confined by impermeable and insulating surroundings. Also, the recent motivation to study this problem has come from the efforts to identify a geologic repository for nuclear waste disposal. The purpose of the present work is to consider the effects of aspect ratio and Rayleigh number on free convection heat transfer from an isothermal heat source centrally located on the bottom surface of a horizontal porous cavity.
Buoyancy-Driven Natural Convection of Liquid Helium in an Electron Bubble Chamber
Ju, Y. L.; Dodd, J. R.; Willis, W. J.
2006-04-27
A small liquid helium test chamber with 1.5 L active volume has been designed and constructed, to make the fundamental measurements of physical properties of electron bubble transports in liquid helium, aimed at developing a new cryogenic neutrino detector, using liquid helium as the detecting medium, for the detection of solar neutrinos. The test chamber is a double-walled cylindrical container equipped with five optical windows and ten high voltage cables. A LN2/LHe cryostat and a needle valve for vapor helium cooling are used to provide a 1.7{approx}4.5 K low temperature environments for the test chamber. One of key issues for the cryogenic design and experimental sensitivity of electron bubble tracking is that of keeping a thermally uniform liquid helium bath. The external heat loads to the chamber will generate a buoyancy-induced convection of liquid helium, which will carry the electron bubbles and accelerate or decelerate their transportation and therefore must be reduced to the minimum, so that the slow motion of the electron bubbles will not be confused by this effect. This paper will present the computational simulation and analysis on thermal convection and uniformity of the test chamber.
Natural convection of a two-dimensional Boussinesq fluid does not maximize entropy production.
Bartlett, Stuart; Bullock, Seth
2014-08-01
Rayleigh-Bénard convection is a canonical example of spontaneous pattern formation in a nonequilibrium system. It has been the subject of considerable theoretical and experimental study, primarily for systems with constant (temperature or heat flux) boundary conditions. In this investigation, we have explored the behavior of a convecting fluid system with negative feedback boundary conditions. At the upper and lower system boundaries, the inward heat flux is defined such that it is a decreasing function of the boundary temperature. Thus the system's heat transport is not constrained in the same manner that it is in the constant temperature or constant flux cases. It has been suggested that the entropy production rate (which has a characteristic peak at intermediate heat flux values) might apply as a selection rule for such a system. In this work, we demonstrate with Lattice Boltzmann simulations that entropy production maximization does not dictate the steady state of this system, despite its success in other, somewhat similar scenarios. Instead, we will show that the same scaling law of dimensionless variables found for constant boundary conditions also applies to this system.
Natural convection of a two-dimensional Boussinesq fluid does not maximize entropy production
NASA Astrophysics Data System (ADS)
Bartlett, Stuart; Bullock, Seth
2014-08-01
Rayleigh-Bénard convection is a canonical example of spontaneous pattern formation in a nonequilibrium system. It has been the subject of considerable theoretical and experimental study, primarily for systems with constant (temperature or heat flux) boundary conditions. In this investigation, we have explored the behavior of a convecting fluid system with negative feedback boundary conditions. At the upper and lower system boundaries, the inward heat flux is defined such that it is a decreasing function of the boundary temperature. Thus the system's heat transport is not constrained in the same manner that it is in the constant temperature or constant flux cases. It has been suggested that the entropy production rate (which has a characteristic peak at intermediate heat flux values) might apply as a selection rule for such a system. In this work, we demonstrate with Lattice Boltzmann simulations that entropy production maximization does not dictate the steady state of this system, despite its success in other, somewhat similar scenarios. Instead, we will show that the same scaling law of dimensionless variables found for constant boundary conditions also applies to this system.
Fox, E.; Visser, A.; Bridges, N.
2011-07-18
This paper presents an experimental study of natural convection heat transfer for an Ionic Liquid. The experiments were performed for 1-butyl-2, 3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide, ([C{sub 4}mmim][NTf{sub 2}]) at a Raleigh number range of 1.26 x 10{sup 7} to 8.3 x 10{sup 7}. In addition to determining the convective heat transfer coefficients, this study also included experimental determination of thermophysical properties of [C{sub 4}mmim][NTf{sub 2}] such as, density, viscosity, heat capacity, and thermal conductivity. The results show that the density of [C{sub 4}mmim][NTf{sub 2}] varies from 1.437-1.396 g/cm{sup 3} within the temperature range of 10-50 C, the thermal conductivity varies from 0.105-0.116 W/m.K between a temperature of 10 to 60 C, the heat capacity varies from 1.015 J/g.K - 1.760 J/g.K within temperature range of 25-340 C and the viscosity varies from 18cp-243cp within temperature range 10-75 C. The results for density, thermal conductivity, heat capacity, and viscosity were in close agreement with the values in the literature. Measured dimensionless Nusselt number was observed to be higher for the ionic liquid than that of DI water. This is expected as Nusselt number is the ratio of heat transfer by convection to conduction and the ionic liquid has lower thermal conductivity (approximately 18%) than DI water.
NASA Astrophysics Data System (ADS)
Rashidi, M. M.; Hayat, T.; Erfani, E.; Mohimanian Pour, S. A.; Hendi, Awatif A.
2011-11-01
The purpose of present research is to derive analytical expressions for the solution of steady MHD convective and slip flow due to a rotating disk. Viscous dissipation and Ohmic heating are taken into account. The nonlinear partial differential equations for MHD laminar flow of the homogeneous fluid are reduced to a system of five coupled ordinary differential equations by using similarity transformation. The derived solution is expressed in series of exponentially-decaying functions using homotopy analysis method (HAM). The convergence of the obtained series solutions is examined. Finally some figures are sketched to show the accuracy of the applied method and assessment of various slip parameter γ, magnetic field parameter M, Eckert Ec, Schmidt Sc and Soret Sr numbers on the profiles of the dimensionless velocity, temperature and concentration distributions. Validity of the obtained results are verified by the numerical results.
Yang, G.Z.; Zabaras, N.
1996-12-31
This paper presents a finite element solution of an inverse solidification design problem. It is based on the previous work on an adjoint method with a functional optimization scheme for the solution of inverse thermal convection problems with overspecified thermal boundary conditions. An inverse calculation is performed here for directional solidification processes to find the optimal heat flux at the mold wall boundary on both the solid and liquid mold sides. The objective is to achieve desired velocity and heat flux histories at the solid-liquid interface. The specification of the growth velocity and freezing interface heat fluxes considers the microstructural implications on the casting product and the morphological stability requirements of the freezing interface. An example of solidification in a rectangular mold with a planar interface growth is shown.
NASA Technical Reports Server (NTRS)
Masiulaniec, K. Cyril; Vanfossen, G. James, Jr.; Dewitt, Kenneth J.; Dukhan, Nihad
1995-01-01
A technique was developed to cast frozen ice shapes that had been grown on a metal surface. This technique was applied to a series of ice shapes that were grown in the NASA Lewis Icing Research Tunnel on flat plates. Nine flat plates, 18 inches square, were obtained from which aluminum castings were made that gave good ice shape characterizations. Test strips taken from these plates were outfitted with heat flux gages, such that when placed in a dry wind tunnel, can be used to experimentally map out the convective heat transfer coefficient in the direction of flow from the roughened surfaces. The effects on the heat transfer coefficient for both parallel and accelerating flow will be studied. The smooth plate model verification baseline data as well as one ice roughened test case are presented.
Natural convection on a vertical plate in a saturated porous medium with internal heat generation
NASA Astrophysics Data System (ADS)
Guedda, M.; Sriti, M.; Achemlal, D.
2014-08-01
The main goal of this paper is to re-exam a class of exact solutions for the two-dimensional free convection boundary layers induced by a heated vertical plate embedded in a saturated porous medium with an exponential decaying heat generation. The temperature distribution of the plate has been assumed to vary as a power of the axial coordinate measured from the leading edge of the plate and subjected to an applied lateral mass flux. The boundary layer equations are solved analytically and numerically using a fifth-order Runge-Kutta scheme coupled with the shooting iteration method. As for the classical problem without internal heat generation, it is proved that multiple (unbounded) solutions arise for any and for any suction/injection parameter. For such solutions, the asymptotic behavior as the similarity variable approaches infinity is determined.
Natural convection between two concentric spheres - Transition toward a multicellular flow
NASA Astrophysics Data System (ADS)
Caltagirone, J.-P.; Mojtabi, A.; Combarnous, M.
1980-03-01
A moderate temperature difference maintained between two concentric spherical surfaces induces, in steady state, unicellular toroidal movements in the enclosed fluid. Beyond a critical temperature difference, the flow becomes unstable and the convective phenomena rearrange into counter-rotating toroidal cells. A two-dimensional axisymmetric numerical model confirms the existence of a unicellular regime and shows that, beyond the critical conditions and for the same set of parameters, two convergent solutions can be obtained. One is unicellular and the other is bicellular; in the latter, the additional cell appears at the top of the layer. The initial conditions determine which one of these two will be established. This transition is investigated as a function of several parameters and the results are compared with the experimental results in the literature.
Schlieren visualization of water natural convection in a vertical ribbed channel
NASA Astrophysics Data System (ADS)
Fossa, M.; Misale, M.; Tanda, G.
2015-11-01
Schlieren techniques are valuable tools for the qualitative and quantitative visualizations of flows in a wide range of scientific and engineering disciplines. A large number of schlieren systems have been developed and documented in the literature; majority of applications involve flows of gases, typically air. In this work, a schlieren technique is applied to visualize the buoyancy-induced flow inside vertical ribbed channels using water as convective fluid. The test section consists of a vertical plate made of two thin sheets of chrome-plated copper with a foil heater sandwiched between them; the external sides of the plate are roughened with transverse, square-cross-sectioned ribs. Two parallel vertical walls, smooth and unheated, form with the heated ribbed plate two adjacent, identical and asymmetrically heated, vertical channels. Results include flow schlieren visualizations with colour-band filters, reconstructions of the local heat transfer coefficient distributions along the ribbed surfaces and comparisons with past experiments performed using air as working fluid.
NASA Astrophysics Data System (ADS)
Rieger, Daniel; Bangert, Max; Vogel, Bernhard
2013-04-01
Shallow postfrontal convective clouds are thought to be sensitive to the aerosol burden. In our case study we present results of model runs, simulating April 25, 2008. On this day a cold front passes Germany from north to south. During this situation the sea salt aerosol transported by the northerly flow into the model domain replaces the preexisting anthropogenic aerosol. We quantify the effect of the aerosol on the microphysical properties of the convective clouds that develop after the passage of the cold front. The model system COSMO-ART (Vogel et al., 2009, Bangert et al., 2010) is a comprehensive online coupled model system to simulate the spatial and temporal distribution of reactive gaseous and particulate matter. It is used to quantify the feedback processes between aerosols and the. state of the atmosphere on the continental to the regional scale with two-way interactions between different atmospheric processes. The model system enables further investigations of the aerosol-cloud-interactions and associated feedback processes. The model framework contains a two-moment cloud microphysics scheme (Seifert and Beheng, 2006) in combination with sophisticated activation parameterizations (Bangert et al., 2012). We carried out sensitivity runs. One applies a bulk microphysics scheme as used in the operational forecasts of the German weather service. In two of them the aerosol was. prescribed (continental, maritime) and kept constant in space and time. In the fourth one we used the full capabilities of COSMO-ART to simulate the dynamic behavior of aerosol and its feedback with radiation and cloud microphysics. We compare our model results with radar data, satellite IR images, and rain gauges.
Cumulus convection and the terrestrial water-vapor distribution
NASA Technical Reports Server (NTRS)
Donner, Leo J.
1988-01-01
Cumulus convection plays a significant role in determining the structure of the terrestrial water vapor field. Cumulus convection acts directly on the moisture field by condensing and precipitating water vapor and by redistributing water vapor through cumulus induced eddy circulations. The mechanisms by which cumulus convection influences the terrestrial water vapor distribution is outlined. Calculations using a theory due to Kuo is used to illustrate the mechanisms by which cumulus convection works. Understanding of these processes greatly aids the ability of researchers to interpret the seasonal and spatial distribution of atmospheric water vapor by providing information on the nature of sources and sinks and the global circulation.
Analysis of Phenix end-of-life natural convection test with the MARS-LMR code
Jeong, H. Y.; Ha, K. S.; Lee, K. L.; Chang, W. P.; Kim, Y. I.
2012-07-01
The end-of-life test of Phenix reactor performed by the CEA provided an opportunity to have reliable and valuable test data for the validation and verification of a SFR system analysis code. KAERI joined this international program for the analysis of Phenix end-of-life natural circulation test coordinated by the IAEA from 2008. The main objectives of this study were to evaluate the capability of existing SFR system analysis code MARS-LMR and to identify any limitation of the code. The analysis was performed in three stages: pre-test analysis, blind posttest analysis, and final post-test analysis. In the pre-test analysis, the design conditions provided by the CEA were used to obtain a prediction of the test. The blind post-test analysis was based on the test conditions measured during the tests but the test results were not provided from the CEA. The final post-test analysis was performed to predict the test results as accurate as possible by improving the previous modeling of the test. Based on the pre-test analysis and blind test analysis, the modeling for heat structures in the hot pool and cold pool, steel structures in the core, heat loss from roof and vessel, and the flow path at core outlet were reinforced in the final analysis. The results of the final post-test analysis could be characterized into three different phases. In the early phase, the MARS-LMR simulated the heat-up process correctly due to the enhanced heat structure modeling. In the mid phase before the opening of SG casing, the code reproduced the decrease of core outlet temperature successfully. Finally, in the later phase the increase of heat removal by the opening of the SG opening was well predicted with the MARS-LMR code. (authors)
Analysis of the Phenix end-of-life natural convection test with SAS4A/SASSYS-1
Thomas, J. W.; Fanning, T. H.; Dunn, F. E.; Sofu, T.
2012-07-01
From a reduced power and flow condition, the 2009 Phenix Natural Convection Test mimics a protected loss-of-heat sink event. The measured transient response of the Phenix reactor to such an event provides an important data set for validating safety analysis codes. A model of the Phenix reactor and primary coolant system was developed using the reactor safety analysis code system SAS4A/SASSYS-1. While the overall global response of the reactor was predicted reasonably well, there were some non-negligible discrepancies in the temperature predictions during the transient and work continues to improve the model. Some modeling issues have been identified, and will be addressed as improvements to the model continue. (authors)
NASA Astrophysics Data System (ADS)
Sahebi, S. A. R.; Pourziaei, H.; Feizi, A. R.; Taheri, M. H.; Rostamiyan, Y.; Ganji, D. D.
2015-12-01
In this paper, natural convection of non-Newtonian bio-nanofluids flow between two vertical flat plates is investigated numerically. Sodium Alginate (SA) and Sodium Carboxymethyl Cellulose (SCMC) are considered as the base non-Newtonian fluid, and nanoparticles such as Titania ( TiO2 and Alumina ( Al2O3 were added to them. The effective thermal conductivity and viscosity of nanofluids are calculated through Maxwell-Garnetts (MG) and Brinkman models, respectively. A fourth-order Runge-Kutta numerical method (NUM) and three Weighted Residual Methods (WRMs), Collocation (CM), Galerkin (GM) and Least-Square Method (LSM) and Finite-Element Method (FEM), are used to solve the present problem. The influence of some physical parameters such as nanofluid volume friction on non-dimensional velocity and temperature profiles are discussed. The results show that SCMC- TiO2 has higher velocity and temperature values than other nanofluid structures.
NASA Astrophysics Data System (ADS)
Chiba, Ryoichi
2016-02-01
The transient natural convection of a viscous fluid in a heated vertical tube is studied using the two-dimensional differential transform method (DTM). A time-dependent Dirichlet boundary condition is imposed for tube wall temperature. The partial differential equations for the velocity and temperature fields within the tube are solved by the DTM while considering temperature-dependent viscosity and thermal conductivity of the fluid. As a result, tractable solutions in double-series form are derived for the temperature and flow velocity. The transformed functions included in the solutions are obtained through a simple recursive procedure. Numerical results illustrate the effects of temperature-dependent properties on transient temperature and flow behaviour, including the Nusselt number and volumetric flow rate. The DTM gives accurate series solutions without any special functions for nonlinear transient heat transfer problems which are advantageous in finding the derivative or integral.
NASA Technical Reports Server (NTRS)
Kulacki, F. A.; Emara, A. A.
1975-01-01
Natural convection energy transport in a horizontal layer of internally heated fluid was measured for Rayleigh numbers from 1890 to 2.17 x 10 to the 12th power. The fluid layer is bounded below by a rigid zero-heat-flux surface and above by a rigid constant-temperature surface. Joule heating by an alternating current passing horizontally through the layer provides the uniform volumetric energy source. The overall steady-state heat transfer coefficient at the upper surface was determined by measuring the temperature difference across the layer and power input to the fluid. The correlation between the Nusselt and Rayleigh numbers for the data of the present study and the data of the Kulacki study is given.
Natural convection flow of Cu-H2O nanofluid along a vertical wavy surface with uniform heat flux
NASA Astrophysics Data System (ADS)
Habiba, Farjana; Molla, Md. Mamun; Khan, M. A. Hakim
2016-07-01
A numerical study on natural convection flow of Cu-Water nanofluid along a vertical wavy surface with uniform heat flux has been carried out. The governing boundary layer equations are transformed into parabolic partial differential equations by applying a suitable set of variables. The resulting nonlinear system of equations are then mapped into a regular rectangular computational domain and solved numerically by using an implicit finite difference method. Numerical results are thoroughly discussed in terms of velocity and temperature distributions, surface temperature distribution, skin friction coefficient and Nusselt number coefficient for selected key parameters such as solid volume fraction of nanofluid (ϕ) and amplitude (α) of surface waviness. In addition, velocity vectors, streamlines and isotherms are plotted to visualize momentum and thermal flow pattern within the boundary layer region.
NASA Astrophysics Data System (ADS)
Seta, Takeshi
2013-06-01
In the present paper, we apply the implicit-correction method to the immersed-boundary thermal lattice Boltzmann method (IB-TLBM) for the natural convection between two concentric horizontal cylinders and in a square enclosure containing a circular cylinder. The Chapman-Enskog multiscale expansion proves the existence of an extra term in the temperature equation from the source term of the kinetic equation. In order to eliminate the extra term, we redefine the temperature and the source term in the lattice Boltzmann equation. When the relaxation time is less than unity, the new definition of the temperature and source term enhances the accuracy of the thermal lattice Boltzmann method. The implicit-correction method is required in order to calculate the thermal interaction between a fluid and a rigid solid using the redefined temperature. Simulation of the heat conduction between two concentric cylinders indicates that the error at each boundary point of the proposed IB-TLBM is reduced by the increment of the number of Lagrangian points constituting the boundaries. We derive the theoretical relation between a temperature slip at the boundary and the relaxation time and demonstrate that the IB-TLBM requires a small relaxation time in order to avoid temperature distortion around the immersed boundary. The streamline, isotherms, and average Nusselt number calculated by the proposed method agree well with those of previous numerical studies involving natural convection. The proposed IB-TLBM improves the accuracy of the boundary conditions for the temperature and velocity using an adequate discrete area for each of the Lagrangian nodes and reduces the penetration of the streamline on the surface of the body.
Seta, Takeshi
2013-06-01
In the present paper, we apply the implicit-correction method to the immersed-boundary thermal lattice Boltzmann method (IB-TLBM) for the natural convection between two concentric horizontal cylinders and in a square enclosure containing a circular cylinder. The Chapman-Enskog multiscale expansion proves the existence of an extra term in the temperature equation from the source term of the kinetic equation. In order to eliminate the extra term, we redefine the temperature and the source term in the lattice Boltzmann equation. When the relaxation time is less than unity, the new definition of the temperature and source term enhances the accuracy of the thermal lattice Boltzmann method. The implicit-correction method is required in order to calculate the thermal interaction between a fluid and a rigid solid using the redefined temperature. Simulation of the heat conduction between two concentric cylinders indicates that the error at each boundary point of the proposed IB-TLBM is reduced by the increment of the number of Lagrangian points constituting the boundaries. We derive the theoretical relation between a temperature slip at the boundary and the relaxation time and demonstrate that the IB-TLBM requires a small relaxation time in order to avoid temperature distortion around the immersed boundary. The streamline, isotherms, and average Nusselt number calculated by the proposed method agree well with those of previous numerical studies involving natural convection. The proposed IB-TLBM improves the accuracy of the boundary conditions for the temperature and velocity using an adequate discrete area for each of the Lagrangian nodes and reduces the penetration of the streamline on the surface of the body.
MHD natural convection in an inclined wavy cavity with corner heater filled with a nanofluid
NASA Astrophysics Data System (ADS)
Sheremet, M. A.; Oztop, H. F.; Pop, I.
2016-10-01
A mathematical modelling of MHD free convection in an inclined wavy enclosure filled with a Cu-water nanofluid in the presence of an isothermal corner heater has been carried out. The cavity is heated from the left bottom corner and cooled from the top wavy wall while the rest walls are adiabatic. Uniform magnetic field affects the heat transfer and fluid flow with an inclination angle to the axis xbar. Wavy cavity is inclined to the horizontal direction. Mathematical model formulated using the single-phase nanofluid approach in dimensionless variables stream function, vorticity and temperature has been solved by finite difference method of the second order accuracy in a wide range of governing parameters: Hartmann number (Ha=0-100), inclination angle of the magnetic field (χ = 0 - π) , undulation number (κ=0-4), inclination angle of the cavity (ζ = 0 - π) , solid volume fraction parameter of nanoparticles (φ=0.0-0.05), and dimensionless time (τ=0-0.27). Main efforts have been focused on the effects of these parameters on the fluid flow and heat transfer inside the cavity. Numerical results have been presented in the form of streamlines, isotherms and average Nusselt numbers.
Liu, Zhongliang; Zhang, Xinghua; Wang, Hongyan; Meng, Sheng; Cheng, Shuiyuan
2007-07-15
Surface hydrophilicity has a strong influence on frost nucleation according to phase transition theory. To study this effect, a close observation of frost formation and deposition processes on a vertical plate was made under free convection conditions. The formation and shape variation of frost crystals during the initial period are described and the frost thickness variation with time on both hydrophobic and plain copper cold surfaces are presented. The various influencing factors are discussed in depth. The mechanism of surface hydrophilicity influence on frost formation was analyzed theoretically. This revealed that increasing the contact angle can increase the potential barrier and restrain crystal nucleation and growth and thus frost deposition. The experimental results show that the initial water drops formed on a hydrophobic surface are smaller and remain in the liquid state for a longer time compared with ones formed on a plain copper surface. It is also observed that the frost layer deposited on a hydrophobic surface is loose and weak. Though the hydrophobic surface can retard frost formation to a certain extent and causes a looser frost layer, our experimental results show that it does not depress the growth of the frost layer. (author)
Woods, A.L.; Senthooran, S.; Parameswaran, S.
1999-01-01
The low {kappa}-{var_epsilon} model proposed by Yang and Shih (1992) is applied to the calculation of the turbulent natural convective boundary layer over a semi-infinite, vertical, isothermal surface. Using {kappa}/{var_epsilon} as the turbulent time scale will introduce a singularity in the {var_epsilon} equation, near the wall. This model uses a modified turbulent time scale near the wall to eliminate this singularity. The constants in the equation for damping function are modified to produce better results for both, natural convection and force convection. The results are compared with available experimental data and the results obtained from Chien`s model and are found to be in reasonable agreement. Here {kappa} represents the turbulent kinetic energy and {var_epsilon} represents the dissipation rate of turbulent kinetic energy.
The Nature of p-Modes and Granulation in Procyon: New MOST Photometry and New Yale Convection Models
NASA Astrophysics Data System (ADS)
Guenther, D. B.; Kallinger, T.; Gruberbauer, M.; Huber, D.; Weiss, W. W.; Kuschnig, R.; Demarque, P.; Robinson, F.; Matthews, J. M.; Moffat, A. F. J.; Rucinski, S. M.; Sasselov, D.; Walker, G. A. H.
2008-11-01
We present new photometry of Procyon, obtained by MOST during a 38 day run in 2007, and frequency analyses of those data. The long time coverage and low point-to-point scatter of the light curve yield an average noise amplitude of about 1.5-2.0 ppm in the frequency range 500-1500 μHz. This is half the noise level obtained from each of the previous two Procyon campaigns by MOST in 2004 and 2005. The 2007 MOST amplitude spectrum shows some evidence for p-mode signal: excess power centered near 1000 μHz and an autocorrelation signal near 55 μHz (suggestive of a mode spacing around that frequency), both consistent with p-mode model predictions. However, we do not see regularly spaced frequencies aligned in common l-valued ridges in echelle diagrams of the most significant peaks in the spectrum unless we select modes from the spectrum using a priori assumptions. The most significant peaks in the spectrum are scattered by more than ±5 μHz about the predicted l-valued ridges, a value that is consistent with the scatter among individually identified frequencies obtained from ground-based radial velocity (RV) observations. We argue that the observed scatter is intrinsic to the star, due to short lifetimes of the modes and the dynamic structure of Procyon's thin convection zone. We compare the MOST Procyon amplitude and power density spectra with preliminary results of three-dimensional numerical models of convection by the Yale group. These models show that, unlike in the Sun, Procyon's granulation signal in luminosity has a peak coinciding with the expected frequency region for p-modes near 1000 μHz. Based on data from the MOST satellite, a Canadian Space Agency mission, jointly operated by Dynacon, Inc., the University of Toronto Institute of Aerospace Studies, and the University of British Columbia, with the assistance of the University of Vienna.
NASA Astrophysics Data System (ADS)
Kamajaya, K.; Umar, E.; Sudjatmi
2015-09-01
Study on convection heat transfer using water-Al2O3 nanofluid as the working fluid in the vertical sub-channel has been conducted. The results of the study have been compared with the water-ZrO2 nanofluid and pure-water as the working fluid. The equipment used in this experiment is a vertical triangular sub-channel, equipped by primary cooling system, heat exchanger and a secondary cooling system. As a heating source used three vertical cylinders that have a uniform heat flux with a pitch to diameter ratio (P/D) 01:16. Cooling is used is water-Al2O3 colloid at 0.05 wt. %. Heat transfer from heating to cooling would occur in natural or forced convection. However, in this study will be discussed only natural convection heat transfer. The results showed that the natural convection heat transfer of water-Al2O3 nanofluid in a triangular sub-channels depending on the position. The results of the correlation as follows,
Routes to Unicellular Convection in a Tilted Rectangular Cavity
NASA Astrophysics Data System (ADS)
Mizushima, Jiro; Hara, Yusuke
2000-08-01
Transitions of natural convection in a tilted rectangular cavity heated from below are investigated numerically and theoretically by assuming two-dimensional and incompressible flow fields. All the boundary walls are assumed to be perfectly thermally conducting. It is known that thermal convection in a horizontal cavity heated from below occurs above a critical Rayleigh number due to an instability and has almost the same integral number of cells as the aspect ratio of the cavity, whereas natural convection in a vertical cavity heated from one side wall occurs even for very small Rayleigh (or Grashof) numbers and exhibits unicellular global circulation. Routes from multicellular convection to unicellular convection when the cavity is inclined gradually from the horizontal plane are explored by bifurcation analyses of the numerical results.
2011-01-01
A boundary layer analysis is presented for the mixed convection past a vertical wedge in a porous medium saturated with a nano fluid. The governing partial differential equations are transformed into a set of non-similar equations and solved numerically by an efficient, implicit, iterative, finite-difference method. A parametric study illustrating the influence of various physical parameters is performed. Numerical results for the velocity, temperature, and nanoparticles volume fraction profiles, as well as the friction factor, surface heat and mass transfer rates have been presented for parametric variations of the buoyancy ratio parameter Nr, Brownian motion parameter Nb, thermophoresis parameter Nt, and Lewis number Le. The dependency of the friction factor, surface heat transfer rate (Nusselt number), and mass transfer rate (Sherwood number) on these parameters has been discussed. PMID:21711715
Gorla, Rama Subba Reddy; Chamkha, Ali Jawad; Rashad, Ahmed Mohamed
2011-01-01
A boundary layer analysis is presented for the mixed convection past a vertical wedge in a porous medium saturated with a nano fluid. The governing partial differential equations are transformed into a set of non-similar equations and solved numerically by an efficient, implicit, iterative, finite-difference method. A parametric study illustrating the influence of various physical parameters is performed. Numerical results for the velocity, temperature, and nanoparticles volume fraction profiles, as well as the friction factor, surface heat and mass transfer rates have been presented for parametric variations of the buoyancy ratio parameter Nr, Brownian motion parameter Nb, thermophoresis parameter Nt, and Lewis number Le. The dependency of the friction factor, surface heat transfer rate (Nusselt number), and mass transfer rate (Sherwood number) on these parameters has been discussed.
NASA Astrophysics Data System (ADS)
Zavala-Guillén, I.; Xamán, J.; Álvarez, G.; Arce, J.; Hernández-Pérez, I.; Gijón-Rivera, M.
2016-03-01
This study reports the modeling of the turbulent natural convection in a double air-channel solar chimney (SC-DC) and its comparison with a single air-channel solar chimney (SC-C). Prediction of the mass flow and the thermal behavior of the SC-DC were obtained under three different climates of Mexico during one summer day. The climates correspond to: tropical savannah (Mérida), arid desert (Hermosillo) and temperate with warm summer (Mexico City). A code based on the Finite Volume Method was developed and a k-ω turbulence model has been used to model air turbulence in the solar chimney (SC). The code was validated against experimental data. The results indicate that during the day the SC-DC extracts about 50% more mass flow than the SC-C. When the SC-DC is located in Mérida, Hermosillo and Mexico City, the air-changes extracted along the day were 60, 63 and 52, respectively. The air temperature at the outlet of the chimney increased up to 33%, 38% and 61% with respect to the temperature it has at the inlet for Mérida, Hermosillo and Mexico City, respectively.
NASA Astrophysics Data System (ADS)
Chowdhury, Raju; Parvin, Salma; Khan, Md. Abdul Hakim
2016-07-01
The problem of natural convective heat and mass transfer in a triangular enclosure filled with nanofluid saturated porous medium in presence of heat generation has been studied in this paper. The bottom wall of the cavity is heated uniformly, the left inclined wall is heated linearly and the right inclined wall is considered to be cold. The concentration is higher at bottom wall, lower at right inclined wall and linearly concentrated at left inclined wall of the cavity. The governing equations are transformed to the dimensionless form and solved numerically using Galerkin weighted residual technique of finite element method. The results are obtained in terms of streamline, isotherms, isoconcentrations, Nusselt number (Nu) and Sherwood number (Sh) for the parameters thermal Rayleigh number (RaT), Heat generation parameter (λ) and Lewis number (Le) while Prandtl number (Pr), Buoyancy ratio (N) and Darcy number (Da) are considered to be fixed. It is observed that flow pattern, temperature fields and concentration fields are affected by the variation of above considered parameters.
NASA Astrophysics Data System (ADS)
Alsabery, Ammar I.; Hussain, Salam H.; Saleh, Habibis; Hashim, Ishak
2015-09-01
The problem of inclination angle effect on natural convection in a square cavity partially filled with non-Newtonian fluid layer is studied numerically using The Finite Volume Method. Governing equations are solved over wide range of Darcy number (10-5 ≤ Da ≤ 10-1), power-law index(0.6 ≤ n ≤ 1.4), the inclination angle of the cavity (0° ≤ ω ≤ 90°), Rayleigh number (Ra = 105) and porous layer thickness (S = 0.5). The results presented for values of the governing parameters in terms of streamlines in both porous/non-Newtonian fluid-layer, isotherms in both porous/non-Newtonian fluid-layer and average Nusselt number. It is shown that the heat transfer has maximum value when the power-law index is less than one (pseudoplastic fluid), and then decreases remarkably as the power-law index increases. The results have possible applications in heat-removal and heat-storage non-Newtonian fluid-saturated porous systems.
NASA Astrophysics Data System (ADS)
Thohura, Sharaban; Molla, Md. Mamun; Sarker, M. M. A.
2016-07-01
A study on the natural convection flow of non-Newtonian fluid along a vertical thin cylinder with constant wall temperature using modified power law viscosity model has been done. The basic equations are transformed to non dimensional boundary layer equations and the resulting systems of nonlinear partial differential equations are then solved employing marching order implicit finite difference method. The evolution of the surface shear stress in terms of local skin-friction, the rate of heat transfer in terms of local Nusselt number, velocity and temperature profiles for shear thinning as well as shear-thickening fluid considering the different values of Prandtl number have been focused. For the Newtonian fluids the present numerical results are compared with available published results which show a good agreement indeed. From the results it can be concluded that, at the leading edge, a Newtonian-like solution exists as the shear rate is not large enough to trigger non-Newtonian effects. Non-Newtonian effects can be found when the shear-rate increases beyond a threshold value.
Experimental study on natural-convection boiling burnout in an annulus. [PWR; BWR
Mishima, K.; Ishii, M.
1982-01-01
An experimental study was performed on burnout heat flux at low flow rates for low-pressure steam-water upward flow in an annulus. The data indicated that a premature burnout occurred due to flow-regime transition from churn-turbulent to annular flow. It is shown that the burnout observed in the experiment is essentially a flooding-limited burnout and the burnout heat flux can be well reproduced by a nondimensional correlation derived from the previously obtained criterion for flow-regime transition. It is also shown that the conventional correlations for burnout heat flux at low mass velocities agree well with the data on circulation and entrainment-limited burnout.
CATHARE thermal-hydraulic system code for HLM preliminary validation in natural convection tests
Polidori, M.; Meloni, P.; Lombardo, C.; Bandini, G.; Geffraye, G.; Kadri, D.
2012-07-01
The innovative nuclear systems cooled by Heavy Liquid Metal (HLM) are the subject of an ongoing interest both in Europe and outside, evidenced by a number of projects in progress. In the frame of the European Framework Programmes have been evidenced the need to adopt a thermalhydraulic system code capable to treat lead and Lead-Bismuth Eutectic (LBE) systems, with a particular interest in developing a 'European' code. Considering this scenario, within a specific collaboration between ENEA and CEA, the CATHARE French system code has been modified extending its capabilities to simulate HLM systems. In the present paper, the state of the validation process of CATHARE-HLM is discussed. The activity aims to assess the capabilities and limitations of the code to simulate the behavior of integral facilities, in particular in natural circulation conditions. The experimental data come from NACIE LBE-cooled facility sited at the ENEA Brasimone laboratories. The results obtained show a good capability in reproducing the systems behavior, despite some uncertainties on the experimental measurements. Future improvements on the code are going to be planned within the collaboration ENEA/CEA. (authors)
Comparison between ionospheric convection vortices and the associated equivalent currents
NASA Astrophysics Data System (ADS)
Liang, J.; Benkevitch, L.; Sofko, G. J.; Koustov, A. V.
2004-12-01
The equivalent current pattern derived from CANOPUS, NRCAN/GSC and MACCS magnetometers has been compared with the ionospheric convection pattern observed by SuperDARN HF radars. The discrepancies between the equivalent convection (EQC) and the SuperDARN-observed convection (SDC) patterns are explained in terms of the effect of day-night photoionization conductance gradient and the coupling between field-aligned currents (FACs) and ionospheric conductances. In particular, the agreement between the EQC and SDC patterns is usually worse for a counterclockwise convection vortex than for a clockwise cell, but a consistent pattern of discrepancy for counterclockwise convection vortices has been found. We suggest that the discrepancies are due to a downward FAC-conductance coupling process. Since the counterclockwise vortices and clockwise vortices occur predominantly in the dawn and dusk sectors, respectively, in accordance with the usual 2-cell global convection pattern, the asymmetry between the EQC and SDC patterns for counterclockwise vortices and clockwise vortices would naturally lead to a dawn-dusk asymmetry as well. This is revealed by a global statistical study of the deviation of direction between the magnetic equivalent convection and the SuperDARN convection in different time sectors and latitudes. In the dawn sector, the statistical results reveal that, at lower latitudes, the EQC direction deviation is slightly counterclockwise with respect to the SDC direction, whereas the deviation is significantly clockwise at high latitudes. These deviations are consistent with the discrepancy pattern for counterclockwise convection vortices, as found in the individual vortex event studies.
