Evaluation of a convective downburst prediction application for India
NASA Astrophysics Data System (ADS)
Pryor, Kenneth L.; Johny, C. J.; Prasad, V. S.
2016-05-01
During the month of June 2015, the South Asian (or Southwest) monsoon advanced steadily from the southern to the northwestern states of India. The progression of the monsoon had an apparent effect on the relative strength of convective storm downbursts that occurred during June and July 2015. A convective downburst prediction algorithm, involving the Microburst Windspeed Potential Index (MWPI) and a satellite-derived three-band microburst risk product, and applied with meteorological geostationary satellite (KALPANA-1 VHRR and METEOSAT-7) and MODIS Aqua data, was evaluated and found to effectively indicate relative downburst intensity in both pre-monsoon and monsoon environments over various regions of India. The MWPI product, derived from T574L64 Global Forecast System (NGFS) model data, is being generated in real-time by National Center for Medium Range Weather Forecasting (NCMRWF), Ministry of Earth Sciences, India. The validation process entailed direct comparison of measured downburst-related wind gusts at airports and India Meteorological Department (IMD) observatories to adjacent MWPI values calculated from GFS and India NGFS model datasets. Favorable results include a statistically significant positive correlation between MWPI values and proximate measured downburst wind gusts with a confidence level near 100%. Case studies demonstrate the influence of the South Asian monsoon on convective storm environments and the response of the downburst prediction algorithm.
Modes of isolated, severe convective storm formation along the dryline
Bluestein, H.B.; Parker, S.S. )
1993-05-01
Patterns of the formation of isolated, severe convective storms along the dryline in the Southern plains of the United States during the spring over a 16-year period were determined from an examination of the evolution of radar echoes as depicted by WSR-57 microfilm data. It was found that in the first 30 min after the first echo, more than half of the radar echoes evolved into isolated storms as isolated cells from the start; others developed either from a pair of cells, from a line segment, from a cluster of cells, from the merger of mature cells, or from a squall line. Proximity soundings were constructed from both standard and special soundings, and from standard surface data. It was found that the estimated convective available potential energy and vertical shear are characteristic of the environment of supercell storms. The average time lag between the first echo and the first occurrence of severe weather of any type, or tornadoes alone, was approximately 2 h. There were no significant differences in the environmental parameters for the different modes of storm formation. 49 refs., 15 figs., 3 tabs.
Dryline on 22 May 2002 During IHOP: Convective Scale Measurements at the Profiling Site
NASA Technical Reports Server (NTRS)
Demoz, Belay; Flamant, Cyrille; Miller, David; Evans, Keith; Fabry, Federic; DiGirolamo, Paolo; Whiteman, David; Geerts, Bart; Weckwerth, Tammy; Brown, William
2004-01-01
A unique set of measurements of wind, water vapor mixing ratio and boundary layer height variability was observed during the first MOP dryline mission of 22 May 2002. Water vapor mixing ratio from the Scanning Raman Lidar (SRL), high-resolution profiles of aerosol backscatter from the HARLIE and wind profiles from the GLOW are combined with the vertical velocity derived from the NCAR/ISS/MAPR and the high-resolution FMCW radar to reveal the convective variability of the cumulus cloud-topped boundary layer. A combined analysis of the in-situ and remote sensing data from aircraft, radiosonde, lidars, and radars reveals moisture variability within boundary layer updraft and downdraft regions as well as characterizes the boundary layer height variability in the dry and moist sides of the dryline. The profiler site measurements will be tied to aircraft data to reveal the relative intensity and location of these updrafts to the dry line. This study provides unprecedented high temporal and spatial resolution measurements of wind, moisture and backscatter within a dryline and the associated convective boundary layer.
The Dryline on 22 May 2002 during IHOP_2002: Convective-Scale Measurements at the Profiling Site
NASA Technical Reports Server (NTRS)
Demoz, Belay; Flamant, Cyrille; Weckwerth, Tammy; Whiteman, David; Evans, Keith; Fabry, Frederic; DiGirolamo, Paolo; Miller, David; Geerts, Bart; Brown, William; Schwemmer, Geary; Gentry, Bruce; Feltz, Wayne; Wang, Zhien
2006-01-01
A detailed analysis of the structure of a double dryline observed over the Oklahoma panhandle during the first International H2O Project (IHOP_2002) convective initiation (CI) mission on 22 May 2002 is presented. A unique and unprecedented set of high temporal and spatial resolution measurements of water vapor mixing ratio, wind, and boundary layer structure parameters were acquired using the National Aeronautics and Space Administration (NASA) scanning Raman lidar (SRL), the Goddard Lidar Observatory for Winds (GLOW), and the Holographic Airborne Rotating Lidar Instrument Experiment (HARLIE), respectively. These measurements are combined with the vertical velocity measurements derived from the National Center for Atmospheric Research (NCAR) Multiple Antenna Profiler Radar (MAPR) and radar structure function from the high-resolution University of Massachusetts frequency-modulated continuous-wave (FMCW) radar to reveal the evolution and structure of the late afternoon double-dryline boundary layer. The eastern dryline advanced and then retreated over the Homestead profiling site in the Oklahoma panhandle, providing conditions ripe for a detailed observation of the small-scale variability within the boundary layer and the dryline. In situ aircraft data, dropsonde and radiosonde data, along with NCAR S-band dual-polarization Doppler radar (S-Pol) measurements, are also used to provide the larger-scale picture of the double-dryline environment. Moisture and temperature jumps of about 3 g kg(sup -1) and 1 -2 K, respectively, were observed across the eastern radar fine line (dryline), more than the moisture jumps (1-2 g kg(sup -1)) observed across the western radar fine line (secondary dryline). Most updraft plumes observed were located on the moist side of the eastern dryline with vertical velocities exceeding 3 m s(sup -1) and variable horizontal widths of 2-5 km, although some were as wide as 7-8 km. These updrafts were up to 1.5 g kg(sup -1) moister than the
Moisture, Wind, and Boundary Layer Evolution During a Dryline in IHOP-2002: May 22, 2002
NASA Technical Reports Server (NTRS)
Demoz, B.; Miller, D.; Evans, K.; Whiteman, D.; DiGirolamo, P.; Schwemmer, G.; Gentry, B.; Starr, D.; Wang, Z.
2003-01-01
As part of the International H2O Project (MOP-2002), three NASNGSFC lidars acquired high-resolution clear air data of wind, CBL evolution, and water vapor mixing ratio profiles during a dryline event that occurred on 22 May 2002. Together with the anciliary suite of instruments deployed in MOP-2002, these lidar data sets offer a unique look into the mesoscale evolution and convective scale dynamics in and around a dryline.
Leonardo da Vinci and the Downburst.
NASA Astrophysics Data System (ADS)
Gedzelman, Stanley David
1990-05-01
Evidence from the drawings, experiments, and writings of Leonardo da Vinci are presented to demonstrate that da Vinci recognized and, possibly, discovered the downburst and understood its associated airflow. Other early references to vortex flows resembling downbursts are mentioned.
Short range prediction and monitoring of downbursts over Indian region
NASA Astrophysics Data System (ADS)
Johny, C. J.; Prasad, V. S.; Singh, S. K.; Basu, Swati
2016-05-01
Convective downdraft motions and related outflow wind considered as an eventual source of potential damage which can be more severe in the aviation sector. A great variety of atmospheric environments can produce these downdraft motions. These events are not easily detectable using conventional weather radar or wind shear alert systems, while Doppler radars are useful for identifying these Downbursts. In order to identify the situations that can cause these downdraft events different diagnostic tools are designed. Recently launched Indian satellite INSAT-3D, with atmospheric sounder and imager on board, is capable of identifying regions of downburst occurrence and can help in monitoring them in real time. Some Downburst events reported over different parts of India, during January-April period is investigated using Microburst Wind Speed Potential Index (MWPI) and thermodynamic characteristics derived from the NCMRWF GFS (NGFS) model. An attempt is made to make a short range prediction of these events using MWPI computed from NGFS model forecasts. The results are validated with in-situ observations and also by employing INSAT-3D data and it is shown that the method has a reasonable success. All the investigated downdraft events are associated with the hybrid Microburst environment.
MCS precipitation and downburst intensity response to increased aerosol concentrations
NASA Astrophysics Data System (ADS)
Clavner, M.; Cotton, W. R.; van den Heever, S. C.
2015-12-01
Mesoscale convective systems (MCSs) are important contributors to rainfall in the High Plains of the United States as well as producers of severe weather such as hail, tornados and straight-line wind events known as derechos. Past studies have shown that changes in aerosol concentrations serving as cloud condensation nuclei (CCN) alter the MCS hydrometeor characteristics which in turn modify precipitation yield, downdraft velocity, cold-pool strength, storm propagation and the potential for severe weather to occur. In this study, the sensitivity of MCS precipitation characteristics and convective downburst velocities associated with a derecho to changes in CCN concentrations were examined by simulating a case study using the Regional Atmospheric Modeling System (RAMS). The case study of the 8 May 2009 "Super-Derecho" MCS was chosen since it produced a swath of widespread wind damage in association with an embedded large-scale bow echo, over a broad region from the High Plains of western Kansas to the foothills of the Appalachians. The sensitivity of the storm to changes in CCN concentrations was examined by conducting a set of three simulations which differed in the initial aerosol concentration based on output from the 3D chemical transport model, GEOS-Chem. Results from this study indicate that while increasing CCN concentrations led to an increase in precipitation rates, the changes to the derecho strength were not linear. A moderate increase in aerosol concentration reduced the derecho strength, while the simulation with the highest aerosol concentrations increased the derecho intensity. These changes are attributed to the impact of enhanced CCN concentration on the production of convective downbursts. An analysis of aerosol loading impacts on these MCS features will be presented.
Manual of downburst identification for Project NIMROD. [atmospheric circulation
NASA Technical Reports Server (NTRS)
Fujita, T. T.
1978-01-01
Aerial photography, Doppler radar, and satellite infrared imagery are used in the two year National Intensive Meteorological Research on Downburst (NIMROD) project to provide large area mapping of strong downdrafts that induce an outward burst of damaging winds over or near the earth. Topics discussed include scales of thunderstorm outflow; aerial photographs of downburst damage; microbursts and aviation hazards; radar echo characteristics; infrared imagery from GOES/SMS; and downburts-tornado relationships. Color maps of downbursts and tornadoes are included.
Spearhead echo and downburst in the crash of an airliner
NASA Technical Reports Server (NTRS)
Fujita, T. T.; Byers, H. R.
1977-01-01
Meteorological conditions leading to the crash of an airliner short of the runway of a New York airport were studied. Thunderstorm downdrafts much stronger than those measured on the 1946-47 Thunderstorm Project were found. These exceptional downdrafts have been designated as 'downbursts'. The violent cloud systems that produce downburst cells can be identified in the form of forward extensions of radar echoes designed as 'spearhead echoes' which move with unusual speed. The development of downburst cells appears to be tied in with overshooting tops of clouds at the anvil level.
Downbursts and microbursts - An aviation hazard. [downdrafts beneath thunderstorms
NASA Technical Reports Server (NTRS)
Fujita, T. T.
1980-01-01
Downburst and microburst phenomena occurring since 1975 are studied, based on meteorological analyses of aircraft accidents, aerial surveys of wind effects left behind downbursts, and studies of sub-mesoscale wind systems. It is concluded that microbursts beneath small, air mass thunderstorms are unpredictable in terms of weather forecast. Most aircraft incidents, however, were found to have occurred in the summer months, June through August. An intense microburst could produce 150 mph horizontal winds as well as 60 fps downflows at the tree-top level. The largest contributing factor to aircraft difficulties seemed to be a combination of the headwind decrease and the downflow. Anemometers and/or pressure sensors placed near runways were found effective for detecting gust fronts, but not for detecting downbursts. It is recommended that new detection systems placed on the ground or airborne, be developed, and that pilots be trained for simulated landing and go-around through microbursts.
Aircraft performance and control in downburst wind shear
NASA Technical Reports Server (NTRS)
Bray, Richard S.
1986-01-01
The methods developed for analyses of the winds and of aircraft performance during an investigation of a downburst wind-shear-induced accident have been utilized in a more general study of aircraft performance in such encounters. The computed responses of a generic, large transport aircraft to take-off and approach encounters with a downburst wind field were used in examining the effects of performance factors and control procedures on the ability of the aircraft to survive. Obvious benefits are seen for higher initial encounter speeds, maximum thrust-weight values typical of two-engined aircraft, and immediacy of pilot response. The results of controlling to a constant, predetermined, pitch attitude are demonstrated. Control algorithms that sacrifice altitude for speed appear to provide a higher level of survivability, but guidance displays more explicitly defining flightpath than those commonly in use might be required.
Comprehensive Analysis of Two Downburst-Related Aircraft Accidents
NASA Technical Reports Server (NTRS)
Shen, J.; Parks, E. K.; Bach, R. E.
1996-01-01
Although downbursts have been identified as the major cause of a number of aircraft takeoff and landing accidents, only the 1985 Dallas/Fort Worth (DFW) and the more recent (July 1994) Charlotte, North Carolina, landing accidents provided sufficient onboard recorded data to perform a comprehensive analysis of the downburst phenomenon. The first step in the present analysis was the determination of the downburst wind components. Once the wind components and their gradients were determined, the degrading effect of the wind environment on the airplane's performance was calculated. This wind-shear-induced aircraft performance degradation, sometimes called the F-factor, was broken down into two components F(sub 1) and F(sub 2), representing the effect of the horizontal wind gradient and the vertical wind velocity, respectively. In both the DFW and Charlotte cases, F(sub 1) was found to be the dominant causal factor of the accident. Next, the aircraft in the two cases were mathematically modeled using the longitudinal equations of motion and the appropriate aerodynamic parameters. Based on the aircraft model and the determined winds, the aircraft response to the recorded pilot inputs showed good agreement with the onboard recordings. Finally, various landing abort strategies were studied. It was concluded that the most acceptable landing abort strategy from both an analytical and pilot's standpoint was to hold constant nose-up pitch attitude while operating at maximum engine thrust.
NASA Technical Reports Server (NTRS)
Fujita, T. T.; Caracena, F.
1977-01-01
Three aircraft accidents are analyzed to gain an understanding of thunderstorm-related downbursts, or extremely rapid downdrafts, which interfered with takeoff or landing maneuvers in each of the three cases. For the purposes of this study, downbursts are defined as having downward speeds greater than 3.6 m/sec at 91 m altitude, and diameters of 800 m or greater. Few of the strongest downdrafts investigated reach the intensity of a downburst. The downburst cells mature about 5-10 minutes after formation, and are generally no more than 3-4 miles in diameter at maturity. A spearhead echo is found to be associated with each of the downburst-caused accidents.
NASA Astrophysics Data System (ADS)
Britz, Dieter
Convection has long been coupled with electrochemistry, and the name hydrodynamic voltammetry has become standard. In electroanalytical chemistry we mainly seek reproducible conditions. These are almost always attained by systems in which a steady convective state is achieved, although not always. Thus, the once popular dropping mercury electrode (see texts such as [74, 257]) has convection around it, but is never in steady state; it might be called a reproducible periodic dynamic state.
Mesoscale aspects of convective storms
NASA Technical Reports Server (NTRS)
Fujita, T. T.
1981-01-01
The structure, evolution and mechanisms of mesoscale convective disturbances are reviewed and observation techniques for "nowcasting" their nature are discussed. A generalized mesometeorological scale is given, classifying both low and high pressure systems. Mesoscale storms are shown often to induce strong winds, but their wind speeds are significantly less than those accompanied by submesoscale disturbances, such as tornadoes, downbursts, and microbursts. Mesoscale convective complexes, severe storm wakes, and flash floods are considered. The understanding of the evolution of supercells is essential for improving nowcasting capabilities and a very accurate combination of radar and satellite measurements is required.
NASA Technical Reports Server (NTRS)
Fujita, T. T.
1976-01-01
Radar echoes of a storm at John F. Kennedy International Airport are examined. Results regarding the phenomena presented suggest the existence of downburst cells. These cells are characterized by spearhead echoes. About 2% of the echoes in the New York area were spearhead echoes. The detection and identification of downburst cells, their potential hazard to approaching and landing aircraft, and communication of this information to the pilots of those aircraft are discussed.
Five scales of airflow associated with a series of downbursts on 16 July 1980
NASA Technical Reports Server (NTRS)
Fujita, T. T.; Wakimoto, R. M.
1981-01-01
An attempt is made to estimate wind speed in a series of windstorms, which occurred in a 50-km wide zone from Chicago to Detroit on July 16, 1980, based on three types of airborne objects: a 180 kg chimney, a 1000 kg corn storage bin, and lumber from damaged roofs. The maximum wind speed obtained is 63 + or - 10 m/sec, or 140 + or - 25 mph. SMS/GOES pictures show that the parent cloud was oval-shaped, with a lifetime in excess of 12 hours. That the downbursts began when overshooting activities subsided is indicated by the rapid shrinking of overshooting areas enclosed by -66 C isotherms at the onset of the Chicago-area downbursts. Cloud-top features and wind effects on the ground are presented with no attempt to relate them, on the basis of current conceptual models.
NASA Technical Reports Server (NTRS)
Forbes, G. S.; Wakimoto, R. M.
1983-01-01
A remarkable case of severe weather occurred near Springfield, Illinois on 6 August 1977. Aerial and ground surveys revealed that 17 cyclonic vortices, an anticyclonic vortex, 10 downbursts and 19 microbursts occurred in a limited (20 km x 40 km) area, associated with a bow-shaped radar echo. About half of the vortices appeared to have occurred along a gust front. Some of the others appear to have occurred within the circulation of a mesocyclone accompanying the bow echo, but these vortices seem to have developed specifically in response to localized boundary-layer vorticity generation associated with horizontal and vertical wind shears on the periphery of microbursts. Some of these vortices, and other destructive vortices in the literature, do not qualify as tornadoes as defined in the Glossary of Meteorology. A more pragmatic definition of a tornado is suggested.
Case study: A severe hailstorm and strong downbursts over northeastern Slovenia on June 16th 2009
NASA Astrophysics Data System (ADS)
Korosec, M.
2009-09-01
a bow echo and also satellite imagery showed signs of extremely severe storm as overshooting tops, "cold ring" and "U-shape" features were observed. References - Skywarn Austria forum: (http://www.skywarn.at/forum/) - EARS radar and SFC observations archive (http://www.arso.gov.si) - EARS article: Porocilo o neurjih 16. junija 2009 - OSMER FVG (http://www.meteo.fvg.it) - ESSL/ESWD database (www.essl.org) - ESTOFEX convective maps (www.estofex.org) - EUMETSAT satellite imagery (www.eumetsat.int) - 24ur.com/RTVSLO web portal (www.24ur.com, www.rtvslo.si) - Sobota Info web portal (www.sobotainfo.com) - Pomurje web portal (www.pomurje.si) - Administration of the Republic of Slovenia for Civil Protection and Disaster Relief, www.sos112.si - Worldwide Skew-t diagrams (http://weather.uwyo.edu/upperair/europe.html)
NASA Astrophysics Data System (ADS)
McCue, Mitchell Hollis
Using a 15-year (1995 to 2009) climatology of 1500 UTC warm-season (May through September) rawinsonde observation (RAOB) data from the Cape Canaveral Air Force Station (CCAFS) Skid Strip (KXMR) and 5 minute wind data from 36 wind towers on CCAFS and Kennedy Space Center (KSC), several convective wind forecasting techniques currently employed by the 45th Weather Squadron (45 WS) were evaluated. Present forecasting methods under evaluation include examining the vertical equivalent potential temperature (theta e) profile, vertical profiles of wind spend and direction, and several wet downburst forecasting indices. Although previous research found that currently used wet downburst forecasting methods showed little promise for forecasting convective winds, it was carried out with a very small sample, limiting the reliability of the results. Evaluation versus a larger 15-year dataset was performed to truly assess the forecasting utility of these methods in the central Florida warm-season convective environment. In addition, several new predictive analytic based forecast methods for predicting the occurrence of warm-season convection and its associated wind gusts were developed and validated. This research was performed in order to help the 45 WS better forecast not only which days are more likely to produce convective wind gusts, but also to better predict which days are more likely to yield warning criteria wind events of 35 knots or greater, should convection be forecasted. Convective wind forecasting is a very challenging problem that requires new statistically based modeling techniques since conventional meteorologically based methods do not perform well. New predictive analytic based forecasting methods were constructed using R statistical software and incorporate several techniques including multiple linear regression, logistic regression, multinomial logistic regression, classification and regression trees (CART), and ensemble CART using bootstrapping. All of
On Vortex Genesis. An Heuristic Model of Convection-Advection Linkages
NASA Astrophysics Data System (ADS)
Bouali, S.
2003-04-01
We connect to a model of convection roll an appropriate feedback loop (i.e., the advection retroaction) which unfolds a wide range of vorticity behavior. The 3D numerical computations display the conservative aerologic flows of waterspouts and tropical cyclones. Moreover, several simulations exhibit the singular topology of the structure of cyclones eventually trapped in period-2 orbit. On the other hand, additional advection linkages provide our dynamical system with an explicative proof of the tornadogenesis. These amendements modify the model. The airflows are now dissipatives and we investigate the kinematics of these short life span phenomena. The extended model leads the trajectories of air pockets to one or more vortices. The downbursts and microbursts described in the Fujita classification are also simulated. Our heuristic dynamical system lays the foundation of an unified modelisation of vortices. Theory and Direct Numerical Simulation of the vortex genesis are associated in a new perspective.
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
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.
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.
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…
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.
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.
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.
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, 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.
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
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
Convective dynamics - Panel report
NASA Technical Reports Server (NTRS)
Carbone, Richard; Foote, G. Brant; Moncrieff, Mitch; Gal-Chen, Tzvi; Cotton, William; Heymsfield, Gerald
1990-01-01
Aspects of highly organized forms of deep convection at midlatitudes are reviewed. Past emphasis in field work and cloud modeling has been directed toward severe weather as evidenced by research on tornadoes, hail, and strong surface winds. A number of specific issues concerning future thrusts, tactics, and techniques in convective dynamics are presented. These subjects include; convective modes and parameterization, global structure and scale interaction, convective energetics, transport studies, anvils and scale interaction, and scale selection. Also discussed are analysis workshops, four-dimensional data assimilation, matching models with observations, network Doppler analyses, mesoscale variability, and high-resolution/high-performance Doppler. It is also noted, that, classical surface measurements and soundings, flight-level research aircraft data, passive satellite data, and traditional photogrammetric studies are examples of datasets that require assimilation and integration.
Deep convection in mesoscale convective systems
NASA Technical Reports Server (NTRS)
Goodman, S. J.
1985-01-01
A study was undertaken to examine the evolution of radar echoes and lightning attending the convective storms in mesoscale convective systems (MCS) and the relationships between the spatial and temporal evolution of deep convection and the storm environment, precipitation, severe weather, and lightning. The total number of ground discharges ranges from 10,000 to 30,000 over the life cycle of the MCS with peak sustained rates (for up to 10 consecutive hours) in excess of 2000 per hour. The peak lightning activity occurs from 5 to 20 hours after the first storms and anywhere from 7 hours prior to 7 hours after the time of the maximum areal extent of the MCS for very similar synoptic environments. Thus, it appears that mesoscale and sub-synoptic scale mechanisms are responsible for these large temporal variation in lightning activity. In addition, we have found that the lightning rates in MCS's are not related to either the size or the duration of the MCS. Preliminary results suggest that the MCA's with embedded squall lines produce the greatest flash rates.
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
Thermodynamics of convective circulations
NASA Astrophysics Data System (ADS)
Adams, D. K.; Renno, N. O.
2003-04-01
The heat engine framework has proven successful for studies of atmospheric phenomena ranging from small to large scales. At large scales, the heat engine framework provides estimates of convective available potential energy, convective velocities, and fractional area covered by convection. At the smaller end of the spectrum, the framework provides estimates of the intensity of convective vortices such as dust devils and waterspouts. The heat engine framework sheds light on the basic physics of planetary atmospheres. In particular, it allows the calculation of their thermodynamic efficiency. Indeed, this is a fundamental number for atmospheric circulations because it quantifies the amount of heat that is converted into kinetic energy. As such, it is a valuable number not only for comparison of models with nature, but also for the intercomparison of models. In the present study, we generalize the heat engine framework to large-scale circulations, both open (e.g., the Hadley circulation) and closed (e.g., the general circulation) and apply it to an idealized global climate model to ascertain the thermodynamic efficiency of model circulations, both global and regional. Our results show that the thermodynamic efficiency is sensitive to model resolution and provides a baseline for minimum model resolution in climate studies. The value of the thermodynamic efficiency of convective circulations in nature is controversial. It has been suggested that both nature and numerical models are extremely irreversible. We show that both the global and the Hadley circulation of the idealized model are, to a first approximation, reversible.
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.
Simulating deep convection with a shallow convection scheme
NASA Astrophysics Data System (ADS)
Hohenegger, C.; Bretherton, C. S.
2011-03-01
Convective processes profoundly affect the global water and energy balance of our planet but remain a challenge for global climate modeling. Here we develop and investigate the suitability of a unified convection scheme, capable of handling both shallow and deep convection, to simulate cases of tropical oceanic convection, mid-latitude continental convection, and maritime shallow convection. To that aim, we employ large-eddy simulations (LES) as a benchmark to test and refine a unified convection scheme implemented in the Single-Column Community Atmosphere Model (SCAM). Our approach is motivated by previous cloud-resolving modeling studies, which have documented the gradual transition between shallow and deep convection and its possible importance for the simulated precipitation diurnal cycle. Analysis of the LES reveals that differences between shallow and deep convection, regarding cloud-base properties as well as entrainment/detrainment rates, can be related to the evaporation of precipitation. Parameterizing such effects and accordingly modifying the University of Washington shallow convection scheme, it is found that the new unified scheme can represent both shallow and deep convection as well as tropical and continental convection. Compared to the default SCAM version, the new scheme especially improves relative humidity, cloud cover and mass flux profiles. The new unified scheme also removes the well-known too early onset and peak of convective precipitation over mid-latitude continental areas.