Lance, Blake W.; Smith, Barton L.
2016-06-23
Transient convection has been investigated experimentally for the purpose of providing Computational Fluid Dynamics (CFD) validation benchmark data. A specialized facility for validation benchmark experiments called the Rotatable Buoyancy Tunnel was used to acquire thermal and velocity measurements of flow over a smooth, vertical heated plate. The initial condition was forced convection downward with subsequent transition to mixed convection, ending with natural convection upward after a flow reversal. Data acquisition through the transient was repeated for ensemble-averaged results. With simple flow geometry, validation data were acquired at the benchmark level. All boundary conditions (BCs) were measured and their uncertainties quantified.more » Temperature profiles on all four walls and the inlet were measured, as well as as-built test section geometry. Inlet velocity profiles and turbulence levels were quantified using Particle Image Velocimetry. System Response Quantities (SRQs) were measured for comparison with CFD outputs and include velocity profiles, wall heat flux, and wall shear stress. Extra effort was invested in documenting and preserving the validation data. Details about the experimental facility, instrumentation, experimental procedure, materials, BCs, and SRQs are made available through this paper. As a result, the latter two are available for download and the other details are included in this work.« less
Birkholzer, J.T.; Webb, S.W.; Halecky, N.; Peterson, P.F.; Bodvarsson, G.S.
2005-12-14
The energy output of the high-level radioactive waste to beemplaced in the proposed geologic repository at Yucca Mountain, Nevada,will strongly affect the thermal-hydrological (TH) conditions in thenear-drift fractured rock. Heating of rock water to above-boilingconditions will induce large water saturation changes and fluxperturbations close to the waste emplacement tunnels (drifts) that willlast several thousand years. Understanding these perturbations isimportant for the performance of the repository, because they couldincrease, for example, the amount of formation water seeping into theopen drifts and contacting waste packages. Recent computational fluiddynamics (CFD) analysis has demonstrated that the drifts will act asimportant conduits for gas flows driven by natural convection. As aresult, vapor generated from boiling of formation water nearelevated-temperature sections of the drifts may effectively betransported to cooler end sections (where no waste is emplaced), wouldcondense there, and subsequently drain into underlying rock units. Thus,natural convection processes have great potential for reducing thenear-drift moisture content in heated drift sections, which has positiveramifications for repository performance. To study these processes, wehave developed a new simulation method that couples existing tools forsimulating TH conditions in the fractured formation with modules thatapproximate natural convection and evaporation conditions in heatedemplacement drifts. The new method is applied to evaluate the future THconditions at Yucca Mountain in a three-dimensional model domaincomprising a representative emplacement drift and the surroundingfractured rock.
Vilim, R .B.; Feldman, E. E.; Nuclear Engineering Division
2007-08-07
Passive safety in the Very High Temperature Reactor (VHTR) is strongly dependent on the thermal performance of the Reactor Cavity Cooling System (RCCS). Scaled experiments performed in the Natural Shutdown Test Facility (NSTF) are to provide data for assessing and/or improving computer code models for RCCS phenomena. Design studies and safety analyses that are to support licensing of the VHTR will rely on these models to achieve a high degree of certainty in predicted design heat removal rate. To guide in the selection and development of an appropriate set of experiments a scaling analysis has been performed for the air-cooled RCCS option. The goals were to (1) determine the phenomena that dominate the behavior of the RCCS, (2) determine the general conditions that must be met so that these phenomena and their relative importance are preserved in the experiments, (3) identify constraints specific to the NSTF that potentially might prevent exact similitude, and (4) then to indicate how the experiments can be scaled to prevent distortions in the phenomena of interest. The phenomena identified as important to RCCS operation were also the subject of a recent PIRT study. That work and the present work collectively indicate that the main phenomena influencing RCCS heat removal capability are (1) radiation heat transport from the vessel to the air ducts, (2) the integral effects of momentum and heat transfer in the air duct, (3) buoyancy at the wall inside the air duct giving rise to mixed convection, and (4) multidimensional effects inside the air duct caused by non-uniform circumferential heat flux and non-circular geometry.
Effective gamma-ray doses due to natural radiation from soils of southeastern Brazil
Silveira, M. A. G.; Moreira, R. H.; Bellini, B. S.; Medina, N. H.; Aguiar, V. A. P.
2010-08-04
We have used gamma-ray spectrometry to study the distribution of natural radiation from soils of southeastern Brazil: Billings reservoir, Sao Bernardo do Campo Parks, Diadema Parks, Interlagos region, Sao Paulo, and soil from Sao Paulo and Rio de Janeiro beaches. In most of the regions studied we have found that the dose due the external exposure to gamma-rays, proceeding from natural terrestrial elements, are between the values 0.3 and 0.6 mSv/year, established by the United Nations Scientific Committee on the Effects of Atomic Radiation.
Tzanos, C.P.; Farmer, M.T.; Nuclear Engineering Division
2007-08-31
-normal operating conditions. The standpipes are headered (in groups of four in the prototype) to water supply (header) tanks that are situated well above the reactor vessel to facilitate natural convection cooling during a loss of forced flow event. During normal operations, the water is pumped from a heat sink located outside the containment to the headered inlets to the standpipes. The water is then delivered to each standpipe through a centrally located downcomer that passes the coolant to the bottom of each pipe. The water then turns 180{sup o} and rises up through the annular gap while extracting heat from the reactor cavity due to a combination of natural convection and radiation across the gap between the reactor vessel and standpipes. The water exits the standpipes at the top where it is headered (again in groups of four) into a return line that passes the coolant to the top of the header tank. Coolant is drawn from each tank through a fitting located near the top of the tank where it flows to the heat rejection system located outside the containment. This completes the flow circuit for normal operations. During off-normal conditions, forced convection water cooling in the RCCS is presumed to be lost, as well as the ultimate heat sink outside the containment. In this case, water is passively drawn from an open line located at the bottom of the header tank. This line is orificed so that flow bypass during normal operations is small, yet the line is large enough to provide adequate flow during passive operations to remove decay heat while maintaining acceptable fuel temperatures. In the passive operating mode, water flows by natural convection from the bottom of the supply tank to the standpipes, and returns through the normal pathway to the top of the tanks. After the water reaches saturation and boiling commences, steam will pass through the top of the tanks and be vented to atmosphere. In the experiment system shown in Fig. 4, a steam condensation and collection system is
Okada, Masashi; Kang, Chaedong; Oyama, Kazuya; Yano, Satoshi
1999-07-01
Experiments of a natural convection of a water-fine particle suspension in a rectangular cell which was headed from a vertical wall and cooled from the opposing vertical wall were carried out. The suspension was a mixture of micro beads and water. Two kinds of particles were used. One was the micro beads made of soda glass of which specific gravity was 2.5. Mean diameter of the particle was 5.85{micro}m and standard deviation was 2.65{micro}m. It was considered that this particle size distribution was wide relatively. The other was micro beads made of SiO{sub 2} of which specific gravity was 2.15. Mean diameter of the particles was 2.97{micro}m and the standard deviation was 0.033{micro}m. This particle size distribution was considered to be narrow relatively. In the case of the suspension with particles whose size distribution is wide, many layers separated by almost-horizontal sharp interfaces were observed. In the beginning many layers appeared, and each interface of the layers fell gradually with a constant velocity, and finally all layers vanished. In the case of the suspension with the particles whose size distribution is narrow many layers were not formed but three layers were. The location of the interface was measured by video camera and at that time the temperature distribution in the vertical direction along the centerline of the test cell was measured. Furthermore the mean diameter and concentration of beads in each layer of the suspension were measured. In the above measurements the following results were obtained. The falling velocity of the interface becomes smaller as the initial concentration of particles becomes larger and the lower interface has the larger falling velocity. Each layer has a circular flow and each flow in a layer does not go into neighboring layers. The concentration of particles in each layer is almost uniform and the lower layer has the larger concentration and the larger mean diameter of particles. When the particle size
NASA Astrophysics Data System (ADS)
Sandholt, P.; Farrugia, C. J.; Andalsvik, Y.
2012-12-01
The aim of this study is to investigate the contributions of substorm processes to temporal structure of polar cap plasma convection. The central parameter is the cross-polar cap potential (CPCP). Selecting a ten hour-long interval of stable interplanetary driving by an interplanetary CME (ICME), we are able to distinguish between the dayside and nightside sources of the convection. The event was initiated by an abrupt enhancement of the magnetopause (MP) reconnection rate triggered by a southward turning of the ICME magnetic field. This was followed by long interval (ten hours) of steady and strong driving. Under the latter condition a long series of electrojet intensifications (polar cap contractions) was observed which recurred at 50 min. intervals. The detailed temporal structure of polar cap convection in relation to the polar cap contraction events is obtained by combining continuous ground observations of convection - related magnetic deflections (including polar cap magnetic indices in the northern and southern hemispheres, PCN and PCS) and the more direct but lower resolution ion drift data obtained from a satellite (DMSP F13) in polar orbit. The observed PCN enhancements combined with satellite observations (DMSP F13 and F15 data) of polar cap contractions during the evolution of selected substorm expansions allowed us to calculate the CPCP enhancements associated with each event in the series.
NASA Astrophysics Data System (ADS)
Sandholt, P. E.; Andalsvik, Y. L.; Farrugia, C. J.
2012-10-01
The aim of this study is to investigate the relative contributions of dayside and nightside processes to the spatial and temporal structure of polar cap plasma convection. The central parameter is the cross-polar cap potential (CPCP). Selecting a 10-h-long interval of stable interplanetary driving by an interplanetary CME (ICME), we are able to distinguish between the dayside and nightside sources of the convection. The event was initiated by an abrupt enhancement of the magnetopause (MP) reconnection rate triggered by a southward turning of the ICME magnetic field. This was followed by a long interval (10 h) of steady and strong driving. Under the latter condition a long series of electrojet intensifications was observed which recurred at 50 min intervals. The detailed temporal structure of polar cap convection in relation to polar cap contraction events is obtained by combining continuous ground observations of convection-related magnetic deflections (including polar cap magnetic indices in the Northern and Southern Hemispheres, PCN and PCS) and the more direct, but lower-resolution ion drift data obtained from a satellite (DMSP F13) in polar orbit. The observed PCN enhancements combined with DMSP satellite observations (F13 and F15 data) of polar cap contractions during the evolution of selected substorm expansions allowed us to estimate the CPCP enhancements (25%) associated with individual events in the series. Ground-satellite conjunctions are further used to investigate the spatial structure of polar cap convection, i.e., the homogeneous plasma flow in the centre (Vi ≤ 1 km s-1) versus channels of enhanced antisunward flows (Vi ≥ 1 km s-1) along the periphery of the polar cap. We emphasise the temporal structure of these polar cap flow phenomena in relation to the prevailing solar wind forcing and the repetitive substorm activity.
Occupational exposure due to naturally occurring radionuclide material in granite quarry industry.
Ademola, J A
2012-02-01
The potential occupational exposure in granite quarry industry due to the presence of naturally occurring radioactive material (NORM) has been investigated. The activity concentrations of (40)K, (226)Ra and (232)Th were determined using gamma-ray spectroscopy method. The annual effective dose of workers through different exposure pathways was determined by model calculations. The total annual effective dose varied from 21.48 to 33.69 μSv y(-1). Inhalation dose contributes the highest to the total effective dose. The results obtained were much lower than the intervention exemption levels (1.0 mSv y(-1)) given in the International Commission on Radiological Protection Publication 82.
NASA Astrophysics Data System (ADS)
Quettier, L.; Vincent-Viry, O.; Mailfert, A.; Juster, F. P.
2003-04-01
This paper presents a novel design of superconducting coils able to generate a micro-gravity environment for protein crystal growth in aqueous solution. The structures have been calculated thanks to a method for “inverse source synthesis problem" developed at the GREEN Choice of the angular offset between the directions of magnetic force field and magnetic field in the working area as well as convection phenomena are also studied.
Hong, Z.C.; Liou, J.H.
1998-02-20
Control volume methods have recently been developed for fluid flow and heat transfer on unstructured meshes. In this study, the authors extend these methods to implement the solution of natural-convection-dominated melting of gallium by a fixed-grid method. A simple, robust, and reliable explicit numerical method (MAC method) is applied for an unstructured triangular grid. This investigation also applies the implicit SIMPLER method for an unstructured triangular grid. Results obtained from the unstructured triangular grid correlate well with the structured mesh computations and experimental data. Also, the feasibility of applying the triangular grid to complex geometric problems is demonstrated by calculating two different triangular domains.
NASA Technical Reports Server (NTRS)
Abramzon, B.; Edwards, D. K.; Sirignano, W. A.
1986-01-01
A numerical study has been made of transient heat transfer and fluid flow in a cylindrical enclosure containing a two-layer gas-and-liquid system. The geometric configuration and the boundary conditions of the problem are relevant to the analysis of the preignition processes during the fire accident situation involving a pool of liquid fuel in the vicinity of an ignition source. It is demonstrated that the effects of the natural and thermocapillary convection, radiative transfer, thermal inertia and conduction of the walls bounding the enclosure, as well as, the magnitude of the gravity field play important roles in the development of the temperature and velocity fields in the container.
Okada, Kazuto . Interdisciplinary Graduate School of Engineering Science); Ozoe, Hiroyuki . Inst. of Advanced Material Study)
1993-03-01
The finite-difference computational scheme is developed for two-dimensional oscillatory natural convection of zero Prandtl number fluid in an open boat heated and cooled from opposing vertical walls. Various computational conditions are tested, such as the initial condition, time step length, finite-difference width, and finite-difference scheme. Instantaneous contour maps and velocity vectors in oscillatory states are presented in a series of maps to represent the fluctuating characteristics of two-dimensional roll cells. The physical conditions are for a boat with aspect ratio A = 3[minus]5 at Pr = 0 and Gr = 14,000-40,000.
NASA Astrophysics Data System (ADS)
Gibanov, N. S.; Sheremet, M. A.
2016-04-01
Numerical analysis of laminar natural convection inside a cubical cavity with a local heat source of triangular cross-section has been conducted. The mathematical model formulated in dimensionless variables such as "vector potential functions - vorticity vector" has been solved by the finite difference method of the second order accuracy. The three-dimensional temperature fields, 2D streamlines and isotherms in a wide range of the Rayleigh number from 104 to 106 have been presented illustrating variations of the fluid flow and heat transfer.
Analysis of hazardous material releases due to natural hazards in the United States.
Sengul, Hatice; Santella, Nicholas; Steinberg, Laura J; Cruz, Ana Maria
2012-10-01
Natural hazards were the cause of approximately 16,600 hazardous material (hazmat) releases reported to the National Response Center (NRC) between 1990 and 2008-three per cent of all reported hazmat releases. Rain-induced releases were most numerous (26 per cent of the total), followed by those associated with hurricanes (20 per cent), many of which resulted from major episodes in 2005 and 2008. Winds, storms or other weather-related phenomena were responsible for another 25 per cent of hazmat releases. Large releases were most frequently due to major natural disasters. For instance, hurricane-induced releases of petroleum from storage tanks account for a large fraction of the total volume of petroleum released during 'natechs' (understood here as a natural hazard and the hazardous materials release that results). Among the most commonly released chemicals were nitrogen oxides, benzene, and polychlorinated biphenyls. Three deaths, 52 injuries, and the evacuation of at least 5,000 persons were recorded as a consequence of natech events. Overall, results suggest that the number of natechs increased over the study period (1990-2008) with potential for serious human and environmental impacts.
Analysis of hazardous material releases due to natural hazards in the United States.
Sengul, Hatice; Santella, Nicholas; Steinberg, Laura J; Cruz, Ana Maria
2012-10-01
Natural hazards were the cause of approximately 16,600 hazardous material (hazmat) releases reported to the National Response Center (NRC) between 1990 and 2008-three per cent of all reported hazmat releases. Rain-induced releases were most numerous (26 per cent of the total), followed by those associated with hurricanes (20 per cent), many of which resulted from major episodes in 2005 and 2008. Winds, storms or other weather-related phenomena were responsible for another 25 per cent of hazmat releases. Large releases were most frequently due to major natural disasters. For instance, hurricane-induced releases of petroleum from storage tanks account for a large fraction of the total volume of petroleum released during 'natechs' (understood here as a natural hazard and the hazardous materials release that results). Among the most commonly released chemicals were nitrogen oxides, benzene, and polychlorinated biphenyls. Three deaths, 52 injuries, and the evacuation of at least 5,000 persons were recorded as a consequence of natech events. Overall, results suggest that the number of natechs increased over the study period (1990-2008) with potential for serious human and environmental impacts. PMID:22329456
Imbalance of Nature due to Contaminant Loads in the Culiacan River Watershed, Sinaloa, México
NASA Astrophysics Data System (ADS)
García Páez, F.; Ley-Aispuro, E.
2013-05-01
The Culiacan River discharges runoff from a large agricultural watershed into the wetlands at Ensenada de Pabellones ranked as a priority marine region of Mexico due to its high biodiversity and the economic importance of its fishing resources. This research estimated potential contaminant loads for BOD5, TSS, N and P from stormwater runoff and associated land use in the watershed. Previous studies had demonstrated the imbalance of nature due to land use change causing contamination by heavy metals, pesticides, sediment, phosphorus and eutrophication (Lopez and Osuna, 2002; Green and Paez, 2004, Gonzalez et al., 2006; Osuna et al., 2007). The methodology included: Characterizing the watershed according to land use, soil, vegetation, annual runoff and population density by sub-watershed; estimating the potential contaminant load and annual average concentrations of contaminants using the PLOAD program, comparing the result with monitored contaminant concentrations; and identifying the impact of pollutant loads in the watershed and coastal ecosystems and proposing management strategies to reduce or reverse the imbalance of nature caused by contamination in the Culiacan River watershed. Calculated contaminant loads in tonne/year were 13,682.4 of BOD5; 503,621.8 of TSS; 5,975.7 of N and 1,789.1 of P. The Tamazula and Humaya rivers watersheds provide 72% of the total load of BOD5, 68.5% of TSS, 77.6% of N and 62.7% of P discharged to the wetlands. Monitored results include: 89% of temperature observations were above 21°C, which is stressful to aquatic life due to a subsequent decrease in dissolved oxygen; 100% of the observations of P exceeded the ecological criteria for water quality; 71.5% of the observations for DO from 2001 to 2011, were above the ecological criteria for protection of aquatic life and 91.5% met the criteria for use in drinking water; 100% of the observations for BOD5 values remained in the range of Excellent to Good; 22% of the observations for the
The 5 key questions coping with risks due to natural hazards, answered by a case study
NASA Astrophysics Data System (ADS)
Hardegger, P.; Sausgruber, J. T.; Schiegg, H. O.
2009-04-01
Based on Maslow's hierarchy of needs, human endeavours concern primarily existential needs, consequently, to be safeguarded against both natural as well as man made threads. The subsequent needs are to realize chances in a variety of fields, as economics and many others. Independently, the 5 crucial questions are the same as for coping with risks due to natural hazards specifically. These 5 key questions are I) What is the impact in function of space and time ? II) What protection measures comply with the general opinion and how much do they mitigate the threat? III) How can the loss be adequately quantified and monetized ? IV) What budget for prevention and reserves for restoration and compensation are to be planned ? V) Which mix of measures and allocation of resources is sustainable, thus, optimal ? The 5 answers, exemplified by a case study, concerning the sustainable management of risk due to the debris flows by the Enterbach / Inzing / Tirol / Austria, are as follows : I) The impact, created by both the propagation of flooding and sedimentation, has been forecasted by modeling (numerical simulation) the 30, 50, 100, 150, 300 and 1000 year debris flow. The input was specified by detailed studies in meteorology, precipitation and runoff, in geology, hydrogeology, geomorphology and slope stability, in hydraulics, sediment transport and debris flow, in forestry, agriculture and development of communal settlement and infrastructure. All investigations were performed according to the method of ETAlp (Erosion and Transport in Alpine systems). ETAlp has been developed in order to achieve a sustainable development in alpine areas and has been evaluated by the research project "nab", within the context of the EU-Interreg IIIb projects. II) The risk mitigation measures of concern are in hydraulics at the one hand and in forestry at the other hand. Such risk management is evaluated according to sustainability, which means economic, ecologic and social, in short, "triple
NASA Astrophysics Data System (ADS)
Hummel, Tobias; Pacheco-Vega, Arturo
2012-11-01
In the present study we use Karhunen-Loève (KL) expansions to model the dynamic behavior of a single-phase natural convection loop. The loop is filled with an incompressible fluid that exchanges heat through the walls of its toroidal shape. Influx and efflux of energy take place at different parts of the loop. The focus here is a sinusoidal variation of the heat flux exchanged with the environment for three different scenarios; i.e., stable, limit cycles and chaos. For the analysis, one-dimensional models, in which the tilt angle and the amplitude of the heat flux are used as parameters, were first developed under suitable assumptions and then solved numerically to generate the data from which the KL-based models could be constructed. The method of snapshots, along with a Galerkin projection, was then used to find the basis functions and corresponding constants of each expansion, thus producing the optimal representation of the system. Results from this study indicate that the dimension of the KL-based dynamical system depends on the linear stability of the steady states; the number of basis functions necessary to describe the system increases with increased complexity of the system operation. When compared to typical dynamical systems based on Fourier expansions the KL-based models are, in general, more compact and equally accurate in the dynamic description of the natural convection loop.
Lisowski, D. D.; Farmer, M. T.; Lomperski, S.; Kilsdonk, D. J.; Bremer, N.; Aeschlimann, R. W.
2014-06-01
The Natural convection Shutdown heat removal Test Facility (NSTF) is a large scale thermal hydraulics test facility that has been built at Argonne National Laboratory (ANL). The facility was constructed in order to carry out highly instrumented experiments that can be used to validate the performance of passive safety systems for advanced reactor designs. The facility has principally been designed for testing of Reactor Cavity Cooling System (RCCS) concepts that rely on natural convection cooling for either air or water-based systems. Standing 25-m in height, the facility is able to supply up to 220 kW at 21 kW/m^{2} to accurately simulate the heat fluxes at the walls of a reactor pressure vessel. A suite of nearly 400 data acquisition channels, including a sophisticated fiber optic system for high density temperature measurements, guides test operations and provides data to support scaling analysis and modeling efforts. Measurements of system mass flow rate, air and surface temperatures, heat flux, humidity, and pressure differentials, among others; are part of this total generated data set. The following report provides an introduction to the top level-objectives of the program related to passively safe decay heat removal, a detailed description of the engineering specifications, design features, and dimensions of the test facility at Argonne. Specifications of the sensors and their placement on the test facility will be provided, along with a complete channel listing of the data acquisition system.
Joule-Thomson Cooling Due to CO2 Injection into Natural GasReservoirs
Oldenburg, Curtis M.
2006-04-21
Depleted natural gas reservoirs are a promising target for Carbon Sequestration with Enhanced Gas Recovery (CSEGR). The focus of this study is on evaluating the importance of Joule-Thomson cooling during CO2 injection into depleted natural gas reservoirs. Joule-Thomson cooling is the adiabatic cooling that accompanies the expansion of a real gas. If Joule-Thomson cooling were extreme, injectivity and formation permeability could be altered by the freezing of residual water,formation of hydrates, and fracturing due to thermal stresses. The TOUGH2/EOS7C module for CO2-CH4-H2O mixtures is used as the simulation analysis tool. For verification of EOS7C, the classic Joule-Thomson expansion experiment is modeled for pure CO2 resulting in Joule-Thomson coefficients in agreement with standard references to within 5-7 percent. For demonstration purposes, CO2 injection at constant pressure and with a large pressure drop ({approx}50 bars) is presented in order to show that cooling by more than 20 C can occur by this effect. Two more-realistic constant-rate injection cases show that for typical systems in the Sacramento Valley, California, the Joule-Thomson cooling effect is minimal. This simulation study shows that for constant-rate injections into high-permeability reservoirs, the Joule-Thomson cooling effect is not expected to create significant problems for CSEGR.
NASA Astrophysics Data System (ADS)
Morris, J.; Roy, P.; Walsh, S.
2015-12-01
Proppant, such as sand, is injected during hydraulic fracturing to maintain fracture aperture and conductivity. Proppant performance is a complex result of fluid flow, discrete particle mechanics and geomechanical deformation. We present investigations into these phenomena at scales ranging from millimeters to meters. Traditionally, the design goal for proppant placement is uniform distribution by using viscous carrier fluids that keep the proppant suspended and maintain conductivity over the full area of the fracture. Large volume hydraulic fracturing in shales typically use low viscosity fluids, resulting in proppant settling out from the carrier fluid. Consequently, the proppant occupies the lower portion of the fracture. In addition, many shale plays host natural fractures that take up injected carrier fluid, but may not develop sufficient aperture to accommodate proppant. We present simulations investigating natural development of heterogeneity in proppant distribution within fracture networks due to settling and network flow. In addition to natural development of heterogeneity, the petroleum industry has sought to engineer heterogeneity to generate isolated propped portions of the fracture that maintain aperture in adjacent, open channels. We present two examples of such heterogeneous proppant placement (HPP) technologies. The first involves pulsating proppant at the wellhead and the second utilizes a homogenous composite fluid that develops heterogeneity spontaneously through hydrodynamic instabilities. We present simulation results that compare these approaches and conclude that spontaneous creation of heterogeneity has distinct geomechanical advantages. Finally, we present simulations at the scale of individual proppant particles that emphasize the complexity of dynamic instabilities and their influence upon proppant fate. Disclaimer: This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under
NASA Astrophysics Data System (ADS)
Udayashankar, Paniveni
2015-12-01
Observation of the Solar photosphere through high resolution instruments have long indicated that the surface of the Sun is not a tranquil, featureless surface but is beset with a granular appearance. These cellular velocity patterns are a visible manifestation of sub- photospheric convection currents which contribute substantially to the outward transport of energy from the deeper layers, thus maintaining the energy balance of the Sun as a whole.Convection is the chief mode of transport in the outer layers of all cool stars such as the Sun (Noyes,1982). Convection zone of thickness 30% of the Solar radius lies in the sub-photospheric layers of the Sun. Here the opacity is so large that heat flux transport is mainly by convection rather than by photon diffusion. Convection is revealed on four scales. On the scale of 1000 km, it is granulation and on the scale of 8-10 arcsec, it is Mesogranulation. The next hierarchial scale of convection , Supergranules are in the range of 30-40 arcsec. The largest reported manifestation of convection in the Sun are ‘Giant Cells’or ‘Giant Granules’, on a typical length scale of about 108 m.'Supergranules' is caused by the turbulence that extends deep into the convection zone. They have a typical lifetime of about 20hr with spicules marking their boundaries. Gas rises in the centre of the supergranules and then spreads out towards the boundary and descends.Broadly speaking supergranules are characterized by the three parameters namely the length L, the lifetime T and the horizontal flow velocity vh . The interrelationships amongst these parameters can shed light on the underlying convective processes and are in agreement with the Kolmogorov theory of turbulence as applied to large scale solar convection (Krishan et al .2002 ; Paniveni et. al. 2004, 2005, 2010).References:1) Noyes, R.W., The Sun, Our Star (Harvard University Press, 1982)2) Krishan, V., Paniveni U., Singh , J., Srikanth R., 2002, MNRAS, 334/1,2303) Paniveni
NASA Astrophysics Data System (ADS)
Randall, D. A.; Branson, M.; Dutta, R.; Jones, T.
2015-12-01
It has been suggested that stochastic fluctuations of convective activity can lead to systematic changes in large-scale weather and climate. We present results of recent ensemble-prediction experiments with the super-parameterized version of CAM that provide a new way to explore such effects, without the need for adhoc assumptions about the nature of the stochastic effects.
Mixed double-diffusive convection in gas-loaded heat pipes
Peterson, P.F. ); Tien, C.L. )
1990-02-01
This study examines mixed double-diffusive convection in gas-loaded heat pipes and two-phase thermosyphons. The numerical simulation and experiments show that steady, laminar natural convection due to the combined effects of temperature and concentration gradients can greatly redistribute the noncondensable gas within the condenser. This change of the gas distribution, however, does not significantly alter the overall condensation heat transfer. This interesting result implies that even with natural convection present, much simpler integral models can still be applied with confidence for the design of variable-conductance heat pipes and thermosyphons.
Goodarzi, M; Safaei, M R; Oztop, Hakan F; Karimipour, A; Sadeghinezhad, E; Dahari, M; Kazi, S N; Jomhari, N
2014-01-01
The effect of radiation on laminar and turbulent mixed convection heat transfer of a semitransparent medium in a square enclosure was studied numerically using the Finite Volume Method. A structured mesh and the SIMPLE algorithm were utilized to model the governing equations. Turbulence and radiation were modeled with the RNG k-ε model and Discrete Ordinates (DO) model, respectively. For Richardson numbers ranging from 0.1 to 10, simulations were performed for Rayleigh numbers in laminar flow (10⁴) and turbulent flow (10⁸). The model predictions were validated against previous numerical studies and good agreement was observed. The simulated results indicate that for laminar and turbulent motion states, computing the radiation heat transfer significantly enhanced the Nusselt number (Nu) as well as the heat transfer coefficient. Higher Richardson numbers did not noticeably affect the average Nusselt number and corresponding heat transfer rate. Besides, as expected, the heat transfer rate for the turbulent flow regime surpassed that in the laminar regime. The simulations additionally demonstrated that for a constant Richardson number, computing the radiation heat transfer majorly affected the heat transfer structure in the enclosure; however, its impact on the fluid flow structure was negligible.
Goodarzi, M.; Safaei, M. R.; Oztop, Hakan F.; Karimipour, A.; Sadeghinezhad, E.; Dahari, M.; Kazi, S. N.; Jomhari, N.
2014-01-01
The effect of radiation on laminar and turbulent mixed convection heat transfer of a semitransparent medium in a square enclosure was studied numerically using the Finite Volume Method. A structured mesh and the SIMPLE algorithm were utilized to model the governing equations. Turbulence and radiation were modeled with the RNG k-ε model and Discrete Ordinates (DO) model, respectively. For Richardson numbers ranging from 0.1 to 10, simulations were performed for Rayleigh numbers in laminar flow (104) and turbulent flow (108). The model predictions were validated against previous numerical studies and good agreement was observed. The simulated results indicate that for laminar and turbulent motion states, computing the radiation heat transfer significantly enhanced the Nusselt number (Nu) as well as the heat transfer coefficient. Higher Richardson numbers did not noticeably affect the average Nusselt number and corresponding heat transfer rate. Besides, as expected, the heat transfer rate for the turbulent flow regime surpassed that in the laminar regime. The simulations additionally demonstrated that for a constant Richardson number, computing the radiation heat transfer majorly affected the heat transfer structure in the enclosure; however, its impact on the fluid flow structure was negligible. PMID:24778601
Probabilistic Forecasting of Life and Economic Losses due to Natural Disasters
NASA Astrophysics Data System (ADS)
Barton, C. C.; Tebbens, S. F.
2014-12-01
The magnitude of natural hazard events such as hurricanes, tornadoes, earthquakes, and floods are traditionally measured by wind speed, energy release, or discharge. In this study we investigate the scaling of the magnitude of individual events of the 20th and 21stcentury in terms of economic and life losses in the United States and worldwide. Economic losses are subdivided into insured and total losses. Some data sets are inflation or population adjusted. Forecasts associated with these events are of interest to insurance, reinsurance, and emergency management agencies. Plots of cumulative size-frequency distributions of economic and life loss are well-fit by power functions and thus exhibit self-similar scaling. This self-similar scaling property permits use of frequent small events to estimate the rate of occurrence of less frequent larger events. Examining the power scaling behavior of loss data for disasters permits: forecasting the probability of occurrence of a disaster over a wide range of years (1 to 10 to 1,000 years); comparing losses associated with one type of disaster to another; comparing disasters in one region to similar disasters in another region; and, measuring the effectiveness of planning and mitigation strategies. In the United States, life losses due to flood and tornado cumulative-frequency distributions have steeper slopes, indicating that frequent smaller events contribute the majority of losses. In contrast, life losses due to hurricanes and earthquakes have shallower slopes, indicating that the few larger events contribute the majority of losses. Disaster planning and mitigation strategies should incorporate these differences.
NASA Astrophysics Data System (ADS)
Liu, Qing; He, Ya-Ling
2015-11-01
In this paper, a double multiple-relaxation-time lattice Boltzmann model is developed for simulating transient solid-liquid phase change problems in porous media at the representative elementary volume scale. The model uses two different multiple-relaxation-time lattice Boltzmann equations, one for the flow field and the other for the temperature field with nonlinear latent heat source term. The model is based on the generalized non-Darcy formulation, and the solid-liquid interface is traced through the liquid fraction which is determined by the enthalpy-based method. The present model is validated by numerical simulations of conduction melting in a semi-infinite space, solidification in a semi-infinite corner, and convection melting in a square cavity filled with porous media. The numerical results demonstrate the efficiency and accuracy of the present model for simulating transient solid-liquid phase change problems in porous media.
NASA Astrophysics Data System (ADS)
Timchenko, V.; Tkachenko, O. A.; Giroux-Julien, S.; Ménézo, C.
2015-05-01
Numerical and experimental investigations of the flow and heat transfer in open-ended channel formed by the double skin façade have been undertaken in order to improve understanding of the phenomena and to apply it to passive cooling of building integrated photovoltaic systems. Both uniform heating and non-uniform heating configurations in which heat sources alternated with unheated zones on both skins were studied. Different periodic and asymmetric heating modes have been considered for the same aspect ratio 1/15 of wall distance to wall height and for periodicity 1/15 and 4/15 of heated/unheated zones and heat input, 220 W/m2. In computational study three dimensional transient LES simulation was carried out. It is shown that in comparison to uniformly heating configuration, non-uniformly heating configuration enhances both convective heat transfer and chimney effect.
Davidson, J.H.
1998-06-01
This report very briefly summarizes project objectives, results, and current activities. The goals of the project are: (1) to develop guidelines for the design and use of thermosypohon side-arm heat exchangers in solar domestic water heating systems, and (2) to establish appropriate modeling and testing criteria for evaluating the performance of systems using this type of heat exchanger. Results include the experimental study of thermosyphon heat exchangers, which led to modeling equations that correlate the overall heat transfer coefficient-area product (UA) to mixed convection regime parameters. Current activities include the development and evaluation of a side-arm heat exchanger computer model and modification of the experimental facility for fundamental heat exchanger studies.
NASA Astrophysics Data System (ADS)
Mobedi, M.; Saidi, A.; Sunden, B.
In this study, a numerical investigation has been carried out to reveal the mechanism of fluid flow and heat transfer from a vertical rectangular fin attached to a partially heated horizontal base. The problem is a conjugate conduction-convection heat transfer problem with open boundaries. The governing equations for the problem are the conservation of mass, momentum and energy equations for the fluid and the heat conduction equation for the fin. The control volume technique based on the SIMPLEC algorithm with a nonstaggerred grid arrangement is employed to solve the governing equations. The effect of the heated base, on the mechanism of the fluid flow and heat transfer, is numerically investigated. Temperature distribution and flow patterns around the fin are plotted to support the discussion. Results are obtained for air at laminar and steady flow.
Erbas, S.; Ece, M.C.
1999-07-01
Fluids such as molten plastics, polymers, pulps, foodstuffs or slurries exhibit non-Newtonian fluid behavior and are increasingly used in various manufacturing and processing industries. Determination of the friction and heat transfer characteristics of non-Newtonian fluids over heated surfaces is important for the design of industrial equipment working with this type of fluids. Steady free convection laminar boundary-layer flow along a heated vertical plate immersed in a quiescent power-law fluid is investigated. Two heating modes are considered by assuming that either surface temperature or heat flux has a power-law variation. Similarity solutions of the boundary-layer equations are obtained numerically for both heating conditions. The skin friction coefficient and Nusselt number are found to be higher in the prescribed temperature case for large Prandtl numbers and increase with the flow behavior index.
Bau, H.H.
1995-12-31
Using stability theory, numerical simulations, and in some instances experiments, it is demonstrated that the critical Rayleigh number for the bifurcation (1) from the no-motion (conduction) state to the motion state and (2) from time-independent convection to time-dependent, oscillatory convection in the thermal convection loop and Rayleigh-Benard problems can be significantly increased or decreased. This is accomplished through the use of a feedback controller effectuating small perturbations in the boundary data. The controller consists of sensors which detect deviations in the fluid`s temperature from the motionless, conductive values and then direct actuators to respond to these deviations in such a way as to suppress the naturally occurring flow instabilities. Actuators which modify the boundary`s temperature/heat flux are considered. The feedback controller can also be used to control flow patterns and generate complex dynamic behavior at relatively low Rayleigh numbers.
NASA Technical Reports Server (NTRS)
Kassemi, M.; Naraghi, M. H. N.
1993-01-01
A new numerical method is presented for the analysis of combined natural convection and radiation heat transfer with applications in many engineering situations such as materials processing, combustion and fire research. Because of the recent interest in the low gravity environment of space, attention is devoted to both 1-g and low-g applications. The two-dimensional mathematical model is represented by a set of coupled nonlinear integro-partial differential equations. Radiative exchange is formulated using the Discrete Exchange Factor method (DEF). This method considers point to point exchange and provides accurate results over a wide range of radiation parameters. Numerical results show that radiation significantly influences the flow and heat transfer in both low-g and 1-g applications. In the low-g environment, convection is weak, and radiation can easily become the dominant heat transfer mode. It is also shown that volumetric heating by radiation gives rise to an intricate cell pattern in the top heated enclosure.
NASA Technical Reports Server (NTRS)
Choudhari, Meelan
1992-01-01
Acoustic receptivity of a Blasius boundary layer in the presence of distributed surface irregularities is investigated analytically. It is shown that, out of the entire spatial spectrum of the surface irregularities, only a small band of Fourier components can lead to an efficient conversion of the acoustic input at any given frequency to an unstable eigenmode of the boundary layer flow. The location, and width, of this most receptive band of wavenumbers corresponds to a relative detuning of O(R sub l.b.(exp -3/8)) with respect to the lower-neutral instability wavenumber at the frequency under consideration, R sub l.b. being the Reynolds number based on a typical boundary-layer thickness at the lower branch of the neutral stability curve. Surface imperfections in the form of discrete mode waviness in this range of wavenumbers lead to initial instability amplitudes which are O(R sub l.b.(exp 3/8)) larger than those caused by a single, isolated roughness element. In contrast, irregularities with a continuous spatial spectrum produce much smaller instability amplitudes, even compared to the isolated case, since the increase due to the resonant nature of the response is more than that compensated for by the asymptotically small band-width of the receptivity process. Analytical expressions for the maximum possible instability amplitudes, as well as their expectation for an ensemble of statistically irregular surfaces with random phase distributions, are also presented.
Prenatal Stress due to a Natural Disaster Predicts Adiposity in Childhood: The Iowa Flood Study
Dancause, Kelsey N.; Laplante, David P.; Hart, Kimberly J.; O'Hara, Michael W.; Brunet, Alain
2015-01-01
Prenatal stress can affect lifelong physical growth, including increased obesity risk. However, human studies remain limited. Natural disasters provide models of independent stressors unrelated to confounding maternal characteristics. We assessed degree of objective hardship and subjective distress in women pregnant during severe flooding. At ages 2.5 and 4 years we assessed body mass index (BMI), subscapular plus triceps skinfolds (SS + TR, an index of total adiposity), and SS : TR ratio (an index of central adiposity) in their children (n = 106). Hierarchical regressions controlled first for several potential confounds. Controlling for these, flood exposure during early gestation predicted greater BMI increase from age 2.5 to 4, as well as total adiposity at 2.5. Greater maternal hardship and distress due to the floods, as well as other nonflood life events during pregnancy, independently predicted greater increase in total adiposity between 2.5 and 4 years. These results support the hypothesis that prenatal stress increases adiposity beginning in childhood and suggest that early gestation is a sensitive period. Results further highlight the additive effects of maternal objective and subjective stress, life events, and depression, emphasizing the importance of continued studies on multiple, detailed measures of maternal mental health and experience in pregnancy and child growth. PMID:25874124
Prenatal stress due to a natural disaster predicts adiposity in childhood: the Iowa Flood Study.
Dancause, Kelsey N; Laplante, David P; Hart, Kimberly J; O'Hara, Michael W; Elgbeili, Guillaume; Brunet, Alain; King, Suzanne
2015-01-01
Prenatal stress can affect lifelong physical growth, including increased obesity risk. However, human studies remain limited. Natural disasters provide models of independent stressors unrelated to confounding maternal characteristics. We assessed degree of objective hardship and subjective distress in women pregnant during severe flooding. At ages 2.5 and 4 years we assessed body mass index (BMI), subscapular plus triceps skinfolds (SS + TR, an index of total adiposity), and SS : TR ratio (an index of central adiposity) in their children (n = 106). Hierarchical regressions controlled first for several potential confounds. Controlling for these, flood exposure during early gestation predicted greater BMI increase from age 2.5 to 4, as well as total adiposity at 2.5. Greater maternal hardship and distress due to the floods, as well as other nonflood life events during pregnancy, independently predicted greater increase in total adiposity between 2.5 and 4 years. These results support the hypothesis that prenatal stress increases adiposity beginning in childhood and suggest that early gestation is a sensitive period. Results further highlight the additive effects of maternal objective and subjective stress, life events, and depression, emphasizing the importance of continued studies on multiple, detailed measures of maternal mental health and experience in pregnancy and child growth.
Imbalance of Nature due to Anthropogenic Activities in the Bay of Bacorehuis, Sinaloa, Mexico
NASA Astrophysics Data System (ADS)
Torrecillas Nunez, C.; Cárdenas Cota, H.
2013-05-01
Pollution is further enhancing water scarcity by reducing water usability downstream, globally the most prevalent water quality problem is eutrophication, a result of high-nutrient loads, which substantially impairs beneficial uses of water. Projected food production needs and increasing wastewater effluents associated with an increasing population over the next three decades suggest a 10%-15% increase in the river input of nitrogen loads into coastal ecosystems (UNO, 2009). Our study in the Bay of Bacorehuis in the State of Sinaloa, which was carried out due to a request from local fishermen who wanted to find out the reason for fishing stocks depletion, confirmed this trend with the consequent imbalance of nature. Sinaloa depends heavily on intensive agricultural production to support its economy which in turn relies on water irrigation and the application of agro-chemicals. The research project included a desk top study of geophysical and environmental factors as well as sampling and testing of the water. In addition we carried out socio-economic research to find out the impact on the local community of the imbalance caused by anthropogenic activities in the watershed upstream from the Bay. Our research established that the Bay of Bacorehuis is contaminated by organic matter, bacteria coliforms, pesticides and mercury due to the discharge of surplus runoff generated by irrigation of farmlands into drainage networks as well as the discharge of untreated industrial and domestic wastewater form more than 24,000 inhabitants. The main contaminants detected in the water bodies were organic matter, faecal coliforms, mercury, dimethoate, endosulfan, heptachlor, DDE, DDT, organonitrogen, synthetic pyrethroid, chlorothalonil, ethion, endosulfan, diazinon, malathion and chlorpyrifos. Contaminants in sediments included the pesticides endosulfan, heptachlor, DDE, DDT, organophosphates, organonitrogen and synthetic pyrethroids. Natural water courses have been highly modified
Mustafa, Meraj; Mushtaq, Ammar; Hayat, Tasawar; Ahmad, Bashir
2014-01-01
The problem of natural convective boundary layer flow of nanofluid past a vertical plate is discussed in the presence of nonlinear radiative heat flux. The effects of magnetic field, Joule heating and viscous dissipation are also taken into consideration. The governing partial differential equations are transformed into a system of coupled nonlinear ordinary differential equations via similarity transformations and then solved numerically using the Runge–Kutta fourth-fifth order method with shooting technique. The results reveal an existence of point of inflection for the temperature distribution for sufficiently large wall to ambient temperature ratio. Temperature and thermal boundary layer thickness increase as Brownian motion and thermophoretic effects intensify. Moreover temperature increases and heat transfer from the plate decreases with an increase in the radiation parameter. PMID:25251242
NASA Astrophysics Data System (ADS)
Moreno, Rafael; Ramaswamy, Balasubramaniam
2003-01-01
Using object-oriented programming (OOP) techniques and philosophies, a collection of C++ tools for the rapid development of finite element applications has been created. The object-oriented finite element analysis (OOFEA) toolkit provides both the geometrical and mathematical management tools necessary for this task in the form of useful class hierarchies. In particular, the OOFEA toolkit features methods for evaluating arbitrary weak forms provided by the user in order to solve particular problems of interest. A description of the underlying concepts, philosophies and techniques used to develop the toolkit are included. A strong effort has been made to concentrate on its possibly beneficial usage in the computational fluid dynamics area. In order to demonstrate the toolkit capabilities of managing complex projects, a simulator for laminar and turbulent natural convective flows in enclosures has been developed and a numerical study of some of these flows has been conducted.
NASA Astrophysics Data System (ADS)
Kochetov, N. A.; Loktionov, V. D.; Sidorov, A. S.
2015-09-01
The possibility of using the Star CCM+ software system for analyzing the thermal state of the melt pool metal layer generated as a result of melt stratification during a severe accident in pressure-vessel nuclear reactors is considered. In order to verify and substantiate the possibility of using this software system for modeling the natural convection processes in the melt at high values of the Rayleigh number, test problems were solved. The obtained results were found to be in good agreement with the known solutions and with the experimental data. The behavior of the melt metal layer was subjected to a parametric analysis for different melt heating conditions, the results of which showed that certain parameters have a determining influence on the so-called focusing effect and on the specific features of current in this layer.
Green, M.A.; Ishimoto, S.; Lau, W.; Yang, S.
2003-09-15
The Muon Ionization Cooling Experiment (MICE) has three 350-mm long liquid hydrogen absorbers to reduce the momentum of 200 MeV muons in all directions. The muons are then re-accelerated in the longitudinal direction by 200 MHz RF cavities. The result is cooled muons with a reduced emittance. The energy from the muons is taken up by the liquid hydrogen in the absorber. The hydrogen in the MICE absorbers is cooled by natural convection to the walls of the absorber that are in turn cooled by helium gas that enters at 14 K. This report describes the MICE liquid hydrogen absorber and the heat exchanger between the liquid hydrogen and the helium gas that flows through passages in the absorber wall.
Mustafa, Meraj; Mushtaq, Ammar; Hayat, Tasawar; Ahmad, Bashir
2014-01-01
The problem of natural convective boundary layer flow of nanofluid past a vertical plate is discussed in the presence of nonlinear radiative heat flux. The effects of magnetic field, Joule heating and viscous dissipation are also taken into consideration. The governing partial differential equations are transformed into a system of coupled nonlinear ordinary differential equations via similarity transformations and then solved numerically using the Runge-Kutta fourth-fifth order method with shooting technique. The results reveal an existence of point of inflection for the temperature distribution for sufficiently large wall to ambient temperature ratio. Temperature and thermal boundary layer thickness increase as Brownian motion and thermophoretic effects intensify. Moreover temperature increases and heat transfer from the plate decreases with an increase in the radiation parameter.
Jeong, Y; Baek, S; Dupriez, P; Maran, J-N; Sahu, J K; Nilsson, J; Lee, B
2008-11-24
We investigate the thermal characteristics of a polymer-clad fiber laser under natural convection when it is strongly pumped up to the damage point of the fiber. For this, we utilize a temperature sensing technique based on a fiber Bragg grating sensor array. We have measured the longitudinal temperature distribution of a 2.4-m length ytterbium-sensitized erbium-doped fiber laser that was end-pumped at approximately 975 nm. The measured temperature distribution decreases exponentially, approximately, decaying away from the pump-launch end. We attribute this to the heat dissipation of absorbed pump power. The maximum temperature difference between the fiber ends was approximately 190 K at the maximum pump power of 60.8 W. From this, we estimate that the core temperature reached approximately 236 degrees C. PMID:19030073
Mustafa, Meraj; Mushtaq, Ammar; Hayat, Tasawar; Ahmad, Bashir
2014-01-01
The problem of natural convective boundary layer flow of nanofluid past a vertical plate is discussed in the presence of nonlinear radiative heat flux. The effects of magnetic field, Joule heating and viscous dissipation are also taken into consideration. The governing partial differential equations are transformed into a system of coupled nonlinear ordinary differential equations via similarity transformations and then solved numerically using the Runge-Kutta fourth-fifth order method with shooting technique. The results reveal an existence of point of inflection for the temperature distribution for sufficiently large wall to ambient temperature ratio. Temperature and thermal boundary layer thickness increase as Brownian motion and thermophoretic effects intensify. Moreover temperature increases and heat transfer from the plate decreases with an increase in the radiation parameter. PMID:25251242
Prueitt, Melvin L.
1996-01-01
Convection towers which are capable of cleaning the pollution from large quantities of air, of generating electricity, and of producing fresh water utilize the evaporation of water sprayed into the towers to create strong airflows and to remove pollution from the air. Turbines in tunnels at the skirt section of the towers generate electricity, and condensers produce fresh water.
Prueitt, Melvin L.
1995-01-01
Convection towers which are capable of cleaning the pollution from large quantities of air, of generating electricity, and of producing fresh water utilize the evaporation of water sprayed into the towers to create strong airflows and to remove pollution from the air. Turbines in tunnels at the skirt section of the towers generate electricity, and condensers produce fresh water.
Prueitt, M.L.
1996-01-16
Convection towers which are capable of cleaning the pollution from large quantities of air, of generating electricity, and of producing fresh water utilize the evaporation of water sprayed into the towers to create strong airflows and to remove pollution from the air. Turbines in tunnels at the skirt section of the towers generate electricity, and condensers produce fresh water. 6 figs.
Prueitt, Melvin L.
1994-01-01
Convection towers which are capable of cleaning the pollution from large quantities of air and of generating electricity utilize the evaporation of water sprayed into the towers to create strong airflows and to remove pollution from the air. Turbines in tunnels at the skirt section of the towers generate electricity. Other embodiments may also provide fresh water, and operate in an updraft mode.
ERIC Educational Resources Information Center
Ebert, James R.; Elliott, Nancy A.; Hurteau, Laura; Schulz, Amanda
2004-01-01
Students must understand the fundamental process of convection before they can grasp a wide variety of Earth processes, many of which may seem abstract because of the scales on which they operate. Presentation of a very visual, concrete model prior to instruction on these topics may facilitate students' understanding of processes that are largely…
Sedahmed, G.H.; Nirdosh, I.
1995-06-01
Many industrial electrochemical processes such as electrowinning of metals, electrochemical pollution control, and electroorganic and electroinorganic syntheses are diffusion-controlled processes whose rates depend on the geometry of the working electrode as well as the prevailing hydrodynamic conditions. Recently much work has been done to develop new electrochemical reactors which are more efficient than the traditional parallel plate electrochemical reactor used in conducting such processes. In line with this, the object of the present work was to study the natural convection mass transfer behavior of a new electrode geometry, namely an array of closely-spaced horizontal tubes. Natural convection mass transfer at a vertical array of closely-spaced horizontal cylinders was studied by an electrochemical technique involving the measurement of the limiting current of the cathodic deposition of copper from acidified copper sulfate solution. Various combinations of solution concentration, cylinder diameter, and number of cylinders per array were used including experiments on single cylinders. The mass transfer coefficient at the array was found to decrease with increasing number of cylinders, pass through a minimum, and then increase with further increase in the number of cylinders per array; the mass transfer coefficient increased with increasing cylinder diameter in the array. Mass transfer data for different arrays were correlated for the range 6.3 {times} 10{sup 9} < ScGr < 3.63 {times} 10{sup 10} by the equation Sh = 0.455(ScGr){sup 0.25} and for the range 6.3 {times} 10{sup 10} < ScGr < 3.63 {times} 10{sup 12} by the equation Sh = 0.0064(ScGr){sup 0.42}. The characteristic length used in the above correlations was obtained by dividing the array area by the perimeter projected onto a horizontal plane. Practical implications of the present results in designing electrochemical reactors with heat transfer facilities are highlighted.
Richard C. Martineau; Ray A. Berry; Aurélia Esteve; Kurt D. Hamman; Dana A. Knoll; Ryosuke Park; William Taitano
2009-01-01
This report illustrates a comparative study to analyze the physical differences between numerical simulations obtained with both the conservation and incompressible forms of the Navier-Stokes equations for natural convection flows in simple geometries. The purpose of this study is to quantify how the incompressible flow assumption (which is based upon constant density advection, divergence-free flow, and the Boussinesq gravitational body force approximation) differs from the conservation form (which only assumes that the fluid is a continuum) when solving flows driven by gravity acting upon density variations resulting from local temperature gradients. Driving this study is the common use of the incompressible flow assumption in fluid flow simulations for nuclear power applications in natural convection flows subjected to a high heat flux (large temperature differences). A series of simulations were conducted on two-dimensional, differentially-heated rectangular geometries and modeled with both hydrodynamic formulations. From these simulations, the selected characterization parameters of maximum Nusselt number, average Nusselt number, and normalized pressure reduction were calculated. Comparisons of these parameters were made with available benchmark solutions for air with the ideal gas assumption at both low and high heat fluxes. Additionally, we generated body force, velocity, and divergence of velocity distributions to provide a basis for further analysis. The simulations and analysis were then extended to include helium at the Very High Temperature gas-cooled Reactor (VHTR) normal operating conditions. Our results show that the consequences of incorporating the incompressible flow assumption in high heat flux situations may lead to unrepresentative results. The results question the use of the incompressible flow assumption for simulating fluid flow in an operating nuclear reactor, where large temperature variations are present. The results show that the use of
Davidson, J.H.
1998-06-01
The goals of this project are: (1) to develop guidelines for the design and use of thermosyphon side-arm heat exchangers in solar domestic water heating systems, and (2) to establish appropriate modeling and testing criteria for evaluating the performance of systems using this type of heat exchanger. The tasks for the project are as follows: (1) Develop a model of the thermal performance of thermosyphon heat exchangers in solar water heating applications. A test protocol will be developed which minimizes the number of tests required to adequately account for mixed convection effects. The TRNSYS component model will be fully integrated in a system component model and will use data acquired with the specified test protocol. (2) Conduct a fundamental study to establish friction and heat transfer correlations for conditions and geometries typical of thermosyphon heat exchangers in solar systems. Data will be obtained as a function of a buoyancy parameter based on Grashof and Reynolds numbers. The experimental domain will encompass the ranges expected in solar water heating systems.
Davidson, J.H.
1998-06-01
The goals of this project are: (1) to develop guidelines for the design and use of thermosyphon side-arm heat exchangers in solar domestic water heating systems, and (2) to establish appropriate modeling and testing criteria for evaluating the performance of systems using this type of heat exchanger. The tasks for the project are as follows: (1) Develop a model of the thermal performance of thermosyphon heat exchangers in solar water heating applications. A test protocol will be developed which minimizes the number of tests required to adequately account for mixed convection effects. The TRNSYS component model will be fully integrated in a system component model and will use data acquired with the specified test protocol. (2) Conduct a fundamental study to establish friction and heat transfer correlations for conditions and geometries typical of thermosyphon heat exchangers in solar systems. Data will be obtained as a function of a buoyancy parameter based on Grashof and Reynolds numbers. The experimental domain will encompass the ranges expected in solar water heating systems.
Davidson, J.H.
1998-06-01
The goals of this project are: (1) to develop guidelines for the design and use of thermosyphon side-arm heat exchangers in solar domestic water heating systems, and (2) to establish appropriate modeling and testing criteria for evaluating the performance of systems using this type of heat exchanger. The tasks for the project are as follows: (1) Develop a model of the thermal performance of thermosyphon heat exchangers in solar water heating applications. A test protocol will be developed which minimizes the number of tests required to adequately account for mixed convection effects. The TRNSYS component model will be fully integrated in a system component model and will use data acquired with the specified test protocol. (2) Conduct a fundamental study to establish friction and heat transfer correlations for conditions and geometries typical of thermosyphon heat exchangers in solar systems. Data will be obtained as a function of a buoyancy parameter based on Grashof and Reynolds numbers. The experimental domain will encompass the ranges expected in solar water heating systems.
Davidson, J.H.
1998-06-01
The goals of this project are: (1) to develop guidelines for the design and use of thermosyphon side-arm heat exchangers in solar domestic water heating systems, and (2) to establish appropriate modeling and testing criteria for evaluating the performance of systems using this type of heat exchanger. The tasks for the project are as follows: (1) Develop a model of the thermal performance of thermosyphon heat exchangers in solar water heating applications. A test protocol will be developed which minimizes the number of tests required to adequately account for mixed convection effects. The TRNSYS component model will be fully integrated in a system component model and will use data acquired with the specified test protocol. (2) Conduct a fundamental study to establish friction and heat transfer correlations for conditions and geometries typical of thermosyphon heat exchangers in solar systems. Data will be obtained as a function of a buoyancy parameter based on Grashof and Reynolds numbers. The experimental domain will encompass the ranges expected in solar water heating systems.
Davidson, J.H.
1998-06-01
This progress report describes the thermodynamic testing and modeling of a thermosyphon heat exchanger used in solar water heating systems. Testing of a four tube-in-shell thermosyphon heat exchanger was performed in two parts. The first portion of the test increased the collector fluid while the storage tank remained isothermal. After the collector fluid temperature was raised to 95 C, the second part of the test allowed the storage tank to gain heat. The test was performed for two collector flow rates. Measured values included collector side forced flow rate, temperature differences across the heat exchanger, vertical temperature distribution in the storage tank, vertical water temperature profile in the heat exchanger, and pressure drop on the thermosyphon side of the heat exchanger. The overall heat transfer coefficient-area product (UA) values obtained confirmed that models which assume UA depends solely on thermosyphon flow rate do not adequately characterize thermosyphon heat exchangers. This is because heat transfer in thermosyphon exchangers occurs in the mixed convection, rather than forced flow, regime. A linear regression equation was developed to better predict UA using the Prandtl, Reynolds, and Grashof numbers and dimensionless parameters based on fluid properties calculated for the average hot and cold leg temperatures. 9 figs.
NASA Astrophysics Data System (ADS)
Angeli, D.; Stalio, E.; Corticelli, M. A.; Barozzi, G. S.
2015-11-01
A parallel algorithm is presented for the Direct Numerical Simulation of buoyancy- induced flows in open or partially confined periodic domains, containing immersed cylindrical bodies of arbitrary cross-section. The governing equations are discretized by means of the Finite Volume method on Cartesian grids. A semi-implicit scheme is employed for the diffusive terms, which are treated implicitly on the periodic plane and explicitly along the homogeneous direction, while all convective terms are explicit, via the second-order Adams-Bashfort scheme. The contemporary solution of velocity and pressure fields is achieved by means of a projection method. The numerical resolution of the set of linear equations resulting from discretization is carried out by means of efficient and highly parallel direct solvers. Verification and validation of the numerical procedure is reported in the paper, for the case of flow around an array of heated cylindrical rods arranged in a square lattice. Grid independence is assessed in laminar flow conditions, and DNS results in turbulent conditions are presented for two different grids and compared to available literature data, thus confirming the favorable qualities of the method.
Thermal convection in vertically suspended soap films
NASA Astrophysics Data System (ADS)
Zhang, Jie
In normal fluids, a temperature difference can create a density difference. In the presence of the gravitational field, denser fluid will fall and lighter fluid will rise, causing fluid motion known as thermal convection. This type of convection can occur on different scales, from a single growing crystal to mantle movement inside the earth. Although many experiments have been conducted in unstably stratified fluids, there have been few laboratory experiments studying convective turbulence in stably stratified fluids, which is more common in nature. Here I present a two-dimensional (2D) convection in a stably stratified vertical soap film. It was found that the interaction between the gravitational potential energy, due to the 2D density fluctuation, and the kinetic energy is important. This interplay between the two energy sources manifests itself in the statistical properties of velocity and 2D density fluctuations in the system. Our experimental findings shed new lights to a turbulent system that strongly couples to a non-passive field.
Formation and dynamics of hazardous convective weather events in Ukraine
NASA Astrophysics Data System (ADS)
Balabukh, Vera; Malytska, Liudmyla; Bazalieieva, Iuliana
2013-04-01
Atmospheric circulation change observed from the middle of the 70s of the twentieth century in the Northern Hemisphere resulted in changes of weather events formation conditions in different regions. The degree of influence of various factors on the formation of weather events also has changed. This eventually led to an increase in number and intensity of weather events and their variations in time and space. Destructions and damages associated with these events have increased recently and the biggest damages are mainly results of complex convective weather events: showers, hail, squall. Therefore, one of the main tasks of climatology is to study the mechanisms of change repeatability and intensity of these events. The paper considers the conditions of formation of hazardous convective weather phenomena (strong showers, hail, squalls, tornadoes) in Ukraine and their spatial and temporal variability during 1981 - 2010. Research of convection processes was based on daily radiosonde data for the warm season (May-September 1981 - 2010s), reanalysis ERA-Interim ECMWF data for 1989 - 2010 years , daily observations at 187 meteorological stations in Ukraine, as well as observations of the natural phenomena in other regions (different from the meteorological stations). Indices of atmospheric instability, the magnitude of the Convective Available Potential Energy (CAPE), the moisture, the height of the condensation and equilibrium level was used to quantify the intensity of convection. The criteria for the intensity of convection for Ukrainian territory were refined on the basis of these data. Features of the development of convection for various hazardous convective weather events were investigated and identified the necessary conditions for the occurrence of showers, hail, tornadoes and squall in Ukraine. Spatio-temporal variability of convection intensity in Ukraine, its regional characteristics and dynamics for the past 30 year was analyzed. Significant tendency to an
Prueitt, M.L.
1994-02-08
Convection towers which are capable of cleaning the pollution from large quantities of air and of generating electricity utilize the evaporation of water sprayed into the towers to create strong airflows and to remove pollution from the air. Turbines in tunnels at the skirt section of the towers generate electricity. Other embodiments may also provide fresh water, and operate in an updraft mode. 5 figures.
Numerical Analysis of Convection/Transpiration Cooling
NASA Technical Reports Server (NTRS)
Glass, David E.; Dilley, Arthur D.; Kelly, H. Neale
1999-01-01
An innovative concept utilizing the natural porosity of refractory-composite materials and hydrogen coolant to provide CONvective and TRANspiration (CONTRAN) cooling and oxidation protection has been numerically studied for surfaces exposed to a high heat flux, high temperature environment such as hypersonic vehicle engine combustor walls. A boundary layer code and a porous media finite difference code were utilized to analyze the effect of convection and transpiration cooling on surface heat flux and temperature. The boundary, layer code determined that transpiration flow is able to provide blocking of the surface heat flux only if it is above a minimum level due to heat addition from combustion of the hydrogen transpirant. The porous media analysis indicated that cooling of the surface is attained with coolant flow rates that are in the same range as those required for blocking, indicating that a coupled analysis would be beneficial.
Numerical Analysis of Convection/Transpiration Cooling
NASA Technical Reports Server (NTRS)
Glass, David E.; Dilley, Arthur D.; Kelly, H. Neale
1999-01-01
An innovative concept utilizing the natural porosity of refractory-composite materials and hydrogen coolant to provide CONvective and TRANspiration (CONTRAN) cooling and oxidation protection has been numerically studied for surfaces exposed to a high heat flux high temperature environment such as hypersonic vehicle engine combustor walls. A boundary layer code and a porous media finite difference code were utilized to analyze the effect of convection and transpiration cooling on surface heat flux and temperature. The boundary layer code determined that transpiration flow is able to provide blocking of the surface heat flux only if it is above a minimum level due to heat addition from combustion of the hydrogen transpirant. The porous media analysis indicated that cooling of the surface is attained with coolant flow rates that are in the same range as those required for blocking, indicating that a coupled analysis would be beneficial.
Prevention and Treatment of Traumatic Brain Injury Due to Rapid-Onset Natural Disasters
Regens, James L.; Mould, Nick
2014-01-01
The prevention and treatment of traumatic brain injury (TBI) attributable to rapid-onset natural disasters is a major challenge confronting disaster preparedness planners and emergency medical personnel responding to those incidents. The kinetic energy released by rapid-onset natural disasters such as earthquakes, hurricanes or typhoons, and tornadoes can cause mild, moderate, or severe TBIs. As a result, neurotrauma is a major risk factor for mortality and morbidity outcomes within the spatial domain impacted by a rapid-onset natural disaster. This review article elucidates major challenges associated with immediate emergency medical response, long-term care, and prevention of post-event increases in pediatric TBIs because of child abuse when rapid-onset natural disasters occur. PMID:24783188
Prevention and treatment of traumatic brain injury due to rapid-onset natural disasters.
Regens, James L; Mould, Nick
2014-01-01
The prevention and treatment of traumatic brain injury (TBI) attributable to rapid-onset natural disasters is a major challenge confronting disaster preparedness planners and emergency medical personnel responding to those incidents. The kinetic energy released by rapid-onset natural disasters such as earthquakes, hurricanes or typhoons, and tornadoes can cause mild, moderate, or severe TBIs. As a result, neurotrauma is a major risk factor for mortality and morbidity outcomes within the spatial domain impacted by a rapid-onset natural disaster. This review article elucidates major challenges associated with immediate emergency medical response, long-term care, and prevention of post-event increases in pediatric TBIs because of child abuse when rapid-onset natural disasters occur.
Prevention and treatment of traumatic brain injury due to rapid-onset natural disasters.
Regens, James L; Mould, Nick
2014-01-01
The prevention and treatment of traumatic brain injury (TBI) attributable to rapid-onset natural disasters is a major challenge confronting disaster preparedness planners and emergency medical personnel responding to those incidents. The kinetic energy released by rapid-onset natural disasters such as earthquakes, hurricanes or typhoons, and tornadoes can cause mild, moderate, or severe TBIs. As a result, neurotrauma is a major risk factor for mortality and morbidity outcomes within the spatial domain impacted by a rapid-onset natural disaster. This review article elucidates major challenges associated with immediate emergency medical response, long-term care, and prevention of post-event increases in pediatric TBIs because of child abuse when rapid-onset natural disasters occur. PMID:24783188
Kasinathan, N.; Rajakumar, A.; Vaidyanathan, G.; Chetal, S.C.
1995-09-01
Post shutdown decay heat removal is an important safety requirement in any nuclear system. In order to improve the reliability of this function, Liquid metal (sodium) cooled fast breeder reactors (LMFBR) are equipped with redundant hot pool dipped immersion coolers connected to natural draught air cooled heat exchangers through intermediate sodium circuits. During decay heat removal, flow through the core, immersion cooler primary side and in the intermediate sodium circuits are also through natural convection. In order to establish the viability and validate computer codes used in making predictions, a 1:20 scale experimental model called RAMONA with water as coolant has been built and experimental simulation of decay heat removal situation has been performed at KfK Karlsruhe. Results of two such experiments have been compiled and published as benchmarks. This paper brings out the results of the numerical simulation of one of the benchmark case through a 1D/2D coupled code system, DHDYN-1D/THYC-2D and the salient features of the comparisons. Brief description of the formulations of the codes are also included.
Transient thermal convection in microgravity
NASA Technical Reports Server (NTRS)
Dressler, R. F.
1981-01-01
The unsteady two-dimensional thermal convection in a cylinder due to a transient acceleration solved for a step-function excitation. From this, the solution was obtained for an arbitrary time-dependent acceleration. The solutions are valied for sufficiently low Rayleigh numbers and therefore, relevant to microgravity fields. As an example, two graphs are presented for he convection resulting from the movement of an astronaut inside the Shuttle. The analysis can be applied to obtain any other convective flows such as those caused by g-jitter or variable rotation of the Shuttle.
[Carbon monoxide poisoning due to lack of maintenance of a natural gas boiler].
Nielsen, H; Johannessen, A C
1994-01-17
Carbon monoxide causes one third of all poisoning deaths in Denmark, but is probably grossly underdiagnosed. We present a case where an elderly couple was admitted on several occasions to local hospitals with a variety of symptoms and signs; e.g. flu-like symptoms, generalized seizures, polycythaemia, chest pain, and ventricular tachycardia. The correct diagnosis, carbon monoxide poisoning, was made when the dog in the family was found dead; examination of the natural gas boiler revealed sooting, clogging of the flue, and a carbon monoxide concentration above 0.2 percent. The natural gas boiler had not been checked after installation five years earlier. Natural gas installations are becoming still more prevalent in Danish homes, but present regulations regarding the installations are apparently not yet able to prevent new incidents of carbon monoxide poisoning. PMID:8296426
[Carbon monoxide poisoning due to lack of maintenance of a natural gas boiler].
Nielsen, H; Johannessen, A C
1994-01-17
Carbon monoxide causes one third of all poisoning deaths in Denmark, but is probably grossly underdiagnosed. We present a case where an elderly couple was admitted on several occasions to local hospitals with a variety of symptoms and signs; e.g. flu-like symptoms, generalized seizures, polycythaemia, chest pain, and ventricular tachycardia. The correct diagnosis, carbon monoxide poisoning, was made when the dog in the family was found dead; examination of the natural gas boiler revealed sooting, clogging of the flue, and a carbon monoxide concentration above 0.2 percent. The natural gas boiler had not been checked after installation five years earlier. Natural gas installations are becoming still more prevalent in Danish homes, but present regulations regarding the installations are apparently not yet able to prevent new incidents of carbon monoxide poisoning.
NASA Technical Reports Server (NTRS)
Antar, Basil N.; Witherow, William K.; Paley, Mark S.; Curreri, Peter A. (Technical Monitor)
2001-01-01
This paper presents results from numerical simulations as well as laboratory experiments of buoyancy driven convection in an ampoule under varying heating and gravitational acceleration loadings. The modeling effort in this work resolves the large scale natural convective motion that occurs in the fluid during photodeposition of polydiacetelene films which is due to energy absorbed by the growth solution from a UV source. Consequently, the growth kinetics of the film are ignored in the model discussed here, and also a much simplified ampoule geometry is considered. The objective of this work is to validate the numerical prediction on the strength and structure of buoyancy driven convection that could occur under terrestrial conditions during nonlinear optical film growth. The validation is used to enable a reliable predictive capability on the nature and strength of the convective motion under low gravity conditions. The ampoule geometry is in the form of a parallelepiped with rectangular faces. The numerical results obtained from the solution to the Boussinesq equations show that natural convection will occur regardless of the orientation of the UV source with respect to the gravity vector. The least strong convective motion occurred with the UV beam directed at the top face of the parallelepiped. The strength of the convective motion was found to be almost linearly proportional to the total power of the UV source. Also, it was found that the strength of the convective motion decreased linearly with the gravity due to acceleration. The pattern of the convective flow on the other hand, depended on the source location.
Energy saving due to natural ventilation in housing blocks in Madrid
NASA Astrophysics Data System (ADS)
González-Lezcano, RA; Hormigos-Jiménez, S.