Simulating deep convection with a shallow convection scheme
NASA Astrophysics Data System (ADS)
Hohenegger, C.; Bretherton, C. S.
2011-10-01
Convective processes profoundly affect the global water and energy balance of our planet but remain a challenge for global climate modeling. Here we develop and investigate the suitability of a unified convection scheme, capable of handling both shallow and deep convection, to simulate cases of tropical oceanic convection, mid-latitude continental convection, and maritime shallow convection. To that aim, we employ large-eddy simulations (LES) as a benchmark to test and refine a unified convection scheme implemented in the Single-column Community Atmosphere Model (SCAM). Our approach is motivated by previous cloud-resolving modeling studies, which have documented the gradual transition between shallow and deep convection and its possible importance for the simulated precipitation diurnal cycle. Analysis of the LES reveals that differences between shallow and deep convection, regarding cloud-base properties as well as entrainment/detrainment rates, can be related to the evaporation of precipitation. Parameterizing such effects and accordingly modifying the University of Washington shallow convection scheme, it is found that the new unified scheme can represent both shallow and deep convection as well as tropical and mid-latitude continental convection. Compared to the default SCAM version, the new scheme especially improves relative humidity, cloud cover and mass flux profiles. The new unified scheme also removes the well-known too early onset and peak of convective precipitation over mid-latitude continental areas.
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.
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 Astrophysics Data System (ADS)
Arnett, W. David
2009-05-01
We review recent progress using numerical simulations as a testbed for development of a theory of stellar convection, much as envisaged by John von Newmann. Necessary features of the theory, non-locality and fluctuations, are illustrated by computer movies. It is found that the common approximation of convection as a diffusive process presents the wrong physical picture, and improvements are suggested. New observational results discussed at the conference are gratifying in their validation of some of our theoretical ideas, especially the idea that SNIb and SNIc events are related to the explosion of massive star cores which have been stripped by mass loss and binary interactions [1
Convection Compensated Electrophoretic NMR
NASA Astrophysics Data System (ADS)
He, Qiuhong; Wei, Zhaohui
2001-06-01
A novel method of convection compensated ENMR (CC-ENMR) has been developed to detect electrophoretic motion of ionic species in the presence of bulk solution convection. This was accomplished using a gradient moment nulling technique to remove spectral artifacts from heat-induced convection and using the polarity switch of the applied electric field to retain spin phase modulations due to electrophoretic flow. Experiments were carried out with a mixture of 100 mM L-aspartic acid and 100 mM 4,9-dioxa-1,12-dodecanediamine to demonstrate this new method of ENMR. CC-ENMR enhances our previously developed capillary array ENMR (CA-ENMR) in solving the convection problem. The combined CA- and CC-ENMR approach strengthens the potential of multidimensional ENMR in simultaneous structural determination of coexisting proteins and protein conformations in biological buffer solutions of high ionic strength. Structural mapping of interacting proteins during biochemical reactions becomes possible in the future using ENMR techniques, which may have a profound impact on the understanding of biological events, including protein folding, genetic control, and signal transduction in general.
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.
Combined buoyancy-thermocapillary convection
NASA Technical Reports Server (NTRS)
Homsy, G. M.
1990-01-01
Combined buoyancy-thermocapillary convection was studied in 2D and 3D. Fluid motion caused by thermally induced tension gradients on the free surface of a fluid is termed thermocapillary convection. It is well-known that in containerless processing of materials in space, thermocapillary convection is a dominant mechanism of fluid flow. Welding and crystal growth processes are terrestrial applications where thermocapillary convection has direct relevance.
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
Interaction Between Convection and Pulsation
NASA Astrophysics Data System (ADS)
Houdek, Günter; Dupret, Marc-Antoine
2015-12-01
This article reviews our current understanding of modelling convection dynamics in stars. Several semi-analytical time-dependent convection models have been proposed for pulsating one-dimensional stellar structures with different formulations for how the convective turbulent velocity field couples with the global stellar oscillations. In this review we put emphasis on two, widely used, time-dependent convection formulations for estimating pulsation properties in one-dimensional stellar models. Applications to pulsating stars are presented with results for oscillation properties, such as the effects of convection dynamics on the oscillation frequencies, or the stability of pulsation modes, in classical pulsators and in stars supporting solar-type oscillations.
Thermocapillary Convection in Liquid Droplets
NASA Technical Reports Server (NTRS)
1986-01-01
The purpose of this video is to understand the effects of surface tension on fluid convection. The fluid system chosen is the liquid sessile droplet to show the importance in single crystal growth, the spray drying and cooling of metal, and the advance droplet radiators of the space stations radiators. A cross sectional representation of a hemispherical liquid droplet under ideal conditions is used to show internal fluid motion. A direct simulation of buoyancy-dominant convection and surface tension-dominant convection is graphically displayed. The clear differences between two mechanisms of fluid transport, thermocapillary convection, and bouncy dominant convection is illustrated.
Oxygen abundance and convection
NASA Astrophysics Data System (ADS)
Van't Veer, C.; Cayrel, R.
The triplet IR lines of O I near 777 nm are computed with the Kurucz's code, modified to accept several convection models. The program has been run with the MLT algorithm, with l/H = 1.25 and 0.5, and with the Canuto-Mazzitelli and Canuto-Goldman-Mazzitelli approaches, on a metal-poor turnoff-star model atmosphere with Teff=6200 K, log g = 4.3, [Fe/H]= -1.5. The results show that the differences in equivalent widths for the 4 cases do not exceed 2 per cent (0.3 mA). The convection treatment is therefore not an issue for the oxygen abundance derived from the permitted lines.
Granular convection in microgravity.
Murdoch, N; Rozitis, B; Nordstrom, K; Green, S F; Michel, P; de Lophem, T-L; Losert, W
2013-01-01
We investigate the role of gravity on convection in a dense granular shear flow. Using a microgravity-modified Taylor-Couette shear cell under the conditions of parabolic flight microgravity, we demonstrate experimentally that secondary, convective-like flows in a sheared granular material are close to zero in microgravity and enhanced under high-gravity conditions, though the primary flow fields are unaffected by gravity. We suggest that gravity tunes the frictional particle-particle and particle-wall interactions, which have been proposed to drive the secondary flow. In addition, the degree of plastic deformation increases with increasing gravitational forces, supporting the notion that friction is the ultimate cause. PMID:23383851
NASA Technical Reports Server (NTRS)
Bachmann, Kurt T.
2000-01-01
I helped to complete a research project with NASA scientists Dr. David Hathaway (my mentor), Rick Bogart, and John Beck from the SOHO/SOI collaboration. Our published paper in 'Solar Physics' was titled 'The Solar Convection Spectrum' (April 2000). Two of my undergraduate students were named on the paper--Gavrav Khutri and Josh Petitto. Gavrav also wrote a short paper for the National Conference of Undergraduate Research Proceedings in 1998 using a preliminary result. Our main result was that we show no evidence of a scale of convection named 'mesogranulation'. Instead, we see only direct evidence for the well-known scales of convection known as graduation and supergranulation. We are also completing work on vertical versus horizontal flow fluxes at the solar surface. I continue to work on phase relationships of solar activity indicators, but I have not yet written a paper with my students on this topic. Along with my research results, I have developed and augmented undergraduate courses at Birmingham-Southern College by myself and with other faculty. We have included new labs and observations, speakers from NASA and elsewhere, new subject material related to NASA and space science. I have done a great deal of work in outreach, mostly as President and other offices in the Birmingham Astronomical Society. My work includes speaking, attracting speakers, giving workshops, and governing.
Albarède, Francis; Van Der Hilst, Rob D
2002-11-15
We review the present state of our understanding of mantle convection with respect to geochemical and geophysical evidence and we suggest a model for mantle convection and its evolution over the Earth's history that can reconcile this evidence. Whole-mantle convection, even with material segregated within the D" region just above the core-mantle boundary, is incompatible with the budget of argon and helium and with the inventory of heat sources required by the thermal evolution of the Earth. We show that the deep-mantle composition in lithophilic incompatible elements is inconsistent with the storage of old plates of ordinary oceanic lithosphere, i.e. with the concept of a plate graveyard. Isotopic inventories indicate that the deep-mantle composition is not correctly accounted for by continental debris, primitive material or subducted slabs containing normal oceanic crust. Seismological observations have begun to hint at compositional heterogeneity in the bottom 1000 km or so of the mantle, but there is no compelling evidence in support of an interface between deep and shallow mantle at mid-depth. We suggest that in a system of thermochemical convection, lithospheric plates subduct to a depth that depends - in a complicated fashion - on their composition and thermal structure. The thermal structure of the sinking plates is primarily determined by the direction and rate of convergence, the age of the lithosphere at the trench, the sinking rate and the variation of these parameters over time (i.e. plate-tectonic history) and is not the same for all subduction systems. The sinking rate in the mantle is determined by a combination of thermal (negative) and compositional buoyancy and as regards the latter we consider in particular the effect of the loading of plates with basaltic plateaux produced by plume heads. Barren oceanic plates are relatively buoyant and may be recycled preferentially in the shallow mantle. Oceanic plateau-laden plates have a more pronounced
Rotating convection in elliptical geometries
NASA Astrophysics Data System (ADS)
Evonuk, M.
2014-12-01
Tidal interactions between hot jupiter planets and their host stars are likely to result in non-spherical geometries. These elliptical instabilities may have interesting effects on interior fluid convective patterns, which in turn influence the nature of the magnetic dynamo within these planets. Simulations of thermal convection in the 2D rotating equatorial plane are conducted to determine to first order the effect of ellipticity on convection for varying density contrasts with differing convective vigor and rotation rate. This survey is conducted in two dimensions in order to simulate a broad range of ellipticities and to maximize the parameter space explored.
An introduction to thermal convection
NASA Astrophysics Data System (ADS)
Rieutord, M.
In this lecture I propose a little tour of thermal convection and its applications in astrophysics. The first part of the lecture is devoted to a qualitative introduction to the convective instability using the Schwarzschild criterion; then, concentrating on the equations governing the fluid motions, I introduce the Boussinesq and anelastic approximations which are so often used in these problems. The following part focuses on the Rayleigh-Bénard model which is worked out in detail up to the Landau equation and the Lorenz strange attractor. Finally, I briefly sketch out some results on turbulent convection and end the lecture with the case of stellar convection.
Modeling ocean deep convection
NASA Astrophysics Data System (ADS)
Canuto, V. M.; Howard, A.; Hogan, P.; Cheng, Y.; Dubovikov, M. S.; Montenegro, L. M.
The goal of this study is to assess models for Deep Convection with special emphasis on their use in coarse resolution ocean general circulation models. A model for deep convection must contain both vertical transport and lateral advection by mesoscale eddies generated by baroclinic instabilities. The first process operates mostly in the initial phases while the second dominates the final stages. Here, the emphasis is on models for vertical mixing. When mesoscales are not resolved, they are treated with the Gent and McWilliams parameterization. The model results are tested against the measurements of Lavender, Davis and Owens, 2002 (LDO) in the Labrador Sea. Specifically, we shall inquire whether the models are able to reproduce the region of " deepest convection," which we shall refer to as DC (mixed layer depths 800-1300 m). The region where it was measured by Lavender et al. (2002) will be referred to as the LDO region. The main results of this study can be summarized as follows. 3° × 3° resolution. A GFDL-type OGCM with the GISS vertical mixing model predicts DC in the LDO region where the vertical heat diffusivity is found to be 10 m 2 s -1, a value that is quite close to the one suggested by heuristic studies. No parameter was changed from the original GISS model. However, the GISS model also predicts some DC in a region to the east of the LDO region. 3° × 3° resolution. A GFDL-type OGCM with the KPP model (everything else being the same) does not predict DC in the LDO region where the vertical heat diffusivity is found to be 0.5 × 10 -4 m 2 s -1 which is the background value. The KPP model yields DC only to the east of the LDO region. 1° × 1° resolution. In this case, a MY2.5 mixing scheme predicts DC in the LDO region. However, it also predicts DC to the west, north and south of it, where it is not observed. The behavior of the KPP and MY models are somewhat anti-symmetric. The MY models yield too low a mixing in stably stratified flows since they
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.
Modeling ocean deep convection
NASA Astrophysics Data System (ADS)
Canuto, V. M.; Howard, A.; Hogan, P.; Cheng, Y.; Dubovikov, M. S.; Montenegro, L. M.
The goal of this study is to assess models for Deep Convection with special emphasis on their use in coarse resolution ocean general circulation models. A model for deep convection must contain both vertical transport and lateral advection by mesoscale eddies generated by baroclinic instabilities. The first process operates mostly in the initial phases while the second dominates the final stages. Here, the emphasis is on models for vertical mixing. When mesoscales are not resolved, they are treated with the Gent and McWilliams parameterization. The model results are tested against the measurements of Lavender, Davis and Owens, 2002 (LDO) in the Labrador Sea. Specifically, we shall inquire whether the models are able to reproduce the region of " deepest convection," which we shall refer to as DC (mixed layer depths 800-1300 m). The region where it was measured by Lavender et al. (2002) will be referred to as the LDO region. The main results of this study can be summarized as follows. 3° × 3° resolution. A GFDL-type OGCM with the GISS vertical mixing model predicts DC in the LDO region where the vertical heat diffusivity is found to be 10 m 2 s -1, a value that is quite close to the one suggested by heuristic studies. No parameter was changed from the original GISS model. However, the GISS model also predicts some DC in a region to the east of the LDO region. 3° × 3° resolution. A GFDL-type OGCM with the KPP model (everything else being the same) does not predict DC in the LDO region where the vertical heat diffusivity is found to be 0.5 × 10 -4 m 2 s -1 which is the background value. The KPP model yields DC only to the east of the LDO region. 1° × 1° resolution. In this case, a MY2.5 mixing scheme predicts DC in the LDO region. However, it also predicts DC to the west, north and south of it, where it is not observed. The behavior of the KPP and MY models are somewhat anti-symmetric. The MY models yield too low a mixing in stably stratified flows since they
NASA Astrophysics Data System (ADS)
Pasetto, Stefano; Chiosi, Cesare; Cropper, Mark; Grebel, Eva K.
2015-08-01
Convection is one of the fundamental mechanism to transport energy, e.g., in planetology, oceanography as well as in astrophysics where stellar structure customarily described by the mixing-length theory, which makes use of the mixing-length scale parameter to express the convective flux, velocity, and temperature gradients of the convective elements and stellar medium. The mixing-length scale is taken to be proportional to the local pressure scale height of the star, and the proportionality factor (the mixing-length parameter) must be determined by comparing the stellar models to some calibrator, usually the Sun.No strong arguments exist to claim that the mixing-length parameter is the same in all stars and all evolutionary phases. Because of this, all stellar models in literature are hampered by this basic uncertainty.In a recent paper (Pasetto et al 2014) we presented the first fully analytical scale-free theory of convection that does not require the mixing-length parameter. Our self-consistent analytical formulation of convection determines all the properties of convection as a function of the physical behaviour of the convective elements themselves and the surrounding medium (being it a either a star, an ocean, a primordial planet). The new theory of convection is formulated starting from a conventional solution of the Navier-Stokes/Euler equations, i.e. the Bernoulli equation for a perfect fluid, but expressed in a non-inertial reference frame co-moving with the convective elements. In our formalism, the motion of convective cells inside convective-unstable layers is fully determined by a new system of equations for convection in a non-local and time dependent formalism.We obtained an analytical, non-local, time-dependent solution for the convective energy transport that does not depend on any free parameter. The predictions of the new theory in astrophysical environment are compared with those from the standard mixing-length paradigm in stars with
α Centauri and convection theories.
NASA Astrophysics Data System (ADS)
Fernandes, J.; Neuforge, C.
1995-03-01
The metallicity of the alpha Centauri system, Z, suffers from uncertainties. For this reason, different methods are used to calibrate the system: calibrations performed in YALE (Edmonds et al. 1992) use a fixed value for Z: Z=0.026 and a convection parameter for each star, while those made in Meudon and Liege (Noels et al. 1991; Neuforge 1993a) make the hypothesis of a unique convection parameter for the two components of the system and consider Z as a free parameter. We discuss these two techniques, both using models calculated with mixing length convection theory, (MLT), and we explain our solution through the behaviour of the convection parameter with chemical composition. We also compare our results with those of Lydon (1993) and find consistency. With a precise observational value of Z, of the effective temperatures and of the luminosities, our results provide a test for the unicity of α, if, in the frame of the same physics, a precise atmosphere treatment can be used and low-temperature opacities are known with sufficient accuracy. Finally, we perform calibrations with models calculated with the convection treatment of Canuto & Mazzitelli (1991, 1992), where we use {LAMBDA}=z, z being the distance to the top of the convective envelope. We avoid thus problems raised by the MLT convection parameter. In this frame, satisfactory solutions can be found for 0.024<=Z<=0.040.
Convection in Type 2 supernovae
Miller, D.S.
1993-10-15
Results are presented here from several two dimensional numerical calculations of events in Type II supernovae. A new 2-D hydrodynamics and neutrino transport code has been used to compute the effect on the supernova explosion mechanism of convection between the neutrinosphere and the shock. This convection is referred to as exterior convection to distinguish it from convection beneath the neutrinosphere. The model equations and initial and boundary conditions are presented along with the simulation results. The 2-D code was used to compute an exterior convective velocity to compare with the convective model of the Mayle and Wilson 1-D code. Results are presented from several runs with varying sizes of initial perturbation, as well as a case with no initial perturbation but including the effects of rotation. The M&W code does not produce an explosion using the 2-D convective velocity. Exterior convection enhances the outward propagation of the shock, but not enough to ensure a successful explosion. Analytic estimates of the growth rate of the neutron finger instability axe presented. It is shown that this instability can occur beneath the neutrinosphere of the proto-neutron star in a supernova explosion with a growth time of {approximately} 3 microseconds. The behavior of the high entropy bubble that forms between the shock and the neutrinosphere in one dimensional calculations of supernova is investigated. It has been speculated that this bubble is a site for {gamma}-process generation of heavy elements. Two dimensional calculations are presented of the time evolution of the hot bubble and the surrounding stellar material. Unlike one dimensional calculations, the 2D code fails to achieve high entropies in the bubble. When run in a spherically symmetric mode the 2-D code reaches entropies of {approximately} 200. When convection is allowed, the bubble reaches {approximately} 60 then the bubble begins to move upward into the cooler, denser material above it.
Nonlinear Convection in Mushy Layers
NASA Technical Reports Server (NTRS)
Worster, M. Grae; Anderson, Daniel M.; Schulze, T. P.
1996-01-01
When alloys solidify in a gravitational field there are complex interactions between solidification and natural, buoyancy-driven convection that can alter the composition and impair the structure of the solid product. The particular focus of this project has been the compositional convection within mushy layers that occurs in situations where the lighter component of the alloy is rejected into the melt during solidification by cooling from below. The linear stability of such a situation was previously described and has been further elucidated in a number of published articles. Here we describe some recent developments in the study of nonlinear evolution of convection in mushy layers.
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.
Convective adjustment in baroclinic atmospheres
NASA Technical Reports Server (NTRS)
Emanuel, Kerry A.
1986-01-01
Local convection in planetary atmospheres is generally considered to result from the action of gravity on small regions of anomalous density. That in rotating baroclinic fluids the total potential energy for small scale convection contains a centrifugal as well as a gravitational contribution is shown. Convective adjustment in such an atmosphere results in the establishment of near adiabatic lapse rates of temperature along suitably defined surfaces of constant angular momentum, rather than in the vertical. This leads in general to sub-adiabatic vertical lapse rates. That such an adjustment actually occurs in the earth's atmosphere is shown by example and the magnitude of the effect for several other planetary atmospheres is estimated.
Dynamics of convective scale interaction
NASA Technical Reports Server (NTRS)
Purdom, James F. W.; Sinclair, Peter C.
1988-01-01
Several of the mesoscale dynamic and thermodynamic aspects of convective scale interaction are examined. An explanation of how sounding data can be coupled with satellite observed cumulus development in the warm sector and the arc cloud line's time evolution to develop a short range forecast of expected convective intensity along an arc cloud line. The formative, mature and dissipating stages of the arc cloud line life cycle are discussed. Specific properties of convective scale interaction are presented and the relationship between arc cloud lines and tornado producing thunderstorms is considered.
Dunn, J.C.; Hardee, H.C.; Striker, R.P.
1984-01-09
A convective heat flow probe device is provided which measures heat flow and fluid flow magnitude in the formation surrounding a borehole. The probe comprises an elongate housing adapted to be lowered down into the borehole; a plurality of heaters extending along the probe for heating the formation surrounding the borehole; a plurality of temperature sensors arranged around the periphery of the probe for measuring the temperature of the surrounding formation after heating thereof by the heater elements. The temperature sensors and heater elements are mounted in a plurality of separate heater pads which are supported by the housing and which are adapted to be radially expanded into firm engagement with the walls of the borehole. The heat supplied by the heater elements and the temperatures measured by the temperature sensors are monitored and used in providing the desired measurements. The outer peripheral surfaces of the heater pads are configured as segments of a cylinder and form a full cylinder when taken together. A plurality of temperature sensors are located on each pad so as to extend along the length and across the width thereof, with a heating element being located in each pad beneath the temperature sensors. An expansion mechanism driven by a clamping motor provides expansion and retraction of the heater pads and expandable packet-type seals are provided along the probe above and below the heater pads.
Dunn, James C.; Hardee, Harry C.; Striker, Richard P.
1985-01-01
A convective heat flow probe device is provided which measures heat flow and fluid flow magnitude in the formation surrounding a borehole. The probe comprises an elongate housing adapted to be lowered down into the borehole; a plurality of heaters extending along the probe for heating the formation surrounding the borehole; a plurality of temperature sensors arranged around the periphery of the probe for measuring the temperature of the surrounding formation after heating thereof by the heater elements. The temperature sensors and heater elements are mounted in a plurality of separate heater pads which are supported by the housing and which are adapted to be radially expanded into firm engagement with the walls of the borehole. The heat supplied by the heater elements and the temperatures measured by the temperature sensors are monitored and used in providing the desired measurements. The outer peripheral surfaces of the heater pads are configured as segments of a cylinder and form a full cylinder when taken together. A plurality of temperature sensors are located on each pad so as to extend along the length and across the width thereof, with a heating element being located in each pad beneath the temperature sensors. An expansion mechanism driven by a clamping motor provides expansion and retraction of the heater pads and expandable packer-type seals are provided along the probe above and below the heater pads.
NASA Astrophysics Data System (ADS)
White, Brian; Scotti, Alberto
2014-11-01
We perform three-dimensional DNS of Horizontal Convection in a rectangular tank with idealized boundary conditions. The flow is driven by imposing the profile for the buoyancy b at the surface, where it ranges from b0 to b0 + Δb and the transition region is confined to a very small area. The Rayleigh based on the domain depth ranges from 105 to 1012. The scaling observed for the Nusselt number and the strength of the circulation is consistent with Rossby's scaling across the range of Rayleigh numbers considered, indicating that the dynamics in the boundary layer under the ``warming'' side throttles the flow. Energetically, we find that Available Potential Energy (APE) is generated along the surface, and converted to Kinetic Energy (KE). Along the descending plume energy goes from APE to KE up to Ra ~1011 . For higher Rayleigh numbers the plume becomes a net sink of APE. When the switch occurs, a stagnant layer develops near the bottom, and the overall circulation becomes characterized by a narrow plume which retroflects rapidly towards the surface, with a shallow recirculation to close the flow. This may indicate the beginning of a Sandström regime characterized by a stagnant abyssal region and a shallow circulation. Work supported by the National Science Foundation.
Realistic Solar Surface Convection Simulations
NASA Technical Reports Server (NTRS)
Stein, Robert F.; Nordlund, Ake
2000-01-01
We perform essentially parameter free simulations with realistic physics of convection near the solar surface. We summarize the physics that is included and compare the simulation results with observations. Excellent agreement is obtained for the depth of the convection zone, the p-mode frequencies, the p-mode excitation rate, the distribution of the emergent continuum intensity, and the profiles of weak photospheric lines. We describe how solar convection is nonlocal. It is driven from a thin surface thermal boundary layer where radiative cooling produces low entropy gas which forms the cores of the downdrafts in which most of the buoyancy work occurs. We show that turbulence and vorticity are mostly confined to the intergranular lanes and underlying downdrafts. Finally, we illustrate our current work on magneto-convection.
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.
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
Natural convection in nonvertical wells
Denbow, D.A.; Murphy, H.D.; McEligot, D.M.
1985-01-01
Convective instabilities and the shapes of the ensuing convection cells were experimentally studied for nonvertical wellbores. Steady-state temperature distributions were measured for three inclination angles over a wide range of heating rates to demonstrate the effects of drilling angle and Rayleigh number. In addition, velocities were estimated by measuring the time-of-flight of tracers formed by the Thymol blue technique. 8 refs., 6 figs.
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.
Observation of deep convection initiation from shallow convection environment
NASA Astrophysics Data System (ADS)
Lothon, Marie; Couvreux, Fleur; Guichard, Françoise; Campistron, Bernard; Chong, Michel; Rio, Catherine; Williams, Earle
2010-05-01
In the afternoon of 10 July 2006, deep convective cells initiated right in the field of view of the Massachusetts Institute Technology (MIT) C-band Doppler radar. This radar, with its 3D exploration at 10 min temporal resolution and 250 m radial resolution, allows us to track the deep convective cells and also provides clear air observations of the boundary layer structure prior to deep convection initiation. Several other observational platforms were operating then which allow us to thoroughly analyse this case: Vertically pointing aerosol lidar, W-band radar and ceilometer from the ARM Mobile Facility, along with radiosoundings and surface measurements enable us to describe the environment, from before their initiation to after the propagation of of one propagating cell that generated a circular gust front very nicely caught by the MIT radar. The systems considered here differ from the mesoscale convective systems which are often associated with African Easterly Waves, increasing CAPE and decreasing CIN. The former have smaller size, and initiate more locally, but there are numerous and still play a large role in the atmospheric circulation and scalar transport. Though, they remain a challenge to model. (See the presentation by Guichard et al. in the same session, for a model set up based on the same case, with joint single-column model and Large Eddy Simulation, which aims at better understanding and improving the parametrisation of deep convection initiation.) Based on the analysis of the observations mentioned above, we consider here the possible sources of deep convection initiation that day, which showed a typical boundary-layer growth in semi-arid environment, with isolated deep convective events.