2016-07-01
Getting a healthy and comfortable indoor environment in homes in southern Europe is a complicated task. In continental climates, with very cold temperatures in winter and very hot in summer, energy consumption greatly increases with air conditioning significant spending. To propose action guidelines for use of natural ventilation and to develop effective design strategies is essential. Therefore, and given a specific building type block of flats in Madrid, this article focuses on establishing what periods of the year natural ventilation is required to reduce energy consumption in air conditioning, also considering the quality of the outdoor environment and the design of the building. To develop this, a statistical study of the chosen type, that allows studying the direction and the wind speed in the area, is performed. Analysis of wind pressures in holes in the facade is performed by means of numerical simulations of fluid flow (CFD) inside to later infer in the natural ventilation rate required within policy parameters. With the data obtained, a study of energy saving is made as a function of natural ventilation rate established for the building type.
Food supply chain disruption due to natural disaster: Entities, risks and strategies for resilience
Technology Transfer Automated Retrieval System (TEKTRAN)
The resilience of food supply chain (FSC) to disruptions has not kept pace with the extended, globalized and complex network of modern food chain. This chapter presents a holistic view of the FSC, interactions among its components, risks and vulnerabilities of disruption in the context of natural d...
Hammond, R.P.; King, L.D.P.
1960-03-22
An homogeneous nuclear power reactor utilizing convection circulation of the liquid fuel is proposed. The reactor has an internal heat exchanger looated in the same pressure vessel as the critical assembly, thereby eliminating necessity for handling the hot liquid fuel outside the reactor pressure vessel during normal operation. The liquid fuel used in this reactor eliminates the necessity for extensive radiolytic gas rocombination apparatus, and the reactor is resiliently pressurized and, without any movable mechanical apparatus, automatically regulates itself to the condition of criticality during moderate variations in temperature snd pressure and shuts itself down as the pressure exceeds a predetermined safe operating value.
Prenatal stress due to a natural disaster predicts insulin secretion in adolescence.
Dancause, Kelsey N; Veru, Franz; Andersen, Ross E; Laplante, David P; King, Suzanne
2013-09-01
Prenatal stress might increase cardiometabolic disease risk. We measured prenatal stress due to an ice storm in 1998, and measured glucose tolerance among a subsample of 32 exposed adolescents in 2011. Severity of stress was positively associated with insulin secretion, suggesting that prenatal stress independently predicts metabolic outcomes in adolescence.
Assessing Intraseasonal Variability Produced by Several Deep Convection Schemes in the NCAR CCM3.6
NASA Astrophysics Data System (ADS)
Maloney, E. D.
2001-05-01
The Hack, Zhang/McFarlane, and McRAS convection schemes produce very different simulations of intraseasonal variability in the NCAR CCM3.6. A robust analysis of simulation performance requires an expanded set of diagnostics. The use of only one criterion to analyze model Madden-Julian oscillation (MJO) variability, such as equatorial zonal wind variability, may give a misleading impression of model performance. Schemes that produce strong variability in zonal winds may sometimes lack a corresponding coherent signal in precipitation, suggesting that model convection and the large-scale circulation are not as strongly coupled as observed. The McRAS scheme, which includes a parametrization of unsaturated convective downdrafts, produces the best simulation of intraseasonal variability of the three schemes used. Downdrafts in McRAS create a moister equatorial troposphere, which increases equatorial convection. Composite analysis indicates a strong dependence of model intraseasonal variability on the frictional convergence mechanism, which may also be important in nature. The McRAS simulation has limitations, however. Indian Ocean variability is weak, and anomalous convection extends too far east across the Pacific. The dependence of convection on surface friction is too strong, and causes enhanced MJO convection to be associated with low-level easterly wind perturbations, unlike observed MJO convection. Anomalous vertical advection associated with surface convergence influences model convection by moistening the lower troposphere. Based on the work of Hendon (2000), coupling to an interactive ocean is unlikely to change the performance of the CCM3 with McRAS, due to the phase relationship between anomalous convection and zonal winds. Use of the analysis tools presented here indicates areas for improvement in the parametrization of deep convection by atmospheric GCMs.
Turbulent Convection: Old and New Models
NASA Astrophysics Data System (ADS)
Canuto, V. M.
1996-08-01
This paper contains (1) a physical argument to show that the one-eddy MLT model underestimates the convective flux Fc in the high-efficiency regime, while it overestimates Fc in the low-efficiency regime, and (2) a new derivation of the Fc(MLT) using a turbulence model in the one-eddy approximation. (3) We forsake the one-eddy approximation and adopt the Kolmogorov spectrum to represent the turbulent energy spectrum. The resulting Fc > Fc(MLT) in the high-efficiency regime, and Fc
Seismic Constraints on Interior Solar Convection
NASA Technical Reports Server (NTRS)
Hanasoge, Shravan M.; Duvall, Thomas L.; DeRosa, Marc L.
2010-01-01
We constrain the velocity spectral distribution of global-scale solar convective cells at depth using techniques of local helioseismology. We calibrate the sensitivity of helioseismic waves to large-scale convective cells in the interior by analyzing simulations of waves propagating through a velocity snapshot of global solar convection via methods of time-distance helioseismology. Applying identical analysis techniques to observations of the Sun, we are able to bound from above the magnitudes of solar convective cells as a function of spatial convective scale. We find that convection at a depth of r/R(solar) = 0.95 with spatial extent l < 30, where l is the spherical harmonic degree, comprise weak flow systems, on the order of 15 m/s or less. Convective features deeper than r/R(solar) = 0.95 are more difficult to image due to the rapidly decreasing sensitivity of helioseismic waves.
NASA Astrophysics Data System (ADS)
Chang, Chandong; Jo, Yeonguk
2015-11-01
Two field examples of hydraulic fracturing stress measurements are reported, in which the determined stress magnitudes exhibit severe variations with depth. The stress measurements were conducted in vertical boreholes drilled in granites in two different locations in South Korea. Several isolated intervals of intact rocks in the boreholes were vertically fractured by injecting water. The magnitudes of the minimum horizontal principal compressive stress (Shmin) were determined from shut-in pressures. The magnitudes of the maximum horizontal principal compressive stress (SHmax) were estimated based on the Kirsch equation using tensile strengths determined from hollow cylinder tests and Brazilian tests, in which pressurization-rate effects on tensile strength were taken into account. The stress states in both locations are in reverse-faulting stress regimes. The magnitudes of SHmax are generally within a stress range defined by frictional limits of favorably oriented fractures having frictional coefficients of 0.6 and 1.0. However, SHmax magnitudes do not increase linearly with depth, but rather scatter quite severely. It is noted that near the depths where the measured stresses are relatively low, natural discontinuities with wide apertures containing weak filling material exist, whereas near the depths of high stress, such wide discontinuities are scarce. Wide aperture discontinuities are predominantly oriented such that their slip tendency is high under the given stress conditions, meaning that if excessive shear stress is exerted, the weak discontinuities would slip to release the excessive stress. Such local processes would restrict SHmax magnitudes within values that can only be sustained by the shear strengths of the discontinuities, leading to severe variations of SHmax with depth. This result suggests that stress magnitudes are controlled quite locally by the frictional property of natural discontinuities, and that the stress state in granitic rock might be
Absorbed Gamma-Ray Doses due to Natural Radionuclides in Building Materials
Aguiar, Vitor A. P.; Medina, Nilberto H.; Moreira, Ramon H.; Silveira, Marcilei A. G.
2010-05-21
This work is devoted to the application of high-resolution gamma-ray spectrometry in the study of the effective dose coming from naturally occurring radionuclides, namely {sup 40}K, {sup 232}Th and {sup 238}U, present in building materials such as sand, cement, and granitic gravel. Four models were applied to estimate the effective dose and the hazard indices. The maximum estimated effective dose coming from the three reference rooms considered is 0.90(45) mSv/yr, and maximum internal hazard index is 0.77(24), both for the compact clay brick reference room. The principal gamma radiation sources are cement, sand and bricks.
Absorbed Gamma-Ray Doses due to Natural Radionuclides in Building Materials
NASA Astrophysics Data System (ADS)
Aguiar, Vitor A. P.; Medina, Nilberto H.; Moreira, Ramon H.; Silveira, Marcilei A. G.
2010-05-01
This work is devoted to the application of high-resolution gamma-ray spectrometry in the study of the effective dose coming from naturally occurring radionuclides, namely 40K, 232Th and 238U, present in building materials such as sand, cement, and granitic gravel. Four models were applied to estimate the effective dose and the hazard indices. The maximum estimated effective dose coming from the three reference rooms considered is 0.90(45) mSv/yr, and maximum internal hazard index is 0.77(24), both for the compact clay brick reference room. The principal gamma radiation sources are cement, sand and bricks.
Convection, nucleosynthesis, and core collapse
NASA Technical Reports Server (NTRS)
Bazan, Grant; Arnett, David
1994-01-01
We use a piecewise parabolic method hydrodynamics code (PROMETHEUS) to study convective burning in two dimensions in an oxygen shell prior to core collapse. Significant mixing beyond convective boundaries determined by mixing-length theory brings fuel (C-12) into the convective regon, causing hot spots of nuclear burning. Plumes dominate the velocity structure. Finite perturbations arise in a region in which O-16 will be explosively burned to Ni-56 when the star explodes; the resulting instabilities and mixing are likely to distribute Ni-56 throughout the supernova envelope. Inhomogeneities in Y(sub e) may be large enough to affect core collapse and will affect explosive nucleosynthesis. The nature of convective burning is dramatically different from that assumed in one-dimensional simulations; quantitative estimates of nucleosynthetic yields, core masses, and the approach to core collapse will be affected.
Chowdhury, Raju; Parvin, Salma; Khan, Md Abdul Hakim
2016-08-01
The problem of double-diffusive natural convection of Al2O3 -water nanofluid in a porous triangular enclosure in presence of heat generation has been studied numerically in this paper. The bottom wall of the cavity is heated isothermally, the left inclined wall is non-isothermal and the right inclined wall is considered to be cold. The concentration is higher at bottom wall, lower at right inclined wall and non-isoconcentration at left inclined wall of the cavity. The governing equations are transformed to the dimensionless form and solved numerically using Galerkin weighted residual technique of finite element method. The results are obtained in terms of streamlines, isotherms, isoconcentrations, average Nueeslt number (Nu) and average Sherwood number (Sh) for the parameters thermal Rayleigh number (RaT ), dimensionless heat generation parameter (λ), solid volume fraction (ϕ) and Lewis number (Le) while Prandtl number (Pr), Buoyancy ratio (N) and Darcy number (Da) are considered to be fixed. It is observed that flow pattern, temperature fields and concentration fields are affected by the variation of above considered parameters. PMID:27579447
NASA Astrophysics Data System (ADS)
Rezaiguia, Issam; Kadja, Mahfoud; mebrouk, Ridha; Belghar, Noureddine
2013-09-01
This paper discusses the results of a study related to natural convection cooling of a heat source located on the bottom wall of an inclined isosceles triangular enclosure filled with a Cu water-nanofluid. The right and left walls of the enclosure are both maintained cold at constant equal temperatures, while the remaining parts of the bottom wall are insulated. The study has been carried out for a Rayleigh number in the range 104 ≤ Ra ≤ 106, for a heat source length in the range 0.2 ≤ ɛ ≤0.8, for a solid volume fraction in the range 0 ≤ ϕ≤0.06 and for an inclination angle in the range 0° ≤ δ≤45°. Results are presented in the form of streamline contours, isotherms, maximum temperature at the heat source surface and average Nusselt number. It is noticed that the addition of Cu nanoparticles enhances the heat transfer rate and therefore cooling effectiveness for all values of Rayleigh number, especially at low values of Ra. The effect of the inclination angle becomes more noticeable as one increases the value of Ra. For high Rayleigh numbers, a critical value for the inclination angle of δ = 15° is found for which the heat source maximum temperature is highest.
Pinchasik, Bat-El; Möhwald, Helmuth; Skirtach, Andre G
2014-07-01
Bubbles are widely used by animals in nature in order to fulfill important functions. They are used by animals in order to walk underwater or to stabilize themselves at the water/air interface. The main aim of this work is to imitate such phenomena, which is the essence of biomimetics. Here, bubbles are used to propel and to control the location of Janus particles in an aqueous medium. The synthesis of Janus SiO2-Ag and polystyrene-Ag (PS-Ag) particles through embedment in Parafilm is presented. The Janus particles, partially covered with catalytically active Ag nanoparticles, are redispersed in water and placed on a glass substrate. The active Ag sites are used for the splitting of H2O2 into water and oxygen. As a result, an oxygen bubble is formed on one side of the particle and promotes its propulsion. Once formed, the bubble-particle complex is stable and therefore, can be manipulated by tuning hydrophilic-hydrophobic interactions with the surface. In this way a transition between two- and three- dimensional motion is possible by changing the hydrophobicity of the substrate. Similar principles are used in nature.
Form drag in rivers due to small-scale natural topographic features: 1. Regular sequences
Kean, J.W.; Smith, J.D.
2006-01-01
Small-scale topographic features are commonly found on the boundaries of natural rivers, streams, and floodplains. A simple method for determining the form drag on these features is presented, and the results of this model are compared to laboratory measurements. The roughness elements are modeled as Gaussian-shaped features defined in terms of three parameters: a protrusion height, H; a streamwise length scale, ??; and a spacing between crests, ??. This shape is shown to be a good approximation to a wide variety of natural topographic bank features. The form drag on an individual roughness element embedded in a series of identical elements is determined using the drag coefficient of the individual element and a reference velocity that includes the effects of roughness elements further upstream. In addition to calculating the drag on each element, the model determines the spatially averaged total stress, skin friction stress, and roughness height of the boundary. The effects of bank roughness on patterns of velocity and boundary shear stress are determined by combining the form drag model with a channel flow model. The combined model shows that drag on small-scale topographic features substantially alters the near-bank flow field. These methods can be used to improve predictions of flow resistance in rivers and to form the basis for fully predictive (no empirically adjusted parameters) channel flow models. They also provide a foundation for calculating the near-bank boundary shear stress fields necessary for determining rates of sediment transport and lateral erosion.
Form drag in rivers due to small-scale natural topographic features: 1. Regular sequences
NASA Astrophysics Data System (ADS)
Kean, Jason W.; Smith, J. Dungan
2006-12-01
Small-scale topographic features are commonly found on the boundaries of natural rivers, streams, and floodplains. A simple method for determining the form drag on these features is presented, and the results of this model are compared to laboratory measurements. The roughness elements are modeled as Gaussian-shaped features defined in terms of three parameters: a protrusion height, H; a streamwise length scale, σ; and a spacing between crests, λ. This shape is shown to be a good approximation to a wide variety of natural topographic bank features. The form drag on an individual roughness element embedded in a series of identical elements is determined using the drag coefficient of the individual element and a reference velocity that includes the effects of roughness elements further upstream. In addition to calculating the drag on each element, the model determines the spatially averaged total stress, skin friction stress, and roughness height of the boundary. The effects of bank roughness on patterns of velocity and boundary shear stress are determined by combining the form drag model with a channel flow model. The combined model shows that drag on small-scale topographic features substantially alters the near-bank flow field. These methods can be used to improve predictions of flow resistance in rivers and to form the basis for fully predictive (no empirically adjusted parameters) channel flow models. They also provide a foundation for calculating the near-bank boundary shear stress fields necessary for determining rates of sediment transport and lateral erosion.
NASA Astrophysics Data System (ADS)
Ram, P. C.; Nath, R.; Agrawal, A. K.
1984-01-01
The flow of an electrically conducting incompressible rarefied gas due to the combined buoyancy effects of thermal and mass diffusion past an infinite vertical porous plate with constant suction has been studied in the presence of uniform transverse magnetic field. The problem has been solved for velocity, temperature, and concentration fields. It has been observed that mean velocity and the mean temperature are affected by the Grashof numbers G1 and G2, the slip parameter h1, temperature jump coefficient h2, concentration jump coefficient h3 and magnetic field parameter M. The amplitude and the phase of skin-friction and the rate of heat transfer are affected by frequency in addition to the above parameters. They are shown graphically. The numerical values of the mean skin- friction and the mean rate of heat transfer are also tabulated
Medhat, M E
2009-02-01
Different types of Egyptian building materials from various locations in Cairo and its suburbs have been analysed for natural radioactivity using gamma ray spectrometry. Concentrations of (226)Ra, (232)Th and (40)K were in the ranges of (12 +/- 2.8-65 +/- 6.5), (5 +/- 1.8-60 +/- 6.7) and (159 +/- 3.8-920 +/- 12.7 Bq kg(-1)), respectively. The minimum concentration of (226)Ra, (232)Th and (40)K was found in gravel samples, whereas the maximum in granite samples. The results are compared with the published data of other countries and with the world average limits. The radiological hazard parameters: radium equivalent activity, gamma index, alpha index, absorbed dose rate and the annual exposure rate, were determined to assess the radiation hazards associated with Egyptian buildings. All studied samples are lower than world average limits.
Differential susceptibility to colorectal cancer due to naturally occurring gut microbiota
Ericsson, Aaron C.; Akter, Sadia; Hanson, Marina M.; Busi, Susheel B.; Parker, Taybor W.; Schehr, Rebecca J.; Hankins, Miriam A.; Ahner, Carin E.; Davis, Justin W.; Franklin, Craig L.; Amos-Landgraf, James M.; Bryda, Elizabeth C.
2015-01-01
Recent studies investigating the human microbiome have identified particular bacterial species that correlate with the presence of colorectal cancer. To evaluate the role of qualitatively different but naturally occurring gut microbiota and the relationship with colorectal cancer development, genetically identical embryos from the Polyposis in Rat Colon (Pirc) rat model of colorectal cancer were transferred into recipients of three different genetic backgrounds (F344/NHsd, LEW/SsNHsd, and Crl:SD). Tumor development in the pups was tracked longitudinally via colonoscopy, and end-stage tumor burden was determined. To confirm vertical transmission and identify associations between the gut microbiota and disease phenotype, the fecal microbiota was characterized in recipient dams 24 hours pre-partum, and in Pirc rat offspring prior to and during disease progression. Our data show that the gut microbiota varies between rat strains, with LEW/SsNHsd having a greater relative abundance of the bacteria Prevotella copri. The mature gut microbiota of pups resembled the profile of their dams, indicating that the dam is the primary determinant of the developing microbiota. Both male and female F344-Pirc rats harboring the Lewis microbiota had decreased tumor burden relative to genetically identical rats harboring F344 or SD microbiota. Significant negative correlations were detected between tumor burden and the relative abundance of specific taxa from samples taken at weaning and shortly thereafter, prior to observable adenoma development. Notably, this naturally occurring variation in the gut microbiota is associated with a significant difference in severity of colorectal cancer, and the abundance of certain taxa is associated with decreased tumor burden. PMID:26378041
Thorogood, Robert M.
1983-01-01
A convective heater for heating fluids such as a coal slurry is constructed of a tube circuit arrangement which obtains an optimum temperature distribution to give a relatively constant slurry film temperature. The heater is constructed to divide the heating gas flow into two equal paths and the tube circuit for the slurry is arranged to provide a mixed flow configuration whereby the slurry passes through the two heating gas paths in successive co-current, counter-current and co-current flow relative to the heating gas flow. This arrangement permits the utilization of minimum surface area for a given maximum film temperature of the slurry consistent with the prevention of coke formation.
Thorogood, Robert M.
1986-01-01
A convective heater for heating fluids such as a coal slurry is constructed of a tube circuit arrangement which obtains an optimum temperature distribution to give a relatively constant slurry film temperature. The heater is constructed to divide the heating gas flow into two equal paths and the tube circuit for the slurry is arranged to provide a mixed flow configuration whereby the slurry passes through the two heating gas paths in successive co-current, counter-current and co-current flow relative to the heating gas flow. This arrangement permits the utilization of minimum surface area for a given maximum film temperature of the slurry consistent with the prevention of coke formation.
Thorogood, R.M.
1983-12-27
A convective heater for heating fluids such as a coal slurry is constructed of a tube circuit arrangement which obtains an optimum temperature distribution to give a relatively constant slurry film temperature. The heater is constructed to divide the heating gas flow into two equal paths and the tube circuit for the slurry is arranged to provide a mixed flow configuration whereby the slurry passes through the two heating gas paths in successive co-current, counter-current and co-current flow relative to the heating gas flow. This arrangement permits the utilization of minimum surface area for a given maximum film temperature of the slurry consistent with the prevention of coke formation. 14 figs.
Change in climate and nature over Toyama prefecture due to global warming
NASA Astrophysics Data System (ADS)
Hatsushika, H.; Kawasaki, K.; Oritani, T.; Kondo, T.; Mizoguchi, T.; Kido, M.; Tsuchihara, Y.; Wada, N.; Horikawa, K.
2007-12-01
Toyama prefecture is located in the center of the mainland of Japan and is surrounded by steep mountains called Tateyama at about 3,000m above sea level and by the deep Toyama bay at about 1,000m depth. In summer, since Pacific high covers mainland of Japan and East Asian summer monsoon brings a lot of rainfalls, climate in Toyama is suitable for cropping the highest qualified rice and vegetables. In winter, the dominant East Asian winter monsoon brings water from the Japan Sea by heavy snowfalls onto Tateyama. As the snow melts gradually from spring to early autumn, the abundant pure water is utilized for generating hydroelectric power and for a variety of other purposes, making it a vital resource for industries, agriculture, fishery, and human life as well as for wildlife on both sides of the plains and the mountains of Toyama. In recent, by the IPCC-AR4, influence of global warming is reported in many aspects of nature and human lives all over the world. However, we have yet to realize whether these signs are also appeared in Toyama. Therefore, we carried out statistical analyses to investigate change of nature, climate, and human lives in Toyama by global warming. Some of main results are as follows. Using the phenological data of a sample maple (Acer palmatum) tree growing at the garden in Toyama Local Meteorological Observatory, we analyzed that the leaf-color-change date is delayed ca. 20 days and the leaf-falling date is delayed ca. 10 days during recent 30 years. Using daily snowfall data between 1958 and 2007, we found that snow amounts and snowfall days are decreased significantly on the plains, while there is no trend on the mountain side. Using AMeDAS's hourly temperature data between 1978 and 2006, we detected increases in winter time minimum temperature, summer time maximum temperature, and "typical summer days" which is defined as total days that the daily maximum temperature exceeds 30 degree C. It can be inferred from these findings that the
Permeability evolution due to dissolution of natural shale fractures reactivated by fracking
NASA Astrophysics Data System (ADS)
Kwiatkowski, Kamil; Kwiatkowski, Tomasz; Szymczak, Piotr
2015-04-01
Investigation of cores drilled from gas-bearing shale formations reveals a relatively large number of calcite-cemented fractures. During fracking, some of these fractures will be reactivated [1-2] and may become important flow paths in the resulting fracture system. In this communication, we investigate numerically the effect of low-pH reactive fluid on such fractures. The low-pH fluids can either be pumped during the initial fracking stage (as suggested e.g. by Grieser et al., [3]) or injected later, as part of enhanced gas recovery (EGR) processes. In particular, it has been suggested that CO2 injection can be considered as a method of EGR [4], which is attractive as it can potentially be combined with simultaneous CO2 sequestration. However, when mixed with brine, CO2 becomes acidic and thus can be a dissolving agent for the carbonate cement in the fractures. The dissolution of the cement leads to the enhancement of permeability and interconnectivity of the fracture network and, as a result, increases the overall capacity of the reservoir. Importantly, we show that the dissolution of such fractures proceeds in a highly non-homogeneous manner - a positive feedback between fluid transport and mineral dissolution leads to the spontaneous formation of pronounced flow channels, frequently referred to as "wormholes". The wormholes carry the chemically active fluid deeper inside the system, which dramatically speeds up the overall permeability increase. If the low-pH fluids are used during fracking, then the non-uniform dissolution becomes important for retaining the fracture permeability, even in the absence of the proppant. Whereas a uniformly etched fracture will close tightly under the overburden once the fluid pressure is removed, the nonuniform etching will tend to maintain the permeability since the less dissolved regions will act as supports to keep more dissolved regions open. [1] Gale, J. F., Reed, R. M., Holder, J. (2007). Natural fractures in the Barnett
Assessing the indirect effects due to natural hazards on a mesoscale
NASA Astrophysics Data System (ADS)
Pfurtscheller, C.; Schwarze, R.
2009-04-01
Measuring indirect economic costs and other effects from natural hazards, especially floods in alpine and other mountainous regions, are a necessary part of a comprehensive economic assessment. Their omission seriously affects the relative economic benefits of structural or non structural measures of flood defence. Surpassing controversial, IO-model-based economic estimates, analysing indirect economic effects lead to the key question of identifying and evaluating the drivers of indirect economic effects and resilience to system effects in the regional economy, i.e. at the meso-level. This investigation takes place for the catastrophic floods in summer 2005 in the provinces of Tyrol and Vorarlberg, Austria, which caused an estimated € 670 Mio direct loss on private and public assets and severe interruptions in lifeline services. The paper starts out with differentiating the concept of indirect economic costs from direct costs, examing different temporal (short vs. long-term) and spatial (macro-, meso- vs. microeconomic) system boundaries. It surveys common theories of economic resilience and vulnerability at the regional economy level. Indirect effects at the regional economy level can be defined as interferences of the economic exchange of goods and services triggered by breakdowns of transport lines and critical production inputs. The extent and persistence of indirect effects of natural hazards is not only by parameters of the extreme event, such as duration and amplitude of the flood, but much more by resilience parameters of the regional economy such as size of enterprises, the network structure (linkages) of the regional economy, availability of insurance and relief funds, and the stock of inventory. These effects can only be dissected by means of expert judgement and event studies. This paper presents the results of a survey conducted among business practioneers, members of chamber of commerce, civil protection agencies to identify and scale the drivers of
Form drag in rivers due to small-scale natural topographic features: 2. Irregular sequences
Kean, J.W.; Smith, J.D.
2006-01-01
The size, shape, and spacing of small-scale topographic features found on the boundaries of natural streams, rivers, and floodplains can be quite variable. Consequently, a procedure for determining the form drag on irregular sequences of different-sized topographic features is essential for calculating near-boundary flows and sediment transport. A method for carrying out such calculations is developed in this paper. This method builds on the work of Kean and Smith (2006), which describes the flow field for the simpler case of a regular sequence of identical topographic features. Both approaches model topographic features as two-dimensional elements with Gaussian-shaped cross sections defined in terms of three parameters. Field measurements of bank topography are used to show that (1) the magnitude of these shape parameters can vary greatly between adjacent topographic features and (2) the variability of these shape parameters follows a lognormal distribution. Simulations using an irregular set of topographic roughness elements show that the drag on an individual element is primarily controlled by the size and shape of the feature immediately upstream and that the spatial average of the boundary shear stress over a large set of randomly ordered elements is relatively insensitive to the sequence of the elements. In addition, a method to transform the topography of irregular surfaces into an equivalently rough surface of regularly spaced, identical topographic elements also is given. The methods described in this paper can be used to improve predictions of flow resistance in rivers as well as quantify bank roughness.
Formation of flammable clouds due to rupture of a natural gas pipeline
Badr, O.; Elsheikh, H.
1997-07-01
Despite all precaution procedures, accidental release of natural gas from its massive pipeline networks may occur. This paper considers the environmental impact of such releases from flammability view point for two specific scenarios. The high initial pressure of the pipeline resulted in a choked flow. Equilibrium between the pressure of the released gas and the ambient one occurs through a series of interacting expansion and shock waves. Transient mass flow rate, temperature, speed, and cross sectional area of the released jet have been calculated using principles of compressible fluid flow. Time-average values of such parameters have been utilized in the EPA-based Screen software to predict the resulting steady state concentration profiles. Flammable clouds with dimensions up to 3,000 x 26 m in the downwind and upward directions, respectively have been predicted for the first scenario. On the other hand, the results of the second scenario have indicated the formation of flammable clouds extending to 14 m and 160 m in the downwind and upward directions, respectively. Moreover, a parametric study of wind speed and atmospheric stability has shown strong effects on the size of the formed dangerous clouds. Such results have been discussed in relation to the involved mixing processes.
The Behaviour of Naturally Debonded Composites Due to Bending Using a Meso-Level Model
NASA Astrophysics Data System (ADS)
Lord, C. E.; Rongong, J. A.; Hodzic, A.
2012-06-01
Numerical simulations and analytical models are increasingly being sought for the design and behaviour prediction of composite materials. The use of high-performance composite materials is growing in both civilian and defence related applications. With this growth comes the necessity to understand and predict how these new materials will behave under their exposed environments. In this study, the displacement behaviour of naturally debonded composites under out-of-plane bending conditions has been investigated. An analytical approach has been developed to predict the displacement response behaviour. The analytical model supports multi-layered composites with full and partial delaminations. The model can be used to extract bulk effective material properties in which can be represented, later, as an ESL (Equivalent Single Layer). The friction between each of the layers is included in the analytical model and is shown to have distinct behaviour for these types of composites. Acceptable agreement was observed between the model predictions, the ANSYS finite element model, and the experiments.
Form drag in rivers due to small-scale natural topographic features: 2. Irregular sequences
NASA Astrophysics Data System (ADS)
Kean, Jason W.; Smith, J. Dungan
2006-12-01
The size, shape, and spacing of small-scale topographic features found on the boundaries of natural streams, rivers, and floodplains can be quite variable. Consequently, a procedure for determining the form drag on irregular sequences of different-sized topographic features is essential for calculating near-boundary flows and sediment transport. A method for carrying out such calculations is developed in this paper. This method builds on the work of Kean and Smith (2006), which describes the flow field for the simpler case of a regular sequence of identical topographic features. Both approaches model topographic features as two-dimensional elements with Gaussian-shaped cross sections defined in terms of three parameters. Field measurements of bank topography are used to show that (1) the magnitude of these shape parameters can vary greatly between adjacent topographic features and (2) the variability of these shape parameters follows a lognormal distribution. Simulations using an irregular set of topographic roughness elements show that the drag on an individual element is primarily controlled by the size and shape of the feature immediately upstream and that the spatial average of the boundary shear stress over a large set of randomly ordered elements is relatively insensitive to the sequence of the elements. In addition, a method to transform the topography of irregular surfaces into an equivalently rough surface of regularly spaced, identical topographic elements also is given. The methods described in this paper can be used to improve predictions of flow resistance in rivers as well as quantify bank roughness.
NASA Astrophysics Data System (ADS)
Walko, R. L.; Medvigy, D.; Avissar, R.
2013-12-01
Variable-resolution computational grids can substantially improve the benefit-to-cost ratio in many environmental modeling applications, but they can also introduce unwanted and unrealistic numerical anomalies if not properly utilized. For example, we showed in previous studies that resolved (non-parameterized) atmospheric convection develops more quickly as resolution increases. Furthermore, on variable grids that transition from resolved to parameterized convection, timing and intensity of the convection in both regimes is generally disparate unless special care is taken to tune the parameterization. In both cases, the convection that develops first (due to purely numerical reasons) tends to suppress convection elsewhere by inducing subsidence in the surrounding environment. This highly nonlinear competition, while desirable when induced by natural causes such as surface inhomogeneity, is highly undesirable when it is a numerical artifact of variable grid resolution and/or selective application of convective parameterization. Our current research is aimed at leveling the playing field for convection across a variable resolution grid so that the above problems are avoided. The underlying idea is to apply the same or very similar 'convective machinery' to all areas of the grid. For convection-resolving regions of the grid, this machinery is simply the model grid itself, along with explicit representation of dynamics and a bulk microphysics parameterization. For coarser regions of the grid, the local environment is sampled from one or more grid columns (depending on local resolution) and fed to a separate 'convective processor', which determines the convective response to that environment and feeds the result back to the host grid. The convective processor chooses to either (1) explicitly resolve convective activity in the given environment on a separate (independent) limited-area 3D computational grid of comparable resolution to the convection-resolving part of the
NASA Astrophysics Data System (ADS)
Polemio, Maurizio; Lonigro, Teresa
2013-04-01
Recent international researches have underlined the evidences of climate changes throughout the world. Among the consequences of climate change, there is the increase in the frequency and magnitude of natural disasters, such as droughts, windstorms, heat waves, landslides, floods and secondary floods (i.e. rapid accumulation or pounding of surface water with very low flow velocity). The Damaging Hydrogeological Events (DHEs) can be defined as the occurrence of one or more simultaneous aforementioned phenomena causing damages. They represent a serious problem, especially in DHE-prone areas with growing urbanisation. In these areas the increasing frequency of extreme hydrological events could be related to climate variations and/or urban development. The historical analysis of DHEs can support decision making and land-use planning, ultimately reducing natural risks. The paper proposes a methodology, based on both historical and time series approaches, used for describing the influence of climatic variability on the number of phenomena observed. The historical approach is finalised to collect phenomenon historical data. The historical flood and landslide data are important for the comprehension of the evolution of a study area and for the estimation of risk scenarios as a basis for civil protection purposes. Phenomenon historical data is useful for expanding the historical period of investigation in order to assess the occurrence trend of DHEs. The time series approach includes the collection and the statistical analysis of climatic and rainfall data (monthly rainfall, wet days, rainfall intensity, and temperature data together with the annual maximum of short-duration rainfall data, from 1 hour to 5 days), which are also used as a proxy for floods and landslides. The climatic and rainfall data are useful to characterise the climate variations and trends and to roughly assess the effects of these trends on river discharge and on the triggering of landslides. The time
NASA Astrophysics Data System (ADS)
Kline, Shaun W.; Adams, Peter N.; Limber, Patrick W.
2014-08-01
Sea cliff retreat is often linked to large waves, heavy precipitation and seismic events, but the specific operative mechanics have not been well constrained. In particular, what is the role of mechanical abrasion by beach sediments in cliff/platform evolution and how does it relate to the episodic nature of cliff retreat observed at certain locations? Here we present a simple, numerical model of sea cliff retreat that incorporates mechanical abrasion of a basal notch, threshold-controlled failure of the cantilevered block, and a feedback mechanism wherein retreat is dependent on the rate of sediment comminution within the surf zone. Using shore platform and cliff characteristics found in two coastal settings (the central California coast and the English North Sea coast), the model produces retreat rates comparable to those observed via field measurements. The highest retreat rates coincide with the steepest shore platforms and increasing wave height. Steeper platforms promote wave access to the cliff toe and, correspondingly, the receding cliff face produces additional accommodation space for the platform beach, preserving the erosive efficacy of the beach sediments. When exposed to energetic wave forcing, the slope of the inner platform segment controls retreat rates for concave platforms, whereas the slope of the outer platform segment exerts greater control for convex platforms. Platform beaches approached a long-term dynamic equilibrium on the concave profiles, leading to more consistent and steady retreat. Platform beaches were ephemeral on convex profiles, mirroring observed sand wave (Ord) migration on the Holderness coast, UK. These findings agree with previous field observations and support mechanical abrasion as a viable cause of temporal heterogeneity in cliff retreat rate for both coastlines.
Chapelle, Francis H.; Lacombe, Pierre J.; Bradley, Paul M.