Convection wave studies over land and sea
NASA Technical Reports Server (NTRS)
Kuettner, Joachim; Grossmann, Robert
1991-01-01
Preliminary results of recent case studies conducted over land and sea are given. Two dimensional convection (roll vortex/cloudstreet) and three dimensional convection in the underlying boundary layer are dealt with. Vertical momentum flux profiles and time series of important parameters and vertical soundings taken in the experiment area are shown. The three cases described show that convection waves occur over land and over ocean, over three dimensional convection and over two dimensional convection.
Year of Tropical Convection (YOTC)
NASA Astrophysics Data System (ADS)
Moncrieff, M. W.; Waliser, D. E.
2009-05-01
Tropical convection and the multi-scale organization of precipitating convection are associated with scale interactions that are fundamental to the atmospheric circulation and its interaction with the ocean. The realistic representation of tropical convection and its multi-scale organization is a long-standing challenge for numerical weather prediction and climate models. Incomplete knowledge and practical issues disadvantage the representation of important phenomena and processes in global models, such as the ITCZ, monsoons, MJO, and easterly waves and tropical cyclones. The tropical-extratropical interactions of tropical convection are key aspects of the Predictability and Dynamical Processes of THORPEX. The WCRP and WWRP/THORPEX are jointly coordinating a year of observing, modeling, and forecasting with a focus on the multi-scale organization of tropical convection, prediction, and predictability: Year of Tropical Convection (YOTC). Satellite, in-situ, and field-campaign measurements (e.g., TPARC), operational prediction, and cloud-system resolving models will be utilized. The temporal scales addressed, up to seasonal, enables the above phenomena to be modeled at high resolution, and seamless prediction issues at the intersection of weather and climate addressed. The 'Year', the period 1 May 2008 - 31 October 2009, began with the archiving of ECMWF T799 (i.e., 25 km) products: i) complete global analysis; ii) deterministic forecasts; and iii) special diagnostics. Plans are underway to obtain similar NCEP and NASA GEOS-5 data, and to integrate various multi-sensor satellite products. The YOTC Science Plan, which is available at http://www.wmo.int/pages/prog/arep/wwrp/new/documents/ YOTC_Science_Plan.pdf, has been published as a WMO Technical Document. The YOTC Implementation Plan, presently being drafted, will be discussed and finalized at an international workshop in July 2009. This talk summarizes programmatic aspects; science issues involving the multiscale
Mantle Convection on Modern Supercomputers
NASA Astrophysics Data System (ADS)
Weismüller, J.; Gmeiner, B.; Huber, M.; John, L.; Mohr, M.; Rüde, U.; Wohlmuth, B.; Bunge, H. P.
2015-12-01
Mantle convection is the cause for plate tectonics, the formation of mountains and oceans, and the main driving mechanism behind earthquakes. The convection process is modeled by a system of partial differential equations describing the conservation of mass, momentum and energy. Characteristic to mantle flow is the vast disparity of length scales from global to microscopic, turning mantle convection simulations into a challenging application for high-performance computing. As system size and technical complexity of the simulations continue to increase, design and implementation of simulation models for next generation large-scale architectures is handled successfully only in an interdisciplinary context. A new priority program - named SPPEXA - by the German Research Foundation (DFG) addresses this issue, and brings together computer scientists, mathematicians and application scientists around grand challenges in HPC. Here we report from the TERRA-NEO project, which is part of the high visibility SPPEXA program, and a joint effort of four research groups. TERRA-NEO develops algorithms for future HPC infrastructures, focusing on high computational efficiency and resilience in next generation mantle convection models. We present software that can resolve the Earth's mantle with up to 1012 grid points and scales efficiently to massively parallel hardware with more than 50,000 processors. We use our simulations to explore the dynamic regime of mantle convection and assess the impact of small scale processes on global mantle flow.
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.
Convective Excitation of Internal Waves
NASA Astrophysics Data System (ADS)
Lecoanet, Daniel; Le Bars, Michael; Burns, Keaton; Vasil, Geoffrey; Quataert, Eliot; Brown, Benjamin; Oishi, Jeffrey
2015-11-01
We will present a joint experimental & computational study of internal wave generation by convection. First we describe an experiment using the peculiar property of water that its density maximum is at 4° C . A tank of water cooled from below and heated from above develops a cold, convective layer near 4° C at the bottom of the tank, adjacent to a hot stably stratified layer at the top of the tank. We simulate this setup in 2D using the open-source Dedalus code (dedalus-project.org). Our simulations show that waves are excited from within the convection zone, opposed to at the interface between the convective and stably stratified regions. Finally, we will present 3D simulations of internal wave excitation by convection in a fully compressible atmosphere with multiple density scaleheights. These simulations provide greater freedom in choosing the thermal equilibrium of the system, and are run at higher Rayleigh number. The simulated waves are then compared to analytic predictions of the bulk excitation model.
Mantle convection on modern supercomputers
NASA Astrophysics Data System (ADS)
Weismüller, Jens; Gmeiner, Björn; Mohr, Marcus; Waluga, Christian; Wohlmuth, Barbara; Rüde, Ulrich; Bunge, Hans-Peter
2015-04-01
Mantle convection is the cause for plate tectonics, the formation of mountains and oceans, and the main driving mechanism behind earthquakes. The convection process is modeled by a system of partial differential equations describing the conservation of mass, momentum and energy. Characteristic to mantle flow is the vast disparity of length scales from global to microscopic, turning mantle convection simulations into a challenging application for high-performance computing. As system size and technical complexity of the simulations continue to increase, design and implementation of simulation models for next generation large-scale architectures demand an interdisciplinary co-design. Here we report about recent advances of the TERRA-NEO project, which is part of the high visibility SPPEXA program, and a joint effort of four research groups in computer sciences, mathematics and geophysical application under the leadership of FAU Erlangen. TERRA-NEO develops algorithms for future HPC infrastructures, focusing on high computational efficiency and resilience in next generation mantle convection models. We present software that can resolve the Earth's mantle with up to 1012 grid points and scales efficiently to massively parallel hardware with more than 50,000 processors. We use our simulations to explore the dynamic regime of mantle convection assessing the impact of small scale processes on global mantle flow.
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.
Penetration below a convective zone
NASA Astrophysics Data System (ADS)
Hurlburt, Neal E.; Toomre, Juri; Massaguer, Josep M.; Zahn, Jean-Paul
1994-01-01
Two-dimensional numerical simulations are used to investigate how fully compressible nonlinear convection penetrates into a stably stratified zone beneath a stellar convection zone. Estimates are obtained of the extent of penetration as the relative stability S of the stable to the unstable zone is varied over a broad range. The model deals with a perfect gas possessing a constant dynamic viscosity. The dynamics is dominated by downward-directed plumes which can extend far into the stable material and which can lead to the excitation of a broad spectrum of internal gravity waves in the lower stable zone. The convection is highly time dependent, with the close coupling between the lateral swaying of the plumes and the internal gravity waves they generate serving to modulate the strength of the convection. The depth of penetration delta, determined by the position where the time-averaged kinetic flux has its first zero in the stable layer, is controlled by a balance between the kinetic energy carried into the stable layer by the plumes and the buoyancy braking they experience there. A passive scalar is introduced into the unstable layer to evaluate the transport of chemical species downward. Such a tracer is effectively mixed within a few convective overturning times down to a depth of delta within the stable layer. Analytical estimates based on simple scaling laws are used to interpret the variation of delta with S, showing that it first involves an interval of adiabatic penetration if the local Peclet number of the convection exceeds unity, followed by a further thermal adjustment layer, the depths of each interval scaling in turn as S-1 and S-1/4. These estimates are in accord with the penetration results from the simulations.
Simulating Convection in Stellar Envelopes
NASA Astrophysics Data System (ADS)
Tanner, Joel
Understanding convection in stellar envelopes, and providing a mathematical description of it, would represent a substantial advance in stellar astrophysics. As one of the largest sources of uncertainty in stellar models, existing treatments of convection fail to account for many of the dynamical effects of convection, such as turbulent pressure and asymmetry in the velocity field. To better understand stellar convection, we must be able to study and examine it in detail, and one of the best tools for doing so is numerical simulation. Near the stellar surface, both convective and radiative process play a critical role in determining the structure and gas dynamics. By following these processes from first principles, convection can be simulated self-consistently and accurately, even in regions of inefficient energy transport where existing descriptions of convection fail. Our simulation code includes two radiative transfer solvers that are based on different assumptions and approximations. By comparing simulations that differ only in their respective radiative transfer methods, we are able to isolate the effect that radiative efficiency has on the structure of the superadiabatic layer. We find the simulations to be in good general agreement, but they show distinct differences in the thermal structure in the superadiabatic layer and atmosphere. Using the code to construct a grid of three-dimensional radiation hydrodynamic simulations, we investigate the link between convection and various chemical compositions. The stellar parameters correspond to main-sequence stars at several surface gravities, and span a range in effective temperatures (4500 < Teff < 6400). Different chemical compositions include four metallicities (Z = 0.040, 0.020, 0.010, 0.001), three helium abundances (Y = 0.1, 0.2, 0.3) and several levels of alpha-element enhancement. Our grid of simulations shows that various convective properties, such as velocity and the degree of superadiabaticity, are
Wavenumber selection in Benard convection
Catton, I.
1988-11-01
The results of three related studies dealing with wavenumber selection in Rayleigh--Benard convection are reported. The first, an extension of the power integral method, is used to argue for the existence of multi-wavenumbers at all supercritical wavenumbers. Most existing closure schemes are shown to be inadequate. A thermodynamic stability criterion is shown to give reasonable results but requires empirical measurement of one parameter for closure. The third study uses an asymptotic approach based in part on geometric considerations and requires no empiricism to obtain good predictions of the wavenumber. These predictions, however, can only be used for certain planforms of convection.
Report of convective phenomena team
NASA Technical Reports Server (NTRS)
Orville, H.; Koenig, R.; Miller, J.; Telford, J.; Jones, B.; Alger, G.; Lee, R.; Boudle, D.
1980-01-01
A group meeting was assembled to focus on the planning of specific experiments, to establish some priorities, identify interested scientists who would like to participate, establish any special requirements, make recommendations on data processing, and to prepare flight plan outlines. Since the number of convective storms in the CCOPE (Cooperative Convective Precipitation Experiment) field experiment area are limited to only a few days during the operational time period the flight plans must be designed with a hierarchy of abort experiments so that the easily identified and lowest probability events should take priority until their quota is filled.
Convective Overshoot in Stellar Interior
NASA Astrophysics Data System (ADS)
Zhang, Q. S.
2015-07-01
In stellar interiors, the turbulent thermal convection transports matters and energy, and dominates the structure and evolution of stars. The convective overshoot, which results from the non-local convective transport from the convection zone to the radiative zone, is one of the most uncertain and difficult factors in stellar physics at present. The classical method for studying the convective overshoot is the non-local mixing-length theory (NMLT). However, the NMLT bases on phenomenological assumptions, and leads to contradictions, thus the NMLT was criticized in literature. At present, the helioseismic studies have shown that the NMLT cannot satisfy the helioseismic requirements, and have pointed out that only the turbulent convection models (TCMs) can be accepted. In the first part of this thesis, models and derivations of both the NMLT and the TCM were introduced. In the second part, i.e., the work part, the studies on the TCM (theoretical analysis and applications), and the development of a new model of the convective overshoot mixing were described in detail. In the work of theoretical analysis on the TCM, the approximate solution and the asymptotic solution were obtained based on some assumptions. The structure of the overshoot region was discussed. In a large space of the free parameters, the approximate/asymptotic solutions are in good agreement with the numerical results. We found an important result that the scale of the overshoot region in which the thermal energy transport is effective is 1 HK (HK is the scale height of turbulence kinetic energy), which does not depend on the free parameters of the TCM. We applied the TCM and a simple overshoot mixing model in three cases. In the solar case, it was found that the temperature gradient in the overshoot region is in agreement with the helioseismic requirements, and the profiles of the solar lithium abundance, sound speed, and density of the solar models are also improved. In the low-mass stars of open
Segregation and convection in dendritic alloys
NASA Technical Reports Server (NTRS)
Poirier, D. R.
1990-01-01
Microsegregation in dentritic alloys is discussed, including solidification with and without thermal gradient, the convection of interdendritic liquid. The conservation of momentum, energy, and solute is considered. Directional solidification and thermosolutal convection are discussed.
Synthesis : Convection, structure and evolution
NASA Astrophysics Data System (ADS)
Schatzman, E.
1997-12-01
Lectures and discussions at the SCORe workshop have given a general idea of our present understanding of convection and oscillations and its application to the special case of the Sun. This {\\it SYNTHESIS} is just an attempt to present what seems to me to be the most important results, to draw attention to forgotten physical processes and to approach some important unsolved questions.
Convection in Uranus and Neptune
NASA Astrophysics Data System (ADS)
Podolak, Morris; Helled, Ravit; Schubert, Gerald
2015-11-01
It is a common assumption of interior models that the outer planets of our solar system are convective, and that the internal temperature distributions are therefore adiabatic. If this assumption is not correct, the inferred internal structures of these planets can be different than typically thought. Therefore, exploring this topic is crucial for planetary characterization. We investigate how the internal temperature profiles of Uranus and Neptune depend on the treatment of layered-convection. We then use a set of possible temperature profiles associated with layered-convection together with density profiles derived from interior models that match the measured gravitational fields to derive the compositions of the planets. We find that the inferred compositions of both Uranus and Neptune are not very sensitive to the thermal profile. In addition, we show that calculating the thermal flux is important for understanding the energy transport mechanism in giant planets. Finally, we suggest that Neptune’s interior is just at the boundary between being convective or conductive and both configurations are consistent with its thermal flux, while Uranus’ interior is mostly conductive. This result is consistent with recent dynamo models and useful for understanding the origin of the magnetic fields of the planets.
Mantle convection, topography and geoid
NASA Astrophysics Data System (ADS)
Golle, Olivia; Dumoulin, Caroline; Choblet, Gaël.; Cadek, Ondrej
2010-05-01
The internal evolution of planetary bodies often include solid-state convection. This phenomenon may have a large impact on the various interfaces of these bodies (dynamic topography occurs). It also affects their gravity field (and the geoid). Since both geoid and topography can be measured by a spacecraft, and are therefore available for several planetary bodies (while seismological measurements are still lacking for all of them but the Moon and the Earth), these are of the first interest for the study of internal structures and processes. While a classical approach now is to combine gravity and altimetry measurements to infer the internal structure of a planet [1], we propose to complement it by the reverse problem, i.e., producing synthetic geoid and dynamic topography from numerical models of convection as proposed by recent studies (e.g. for the CMB topography of the Earth,[2]). This procedure first include a simple evaluation of the surface topography and geoid from the viscous flow obtained by the 3D numerical tool OEDIPUS [3] modeling convection in a spherical shell. An elastic layer will then be considered and coupled to the viscous model - one question being whether the elastic shell shall be included 'on top' of the convective domain or within it, in the cold 'lithospheric' outer region. What we will present here corresponds to the first steps of this work: the comparison between the response functions of the topography and the geoid obtained from the 3D convection program to the results evaluated by a spectral method handling radial variations of viscosity [4]. We consider the effect of the elastic layer whether included in the convective domain or not. The scale setting in the context of a full thermal convection model overlaid by an elastic shell will be discussed (thickness of the shell, temperature at its base...). References [1] A.M. Wieczorek, (2007), The gravity and topography of the terrestrial planets, Treatise on Geophysics, 10, 165-206. [2
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
Convective storms in planetary atmospheres
NASA Astrophysics Data System (ADS)
Hueso, R.; Sánchez-Lavega, A.
2013-05-01
The atmospheres of the planets in the Solar System have different physical properties that in some cases can be considered as extreme when compared with our own planet's more familiar atmosphere. From the tenuous and cold atmosphere of Mars to the dense and warm atmosphere of Venus in the case of the terrestrial planets, to the gigantic atmospheres of the outer planets, or the nitrogen and methane atmosphere of Saturn's moon Titan, we can find a large variety of physical environments. The comparative study of these atmospheres provides a better understanding of the physics of a geophysical fluid. In many of these worlds convective storms of different intensity appear. They are analogous to terrestrial atmospheres fed by the release of latent heat when one of the gases in the atmosphere condenses and they are therefore called moist convective storms. In many of these planets they can produce severe meteorological phenomena and by studying them in a comparative way we can aspire to get a further insight in the dynamics of these atmospheres even beyond the scope of moist convection. A classical example is the structure of the complex systems of winds in the giant planets Jupiter and Saturn. These winds are zonal and alternate in latitude but their deep structure is not accessible to direct observation. However the behaviour of large--scale convective storms vertically extending over the "weather layer" allows to study the buried roots of these winds. Another interesting atmosphere with a rather different structure of convection is Titan, a world where methane is close to its triple point in the atmosphere and can condense in bright clouds with large precipitation fluxes that may model part of the orography of the surface making Titan a world with a methane cycle similar to the hydrological cycle of Earth's atmosphere.
Tropical Convection's Roles in Tropical Tropopause Cirrus
NASA Technical Reports Server (NTRS)
Boehm, Matthew T.; Starr, David OC.; Verlinde, Johannes; Lee, Sukyoung
2002-01-01
The results presented here show that tropical convection plays a role in each of the three primary processes involved in the in situ formation of tropopause cirrus. First, tropical convection transports moisture from the surface into the upper troposphere. Second, tropical convection excites Rossby waves that transport zonal momentum toward the ITCZ, thereby generating rising motion near the equator. This rising motion helps transport moisture from where it is detrained from convection to the cold-point tropopause. Finally, tropical convection excites vertically propagating tropical waves (e.g. Kelvin waves) that provide one source of large-scale cooling near the cold-point tropopause, leading to tropopause cirrus formation.
A transilient matrix for moist convection
Romps, D.; Kuang, Z.
2011-08-15
A method is introduced for diagnosing a transilient matrix for moist convection. This transilient matrix quantifies the nonlocal transport of air by convective eddies: for every height z, it gives the distribution of starting heights z{prime} for the eddies that arrive at z. In a cloud-resolving simulation of deep convection, the transilient matrix shows that two-thirds of the subcloud air convecting into the free troposphere originates from within 100 m of the surface. This finding clarifies which initial height to use when calculating convective available potential energy from soundings of the tropical troposphere.
Ionospheric convection signatures and magnetic field topology
NASA Technical Reports Server (NTRS)
Coley, W. R.; Heelis, R. A.; Hanson, W. B.; Reiff, P. H.; Sharber, J. R.
1987-01-01
A statistical study of signatures of the high-latitude ionospheric convection pattern and the simultaneously observed energetic electron precipitation is presented. Most often found are convection cells in which the sunward flowing region contains auroral particle precipitation but the antisunward flowing region does not. However, observations also show the frequent occurrence of convection cells in which neither the antisunward nor the sunward flowing plasma region contains auroral particle precipitation. These findings may appear within the dawnside or duskside convection pattern and strongly suggest that such convection cells may be associated with open magnetic field lines that thread the magnetotail lobes.
Granular convection observed by magnetic resonance imaging
NASA Astrophysics Data System (ADS)
Ehrichs, E. E.; Jaeger, H. M.; Karczmar, Greg S.; Knight, James B.; Kuperman, Vadim Yu.; Nagel, Sidney R.
1995-03-01
Vibrations in a granular material can spontaneously produce convection rolls reminiscent of those seen in fluids. Magnetic resonance imaging provides a sensitive and noninvasive probe for the detection of these convection currents, which have otherwise been difficult to observe. A magnetic resonance imaging study of convection in a column of poppy seeds yielded data about the detailed shape of the convection rolls and the depth dependence of the convection velocity. The velocity was found to decrease exponentially with depth; a simple model for this behavior is presented here.
Granular convection observed by magnetic resonance imaging
Ehrichs, E.E.; Jaeger, H.M.; Knight, J.B.; Nagel, S.R.; Karczmar, G.S.; Kuperman, V.Yu.
1995-03-17
Vibrations in a granular material can spontaneously produce convection rolls reminiscent of those seen in fluids. Magnetic resonance imaging provides a sensitive and noninvasive probe for the detection of these convection currents, which have otherwise been difficult to observe. A magnetic resonance imaging study of convection in a column of poppy seeds yielded data about the detailed shape of the convection rolls and the depth dependence of the convection velocity. The velocity was found to decrease exponentially with depth; a simple model for this behavior is presented here. 31 refs., 4 figs.
The shadowgraph method in convection experiments
NASA Astrophysics Data System (ADS)
Rasenat, S.; Hartung, G.; Winkler, B. L.; Rehberg, I.
1989-06-01
The shadowgraph method is applied to thermal convection experiments and electro-hydrodynamic convection (EHC) in nematic liquid crystals. In both cases convection leads to a spatially periodic field of the refractive index causing a spatially periodic intensity modulation of parallel light passing the cell. Close to the onset of convection the temperature or director field is given by linear stability analysis. Knowing these functions the determination of their amplitudes becomes possible by means of the shadowgraph method. The method is demostrated using various examples of thermal and EHC convection experiments.
Modelling natural convection of fluid in cuvette
NASA Astrophysics Data System (ADS)
Kucher, D.; Manukhin, B.; Andreeva, O.; Chivilikhin, S.
2014-09-01
Convection is a process of transfer liquid from a hot region to a cool region. This phenomenon is involved in many physical processes. The main characteristic of convection is a temperature field. Modelling of convection allows to get the information about temperature field at any time of process. In this paper the results of modelling natural convection of fluid in cuvette are presented. All results are approved by experimental data. For modelling the process of natural convection Navier-Stokes equations under Boussinesq approximation were used. An experimental setup based on digital holographic interferometry was developedin order to make an experiment. The results for three stadiums of convection, such as: jet initiation, initial jet formation, jet development with formation of mushroom-shaped convective stream, are presented.
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.
Seismology of Convection in the Sun
NASA Astrophysics Data System (ADS)
Hanasoge, Shravan
2015-08-01
Solar convection lies in extraordinary regime of dynamical parameters. Convective processes in the Sun drive global fluid circulations and magnetic fields, which in turn affect its visible outer layers (solar activity) and, more broadly, the heliosphere (space weather). The precise determination of the depth of solar convection zone, departures from adiabaticity of the temperature gradient, and the internal rotation rate as a function of latitude and depth are among the seminal contributions of helioseismology towards understanding convection in the Sun. Contemporary helioseismology, which is focused on inferring the properties of three-dimensional convective features, suggests that transport velocities are substantially smaller than theoretical predictions. Furthermore, helioseismology provides important constraints on the anisotropic Reynolds stresses that control the global dynamics of the solar convection zone. In this review, I will discuss the state of our understanding of convection in the Sun, with a focus on helioseismic diagnostics.
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.
The continental drift convection cell
NASA Astrophysics Data System (ADS)
Whitehead, J. A.; Behn, Mark D.
2015-06-01
Continents on Earth periodically assemble to form supercontinents and then break up again into smaller continental blocks (the Wilson cycle). Previous highly developed numerical models incorporate fixed continents while others indicate that continent movement modulates flow. Our simplified numerical model suggests that continental drift is fundamental. A thermally insulating continent is anchored at its center to mantle flow on an otherwise stress-free surface for infinite Prandtl number cellular convection with constant material properties. Rayleigh numbers exceed 107, while continent widths and chamber lengths approach Earth's values. The Wilson cycle is reproduced by a unique, rugged monopolar "continental drift convection cell." Subduction occurs at the cell's upstream end with cold slabs dipping at an angle beneath the moving continent (as found in many continent/subduction regions on Earth). Drift enhances vertical heat transport up to 30%, especially at the core-mantle boundary, and greatly decreases lateral mantle temperature differences.
Fluid convection, constraint and causation
Bishop, Robert C.
2012-01-01
Complexity—nonlinear dynamics for my purposes in this essay—is rich with metaphysical and epistemological implications but is receiving sustained philosophical analysis only recently. I will explore some of the subtleties of causation and constraint in Rayleigh–Bénard convection as an example of a complex phenomenon, and extract some lessons for further philosophical reflection on top-down constraint and causation particularly with respect to causal foundationalism. PMID:23386955
Magnetohydrodynamic convection in liquid gallium.
NASA Astrophysics Data System (ADS)
Juel, Anne; Mullin, Tom
1996-11-01
Results are presented from a study of convective flow of liquid gallium confined in a rectangular cavity of length/depth ratio 4, subject to a horizontal temperature gradient. The origin of the problem lies in the area of crystal growth, where it is known that the dynamics of the fluid flow in semiconductor geometries are of vital importance in determining the quality of the crystal. Application of a magnetic field, for instance, damps out the time-dependent convection in the liquid phase that creates striations in the crystal and reduces its quality. Prior to the study of dynamical phenomena, the nature of the steady flow is investigated. In the absence of a magnetic field, a direct comparison between experimental results, the Hadley cell model and two and three-dimensional numerical simulations clearly shows that the flow is three-dimensional in nature. The effect of a uniform transverse magnetic field is then examined. Direct comparison between experimental results and three dimensional simulations shows identical damping of the convective circulation. Numerically, it is found that the magnetic field restricts the flow to 2d motion. Experimentally, this is confirmed from the measurement of isotherms. Hence, the detailed knowledge of the steady flow provides us with a robust basis for studies of time dependent behaviour.
Bifurcation phenomena in cylindrical convection
NASA Astrophysics Data System (ADS)
Tuckerman, Laurette; Boronska, K.; Bordja, L.; Martin-Witkowski, L.; Navarro, M. C.
2008-11-01
We present two bifurcation scenarios occurring in Rayleigh-Benard convection in a small-aspect-ratio cylinder. In water (Pr=6.7) with R/H=2, Hof et al. (1999) observed five convective patterns at Ra=14200. We have computed 14 stable and unstable steady branches, as well as novel time-dependent branches. The resulting complicated bifurcation diagram, can be partitioned according to azimuthal symmetry. For example, three-roll and dipole states arise from an m=1 bifurcation, four-roll and ``pizza'' branches from m=2, and the ``mercedes'' state from an m=3 bifurcation after successive saddle-node bifurcations via ``marigold'', ``mitsubishi'' and ``cloverleaf'' states. The diagram represents a compromise between the physical tendency towards parallel rolls and the mathematical requirement that primary bifurcations be towards trigonometric states. Our second investigation explores the effect of exact counter-rotation of the upper and lower bounding disks on axisymmetric flows with Pr=1 and R/H=1. The convection threshold increases and, for sufficiently high rotation, the instability becomes oscillatory. Limit cycles originating at the Hopf bifurcation are annihilated when their period becomes infinite at saddle-node-on-periodic-orbit (SNOPER) bifurcations.
Atmospheric Vortices in Shallow Convection.
NASA Astrophysics Data System (ADS)
Hess, G. D.; Spillane, K. T.; Lourensz, R. S.
1988-03-01
Observations of funnel clouds over Port Phillip Bay, Victoria, Australia, indicate that they occur during outbreaks of cool air from the Southern Ocean advecting over the relatively warm bay waters. These clouds act as tracers for shallow convection vortices with dynamics similar to large dust devils. The related phenomena of waterspouts and tornadoes differ from these vortices by requiring deep convection and downdraft and updraft interactions associated with rain processes.Deardorff (1978a) suggests that a necessary condition for the formation of dust devils is /L of the order of 100 or more, where h is the convective boundary layer height and L the Obukhov length. Calculations of /L over the bay and over land for the days of observation are consistent with this suggestion. They indicate that significant rotation may occur at /L as low as 50. This information, if confirmed, may make it possible to use boundary layer numerical models to forecast likely conditions of dust devil occurrence over mesoscale regions, which would be of benefit to pilots of light aircraft and helicopters.
Influence of convection on microstructure
NASA Technical Reports Server (NTRS)
Wilcox, William R.; Caram, Rubens; Mohanty, A. P.; Seth, Jayshree
1990-01-01
The mechanism responsible for the difference in microstructure caused by solidifying the MnBi-Bi eutectic in space is sought. The objectives for the three year period are as follows: (1) completion of the following theoretical analyses - determination of the influence of the Soret effect on the average solid composition versus distance of off-eutectic mixtures directionally solidified in the absence of convection, determination of the influence of convection on the microstructure of off-eutectic mixtures using a linear velocity profile in the adjacent melt, determination of the influence of volumetric changes during solidification on microconvection near the freezing interface and on microstructure, and determination of the influence of convection on microstructure when the MnBi fibers project out in front of the bismuth matrix; (2) search for patterns in the effect of microgravity on different eutectics (for example, eutectic composition, eutectic temperature, usual microstructure, densities of pure constituents, and density changes upon solidification); and (3) determination of the Soret coefficient and the diffusion coefficient for Mn-Bi melts near the eutectic composition, both through laboratory experiements to be performed here and from data from Shuttle experiments.
Instability of spiral convective vortex
NASA Astrophysics Data System (ADS)
Evgrafova, Anna; Andrey, Sukhanovsky; Elena, Popova
2014-05-01
Formation of large-scale vortices in atmosphere is one of the interesting problems of geophysical fluid dynamics. Tropical cyclones are examples of atmospheric spiral vortices for which convection plays an important role in their formation and evolution. Our study is focused on intensive cyclonic vortex produced by heating in the central part of the rotating layer. The previous studies made by Bogatyrev et al, showed that structure of such vortex is very similar to the structure of tropical cyclones. Qualitative observations described in (Bogatyrev, 2009) showed that the evolution of large-scale vortex in extreme regimes can be very complicated. Our main goal is the study of evolution of convective cyclonic vortex at high values of Grasshof number by PIV system. Experimental setup is a rotating cylindrical tank of fluid (radius 150 mm, depth 30 mm, free upper surface). Velocity fields for different values of heat flux were obtained and temporal and spatial structure of intensive convective vortex were studied in details. With the use of PIV data vorticity fields were reconstructed in different horizontal cross-sections. Physical interpretation of mechanisms that lead to the crucial change in the vortex structure with the growth of heat rate is described. Financial support from program of UD RAS, the International Research Group Program supported by Perm region Government is gratefully acknowledged.
Thermosolutal convection during dendritic solidification
NASA Technical Reports Server (NTRS)
Heinrich, J. C.; Nandapurkar, P.; Poirier, D. R.; Felicelli, S.
1989-01-01
This paper presents a mathematical model for directional solidification of a binary alloy including a dendritic region underlying an all-liquid region. It is assumed initially that there exists a nonconvecting state with planar isotherms and isoconcentrates solidifying at a constant velocity. The stability of this system has been analyzed and nonlinear calculations are performed that show the effect of convection in the solidification process when the system is unstable. Results of calculations for various cases defined by the initial temperature gradient at the dendrite tips and varying strength of the gravitational field are presented for systems involving lead-tin alloys. The results show that the systems are stable for a gravitational constant of 0.0001 g(0) and that convection can be suppressed by appropriate choice of the container's size for higher values of the gravitational constant. It is also concluded that for the lead-tin systems considered, convection in the mushy zone is not significant below the upper 20 percent of the dendritic zone, if al all.
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.
Vegetation forcing and convective motion
Hong, X.; Leach, M.J.; Raman, S.
1995-04-01
A large irrigated vegetation area in a semiarid or relatively dry location is a strong surface forcing of thermal circulations. Several observational studies have found that such thermally induced mesoscale circulation may contribute to the triggering and development of convective clouds. In the western United States, extensive areas of irrigated farmland are surrounded by hot, dry surfaces, such as a steppe. Substantial gradients of sensible heating in the horizontal direction lead to a {open_quotes}farm breeze{close_quotes} circulation from the cooler agricultural area to the warmer steppes found at Boardman, Oregon. These thermally forced circulations may trigger convection by the related convergence and updraft motion under favorable atmospheric conditions. The role of vegetative covering in convective motion is investigated using a mesoscale numerical model. Two- and three-dimensional simulations are described. The effects of atmospheric stability, moisture in the lower atmosphere, moisture in the upper atmosphere, and horizontal heating scale on thermally induced clouds are studied. The horizontal scale of inhomogeneity is also studied using the two-dimensional model. Finally, a realistic vegetation distribution similar to that of the Boardman Regional Flux Experiment is used in the three-dimensional simulations.
Three types of critical convection patterns in two-layer fluid Bénard-Marangoni convection
NASA Astrophysics Data System (ADS)
Kang, Qi; Li, Lujun; Hu, Liang; Duan, Li
This paper presents the experiment on Benard-Marangoni convection of two-layer fluid. Compared with the single layer fluid convection, the two layers fluid convection depend on not only the Rayleigh number (Ra) and the Marangoni number (Ma) in each layer, but also the ratio of their depth. The theoretical investigations have shown there are three types of convection patterns at or near the convection onset, which are the mechanically coupled stationary convection, the thermally coupled stationary convection, and the time-dependent convection. In the present experiment, the high resolution PIV is used in this experiment and the convection pattern were recognized directly by the velocity field distribution. The experimental cell is rectangular cavity in our experiment. The cell is heated from below and cooled from top. Liquid FC70 and the silicon oil KF96-10CS are used. Their densities are 1.93xchmetcnvTCSC0NumberType1NegativeFalseHasSpaceTrueSourceValue103UnitNamekg103 kg/m3 and chmetcnvTCSC0NumberType1NegativeFalseHasSpaceFalseSourceValue935UnitNamekg935kg/ respectively. The rate of change of interface tension with temperature is -4.46x10-5 N/mK. Our experiment obtained the three types of critical convection patterns as predicted by theory. And the structures of convection cell are learned. The time-dependent convection is validated from experiment. The experimental results show the convection pattern at or near convection onset depends on the depth ratio strongly. When the depth ratio Hr is smaller, such as Hr=1.60, the convection style of two layer fluid is mechanically coupled. When the depth ratio is bigger, such as Hr=3.72, the convection style is thermally coupled. When the depth ratio is intermediate, such as Hr=2.13, the convection style will be time-dependent directly.
How cold pool triggers deep convection?
NASA Astrophysics Data System (ADS)
Yano, Jun-Ichi
2014-05-01
The cold pool in the boundary layer is often considered a major triggering mechanism of convection. Here, presented are basic theoretical considerations on this issue. Observations suggest that cold pool-generated convective cells is available for shallow maritime convection (Warner et al. 1979; Zuidema et al. 2012), maritime deep convection (Barnes and Garstang 1982; Addis et al. 1984; Young et al. 1995) and continental deep convection (e.g., Lima and Wilson 2008; Flamant 2009; Lothon et al. 2011; Dione et al. 2013). Moreover, numerical studies appear to suggest that cold pools promote the organization of clouds into larger structures and thereby aid the transition from shallow to deep convection (Khairoutdinov and Randall 2006, Boing et al. 2012, Schlemmer and Hohenegger, 2014). Even a cold--pool parameterization coupled with convection is already proposed (Grandpeix and Lafore 2010: but see also Yano 2012). However, the suggested link between the cold pool and deep convection so far is phenomenological at the best. A specific process that the cold pool leads to a trigger of deep convection must still to be pinned down. Naively, one may imagine that a cold pool lifts up the air at the front as it propagates. Such an uplifting leads to a trigger of convection. However, one must realize that a shift of air along with its propagation does not necessarily lead to an uplifting, and even if it may happen, it would not far exceed a depth of the cold pool itself. Thus, the uplifting can never be anything vigorous. Its thermodynamic characteristics do help much either for inducing convection. The cold-pool air is rather under rapid recovering process before it can induce convection under a simple parcel-lifting argument. The most likely reason that the cold pool may induce convection is its gust winds that may encounter an air mass from an opposite direction. This induces a strong convergence, also leading to a strong uplifting. This is an argument essentially developed
Convection Design of Cryogenic Piping and Components
NASA Astrophysics Data System (ADS)
McIntosh, G. E.
2006-04-01
Poor thermal performance of dewars, magnet cryostats, and other cryogenic equipment is often caused by failure of the designer to recognize the impact of enclosed free convection heat transfer. This paper describes the mechanism of internal convection in piping, vapor-cooled leads, bayonets and specialty dewars. Specific examples are given in each category. Conclusions include guidelines to avoid convection heat transfer problems and rules for correctly calculating heat leak of cryogenic piping.
Some properties of convection in hybrid stars
NASA Astrophysics Data System (ADS)
Yudin, A. V.; Hempel, M.; Nadyozhin, D. K.; Razinkova, T. L.
2016-02-01
It is shown that the unusual thermodynamic properties of matter within the region of two-phase coexistence in hybrid stars result in a change of the standard condition for beginning of convection. In particular, the thermal flux transported by convection may be directed towards the stellar centre. We discuss favourable circumstances leading to such an effect of `inverse convection' and its possible influence on the thermal evolution of hybrid stars.
Convective cell formation in a Z pinch
NASA Astrophysics Data System (ADS)
Kesner, J.
2003-03-01
Closed field line confinement systems can develop convective cells when the magnetohydrodynamic interchange stability criterion is violated. Using a previously derived set of reduced equations [V. P. Pastukhov and N. V. Chudin, Plasma Phys. Rep. 27, 907 (2001)] it is shown that a true steady state solution can exist. For an assumed large-scale vortex pattern, the plasma pressure profile that is implied by these convective flows as well as the nonlocal heat flux resulting from the convective flows is calculated.
Dust devil vortex generation from convective cells
NASA Astrophysics Data System (ADS)
Onishchenko, O.; Pokhotelov, O.; Horton, W.; Fedun, V.
2015-11-01
We have developed a hydrodynamic theory of the nonlinear stage of dust devil generation in a convectively unstable atmosphere with large-scale seed vertical vorticity. It is shown that convective motion in such an atmosphere transforms into dust devils extremely fast. The strong vortical structure of the dust devils can be formed in a few minutes or even in a fraction of a minute. The formation process strongly depends on the convective instability growth rate and horizontal vorticity.
Convective Instabilities in Liquid Foams
NASA Technical Reports Server (NTRS)
Veretennikov, Igor; Glazier, James A.
2004-01-01
The main goal of this work is to better understand foam behavior both on the Earth and in microgravity conditions and to determine the relation between a foam's structure and wetness and its rheological properties. Our experiments focused on the effects of the bubble size distribution (BSD) on the foam behavior under gradual or stepwise in the liquid flow rate and on the onset of the convective instability. We were able to show experimentally, that the BSD affects foam rheology very strongly so any theory must take foam texture into account.
Moisture processes accompanying convective activity
NASA Technical Reports Server (NTRS)
Sienkiewicz, M. E.; Scoggins, J. R.
1982-01-01
A moisture budget analysis was performed on data collected during the AVE 7 (May 2 to 3, 1978) and AVE-SESAME1 (April 10 to 11, 1979) experiments. Local rates-of-change of moisture were compared with average moisture divergence in the same time period. Results were presented as contoured plots in the horizontal and as vertical cross sections. These results were used to develop models of the distribution of moisture processes in the vicinity of convective areas in two layers representing lower and middle tropospheric conditions. Good correspondence was found between the residual term of the moisture budget and actual precipitation.
A Study of Detrainment from Deep Convection
NASA Astrophysics Data System (ADS)
Glenn, I. B.; Krueger, S. K.
2014-12-01
Uncertainty in the results of Global Climate Model simulations has been attributed to errors and simplifications in how parameterizations of convection coarsely represent the processes of entrainment, detrainment, and mixing between convective clouds and their environment. Using simulations of convection we studied these processes at a resolution high enough to explicitly resolve them. Two of several recently developed analysis techniques that allow insight into these processes at their appropriate scale are an Eulerian method of directly measuring entrainment and detrainment, and a Lagrangian method that uses particle trajectories to map convective mass flux over height and a cloud variable of interest. The authors of the Eulerian technique used it to show that the dynamics of shells of cold, humid air that surround shallow convective updrafts have important effects on the properties of air entrained and detrained from the updrafts. There is some evidence for the existence of such shells around deep convective updrafts as well, and that detrainment is more important than entrainment in determining the ultimate effect of the deep convection on the large scale environment. We present results from analyzing a simulation of deep convection through the Eulerian method as well as using Lagrangian particle trajectories to illustrate the role of the shell in the process of detrainment and mixing between deep convection and its environment.
A Dynamically Computed Convective Time Scale for the Kain–Fritsch Convective Parameterization Scheme
Many convective parameterization schemes define a convective adjustment time scale τ as the time allowed for dissipation of convective available potential energy (CAPE). The Kain–Fritsch scheme defines τ based on an estimate of the advective time period for deep con...
Properties of semi-convection and convective overshooting for massive stars
NASA Astrophysics Data System (ADS)
Ding, C. Y.; Li, Y.
2014-02-01
The properties of semi-convection and core convective overshooting of stars with masses of 15 and 30 M⊙ are calculated in the present article. New methods are used to deal with semi-convection. Different entropy gradients are used when adopting the Schwarzschild and Ledoux methods, which are used to confine the convective boundary and calculate the turbulent quantities: {{partial } overline{s}}/{{partial } r}=-({c_p}/{H_P})(nabla -nabla _ad) when the Schwarzschild method is adopted and {{partial } overline{s}}/{{partial } r}=-({c_p}/{H_P})(nabla -nabla _ad-nabla _{μ }) when the Ledoux method is adopted. Core convective overshooting and semi-convection are treated as a whole and their development is found to present almost opposing tendencies: more intensive core convective overshooting leads to weaker semi-convection. The influence of different parameters and convection processing methods on the turbulent quantities is analysed in this article. Increasing the mixing-length parameter α leads to more turbulent dynamic energy in the convective core and prolongs the overshooting distance but depresses the development of semi-convection. Adoption of the Ledoux method leads to overshooting extending further and semi-convection development being suppressed.
Transitions in turbulent rotating convection
NASA Astrophysics Data System (ADS)
Rajaei, Hadi; Alards, Kim; Kunnen, Rudie; Toschi, Federico; Clercx, Herman; Fluid Dynamics Lab Team
2015-11-01
This study aims to explore the flow transition from one state to the other in rotating Rayleigh-Bènard convection using Lagrangian acceleration statistics. 3D particle tracking velocimetry (3D-PTV) is employed in a water-filled cylindrical tank of equal height and diameter. The measurements are performed at the center and close to the top plate at a Rayleigh number Ra = 1.28e9 and Prandtl number Pr = 6.7 for different rotation rates. In parallel, direct numerical simulation (DNS) has been performed to provide detailed information on the boundary layers. We report the acceleration pdfs for different rotation rates and show how the transition from weakly to strongly rotating Rayleigh-Bènard affects the acceleration pdfs in the bulk and boundary layers. We observe that the shapes of the acceleration PDFs as well as the isotropy in the cell center are largely unaffected while crossing the transition point. However, acceleration pdfs at the top show a clear change at the transition point. Using acceleration pdfs and DNS data, we show that the transition between turbulent states is actually a boundary layer transition between Prandtl-Blasius type (typical of non-rotating convection) and Ekman type.
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)
Khan, B. A.; Stenchikov, G. L.; Abualnaja, Y.
2014-12-01
Shallow convection has been studied in the sea breeze frontal zone along the Arabian Red Sea coast. This convection is forced by thermal and dynamic instabilities and generally is capped below 500 hPa. The thermally induced sea breeze modifies the desert Planetary Boundary Layer (PBL) and propagates inland as a density current. The leading edge of the denser marine air rapidly moves inland undercutting the hot and dry desert air mass. The warm air lifts up along the sea breeze front (SBF). Despite large moisture flux from the sea, the shallow convection in SBF does not cause precipitation on the most part of the Arabian coastal plane. The main focus of this research is to study the vertical structure and extent of convective activity in SBF and to differentiate flow regimes that lead to dry and wet convection. The Weather Research and Forecasting Model (WRF) has been employed at a high spatial resolution of 500 m to investigate the thermodynamic structure of the atmospheric column along the SBF. We found that convection occurs during offshore and cross-shore mean wind conditions; precipitation in SBF frequently develops in the southern region of the Red Sea along the high terrain of Al-Sarawat Mountains range, while on most of the days convection is dry in the middle region and further north of the Red Sea. The coherent structures in the PBL, horizontal convective rolls (HCRs) and open convective cells (OCCs), play an important role shaping interaction of SBF with the desert boundary layer. The HCRs develop in the midmorning along the mean wind vector and interact with the SBF. Later in the afternoon HCRs evolve into OCCs. The convection is strongest, where the HCR and OCC updrafts overlap with SBF and is weakest in their downdraft regions.
Convection heat transfer coefficients at convective drying of porous materials
Szentgyoergyi, S.; Toemoesy, L.; Molnar, O.
2000-07-01
Measurements proved that the convective heat transfer coefficient (h) has a larger value h{sub wet} at the constant drying rate period and after that it falls down to a minimum one: h{sub dry} in the equilibrium dried state. Measurements showed also that the heat of vaporization in the last phase of the falling drying rate period is far greater than it was in the constant drying rate period. The first measurements were made on a gypsum plate. Afterwards the authors carried out measurement research with fine glass powder and cement-perlite plate and determined h{sub wet} and h{sub dry} heat transfer coefficients as a function of Reynolds number. All of these measurements confirmed the conclusion that h{sub wet} is far greater than h{sub dry}.
OBSERVATIONS OF TRANSPORT OF TRACE GASES BY VIGOROUS CONVECTIVE CLOUDS
Cumulus convective clouds provide an important link between the mixed layer and the upper levels of the troposphere. resh boundary layer pollutants emitted naturally and anthropogenically can be transported to high altitudes during deep convective activity. he convective transpor...
Introductory Analysis of Benard-Marangoni Convection
ERIC Educational Resources Information Center
Maroto, J. A.; Perez-Munuzuri, V.; Romero-Cano, M. S.
2007-01-01
We describe experiments on Benard-Marangoni convection which permit a useful understanding of the main concepts involved in this phenomenon such as, for example, Benard cells, aspect ratio, Rayleigh and Marangoni numbers, Crispation number and critical conditions. In spite of the complexity of convection theory, we carry out a simple and…
Extremely tall convection: characteristics and controls
NASA Astrophysics Data System (ADS)
Nesbitt, S. W.; Rasmussen, K. L.
2015-12-01
Tall continental convective structures are observed in several climatological regions, and have been shown to be related with severe weather and extreme hydrologic events. Recent work has defined tall convection as regions with precipitation structures observed with spaceborne radar echo extending into the upper troposphere/lower stratosphere. While these climatological regions are known for these tall convective structures (subtropical South America, equatorial Africa, southcentral USA, South Asia), not all observed convective eventsin these regions contain strong structures, and the characteristics of the meteorological environments, including sounding profiles, that dictate the strength of the spectrum of convective systems are poorly constrained. In this study, precipitation radar (PR) data from the Tropical Rainfall Measuring Mission (TRMM) and dual-frequency precipitation radar (DPR) from the Global Precipitation Measurement (GPM) satellites will be examined alongside composites of atmospheric reanalysis data to examine the structural and meteorological environments surrounding observed tall convective systems. Environments of convective systems of various vertical extents will be contrasted with less extreme convection to infer physical causal mechanisms and to examine issues of predictability of these events.
Generalized convective quasi-equilibrium principle
NASA Astrophysics Data System (ADS)
Yano, Jun-Ichi; Plant, Robert S.
2016-03-01
A generalization of Arakawa and Schubert's convective quasi-equilibrium principle is presented for a closure formulation of mass-flux convection parameterization. The original principle is based on the budget of the cloud work function. This principle is generalized by considering the budget for a vertical integral of an arbitrary convection-related quantity. The closure formulation includes Arakawa and Schubert's quasi-equilibrium, as well as both CAPE and moisture closures as special cases. The formulation also includes new possibilities for considering vertical integrals that are dependent on convective-scale variables, such as the moisture within convection. The generalized convective quasi-equilibrium is defined by a balance between large-scale forcing and convective response for a given vertically-integrated quantity. The latter takes the form of a convolution of a kernel matrix and a mass-flux spectrum, as in the original convective quasi-equilibrium. The kernel reduces to a scalar when either a bulk formulation is adopted, or only large-scale variables are considered within the vertical integral. Various physical implications of the generalized closure are discussed. These include the possibility that precipitation might be considered as a potentially-significant contribution to the large-scale forcing. Two dicta are proposed as guiding physical principles for the specifying a suitable vertically-integrated quantity.
Traveling waves and chaos in thermosolutal convection
NASA Technical Reports Server (NTRS)
Deane, A. E.; Toomre, J.; Knobloch, E.
1987-01-01
Numerical experiments on two-dimensional thermosolutal convection reveal oscillations in the form of traveling, standing, modulated, and chaotic waves. Transitions between these wave forms and steady convection are investigated and compared with theory. Such rich nonlinear behavior is possible in fluid layers of wide horizontal extent, and provides an explanation for waves observed in recent laboratory experiments with binary fluid mixtures.
Convection in stars and heating of coronae
NASA Technical Reports Server (NTRS)
Mullan, D. J.
1991-01-01
The properties of convection in the sun and other cool stars are summarized. Recent studies of convection which have involved the use of supercomputers to model the flow of compressible gas in three dimensions are discussed. It is shown how the results of these computations may eventualy provide an understanding of how nonthermal processes heat coronal gas to temperatures of millions of degrees.
Spatial localization in rotating convection and magnetoconvection
NASA Astrophysics Data System (ADS)
Kao, H.-C.; Knobloch, E.
2014-01-01
Stationary spatially localized states are present in both rotating convection and magnetoconvection. In two-dimensional convection with stress-free boundary conditions, the formation of such states is due to the interaction between convection and a large scale mode: zonal velocity in rotating convection and magnetic potential in magnetoconvection. We develop a higher order theory, a nonlocal fifth order Ginzburg-Landau equation, to describe the effects of spatial modulation near a codimension-two point. Two different bifurcation scenarios are identified. Our results shed light on numerical studies of two-dimensional convective systems with stress-free boundary conditions. This paper is dedicated to Professor Helmut Brand on the occasion of his 60th birthday.
Collective phase description of oscillatory convection
Kawamura, Yoji; Nakao, Hiroya
2013-12-15
We formulate a theory for the collective phase description of oscillatory convection in Hele-Shaw cells. It enables us to describe the dynamics of the oscillatory convection by a single degree of freedom which we call the collective phase. The theory can be considered as a phase reduction method for limit-cycle solutions in infinite-dimensional dynamical systems, namely, stable time-periodic solutions to partial differential equations, representing the oscillatory convection. We derive the phase sensitivity function, which quantifies the phase response of the oscillatory convection to weak perturbations applied at each spatial point, and analyze the phase synchronization between two weakly coupled Hele-Shaw cells exhibiting oscillatory convection on the basis of the derived phase equations.
Magnetospheric convection pattern and its implications
NASA Technical Reports Server (NTRS)
Zhu, Xiaoming
1993-01-01
When we use 14 months of the Fast Plasma Experiment ion velocity measurements, the mean magnetospheric circulation pattern is constructed. It is shown that the magnetospheric convection velocity is of the order tens of kilometers per second. The convection is largely restricted to the outer magnetosphere. During magnetically active periods the convection velocity increases and the convection boundary extends to the region closer to the Earth, indicating more magnetic field flux is being transported to the dayside magnetosphere. It is also shown that the convective flows tend to follow contours of constant unit flux volume as they move around the Earth, especially on the duskside of the magnetosphere. This helps to avoid the pressure balance inconsistency often found in two-dimensional magnetotail models.