2012-01-01
Rates of trichloroethene (TCE) mass transformed by naturally occurring biodegradation processes in a fractured rock aquifer underlying a former Naval Air Warfare Center (NAWC) site in West Trenton, New Jersey, were estimated. The methodology included (1) dividing the site into eight elements of equal size and vertically integrating observed concentrations of two daughter products of TCE biodegradation–cis-dichloroethene (cis-DCE) and chloride–using water chemistry data from a network of 88 observation wells; (2) summing the molar mass of cis-DCE, the first biodegradation product of TCE, to provide a probable underestimate of reductive biodegradation of TCE, (3) summing the molar mass of chloride, the final product of chlorinated ethene degradation, to provide a probable overestimate of overall biodegradation. Finally, lower and higher estimates of aquifer porosities and groundwater residence times were used to estimate a range of overall transformation rates. The highest TCE transformation rates estimated using this procedure for the combined overburden and bedrock aquifers was 945 kg/yr, and the lowest was 37 kg/yr. However, hydrologic considerations suggest that approximately 100 to 500 kg/yr is the probable range for overall TCE transformation rates in this system. Estimated rates of TCE transformation were much higher in shallow overburden sediments (approximately 100 to 500 kg/yr) than in the deeper bedrock aquifer (approximately 20 to 0.15 kg/yr), which reflects the higher porosity and higher contaminant mass present in the overburden. By way of comparison, pump-and-treat operations at the NAWC site are estimated to have removed between 1,073 and 1,565 kg/yr of TCE between 1996 and 2009.
Heat transport in bubbling turbulent convection.
Lakkaraju, Rajaram; Stevens, Richard J A M; Oresta, Paolo; Verzicco, Roberto; Lohse, Detlef; Prosperetti, Andrea
2013-06-01
Boiling is an extremely effective way to promote heat transfer from a hot surface to a liquid due to numerous mechanisms, many of which are not understood in quantitative detail. An important component of the overall process is that the buoyancy of the bubble compounds with that of the liquid to give rise to a much-enhanced natural convection. In this article, we focus specifically on this enhancement and present a numerical study of the resulting two-phase Rayleigh-Bénard convection process in a cylindrical cell with a diameter equal to its height. We make no attempt to model other aspects of the boiling process such as bubble nucleation and detachment. The cell base and top are held at temperatures above and below the boiling point of the liquid, respectively. By keeping this difference constant, we study the effect of the liquid superheat in a Rayleigh number range that, in the absence of boiling, would be between 2 × 10(6) and 5 × 10(9). We find a considerable enhancement of the heat transfer and study its dependence on the number of bubbles, the degree of superheat of the hot cell bottom, and the Rayleigh number. The increased buoyancy provided by the bubbles leads to more energetic hot plumes detaching from the cell bottom, and the strength of the circulation in the cell is significantly increased. Our results are in general agreement with recent experiments on boiling Rayleigh-Bénard convection.
Laboratory evidence of natural remobilization of multicomponent DNAPL pools due to dissolution.
Roy, J W; Smith, J E; Gillham, R W
2004-10-01
, shape. The experimental results were simulated using the model developed by Roy et al. [J. Contam. Hydrol. 2002 (59) 163]. The model matched the observations well, suggesting that it accurately represents the primary mechanisms involved with natural remobilization under the conditions of the study.
McDonnell, Aoibhinn; Schulman, Betsy; Ali, Zahid; Dib-Hajj, Sulayman D; Brock, Fiona; Cobain, Sonia; Mainka, Tina; Vollert, Jan; Tarabar, Sanela; Waxman, Stephen G
2016-04-01
Inherited erythromelalgia, the first human pain syndrome linked to voltage-gated sodium channels, is widely regarded as a genetic model of human pain. Because inherited erythromelalgia was linked to gain-of-function changes of sodium channel Na(v)1.7 only a decade ago, the literature has mainly consisted of reports of genetic and/or clinical characterization of individual patients. This paper describes the pattern of pain, natural history, somatosensory profile, psychosocial status and olfactory testing of 13 subjects with primary inherited erythromelalgia with mutations of SCN9A, the gene encoding Na(v)1.7. Subjects were clinically profiled using questionnaires, quantitative sensory testing and olfaction testing during the in-clinic phase of the study. In addition, a detailed pain phenotype for each subject was obtained over a 3-month period at home using diaries, enabling subjects to self-report pain attacks, potential triggers, duration and severity of pain. All subjects reported pain and heat in the extremities (usually feet and/or hands), with pain attacks triggered by heat or exercise and relieved mainly by non-pharmacological manoeuvres such as cooling. A large proportion of pain attacks (355/1099; 32%) did not involve a specific trigger. There was considerable variability in the number, duration and severity of pain attacks between subjects, even those carrying the same mutation within a family, and within individuals over the 12-13 week observation period. Most subjects (11/13) had pain between attacks. For these subjects, mean pain severity between pain attacks was usually lower than that during an attack. Olfaction testing using the Sniffin'T test did not demonstrate hyperosmia. One subject had evidence of orthostatic hypotension. Overall, there was a statistically significant correlation between total Hospital Anxiety and Depression Scale scores (P= 0.005) and pain between attacks and for Hospital Anxiety and Depression Scale Depression scores and pain
Distribution of natural disturbance due to wave and tidal bed currents around the UK
NASA Astrophysics Data System (ADS)
Bricheno, Lucy M.; Wolf, Judith; Aldridge, John
2015-10-01
The UK continental shelf experiences large tidal ranges and winter storm events, which can both generate strong near-bed currents. The regular tidal bottom currents from tides plus wind driven 'benthic storms' (dominated by wave-driven oscillatory currents in shallow water) are a major source of disturbance to benthic communities, particularly in shallow waters. We aim to identify and map the relative impact of the tides and storm events on the shallower parts of the North West European continental shelf. A 10-year simulation of waves, tides and surges on the continental shelf was performed. The shelf model was validated against current meter observations and the Centre for Environmental, Fisheries and Aquaculture Science (CEFAS) network of SmartBuoys. Next, the model performance was assessed against seabed lander data from two sites in the Southern North Sea; one in deep water and another shallow water site at Sea Palling, and a third in Liverpool Bay. Both waves and currents are well simulated at the offshore Southern North Sea site. A large storm event was also well captured, though the model tends to underpredict bottom orbital velocity. Poorer results were achieved at the Sea Palling site, thought to be due to an overly deep model water depth, and missing wave-current interactions. In Liverpool Bay tides were well modelled and good correlations (average R2=0.89) are observed for significant wave height, with acceptable values (average R2=0.79) for bottom orbital velocity. Using the full 10-year dataset, return periods can be calculated for extreme waves and currents. Mapping these return periods presents a spatial picture of extreme bed disturbance, highlighting the importance of rare wave disturbances (e.g. with a return period of 1 in 10 years). Annual maximum currents change little in their magnitude and distribution from year to year, with mean speeds around 0.04 m s-1, and maximums exceeding 3 m s-1. Wave conditions however are widely variable throughout
Pattern Formation in Convective Instabilities
NASA Astrophysics Data System (ADS)
Friedrich, R.; Bestehorn, M.; Haken, H.
The present article reviews recent progress in the study of pattern formation in convective instabilities. After a brief discussion of the relevant basic hydrodynamic equations as well as a short outline of the mathematical treatment of pattern formation in complex systems the self-organization of spatial and spatio-temporal structures due to convective instabilities is considered. The formation of various forms of convective patterns arising in the Bénard experiment, i.e. in a horizontal fluid layer heated from below, is discussed. Then the review considers pattern formation in the Bénard instability in spherical geometries. In that case it can be demonstrated how the interaction among several convective cells may lead to time dependent as well as chaotic evolution of the spatial structures. Finally, the convective instability in a binary fluid mixture is discussed. In contrast to the instability in a single component fluid the instability may be oscillatory. In that case convection sets in in the form of travelling wave patterns which in addition to a complicated and chaotic temporal behaviour exhibit more or less spatial irregularity already close to threshold.
Isentropic Analysis of Convective Motions
NASA Technical Reports Server (NTRS)
Pauluis, Olivier M.; Mrowiec, Agnieszka A.
2013-01-01
This paper analyzes the convective mass transport by sorting air parcels in terms of their equivalent potential temperature to determine an isentropic streamfunction. By averaging the vertical mass flux at a constant value of the equivalent potential temperature, one can compute an isentropic mass transport that filters out reversible oscillatory motions such as gravity waves. This novel approach emphasizes the fact that the vertical energy and entropy transports by convection are due to the combination of ascending air parcels with high energy and entropy and subsiding air parcels with lower energy and entropy. Such conditional averaging can be extended to other dynamic and thermodynamic variables such as vertical velocity, temperature, or relative humidity to obtain a comprehensive description of convective motions. It is also shown how this approach can be used to determine the mean diabatic tendencies from the three-dimensional dynamic and thermodynamic fields. A two-stream approximation that partitions the isentropic circulation into a mean updraft and a mean downdraft is also introduced. This offers a straightforward way to identify the mean properties of rising and subsiding air parcels. The results from the two-stream approximation are compared with two other definitions of the cloud mass flux. It is argued that the isentropic analysis offers a robust definition of the convective mass transport that is not tainted by the need to arbitrarily distinguish between convection and its environment, and that separates the irreversible convective overturning fromoscillations associated with gravity waves.
Tropical errors and convection
NASA Astrophysics Data System (ADS)
Bechtold, P.; Bauer, P.; Engelen, R. J.
2012-12-01
Tropical convection is analysed in the ECMWF Integrated Forecast System (IFS) through tropical errors and their evolution during the last decade as a function of model resolution and model changes. As the characterization of these errors is particularly difficult over tropical oceans due to sparse in situ upper-air data, more weight compared to the middle latitudes is given in the analysis to the underlying forecast model. Therefore, special attention is paid to available near-surface observations and to comparison with analysis from other Centers. There is a systematic lack of low-level wind convergence in the Inner Tropical Convergence Zone (ITCZ) in the IFS, leading to a spindown of the Hadley cell. Critical areas with strong cross-equatorial flow and large wind errors are the Indian Ocean with large interannual variations in forecast errors, and the East Pacific with persistent systematic errors that have evolved little during the last decade. The analysis quality in the East Pacific is affected by observation errors inherent to the atmospheric motion vector wind product. The model's tropical climate and its variability and teleconnections are also evaluated, with a particular focus on the Madden-Julian Oscillation (MJO) during the Year of Tropical Convection (YOTC). The model is shown to reproduce the observed tropical large-scale wave spectra and teleconnections, but overestimates the precipitation during the South-East Asian summer monsoon. The recent improvements in tropical precipitation, convectively coupled wave and MJO predictability are shown to be strongly related to improvements in the convection parameterization that realistically represents the convection sensitivity to environmental moisture, and the large-scale forcing due to the use of strong entrainment and a variable adjustment time-scale. There is however a remaining slight moistening tendency and low-level wind imbalance in the model that is responsible for the Asian Monsoon bias and for too
Parameterization of precipitating shallow convection
NASA Astrophysics Data System (ADS)
Seifert, Axel
2015-04-01
Shallow convective clouds play a decisive role in many regimes of the atmosphere. They are abundant in the trade wind regions and essential for the radiation budget in the sub-tropics. They are also an integral part of the diurnal cycle of convection over land leading to the formation of deeper modes of convection later on. Errors in the representation of these small and seemingly unimportant clouds can lead to misforecasts in many situations. Especially for high-resolution NWP models at 1-3 km grid spacing which explicitly simulate deeper modes of convection, the parameterization of the sub-grid shallow convection is an important issue. Large-eddy simulations (LES) can provide the data to study shallow convective clouds and their interaction with the boundary layer in great detail. In contrast to observation, simulations provide a complete and consistent dataset, which may not be perfectly realistic due to the necessary simplifications, but nevertheless enables us to study many aspects of those clouds in a self-consistent way. Today's supercomputing capabilities make it possible to use domain sizes that not only span several NWP grid boxes, but also allow for mesoscale self-organization of the cloud field, which is an essential behavior of precipitating shallow convection. By coarse-graining the LES data to the grid of an NWP model, the sub-grid fluctuations caused by shallow convective clouds can be analyzed explicitly. These fluctuations can then be parameterized in terms of a PDF-based closure. The necessary choices for such schemes like the shape of the PDF, the number of predicted moments, etc., will be discussed. For example, it is shown that a universal three-parameter distribution of total water may exist at scales of O(1 km) but not at O(10 km). In a next step the variance budgets of moisture and temperature in the cloud-topped boundary layer are studied. What is the role and magnitude of the microphysical correlation terms in these equations, which
2D Mixed Convection Thermal Incompressible Viscous Flows
NASA Astrophysics Data System (ADS)
Bermudez, Blanca; Nicolas, Alfredo
2005-11-01
Mixed convection thermal incomprressible viscous fluid flows in rectangular cavities are presented. These kind of flows may be governed by the time-dependent Boussinesq approximation in terms of the stream function-vorticity variables formulation. The results are obtained with a simple numerical scheme based mainly on a fixed point iterative process applied to the non-linear system of elliptic equations that is obtained after a second order time discretization. Numerical experiments are reported for the problem of a cavity with fluid boundary motion on the top. Some results correspond to validation examples and others, to the best of our knowledge, correspond to new results. To show that the new results are correct, a mesh size and time independence studies are carried out, and the acceptable errors are measured point-wise. For the optimal mesh size and time step the final times when the steady state is reached, as solution from the unsteady problem, are reported; it should be seen that they are larger than the ones for natural convection which, physically speaking, show the agreement that mixed convection flows are more active than those of natural convection due to the fluid boundary motion on the top of the cavity. The flow parameters are: the Reynolds number, the Grashof number and the aspect ratio.
NASA Astrophysics Data System (ADS)
Carey, Michael Richard
Binary porous convection falls into the larger category of pattern formation---a symmetry breaking instability which creates a spatially periodic structure within a homogeneous system. The experiments and model presented in this dissertation indicate that an essential piece of physics is missing from the standard Darcian picture used to describe pattern formation in a porous medium convection system. Present theory predicts a bifurcation to an oscillatory state at onset for a binary mixture in a porous media over a wide range of experimental parameters (Brand and Steinberg, Physics Letters 93A 333 (1983)). This theory is inadequate in explaining the predominant large amplitude, backward, stationary overturning convection state observed in our experiments after transients have decayed. Convection experiments were visualized with magnetic resonance imaging and performed with a foam medium in slot and cylindrical geometries as well as a rectangular, packed bead system with water-ethanol mixtures. We explore the possibility that the difference between theory and experiment is due to enhanced solutal mixing not included in previous theories. The enhanced mixing of the fluid produces an effective diffusion coefficient that largely suppresses gradients in the concentration field, resulting in single-fluid like behavior. We model the experimental system with a Lorenz truncation of the binary Darcy equations with enhanced mixing. This model predicts results qualitatively similar to experiments: a forward bifurcation to small amplitude oscillations with a secondary backward bifurcation to large amplitude stationary convection. We have also developed an experimental nuclear magnetic resonance technique that measures the effective diffusion coefficient, D = D(v), as a function of velocity, v, for the individual species of the binary mixture simultaneously. However, the mixing effect measured in plug flow experiments is roughly two to three orders of magnitude too small to have
Tropical deep convective cloud morphology
NASA Astrophysics Data System (ADS)
Igel, Matthew R.
A cloud-object partitioning algorithm is developed. It takes contiguous CloudSat cloudy regions and identifies various length scales of deep convective clouds from a tropical, oceanic subset of data. The methodology identifies a level above which anvil characteristics become important by analyzing the cloud object shape. Below this level in what is termed the pedestal region, convective cores are identified based on reflectivity maxima. Identifying these regions allows for the assessment of length scales of the anvil and pedestal of the deep convective clouds. Cloud objects are also appended with certain environmental quantities from the ECMWF reanalysis. Simple geospatial and temporal assessments show that the cloud object technique agrees with standard observations of local frequency of deep-convective cloudiness. Additionally, the nature of cloud volume scale populations is investigated. Deep convection is seen to exhibit power-law scaling. It is suggested that this scaling has implications for the continuous, scale invariant, and random nature of the physics controlling tropical deep convection and therefore on the potentially unphysical nature of contemporary convective parameterizations. Deep-convective clouds over tropical oceans play important roles in Earth's climate system. The response of tropical, deep convective clouds to sea surface temperatures (SSTs) is investigated using this new data set. Several previously proposed feedbacks are examined: the FAT hypothesis, the Iris hypothesis, and the Thermostat hypothesis. When the data are analyzed per cloud object, each hypothesis is broadly found to correctly predict cloud behavior in nature, although it appears that the FAT hypothesis needs a slight modification to allow for cooling cloud top temperatures with increasing SSTs. A new response that shows that the base temperature of deep convective anvils remains approximately constant with increasing SSTs is introduced. These cloud-climate feedbacks are
Microstructural indicators of convection in sills and dykes
NASA Astrophysics Data System (ADS)
Holness, Marian; Neufeld, Jerome; Gilbert, Andrew
2016-04-01
. In mafic sills, the average apparent aspect ratio (AR), as measured in thin-section, varies smoothly with model crystallization times (calculated assuming diffusive heat loss), consistent with in situ growth in solidification fronts. However, AR is invariant across individual mafic dykes, with decreasing values (i.e. more blocky grains) as the dyke width increases. This difference can be accounted for by the plagioclase in dykes growing as individual grains and clusters suspended in a convecting magma. Cooling at a vertical wall, as is the case for dykes, will always result in a gravitational convective instability, and therefore crystal-poor magma in dykes will always convect. As solidification proceeds, the increasing volume fraction of suspended crystals will eventually damp convection: the final stages of solidification occur in static crystal-rich magma, containing a well-mixed grain population. That the Shiant Isles Main Sill exhibits evidence for prolonged convection of sufficient vigour to suspend 5 mm olivine clusters, while other sills of comparable thickness contain plagioclase with grain shapes indicative of growth predominantly in solidification fronts, is most likely due to the composite nature of the Shiant. The 140m unit is underlain by 23m of picrite which intruded shortly before - this heat source would have acted as a strong driver for convection.
A stochastic parameterization for deep convection using cellular automata
NASA Astrophysics Data System (ADS)
Bengtsson, L.; Steinheimer, M.; Bechtold, P.; Geleyn, J.
2012-12-01
Cumulus parameterizations used in most operational weather and climate models today are based on the mass-flux concept which took form in the early 1970's. In such schemes it is assumed that a unique relationship exists between the ensemble-average of the sub-grid convection, and the instantaneous state of the atmosphere in a vertical grid box column. However, such a relationship is unlikely to be described by a simple deterministic function (Palmer, 2011). Thus, because of the statistical nature of the parameterization challenge, it has been recognized by the community that it is important to introduce stochastic elements to the parameterizations (for instance: Plant and Craig, 2008, Khouider et al. 2010, Frenkel et al. 2011, Bentsson et al. 2011, but the list is far from exhaustive). There are undoubtedly many ways in which stochastisity can enter new developments. In this study we use a two-way interacting cellular automata (CA), as its intrinsic nature possesses many qualities interesting for deep convection parameterization. In the one-dimensional entraining plume approach, there is no parameterization of horizontal transport of heat, moisture or momentum due to cumulus convection. In reality, mass transport due to gravity waves that propagate in the horizontal can trigger new convection, important for the organization of deep convection (Huang, 1988). The self-organizational characteristics of the CA allows for lateral communication between adjacent NWP model grid-boxes, and temporal memory. Thus the CA scheme used in this study contain three interesting components for representation of cumulus convection, which are not present in the traditional one-dimensional bulk entraining plume method: horizontal communication, memory and stochastisity. The scheme is implemented in the high resolution regional NWP model ALARO, and simulations show enhanced organization of convective activity along squall-lines. Probabilistic evaluation demonstrate an enhanced spread in
ARM - Midlatitude Continental Convective Clouds
Jensen, Mike; Bartholomew, Mary Jane; Genio, Anthony Del; Giangrande, Scott; Kollias, Pavlos
2012-01-19
Convective processes play a critical role in the Earth's energy balance through the redistribution of heat and moisture in the atmosphere and their link to the hydrological cycle. Accurate representation of convective processes in numerical models is vital towards improving current and future simulations of Earths climate system. Despite improvements in computing power, current operational weather and global climate models are unable to resolve the natural temporal and spatial scales important to convective processes and therefore must turn to parameterization schemes to represent these processes. In turn, parameterization schemes in cloud-resolving models need to be evaluated for their generality and application to a variety of atmospheric conditions. Data from field campaigns with appropriate forcing descriptors have been traditionally used by modelers for evaluating and improving parameterization schemes.
NASA Astrophysics Data System (ADS)
Tuan, L.; Abd Razak, A.; Zaki, S. A.; Mohammad, A. F.; Hassan, M. K.
2015-09-01
Similar to most tropical countries, Malaysia have low wind speed and airflow characteristics to provide an effective natural ventilation system for comfortable living especially in terrace houses. Even so, by designing them with suitable threshold height/width, H/W, ratio may help reduce heat sink, or even the accumulation of contaminants within the setback distance. Through this study, the downstream building of these terrace houses will be investigated due to the effects from an upstream building. With the use of Large-Eddy Simulation (LES) method, the formation of the vortex within the threshold H/W ratio will be clearly simulated and allow the observation of flow regimes developed by each model. With increasing threshold H/W ratios the models will exhibit some wake interference flow and skimming flow which will determine the negative or positive effect of ventilation from the upstream building towards the downstream building. The airflow characteristics of the downstream house will also be analysed and the most effective layout in providing a better air circulation may be determined. Improving the natural ventilation of such houses could significantly reduce these negative effects such as the accumulation of dust, smoke or bacteria. In turn, with the alarming rate of depletion in natural resources and its effects to the environment, this study can significantly reduce energy usage for ventilation and space cooling.
Al-Jundi, J; Ulanovsky, A; Pröhl, G
2009-10-01
The use of building materials containing naturally occurring radionuclides as (40)K, (232)Th, and (238)U and their progeny results in external exposures of the residents of such buildings. In the present study, indoor dose rates for a typical Jordan concrete room are calculated using Monte Carlo method. Uniform chemical composition of the walls, floor and ceiling as well as uniform mass concentrations of the radionuclides in walls, floor and ceiling are assumed. Using activity concentrations of natural radionuclides typical for the Jordan houses and assuming them to be in secular equilibrium with their progeny, the maximum annual effective doses are estimated to be 0.16, 0.12 and 0.22 mSv a(-1) for (40)K, (232)Th- and (238)U-series, respectively. In a total, the maximum annual effective indoor dose due to external gamma-radiation is 0.50 mSv a(-1). Additionally, organ dose coefficients are calculated for all organs considered in ICRP Publication 74. Breast, skin and eye lenses have the maximum equivalent dose rate values due to indoor exposures caused by the natural radionuclides, while equivalent dose rates for uterus, colon (LLI) and small intestine are found to be the smallest. More specifically, organ dose rates (nSv a(-1)per Bq kg(-1)) vary from 0.044 to 0.060 for (40)K, from 0.44 to 0.60 for radionuclides from (238)U-series and from 0.60 to 0.81 for radionuclides from (232)Th-series. The obtained organ and effective dose conversion coefficients can be conveniently used in practical dose assessment tasks for the rooms of similar geometry and varying activity concentrations and local-specific occupancy factors. PMID:19628312
The potential for free and mixed convection in sedimentary basins
Raffensperger, J.P.; Vlassopoulos, D.
1999-01-01
Free thermal convection and mixed convection are considered as potential mechanisms for mass and heat transport in sedimentary basins. Mixed convection occurs when horizontal flows (forced convection) are superimposed on thermally driven flows. In cross section, mixed convection is characterized by convection cells that migrate laterally in the direction of forced convective flow. Two-dimensional finite-element simulations of variable-density groundwater flow and heat transport in a horizontal porous layer were performed to determine critical mean Rayleigh numbers for the onset of free convection, using both isothermal and semi-conductive boundaries. Additional simulations imposed a varying lateral fluid flux on the free-convection pattern. Results from these experiments indicate that forced convection becomes dominant, completely eliminating buoyancy-driven circulation, when the total forced-convection fluid flux exceeds the total flux possible due to free convection. Calculations of the thermal rock alteration index (RAI=q????T) delineate the patterns of potential diagenesis produced by fluid movement through temperature gradients. Free convection produces a distinct pattern of alternating positive and negative RAIs, whereas mixed convection produces a simpler layering of positive and negative values and in general less diagenetic alteration. ?? Springer-Verlag.
Influence of convection on microstructure
NASA Technical Reports Server (NTRS)
Wilcox, William R.; Eisa, Gaber Faheem; Chandrasekhar, S.; Larrousse, Mark; Banan, Mohsen
1988-01-01
The influence was studied of convection during directional solidification on the resulting microstructure of eutectics, specifically lead/tin and manganese/bismuth. A theory was developed for the influence of convection on the microstructure of lamellar and fibrous eutectics, through the effect of convection on the concentration field in the melt in front of the growing eutectic. While the theory agrees with the experimental spin-up spin-down results, it predicts that the weak convection expected due to buoyancy will not produce a measurable change in eutectic microstructure. Thus, this theory does not explain the two fold decrease in MnBi fiber size and spacing observed when MnBi-Bi is solidified in space or on Earth with a magnetic field applied. Attention was turned to the morphology of the MnBi-Bi interface and to the generation of freezing rate fluctuations by convection. Decanting the melt during solidification of MnBi-Bi eutectic showed that the MnBi phase projects into the melt ahead of the Bi matrix. Temperature measurements in a Bi melt in the vertical Bridgman-Stockbarger configuration showed temperature variations of up to 25 C. Conclusions are drawn and discussed.
Marangoni Convection and Deviations from Maxwells' Evaporation Model
NASA Technical Reports Server (NTRS)
Segre, P. N.; Snell, E. H.; Adamek, D. H.
2003-01-01
We investigate the convective dynamics of evaporating pools of volatile liquids using an ultra-sensitive thermal imaging camera. During evaporation, there are significant convective flows inside the liquid due to Marangoni forces. We find that Marangoni convection during evaporation can dramatically affect the evaporation rates of volatile liquids. A simple heat balance model connects the convective velocities and temperature gradients to the evaporation rates.
Tzanos, C. P.
2007-05-16
The Very High Temperature gas cooled reactor (VHTR) is one of the GEN IV reactor concepts that have been proposed for thermochemical hydrogen production and other process-heat applications like coal gasification. The United States Department of Energy has selected the VHTR for further research and development, aiming to demonstrate emissions-free electricity and hydrogen production at a future time. One of the major safety advantages of the VHTR is the potential for passive decay heat removal by natural circulation of air in a Reactor Cavity Cooling System (RCCS). The air-side of the RCCS is very similar to the Reactor Vessel Auxiliary Cooling System (RVACS) that has been proposed for the PRISM reactor design. The design and safety analysis of the RVACS have been based on extensive analytical and experimental work performed at ANL. The Natural Convection Shutdown Heat Removal Test Facility (NSTF) at ANL that simulates at full scale the air-side of the RVACS was built to provide experimental support for the design and analysis of the PRISM RVACS system. The objective of this work is to demonstrate that the NSTF facility can be used to generate RCCS experimental data: to validate CFD and systems codes for the analysis of the RCCS; and to support the design and safety analysis of the RCCS. At this time no reference design is available for the NGNP. The General Atomics (GA) gas turbine - modular helium reactor (GT-MHR) has been used in many analyses as a starting reference design. In the GT-MHR the reactor outlet temperature is 850 C, while the target outlet reactor temperature in VHTR is 1000 C. VHTR scoping studies with a reactor outlet temperature of 1000 C have been performed at GA and INEL. Although the reactor outlet temperature in the VHTR is significantly higher than in the GT-MHR, the peak temperature in the reactor vessel (which is the heat source for the RCCS) is not drastically different. In this work, analyses have been performed using reactor vessel
Krstic, D; Nikezic, D
2009-10-01
In this paper the effective dose in the age-dependent ORNL phantoms series, due to naturally occurring radionuclides in building materials, was calculated. The absorbed doses for various organs or human tissues have been calculated. The MCNP-4B computer code was used for this purpose. The effective dose was calculated according to ICRP Publication 74. The obtained values of dose conversion factors for a standard room are: 1.033, 0.752 and 0.0538 nSv h-1 per Bq kg-1 for elements of the U and Th decay series and for the K isotope, respectively. The values of effective dose agreed generally with those found in the literature, although the values estimated here for elements of the U series were higher in some cases.
Krstic, D; Nikezic, D
2009-10-01
In this paper the effective dose in the age-dependent ORNL phantoms series, due to naturally occurring radionuclides in building materials, was calculated. The absorbed doses for various organs or human tissues have been calculated. The MCNP-4B computer code was used for this purpose. The effective dose was calculated according to ICRP Publication 74. The obtained values of dose conversion factors for a standard room are: 1.033, 0.752 and 0.0538 nSv h-1 per Bq kg-1 for elements of the U and Th decay series and for the K isotope, respectively. The values of effective dose agreed generally with those found in the literature, although the values estimated here for elements of the U series were higher in some cases. PMID:19741358
Steady thermal convection in multiple liquid layers
NASA Astrophysics Data System (ADS)
Prakash, Ajay
1993-03-01
Convective flow in multiple liquid layers confined in a rectangular cavity is investigated using analytical, numerical, and experimental techniques. The cavity is subjected to differential heating, either parallel to or perpendicular to the interfaces between liquid layers. Thermal convection in the liquid layers results from buoyancy and from temperature induced changes in interfacial tension. Since the genesis of buoyancy is gravity, buoyancy effects are significantly reduced in a low-gravity environment. Definition of a space flight experiment aboard the upcoming IML-2 mission along with validation of fluid dynamical models with ground based experimentation are the objectives of this investigation. Flow in shallow cavities subjected to differential heating parallel to the interfaces is analytically investigated using the method of matched asymptotic expansions. Natural convection, without the influence of thermocapillary forces, is investigated in two and three layer systems. In low-gravity environments, thermocapillary convection with deformable interfaces is also studied. Ground based experiments to visualize the flow field are conducted. Particle streak line photography is used to visualize the flow. Particle displacement tracking is used to evaluate the velocity vector field, and holographic interferometry is used to visualize the temperature field. Numerical simulation is performed with the computer code FIDAP. Convection due to differential heating perpendicular to the interfaces is investigated using a linear stability analysis. Two and three layers of infinite horizontal extent are considered. Ground based experiments are conducted to visualize the temperature field in two and three layer systems confined in a box. Fluid dynamical models relying on mechanical coupling are experimentally validated for certain fluid combinations, while for other fluid combinations significant disparity is observed. An immobile interface is observed in the experiments
NASA Astrophysics Data System (ADS)
Guzman Galindo, T. D.; Plata Rocha, W. D.; Aguilar-Villegas, J. M.
2013-05-01
The imbalance of nature in recent years has been highlighted throughout the world due to the consequences of population and economic growth and changes land use in general. These changes are the result of complex processes between the human and natural environment. This is a very important phenomenon, especially from the point of view of sustainability, as these changes have been considered as one of the most important components of global change (Plata et al., 2009). In the same way the process of deforestation and forest degradation as a result of human activities are a major source of emissions of greenhouse gases in Mexico (Masera et al., 1997). However, forests in Mexico have great potential to become carbon sinks by adopting appropriate support policies, and implementation of sustainable forestry management techniques to improve their production. From this perspective, forest management and reforestation of forests are presented as options for short and medium term climate change mitigation (Sheinbaum and Masera, 2000). Based on the foregoing, the research updates emissions from the Land-Cover and Land-Use Change (LCLUC) for the period 2000 to 2005 for the State of Sinaloa, Mexico, from activity data and national emission factors, reliable and updated to improve certainty and to determine the emissions of greenhouse gases for the sector. This paper examines the updated statewide LCLUC inventory using the gradation level 2 of the IPCC and recommends climate change mitigation and adaptation strategies.t;
How stratified is mantle convection?
NASA Astrophysics Data System (ADS)
Puster, Peter; Jordan, Thomas H.
1997-04-01
We quantify the flow stratification in the Earth's mid-mantle (600-1500 km) in terms of a stratification index for the vertical mass flux, Sƒ (z) = 1 - ƒ(z) / ƒref (z), in which the reference value ƒref(z) approximates the local flux at depth z expected for unstratified convection (Sƒ=0). Although this flux stratification index cannot be directly constrained by observations, we show from a series of two-dimensional convection simulations that its value can be related to a thermal stratification index ST(Z) defined in terms of the radial correlation length of the temperature-perturbation field δT(z, Ω). ST is a good proxy for Sƒ at low stratifications (Sƒ<0.2), where it rises with stratification strength much more rapidly than Sƒ. Assuming that the shear-speed variations δβ(z, Ω) imaged by seismic tomography are primarily due to convective temperature fluctuations, we can approximate ST by Sβ, the analogous index for the radial correlation length of δβ, and thereby construct bounds on Sƒ. We discuss several key issues regarding the implementation of this strategy, including finite resolution of the seismic data, biases due to the parameterization of the tomographic models, and the bias and variance due to noise. From the comparison of the numerical simulations with recent tomographic structures, we conclude that it is unlikely that convection in the Earth's mantle has Sƒ≳0.15. We consider the possibility that this estimate is biased because mantle convection is intermittent and therefore that the present-day tomographic snapshot may differ from its time average. Although this possibility cannot be dismissed completely, we argue that values of Sƒ≳0.2 can be discounted under a weak version of the Uniformitarian Principle. The bound obtained here from global tomography is consistent with local seismological evidence for slab flux into the lower mantle; however, the total material flux has to be significantly greater (by a factor of 2-3) than that
NASA Astrophysics Data System (ADS)
Berthold, Susann
2013-04-01
exceed the velocity of the horizontal natural groundwater (through) flow. Density-driven convection causes anomalies and patterns in temperature and water quality logs. These include e.g., stratification and staircase structures or oscillations caused by convection cells. Overturning thermal, solutale or thermosolutale convection leads to spatially and temporarily oscillations in the otherwise smooth temperature log. The affected water column acts as a mixing reactor. Due to the high velocity of the revolving convection, this mixing is very effective. The so-called double-diffusion is accompanied by the formation of typical step structures in temperature logs which originate from the change from convective to well mixed layers with intervening diffusive transitional layers. The layers of convective transport are characterized by relatively steady values and the layers by diffusive transport are characterized by jump-like changes in temperature, forming the staircase structures. These anomalies and patterns can be qualitatively evaluated (e.g., by the Synthetic Convection Log) to assess falsifying effects on temperature logs due to mixture, heat and mass transport.
NASA Astrophysics Data System (ADS)
Venturi, Daniele
2005-11-01
Stochastic bifurcations and stability of natural convective flows in 2d and 3d enclosures are investigated by the multi-element generalized polynomial chaos (ME-gPC) method (Xiu and Karniadakis, SISC, vol. 24, 2002). The Boussinesq approximation for the variation of physical properties is assumed. The stability analysis is first carried out in a deterministic sense, to determine steady state solutions and primary and secondary bifurcations. Stochastic simulations are then conducted around discontinuities and transitional regimes. It is found that these highly non-linear phenomena can be efficiently captured by the ME-gPC method. Finally, the main findings of the stochastic analysis and their implications for heat transfer will be discussed.
Regional Variability in Convection and Rain Retrievals from the TRMM Microwave Imager (TMI)
NASA Technical Reports Server (NTRS)
Prabhakara, C.; Iacovazzi, R., Jr.
2003-01-01
Precipitation Radar (PR) on board the TRMM satellite shows that the average height of 30 dBz in convective rain areas of the tropics varies significantly from one region to the other. When the convection is weak this height is shallow and when convection is strong this height extends deeper into the troposphere. The brightness temperature (Tb) measured by the microwave radiometer by itself does not reflect this nature of convection satisfactorily. Radiative transfer simulations of Tbs reveal that this could be due to the variations in the vertical distribution of optically active water and ice hydrometeors and their density, shape, and size. These variations are not coupled uniquely to the strength of the convective updrafts, and as a result the Tbs do not reflect properly the convective strength indicated by PR. Because of this deficiency in the Tbs the rain rate deduced from them differs from that of PR. For this reason, to improve the estimation of rain rate we have developed an empirical method. In this method a parameter based on the areal extent of the Tbs that exceed a certain magnitude is included along with the Tbs. Rain rate deduced with this approach is better correlated with that of PR when compared to the current Version 5 operational algorithm. The percentage of rain volume as a function of rain rate, for a given region of 5deg lat. X 5deg long. over a period of three months, deduced from this method, is also in better agreement with that of the PR.
NASA Astrophysics Data System (ADS)
Nee, Alexander
2016-02-01
Mathematical modeling of conjugate natural convection in a closed rectangular cavity with a radiant energy source in conditions of convective-radiative heat exchange at the external boundary was conducted. The radiant energy distribution was set by the Lambert's law. Conduction and convection processes analysis showed that the air masses flow pattern is modified slightly over the time. The temperature increases in the gas cavity, despite the heat removal from the one of the external boundary. According to the results of the integral heat transfer analysis were established that the average Nusselt number (Nuav) increasing occurs up to τ = 200 (dimensionless time). Further Nuav has changed insignificantly due to the temperature field equalization near the interfaces "gas - wall".
Studies of Forced-Convection Heat Transfer Augmentation in Large Containment Enclosures
Kuhn, S.Z.; Peterson, P.F.
2001-06-17
Heat transfer enhancement due to jet mixing inside a cylindrical enclosure is discussed. This work addresses conservative heat transfer assumptions regarding mixing and condensation that have typically been incorporated into passive containment design analyses. This research presents the possibility for increasing decay heat removal of passive containment systems under combined natural and forced convection. Eliminating these conservative assumptions could result in a changed containment design and reduce the construction cost. It is found that the ratio of forced- and free-convection Nusselt numbers can be predicted as a function of the Archimedes number and a correlated factor accounting for jet orientation and enclosure geometry.
Using Jupiter's gravitational field to probe the Jovian convective dynamo.
Kong, Dali; Zhang, Keke; Schubert, Gerald
2016-03-23
Convective motion in the deep metallic hydrogen region of Jupiter is believed to generate its magnetic field, the strongest in the solar system. The amplitude, structure and depth of the convective motion are unknown. A promising way of probing the Jovian convective dynamo is to measure its effect on the external gravitational field, a task to be soon undertaken by the Juno spacecraft. We calculate the gravitational signature of non-axisymmetric convective motion in the Jovian metallic hydrogen region and show that with sufficiently accurate measurements it can reveal the nature of the deep convection.
Using Jupiter's gravitational field to probe the Jovian convective dynamo.
Kong, Dali; Zhang, Keke; Schubert, Gerald
2016-01-01
Convective motion in the deep metallic hydrogen region of Jupiter is believed to generate its magnetic field, the strongest in the solar system. The amplitude, structure and depth of the convective motion are unknown. A promising way of probing the Jovian convective dynamo is to measure its effect on the external gravitational field, a task to be soon undertaken by the Juno spacecraft. We calculate the gravitational signature of non-axisymmetric convective motion in the Jovian metallic hydrogen region and show that with sufficiently accurate measurements it can reveal the nature of the deep convection. PMID:27005472
Effects of convection and solid wall on the diffusion in microscale convection flows
NASA Astrophysics Data System (ADS)
Zhang, Jun; Fan, Jing; Fei, Fei
2010-12-01
The diffusive transport properties in microscale convection flows are studied by using the direct simulation Monte Carlo method. The effective diffusion coefficient D∗ is computed from the mean square displacements of simulated molecules based on the Einstein diffusion equation D∗=⟨Δx2(t )⟩/2t. Two typical convection flows, namely, thermal creep convection and Rayleigh-Bénard convection, are investigated. The thermal creep convection in our simulation is in the noncontinuum regime, with the characteristic scale of the vortex varying from 1 to 100 molecular mean free paths. The diffusion is shown to be enhanced only when the vortex scale exceeds a certain critical value, while the diffusion is reduced when the vortex scale is less than the critical value. The reason for phenomenon of diffusion reduction in the noncontinuum regime is that the reduction effect due to solid wall is dominant while the enhancement effect due to convection is negligible. A molecule will lose its memory of macroscopic velocity when it collides with the walls, and thus molecules are hard to diffuse away if they are confined between very close walls. The Rayleigh-Bénard convection in our simulation is in the continuum regime, with the characteristic length of 1000 molecular mean free paths. Under such condition, the effect of solid wall on diffusion is negligible. The diffusion enhancement due to convection is shown to scale as the square root of the Péclet number in the steady convection regime, which is in agreement with previous theoretical and experimental results. In the oscillation convection regime, the diffusion is more strongly enhanced because the molecules can easily advect from one roll to its neighbor due to an oscillation mechanism.
Boundary Layer Control of Rotating Convection Systems
NASA Astrophysics Data System (ADS)
King, E. M.; Stellmach, S.; Noir, J.; Hansen, U.; Aurnou, J. M.
2008-12-01
Rotating convection is ubiquitous in the natural universe, and is likely responsible for planetary processes such magnetic field generation. Rapidly rotating convection is typically organized by the Coriolis force into tall, thin, coherent convection columns which are aligned with the axis of rotation. This organizational effect of rotation is thought to be responsible for the strength and structure of magnetic fields generated by convecting planetary interiors. As thermal forcing is increased, the relative influence of rotation weakens, and fully three-dimensional convection can exist. It has long been assumed that rotational effects will dominate convection dynamics when the ratio of buoyancy to the Coriolis force, the convective Rossby number, Roc, is less than unity. We investigate the influence of rotation on turbulent Rayleigh-Benard convection via a suite of coupled laboratory and numerical experiments over a broad parameter range: Rayleigh number, 10310; Ekman number, 10-6≤ E ≤ ∞; and Prandtl number, 1≤ Pr ≤ 100. In particular, we measure heat transfer (as characterized by the Nusselt number, Nu) as a function of the Rayleigh number for several different Ekman and Prandtl numbers. Two distinct heat transfer scaling regimes are identified: non-rotating style heat transfer, Nu ~ Ra2/7, and quasigeostrophic style heat transfer, Nu~ Ra6/5. The transition between the non-rotating regime and the rotationally dominant regime is described as a function of the Ekman number, E. We show that the regime transition depends not on the global force balance Roc, but on the relative thicknesses of the thermal and Ekman boundary layers. The transition scaling provides a predictive criterion for the applicability of convection models to natural systems such as Earth's core.
Vigorous convection in a sunspot granular light bridge
NASA Astrophysics Data System (ADS)
Lagg, Andreas; Solanki, Sami K.; van Noort, Michiel; Danilovic, Sanja
2014-08-01
Context. Light bridges are the most prominent manifestation of convection in sunspots. The brightest representatives are granular light bridges composed of features that appear to be similar to granules. Aims: An in-depth study of the convective motions, temperature stratification, and magnetic field vector in and around light bridge granules is presented with the aim of identifying similarities and differences to typical quiet-Sun granules. Methods: Spectropolarimetric data from the Hinode Solar Optical Telescope were analyzed using a spatially coupled inversion technique to retrieve the stratified atmospheric parameters of light bridge and quiet-Sun granules. Results: Central hot upflows surrounded by cooler fast downflows reaching 10 km s-1 clearly establish the convective nature of the light bridge granules. The inner part of these granules in the near surface layers is field free and is covered by a cusp-like magnetic field configuration. We observe hints of field reversals at the location of the fast downflows. The quiet-Sun granules in the vicinity of the sunspot are covered by a low-lying canopy field extending radially outward from the spot. Conclusions: The similarities between quiet-Sun and light bridge granules point to the deep anchoring of granular light bridges in the underlying convection zone. The fast, supersonic downflows are most likely a result of a combination of invigorated convection in the light bridge granule due to radiative cooling into the neighboring umbra and the fact that we sample deeper layers, since the downflows are immediately adjacent to the slanted walls of the Wilson depression. The two movies are available in electronic form at http://www.aanda.org
Turbulent convective flows in the solar photospheric plasma
NASA Astrophysics Data System (ADS)
Caroli, A.; Giannattasio, F.; Fanfoni, M.; Del Moro, D.; Consolini, G.; Berrilli, F.
2015-10-01
> The origin of the 22-year solar magnetic cycle lies below the photosphere where multiscale plasma motions, due to turbulent convection, produce magnetic fields. The most powerful intensity and velocity signals are associated with convection cells, called granules, with a scale of typically 1 Mm and a lifetime of a few minutes. Small-scale magnetic elements (SMEs), ubiquitous on the solar photosphere, are passively transported by associated plasma flows. This advection makes their traces very suitable for defining the convective regime of the photosphere. Therefore the solar photosphere offers an exceptional opportunity to investigate convective motions, associated with compressible, stratified, magnetic, rotating and large Rayleigh number stellar plasmas. The magnetograms used here come from a Hinode/SOT uninterrupted 25-hour sequence of spectropolarimetric images. The mean-square displacement of SMEs has been modelled with a power law with spectral index . We found for times up to and for times up to . An alternative way to investigate the advective-diffusive motion of SMEs is to look at the evolution of the two-dimensional probability distribution function (PDF) for the displacements. Although at very short time scales the PDFs are affected by pixel resolution, for times shorter than the PDFs seem to broaden symmetrically with time. In contrast, at longer times a multi-peaked feature of the PDFs emerges, which suggests the non-trivial nature of the diffusion-advection process of magnetic elements. A Voronoi distribution analysis shows that the observed small-scale distribution of SMEs involves the complex details of highly nonlinear small-scale interactions of turbulent convective flows detected in solar photospheric plasma.
Latent Heating Processes within Tropical Deep Convection
NASA Astrophysics Data System (ADS)
van den Heever, S. C.; Mcgee, C. J.
2013-12-01
It has been suggested that latent heating above the freezing level plays an important role in reconciling Riehl and Malkus' Hot Tower Hypothesis (HTH) with observational evidence of diluted tropical deep convective cores. In this study, recent modifications to the HTH have been evaluated through the use of Lagrangian trajectory analysis of deep convective cores simulated using the Regional Atmospheric Modeling System (RAMS), a cloud-resolving model (CRM) with sophisticated microphysical, surface and radiation parameterization schemes. Idealized, high-resolution simulations of a line of tropical convective cells have been conducted. A two-moment microphysical scheme was utilized, and the initial and lateral boundary grid conditions were obtained from a large-domain CRM simulation approaching radiative convective equilibrium. As the tropics are never too far from radiative convective equilibrium, such a framework is useful for investigating the relationships between radiation, thermodynamics and microphysics in tropical convection. Microphysical impacts on latent heating and equivalent potential temperature (θe) have been analyzed along trajectories ascending within convective regions. Changes in θe along backward trajectories are partitioned into contributions from latent heating due to ice processes and a residual term that is shown to be an approximate representation of mixing. It is apparent from the CRM simulations that mixing with dry environmental air decreases θe along ascending trajectories below the freezing level, while latent heating due to freezing and vapor deposition increase θe above the freezing level. The along-trajectory contributions to latent heating from cloud nucleation, condensation, evaporation, freezing, deposition, and sublimation have also been quantified. Finally, the source regions of trajectories reaching the upper troposphere have been identified. The analysis indicates that while much of the air ascending within convective
Magnetic Control of Solutal Buoyancy Driven Convection
NASA Technical Reports Server (NTRS)
Ramachandran, N.; Leslie, F. W.
2003-01-01
Volumetric forces resulting from local density variations and gravitational acceleration cause buoyancy induced convective motion in melts and solutions. Solutal buoyancy is a result of concentration differences in an otherwise isothermal fluid. If the fluid also exhibits variations in magnetic susceptibility with concentration then convection control by external magnetic fields can be hypothesized. Magnetic control of thermal buoyancy induced convection in ferrofluids (dispersions of ferromagnetic particles in a carrier fluid) and paramagnetic fluids have been demonstrated. Here we show the nature of magnetic control of solutal buoyancy driven convection of a paramagnetic fluid, an aqueous solution of Manganese Chloride hydrate. We predict the critical magnetic field required for balancing gravitational solutal buoyancy driven convection and validate it through a simple experiment. We demonstrate that gravity driven flow can be completely reversed by a magnetic field but the exact cancellation of the flow is not possible. This is because the phenomenon is unstable. The technique can be applied to crystal growth processes in order to reduce convection and to heat exchanger devices for enhancing convection. The method can also be applied to impose a desired g-level in reduced gravity applications.
Near-surface physics during convection affecting air-water gas transfer
NASA Astrophysics Data System (ADS)
Fredriksson, S. T.; Arneborg, L.; Nilsson, H.; Handler, R. A.
2016-05-01
The gas flux at the water surface is affected by physical processes including turbulence from wind shear, microscale wave breaking, large-scale breaking, and convection due to heat loss at the surface. The main route in the parameterizations of the gas flux has been to use the wind speed as a proxy for the gas flux velocity, indirectly taking into account the dependency of the wind shear and the wave processes. The interest in the contributions from convection processes has increased as the gas flux from inland waters (with typically lower wind and sheltered conditions) now is believed to play a substantial role in the air-water gas flux budget. The gas flux is enhanced by convection through the mixing of the mixed layer as well as by decreasing the diffusive boundary layer thickness. The direct numerical simulations performed in this study are shown to be a valuable tool to enhance the understanding of this flow configuration often present in nature.
Cloud formation, convection, and stratospheric dehydration
NASA Astrophysics Data System (ADS)
Schoeberl, Mark R.; Dessler, Andrew E.; Wang, Tao; Avery, Melody A.; Jensen, Eric J.
2014-12-01
Using the Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis winds, temperatures, and anvil cloud ice, we use our domain-filling, forward trajectory model combined with a new cloud module to show that convective transport of saturated air and ice to altitudes below the tropopause has a significant impact on stratospheric water vapor and upper tropospheric clouds. We find that including cloud microphysical processes (rather than assuming that parcel water vapor never exceeds saturation) increases the lower stratospheric average H2O by 10-20%. Our model-computed cloud fraction shows reasonably good agreement with tropical upper troposphere (TUT) cloud frequency observed by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument in boreal winter with poorer agreement in summer. Our results suggest that over 40% of TUT cirrus is due to convection, and it is the saturated air from convection rather than injected cloud ice that primarily contributes to this increase. Convection can add up to 13% more water to the stratosphere. With just convective hydration (convection adds vapor up to saturation), the global lower stratospheric modeled water vapor is close to Microwave Limb Sounder observations. Adding convectively injected ice increases the modeled water vapor to ~8% over observations. Improving the representation of MERRA tropopause temperatures fields reduces stratospheric water vapor by ~4%.
Transition in Internally Heated Convection
NASA Astrophysics Data System (ADS)
Tasaka, Yuji; Yanagisawa, Takatoshi
2005-11-01
Natural convection induced by internal heat generation in a shallow fluid layer was investigated experimentally. Internal heat generation was realized by passing electric current through ionic liquid. Kalliroscope flakes and thermo-chromic liquid crystal were utilized to clarify a transition of the convection with respect to the Rayleigh number, RI. Visualized flow pattern at higher Rayleigh number show two types of deformed cell shape, double cell structure, which has a small cell in a large cell, and spoke like cell structure, where descending flow neat the center of a cell spread like a spoke. Visualized temperature field was converted to temperature field in order to investigate the transition quantitatively. Variation of horizontal temperature fluctuation with respect to RI may show critical Rayleigh number for the transition.
Convective Available Potential Energy of World Ocean
NASA Astrophysics Data System (ADS)
Su, Z.; Ingersoll, A. P.; Thompson, A. F.
2012-12-01
-ocean convection may arise through strong surface buoyancy fluxes (Schott et al. 1996), or by thermobaric instability (Akitomo 1999a, b). Ingersoll (2005) demonstrated that thermobaric-induced deep convection is due to the abrupt release of ocean potential energy into kinetic energy. In atmospheric dynamics, Convective Available Potential Energy (CAPE) has long been an important thermodynamic variable (Arakawa and Schubert 1974) that has been used to forecast moist convection (Doswell and Rasmussen 1994) and to test the performance of GCMs (Ye et al. 1998). However, the development of a similar diagnostic in the ocean has received little attention.; World Ocean Convective Available Potential Energy distribution in North-Hemisphere Autumn (J/kg)
Ice Nucleation in Deep Convection
NASA Technical Reports Server (NTRS)
Jensen, Eric; Ackerman, Andrew; Stevens, David; Gore, Warren J. (Technical Monitor)
2001-01-01
The processes controlling production of ice crystals in deep, rapidly ascending convective columns are poorly understood due to the difficulties involved with either modeling or in situ sampling of these violent clouds. A large number of ice crystals are no doubt generated when droplets freeze at about -40 C. However, at higher levels, these crystals are likely depleted due to precipitation and detrainment. As the ice surface area decreases, the relative humidity can increase well above ice saturation, resulting in bursts of ice nucleation. We will present simulations of these processes using a large-eddy simulation model with detailed microphysics. Size bins are included for aerosols, liquid droplets, ice crystals, and mixed-phase (ice/liquid) hydrometers. Microphysical processes simulated include droplet activation, freezing, melting, homogeneous freezing of sulfate aerosols, and heterogeneous ice nucleation. We are focusing on the importance of ice nucleation events in the upper part of the cloud at temperatures below -40 C. We will show that the ultimate evolution of the cloud in this region (and the anvil produced by the convection) is sensitive to these ice nucleation events, and hence to the composition of upper tropospheric aerosols that get entrained into the convective column.
Polar Cap Plasma and Convection
NASA Technical Reports Server (NTRS)
Elliott, Heather A.; Craven, Paul D.; Comfort, Richard H.; Chandler, Michael O.; Moore, Thomas E.; Ruohoniemi, J. M.
1998-01-01
This presentation will describe the character of the polar cap plasma in 10% AGU Spring 1998 particular the convection velocities at the perigee (about 1.8 Re) and apogee( about 8.9 Re) of Polar in relationship to Interplanetary Magnetic Field (IMF) and solar wind parameters. This plasma is thought to be due to several sources; the polar wind, cleft ion fountain, and auroral outflow. The plasma in the polar cap tends to be mostly field-aligned. At any given point in the polar cap, this plasma could be from a different regions since convection of magnetic field lines can transport this material. it is quite difficult to study such a phenomena with single point measurements. Current knowledge of the polar cap plasma obtained by in situ measurements will be presented along with recent results from the Polar mission. This study also examines the direct electrical coupling between the magnetosphere and ionosphere by comparing convection velocities measured by the Thermal Ion Dynamics Experiment (TIDE) and Magnetic Field Experiment (MFE) instruments in magnetosphere and measurements of the ionosphere by ground-based radars. At times such a comparison is difficult because the Polar satellite at apogee spends a large amount of time in the polar cap which is a region that is not coverage well by the current SuperDam coherent radars. This is impart due to the lack of irregularities that returns the radar signal.
Stochastic Convection Parameterizations
NASA Technical Reports Server (NTRS)
Teixeira, Joao; Reynolds, Carolyn; Suselj, Kay; Matheou, Georgios
2012-01-01
computational fluid dynamics, radiation, clouds, turbulence, convection, gravity waves, surface interaction, radiation interaction, cloud and aerosol microphysics, complexity (vegetation, biogeochemistry, radiation versus turbulence/convection stochastic approach, non-linearities, Monte Carlo, high resolutions, large-Eddy Simulations, cloud structure, plumes, saturation in tropics, forecasting, parameterizations, stochastic, radiation-clod interaction, hurricane forecasts
Marangoni convection in an open boat
NASA Astrophysics Data System (ADS)
Schwabe, D.; Lamprecht, R.; Scharmann, A.
The effect of microgravity on thermal Marangoni convection in an open cavity is examined, comparing the results of experiments conducted during Spacelab mission D1 with ground experiments. Molten tetracosane (with added tracer particles) contained in a quartz glass cuvette between two Al-block heaters was observed as the temperature gradient between the heaters (DeltaT) was varied from 0 to 60 C. In the D1 experiment, Marangoni convection at free-surface velocity 20 mm/sec was detected, but only at DeltaT = 60 C; the suppression of Marangoni convection at lower DeltaT is attributed to the formation of a stable surface skin of contaminant particles (not formed in the ground experiments due to buoyant convection). Calculations show that thermocapillary forces are about equal to buoyant forces at 1 g but are dominant under microgravity.
NASA Technical Reports Server (NTRS)
Stewart, R. B.
1972-01-01
Numberical solutions are obtained for the quasi-compressible Navier-Stokes equations governing the time dependent natural convection flow within a horizontal cylinder. The early time flow development and wall heat transfer is obtained after imposing a uniformly cold wall boundary condition on the cylinder. Solutions are also obtained for the case of a time varying cold wall boundary condition. Windware explicit differ-encing is used for the numerical solutions. The viscous truncation error associated with this scheme is controlled so that first order accuracy is maintained in time and space. The results encompass a range of Grashof numbers from 8.34 times 10,000 to 7 times 10 to the 7th power which is within the laminar flow regime for gravitationally driven fluid flows. Experiments within a small scale instrumented horizontal cylinder revealed the time development of the temperature distribution across the boundary layer and also the decay of wall heat transfer with time.
Geothermal reservoirs in hydrothermal convection systems
Sorey, M.L.
1982-01-01
Geothermal reservoirs commonly exist in hydrothermal convection systems involving fluid circulation downward in areas of recharge and upwards in areas of discharge. Because such reservoirs are not isolated from their surroundings, the nature of thermal and hydrologic connections with the rest of the system may have significant effects on the natural state of the reservoir and on its response to development. Conditions observed at numerous developed and undeveloped geothermal fields are discussed with respect to a basic model of the discharge portion of an active hydrothermal convection system. Effects of reservoir development on surficial discharge of thermal fluid are also delineated.
NASA Astrophysics Data System (ADS)
Mojumder, Satyajit; Rabbi, Khan Md.; Saha, Sourav; Hasan, MN; Saha, Suvash C.
2016-06-01
In this study magneto-hydrodynamic convection in a half-moon shaped cavity filled with ferrofluid has been analyzed numerically. The cavity has two semi-circular bottom heaters and effect of the distance between these two heaters (λ = 0.1 , 0.4) has been thoroughly investigated. Numerical simulation has been carried out for a wide range of Rayleigh number (Ra =103 ∼107), Hartmann number (Ha = 0 ∼ 100) and inclination angle of magnetic field (γ = 0 ° ∼ 90 °) to understand the flow field, thermal field and entropy generation respectively. Cobalt-kerosene and Fe3 O4 -water ferrofluids are used for the present investigation and considered as a single phase fluid. Galerkin weighted residual method of finite element analysis has been used for numerical solution. The code validation and grid independency test have been carried out to justify the numerical accuracy. It has been observed that increment of magnetic field reduces the heat transfer rate, whereas increment of heater distance augments the heat transfer rate significantly. Results are discussed on the basis of Nusselt number (Nu), Bejan number (Be) and shown by contours and 3D plots. It has also been found that λ = 0.4 always shows better heat transfer rate and entropy optimization.
Effects of Deep Convection on Atmospheric Chemistry
NASA Technical Reports Server (NTRS)
Pickering, Kenneth E.
2007-01-01
This presentation will trace the important research developments of the last 20+ years in defining the roles of deep convection in tropospheric chemistry. The role of deep convection in vertically redistributing trace gases was first verified through field experiments conducted in 1985. The consequences of deep convection have been noted in many other field programs conducted in subsequent years. Modeling efforts predicted that deep convection occurring over polluted continental regions would cause downstream enhancements in photochemical ozone production in the middle and upper troposphere due to the vertical redistribution of ozone precursors. Particularly large post-convective enhancements of ozone production were estimated for convection occurring over regions of pollution from biomass burning and urban areas. These estimates were verified by measurements taken downstream of biomass burning regions of South America. Models also indicate that convective transport of pristine marine boundary layer air causes decreases in ozone production rates in the upper troposphere and that convective downdrafts bring ozone into the boundary layer where it can be destroyed more rapidly. Additional consequences of deep convection are perturbation of photolysis rates, effective wet scavenging of soluble species, nucleation of new particles in convective outflow, and the potential fix stratosphere-troposphere exchange in thunderstorm anvils. The remainder of the talk will focus on production of NO by lightning, its subsequent transport within convective clouds . and its effects on downwind ozone production. Recent applications of cloud/chemistry model simulations combined with anvil NO and lightning flash observations in estimating NO Introduction per flash will be described. These cloud-resolving case-study simulations of convective transport and lightning NO production in different environments have yielded results which are directly applicable to the design of lightning
Stein, Robert F
2012-07-13
Convection is the transport of energy by bulk mass motions. Magnetic fields alter convection via the Lorentz force, while convection moves the fields via the curl(v×B) term in the induction equation. Recent ground-based and satellite telescopes have increased our knowledge of the solar magnetic fields on a wide range of spatial and temporal scales. Magneto-convection modelling has also greatly improved recently as computers become more powerful. Three-dimensional simulations with radiative transfer and non-ideal equations of state are being performed. Flux emergence from the convection zone through the visible surface (and into the chromosphere and corona) has been modelled. Local, convectively driven dynamo action has been studied. The alteration in the appearance of granules and the formation of pores and sunspots has been investigated. Magneto-convection calculations have improved our ability to interpret solar observations, especially the inversion of Stokes spectra to obtain the magnetic field and the use of helioseismology to determine the subsurface structure of the Sun. PMID:22665893
3D Convection-pulsation Simulations with the HERACLES Code
NASA Astrophysics Data System (ADS)
Felix, S.; Audit, E.; Dintrans, B.
2015-10-01
We present 3D simulations of the coupling between surface convection and pulsations due to the κ-mechanism in classical Cepheids of the red edge of Hertzsprung-Russell diagram's instability strip. We show that 3D convection is less powerful than 2D convection and does not quench the radiative pulsations, leading to an efficient 3D κ-mechanism. Thus, the 3D instability strip is closer to the observed one than the 1D or 2D were.
NASA Astrophysics Data System (ADS)
Mertes, Stephan; Kästner, Udo; Schulz, Christiane; Klimach, Thomas; Krüger, Mira; Schneider, Johannes
2015-04-01
Airborne sampling of cloud particles inside different cirrus cloud types and inside deep convective clouds was conducted during the HALO missions ML-CIRRUS over Europe in March/April 2014 and ACRIDICON over Amazonia in September 2014. ML-CIRRUS aims at the investigation of the for-mation, evolution, microphysical state and radiative effects of different natural and aviation-induced cirrus clouds in the mid-latitudes. The main objectives of ACRIDICON are the microphysical vertical profiling, vertical aerosol transport and the cloud processing of aerosol particles (compari-son in- and outflow) of tropical deep convective cloud systems in clean and polluted air masses and over forested and deforested regions. The hydrometeors (drops and ice particles) are sampled by a counterflow virtual impactor (CVI) which has to be installed in the front part of the upper fuselage of the HALO aircraft. Such an intake position implies a size dependent abundance of cloud particles with respect to ambient conditions that was studied by particle trajectory simulations (Katrin Witte, HALO Technical Note 2008-003-A). On the other hand, this sampling location avoids that large ice crystals which could potentially bias the cloud particle sampling by shattering and break-up at the inlet shroud and tip enter the inlet. Both aspects as well as the flight conditions of HALO were taken into account for an optimized CVI design for HALO (HALO-CVI). Interstitial particles are pre-segregated and the condensed phase is evaporated/sublimated by the CVI, such that the residuals from cloud droplets and ice particles (CDR and IPR) can be microphysically and chemically analyzed by respective aerosol sensors located in the cabin. Although an even more comprehensive characterization of CDR and IPR was carried out, we like to report on the following measurements of certain aerosol properties. Particle number concentra-tion and size distribution are measured by a condensation particle counter (CPC) and an
Pulsation driving and convection
NASA Astrophysics Data System (ADS)
Antoci, Victoria
2015-08-01
Convection in stellar envelopes affects not only the stellar structure, but has a strong impact on different astrophysical processes, such as dynamo-generated magnetic fields, stellar activity and transport of angular momentum. Solar and stellar observations from ground and space have shown that the turbulent convective motion can also drive global oscillations in many type of stars, allowing to study stellar interiors at different evolutionary stages. In this talk I will concentrate on the influence of convection on the driving of stochastic and coherent pulsations across the Hertzsprung-Russell diagram and give an overview of recent studies.
ERIC Educational Resources Information Center
Kaun, Karla R.; Hendel, Thomas; Gerber, Bertram; Sokolowski, Marla B.
2007-01-01
Animals must be able to find and evaluate food to ensure survival. The ability to associate a cue with the presence of food is advantageous because it allows an animal to quickly identify a situation associated with a good, bad, or even harmful food. Identifying genes underlying these natural learned responses is essential to understanding this…
NASA Astrophysics Data System (ADS)
Helmig, Detlev; Rossabi, Samuel; Hueber, Jacques; Tans, Pieter; Montzka, Stephen A.; Masarie, Ken; Thoning, Kirk; Plass-Duelmer, Christian; Claude, Anja; Carpenter, Lucy J.; Lewis, Alastair C.; Punjabi, Shalini; Reimann, Stefan; Vollmer, Martin K.; Steinbrecher, Rainer; Hannigan, James W.; Emmons, Louisa K.; Mahieu, Emmanuel; Franco, Bruno; Smale, Dan; Pozzer, Andrea
2016-07-01
Non-methane hydrocarbons such as ethane are important precursors to tropospheric ozone and aerosols. Using data from a global surface network and atmospheric column observations we show that the steady decline in the ethane mole fraction that began in the 1970s halted between 2005 and 2010 in most of the Northern Hemisphere and has since reversed. We calculate a yearly increase in ethane emissions in the Northern Hemisphere of 0.42 (+/-0.19) Tg yr-1 between mid-2009 and mid-2014. The largest increases in ethane and the shorter-lived propane are seen over the central and eastern USA, with a spatial distribution that suggests North American oil and natural gas development as the primary source of increasing emissions. By including other co-emitted oil and natural gas non-methane hydrocarbons, we estimate a Northern Hemisphere total non-methane hydrocarbon yearly emission increase of 1.2 (+/-0.8) Tg yr-1. Atmospheric chemical transport modelling suggests that these emissions could augment summertime mean surface ozone by several nanomoles per mole near oil and natural gas production regions. Methane/ethane oil and natural gas emission ratios could suggest a significant increase in associated methane emissions; however, this increase is inconsistent with observed leak rates in production regions and changes in methane's global isotopic ratio.
Dawson-Scully, Ken; Armstrong, Gary A.B.; Kent, Clement; Robertson, R. Meldrum; Sokolowski, Marla B.
2007-01-01
Although it is acknowledged that genetic variation contributes to individual differences in thermotolerance, the specific genes and pathways involved and how they are modulated by the environment remain poorly understood. We link natural variation in the thermotolerance of neural function and behavior in Drosophila melanogaster to the foraging gene (for, which encodes a cGMP-dependent protein kinase (PKG)) as well as to its downstream target, protein phosphatase 2A (PP2A). Genetic and pharmacological manipulations revealed that reduced PKG (or PP2A) activity caused increased thermotolerance of synaptic transmission at the larval neuromuscular junction. Like synaptic transmission, feeding movements were preserved at higher temperatures in larvae with lower PKG levels. In a comparative assay, pharmacological manipulations altering thermotolerance in a central circuit of Locusta migratoria demonstrated conservation of this neuroprotective pathway. In this circuit, either the inhibition of PKG or PP2A induced robust thermotolerance of neural function. We suggest that PKG and therefore the polymorphism associated with the allelic variation in for may provide populations with natural variation in heat stress tolerance. for's function in behavior is conserved across most organisms, including ants, bees, nematodes, and mammals. PKG's role in thermotolerance may also apply to these and other species. Natural variation in thermotolerance arising from genes involved in the PKG pathway could impact the evolution of thermotolerance in natural populations. PMID:17712421
Multistation measurements of high-latitude ionospheric convection
NASA Astrophysics Data System (ADS)
Heelis, R. A.; Foster, J. C.; Holt, J.; de La Beaujardiere, O.
1983-12-01
Satellite and ground-based observations of the ionospheric drift velocity taken during a MITHRAS campaign have been combined to determine instantaneous pictures of the high-latitude convection pattern. These data, taken when the interplanetary magnetic field has a relatively stable southward/away orientation, show the existence of an asymmetric convection pattern under these conditions. A stability in the high latitude convection geometry can also be seen and changes in response to magnetic disturbances are inferred. Changes in the convection pattern as the interplanetary field turns northward possibly provide some information about the nature of the magnetosphere-solar wind interaction.
Multistation measurements of high-latitude ionospheric convection
NASA Technical Reports Server (NTRS)
Heelis, R. A.; Foster, J. C.; Holt, J.; De La Beaujardiere, O.
1983-01-01
Satellite and ground-based observations of the ionospheric drift velocity taken during a MITHRAS campaign have been combined to determine instantaneous pictures of the high-latitude convection pattern. These data, taken when the interplanetary magnetic field has a relatively stable southward/away orientation, show the existence of an asymmetric convection pattern under these conditions. A stability in the high latitude convection geometry can also be seen and changes in response to magnetic disturbances are inferred. Changes in the convection pattern as the interplanetary field turns northward possibly provide some information about the nature of the magnetosphere-solar wind interaction.
Mantle Convection in a Microwave Oven: New Perspectives for the Internally Heated Convection
NASA Astrophysics Data System (ADS)
Limare, A.; Fourel, L.; Surducan, E.; Neamtu, C.; Surducan, V.; Vilella, K.; Farnetani, C. G.; Kaminski, E. C.; Jaupart, C. P.
2015-12-01
The thermal evolution of silicate planets is primarily controlled by the balance between internal heating - due to radioactive decay - and heat transport by mantle convection. In the Earth, the problem is particularly complex due to the heterogeneous distribution of heat sources in the mantle and the non-linear coupling between this distribution and convective mixing. To investigate the behaviour of such systems, we have developed a new technology based on microwave absorption to study internally-heated convection in the laboratory. This prototype offers the ability to reach the high Rayleigh-Roberts and Prandtl numbers that are relevant for planetary convection. Our experimental results obtained for a uniform distribution of heat sources were compared to numerical calculations reproducing exactly experimental conditions (3D Cartesian geometry and temperature-dependent physical properties), thereby providing the first cross validation of experimental and numerical studies of convection in internally-heated systems. We find that the thermal boundary layer thickness and interior temperature scale with RaH-1/4, where RaH is the Rayleigh-Roberts number, as theoretically predicted by scaling arguments on the dissipation of kinetic energy. Our microwave-based method offers new perspectives for the study of internally-heated convection in heterogeneous systems which have been out of experimental reach until now. We are able to selectively heat specific regions in the convecting layer, through the careful control of the absorption properties of different miscible fluids. This is analogous to convection in the presence of chemical reservoirs with different concentration of long-lived radioactive isotopes. We shall show results for two different cases: the stability of continental lithosphere over a convective fluid and the evolution of a hidden enriched reservoir in the lowermost mantle.
Global aerosol effects on convective clouds
NASA Astrophysics Data System (ADS)
Wagner, Till; Stier, Philip
2013-04-01
Atmospheric aerosols affect cloud properties, and thereby the radiation balance of the planet and the water cycle. The influence of aerosols on clouds is dominated by increase of cloud droplet and ice crystal numbers (CDNC/ICNC) due to enhanced aerosols acting as cloud condensation and ice nuclei. In deep convective clouds this increase in CDNC/ICNC is hypothesised to increase precipitation because of cloud invigoration through enhanced freezing and associated increased latent heat release caused by delayed warm rain formation. Satellite studies robustly show an increase of cloud top height (CTH) and precipitation with increasing aerosol optical depth (AOD, as proxy for aerosol amount). To represent aerosol effects and study their influence on convective clouds in the global climate aerosol model ECHAM-HAM, we substitute the standard convection parameterisation, which uses one mean convective cloud for each grid column, with the convective cloud field model (CCFM), which simulates a spectrum of convective clouds, each with distinct values of radius, mixing ratios, vertical velocity, height and en/detrainment. Aerosol activation and droplet nucleation in convective updrafts at cloud base is the primary driver for microphysical aerosol effects. To produce realistic estimates for vertical velocity at cloud base we use an entraining dry parcel sub cloud model which is triggered by perturbations of sensible and latent heat at the surface. Aerosol activation at cloud base is modelled with a mechanistic, Köhler theory based, scheme, which couples the aerosols to the convective microphysics. Comparison of relationships between CTH and AOD, and precipitation and AOD produced by this novel model and satellite based estimates show general agreement. Through model experiments and analysis of the model cloud processes we are able to investigate the main drivers for the relationship between CTH / precipitation and AOD.