Natural convection around the human head.
Clark, R P; Toy, N
1975-01-01
1. Factors determining the convective flow patterns around the human head in 'still' conditions are discussed in relation to body posture. 2. The flow patterns have been visualized using a schlieren optical system which reveals that the head has a thicker 'insulating' layer of convecting air in the erect posture than in the supine position. 3. Local convective and radiative heat transfer measurements from the head have been using surface calorimeters. These results are seen to be closely related to the thickness of the convective boundary layer flows. 4. The total convective and radiative heat loss from the head of a subject in the erect and supine position has been evaluated from the local measurements. For the head of the supine subject the heat loss was found to be 30% more than when the subject was standing. PMID:1142118
Phase transitions and convection in icy satellites
NASA Technical Reports Server (NTRS)
Bercovici, D.; Schubert, G.; Reynolds, R. T.
1986-01-01
The effects of solid-solid phase changes on subsolidus convection in the large icy moons of the outer solar system are considered. Phase transitions affect convection via processes that distort the phase change boundary and/or influence buoyancy through thermal expansion. Linear stability analyses are carried out for ice layers with a phase change at the midplane. Two exothermic phase transitions (ice I - ice II, ice VI - ice VIII) and two endothermic transitions (ice I - ice III, ice II - ice V) are considered. For the exothermic cases, the phase change can either impede or enhance whole-layer convection. For the endothermic cases, the phse change always inhibits whole-layer convective overturn and tends to enforce two-layer convection. These results play some constraints on possible models of icy satellite evolution and structure.
Penetrative Convection and Zonal Flow on Jupiter
Zhang; Schubert
1996-08-16
Measurements by the Galileo probe support the possibility that the zonal winds in Jupiter's atmosphere originate from convection that takes place in the deep hydrogen-helium interior. However, according to models based on recent opacity data and the probe's temperature measurements, there may be radiative and nonconvective layers in the outer part of the jovian interior, raising the question of how deep convection could extend to the surface. A theoretical model is presented to demonstrate that, because of predominant rotational effects and spherical geometry, thermal convection in the deep jovian interior can penetrate into any outer nonconvective layer. These penetrative convection rolls interact nonlinearly and efficiently in the model to generate and sustain a mean zonal wind with a larger amplitude than that of the nonaxisymmetric penetrative convective motions, a characteristic of the wind field observed at the cloud level on Jupiter. PMID:8688074
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.
Two-dimensional convective turbulence
Gruzinov, A.V.; Kukharkin, N.; Sudan, R.N.
1996-02-01
We show that 2D {bold E{times}B} ionospheric turbulence of the electron density in the equatorial electrojet is isomorphic to the viscous convection of an ordinary fluid in a porous medium due to temperature gradients. Numerical simulations reveal the strong anisotropy in the turbulence, which consists of rising hot bubbles and falling cool bubbles. These bubbles break up into fingers leading to the formation of stable shear flows. After reaching a quasisteady state, the omnidirectional energy spectrum approaches a {ital k}{sup {minus}2} behavior, rather than {ital k}{sup {minus}5/3} as expected from isotropic turbulence. Physical mechanisms that lead to anisotropy are analyzed. {copyright} {ital 1996 The American Physical Society.}
New Approaches to Parameterizing Convection
NASA Technical Reports Server (NTRS)
Randall, David A.; Lappen, Cara-Lyn
1999-01-01
Many general circulation models (GCMs) currently use separate schemes for planetary boundary layer (PBL) processes, shallow and deep cumulus (Cu) convection, and stratiform clouds. The conventional distinctions. among these processes are somewhat arbitrary. For example, in the stratocumulus-to-cumulus transition region, stratocumulus clouds break up into a combination of shallow cumulus and broken stratocumulus. Shallow cumulus clouds may be considered to reside completely within the PBL, or they may be regarded as starting in the PBL but terminating above it. Deeper cumulus clouds often originate within the PBL with also can originate aloft. To the extent that our models separately parameterize physical processes which interact strongly on small space and time scales, the currently fashionable practice of modularization may be doing more harm than good.
Steady Magnetospheric Convection: A Review
NASA Astrophysics Data System (ADS)
Fairfield, D. H.
2004-12-01
On occasion, solar wind energy enters Earth's magnetosphere yet the common discrete energy-dissipation events known as magnetospheric substorms fail to occur. During these times, the magnetotail assumes a configuration where earthward of about 12 Re the tail remains in a stretched tail-like state with a thin current sheet similar to the substorm growth phase. At the same time the more distant tail attains a more relaxed configuration with a thick plasma sheet, weak lobe field and enhanced northward Bz, similar to the substorm recovery phase. Simultaneously, (1) auroral zone currents remain strong and assume a two cell DP 2 convection pattern; (2) the auroral oval is wide and optically active, particularly at its poleward and equatorward edges; (3) polar cap area remains constant and energetic particle boundaries are stable, (4) earthward plasma flow persists near the center of the tail as implied by the name steady magnetospheric convection (SMC) except that it occurs on a time scale of minutes and the flow remains bursty. These small scale flows in the tail correspond to auroral streamers that form near the poleward boundary of the oval and propagate equatorward in a few minutes time. Although SMC events have some substorm-like characteristics, such as Pi2's, particle injections and region 1-type field aligned currents with their associated westward ionospheric currents, such phenomena occur on much shorter time and spatial scales and with much smaller amplitudes than actual substorms. Modeling the global magnetic field for several specific SMC events suggest that a minimum in the equatorial tail field Bz magnitude exists near 12 Re which may correspond to the one known equilibrium field configuration that can avoid the pressure catastrophe that may correspond to substorms. This unique field configuration may permit the return of magnetic flux to the dayside that allows the persistence of the steady state field configuration.
Actively convected liquid metal divertor
NASA Astrophysics Data System (ADS)
Shimada, Michiya; Hirooka, Yoshi
2014-12-01
The use of actively convected liquid metals with j × B force is proposed to facilitate heat handling by the divertor, a challenging issue associated with magnetic fusion experiments such as ITER. This issue will be aggravated even more for DEMO and power reactors because the divertor heat load will be significantly higher and yet the use of copper would not be allowed as the heat sink material. Instead, reduced activation ferritic/martensitic steel alloys with heat conductivities substantially lower than that of copper, will be used as the structural materials. The present proposal is to fill the lower part of the vacuum vessel with liquid metals with relatively low melting points and low chemical activities including Ga and Sn. The divertor modules, equipped with electrodes and cooling tubes, are immersed in the liquid metal. The electrode, placed in the middle of the liquid metal, can be biased positively or negatively with respect to the module. The j × B force due to the current between the electrode and the module provides a rotating motion for the liquid metal around the electrodes. The rise in liquid temperature at the separatrix hit point can be maintained at acceptable levels from the operation point of view. As the rotation speed increases, the current in the liquid metal is expected to decrease due to the v × B electromotive force. This rotating motion in the poloidal plane will reduce the divertor heat load significantly. Another important benefit of the convected liquid metal divertor is the fast recovery from unmitigated disruptions. Also, the liquid metal divertor concept eliminates the erosion problem.
Mesoscale convective complexes in Africa
Laing, A.G.; Fritsch, J.M. )
1993-08-01
Digitized full-disk infrared satellite imagery from the European geostationary satellite (Meteosat) for 1986 and 1987 was used to construct a climatology of mesoscale convective complexes (MCCs) in Africa. One hundred ninety-five systems formed over Africa and its near vicinity during the two-year study period. From this database, characteristics of Africa MCCs were calculated. The results indicate that these MCCs display many of the same characteristics as those found in the Americas, the Indian subcontinent, and the western Pacific region. The systems are predominantly nocturnal and tend to form over or in the immediate vicinity of land. The average lifetime of African MCCs is about 11.5 h. The size distributions of the African systems are also extremely similar to those of the Americas, the Indian subcontinent, and the western Pacific region with most systems exhibiting areas between 2 [times] 10[sup 5] and 3 [times] 10[sup 5] km[sup 2]. The monthly frequency distribution of African systems indicates that peak activity tends to occur during the period of most intense insolation. Like the MCCs in the western Pacific region and the Americas, the African MCCs tend to propagate toward the low-level high-[theta][sub e] air that feeds the convective systems. Systems over northern Africa moved toward the west-southwest, with a few developing into tropical cyclones over the Atlantic. Systems over southeastern Africa generally moved toward the northeast and east. It is concluded that the satellite-observed systems over Africa are essentially the same phenomena as the MCC populations observed over the Americas, the Indian monsoon region, and the western Pacific region. In addition, the large number of MCCs found worldwide (approximately 300-400 per year) indicate that they may be significant contributors to the global tropospheric energy budget and hydrological cycle. 46 refs., 9 figs., 1 tab.
Convective Available Potential Energy of World Ocean
NASA Astrophysics Data System (ADS)
Su, Z.; Ingersoll, A. P.; Thompson, A. F.
2012-12-01
Here, for the first time, we propose the concept of Ocean Convective Available Potential Energy (OCAPE), which is the maximum kinetic energy (KE) per unit seawater mass achievable by ocean convection. OCAPE occurs through a different mechanism from atmospheric CAPE, and involves the interplay of temperature and salinity on the equation of state of seawater. The thermobaric effect, which arises because the thermal coefficient of expansion increases with depth, is an important ingredient of OCAPE. We develop an accurate algorithm to calculate the OCAPE for a given temperature and salinity profile. We then validate our calculation of OCAPE by comparing it with the conversion of OCAPE to KE in a 2-D numerical model. We propose that OCAPE is an important energy source of ocean deep convection and contributes to deep water formation. OCAPE, like Atmospheric CAPE, can help predict deep convection and may also provide a useful constraint for modelling deep convection in ocean GCMs. We plot the global distribution of OCAPE using data from the World Ocean Atlas 2009 (WOA09) and see many important features. These include large values of OCAPE in the Labrador, Greenland, Weddell and Mediterranean Seas, which are consistent with our present observations and understanding, but also identify some new features like the OCAPE pattern in the Antarctic Circumpolar Current (ACC). We propose that the diagnosis of OCAPE can improve our understanding of global patterns of ocean convection and deep water formation as well as ocean stratification, the meridional overturning circulation and mixed layer processes. The background of this work is briefly introduced as below. Open-ocean deep convection can significantly modify water properties both at the ocean surface and throughout the water column (Gordon 1982). Open-ocean convection is also an important mechanism for Ocean Deep Water formation and the transport of heat, freshwater and nutrient (Marshall and Schott 1999). Open
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%.
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.
Seismic sounding of convection in the Sun
NASA Astrophysics Data System (ADS)
Sreenivasan, Katepalli R.
2015-11-01
Thermal convection is the dominant mechanism of energy transport in the outer envelope of the Sun (one-third by radius). It drives global fluid circulations and magnetic fields observed on the solar surface. Convection excites a broadband spectrum of acoustic waves that propagate within the interior and set up modal resonances. These acoustic waves, also called seismic waves, are observed at the surface of the Sun by space- and ground-based telescopes. Seismic sounding, the study of these seismic waves to infer the internal properties of the Sun, constitutes helioseismology. Here we review our knowledge of solar convection, especially that obtained through seismic inference. Several characteristics of solar convection, such as differential rotation, anisotropic Reynolds stresses, the influence of rotation on convection and supergranulation, are considered. On larger scales, several inferences suggest that convective velocities are substantially smaller than those predicted by theory and simulations. This discrepancy challenges the models of internal differential rotation that rely on convective stresses as a driving mechanism and provide an important benchmark for numerical simulations. In collaboration with Shravan Hanasoge, Tata Institute of Fundamental Research, Mumbai and Laurent Gizon, Max-Planck-Institut fuer Sonnensystemforschung, Goettingen.
Seismic Sounding of Convection in the Sun
NASA Astrophysics Data System (ADS)
Hanasoge, Shravan; Gizon, Laurent; Sreenivasan, Katepalli R.
2016-01-01
Thermal convection is the dominant mechanism of energy transport in the outer envelope of the Sun (one-third by radius). It drives global fluid circulations and magnetic fields observed on the solar surface. Vigorous surface convection excites a broadband spectrum of acoustic waves that propagate within the interior and set up modal resonances. These acoustic waves, also called seismic waves in this context, are observed at the surface of the Sun by space- and ground-based telescopes. Seismic sounding, the study of these seismic waves to infer the internal properties of the Sun, constitutes helioseismology. Here we review our knowledge of solar convection, especially that obtained through seismic inference. Several characteristics of solar convection, such as differential rotation, anisotropic Reynolds stresses, the influence of rotation on convection, and supergranulation, are considered. On larger scales, several inferences suggest that convective velocities are substantially smaller than those predicted by theory and simulations. This discrepancy challenges the models of internal differential rotation that rely on convective stresses as a driving mechanism and provide an important benchmark for numerical simulations.
Transition to chaos of thermocapillary convection
NASA Astrophysics Data System (ADS)
Li, Kai; Tang, Ze Mei; Aa, Yan; Hu, Wen-Rui
Transition of fluid convection to chaos in dissipative dynamical systems is a subject of great interest for both its theoretical and practical aspects in the fluid mechanics. Extensive studies have shown that there are several routes of the buoyant natural convection to chaos depending on parameters of the dissipative dynamical systems such as the Rayleigh number, the Prandtl number and geometry aspect. Another important type of natural convection is thermocapillary convection driven by the surface-tension gradient prominent in fluid systems with interface in the microgravity condition or in small-scaled terrestrial configurations (The relative importance of the gravity effect to the capillary effect is scaled by the static Bond number, , and the dynamic Bond number, , the geometrical scale of the system in the terrestrial experiments, therefore, was significantly reduced to make the capillary effect dominant). The thermocapillary convection has become one of the fundamental subjects in the microgravity fluid physics and space fluid/heat management. However, most studies now available were focused on the onset of oscillatory thermocapillary convection, the initial regime of the route to chaos. A complete route to chaos in such a new sort of dissipative system is still an attractive open question, especially in the experimental study. In present study, the route to chaos of the thermocapillary convection has been investigated. Several routes to chaos, e.g. period oscillatory convection to quasi-period oscillatory convection with 2 to 3 major frequencies, a series of successive period doubling bifurcations and their combination, of the thermocapillary flow is reported through the temperature measurements and the corresponding real time analysis of frequency spectra accomplished by Fast-Fourier-Transformation (FFT) or numerically. The corresponding phase diagrams are also provided.
Convective cell development and propagation in a mesoscale convective complex
NASA Technical Reports Server (NTRS)
Ahn, Yoo-Shin; Brundidge, Kenneth C.
1987-01-01
A case study was made of the mesoscale convective complex (MCC) which occurred over southern Oklahoma and northern Texas on 27 May 1981. This storm moved in an eastsoutheasterly direction and during much of its lifetime was observable by radars at Oklahoma City, Ok. and Stephenville, Tx. It was found that the direction of cell (VIP level 3 or more reflectivity) propagation was somewhat erratic but approximately the same as the system (VIP level 1 reflectivity) movement and the ambient wind. New cells developed along and behind the gust front make it appear that once the MCC is initiated, a synergistic relationship exists between the gust front and the MCC. The relationship between rainfall patterns and amounts and the infrared (IR) temperature field in the satellite imagery were examined. The 210 K isotherm of GOES IR imagery was found to encompass the rain area of the storm. The heaviest rainfall was in the vicinity of the VIP level 3 cells and mostly contained within the 205 K isotherm of GOES IR imagery.
Importance of combining convection with film cooling.
NASA Technical Reports Server (NTRS)
Colladay, R. S.
1972-01-01
The interaction of film and convection cooling and its effect on wall cooling efficiency is investigated analytically for two cooling schemes for advanced gas turbine applications. The two schemes are full coverage- and counterflow-film cooling. In full coverage film cooling, the cooling air issues from a large number of small discrete holes in the surface. Counterflow film cooling is a film-convection scheme with film injection from a slot geometry. The results indicate that it is beneficial to utilize as much of the cooling air heat sink as possible for convection cooling prior to ejecting it as a film.
Importance of combining convection with film cooling
NASA Technical Reports Server (NTRS)
Colladay, R. S.
1971-01-01
The interaction of film and convection cooling and its effect on wall cooling efficiency is investigated analytically for two cooling schemes for advanced gas turbine applications. The two schemes are full coverage- and counterflow-film cooling. In full coverage film cooling, the cooling air issues from a large number of small discrete holes in the surface. Counterflow film cooling is a film-convection scheme with film injection from a slot geometry. The results indicate that it is beneficial to utilize as much of the cooling air heat sink as possible for convection cooling prior to ejecting it as a film.
Transient Mixed Convection Validation for NGNP
Smith, Barton; Schultz, Richard
2015-10-19
The results of this project are best described by the papers and dissertations that resulted from the work. They are included in their entirety in this document. They are: (1) Jeff Harris PhD dissertation (focused mainly on forced convection); (2) Blake Lance PhD dissertation (focused mainly on mixed and transient convection). This dissertation is in multi-paper format and includes the article currently submitted and one to be submitted shortly; and, (3) JFE paper on CFD Validation Benchmark for Forced Convection.
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
Skylab M518 multipurpose furnace convection analysis
NASA Technical Reports Server (NTRS)
Bourgeois, S. V.; Spradley, L. W.
1975-01-01
An analysis was performed of the convection which existed on ground tests and during skylab processing of two experiments: vapor growth of IV-VI compounds growth of spherical crystals. A parallel analysis was also performed on Skylab experiment indium antimonide crystals because indium antimonide (InSb) was used and a free surface existed in the tellurium-doped Skylab III sample. In addition, brief analyses were also performed of the microsegregation in germanium experiment because the Skylab crystals indicated turbulent convection effects. Simple dimensional analysis calculations and a more accurate, but complex, convection computer model, were used in the analysis.
Convective drying of sludge cake
NASA Astrophysics Data System (ADS)
Chen, Jianbo; Peng, Xiaofeng; Xue, Yuan; Lee, Duujong; Chu, Chingping
2002-08-01
This paper presented an experimental study on convective drying of waste water sludge collected from Beijing GaoBeiDian Sewage Treatment Plant, particularly on the correlation between the observed shrinkage dynamics of sludge cake and the drying curve. During the initial stage of drying the process resembles to that of a particulate bed, in which moisture diffuses and evaporates at the upper surface. Conventional drying theory assuming a diffusion-evaporating front interprets this period of drying. Consequently, owing to the very large shrinkage ratio of the dried cake, cracks emerges and propagates on and within the cake body, whence inducing evaporating channel that facilitates the water removal. This occurrence compensates the reduction of surface area for evaporation, whence extending the constant-rate period during the test. Afterwards, the cracks meet with each other and form isolated cake piles, while the subsequent drying occur mainly within these piles and the conventional theory fails. The transition between the drying on a plain cake layer and that on the isolated piles demonstrates the need to adopt distinct descriptions on these two regimes of drying for the sludge cake.
Influence of convection on microstructure
NASA Technical Reports Server (NTRS)
Wilcox, William R.; Caram, Rubens; Mohanty, A. P.; Seth, Jayshree
1990-01-01
In eutectic growth, as the solid phases grow they reject atoms to the liquid. This results in a variation of melt composition along the solid/liquid interface. In the past, mass transfer in eutectic solidification, in the absence of convection, was considered to be governed only by the diffusion induced by compositional gradients. However, mass transfer can also be generated by a temperature gradient. This is called thermotransport, thermomigration, thermal diffusion or the Soret effect. A theoretical model of the influence of the Soret effect on the growth of eutectic alloys is presented. A differential equation describing the compositional field near the interface during unidirectional solidification of a binary eutectic alloy was formulated by including the contributions of both compositional and thermal gradients in the liquid. A steady-state solution of the differential equation was obtained by applying appropriate boundary conditions and accounting for heat flow in the melt. Following that, the average interfacial composition was converted to a variation of undercooling at the interface, and consequently to microstructural parameters. The results obtained show that thermotransport can, under certain circumstances, be a parameter of paramount importance.
Tachocline dynamics: convective overshoot at stiff interfaces
NASA Astrophysics Data System (ADS)
Brown, Benjamin; Lecoanet, Daniel; Oishi, Jeffrey S.; Burns, Keaton; Vasil, Geoffrey M.
2016-05-01
The solar tachocline lies at the base of the solar convection zone. At this internal interface, motions from the unstable convection zone above overshoot and penetrate downward into the stiffly stable radiative zone below, driving gravity waves, mixing, and possibly pumping and storing magnetic fields. Here we study the dynamics of convective overshoot across very stiff interfaces with some properties similar to the internal boundary layer within the Sun. We use the Dedalus pseudospectral framework and study fully compressible dynamics at moderate to high Peclet number and low Mach number, probing a regime where turbulent transport is important. In this preliminary work, we find that the depth of convective overshoot is well described by a simple buoyancy equilibration model, and we consider implications for dynamics at the solar tachocline.
Double-diffusive convection with sidewalls
NASA Technical Reports Server (NTRS)
Mcfadden, G. B.; Coriell, S. R.; Boisvert, R. F.
1985-01-01
Stommel et al. (1956) have first described an instability, known as thermosolutal convection, thermohaline convection, or double-diffusive convection. This instability may occur in the case of a fluid in a gravitational field with two diffusing components present. The present study is concerned with the effect of sidewalls on flow in the fingering regime in the absence of applied horizontal gradients. The work was motivated by numerical results obtained on the basis of a simulation of thermosolutal convection occurring during the unidirectional solidification of a binary alloy. In this case, the unperturbed solute field in the liquid ahead of the solidifying planar interface has an exponential vertical profile because of the rejection or preferential incorporation of solute by the solid phase.
Electrodynamics of convection in the inner magnetosphere
NASA Technical Reports Server (NTRS)
Spiro, R. W.; Wolf, R. A.
1984-01-01
During the past ten years, substantial progress has been made in the development of quantitative models of convection in the magnetosphere and of the electrodynamic processes that couple that magnetosphere and ionosphere. Using a computational scheme first proposed by Vasyliunas, the convection models under consideration separate the three-dimensional problem of convection in the inner magnetosphere/ionosphere into a pair of two-dimensional problems coupled by Birkeland currents flowing between the two regions. The logic, development, and major results of the inner magnetosphere convection model are reviewed with emphasis on ionospheric and magnetospheric currents. A major theoretical result of the models has been the clarification of the relationship between the region 1/region 2 picture of field-aligned currents and the older partial ring current/tail current interruption picture of substorm dynamics.
Criterion for convection in an inhomogeneous star
NASA Technical Reports Server (NTRS)
Stothers, Richard B.; Chin, Chao-Wen
1992-01-01
To resolve the question of whether the Schwarzschild criterion or the Ledoux criterion should be used to test for convective instability in a star, a well-observed cluster of chemically inhomogeneous massive stars, in which the choice of the criterion for convection makes a crucial and easily observable difference, is required. NGC 330, a metal-poor cluster in the Small Magellanic Cloud, is ideal for this test. Its large evolved stellar population contains both blue and red supergiants, of which its many red supergiants should be absent if a gradient of mean molecular weight did not choke off rapid convective motions in the inhomogeneous region connecting the envelope and core. Thus the Ledoux criterion for convection is strongly indicated as being correct.
SURFACE INHOMOGENEITY EFFECTS ON CONVECTIVE DIFFUSION
It is suggested that convectlve scaling, with appropriate extensions, provides the most useful framework for estimating the effects of urban-scale surface inhomogeneities on diffusion in convective conditions. trong contrasts in surface heat flux exist between cropland, forests, ...
Internal Wave Generation by Turbulent Convection
NASA Astrophysics Data System (ADS)
Lecoanet, D.; Le Bars, M.; Burns, K. J.; Vasil, G. M.; Quataert, E.; Brown, B. P.; Oishi, J.
2015-12-01
Recent measurements suggest that a portion of the Earth's core may be stably stratified. If this is the case, then the Earth's core joins the many planetary and stellar objects which have a stably stratified region adjacent to a convective region. The stably stratified region admits internal gravity waves which can transport angular momentum, energy, and affect magnetic field generation. We describe experiments & simulations of convective excitation of internal waves in water, exploiting its density maximum at 4C. The simulations show that waves are excited within the bulk of the convection zone, opposed to at the interface between the convective and stably stratified regions. We will also present 3D simulations using a compressible fluid. These simulations provide greater freedom in choosing the thermal equilibrium of the system, and are run at higher Rayleigh number.
Supercritical droplet gasification experiments with forced convection
NASA Technical Reports Server (NTRS)
Litchford, Ron; Parigger, Chris; Jeng, San-Mou
1992-01-01
Preliminary results of a comprehensive experimental program are presented which offer the first direct observations of suspended n-heptane droplet gasifications in pure nitrogen with forced convection without the interference to optical probing associated with a flame. Measurements show attainment of a wet-bulb temperature until reduced pressures exceed about 1.0 under supercritical gas temperatures. Thereafter, temperature measurements indicate fully transient heat-up through the critical temperature. The surface is found to regress in a continuous manner with the measured temperature approaching the critical value at the end of the droplet lifetime under supercritical conditions with very mild level of convection. At increased level of convection for the same ambient conditions, similar sized droplets will undergo significant deformation during the gasification process until partially convected away as a dense vapor cloud as the critical temperature is approached.
Understanding and controlling plasmon-induced convection.
Roxworthy, Brian J; Bhuiya, Abdul M; Vanka, Surya P; Toussaint, Kimani C
2014-01-01
The heat generation and fluid convection induced by plasmonic nanostructures is attractive for optofluidic applications. However, previously published theoretical studies predict only nanometre per second fluid velocities that are inadequate for microscale mass transport. Here we show both theoretically and experimentally that an array of plasmonic nanoantennas coupled to an optically absorptive indium-tin-oxide (ITO) substrate can generate >micrometre per second fluid convection. Crucially, the ITO distributes thermal energy created by the nanoantennas generating an order of magnitude increase in convection velocities compared with nanoantennas on a SiO2 base layer. In addition, the plasmonic array alters absorption in the ITO, causing a deviation from Beer-Lambert absorption that results in an optimum ITO thickness for a given system. This work elucidates the role of convection in plasmonic optical trapping and particle assembly, and opens up new avenues for controlling fluid and mass transport on the micro- and nanoscale. PMID:24445431
Understanding and controlling plasmon-induced convection
NASA Astrophysics Data System (ADS)
Roxworthy, Brian J.; Bhuiya, Abdul M.; Vanka, Surya P.; Toussaint, Kimani C.
2014-01-01
The heat generation and fluid convection induced by plasmonic nanostructures is attractive for optofluidic applications. However, previously published theoretical studies predict only nanometre per second fluid velocities that are inadequate for microscale mass transport. Here we show both theoretically and experimentally that an array of plasmonic nanoantennas coupled to an optically absorptive indium-tin-oxide (ITO) substrate can generate >micrometre per second fluid convection. Crucially, the ITO distributes thermal energy created by the nanoantennas generating an order of magnitude increase in convection velocities compared with nanoantennas on a SiO2 base layer. In addition, the plasmonic array alters absorption in the ITO, causing a deviation from Beer-Lambert absorption that results in an optimum ITO thickness for a given system. This work elucidates the role of convection in plasmonic optical trapping and particle assembly, and opens up new avenues for controlling fluid and mass transport on the micro- and nanoscale.
On-off convection: Noise-induced intermittency near the convection threshold.
Fujisaka, H; Ouchi, K; Ohara, H
2001-09-01
A phenomenological nonlinear stochastic model of intermittency experimentally observed by Behn, Lange, and John [Phys. Rev. E 58, 2047 (1998)] in the electrohydrodynamic convection in nematics under dichotomous noise is proposed. This has the structure of the two-dimensional Swift-Hohenberg equation for local convection variable with fluctuating threshold. Numerical integration of the model equation shows intermittent emergence of convective pattern. Its statistics are found to obey those known, so far, for on-off intermittency. In the course of time, although the pattern intensity changes intermittently, no evident pattern change is observed. Adding additive noise, we observe an intermittent change of convective pattern. PMID:11580416
Absolute/convective instabilities and the convective Mach number in a compressible mixing layer
NASA Technical Reports Server (NTRS)
Jackson, T. L.; Grosch, C. E.
1989-01-01
Two aspects of the stability of a compressible mixing layer: Absolute/Convective instability and the convective Mach number were considered. It was shown that, for Mach numbers less than one, the compressible mixing layer is convectively unstable unless there is an appreciable amount of backflow. Also presented was a rigorous derivation of a convective Mach number based on linear stability theory for the flow of a multi-species gas in a mixing layer. The result is compared with the heuristic definitions of others and to selected experimental results.
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.
Subcooled forced convection boiling of trichlorotrifluoroethane
NASA Technical Reports Server (NTRS)
Dougall, R. S.; Panian, D. J.
1972-01-01
Experimental heat-transfer data were obtained for the forced-convection boiling of trichlorotrifluoroethane (R-113 or Freon-113) in a vertical annular test annular test section. The 97 data points obtained covered heat transfer by forced convection, local boiling, and fully-developed boiling. Correlating methods were obtained which accurately predicted the heat flux as a function of wall superheat (boiling curve) over the range of parameters studied.
Numerical Study of a Convective Turbulence Encounter
NASA Technical Reports Server (NTRS)
Proctor, Fred H.; Hamilton, David W.; Bowles, Roland L.
2002-01-01
A numerical simulation of a convective turbulence event is investigated and compared with observational data. The specific case was encountered during one of NASA's flight tests and was characterized by severe turbulence. The event was associated with overshooting convective turrets that contained low to moderate radar reflectivity. Model comparisons with observations are quite favorable. Turbulence hazard metrics are proposed and applied to the numerical data set. Issues such as adequate grid size are examined.
Theory and Simulations of Rotating Convection
NASA Astrophysics Data System (ADS)
Barker, Adrian J.; Dempsey, Adam M.; Lithwick, Yoram
2014-08-01
We study thermal convection in a rotating fluid in order to better understand the properties of convection zones in rotating stars and planets. We first derive a mixing-length theory for rapidly rotating convection, arriving at the results of Stevenson via simple physical arguments. The theory predicts the properties of convection as a function of the imposed heat flux and rotation rate, independent of microscopic diffusivities. In particular, it predicts the mean temperature gradient, the rms velocity and temperature fluctuations, and the size of the eddies that dominate heat transport. We test all of these predictions with high resolution three-dimensional hydrodynamical simulations of Boussinesq convection in a Cartesian box. The results agree remarkably well with the theory across more than two orders of magnitude in rotation rate. For example, the temperature gradient is predicted to scale as the rotation rate to the four-fifths power at fixed flux, and the simulations yield 0.75 ± 0.06. We conclude that the mixing-length theory is a solid foundation for understanding the properties of convection zones in rotating stars and planets.
Driving forces: Slab subduction and mantle convection
NASA Technical Reports Server (NTRS)
Hager, Bradford H.
1988-01-01
Mantle convection is the mechanism ultimately responsible for most geological activity at Earth's surface. To zeroth order, the lithosphere is the cold outer thermal boundary layer of the convecting mantle. Subduction of cold dense lithosphere provides tha major source of negative buoyancy driving mantle convection and, hence, surface tectonics. There are, however, importnat differences between plate tectonics and the more familiar convecting systems observed in the laboratory. Most important, the temperature dependence of the effective viscosity of mantle rocks makes the thermal boundary layer mechanically strong, leading to nearly rigid plates. This strength stabilizes the cold boundary layer against small amplitude perturbations and allows it to store substantial gravitational potential energy. Paradoxically, through going faults at subduction zones make the lithosphere there locally weak, allowing rapid convergence, unlike what is observed in laboratory experiments using fluids with temperature dependent viscosities. This bimodal strength distribution of the lithosphere distinguishes plate tectonics from simple convection experiments. In addition, Earth has a buoyant, relatively weak layer (the crust) occupying the upper part of the thermal boundary layer. Phase changes lead to extra sources of heat and bouyancy. These phenomena lead to observed richness of behavior of the plate tectonic style of mantle convection.
EFFECTS OF PENETRATIVE CONVECTION ON SOLAR DYNAMO
Masada, Youhei; Yamada, Kohei; Kageyama, Akira
2013-11-20
Spherical solar dynamo simulations are performed. A self-consistent, fully compressible magnetohydrodynamic system with a stably stratified layer below the convective envelope is numerically solved with a newly developed simulation code based on the Yin-Yang grid. The effects of penetrative convection are studied by comparing two models with and without the stable layer. The differential rotation profile in both models is reasonably solar-like with equatorial acceleration. When considering the penetrative convection, a tachocline-like shear layer is developed and maintained beneath the convection zone without assuming any forcing. While the turbulent magnetic field becomes predominant in the region where the convective motion is vigorous, mean-field components are preferentially organized in the region where the convective motion is less vigorous. Particularly in the stable layer, the strong, large-scale field with a dipole symmetry is spontaneously built up. The polarity reversal of the mean-field component takes place globally and synchronously throughout the system regardless of the presence of the stable layer. Our results suggest that the stably stratified layer is a key component for organizing the large-scale strong magnetic field, but is not essential for the polarity reversal.
Theory and simulations of rotating convection
Barker, Adrian J.; Dempsey, Adam M.; Lithwick, Yoram
2014-08-10
We study thermal convection in a rotating fluid in order to better understand the properties of convection zones in rotating stars and planets. We first derive a mixing-length theory for rapidly rotating convection, arriving at the results of Stevenson via simple physical arguments. The theory predicts the properties of convection as a function of the imposed heat flux and rotation rate, independent of microscopic diffusivities. In particular, it predicts the mean temperature gradient, the rms velocity and temperature fluctuations, and the size of the eddies that dominate heat transport. We test all of these predictions with high resolution three-dimensional hydrodynamical simulations of Boussinesq convection in a Cartesian box. The results agree remarkably well with the theory across more than two orders of magnitude in rotation rate. For example, the temperature gradient is predicted to scale as the rotation rate to the four-fifths power at fixed flux, and the simulations yield 0.75 ± 0.06. We conclude that the mixing-length theory is a solid foundation for understanding the properties of convection zones in rotating stars and planets.
Convection in Condensible-rich Atmospheres
NASA Astrophysics Data System (ADS)
Ding, F.; Pierrehumbert, R. T.
2016-05-01
Condensible substances are nearly ubiquitous in planetary atmospheres. For the most familiar case—water vapor in Earth’s present climate—the condensible gas is dilute, in the sense that its concentration is everywhere small relative to the noncondensible background gases. A wide variety of important planetary climate problems involve nondilute condensible substances. These include planets near or undergoing a water vapor runaway and planets near the outer edge of the conventional habitable zone, for which CO2 is the condensible. Standard representations of convection in climate models rely on several approximations appropriate only to the dilute limit, while nondilute convection differs in fundamental ways from dilute convection. In this paper, a simple parameterization of convection valid in the nondilute as well as dilute limits is derived and used to discuss the basic character of nondilute convection. The energy conservation properties of the scheme are discussed in detail and are verified in radiative-convective simulations. As a further illustration of the behavior of the scheme, results for a runaway greenhouse atmosphere for both steady instellation and seasonally varying instellation corresponding to a highly eccentric orbit are presented. The latter case illustrates that the high thermal inertia associated with latent heat in nondilute atmospheres can damp out the effects of even extreme seasonal forcing.
Convection automated logic oven control
Boyer, M.A.; Eke, K.I.
1998-03-01
For the past few years, there has been a greater push to bring more automation to the cooling process. There have been attempts at automated cooking using a wide range of sensors and procedures, but with limited success. The authors have the answer to the automated cooking process; this patented technology is called Convection AutoLogic (CAL). The beauty of the technology is that it requires no extra hardware for the existing oven system. They use the existing temperature probe, whether it is an RTD, thermocouple, or thermistor. This means that the manufacturer does not have to be burdened with extra costs associated with automated cooking in comparison to standard ovens. The only change to the oven is the program in the central processing unit (CPU) on the board. As for its operation, when the user places the food into the oven, he or she is required to select a category (e.g., beef, poultry, or casseroles) and then simply press the start button. The CAL program then begins its cooking program. It first looks at the ambient oven temperature to see if it is a cold, warm, or hot start. CAL stores this data and then begins to look at the food`s thermal footprint. After CAL has properly detected this thermal footprint, it can calculate the time and temperature at which the food needs to be cooked. CAL then sets up these factors for the cooking stage of the program and, when the food has finished cooking, the oven is turned off automatically. The total time for this entire process is the same as the standard cooking time the user would normally set. The CAL program can also compensate for varying line voltages and detect when the oven door is opened. With all of these varying factors being monitored, CAL can produce a perfectly cooked item with minimal user input.
Entropy Production in Convective Hydrothermal Systems
NASA Astrophysics Data System (ADS)
Boersing, Nele; Wellmann, Florian; Niederau, Jan
2016-04-01
Exploring hydrothermal reservoirs requires reliable estimates of subsurface temperatures to delineate favorable locations of boreholes. It is therefore of fundamental and practical importance to understand the thermodynamic behavior of the system in order to predict its performance with numerical studies. To this end, the thermodynamic measure of entropy production is considered as a useful abstraction tool to characterize the convective state of a system since it accounts for dissipative heat processes and gives insight into the system's average behavior in a statistical sense. Solving the underlying conservation principles of a convective hydrothermal system is sensitive to initial conditions and boundary conditions which in turn are prone to uncertain knowledge in subsurface parameters. There exist multiple numerical solutions to the mathematical description of a convective system and the prediction becomes even more challenging as the vigor of convection increases. Thus, the variety of possible modes contained in such highly non-linear problems needs to be quantified. A synthetic study is carried out to simulate fluid flow and heat transfer in a finite porous layer heated from below. Various two-dimensional models are created such that their corresponding Rayleigh numbers lie in a range from the sub-critical linear to the supercritical non-linear regime, that is purely conductive to convection-dominated systems. Entropy production is found to describe the transient evolution of convective processes fairly well and can be used to identify thermodynamic equilibrium. Additionally, varying the aspect ratio for each Rayleigh number shows that the variety of realized convection modes increases with both larger aspect ratio and higher Rayleigh number. This phenomenon is also reflected by an enlarged spread of entropy production for the realized modes. Consequently, the Rayleigh number can be correlated to the magnitude of entropy production. In cases of moderate
Tropical Convection's Roles in Tropical Tropopause Cirrus
NASA Technical Reports Server (NTRS)
Boehm, Matthew T.; Starr, David OC.; Verlinde, Johannes; Lee, Sukyoung
2002-01-01
Remote sensing observations reveal the frequent occurrence of tropopause cirrus, thin cirrus layers located near the tropical cold-point tropopause. Here, we present a theory in which tropical convection plays several important roles in tropopause cirrus formation. First, tropical convection is the primary means by which the moisture required for tropopause cirrus formation is transported into the upper troposphere. However, previous studies suggest that this convection rarely penetrates to the altitudes at which tropopause cirrus layers are observed, suggesting that additional vertical moisture transport is required to explain tropopause cirrus formation. We propose a mechanism for explaining this transport in which tropical convection plays the key role. According to this hypothesis, the transport is accomplished by meridional circulations that develop within the tropopause transition layer (TTL) in response to momentum transport by Rossby waves generated by tropical convection. Results of a series of global scale model runs designed to test this hypothesis will be presented. In addition, reanalyses vertical velocity data will be examined for evidence of the expected correlation between large-scale rising motion within the TTL and tropical convection. Once moisture is present near the cold-point tropopause, large-scale cooling is required to initiate tropopause cirrus formation. One source of this cooling is stratospheric tropical waves induced by tropical convection, as we will show using a time series of radiosonde temperature data superimposed with data on cloud occurrence from the DOE ARM Nauru99 field experiment. Observations of the global characteristics of these waves from a longer time series of reanalysis data will also be presented.
Tornadoes and downbursts in the context of generalized planetary scales
NASA Technical Reports Server (NTRS)
Fujita, T. T.
1981-01-01
In order to cover a wide range of horizontal dimensions of airflow, the paper proposes a series of five scales, maso, meso, miso (to be read as my-so), moso and muso arranged in the order of the vowels, A, E, I, O, U. The dimensions decrease by two orders of magnitude per scale, beginning with the planet's equator length chosen to be the maximum dimension of masoscale for each planet. Mesoscale highs and lows were described on the basis of mesoanalyses, while sub-mesoscale disturbances were depicted by cataloging over 20,000 photographs of wind effects taken from low-flying aircraft during the past 15 years. Various motion thus classified into these scales led to a conclusion that extreme winds induced by thunderstorms are associated with misoscale and mososcale airflow spawned by the parent, mesoscale disturbances.
Tornadoes and Downbursts in the Context of Generalized Planetary Scales.
NASA Astrophysics Data System (ADS)
Fujita, T. Theodore
1981-08-01
In order to cover a wide range of horizontal dimensions of airflow, the author proposes a series of five scales, maso, meso, miso (to be read as my-so), moso and muso arranged in the order of the vowels, A, E, 1, O, U. The dimensions decrease by two orders of magnitude per scale, beginning with the planet's equator length chosen to be the maximum dimension of masoscale for each planet.Mesoscale highs and lows were described on the basis of mesoanalyses, while sub-mesoscale disturbances were depicted by cataloging over 20 000 photographs of wind effects taken from low-flying aircraft during the past 15 years. Various motion thus classified into these scales led to a conclusion that extreme winds induced by thunderstorms are associated with misoscale and mososcale airflow spawned by the parent. mesoscale disturbances.
NASA Astrophysics Data System (ADS)
Majda, Andrew J.; Khouider, Boualem; Frenkel, Yevgeniy
2015-02-01
Atmospheric convection has the striking capability to organize itself into a hierarchy of cloud clusters and super-clusters on scales ranging from the convective cell of a few kilometres to planetary scale disturbances such as the Madden-Julian oscillation. It is widely accepted that this phenomenon is due in large part to the two-way coupling between convective processes and equatorially trapped waves and planetary scale flows in general. However, the physical mechanisms responsible for this multiscale organization and the associated across-scale interactions are poorly understood. The two main peculiarities of the tropics are the vanishing of the Coriolis force at the equator and the abundance of mid-level moisture. Here we test the effect of these two physical properties on the organization of convection and its interaction with gravity waves in a simplified primitive equation model for flows parallel to the equator. Convection is represented by deterministic as well as stochastic multicloud models that are known to represent organized convection and convectively coupled waves quite well. It is found here that both planetary rotation and mid-troposphere moisture are important players in the diminishing of organized convection and convectively coupled gravity wave activity in the subtropics and mid-latitudes. The meridional mean circulation increases with latitude while the mean zonal circulation is much shallower and is dominated by mid-level jets, reminiscent of a second baroclinic mode circulation associated with a congestus mode instability in the model. This is consistent with the observed shallow Hadley and Walker circulations accompanied by congestus cloud decks in the higher latitude tropics and sub-tropics. Moreover, deep convection activity in the stochastic model simulations becomes very patchy and unorganized as the computational domain is pushed towards the subtropics and mid-latitudes. This is consistent with previous work based on cloud resolving
Driving mechanism of the nightside ionospheric convection
NASA Astrophysics Data System (ADS)
Kikuchi, T.
2001-12-01
Magnetometer and SuperDARN observations provided evidence of the instantaneous reaction of ionospheric convection on the dayside and nightside. The AMIE analyses revealed that the potential pattern did not move but remained nearly at fixed locations. SuperDARN observations demonstrated that the plasma motion in the nightside ionosphere was intensified immediately after the motion of dayside ionospheric plasma was intensified within a resolution of the measurement (2 min). The convection in the night-side polar ionosphere would cause the plasma convection in the near-earth magnetotail. In the companion paper (Hashimoto and Kikuchi, this meeting) we demonstrate that the growth phase signature at the geosynchronous orbit and the ground magnetic signatures of the partial ring currents developed several minutes after the magnetic reconnection at the dayside magnetopause. These results suggest that the electric field responsible for the convection in the near-Earth magnetotail propagated from the night-side polar ionosphere after having propagated from the magnetosphere to the polar ionosphere on the dayside. In order to explain the quick response of the nightside ionospheric convection, we examine possible propagation modes that could transmit the convection electric field from the dayside outer magnetosphere to the nightside ionosphere. The magnetospheric convection may be generated either by accumulation of the FTEs or by the dynamo action in the cusp and the HLBL. In either case, the electric field propagates from the dayside magnetosphere to the nightside ionosphere within a few minutes. One possible propagation mode would be the magnetosonic wave propagating across the geomagnetic field and the other is the shear Alfvén mode propagating parallel to the geomagnetic field. The magnetosonic waves would be totally reflected at the ionosphere and the resultant electric field would be vanished almost completely. On the other hand, the convective motion of the plasma can
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
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.
Convection in Oblate Late-Type Stars
NASA Astrophysics Data System (ADS)
Wang, Junfeng
2015-08-01
In this talk, we present recent investigations of the convection, oblateness and differential rota-tion in rapidly rotating late-type stars with a novel and powerful Compressible High-ORder Un-structured Spectral-difference (CHORUS) code (J. Comput. Physics Vol. 290, 190-211, 2015). Recent observations have revealed the drastic effects of rapid rotation on stellar structure, including centrifugal deformation and gravity darkening. The centrifugal force counteracts gravity, causing the equatorial region to expand. Consequently, rapidly rotating stars are oblate and cannot be described by an one-dimensional spherically symmetric model. If convection establishes a substantial differential rotation, as in the envelopes of late-type stars, this can considerably increase the oblateness. We have successfully extended the CHORUS code to model rapidly rotating stars on fixed unstructured grids. In the CHORUS code, the hydrodynamic equations are discretized by a robust and efficient high-order Spectral Difference Method (SDM). The discretization stencil of the spectral difference method is compact and advantageous for parallel processing. CHORUS has been verified by comparing to spherical anelastic convection simulations on benchmark problems. This talk will be centred on the first global simulations by CHORUS for convection in oblate stars with different rotating rates. We quantify the influence of the oblateness on the mean flows and the thermal structure of the convection zone through these new simulations and implications of these results for stellar observations will be discussed.
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
Heterogeneous nanofluids: natural convection heat transfer enhancement.
Oueslati, Fakhreddine Segni; Bennacer, Rachid
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
Heterogeneous nanofluids: natural convection heat transfer enhancement
NASA Astrophysics Data System (ADS)
Oueslati, Fakhreddine Segni; Bennacer, Rachid
2011-12-01
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.
CONVECTIVE BABCOCK-LEIGHTON DYNAMO MODELS
Miesch, Mark S.; Brown, Benjamin P.
2012-02-20
We present the first global, three-dimensional simulations of solar/stellar convection that take into account the influence of magnetic flux emergence by means of the Babcock-Leighton (BL) mechanism. We have shown that the inclusion of a BL poloidal source term in a convection simulation can promote cyclic activity in an otherwise steady dynamo. Some cycle properties are reminiscent of solar observations, such as the equatorward propagation of toroidal flux near the base of the convection zone. However, the cycle period in this young sun (rotating three times faster than the solar rate) is very short ({approx}6 months) and it is unclear whether much longer cycles may be achieved within this modeling framework, given the high efficiency of field generation and transport by the convection. Even so, the incorporation of mean-field parameterizations in three-dimensional convection simulations to account for elusive processes such as flux emergence may well prove useful in the future modeling of solar and stellar activity cycles.
Are steady magnetospheric convection events prolonged substorms?
NASA Astrophysics Data System (ADS)
Walach, M.-T.; Milan, S. E.
2015-03-01
Magnetospheric modes, including substorms, sawtooth events, and steady magnetospheric convection events, have in the past been described as different responses of the magnetosphere to coupling with the solar wind. Using previously determined event lists for sawtooth events, steady magnetospheric convection events, and substorms, we produce a statistical study of these event types to examine their similarities and behavior in terms of solar wind parameters, auroral brightness, open magnetospheric flux, and geomagnetic indices. A superposed epoch analysis shows that individual sawteeth show the same signatures as substorms but occur during more extreme cases of solar wind driving as well as geomagnetic activity. We also explore the limitations of current methods of identifying steady magnetospheric convection events and explain why some of those events are flagged inappropriately. We show that 58% of the steady magnetospheric convection events considered, as identified by criteria defined in previous studies are part of a prolonged version of substorms due to continued dayside driving during expansion phase. The remaining 42% are episodes of enhanced magnetospheric convection, occurring after extended periods of dayside driving.
Onset of Natural Convection in Saline Aquifers
NASA Astrophysics Data System (ADS)
Riaz, A.
2013-05-01
Sequestration of carbon dioxide in saline aquifers has emerged as the preferred method of permanently storing CO2 in the subsurface. In order to be successful over geologic time scales, sequestration in saline aquifers relies upon enhanced dissolution of CO2 in brine by natural convection. In this talk we review the progress made thus far towards the modeling and prediction of the onset time for natural convection that occurs due to an unstable stratification of aqueous CO2. We show how the onset of natural convection is connected to a preceding event of the onset of instability with respect to small amplitude perturbations that originate within the aqueous boundary layer. Our analysis indicates that the onset time for instability is uncertain within an initial transient period where perturbation growth depends on the specific form of the initial condition. A constrained adjoint based optimization is employed to determine the upper bound and the mean of perturbation growth. With the help of a weakly nonlinear analysis, we show that the time at which convection initiates is associated with fixed perturbation amplitude. The influence of permeability heterogeneity is studied with this approach. For certain permeability structures, the marginal stability curve bifurcates to form multiple stable and unstable zones in the space of the perturbation wavenumber and time. The transition toward bifurcation governs the behavior of the most dangerous mode in the linear regime and determines the route to the onset of natural convection.
Eclogites, pyroxene geotherm, and layered mantle convection.
Basu, A R; Ongley, J S; Macgregor, I D
1986-09-19
Temperatures of equilibration for the majority (81 percent) of the eclogite xenoliths of the Roberts Victor kimberlite pipe in South Africa range between 1000 degrees and 1250 degrees C, falling essentially on the gap of the lower limb of the subcontinental inflected geotherm derived from garnet peridotite xenoliths. In view of the Archean age (>2.6 x 10(9) years) of these eclogites and their stratigraphic position on the geotherm, it is proposed that the inflected part of the geotherm represents the convective boundary layer beneath the conductive lid of the lithospheric plate. The gradient of 8 Celsius degrees per kilometer for the inflection is characteristic of a double thermal boundary layer and suggests layered convection rather than whole mantle convection for the earth. PMID:17843357
A new conceptual model of convection
Walcek, C.
1995-09-01
Classical cumulus parameterizations assume that cumulus clouds are entraining plumes of hot air rising through the atmosphere. However, ample evidence shows that clouds cannot be simulated using this approach. Dr. Walcek suggests that cumulus clouds can be reasonably simulated by assuming that buoyant plumes detrain mass as they rise through the atmosphere. Walcek successfully simulates measurements of tropical convection using this detraining model of cumulus convection. Comparisons with measurements suggest that buoyant plumes encounter resistance to upward movement as they pass through dry layers in the atmosphere. This probably results from turbulent mixing and evaporation of cloud water, which generates negatively buoyant mixtures which detrain from the upward moving plume. This mass flux model of detraining plumes is considerably simpler than existing mass flux models, yet reproduces many of the measured effects associated with convective activity. 1 fig.
Numerical Archetypal Parameterization for Mesoscale Convective Systems
NASA Astrophysics Data System (ADS)
Yano, J. I.