Stochasticity of convection in Giga-LES data
NASA Astrophysics Data System (ADS)
De La Chevrotière, Michèle; Khouider, Boualem; Majda, Andrew J.
2016-09-01
The poor representation of tropical convection in general circulation models (GCMs) is believed to be responsible for much of the uncertainty in the predictions of weather and climate in the tropics. The stochastic multicloud model (SMCM) was recently developed by Khouider et al. (Commun Math Sci 8(1):187-216, 2010) to represent the missing variability in GCMs due to unresolved features of organized tropical convection. The SMCM is based on three cloud types (congestus, deep and stratiform), and transitions between these cloud types are formalized in terms of probability rules that are functions of the large-scale environment convective state and a set of seven arbitrary cloud timescale parameters. Here, a statistical inference method based on the Bayesian paradigm is applied to estimate these key cloud timescales from the Giga-LES dataset, a 24-h large-eddy simulation (LES) of deep tropical convection (Khairoutdinov et al. in J Adv Model Earth Syst 1(12), 2009) over a domain comparable to a GCM gridbox. A sequential learning strategy is used where the Giga-LES domain is partitioned into a few subdomains, and atmospheric time series obtained on each subdomain are used to train the Bayesian procedure incrementally. Convergence of the marginal posterior densities for all seven parameters is demonstrated for two different grid partitions, and sensitivity tests to other model parameters are also presented. A single column model simulation using the SMCM parameterization with the Giga-LES inferred parameters reproduces many important statistical features of the Giga-LES run, without any further tuning. In particular it exhibits intermittent dynamical behavior in both the stochastic cloud fractions and the large scale dynamics, with periods of dry phases followed by a coherent sequence of congestus, deep, and stratiform convection, varying on timescales of a few hours consistent with the Giga-LES time series. The chaotic variations of the cloud area fractions were
Turco, Claudia Del; La Spina, Carlo; Mantovani, Elena; Gagliardi, Marco; Lattanzio, Rosangela; Pierro, Luisa
2014-01-01
Premacular retrohyaloid hemorrhage is a rare complication of acute severe anemia. The authors report two cases of premacular hemorrhage in which no treatment other than clinical and spectral-domain optical coherence tomography observation was performed. The natural history of this condition reveals that complete clinical resolution is not accompanied by full anatomical restoration. [Ophthalmic Surg Lasers Imaging Retina. 2014;45:E5-E7.]. PMID:24496165
Heterogeneity in diurnal variation of tropospheric convection over Indian region
NASA Astrophysics Data System (ADS)
Muhammed, Muhsin; Sunilkumar, S. V.
2016-07-01
The tropical Tropopause and the features of the Tropical Tropopause Layer (TTL) are governed by troposheric convection from below and radiative heating from above (stratosphere). The brightness temperature in the thermal infrared channel (IRBT) is used as a proxy for identifying tropospheric convection and deep convective clouds. IRBT from Very High Resolution Radiometer (VHRR) onboard KALPANA-1 during different seasons of 2008 to 2014 is being used to examine the heterogeneity of tropospheric convection. Over Indian peninsula, 36 regions have been identified with a spatial resolution of ±0.7° (81 pixels) with equal distance in both longitude and latitude. During monsoon season, a clear diurnal variation in convection is noticed over land when compared with over ocean. Over inland regions, the occurrence of deeper convection occurs during evening and early morning with different diurnal patterns. This can be due to the inhomogeneity of the terrain. It can be noted that the diurnal convection pattern over Arabian Sea is different than Bay of Bengal diurnal convection pattern. Regions near to the western-ghat do not show a clear diurnal variation and shows high occurrence of midlevel clouds (IRBT<265K). During winter (DJF), the occurrence of IRBT below 280K is very less at any time of the day over both land and ocean, which indicates the occurrence of deeper convection is rare. Hence, during winter, the diurnal variations of convection over both land and ocean has insignificant diurnal pattern.
Benard and Marangoni convection in multiple liquid layers
NASA Technical Reports Server (NTRS)
Koster, Jean N.; Prakash, A.; Fujita, D.; Doi, T.
1992-01-01
Convective fluid dynamics of immiscible double and triple liquid layers are considered. First results on multilayer convective flow, in preparation for spaceflight experiment aboard IML-2 (International Microgravity Laboratory), are discussed. Convective flow in liquid layers with one or two horizontal interfaces with heat flow applied parallel to them is one of the systems investigated. The second system comprises two horizontally layered immiscible liquids heated from below and cooled from above, that is, heat flow orthogonal to the interface. In this system convection results due to the classical Benard instability.
Regional analysis of convective systems during the West African monsoon
NASA Astrophysics Data System (ADS)
Guy, Bradley Nicholas
characteristics (e.g. total precipitation and vertical reflectivity profiles) at the inland and maritime sites. The wave regime also resulted in an increased population of the largest observed mesoscale convective systems observed near the coast, which led to an increase in stratiform precipitation. Despite this increase, differentiation of convective strength characteristics was less obvious between wave and no-wave regimes at the coast. Due to the propagating nature of these advecting mesoscale convective systems, interaction with the regional thermodynamic and dynamic environment appears to result in more variability than enhancements due to the wave regime, independent of location. A 13-year (1998-2010) climatology of mesoscale convective characteristics associated with the West African monsoon are also investigated using precipitation radar and passive microwave data from the NASA Tropical Rainfall Measuring Mission satellite. Seven regions defined as continental northeast and northwest, southeast and southwest, coastal, and maritime north and south are compared to analyze zonal and meridional differences. Data are categorized according to identified African easterly wave (AEW) phase and when no wave is present. While some enhancements are observed in association with AEW regimes, regional differences were generally more apparent than wave vs. no-wave differences. Convective intensity metrics confirm that land-based systems exhibit stronger characteristics, such as higher storm top and maximum 30-dBZ heights and significant 85-GHz brightness temperature depressions. Continental systems also contain a lower fraction of points identified as stratiform. Results suggest that precipitation processes also varied depending upon region and AEW regime, with warm-rain processes more apparent over the ocean and the southwest continental region and ice-based microphysics more dominant over land, including mixed-phase processes. AEW regimes did show variability in stratiform fraction and
Hlaing Myat Thu; Lowry, Kym; Jiang Limin; Thaung Hlaing; Holmes, Edward C.; Aaskov, John . E-mail: j.aaskov@qut.edu.au
2005-06-05
Between 1996 and 1998, two clades (B and C; genotype I) of dengue virus type 1 (DENV-1) appeared in Myanmar (Burma) that were new to that location. Between 1998 and 2000, a third clade (A; genotype III) of DENV-1, which had been circulating at that locality for at least 25 years, became extinct. These changes preceded the largest outbreak of dengue recorded in Myanmar, in 2001, in which more than 95% of viruses recovered from patients were DENV-1, but where the incidence of severe disease was much less than in previous years. Phylogenetic analyses of viral genomes indicated that the two new clades of DENV-1 did not arise from the, now extinct, clade A viruses nor was the extinction of this clade due to differences in the fitness of the viral populations. Since the extinction occurred during an inter-epidemic period, we suggest that it was due to a stochastic event attributable to the low rate of virus transmission in this interval.
Layer Formation in Convective Magma Chambers
NASA Astrophysics Data System (ADS)
Höink, T.; Schmalzl, J.; Hansen, U.
2004-12-01
The dynamics of a convective magma chamber is crucially influenced by the competetion between sedimentation and convective suspension of crystals. Crystal settling combined with the crystal's density contribution is a possible mechanism leading to differentiation and layer formation. Here we address the question whether crystals can remain suspended or whether they are able to dynamically form a layered structure within the convective lifetime of a magma chamber. We employ an existing numerical method that, by means of a finite volume scheme, discretizes the equations for thermally driven convection in an infinite Prandtl-number Boussinesq fluid in Cartesian geometry. We implement a newly developed settling algorithm for the numerical study of finite-sized-particle settling in a non-dilute convective suspension. Our approach considers a consistent settling velocity and the density contribution due to particle mass. The buoyancy ratio B, which is the ratio of the density variation due to crystal mass to the thermal density variation, is varied for five different Rayleigh numbers, covering a range of four orders of magnitude. We find B to be a critical parameter and its critical value to depend on the Rayleigh number. For subcritical values we observe that the presence of a crystal phase reduces convective vigor and most crystals stay suspended. When a critical buoyancy ratio is exceeded, the presence of crystals can significantly alter convective motion. For all investigated Rayleigh numbers we find a critical buoyancy ratio, above which layering can be achieved from an initially unstratified fluid. Most of the crystal mass collects in the dynamically created bottom layer, even for cases where the average settling velocity is three orders of magnitude smaller than the root mean square convective velocity. The time it takes a crystal to travel across the height of the cell with the full settling velocity in the absence of a thermal gradient defines the settling
Govind, Ajit
2014-10-01
The nature of canopy radiative transfer mechanism (CRTM) describes the amount of beam penetration through a canopy and governs the nature of canopy illumination, i.e. the abundance of sunlit and shaded portions. Realistic representation of canopy illumination is critical for simulating various canopy biophysical processes associated with vegetated land surfaces. The adequate representation of CRTM can be attributed to the parameterizations of the two main canopy characteristics: the foliage projection (G-function) and the clumping effect (Ω function). Herein, using various types of G and Ω functions developed in a previous study, I tested 15 CRTM scenarios that combine different types of G and Ω functions to predict the dynamics of sunlit fraction (ε) of canopies having a wide range of plant area index (Ptotal) at various solar zenith angles (SZAs). It was observed that, for a given Ptotal, ε decreases as the SZA increases. However, ε significantly changed in accordance with the type of G and Ω functions used. Scenarios that employed random distribution of elements in space (S-4, S-9, and S-14) consistently returned larger ε values even at lower SZAs. This means that ignoring the clumping behavior of canopies could result in greater proportion of sunlit elements thereby reducing the beam penetration deeper into the canopy as opposed to those canopies where the elements are more aggregated. Beyond 70° SZA, almost all the scenarios returned similar ε values for a given Ptotal, which implied that the methods used is less sensitive at higher SZAs. The values of ε calculated by all the scenarios were significantly different from the S-6 (the ideal case). This observation highlights the importance of explicitly describing the G and Ω functions to adequately depict canopy illumination conditions.
Francisco Valentin; Narbeh Artoun; Masahiro Kawaji; Donald M. McEligot
2015-08-01
Fundamental high pressure/high temperature forced convection experiments have been conducted in support of the development of a Very High Temperature Reactor (VHTR) with a prismatic core. The experiments utilize a high temperature/high pressure gas flow test facility constructed for forced convection and natural circulation experiments. The test section has a single 16.8 mm ID flow channel in a 2.7 m long, 108 mm OD graphite column with four 2.3kW electric heater rods placed symmetrically around the flow channel. This experimental study presents the role of buoyancy forces in enhancing or reducing convection heat transfer for helium at high pressures up to 70 bar and high temperatures up to 873 degrees K. Wall temperatures have been compared among 10 cases covering the inlet Re numbers ranging from 500 to 3,000. Downward flows display higher and lower wall temperatures in the upstream and downstream regions, respectively, than the upward flow cases due to the influence of buoyancy forces. In the entrance region, convection heat transfer is reduced due to buoyancy leading to higher wall temperatures, while in the downstream region, buoyancyinduced mixing causes higher convection heat transfer and lower wall temperatures. However, their influences are reduced as the Reynolds number increases. This experimental study is of specific interest to VHTR design and validation of safety analysis codes.
Convective, intrusive geothermal plays: what about tectonics?
NASA Astrophysics Data System (ADS)
Santilano, A.; Manzella, A.; Gianelli, G.; Donato, A.; Gola, G.; Nardini, I.; Trumpy, E.; Botteghi, S.
2015-09-01
We revised the concept of convective, intrusive geothermal plays, considering that the tectonic setting is not, in our opinion, a discriminant parameter suitable for a classification. We analysed and compared four case studies: (i) Larderello (Italy), (ii) Mt Amiata (Italy), (iii) The Geysers (USA) and (iv) Kizildere (Turkey). The tectonic settings of these geothermal systems are different and a matter of debate, so it is hard to use this parameter, and the results of classification are ambiguous. We suggest a classification based on the age and nature of the heat source and the related hydrothermal circulation. Finally we propose to distinguish the convective geothermal plays as volcanic, young intrusive and amagmatic.
Supergranulation, a convective phenomenon
NASA Astrophysics Data System (ADS)
Udayashankar, Paniveni
2015-08-01
Observation of the Solar photosphere through high resolution instruments have long indicated that the surface of the Sun is not a tranquil, featureless surface but is beset with a granular appearance. These cellular velocity patterns are a visible manifestation of sub- photospheric convection currents which contribute substantially to the outward transport of energy from the deeper layers, thus maintaining the energy balance of the Sun as a whole.Convection is the chief mode of transport in the outer layers of all cool stars such as the Sun (Noyes,1982). Convection zone of thickness 30% of the Solar radius lies in the sub-photospheric layers of the Sun. Convection is revealed on four scales. On the scale of 1000 km, it is granulation and on the scale of 8-10 arcsec, it is Mesogranulation. The next hierarchial scale of convection ,Supergranules are in the range of 30-40 arcsec. The largest reported manifestation of convection in the Sun are ‘Giant Cells’or ‘Giant Granules’, on a typical length scale of about 108 m.'Supergranules' is caused by the turbulence that extends deep into the convection zone. They have a typical lifetime of about 20hr with spicules marking their boundaries. Gas rises in the centre of the supergranules and then spreads out towards the boundary and descends.Broadly speaking supergranules are characterized by the three parameters namely the length L, the lifetime T and the horizontal flow velocity vh . The interrelationships amongst these parameters can shed light on the underlying convective processes and are in agreement with the Kolmogorov theory of turbulence as applied to large scale solar convection (Krishan et al .2002 ; Paniveni et. al. 2004, 2005, 2010).References:1) Noyes, R.W., The Sun, Our Star (Harvard University Press, 1982)2) Krishan, V., Paniveni U., Singh , J., Srikanth R., 2002, MNRAS, 334/1,2303) Paniveni , U., Krishan, V., Singh, J., Srikanth, R., 2004, MNRAS, 347, 1279-12814) Paniveni , U., Krishan, V., Singh, J
The influence of convection parameterisations under alternate climate conditions
NASA Astrophysics Data System (ADS)
Rybka, Harald; Tost, Holger
2013-04-01
In the last decades several convection parameterisations have been developed to consider the impact of small-scale unresolved processes in Earth System Models associated with convective clouds. Global model simulations, which have been performed under current climate conditions with different convection schemes, significantly differ among each other in the simulated precipitation patterns due to the parameterisation assumptions and formulations, e.g. the simplified treatment of the cloud microphysics. Additionally, the simulated transport of short-lived trace gases strongly depends on the chosen convection parameterisation due to the differences in the vertical redistribution of mass. Furthermore, other meteorological parameters like the temperature or the specific humidity show substantial differences in convectively active regions. This study presents uncertainties of climate change scenarios caused by different convection parameterisations. For this analysis two experiments (reference simulation with a CO2 concentration of 348 ppm; 2xCO2-simulation with a CO2 concentration of 696 ppm) are calculated with the ECHAM/MESSy atmospheric chemistry (EMAC) model applying four different convection schemes (Tiedtke, ECMWF, Emanuel and Zhang-McFarlane - Hack) and two resolutions (T42 and T63), respectively. The results indicate that the equilibrium climate sensitivity is independent of the chosen convection parameterisation. However, the regional temperature increase, induced by a doubling of the carbon dioxide concentration, demonstrates differences of up to a few Kelvin at the surface as well as in the UTLS for the ITCZ region depending on the selected convection parameterisation. The interaction between cloud and convection parameterisations results in a large disagreement of precipitation patterns. Although every 2xCO2 -experiment simulates an increase in global mean precipitation rates, the change of regional precipitation patterns differ widely. Finally, analysing
Mechanisms initiating deep convection over complex terrain during COPS.
Kottmeier, C.; Kalthoff, N.; Barthlott, C.; Corsmeier, U.; Van Baelen, J.; Coulter, R.; Environmental Science Division; Inst. for Meteorology and Climate Research; Lab. de Meteorologie Physique; Inst. of Physics and Meteorology
2008-12-01
Precipitating convection in a mountain region of moderate topography is investigated, with particular emphasis on its initiation in response to boundary-layer and mid- and upper-tropospheric forcing mechanisms. The data used in the study are from COPS (Convective and Orographically-induced Precipitation Study) that took place in southwestern Germany and eastern France in the summer of 2007. It is found that the initiation of precipitating convection can be roughly classified as being due to either: (i) surface heating and low-level flow convergence; (ii) surface heating and moisture supply overcoming convective inhibition during latent and/or potential instability; or (iii) mid-tropospheric dynamical processes due to mesoscale convergence lines and forced mean vertical motion. These phenomena have to be adequately represented in models in order to improve quantitative precipitation forecast. Selected COPS cases are analyzed and classified into these initiation categories. Although only a subset of COPS data (mainly radiosondes, surface weather stations, radar and satellite data) are used here, it is shown that convective systems are captured in considerable detail by sensor synergy. Convergence lines were observed by Doppler radar in the location where deep convection is triggered several hours later. The results suggest that in many situations, observations of the location and timing of convergence lines will facilitate the nowcasting of convection. Further on, forecasting of the initiation of convection is significantly complicated if advection of potentially convective air masses over changing terrain features plays a major role. The passage of a frontal structure over the Vosges - Rhine valley - Black Forest orography was accompanied by an intermediate suppression of convection over the wide Rhine valley. Further downstream, an intensification of convection was observed over the Black Forest due to differential surface heating, a convergence line, and the flow
Anomalously weak solar convection.
Hanasoge, Shravan M; Duvall, Thomas L; Sreenivasan, Katepalli R
2012-07-24
Convection in the solar interior is thought to comprise structures on a spectrum of scales. This conclusion emerges from phenomenological studies and numerical simulations, though neither covers the proper range of dynamical parameters of solar convection. Here, we analyze observations of the wavefield in the solar photosphere using techniques of time-distance helioseismology to image flows in the solar interior. We downsample and synthesize 900 billion wavefield observations to produce 3 billion cross-correlations, which we average and fit, measuring 5 million wave travel times. Using these travel times, we deduce the underlying flow systems and study their statistics to bound convective velocity magnitudes in the solar interior, as a function of depth and spherical-harmonic degree ℓ. Within the wavenumber band ℓ < 60, convective velocities are 20-100 times weaker than current theoretical estimates. This constraint suggests the prevalence of a different paradigm of turbulence from that predicted by existing models, prompting the question: what mechanism transports the heat flux of a solar luminosity outwards? Advection is dominated by Coriolis forces for wavenumbers ℓ < 60, with Rossby numbers smaller than approximately 10(-2) at r/R([symbol: see text]) = 0.96, suggesting that the Sun may be a much faster rotator than previously thought, and that large-scale convection may be quasi-geostrophic. The fact that isorotation contours in the Sun are not coaligned with the axis of rotation suggests the presence of a latitudinal entropy gradient.
Anomalously Weak Solar Convection
NASA Technical Reports Server (NTRS)
Hanasoge, Shravan M.; Duvall, Thomas L.; Sreenivasan, Katepalli R.
2012-01-01
Convection in the solar interior is thought to comprise structures on a spectrum of scales. This conclusion emerges from phenomenological studies and numerical simulations, though neither covers the proper range of dynamical parameters of solar convection. Here, we analyze observations of the wavefield in the solar photosphere using techniques of time-distance helioseismology to image flows in the solar interior. We downsample and synthesize 900 billion wavefield observations to produce 3 billion cross-correlations, which we average and fit, measuring 5 million wave travel times. Using these travel times, we deduce the underlying flow systems and study their statistics to bound convective velocity magnitudes in the solar interior, as a function of depth and spherical- harmonic degree l..Within the wavenumber band l < 60, convective velocities are 20-100 times weaker than current theoretical estimates. This constraint suggests the prevalence of a different paradigm of turbulence from that predicted by existing models, prompting the question: what mechanism transports the heat flux of a solar luminosity outwards? Advection is dominated by Coriolis forces for wavenumbers l < 60, with Rossby numbers smaller than approximately 10(exp -2) at r/R-solar = 0.96, suggesting that the Sun may be a much faster rotator than previously thought, and that large-scale convection may be quasi-geostrophic. The fact that isorotation contours in the Sun are not coaligned with the axis of rotation suggests the presence of a latitudinal entropy gradient.
Drogowski, Marian
2010-01-01
Since at least 20 years, most of the legal proceedings of marital incapacity are carried on by the church tribunals mainly of no 3 canon 1095 new law code, that is of mental incapacity to undertaking and performing the essential marital duties. Very often the legal proceedings are very difficult and the canonical lawyers need the forensic psychiatric or psychological opinions. The aim of this paper is to give the forensic psychiatrists and psychologists the essential knowledge connected with the new regulations listed in canon 1095 no 3 of the actual canon law code. According to the opinions of the notable canonical lawyers the author describes the legal and marital prerequisites for valid marriage, the ability for undertaking and performing the essential marital duties and the mental reasons for marital incapacity. The author points out several psychiatric reasons underlying the mental incapacity for undertaking and performing essential marital duties: personality disorders and alcoholism which preclude adequate interpersonal relations and sexually related personality disorders which preclude the exclusiveness and natural consumption of the marital agreement. At the end the author points out the conditions which should have good forensic opinion.
Leonardelli, Florencia; Macedo, Daiana; Dudiuk, Catiana; Cabeza, Matias S; Gamarra, Soledad; Garcia-Effron, Guillermo
2016-09-01
Aspergillus fumigatus intrinsic fluconazole resistance has been demonstrated to be linked to the CYP51A gene, although the precise molecular mechanism has not been elucidated yet. Comparisons between A. fumigatus Cyp51Ap and Candida albicans Erg11p sequences showed differences in amino acid residues already associated with fluconazole resistance in C. albicans The aim of this study was to analyze the role of the natural polymorphism I301 in Aspergillus fumigatus Cyp51Ap in the intrinsic fluconazole resistance phenotype of this pathogen. The I301 residue in A. fumigatus Cyp51Ap was replaced with a threonine (analogue to T315 at Candida albicans fluconazole-susceptible Erg11p) by changing one single nucleotide in the CYP51A gene. Also, a CYP51A knockout strain was obtained using the same parental strain. Both mutants' antifungal susceptibilities were tested. The I301T mutant exhibited a lower level of resistance to fluconazole (MIC, 20 μg/ml) than the parental strain (MIC, 640 μg/ml), while no changes in MIC were observed for other azole- and non-azole-based drugs. These data strongly implicate the A. fumigatus Cyp51Ap I301 residue in the intrinsic resistance to fluconazole.
Experimental Study of Convective Dissolution of Carbon Dioxide in Heterogeneous Media
NASA Astrophysics Data System (ADS)
Liang, Y.; DiCarlo, D. A.; Hesse, M. A.
2013-12-01
Carbon capture and storage in deep geological formations has the potential to reduce anthropogenic carbon dioxide (CO2) emissions from industrial point sources. The technology is only viable, if the long-term security of the geological CO2 storage can be demonstrated. Dissolution of CO2 into the brine, resulting in stable stratification, has been identified as the key to long-term storage security. Here we present new analogue laboratory experiments to characterize convective dissolution and to study the effect of porosity and permeability heterogeneity on the CO2 dissolution rate. Understanding the effect of heterogeneity is essential to evaluate if convective dissolution occurs in the field and, in turn, to estimate the security of geological CO2 storage fields. In particular we want to test if the strong heterogeneity observed at the Bravo Dome natural CO2 field can prevent convective currents, which may explain the persistence of free phase CO2 over millennia. Initial laboratory experiments in homogeneous media confirm that the non-classical scaling of the convective flux scales with the 4/5 power of the Rayleigh number that has recently been reported. The large experimental assembly will allow us to quantify for the first time the relationship between wavenumber of the convective motion and the Rayleigh number of the system, which could be essential to trapping process at Bravo Dome. Figure 1 shows the number of fingers that we can observe in our new experimental setup. Figure 2 shows the same photograph that has been processed to enhance the visibility of the dense plumes descending from the interface. Also we plan to complement the homogeneous experiments with a detailed study of the scaling law of the convective flux in heterogeneous, layered media; in particular. Low permeability layers are ubiquitous in geological storage formations and have been observed at Bravo Dome. We plan to measure the reduction in the convective flux due to these barriers compared
Intermittent flow regimes near the convection threshold in ferromagnetic nanofluids.
Krauzina, Marina T; Bozhko, Alexandra A; Putin, Gennady F; Suslov, Sergey A
2015-01-01
The onset and decay of convection in a spherical cavity filled with ferromagnetic nanofluid and heated from below are investigated experimentally. It is found that, unlike in a single-component Newtonian fluid where stationary convection sets in as a result of supercritical bifurcation and where convection intensity increases continuously with the degree of supercriticality, convection in a multicomponent ferromagnetic nanofluid starts abruptly and has an oscillatory nature. The hysteresis is observed in the transition between conduction and convection states. In moderately supercritical regimes, the arising fluid motion observed at a fixed temperature difference intermittently transitions from quasiharmonic to essentially irregular oscillations that are followed by periods of a quasistationary convection. The observed oscillations are shown to result from the precession of the axis of a convection vortex in the equatorial plane. When the vertical temperature difference exceeds the convection onset value by a factor of 2.5, the initially oscillatory convection settles to a steady-state regime with no intermittent behavior detected afterward. The performed wavelet and Fourier analyses of thermocouple readings indicate the presence of various oscillatory modes with characteristic periods ranging from one hour to several days. PMID:25679711
Boundary layer control of rotating convection systems.
King, Eric M; Stellmach, Stephan; Noir, Jerome; Hansen, Ulrich; Aurnou, Jonathan M
2009-01-15
Turbulent rotating convection controls many observed features of stars and planets, such as magnetic fields, atmospheric jets and emitted heat flux patterns. It has long been argued that the influence of rotation on turbulent convection dynamics is governed by the ratio of the relevant global-scale forces: the Coriolis force and the buoyancy force. Here, however, we present results from laboratory and numerical experiments which exhibit transitions between rotationally dominated and non-rotating behaviour that are not determined by this global force balance. Instead, the transition is controlled by the relative thicknesses of the thermal (non-rotating) and Ekman (rotating) boundary layers. We formulate a predictive description of the transition between the two regimes on the basis of the competition between these two boundary layers. This transition scaling theory unifies the disparate results of an extensive array of previous experiments, and is broadly applicable to natural convection systems.
Boundary layer control of rotating convection systems.
King, Eric M; Stellmach, Stephan; Noir, Jerome; Hansen, Ulrich; Aurnou, Jonathan M
2009-01-15
Turbulent rotating convection controls many observed features of stars and planets, such as magnetic fields, atmospheric jets and emitted heat flux patterns. It has long been argued that the influence of rotation on turbulent convection dynamics is governed by the ratio of the relevant global-scale forces: the Coriolis force and the buoyancy force. Here, however, we present results from laboratory and numerical experiments which exhibit transitions between rotationally dominated and non-rotating behaviour that are not determined by this global force balance. Instead, the transition is controlled by the relative thicknesses of the thermal (non-rotating) and Ekman (rotating) boundary layers. We formulate a predictive description of the transition between the two regimes on the basis of the competition between these two boundary layers. This transition scaling theory unifies the disparate results of an extensive array of previous experiments, and is broadly applicable to natural convection systems. PMID:19148097
Mansour, A; Shaheen, H I; Amine, M; Hassan, K; Sanders, J W; Riddle, M S; Armstrong, A W; Svennerholm, A M; Sebeny, P J; Klena, J D; Young, S Y N; Frenck, R W
2014-07-01
Enterotoxigenic Escherichia coli (ETEC) is commonly associated with diarrhea in Egyptian children. Children less than 3 years old in Abu Homos, Egypt, had approximately five diarrheal episodes per child every year, and at least one of these episodes was due to ETEC. The epidemiology of ETEC diarrhea among children living in a rural Egyptian community was further evaluated in this study. Between January 2004 and April 2007, 348 neonates were enrolled and followed for 2 years. Children were visited twice weekly, and a stool sample was obtained every 2 weeks regardless of symptomatology. A stool sample was obtained whenever a child had diarrhea. From the routine stool culture, five E. coli-like colonies were selected and screened for heat-labile and heat-stable toxins by GM1 enzyme-linked immunosorbent assay (ELISA) and further typed for colonization factor antigens by dot blot assay. Incidence of ETEC infection was estimated among children with diarrhea (symptomatic) and without diarrhea (asymptomatic). Incidence of diarrhea and ETEC-associated diarrhea was 7.8 and 1.48 per child-year, respectively. High risk of repeated ETEC diarrhea was associated with being over 6 months of age, warm season, male gender, and crowded sleeping conditions. Exclusive breast-feeding was protective for repeated ETEC infection. ETEC-associated diarrhea remains common among children living in the Nile Delta. The protective role of breast-feeding demonstrates the importance of promoting exclusive breast-feeding during, at least, the first 6 months of life.
Mansour, A; Shaheen, H I; Amine, M; Hassan, K; Sanders, J W; Riddle, M S; Armstrong, A W; Svennerholm, A M; Sebeny, P J; Klena, J D; Young, S Y N; Frenck, R W
2014-07-01
Enterotoxigenic Escherichia coli (ETEC) is commonly associated with diarrhea in Egyptian children. Children less than 3 years old in Abu Homos, Egypt, had approximately five diarrheal episodes per child every year, and at least one of these episodes was due to ETEC. The epidemiology of ETEC diarrhea among children living in a rural Egyptian community was further evaluated in this study. Between January 2004 and April 2007, 348 neonates were enrolled and followed for 2 years. Children were visited twice weekly, and a stool sample was obtained every 2 weeks regardless of symptomatology. A stool sample was obtained whenever a child had diarrhea. From the routine stool culture, five E. coli-like colonies were selected and screened for heat-labile and heat-stable toxins by GM1 enzyme-linked immunosorbent assay (ELISA) and further typed for colonization factor antigens by dot blot assay. Incidence of ETEC infection was estimated among children with diarrhea (symptomatic) and without diarrhea (asymptomatic). Incidence of diarrhea and ETEC-associated diarrhea was 7.8 and 1.48 per child-year, respectively. High risk of repeated ETEC diarrhea was associated with being over 6 months of age, warm season, male gender, and crowded sleeping conditions. Exclusive breast-feeding was protective for repeated ETEC infection. ETEC-associated diarrhea remains common among children living in the Nile Delta. The protective role of breast-feeding demonstrates the importance of promoting exclusive breast-feeding during, at least, the first 6 months of life. PMID:24829232
Kolo, Matthew Tikpangi; Aziz, Siti Aishah Binti Abdul; Khandaker, Mayeen Uddin; Asaduzzaman, Khandoker; Amin, Yusoff Mohd
2015-09-01
Understanding the public awareness concerning the Lynas Advanced Material Plant (LAMP), an Australian rare earths processing plant located in Malaysia, a radiological study in soil and water samples collected at random surrounding the LAMP environment was undertaken using HPGe gamma-ray spectrometry. The mean soil activities for (226)Ra, (232)Th, and (40)K were found to be 6.56 ± 0.20, 10.62 ± 0.42, and 41.02 ± 0.67 Bq/kg, respectively, while for water samples were 0.33 ± 0.05, 0.18 ± 0.04, and 4.72 ± 0.29 Bq/l, respectively. The studied areas show typical local level of radioactivity from natural background radiation. The mean gamma absorbed dose rate in soils at 1 m above the ground was found to be 11.16 nGy/h. Assuming a 20 % outdoor occupancy factor, the corresponding annual effective dose showed a mean value of 0.014 mSv year(-1), significantly lower than the worldwide average value of 0.07 mSv year(-1) for the annual outdoor effective dose as reported by UNSCEAR (2000). Some other representative radiation indices such as activity utilization index (AUI), H ex, H in, excess lifetime cancer risk (ELCR), and annual gonadal dose equivalent (AGDE) were derived and also compared with the world average values. Statistical analysis performed on the obtained data showed a strong positive correlation between the radiological variables and (226)Ra and (232)Th. PMID:25925148
Thermal convection in a liquid metal battery
NASA Astrophysics Data System (ADS)
Shen, Yuxin; Zikanov, Oleg
2016-08-01
Generation of thermal convection flow in the liquid metal battery, a device recently proposed as a promising solution for the problem of the short-term energy storage, is analyzed using a numerical model. It is found that convection caused by Joule heating of electrolyte during charging or discharging is virtually unavoidable. It exists in laboratory prototypes larger than a few centimeters in size and should become much stronger in larger-scale batteries. The phenomenon needs further investigation in view of its positive (enhanced mixing of reactants) and negative (loss of efficiency and possible disruption of operation due to the flow-induced deformation of the electrolyte layer) effects.
Gravity wave initiated convection
NASA Technical Reports Server (NTRS)
Hung, R. J.
1990-01-01
The vertical velocity of convection initiated by gravity waves was investigated. In one particular case, the convective motion-initiated and supported by the gravity wave-induced activity (excluding contributions made by other mechanisms) reached its maximum value about one hour before the production of the funnel clouds. In another case, both rawinsonde and geosynchronous satellite imagery were used to study the life cycles of severe convective storms. Cloud modelling with input sounding data and rapid-scan imagery from GOES were used to investigate storm cloud formation, development and dissipation in terms of growth and collapse of cloud tops, as well as, the life cycles of the penetration of overshooting turrets above the tropopause. The results based on these two approaches are presented and discussed.
Convective Regimes in Crystallizing Basaltic Magma Chambers
NASA Astrophysics Data System (ADS)
Gilbert, A. J.; Neufeld, J. A.; Holness, M. B.
2015-12-01
Cooling through the chamber walls drives crystallisation in crustal magma chambers, resulting in a cumulate pile on the floor and mushy regions at the walls and roof. The liquid in many magma chambers, either the bulk magma or the interstitial liquid in the mushy regions, may convect, driven either thermally, due to cooling, or compositionally, due to fractional crystallization. We have constructed a regime diagram of the possible convective modes in a system containing a basal mushy layer. These modes depend on the large-scale buoyancy forcing characterised by a global Rayleigh number and the proportion of the chamber height constituting the basal mushy region. We have tested this regime diagram using an analogue experimental system composed of a fluid layer overlying a pile of almost neutrally buoyant inert particles. Convection in this system is driven thermally, simulating magma convection above and within a porous cumulate pile. We observe a range of possible convective regimes, enabling us to produce a regime diagram. In addition to modes characterised by convection of the bulk and interstitial fluid, we also observe a series of regimes where the crystal pile is mobilised by fluid motions. These regimes feature saltation and scouring of the crystal pile by convection in the bulk fluid at moderate Rayleigh numbers, and large crystal-rich fountains at high Rayleigh numbers. For even larger Rayleigh numbers the entire crystal pile is mobilised in what we call the snowglobe regime. The observed mobilisation regimes may be applicable to basaltic magma chambers. Plagioclase in basal cumulates crystallised from a dense magma may be a result of crystal mobilisation from a plagioclase-rich roof mush. Compositional convection within such a mush could result in disaggregation, enabling the buoyant plagioclase to be entrained in relatively dense descending liquid plumes and brought to the floor. The phenocryst load in porphyritic lavas is often interpreted as a
NASA Astrophysics Data System (ADS)
Crowley, J. W.; O'Connell, R. J.; Höink, T.