2015-12-01
Vertical shear tends to organize atmospheric moist convection into multiscale coherent structures. Especially, the counter-gradient vertical transport of horizontal momentum by organized convection can enhance the wind shear and transport kinetic energy upscale. However, this process is not represented by traditional parameterizations. The present paper sets the archetypal dynamical models, originally formulated by the second author, into a parameterization context by utilizing a nonhydrostatic anelastic model with segmentally-constant approximation (NAM-SCA). Using a two-dimensional framework as a starting point, NAM-SCA spontaneously generates propagating tropical squall-lines in a sheared environment. A high numerical efficiency is achieved through a novel compression methodology. The numerically-generated archetypes produce vertical profiles of convective momentum transport that are consistent with the analytic archetype.
Entropy in Adiabatic Regions of Convection Simulations
NASA Astrophysics Data System (ADS)
Tanner, Joel D.; Basu, Sarbani; Demarque, Pierre
2016-05-01
One of the largest sources of uncertainty in stellar models is caused by the treatment of convection in stellar envelopes. One-dimensional stellar models often make use of the mixing length or equivalent approximations to describe convection, all of which depend on various free parameters. There have been attempts to rectify this by using 3D radiative-hydrodynamic simulations of stellar convection, and in trying to extract an equivalent mixing length from the simulations. In this Letter, we show that the entropy of the deeper, adiabatic layers in these simulations can be expressed as a simple function of {log}g and {log}{T}{{eff}}, which holds potential for calibrating stellar models in a simple and more general manner.
Buoyant currents arrested by convective dissolution
NASA Astrophysics Data System (ADS)
MacMinn, Christopher W.; Juanes, Ruben
2013-05-01
When carbon dioxide (CO2) dissolves into water, the density of water increases. This seemingly insubstantial phenomenon has profound implications for geologic carbon sequestration. Here we show, by means of laboratory experiments with analog fluids, that the up-slope migration of a buoyant current of CO2 is arrested by the convective dissolution that ensues from a fingering instability at the moving CO2-groundwater interface. We consider the effectiveness of convective dissolution as a large-scale trapping mechanism in sloping aquifers, and we show that a small amount of slope is beneficial compared to the horizontal case. We study the development and coarsening of the fingering instability along the migrating current and predict the maximum migration distance of the current with a simple sharp-interface model. We show that convective dissolution exerts a powerful control on CO2 plume dynamics and, as a result, on the potential of geologic carbon sequestration.
Thermoelectric convection in a variable temperature field
NASA Astrophysics Data System (ADS)
Belyaev, A. V.; Smorodin, B. L.
2008-03-01
We have studied the convective instability of a horizontal layer of a liquid semiconductor or an ionic melt in the presence of a variable temperature gradient under microgravity conditions in the case where an excess charge can appear only as a result of the thermostimulated diffusion. Thresholds for the onset of thermoelectric convective instability are determined. It is established that, under variable thermal action with a zero mean value, perturbations of a subharmonic response are absent. Depending on the amplitude and frequency of modulation and on the physical properties of the semiconductor (or the melt), synchronous perturbations are differently manifested and belong to various classes. The amplitudes and frequencies of an external action necessary for the effective suppression of thermoelectric convection are determined.
Viscous stratification of the earth and convection
NASA Technical Reports Server (NTRS)
Elsasser, W. M.
1972-01-01
The shallow model of the earth's mantle is discussed along with a variety of geophysical arguments for its correctness and against the existence of deep convection. The main agrument is summarized in the proposal that the astheno sphere is less viscous (by a factor of 10 to 100) than has generally been assumed. In this shallow model, the return flow is essentially through the asthenosphere. The dynamical agent is the steep temperature gradient in the upper mantle. Speculations as to the historical variation of this gradient are advanced. The effects on the model of a nonuniform earth aggregation are considered and shown to favor shallow convection as well as a top convective layer (lithosphere plus asthenosphere) whose depth increases slowly over the earth's life, leading to a tectonic activity that increases gradually with time.
Purple fingers: convection in carbon sequestration
NASA Astrophysics Data System (ADS)
Slim, Anja; Bandi, Mahesh; Mahadevan, L.
2011-11-01
In geological carbon sequestration, CO2 injected into a saline formation is less dense than the resident brine and floats above it. It is slightly soluble in brine and progressively dissolves. Brine with dissolved CO2 is slightly denser than ``pure'' brine giving the potential for convective overturning. We experimentally study the dissolution process from first contact between CO2 and brine through onset of convection to almost complete saturation for Rayleigh numbers between 80 and 1200. We present an analogue experiment using potassium permanganate as our solute, which closely mimics relevant CO2 properties. We describe the different dynamical regimes and connect these with trends in several global measures, including the dissolution flux. We find onset of convection using both amplitude and flux deviations from pure diffusion.
Mineral Dust Impacts on Organized Convection Anvils
NASA Astrophysics Data System (ADS)
Seigel, R. B.; van den Heever, S. C.; Saleeby, S.
2012-12-01
Mineral dust in the atmosphere impacts both radiative and microphysical processes. As it is arguably the most abundant aerosol species in the world, dust plays a large role in the global energy budget. In order to understand its global distribution through transport, we must first understand how deep convective clouds microphysically process and subsequently vent mineral dust. This research utilizes a numerically simulated idealized squall line to (1) investigate the impact of mineral dust on convective anvils and aerosol venting, and (2) assess the aerosol indirect effect. To accomplish these tasks, we use the Regional Atmospheric Modeling System (RAMS) set up as a convection-resolving model (CRM). The CRM contains aerosol and microphysical schemes that allow radiatively active mineral dust particles to nucleate as cloud drops and ice crystals, replenish upon evaporation and sublimation, be tracked throughout hydrometeor transition, and scavenge by precipitation and dry sedimentation. Four simulations of the squall line are performed in order to directly assess the individual contributions of radiation and microphysics to the aerosol indirect effects from mineral dust. After three hours into the simulation of a squall line, the four sensitivity simulations are performed by toggling: (1) radiation off and dust not microphysically active; (2) radiation on and dust not microphysically active; (3) radiation off and dust microphysically active; and (4) radiation on and dust microphysically active. The systematic toggling between radiation on and dust being microphysically active allows for direct quantification of mineral dust impacts on various convective and radiative processes governing the squall line. As expansive organized convection anvils are greatly important for both regional and global radiation budgets, this research will highlight both mineral dusts impacts on the anvil region and the venting process of dust in the wake of deep convection.
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
Impacts of Convective Triggering on Convective Variability in a Climate Model
NASA Astrophysics Data System (ADS)
Wang, Y. C.
2015-12-01
In this study, we investigated the impacts of the triggering designs of the deep convection scheme on convective variability from diurnal rainfall cycle to intraseasonal rainfall variability by using NCAR CAM5 model. Using single-column simulations at the Southern Great Plains site, we found that the underestimated nighttime rainfall of diurnal cycle can be greatly improved when two convective triggering designs from the Simplified Arakawa-Schubert scheme (SAS) are implemented into the default Zhang-Mcfarlane (ZM) scheme. We further conducted AMIP-type climate simulations with this modified ZM scheme (ZMMOD), and found that improvements can also be seen for the diurnally propagating convection over topographical regions, such as Maritime Continent and the western coast of Columbia. We further examined the rainfall variability from synoptic to intraseasonal scales, and found that using ZMMOD scheme increases rainfall variability of 2-10-day over South America and Africa land regions. However, this improvement does not seem to transfer to the intraseasonal convective organization (20-100 days), such as the MJO. This study demonstrates the importance of convective triggering and its impacts on convective variability. This work is still on-going to understand the physical processes of such impacts and how they might affect climate systems through multiscale interactions.
Towards high-resolution mantle convection simulations
NASA Astrophysics Data System (ADS)
Höink, T.; Richards, M. A.; Lenardic, A.
2009-12-01
The motion of tectonic plates at the Earth’s surface, earthquakes, most forms of volcanism, the growth and evolution of continents, and the volatile fluxes that govern the composition and evolution of the oceans and atmosphere are all controlled by the process of solid-state thermal convection in the Earth’s rocky mantle, with perhaps a minor contribution from convection in the iron core. Similar processes govern the evolution of other planetary objects such as Mars, Venus, Titan, and Europa, all of which might conceivably shed light on the origin and evolution of life on Earth. Modeling and understanding this complicated dynamical system is one of the true “grand challenges” of Earth and planetary science. In the past three decades much progress towards understanding the dynamics of mantle convection has been made, with the increasing aid of computational modeling. Numerical sophistication has evolved significantly, and a small number of independent codes have been successfully employed. Computational power continues to increase dramatically, and with it the ability to resolve increasingly finer fluid mechanical structures. Yet, the perhaps most often cited limitation in numerical modeling based publications is still the limitation of computing power, because the ability to resolve thermal boundary layers within the convecting mantle (e.g., lithospheric plates), requires a spatial resolution of ~ 10 km. At present, the largest supercomputing facilities still barely approach the power to resolve this length scale in mantle convection simulations that include the physics necessary to model plate-like behavior. Our goal is to use supercomputing facilities to perform 3D spherical mantle convection simulations that include the ingredients for plate-like behavior, i.e. strongly temperature- and stress-dependent viscosity, at Earth-like convective vigor with a global resolution of order 10 km. In order to qualify to use such facilities, it is also necessary to
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.
Double-diffusive inner core convective translation
NASA Astrophysics Data System (ADS)
Deguen, Renaud; Alboussière, Thierry; Labrosse, Stéphane
2016-04-01
The hemispherical asymmetry of the inner core has been interpreted as resulting form a high-viscosity mode of inner core convection, consisting in a translation of the inner core. With melting on one hemisphere and crystallization on the other one, inner core translation would impose a strongly asymmetric buoyancy flux at the bottom of the outer core, with likely strong implications for the dynamics of the outer core and the geodynamo. The main requirement for convective instability in the inner core is an adverse radial density gradient. While older estimates of the inner core thermal conductivity favored a superadiabatic temperature gradient and the existence of thermal convection, the much higher values recently proposed makes thermal convection very unlikely. Compositional convection might be a viable alternative to thermal convection: an unstable compositional gradient may arise in the inner core either because the light elements present in the core are predicted to become increasingly incompatible as the inner core grows (Gubbins et al. 2013), or because of a possibly positive feedback of the development of the F-layer on inner core convection. Though the magnitude of the destabilizing effect of the compositional field is predicted to be similar to or smaller than the stabilizing effect of the thermal field, the huge difference between thermal and chemical diffusivities implies that double-diffusive instabilities can still arise even if the net density decreases upward. We propose here a theoretical and numerical study of double diffusive convection in the inner core that demonstrate that a translation mode can indeed exist if the compositional field is destabilizing, even if the temperature profile is subadiabatic, and irrespectively of the relative magnitude of the destabilizing compositional gradient and stabilizing temperature field. The predicted inner core translation rate is similar to the mean inner core growth rate, which is more consistent with
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.
A high-latitude convective cloud feedback
NASA Astrophysics Data System (ADS)
Abbot, Dorian Schuyler
Available data suggest that during the late Cretaceous and early Paleogene (˜100 to ˜35 million years ago) Earth had an "equable" climate: the equator to pole surface temperature difference and seasonal cycle in high-latitude surface temperature were both much smaller than they are today. The combination of much warmer high latitudes, particularly during winter, and only somewhat warmer tropics has traditionally been difficult to model in global climate models and to explain physically. In this thesis a positive feedback on high-latitude surface temperatures based on the onset of convection and convective clouds is proposed and investigated using a hierarchy of climate models. This feedback mechanism is based on an initial warming leading to destabilization of the high-latitude atmosphere to convection, causing convection, which results in convective clouds and increased atmospheric moisture, both of which trap outgoing longwave radiation and lead to further warming. It is also shown that this convective cloud feedback could be active in a future atmosphere with increased greenhouse gasses, increasing high-latitude climate uncertainty under global warming scenarios. A variety of climate models are used to understand the convective cloudfeedback in this thesis. Simple, analytical models are used in two chapters to understand more complex models and their underlying physics. A zonally-averaged, two-level model of the atmosphere without a seasonal cycle, but containing a hydrological cycle and parameterizations of convection, precipitation, and clouds, and a longwave radiation scheme that explicitly depends on CO 2, water vapor, and cloud fraction is constructed and used as an initial test of the feedback mechanism. The National Center for Atmospheric Research (NCAR) single column model (SCAM), which contains state-of-the-art atmospheric physics parameterizations, high vertical resolution, a full seasonal cycle, a thermodynamic sea ice model, and a mixed layer ocean
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.
NASA Astrophysics Data System (ADS)
Förster, M.; Feldstein, Y. I.; Haaland, S. E.; Dremukhina, L. A.; Gromova, L. I.; Levitin, A. E.
2009-08-01
Cluster/EDI electron drift observations above the Northern and Southern polar cap areas for more than seven and a half years (2001-2008) have been used to derive a statistical model of the high-latitude electric potential distribution for summer conditions. Based on potential pattern for different orientations of the interplanetary magnetic field (IMF) in the GSM y-z-plane, basic convection pattern (BCP) were derived, that represent the main characteristics of the electric potential distribution in dependence on the IMF. The BCPs comprise the IMF-independent potential distribution as well as patterns, which describe the dependence on positive and negative IMFBz and IMFBy variations. The full set of BCPs allows to describe the spatial and temporal variation of the high-latitude electric potential (ionospheric convection) for any solar wind IMF condition near the Earth's magnetopause within reasonable ranges. The comparison of the Cluster/EDI model with the IZMEM ionospheric convection model, which was derived from ground-based magnetometer observations, shows a good agreement of the basic patterns and its variation with the IMF. According to the statistical models, there is a two-cell antisunward convection within the polar cap for northward IMFBz+≤2 nT, while for increasing northward IMFBz+ there appears a region of sunward convection within the high-latitude daytime sector, which assumes the form of two additional cells with sunward convection between them for IMFBz+≍4-5 nT. This results in a four-cell convection pattern of the high-latitude convection. In dependence of the ±IMFBy contribution during sufficiently strong northward IMFBz conditions, a transformation to three-cell convection patterns takes place.
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.
The structure and evolution of boundary layers in stratified convection
NASA Astrophysics Data System (ADS)
Anders, Evan H.; Brown, Benjamin; Brandenburg, Axel; Rast, Mark
2016-05-01
Solar convection is highly stratified, and the density in the Sun increases by many orders of magnitude from the photosphere to the base of the convection zone. The photosphere is an important boundary layer, and interactions between the surface convection and deep convection may lie at the root of the solar convection conundrum, where observed large-scale velocities are much lower than predicted by full numerical simulations. Here, we study the structure and time evolution of boundary layers in numerical stratified convection. We study fully compressible convection within plane-parallel layers using the Dedalus pseudospectral framework. Within the context of polytropic stratification, we study flows from low (1e-3) to moderately high (0.1) Mach number, and at moderate to high Rayleigh number to study both laminar and turbulent convective transport. We aim to characterize the thickness and time variation of velocity and thermal (entropy) boundary layers at the top and bottom boundaries of the domain.
Implementing New Semi-Convection and Overshooting Prescriptions in KEPLER
NASA Astrophysics Data System (ADS)
Brown, Justin; Garaud, P.; Woosley, S. E.
2014-01-01
The processes of semi-convection and overshooting convection have been shown to have drastic impacts on the evolution of stars; in particular, the pre-supernova structure of massive stars depends strongly on the mixing prescription of semi-convection used in 1D stellar models. This has a significant impact on the properties of the supernova and the produced nucleosynthesis (Woosley & Heger 2002). There is currently little consensus on the most appropriate 1D models of semi-convection and overshooting convection, so we have implemented the physically-motivated semi-convection prescriptions from Wood, Garaud, & Stellmach (2013) and Moll, Garaud, & Stellmach (in preparation) and overshooting convection prescription from Rempel (2004) into KEPLER. We present the comparisons of these implementations with the previous semi-convection treatment from Langer, Fricke, & Sugimoto (1983) for a range of massive stars.
Seafloor dynamics in mantle convection models
NASA Astrophysics Data System (ADS)
Coltice, N.; Rolf, T.; Tackley, P. J.; Labrosse, S.
2012-12-01
The distribution of seafloor ages determines fundamental characteristics of our planet: sea-level, ocean chemistry,tectonic forces and heat loss. The present-day distribution suggests that subduction affects lithosphere of all ageswith the same probability (B. Parsons, J. Geophys. Res 87, 289-302, 1982). This is at odds with the theory of thermal convection which predicts that subduction should happen once a critical age has been reached. So far, the area-age distribution remains a primary constraint, which convection models have failed to satisfy (S. Labrosse and C. Jaupart, Earth Planet. Sci. Lett. 260, 465-481, 2007). We will show that combined action of plate-like behavior and continents causes the seafloor area-age distribution in spherical models of mantle convection to be Earth-like (Coltice et al., Science 336, 335-338, 2012). Our simulations suggest that the seafloor age distribution on Earth evolves over a time-scale of several 100Myrs. Depending on the parameters of the convective flow, strong variations of the production rate of new ocean floor and of the length of ridges are obtained.
Jupiter's Convection and Its Red Spot.
Smoluchowski, R
1970-06-12
Physical properties of the liquid hydrogen-helium layer of Jupiter are calculated and used in evaluating convection and in interpreting the approximately constant rate of longitudinal motion of the Red Spot on the basis of the Hide-Streett model. PMID:17731041
The Extension of Convective Boundedness Criterion
NASA Astrophysics Data System (ADS)
Jian, Wu; Traoré, Philippe; Hubert, Romat
2010-09-01
The paper describes an extension of the well-known Convective Boundedness Criterion (CBC). It is shown that the newly proposed criterion is a combination of the CBC and the extended convective boundedness criterion (ECBC), as shown in Fig.1. A new scheme (NECBC1) based on the new criterion is designed and tested by two problems: (1) convection of a stepwise profile in an oblique uniform velocity field and (2) convection of an elliptical profile in a stagnation point flow. The numerical tests show the effectiveness of the new criterion and reveal the limitation of the CBC and the ECBC. Moreover, some numerical experiments of some specially-designed schemes and two TVD-Type schemes: the van Albada scheme and Miroslav Čada & Manuel Torrilhon's new third-order scheme, are carried out. Through these numerical experiments, some extra constraints for the new criterion are observed and in the meantime some other possible regions in the normalized diagram (NV) for high-resolution schemes reveal themselves.
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.
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)
Convectively driven PCR thermal-cycling
Benett, William J.; Richards, James B.; Milanovich, Fred P.
2003-07-01
A polymerase chain reaction system provides an upper temperature zone and a lower temperature zone in a fluid sample. Channels set up convection cells in the fluid sample and move the fluid sample repeatedly through the upper and lower temperature zone creating thermal cycling.
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).
Extreme Convective Weather in Future Decades
NASA Astrophysics Data System (ADS)
Gadian, Alan; Burton, Ralph; Groves, James; Blyth, Alan; Warner, James; Holland, Greg; Bruyere, Cindy; Done, James; Thielen, Jutta
2016-04-01
WISER (Weather Climate Change Impact Study at Extreme Resolution) is a project designed to analyse changes in extreme weather events in a future climate, using a weather model (WRF) which is able to resolve small scale processes. Use of a weather model is specifically designed to look at convection which is of a scale which cannot be resolved by climate models. The regional meso-scale precipitation events, which are critical in understanding climate change impacts will be analysed. A channel domain outer model, with a resolution of ~ 20km in the outer domain drives an inner domain of ~ 3 km resolution. Results from 1989-1994 and 2020-2024 and 2030-2034 will be presented to show the effects of extreme convective events over Western Europe. This presentation will provide details of the project. It will present data from the 1989-1994 ERA-interim and CCSM driven simulations, with analysis of the future years as defined above. The representation of pdfs of extreme precipitation, Outgoing Longwave Radiation and wind speeds, with preliminary comparison with observations will be discussed. It is also planned to use the output to drive the EFAS (European Flood model) to examine the predicted changes in quantity and frequency of severe and hazardous convective rainfall events and leading to the frequency of flash flooding due to heavy convective precipitation.
Global tectonics from mantle convection models
NASA Astrophysics Data System (ADS)
Coltice, N.
2015-12-01
The motions of the surface of the Earth are described using the theory of Plate Tectonics. Despite the fact that this theory has shaped modern geosciences it has some limitations, and among them the impossibility to evaluate the forces at the origin of the surface displacements and deformations. Hence important questions remain difficult to solve like the origin of the sizes of plates, forces driving mountain building or supercontinent dispersal... Tremendous progresses have been made in the past 15 years in mantle convection modelling. Especially, modern convection codes can solve for motion equations with complex material properties. Since the early 2000's, the development of pseudo-plastic rheologies contributed to produce convection models with plate-like behaviour: plates naturally emerge and interact with the flow in a self-organized manner. Using such models in 3D spherical geometry (computed with StagYY - Tackley, 2008), I will show that important questions on the global tectonics of the planet can be addressed now: the distribution of seafloor ages, the distribution of plate area, the lifetime of small and large plates or modes of plate reorganizations. Tackley, P.J., Modellng compressible mantle convection with large viscosity contrasts in a three-dimensional spherical shell using the yin-yang grid, Phys. Earth Planet. Inter, 171, 7-18 (2008).
Forced Convection Heat Transfer in Circular Pipes
ERIC Educational Resources Information Center
Tosun, Ismail
2007-01-01
One of the pitfalls of engineering education is to lose the physical insight of the problem while tackling the mathematical part. Forced convection heat transfer (the Graetz-Nusselt problem) certainly falls into this category. The equation of energy together with the equation of motion leads to a partial differential equation subject to various…
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.
Convection, helioseismology and solar energy: personal reminiscence
NASA Astrophysics Data System (ADS)
Unno, Wasaburo
2014-08-01
This article is a brief history of my life from childhood and describes how I became interested in astronomy. Starting from researches using radiative transfer as a main tool, I gradually expanded my research field to hydrodynamics (particularly convection, turbulence, pulsation, waves and helioseismology), magnetohydrodynamics and chaotic systems. My recent interest is to develop a sustainable society using solar energy.
Parameterization of sub-grid scale convection
NASA Technical Reports Server (NTRS)
Frank, William; Molinari, John; Kain, Jack; Moncrieff, Mitch; Karyampudi, Mohan; Grell, Georg
1993-01-01
The following topics are discussed: an overview of the cumulus parameterization problem; interactions between explicit and implicit processes in mesoscale models; effects of model grid size on the cumulus parameterization problem; parameterizing convective effects on momentum fields in mesoscale models; differences between slantwise and vertical cumulus parameterization; experiments with different closure hypotheses; and coupling cumulus parameterizations to boundary layer, stable cloud, and radiation schemes.
White Dwarf Convection Preceding Type Ia Supernovae
NASA Astrophysics Data System (ADS)
Zingale, Michael; Almgren, A. S.; Bell, J. B.; Malone, C. M.; Nonaka, A.; Woosley, S. E.
2010-01-01
In the single degenerate scenario for Type Ia supernovae, a Chandrasekhar mass white dwarf `simmers' for centuries preceding the ultimate explosion. During this period, reactions near the center drive convection throughout most of the interior of the white dwarf. The details of this convective flow determine how the first flames in the white dwarf ignite. Simulating this phase is difficult because the flows are highly subsonic. Using the low Mach number hydrodynamics code, MAESTRO, we present 3-d, full star models of the final hours of this convective phase, up to the point of ignition of a Type Ia supernova. We discuss the details of the convective velocity field and the locations of the initial hot spots. Finally, we show some preliminary results with rotation. Support for this work came from the DOE/Office of Nuclear Physics, grant No. DE-FG02-06ER41448 (Stony Brook), the SciDAC Program of the DOE Office of Mathematics, Information, and Computational Sciences under the DOE under contract No. DE-AC02-05CH11231 (LBNL), and the DOE SciDAC program, under grant No. DE-FC02-06ER41438 (UCSC). We made use of the jaguar machine via a DOE INCITE allocation at the Oak Ridge Leadership Computational Facility.
Convection in Slab and Spheroidal Geometries
NASA Technical Reports Server (NTRS)
Porter, David H.; Woodward, Paul R.; Jacobs, Michael L.
2000-01-01
Three-dimensional numerical simulations of compressible turbulent thermally driven convection, in both slab and spheroidal geometries, are reviewed and analyzed in terms of velocity spectra and mixing-length theory. The same ideal gas model is used in both geometries, and resulting flows are compared. The piecewise-parabolic method (PPM), with either thermal conductivity or photospheric boundary conditions, is used to solve the fluid equations of motion. Fluid motions in both geometries exhibit a Kolmogorov-like k(sup -5/3) range in their velocity spectra. The longest wavelength modes are energetically dominant in both geometries, typically leading to one convection cell dominating the flow. In spheroidal geometry, a dipolar flow dominates the largest scale convective motions. Downflows are intensely turbulent and up drafts are relatively laminar in both geometries. In slab geometry, correlations between temperature and velocity fluctuations, which lead to the enthalpy flux, are fairly independent of depth. In spheroidal geometry this same correlation increases linearly with radius over the inner 70 percent by radius, in which the local pressure scale heights are a sizable fraction of the radius. The effects from the impenetrable boundary conditions in the slab geometry models are confused with the effects from non-local convection. In spheroidal geometry nonlocal effects, due to coherent plumes, are seen as far as several pressure scale heights from the lower boundary and are clearly distinguishable from boundary effects.