2010-12-01
Plates and mantle convection experience resistance to motion due to deformation associated with subduction. Parameterized models for thermal convection have attempted to incorporate this through the addition of a plate bending dissipation term in the energy balance (e.g. Conrad and Hager [1999, 2001]). However, these models assume that the plate velocity is approximately equal to the mantle flow velocity. This assumption loses validity for non-isoviscous convection. It leads to unreasonably high dissipation rates for plate bending in cases of strongly temperature dependent viscosity in which a sluggish or stagnant lid exists above a rapidly convecting mantle. We present an analytic parameterized model for thermal convection with a finite-strength plate and depth-dependent viscosity. The model allows for a convective flow in which the plate velocity can become small compared to mantle flow velocities. Our model predicts the plate velocity, plate thickness and heat flow, as well as the laterally averaged horizontal flow profile for a convective cell. The model reproduces the classic scaling laws when the plate is weak and the mantle isoviscous. The presence of an asthenosphere in our model allows for the asthenospheric flow channelization observed by Höink and Lenardic [2010]. Furthermore, our model is able to reproduce the plate velocity and flow profiles from their 2D and 3D numerical convection simulations. We find that the introduction of a strong plate has a significant impact on the behavior of the system. Solutions predicted by our model span several convective regimes: the mobile-lid, sluggish-lid and near stagnant-lid regimes. For some model parameters (mantle temperature, viscosities, material properties, etc) multiple solutions can occur. The existence of multiple solutions suggests that the system can move between states and/or that the convective state of the system is history dependent. Preliminary comparisons with 3D spherical numerical simulations
Structures, profile consistency, and transport scaling in electrostatic convection
Bian, N.H.; Garcia, O.E.
2005-04-15
Two mechanisms at the origin of profile consistency in models of electrostatic turbulence in magnetized plasmas are considered. One involves turbulent diffusion in collisionless plasmas and the subsequent turbulent equipartition of Lagrangian invariants. By the very nature of its definition, this state can only be reached in the absence of imposed fluxes of the transported quantities. As such, the concept of turbulent equipartition cannot be used to interpret profiles in numerical simulations of interchange modes, as it has nevertheless been done in the past. It is shown in this article that for interchange modes, profile consistency is in fact due to mixing by persistent large-scale convective cells. This mechanism is not a turbulent diffusion, cannot occur in collisionless systems, and is the analog of the well-known laminar 'magnetic flux expulsion' in magnetohydrodynamics. This expulsion process involves a 'pinch' across closed streamlines and further results in the formation of pressure fingers along the separatrix of the convective cells. By nature, these coherent structures are dissipative because the mixing process that leads to their formation relies on a finite amount of collisional diffusion. Numerical simulations of two-dimensional interchange modes confirm the role of laminar expulsion by convective cells for profile consistency and structure formation. They also show that the fingerlike pressure structures ultimately control the rate of heat transport across the plasma layer and thus the transport scaling at large Rayleigh numbers. This contradicts mixing-length arguments which do not account for collisional processes. For interchange modes, the problem of coherent structure formation, profile consistency, and transport scaling are thus intimately linked.
NASA Astrophysics Data System (ADS)
Touratier, F.; Goyet, C.; Houpert, L.; de Madron, X. Durrieu; Lefèvre, D.; Stabholz, M.; Guglielmi, V.
2016-07-01
The most active deep convection area in the western Mediterranean Sea is located in the Gulf of Lions. Recent studies in this area provides some insights on the complexity of the physical dynamics of convective regions, but very little is known about their impacts on the biogeochemical properties. The CASCADE (CAscading, Surge, Convection, Advection and Downwelling Events) cruise, planed in winter 2011, give us the opportunity to compare vertical profiles of properties sampled either during stratified conditions or after/during a convection event. In the present study, we focus on the distributions of the carbonate system properties (mainly total alkalinity, AT; and total dissolved inorganic carbon, CT) because, in the context of the climate change, deep convection areas are suspected to significantly increase the sequestration of anthropogenic CO2 (CANT). Given its limited size, the impact of the Mediterranean Sea on the global carbon budget is probably minor but this marginal sea can be used as a laboratory to better understand carbon sequestration and its transfer to the basin interior by deep convection processes. Distributions of AT and CT, both measured from bottle samples, and that of CANT (estimated with the TrOCA approach) are first analyzed in the light of other key properties (salinity, temperature, and dissolved oxygen). An objective interpolation procedure is then applied to estimate CT and AT from CTD measured properties. With this procedure, the vertical resolution goes from a maximum of 32 samples per station to one property estimate every meter (more detailed distributions are obtained). Results provide arguments to conclude that CANT is rapidly transferred to the deepest layer due to deep convection events. During deep convection events, the increase of CANT in the water column is positively correlated to that of potential density and oxygen content. The challenge of quantifying the amount of sequestered carbon is however not resolved due to the
A comparison of mantle convection models featuring plates
NASA Astrophysics Data System (ADS)
Stein, C.; Lowman, J. P.; Hansen, U.
2014-06-01
plates are an integral part of the Earth's mantle and thus play an important role in its dynamics and evolution. To allow plate behavior to arise naturally in numerical mantle convection models, self-consistent plate generation methods apply a fully rheological approach (featuring a temperature-, pressure- and stress-dependent viscosity). However, due to the extreme local viscosity changes that the self-generation of model plates entails, their computational requirements are demanding. Alternative plate modeling methods specify the existence of plates explicitly but can also obtain dynamically determined velocities (e.g., by employing a force-balance method). Here we present modifications to a force-balance model by utilizing a rheology-dependent viscosity profile. Accordingly, plate viscosity and plate thickness are no longer prescribed by the modeler but now follow as a dynamic consequence of the temperature and stress dependence of the viscosity and the model's evolution. We describe the new method and present benchmark results for a rheologically self-consistent mantle convection model and the modified force-balance plate model. Our results show that both plate modeling methods lead to the same system behavior for a wide range of system parameters making the new method a powerful tool to also achieve plate-like surface motion naturally.
Aliyu, Abubakar Sadiq; Ibrahim, Umar; Akpa, Chidozie Timothy; Garba, Nuraddeen Nasiru; Ramli, Ahmad Termizi
2015-01-01
terrestrial reference organisms are lichen and bryophytes. In all cases, the radio ecological risks are not likely to be discernible. This paper presents a pioneer data for ecological risk from ionizing contaminants due to mining activity in Nasarawa State, Nigeria. Its methodology could be adopted for future work on radioecology of mining. PMID:25848858
Aliyu, Abubakar Sadiq; Ibrahim, Umar; Akpa, Chidozie Timothy; Garba, Nuraddeen Nasiru; Ramli, Ahmad Termizi
2015-01-01
terrestrial reference organisms are lichen and bryophytes. In all cases, the radio ecological risks are not likely to be discernible. This paper presents a pioneer data for ecological risk from ionizing contaminants due to mining activity in Nasarawa State, Nigeria. Its methodology could be adopted for future work on radioecology of mining.
Current convective instability in detached divertor plasma
NASA Astrophysics Data System (ADS)
Krasheninnikov, S. I.; Smolyakov, A. I.
2016-09-01
The asymmetry of inner and outer divertors, which cause the inner divertor to detach first, while the outer one is still attached, results in the large temperature difference between the vicinities of inner and outer targets and the onset of large electric potential drop through detached plasma of the inner divertor. A large potential drop along with the inhomogeneity of the resistivity of detached plasma across the divertor leg drives the current convective instability in the inner divertor and subsequent fluctuations of radiation loss similar to that observed in experiments. The estimates of the frequency of plasma parameter fluctuations due to the current convective instability are in a reasonable agreement with experimental data. Once the outer divertor also detaches, the temperature difference between the vicinities of inner and outer targets disappears, and the driving force for the current convective instability, and resulting oscillations of radiation loss, vanishes. This feature is indeed observed in experiments.
Morris, D.G.; Wendel, M.W.; Chen, N.C.J.; Ruggles, A.E.; Cook, D.H.
1989-01-01
A study was conducted to examine decay heat removal requirements in the High Flux Isotope Reactor (HFIR) following shutdown from 85 MW. The objective of the study was to determine when forced flow through the core could be terminated without causing the fuel to melt. This question is particularly relevant when a station blackout caused by an external event is considered. Analysis of natural circulation in the core, vessel upper plenum, and reactor pool indicates that 12 h of forced flow will permit a safe shutdown with some margin. However, uncertainties in the analysis preclude conclusive proof that 12 h is sufficient. As a result of the study, two seismically qualified diesel generators were installed in HFIR. 9 refs., 4 figs.
Fingering convection in red giants revisited
NASA Astrophysics Data System (ADS)
Wachlin, F. C.; Vauclair, S.; Althaus, L. G.
2014-10-01
Context. Fingering (thermohaline) convection has been invoked for several years as a possible extra-mixing which could occur in red giant stars; it is due to the modification of the chemical composition induced by nuclear reactions in the hydrogen burning zone. Recent studies show, however, that this mixing is not sufficient to account for the needed surface abundances. Aims: A new prescription for fingering convection, based on 3D numerical simulations has recently been proposed. The resulting mixing coefficient is larger than those previously given in the literature. We compute models using this new coefficient and compare them to previous studies. Methods: We used the LPCODE stellar evolution code with a generalized version of the mixing length theory to compute red giant models and we introduce fingering convection using the BGS prescription. Results: The results show that, although the fingering zone now reaches the outer dynamical convective zone, the efficiency of the mixing is not enough to account for the observations. The fingering mixing coefficient should be increased by two orders of magnitude for the needed surface abundances to be reached. Conclusions: We confirm that fingering convection cannot be the mixing process needed to account for surface abundances in red giant branch stars.
Le couplage pulsation-convection.
NASA Astrophysics Data System (ADS)
Poyet, J.-P.
Contents: Quelques problèmes Boussinesq bien definis. Les théories de couplage pulsation radiale-convection. Quelques pas dans le domaine du couplage des pulsations non radiales avec la convection. Conclusion.
Dehbi, A.; Badreddine, H.
2012-07-01
In CFD simulations of flow mixing in a steam generator (SG) during natural circulation, one is faced with the problem of representing the thousands of SG U-tubes. Typically simplifications are made to render the problem computationally tractable. In particular, one or a number of tubes are lumped in one volume which is treated as a single porous medium. This approach dramatically reduces the computational size of the problem and hence simulation time. In this work, we endeavor to investigate the adequacy of this approach by performing two separate simulations of flow in a mock-up with 262 U-tubes, i.e. one in which the porous media model is used for the tube bundle, and another in which the full geometry is represented. In both simulations, the Reynolds Stress (RMS) model of turbulence is used. We show that in steady state conditions, the porous media treatment yields results which are comparable to those of the full geometry representation (temperature distribution, recirculation ratio, hot plume spread, etc). Hence, the porous media approach can be extended with a good degree of confidence to the full scale SG. (authors)
Marangoni convection and thermo-vibrational convection in two-layer liquid systems
NASA Astrophysics Data System (ADS)
Liu, Q. S.; Wang, A.; Zhou, J. Y.; Polezhaev, V. I.; Fedyushkin, A.; Yaremchuk, V. P.
The onset-instability and the generation of Marangoni convection and thermolvibrational convection induced simultaneously by thermocapillary force and high-frequency vibration in two-layer systems are investigated theoretically and numerically The effect of high-frequency translational harmonic vibrations on the onset of Marangoni convection in the system of two liquid layer with a non-deformable interface bounded by upper and lower solid walls maintained at constant temperatures is studied by using the methods of the linear stability analysis and the averaged convection equations The Krylov-Bogolyubov averaging method is applied to the generalized heat transport equation and Navier-Stokes equation with the Boussinesq approximation on the assumption that the vibration frequency is high and the velocity amplitude is finite A spectral numerical method Tau-Chebychev was used to resolve the eigenvalue problem for the linearized governing equations together with its boundary conditions of the two-layer Marangoni system with vibration In a two-layer Marangoni system with vibration the convection arises due to vibration and temperature dependence of the interfacial tension and their contributions are estimated by two important non-dimension parameters the vibration Rayleigh number Ra V and the Marangoni number Ma At onset of convection these parameters correspond to the critical values Ra V C Ma C with the critical temperature difference T C and the critical vibration amplitudes and frequency In this study we found some
Effects of chemistry on convective and non-convective precipitation over North Eastern North America
NASA Astrophysics Data System (ADS)
Mashayekhi, R.; Sloan, J. J.
2013-12-01
The change in convective and non-convective (microphysically-induced) precipitation due to the influence of chemistry - and particularly that of anthropogenic aerosols - is investigated in this study. The overall effect of chemistry is deduced from a comparison of the results from the Weather Research and Forecasting (WRF v3.4) model and its corresponding chemistry version (WRF/Chem v3.4). Simulations are conducted for a five-month period from April to August 2009 in a domain covering North Eastern North America with 12 km grid spacing. We created the temporally and spatially distributed anthropogenic emissions from area, point and mobile sources using the Sparse Matrix Operator Kernel Emissions (SMOKE v2.7) modeling system by processing the total annual county or province-based inventories for the U.S. and Canada using the appropriate temporal, chemical speciation and spatial surrogate cross-reference files. This study shows that convective precipitation dominates in the summer and in the southern part of the domain due to greater tropospheric instability in warmer periods. Non-convective precipitation becomes more significant during the spring, but it contributes much less in total rain. Both WRF and WRF/Chem models overpredict the mean total daily precipitation, with a positive bias that increases as the convective precipitation increases in warmer months. This appears to be a common problem with the prediction of convective precipitation; it is associated with its high spatial variability. The comparison of WRF/Chem results with those of WRF shows that a non-negligible change in both convective and cloud-resolved (non-convective) precipitation is caused by chemistry (including aerosols) over most parts of the domain. These changes can be attributed to both radiative and microphysical causes. A chemistry-induced change of approximately 15% is found in the five-month mean daily convective precipitation over areas with high convective rain. This can be traced to
NASA Astrophysics Data System (ADS)
Ezzat, M. A.; El-Bary, A. A.; Hatem, A. S.
2014-07-01
A technique of the state space approach and the inversion of the Laplace transform method are applied to dimensionless equations of an unsteady one-dimensional boundary-layer flow due to heat and mass transfer through a porous medium saturated with a viscoelastic fluid bounded by an infinite vertical plate in the presence of a uniform magnetic field is described. Complete analytical solutions for the temperature, concentration, velocity, and induced magnetic and electric fields are presented. The inversion of the Laplace transforms is carried out by using a numerical approach. The proposed method is used to solve two problems: boundary-layer flow in a viscoelastic fluid near a vertical wall subjected to the initial conditions of a stepwise temperature and concentration and viscoelastic fluid flow between two vertical walls. The solutions are found to be dependent on the governing parameters including the Prandtl number, the Schmidt number, the Grashof number, reaction rate coefficient, viscoelastic parameter, and permeability of the porous medium. Effects of these major parameters on the transport behavior are investigated methodically, and typical results are illustrated to reveal the tendency of the solutions. Representative results are presented for the velocity, temperature, concentration, and induced magnetic and electric field distributions, as well as the local skin-friction coefficient and the local Nusselt and Sherwood numbers.
Examining the Impact of Prandtl Number and Surface Convection Models on Deep Solar Convection
NASA Astrophysics Data System (ADS)
O'Mara, B. D.; Augustson, K.; Featherstone, N. A.; Miesch, M. S.
2015-12-01
effective turbulent Prandtl number of order unity. These results are found to be insensitive to the nature of the surface convection model.
Wave generation by turbulent convection
NASA Technical Reports Server (NTRS)
Goldreich, Peter; Kumar, Pawan
1990-01-01
Wave generation by turbulent convection in a plane parallel, stratified atmosphere lying in a gravitational field is studied. The turbulent spectrum is related to the convective energy flux via the Kolmogorov scaling and the mixing length hypothesis. Efficiencies for the conversion of the convective energy flux into both trapped and propagating waves are estimated.
Osmium isotopes and mantle convection.
Hauri, Erik H
2002-11-15
The decay of (187)Re to (187)Os (with a half-life of 42 billion years) provides a unique isotopic fingerprint for tracing the evolution of crustal materials and mantle residues in the convecting mantle. Ancient subcontinental mantle lithosphere has uniquely low Re/Os and (187)Os/(188)Os ratios due to large-degree melt extraction, recording ancient melt-depletion events as old as 3.2 billion years. Partial melts have Re/Os ratios that are orders of magnitude higher than their sources, and the subduction of oceanic or continental crust introduces into the mantle materials that rapidly accumulate radiogenic (187)Os. Eclogites from the subcontinental lithosphere have extremely high (187)Os/(188)Os ratios, and record ages as old as the oldest peridotites. The data show a near-perfect partitioning of Re/Os and (187)Os/(188)Os ratios between peridotites (low) and eclogites (high). The convecting mantle retains a degree of Os-isotopic heterogeneity similar to the lithospheric mantle, although its amplitude is modulated by convective mixing. Abyssal peridotites from the ocean ridges have low Os isotope ratios, indicating that the upper mantle had undergone episodes of melt depletion prior to the most recent melting events to produce mid-ocean-ridge basalt. The amount of rhenium estimated to be depleted from the upper mantle is 10 times greater than the rhenium budget of the continental crust, requiring a separate reservoir to close the mass balance. A reservoir consisting of 5-10% of the mantle with a rhenium concentration similar to mid-ocean-ridge basalt would balance the rhenium depletion of the upper mantle. This reservoir most likely consists of mafic oceanic crust recycled into the mantle over Earth's history and provides the material that melts at oceanic hotspots to produce ocean-island basalts (OIBs). The ubiquity of high Os isotope ratios in OIB, coupled with other geochemical tracers, indicates that the mantle sources of hotspots contain significant quantities
Anomalously weak solar convection
Hanasoge, Shravan M.; Duvall, Thomas L.
2012-01-01
Convection in the solar interior is thought to comprise structures on a spectrum of scales. This conclusion emerges from phenomenological studies and numerical simulations, though neither covers the proper range of dynamical parameters of solar convection. Here, we analyze observations of the wavefield in the solar photosphere using techniques of time-distance helioseismology to image flows in the solar interior. We downsample and synthesize 900 billion wavefield observations to produce 3 billion cross-correlations, which we average and fit, measuring 5 million wave travel times. Using these travel times, we deduce the underlying flow systems and study their statistics to bound convective velocity magnitudes in the solar interior, as a function of depth and spherical-harmonic degree ℓ. Within the wavenumber band ℓ < 60, convective velocities are 20–100 times weaker than current theoretical estimates. This constraint suggests the prevalence of a different paradigm of turbulence from that predicted by existing models, prompting the question: what mechanism transports the heat flux of a solar luminosity outwards? Advection is dominated by Coriolis forces for wavenumbers ℓ < 60, with Rossby numbers smaller than approximately 10-2 at r/R⊙ = 0.96, suggesting that the Sun may be a much faster rotator than previously thought, and that large-scale convection may be quasi-geostrophic. The fact that isorotation contours in the Sun are not coaligned with the axis of rotation suggests the presence of a latitudinal entropy gradient. PMID:22665774
Thermal Boundary Layer Equation for Turbulent Rayleigh-Bénard Convection
NASA Astrophysics Data System (ADS)
Ching, Emily Sc; Shishkina, Olga; Horn, Susanne; Wagner, Sebastian
Turbulent Rayleigh-Bénard convection, consisting of a fluid confined between two horizontal plates, heated from below and cooled from above, is a paradigm system for studying turbulent thermal convection, which is ubiquitous in nature. In turbulent Rayleigh-Bénard convection, there are viscous boundary layers near all rigid walls and two thermal boundary layers, one above the bottom plate and one below the top plate. The classical Prandtl-Blasius-Pohlhausen theory has often been used to describe the mean velocity and temperature boundary layer profiles but systematic deviations are known to exist. These deviations are due to turbulent fluctuations. In this talk, we report a new thermal boundary layer equation for turbulent Rayleigh-Bénard convection derived for Prandtl number (Pr) greater than 1, which takes into account the effects of turbulent fluctuations by using the idea of an eddy thermal diffusivity. Solving this equation, we have obtained two analytical mean temperature profiles for Pr ~ 1 and Pr >> 1 . These two theoretical predictions are shown to be in excellent agreement with the results of our direct numerical simulations for Pr=4.38 (water) and Pr=2547.9 (glycerol). Work of ESCC was supported by the Hong Kong Research Grants Council under Grant No. CUHK-400311.
Pattern of lobate scarps on Mercury's surface reproduced by a model of mantle convection
NASA Astrophysics Data System (ADS)
King, Scott D.
2008-04-01
Mercury is the smallest and least tectonically active of the terrestrial planets. Although Mercury's ancient, cratered surface resembles the Moon, it has the largest ratio of metallic core to silicate mantle among the terrestrial planets as well as an internal magnetic field. Images from the Mariner 10 spacecraft reveal lobate scarps, so called because of their curved or scalloped edges, which have been interpreted to be high-angle thrust faults resulting from a period of global contraction. A range of mechanisms has been invoked to explain the stresses leading to global contraction, including cooling and core formation, tidal effects due to gravitational interactions with the Sun, mantle convection and the impact that formed the Caloris basin. Here I present numerical simulations of the three-dimensional nature of convection within Mercury's silicate mantle. The model yields a regularly spaced pattern of convection, in which upwelling regions of the mantle assume linear, sheet-like shapes at low latitudes and a nearly hexagonal pattern near the poles. The distribution of resultant surface stresses is consistent with the observed pattern of lobate scarps, suggesting that the compressive features record an ancient pattern of mantle convection, in addition to global contraction. The gravity and topographic data returned from the MESSENGER 11 mission will help test this hypothesis.
Structure in turbulent thermal convection
NASA Astrophysics Data System (ADS)
Balachandar, S.
1992-12-01
Small-scale features of vorticity, strain rate, and temperature gradients are considered in a Rayleigh-Bénard convection. The results reported are from a direct numerical simulation of turbulent convection performed in a rectangular box of aspect ratio 2√2 at a Rayleigh number of 6.5×106 and a Prandtl number of 0.72. In agreement with earlier results [Ashurst et al., Phys. Fluids 30, 2343 (1987) and Ruetsch and Maxey, Phys. Fluids A 3, 1587 (1991)], the intermediate strain rate is on an average positive, but the ratio of alpha, beta, and gamma strain rates are measured to be 5.3:1.0:-6.3. This result differs from the earlier result of 3:1:-4 obtained in homogeneous isotropic and shear turbulences. Buoyancy-induced vorticity production makes significant contribution to the overall enstrophy balance, especially close to the boundaries. Vorticity production by buoyancy is exclusively in the horizontal direction and is balanced by preferred production by stretching and tilting in the vertical direction, due to the preferred alignment of extensional alpha strain rate with the vertical direction. Such directional alignment of vorticity, strain rate, and scalar gradient is explained on the basis of preferred spatial orientation of coherent structures in thermal turbulence.
NASA Astrophysics Data System (ADS)
Savarin, A.; Chen, S. S.
2014-12-01
The Madden-Julian Oscillation (MJO) is a dominant mode of intraseasonal variability in the tropics. It has wide-ranging impacts on global patterns of precipitation and surface temperature, and it has been suggested that it has an effect on ENSO. Large-scale convection fueling the MJO is initiated over the tropical Indian Ocean and propagates eastward across the Maritime Continent (MC) and into the western Pacific as a pattern of alternating phases of active and suppressed convection. As an eastward-propagating MJO convective event encounters the MC, its nature is altered due to the complex interactions with the landmass and topography as well as the warm coastal ocean. Previous studies have shown strong diurnal cycles over land and ocean, with distinct diurnal maxima of convection during the day and night, respectively. This complex air-sea-land interaction over the MC region and its impact on the large-scale convection and the MJO are still not well understood. This study aims to improve our understanding of the effects of the MC on the MJO as it propagates eastward from the Indian Ocean to the Pacific. We use the University of Miami Coupled Model (UMCM), a fully coupled atmosphere-wave-ocean model, to investigate the air-sea-land interaction processes and their impact on the large-scale convection associated with MJO. A control simulation of an MJO is carried out first using the DYNAMO observations from 2011 as model verification. To understand the effects of the MC on the MJO convection, coupled model experiments will be conducted by varying the island topography and/or altering the landmass with water surface. Analysis of the coupled model simulations will be compared with satellite observations such as the TRMM precipitation and cloud cluster tracking using hourly IR data. The outcome of this study is expected to provide some insights into important mechanisms that underlie the complex phenomenon.
Free convection in the Matian atmosphere
NASA Technical Reports Server (NTRS)
Clow, G. D.; Haberle, R. M.
1990-01-01
The 'free convective' regime for the Martian atmospheric boundary layer (ABL) was investigated. This state occurs when the mean windspeed at the top of the ABL drops below some critical value U(sub c) and positive buoyant forces are present. Such forces can arise either from vertical temperature or water vapor gradients across the atmospheric surface layer. During free convection, buoyant forces drive narrow plumes that ascend to the inversion height with a return circulation consisting of broad slower-moving downdraughts. Horizontal pressure, temperature, windspeed, and water vapor fluctuations resulting form this circulation pattern can be quite large adjacent to the ground (within the surface layer). The local turbulent fluctuations cause non-zero mean surface stresses, sensible heat fluxes, and latent heat fluxes, even when the mean regional windspeed is zero. Although motions above the surface layer are insensitive to the nature of the surface, the sensible and latent heat fluxes are primarily controlled by processes within the interfacial sublayer immediately adjacent to the ground during free convection. Thus the distinction between aerodynamically smooth and rough airflow within the interfacial sublayer is more important than for the more typical situation where the mean regional windspeed is greater than U(sub c). Buoyant forces associated with water vapor gradients are particularly large on Mars at low pressures and high temperatures when the surface relative humidity is 100 percent, enhancing the likelihood of free convection under these conditions. On this basis, Ingersol postulated the evaporative heat losses from an icy surface on Mars at 237 K and current pressures would exceed the available net radiative flux at the surface, thus prohibiting ice from melting at low atmospheric pressures. Schumann has developed equations describing the horizontal fluctuations and mean vertical gradients occurring during free convection. Schumann's model was
Finding the patterns in mantle convection
NASA Astrophysics Data System (ADS)
Atkins, Suzanne; Rozel, Antoine; Valentine, Andrew; Tackley, Paul; Trampert, Jeannot
2016-04-01
Inverting mantle flow for past configurations is one of the great outstanding problems in geodynamics. We demonstrate a new method for probabilistic inversion of present-day Earth observations for mantle properties and history. Convection is a non-linear and chaotic, thwarting most standard inversion methods. Because of its chaotic and unpredictable nature, small errors in initial conditions, parameter selection, and computational precision can all significantly change the results produced by mantle convection simulations. However, some patterns and statistics of convection contain the signature of the parameters used in the simulations over long time-scales. Geodynamical studies often vary these parameters to investigate their effects on the patterns produced. We show that with a large enough set of simulations, we can investigate the relationship between input parameters and convection patterns in a more rigorous way. Probabilistic inversion is the only way to approach highly non-linear problems. We use neural networks to represent the probability density function linking convection simulation input parameters and the patterns they produce. This allows us to find input parameters, whilst taking into account all of the uncertainties that are inherent in the inversion of any Earth system: how well do we understand the physics of the process; what do we already know about the input parameters; and how certain are our observations? We show that the mantle structures produced by 4.5 Gyr of convection simulations contain enough information on yield stress, viscosity coefficients, mantle heating rate, and the initial state of primordial material that we can infer them directly without requiring any other information, such as plate velocity.
Meniscus height controlled convective self-assembly
NASA Astrophysics Data System (ADS)
Choudhary, Satyan; Crosby, Alfred
Convective self-assembly techniques based on the 'coffee-ring effect' allow for the fabrication of materials with structural hierarchy and multi-functionality across a wide range of length scales. The coffee-ring effect describes deposition of non-volatiles at the edge of droplet due to capillary flow and pattern formations due to pinning and de-pinning of meniscus with the solvent evaporation. We demonstrate a novel convective self-assembly method which uses a piezo-actuated bending motion for driving the de-pinning step. In this method, a dilute solution of nanoparticles or polymers is trapped by capillary forces between a blade and substrate. As the blade oscillates with a fixed frequency and amplitude and the substrate translates at a fixed velocity, the height of the capillary meniscus oscillates. The meniscus height controls the contact angle of three phase contact line and at a critical angle de-pinning occurs. The combination of convective flux and continuously changing contact angle drives the assembly of the solute and subsequent de-pinning step, providing a direct means for producing linear assemblies. We demonstrate a new method for convective self-assembly at an accelerated rate when compared to other techniques, with control over deposit dimensions. Army Research Office (W911NF-14-1-0185).
Stellar evolution at high mass with convective core overshooting
NASA Technical Reports Server (NTRS)
Stothers, R. B.; Chin, C.-W.
1985-01-01
The transition from stellar evolution models with no convective core overshooting (CCO) at all to models in which homogeneous mixing due to CCO reaches far beyond the formal convective core boundary is systematically explored. Overshooting is parameterized in terms of the ratio d/H(p), where d is the distance of convective overshoot beyond the formal convective core boundary and H(p) is the local pressure scale height. It is concluded that CCO in very massive main sequence stars produces a great expansion of the stellar envelope if d/H(p) is large but not excessively large. CCO does not entirely suppress convective instability above the overshoot zone in the envelopes of main sequence stars more massive than about 15 solar masses. A general comparison of theoretically constructed isochrones for young stars with observed main sequence turnups indicates that the observed turnups are longer, brighter, and cooler at the tip than those expected on thfe basis of standard evolutionary theory.
NASA Astrophysics Data System (ADS)
Casey, James Kendall
Natural gas exploration and production has caused large scale changes to portions of the Arkansas landscape. Well pad site construction, access roads, and pipelines utilized to extract and transport natural gas have fragmented forested areas. The forest fragmentation resulting from these rapid changes could be contributing to the documented decline in nesting success of the wild turkey (Meleagris gallopavo). This study quantified temporal changes in forest fragmentation in terms of the number of forest patches, mean forest patch area, and forest edge length. The correlation between these fragmentation variables and nesting success data was explored to test the hypotheses of this study that 1) the number of forest patches is negatively correlated to nesting success, that 2) forest patch size is positively correlated to nesting success, and that 3) forest edge habitat length is negatively correlated to nesting success. There were 838 wells added within Van Buren County during the years 2000 through 2009. These wells resulted in a total forest loss of about 1.5% area from the initial inventory of forest in 2000. Pearson product moment correlation (PPMC) values ranging from -0.19 to 0.17 suggests relationships exist between poults per hen and forest fragmentation due to natural gas development. These PPMC values and their respective directions confirm the hypothesis. However, their p-values were all greater than 0.5 which suggests the correlations may not be statistically significant. A stronger regression model, giving adjusted R squared value of 0.766, was constructed which takes into account annual precipitation, previous year's wild turkey harvest, along with the number of conifer forest patches. This study concludes that the low wild turkey nesting success may not be directly influenced by forests lost due to natural gas development within the study area Van Buren County Arkansas.
Temperature-driven groundwater convection in cold climates
NASA Astrophysics Data System (ADS)
Engström, Maria; Nordell, Bo
2016-08-01
The aim was to study density-driven groundwater flow and analyse groundwater mixing because of seasonal changes in groundwater temperature. Here, density-driven convection in groundwater was studied by numerical simulations in a subarctic climate, i.e. where the water temperature was <4 °C. The effects of soil permeability and groundwater temperature (i.e. viscosity and density) were determined. The influence of impermeable obstacles in otherwise homogeneous ground was also studied. An initial disturbance in the form of a horizontal groundwater flow was necessary to start the convection. Transient solutions describe the development of convective cells in the groundwater and it took 22 days before fully developed convection patterns were formed. The thermal convection reached a maximum depth of 1.0 m in soil of low permeability (2.71 · 10-9 m2). At groundwater temperature close to its density maximum (4 °C), the physical size (in m) of the convection cells was reduced. Small stones or frost lenses in the ground slightly affect the convective flow, while larger obstacles change the size and shape of the convection cells. Performed simulations show that "seasonal groundwater turnover" occurs. This knowledge may be useful in the prevention of nutrient leakage to underlying groundwater from soils, especially in agricultural areas where no natural vertical groundwater flow is evident. An application in northern Sweden is discussed.
Idealized studies of convective summer precipitation in a cloud-resolving model
NASA Astrophysics Data System (ADS)
Schlemmer, Linda; Hohenegger, Cathy; Bretherton, Christopher; Schmidli, Jürg; Schär, Christoph
2010-05-01
Climate change is expected to moisten the atmosphere and to intensify the hydrological cycle. In the global mean, precipitation is projected to increase, but for Europe climate models suggest that mean summer precipitation will decrease. However, despite this decrease in mean, heavy precipitation events are projected to occur more frequently. The credibility of these projections, with decreases in mean amounts but increases in peak intensity, is somewhat limited, as convection is parameterized in current climate models due to its small-scale nature. Differences between climate models are especially large in summer, when synoptic-scale forcing is weak and the chosen model formulation has a great influence. Here we investigate the sensitivity of convection to ambient temperature and humidity profiles in a cloud-resolving model (CRM), using a spatial resolution of 500 m - 2 km. The modeling strategy includes an idealized set-up with explicit convection and a full set of parameterizations. The variables are relaxed towards the prescribed profiles and soil conditions, but the relaxation is weak in the lower troposphere and upper soil, such as to allow the development of a diurnal boundary layer. The model is run for 30 days, after which the diurnal cycle approximately repeats itself. Analysis is conducted for the last 15 days of the simulations. A systematic set of experiments with different stratification and humidity profiles is performed. We confirm that the temperature stratification of the environment has a dominant influence on the amount of precipitation by modifying the stability of the atmosphere and thereby the depth and intensity of convection. A more unstable stratification leads to deeper convective clouds and increased amounts of precipitation. In a more stable atmosphere convection remains shallow and precipitation amounts are small. The moisture profile influences the timing and duration of the precipitation period. Simulations with a drier atmosphere
Turbulent plumes in stellar convective envelopes.
NASA Astrophysics Data System (ADS)
Rieutord, M.; Zahn, J.-P.
1995-04-01
Recent numerical simulations of compressible convection in a stratified medium suggest that strong downwards directed flows may play an important role in stellar convective envelopes, both in the dynamics and in the energy transport. We transpose this idea to stellar convective envelopes by assuming that these plumes are turbulent plumes which may be described by Taylor's entrainment hypothesis, whose validity is well established in various geophysical conditions. We consider first the ideal case of turbulent plumes occurring in an isentropic atmosphere, and ignore all types of feedback. Thereafter we include the effect of the backflow generated by the plumes, and take into account the contribution of the radiative flux. The main result is that plumes originating from the upper layers of a star are able to reach the base of its convective envelope. Their number is necessarily limited because of their conical shape; the backflow further reduces their number to a maximum of about 1000. In these plumes the flux of kinetic energy is directed downwards, but it is less than the upwards directed enthalpy flux, so that the plumes always carry a net energy flux towards the surface. Our plume model is not applicable near the surface, where the departures from adiabaticity become important due to radiative leaking; therefore it cannot predict the depth of the convection zone, which is determined mainly by the transition from the radiative regime above to the nearly adiabatic conditions below. Neither does it permit to evaluate the extent of penetration, which strongly depends on the (unknown) number of plumes. We conclude that, to be complete, a phenomenological model of stellar convection must have a dual character: it should include both the advective transport through diving plumes, which is outlined in this paper, and the turbulent diffusion achieved by the interstitial medium. Only the latter process is apprehended by the familiar mixing-length treatment.