MAGNETIC WREATHS AND CYCLES IN CONVECTIVE DYNAMOS
Nelson, Nicholas J.; Toomre, Juri; Brown, Benjamin P.; Brun, Allan Sacha
2013-01-10
Solar-type stars exhibit a rich variety of magnetic activity. Seeking to explore the convective origins of this activity, we have carried out a series of global three-dimensional magnetohydrodynamic simulations with the anelastic spherical harmonic code. Here we report on the dynamo mechanisms achieved as the effects of artificial diffusion are systematically decreased. The simulations are carried out at a nominal rotation rate of three times the solar value (3 {Omega}{sub Sun }), but similar dynamics may also apply to the Sun. Our previous simulations demonstrated that convective dynamos can build persistent toroidal flux structures (magnetic wreaths) in the midst of a turbulent convection zone and that high rotation rates promote the cyclic reversal of these wreaths. Here we demonstrate that magnetic cycles can also be achieved by reducing the diffusion, thus increasing the Reynolds and magnetic Reynolds numbers. In these more turbulent models, diffusive processes no longer play a significant role in the key dynamical balances that establish and maintain the differential rotation and magnetic wreaths. Magnetic reversals are attributed to an imbalance in the poloidal magnetic induction by convective motions that is stabilized at higher diffusion levels. Additionally, the enhanced levels of turbulence lead to greater intermittency in the toroidal magnetic wreaths, promoting the generation of buoyant magnetic loops that rise from the deep interior to the upper regions of our simulated domain. The implications of such turbulence-induced magnetic buoyancy for solar and stellar flux emergence are also discussed.
Convective towers detection using GPS radio occultations
NASA Astrophysics Data System (ADS)
Biondi, R.; Neubert, T.; Syndergaard, S.; Nielsen, J.
2010-12-01
The tropical deep convection affects the radiation balance of the atmosphere changing the water vapour mixing ratio and the temperature of the upper troposphere and lower stratosphere. To gain a better understanding of deep convective processes, the study of tropical cyclones could play an important role since they lead to deep convective activity. With this work we want to investigate if severe storms leave a significant signature in radio occultation profiles in the tropical tropopause layer. The GPS radio occultation (RO) technique is useful for studying severe weather phenomena because the GPS signals penetrate through clouds and allow measurements of atmospheric profiles related to temperature, pressure, and water vapour with high vertical resolution. Using tropical cyclone best track database and data from different GPS RO missions (COSMIC, GRACE, CHAMP, SACC and GPSMET), we selected 1194 profiles in a time window of 3 hours and a space window of 300 km from the eye of the cyclone. We show that the bending angle anomaly of a GPS RO signal is typically larger than the climatology above the tropopause. Comparisons with co-located radiosondes, climatology of tropopause altitudes and GOES analyses will also be shown to support our hypothesis and to corroborate the idea that the bending angle anomaly can be used as an indicator of convective towers. The results are discussed in connection to the GPS radio occultation receiver which will be part of the Atomic Clock Ensemble in Space (ACES) payload on the International Space Station.
Probability distribution functions in turbulent convection
NASA Technical Reports Server (NTRS)
Balachandar, S.; Sirovich, L.
1991-01-01
Results of an extensive investigation of probability distribution functions (pdfs) for Rayleigh-Benard convection, in hard turbulence regime, are presented. It is shown that the pdfs exhibit a high degree of internal universality. In certain cases this universality is established within two Kolmogorov scales of a boundary. A discussion of the factors leading to the universality is presented.
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.
Mapping high-latitude plasma convection with coherent HF radars
NASA Technical Reports Server (NTRS)
Ruohoniemi, J. M.; Greenwald, R. A.; Baker, K. B.; Villain, J.-P.; Hanuise, C.
1989-01-01
Several methods developed for mapping high-latitude plasma convection with a high-latitude HF radar are described, which utilize coherent backscatter from electron density irregularities at F-region altitudes to observe convective plasma motion. Several examples of two-dimensional convection-velocity maps are presented, showing instances of L-shell-aligned flow in the dusk sector, the reversal of convection near magnetic midnight, and counterstreaming in the dayside cleft.
NASA Astrophysics Data System (ADS)
Dahley, N.; Futterer, B.; Egbers, C.; Crumeyrolle, O.; Mutabazi, I.
2011-12-01
Within the project "Convection in a Cylinder" (CiC) heat transfer enhancement is studied for the case of two concentric, vertically aligned cylinders. The cylindrical gap is filled with a dielectric liquid, which viscosity is just few times higher than that of water. The inner cylinder is heated and the outer one is cooled. This setup in a gravitational buoyancy field leads to a fluid movement in a single convective cell with hot fluid rising at the inner boundary and cold fluid sinking at the outer boundary. The top and bottom part of the system shows horizontal movement, again in boundary layers. The strengthening of temperature gradient induces instabilities of that convective motion. If we vary the buoyancy force by means of electro-hydrodynamic effects, the patterns of convection differ from those instabilities rising only from variation of the temperature gradient.
Convective effects during the physical vapor transport process. I - Thermal convection
NASA Technical Reports Server (NTRS)
Duval, Walter M. B.
1992-01-01
The effects of convection on diffusive-convective physical vapor transport process are examined computationally. We analyze conditions ranging from typical laboratory conditions to conditions achievable only in a low gravity environment. This corresponds to thermal Rayleigh numbers Ra(T) ranging from 1.80 to 1.92 x 10 exp 6. Our results indicate that the effect of the sublimation and condensation fluxes at the boundaries is 10 increase the threshold of instability. For typical ground based conditions time dependent oscillatory convection can occur. This results in nonuniform temperature and concentration gradients at the crystal interface. Spectral analysis of the flow field shows regions of both periodic and quasi-periodic states. Low gravity conditions can effectively reduce convective effects, thus resulting in uniform temperature and concentration gradients at the interface, a desirable condition for crystal growth.
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
Precipitation Characteristics in Warm Convective Clouds
NASA Astrophysics Data System (ADS)
Xue, H.; Ma, Y.; Feingold, G.
2011-12-01
The relationship between radar reflectivity factor Z at 9.6 GHz (3 cm) and rain rate R for warm convective clouds is studied. The objectives are to obtain a reasonable Z-R relationship for use in weather radar observation of warm convective precipitation, and to analyze factors that affect the Z-R relationship. Rain rate R is calculated from the drop size distributions in a large eddy simulation (LES); the drop size distributions from LES are also used as inputs into Quickbeam, a software package for simulating atmospheric radiative characteristics, to get radar reflectivity factor Z. It is found that a uniform Z-R relationship is not valid for the cumulus cloud population that develops for several hours. The Z-R relationship depends on the stage of cloud development and the height relative to cloud base. As expected, a range of R values can all lead to the same Z. This is due to the complicated drop size distributions and may cause large uncertainty in precipitation measurement in warm convective clouds using radar data. This study also investigates the Z-R relationship at 94 GHz (3 mm) to evaluate the possibility of measuring precipitation in warm convective clouds using current millimeter wave cloud radars. Results show that a well-defined Z-R relationship at 94 GHz generally exists when the local rain rate is smaller than 1 mm hour-1. This indicates that a millimeter wave cloud radar can be used to measure light precipitation in warm convective clouds. When precipitation is stronger, the attenuation of the signal due to precipitation particles is significant and the estimation of R from the reflectivity factor Z has bigger uncertainty. The domain-averaged rain rate R can be parameterized as a function of domain-averaged liquid water path and cloud drop concentration for the LES clouds. The result for warm convective clouds in this study is consistent with previous findings for stratiform clouds. This may help to better parameterize the warm convective
The Dynamics of Titan's Convective Clouds
NASA Astrophysics Data System (ADS)
Rafkin, S. C.
2012-12-01
Titan's deep convective clouds are the most dynamic phenomena known to operate within the atmosphere of the moon. Previous studies have focused primarily on the control of these storms by the large scale thermodynamic environment, especially methane abundance, which determines the amount of convective available potential energy (CAPE). This study looks at factors in addition to the thermodynamic environment that may have a first order impact on the evolution and structure of Titan's deep convective clouds. To the extent that thunderstorms on Earth provide a reasonable analog to the storms on Titan, it is well established that CAPE alone is insufficient to determine the structure and behavior of deep convection. Wind shear—both directional and speed—is also known to exert a first order effect. The influence of both CAPE and wind speed shear is typically expressed as the ratio of the two parameters in the form of the Bulk Richardson Number. On Earth, for a fixed value of CAPE, the addition of wind speed shear (i.e., the reduction of the Bulk Richardson Number) will tend to produce storms that are longer lived, tilted upshear with height, and multi-cellular in nature. These multi-cellular storms also tend to be more violent than storms generated in low wind speed shear environments: strong winds and large hail are common. The addition of directional shear (i.e., helicity) can transform the multi-cell storms into single, intense supercell storms. These are the storms associated typically associated with tornadoes. With respect to Titan, if there is a similar dependence on the Bulk Richardson Number, then this would have implications for how long Titan's storms live, how much precipitation they can produce, the area they cover, and the strength and duration of winds. A series of numerical simulations of Titan's deep convective clouds from the Titan Regional Atmospheric Modeling System are presented. A reasonable sweep of the parameter space of CAPE and shear for
Testing particle filters on convective scale dynamics
NASA Astrophysics Data System (ADS)
Haslehner, Mylene; Craig, George. C.; Janjic, Tijana
2014-05-01
Particle filters have been developed in recent years to deal with highly nonlinear dynamics and non Gaussian error statistics that also characterize data assimilation on convective scales. In this work we explore the use of the efficient particle filter (P.v. Leeuwen, 2011) for convective scale data assimilation application. The method is tested in idealized setting, on two stochastic models. The models were designed to reproduce some of the properties of convection, for example the rapid development and decay of convective clouds. The first model is a simple one-dimensional, discrete state birth-death model of clouds (Craig and Würsch, 2012). For this model, the efficient particle filter that includes nudging the variables shows significant improvement compared to Ensemble Kalman Filter and Sequential Importance Resampling (SIR) particle filter. The success of the combination of nudging and resampling, measured as RMS error with respect to the 'true state', is proportional to the nudging intensity. Significantly, even a very weak nudging intensity brings notable improvement over SIR. The second model is a modified version of a stochastic shallow water model (Würsch and Craig 2013), which contains more realistic dynamical characteristics of convective scale phenomena. Using the efficient particle filter and different combination of observations of the three field variables (wind, water 'height' and rain) allows the particle filter to be evaluated in comparison to a regime where only nudging is used. Sensitivity to the properties of the model error covariance is also considered. Finally, criteria are identified under which the efficient particle filter outperforms nudging alone. References: Craig, G. C. and M. Würsch, 2012: The impact of localization and observation averaging for convective-scale data assimilation in a simple stochastic model. Q. J. R. Meteorol. Soc.,139, 515-523. Van Leeuwen, P. J., 2011: Efficient non-linear data assimilation in geophysical
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
Turbulent motions within the solar convection zone play a central role in the generation and maintenance of the Sun's magnetic field. This magnetic field reverses its polarity every 11 years and serves as the source of powerful space weather events, such as solar flares and coronal mass ejections, which can affect artificial satellites and power grids. The structure and inductive properties are linked to the amplitude (i.e. speed) of convective motion. Using the NASA Pleiades supercomputer, a 3D fluids code simulates these processes by evolving the Navier-Stokes equations in time and under an anelastic constraint. This code simulates the fluxes describing heat transport in the sun in a global spherical-shell geometry. Such global models can explicitly capture the large-scale motions in the deep convection zone but heat transport from unresolved small-scale convection in the surface layers must be parameterized. Here we consider two models for heat transport by surface convection, including a conventional turbulent thermal diffusion as well as an imposed flux that carries heat through the surface in a manner that is independent of the deep convection and the entropy stratification it establishes. For both models, we investigate the scaling of convective amplitude with decreasing diffusion (increasing Rayleigh number). If the Prandtl number is fixed, we find that the amplitude of convective motions increases with decreasing diffusion, possibly reaching an asymptotic value in the low diffusion limit. However, if only the thermal diffusion is decreased (keeping the viscosity fixed), we find that the amplitude of convection decreases with decreasing diffusion. Such a high-Prandtl-number, high-Peclet-number limit may be relevant for the Sun if magnetic fields mix momentum, effectively acting as an enhanced viscosity. In this case, our results suggest that the amplitude of large-scale convection in the Sun may be substantially less than in current models that employ an
CONVECTION THEORY AND SUB-PHOTOSPHERIC STRATIFICATION
Arnett, David; Meakin, Casey; Young, Patrick A. E-mail: casey.meakin@gmail.co
2010-02-20
As a preliminary step toward a complete theoretical integration of three-dimensional compressible hydrodynamic simulations into stellar evolution, convection at the surface and sub-surface layers of the Sun is re-examined, from a restricted point of view, in the language of mixing-length theory (MLT). Requiring that MLT use a hydrodynamically realistic dissipation length gives a new constraint on solar models. While the stellar structure which results is similar to that obtained by Yale Rotational Evolution Code (Guenther et al.; Bahcall and Pinsonneault) and Garching models (Schlattl et al.), the theoretical picture differs. A new quantitative connection is made between macro-turbulence, micro-turbulence, and the convective velocity scale at the photosphere, which has finite values. The 'geometric parameter' in MLT is found to correspond more reasonably with the thickness of the superadiabatic region (SAR), as it must for consistency in MLT, and its integrated effect may correspond to that of the strong downward plumes which drive convection (Stein and Nordlund), and thus has a physical interpretation even in MLT. If we crudely require the thickness of the SAR to be consistent with the 'geometric factor' used in MLT, there is no longer a free parameter, at least in principle. Use of three-dimensional simulations of both adiabatic convection and stellar atmospheres will allow the determination of the dissipation length and the geometric parameter (i.e., the entropy jump) more realistically, and with no astronomical calibration. A physically realistic treatment of convection in stellar evolution will require substantial additional modifications beyond MLT, including nonlocal effects of kinetic energy flux, entrainment (the most dramatic difference from MLT found by Meakin and Arnett), rotation, and magnetic fields.
Convective dynamo and emerging flux in the solar convective envelope (Invited)
NASA Astrophysics Data System (ADS)
Fan, Y.
2013-12-01
We present MHD simulations of a convective dynamo in the rotating solar convective envelope driven by the radiative diffusive heat flux in the solar convection zone. Our simulations have obtained a quasi-cyclic large-scale mean field with a period of roughly 10 years. The mean axisymmetric toroidal magnetic field peaks at the bottom of the convection zone, reaching a value of about 7000 G. Including the fluctuating component, individual channels of strong field reaching about 30kG are present. The axisymmetric mean field shows a current helicity that is predominantly negative in the northern hemisphere (consistent with the sense of twist of the solar active regions), but the current helicity of the fluctuating component is opposite, and is of a much larger amplitude. The presence of the turbulent magnetic fields appears to be important for maintaining a solar-like differential rotation. Without the magnetic fields, the convective flows drive a differential rotation with a faster rotating polar-region. We also present simulations where strong buoyant toroidal flux tubes are inserted into the rotating solar convective flows to study their buoyant rise and the properties of the emerging flux they produce.
Convective scale interaction: Arc cloud lines and the development and evolution of deep convection
NASA Technical Reports Server (NTRS)
Purdom, James Francis Whitehurst
1986-01-01
Information is used from satellite data and research aircraft data to provide new insights concerning the mesoscale development and evolution of deep convection in an atmosphere typified by weak synoptic-scale forcing. The importance of convective scale interaction in the development and evolution of deep convection is examined. This interaction is shown to manifest itself as the merger and intersection of thunderstorm outflow boundaries (arc cloud lines) with other convective lines, areas or boundaries. Using geostationary satellite visible and infrared data convective scale interaction is shown to be responsible for over 85 percent of the intense convection over the southeast U.S. by late afternoon, and a majority of that area's afternoon rainfall. The aircraft observations provided valuable information concerning critically important regions of the arc cloud line: (1) the cool outflow region, (2) the density surge line interface region; and (3) the sub-cloud region above the surge line. The observations when analyzed with rapid scan satellite data, helped in defining the arc cloud line's life cycle as 3 evolving stages.
A continuous and prognostic convection scheme based on buoyancy, PCMT
NASA Astrophysics Data System (ADS)
Guérémy, Jean-François; Piriou, Jean-Marcel
2016-04-01
A new and consistent convection scheme (PCMT: Prognostic Condensates Microphysics and Transport), providing a continuous and prognostic treatment of this atmospheric process, is described. The main concept ensuring the consistency of the whole system is the buoyancy, key element of any vertical motion. The buoyancy constitutes the forcing term of the convective vertical velocity, which is then used to define the triggering condition, the mass flux, and the rates of entrainment-detrainment. The buoyancy is also used in its vertically integrated form (CAPE) to determine the closure condition. The continuous treatment of convection, from dry thermals to deep precipitating convection, is achieved with the help of a continuous formulation of the entrainment-detrainment rates (depending on the convective vertical velocity) and of the CAPE relaxation time (depending on the convective over-turning time). The convective tendencies are directly expressed in terms of condensation and transport. Finally, the convective vertical velocity and condensates are fully prognostic, the latter being treated using the same microphysics scheme as for the resolved condensates but considering the convective environment. A Single Column Model (SCM) validation of this scheme is shown, allowing detailed comparisons with observed and explicitly simulated data. Four cases covering the convective spectrum are considered: over ocean, sensitivity to environmental moisture (S. Derbyshire) non precipitating shallow convection to deep precipitating convection, trade wind shallow convection (BOMEX) and strato-cumulus (FIRE), together with an entire continental diurnal cycle of convection (ARM). The emphasis is put on the characteristics of the scheme which enable a continuous treatment of convection. Then, a 3D LAM validation is presented considering an AMMA case with both observations and a CRM simulation using the same initial and lateral conditions as for the parameterized one. Finally, global
Pattern formation in solutal convection: vermiculated rolls and isolated cells
NASA Astrophysics Data System (ADS)
Cartwright, Julyan H. E.; Piro, Oreste; Villacampa, Ana I.
2002-11-01
Observations of the peculiar behaviour of a drink of liqueur topped with cream led us to perform experiments showing that the instability is a convection phenomenon that arises through destabilizing surface-tension forces. The convection is solutal: driven by gradients of concentration of a solute, rather than by heat gradients as in the more commonly studied thermal convection. The convective patterns, vermiculated rolls and isolated cells, are quite unlike the usual planforms. They are associated with an elastic surface film, and the Marangoni number is high, characteristic of solutal convection. We have conducted further experiments that reproduce these patterns in simpler working fluids.
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
Magnetic Control of Convection during Protein Crystallization
NASA Technical Reports Server (NTRS)
Ramachandran, N.; Leslie, F. W.
2004-01-01
An important component in biotechnology, particularly in the area of protein engineering and rational drug design is the knowledge of the precise three-dimensional molecular structure of proteins. The quality of structural information obtained from X-ray diffraction methods is directly dependent on the degree of perfection of the protein crystals. As a consequence, the growth of high quality macromolecular Crystals for diffraction analyses has been the central focus for bio-chemists, biologists, and bioengineers. Macromolecular crystals are obtained from solutions that contain the crystallizing species in equilibrium with higher aggregates, ions, precipitants, other possible phases of the protein, foreign particles, the walls of container, and a likely host of other impurities. By changing transport modes in general, i.e., reduction of convection and Sedimentation as is achieved in "microgravity", we have been able to dramatically affect the movement and distribution of macromolecules in the fluid, and thus their transport, f o d o n of crystal nuclei, and adsorption to the crystal surface. While a limited number of high quality crystals from space flights have been obtained, as the recent National Research Council (NRC) review of the NASA microgravity crystallization program pointed out, the scientific approach and research in crystallization of proteins has been mainly empirical yielding inconclusive results. We postulate that we can reduce convection in ground-based experiments and we can understand the different aspects of convection control through the use of strong magnetic fields and field gradients. We postulate that limited convection in a magnetic field will provide the environment for the growth of high quality crystals. The approach exploits the variation of fluid magnetic susceptibility with counteracts on for this purpose and the convective damping is realized by appropriately positioning the crystal growth cell so that the magnetic susceptibility
Limit of Predictability in Mantle Convection
NASA Astrophysics Data System (ADS)
Bello, L.; Coltice, N.; Rolf, T.; Tackley, P. J.
2013-12-01
Linking mantle convection models with Earth's tectonic history has received considerable attention in recent years: modeling the evolution of supercontinent cycles, predicting present-day mantle structure or improving plate reconstructions. Predictions of future supercontinents are currently being made based on seismic tomography images, plate motion history and mantle convection models, and methods of data assimilation for mantle flow are developing. However, so far there are no studies of the limit of predictability these models are facing. Indeed, given the chaotic nature of mantle convection, we can expect forecasts and hindcasts to have a limited range of predictability. We propose here to use an approach similar to those used in dynamic meteorology, and more recently for the geodynamo, to evaluate the predictability limit of mantle dynamics forecasts. Following the pioneering works in weather forecast (Lorenz 1965), we study the time evolution of twin experiments, started from two very close initial temperature fields and monitor the error growth. We extract a characteristic time of the system, known as the e-folding timescale, which will be used to estimate the predictability limit. The final predictability time will depend on the imposed initial error and the error tolerance in our model. We compute 3D spherical convection solutions using StagYY (Tackley, 2008). We first evaluate the influence of the Rayleigh number on the limit of predictability of isoviscous convection. Then, we investigate the effects of various rheologies, from the simplest (isoviscous mantle) to more complex ones (plate-like behavior and floating continents). We show that the e-folding time increases with the wavelength of the flow and reaches 10Myrs with plate-like behavior and continents. Such an e-folding time together with the uncertainties in mantle temperature distribution suggests prediction of mantle structure from an initial given state is limited to <50 Myrs. References: 1
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
Physics of Multi-scale Convection In The Earth's Mantle
NASA Astrophysics Data System (ADS)
Korenaga, J.; Jordan, T. H.
We investigate the physics of multi-scale convection in the Earth's mantle, character- ized by the coexistence of large-scale mantle circulation associated plate tectonics and small-scale sublithospheric convection. Several basic scaling laws are derived, using a series of 2-D numerical modeling and 3-D linear stability analyses, for the following three distinct phases of sublithospheric convection: (1) onset of convection, (2) lay- ered convection in the upper mantle, and (3) breakdown of layered convection. First, the onset of convection with temperature-dependent viscosity is studied with 2-D con- vection models. A robust scaling law for onset time is derived by a nonlinear scaling analysis based on the concept of the differential Rayleigh number. Next, the planform of sublithospheric convection is studied by a 3-D linear stability analysis of longitu- dinal rolls in the presence of vertical shear. Finally, the temporal and spatial evolu- tion of sublithospheric convection is studied by 2-D whole-mantle convection models with temperature- and depth-dependent viscosity and an endothermic phase transition. Scaling laws for the breakdown of layered convection as well as the strength of con- vection are derived as a function of viscosity layering, the phase buoyancy parameter, and the thermal Rayleigh number. All of these scaling laws are combined to delineate possible dynamic regimes beneath evolving lithosphere.
The Role of Radiation in Organizing Tropical Convection
NASA Astrophysics Data System (ADS)
Sessions, Sharon; Sentic, Stipo; Herman, Michael; Raymond, David
2015-03-01
Convective organization regulates the radiation emitted to space, and therefore is important for the global heat budget. Organized convection-regions of intense convection surrounded by large cloud-free regions-permit more longwave radiation to escape and therefore may constitute a net cooling effect, while more scattered convection promotes greenhouse warming. Models which simulate the spontaneous organization of deep tropical convection-self-aggregation-suggest that radiative cooling in response to water vapor content is essential for convection to spontaneously organize. Multiple equilibria-steady states which maintain persistent precipitating convection or are completely dry-in small domains with weak temperature gradients (WTG) are analogous to dry and moist regions in larger scale simulations of convective self aggregation. We explore the role of radiative cooling in multiple equilibria. Interactive radiative cooling suppresses convection in the dry state and it permits multiple equilibria over a larger parameter range. However, multiple equilibria still exist with fixed radiative cooling. This suggests that while interactive radiation is conducive for organizing convection, it is not essential. This study elucidates radiation's role in convective organization. Work supported by the NSF.
A Convection Superparameterization Method for Variable-Resolution Meshes
NASA Astrophysics Data System (ADS)
Walko, R. L.; Medvigy, D.; Avissar, R.
2011-12-01
This talk considers the problem of how to represent convection in a model with highly variable horizontal resolution. Ideally, convection should be resolved where the grid is fine enough, while it must be parameterized elsewhere. However, idealized simulations readily demonstrate that transitioning smoothly between these two regimes is highly challenging. The simulations further show that even when resolved, convection can be sensitive to grid scale and to its gradient. We describe a new superparameterization method that is designed to overcome or mitigate these problems. The method uses a small 3D cluster of grid cells, some fine enough to resolve convection and some coarser for representing a convective cell's mesoscale environment, that follow the horizontal motion of a convective cell as a Lagrangian entity. The cluster configuration is independent of the underlying local resolution of the parent model grid, thus helping to isolate the convective response from local gradients of grid resolution. The moist physics parameterization and dynamical core governing convection on the superparameterization grid are identical with those used on regions of the parent model grid that directly resolve convection (and where the superparameterization is therefore not applied), enabling a unified representation of convection and a uniform convective response. We will present early test results from a prototype implementation, and will discuss ongoing work to refine the method.
Convective equilibrium and mixing-length theory for stellarator reactors
Ho, D.D.M.; Kulsrud, R.M.
1985-09-01
In high ..beta.. stellarator and tokamak reactors, the plasma pressure gradient in some regions of the plasma may exceed the critical pressure gradient set by ballooning instabilities. In these regions, convective cells break out to enhance the transport. As a result, the pressure gradient can rise only slightly above the critical gradient and the plasma is in another state of equilibrium - ''convective equilibrium'' - in these regions. Although the convective transport cannot be calculated precisely, it is shown that the density and temperature profiles in the convective region can still be estimated. A simple mixing-length theory, similar to that used for convection in stellar interiors, is introduced in this paper to provide a qualitative description of the convective cells and to show that the convective transport is highly efficient. A numerical example for obtaining the density and temperature profiles in a stellarator reactor is given.
Convective dissolution of carbon dioxide in saline aquifers
NASA Astrophysics Data System (ADS)
Neufeld, Jerome A.; Hesse, Marc A.; Riaz, Amir; Hallworth, Mark A.; Tchelepi, Hamdi A.; Huppert, Herbert E.
2010-11-01
Geological carbon dioxide (CO2) storage is a means of reducing anthropogenic emissions. Dissolution of CO2 into the brine, resulting in stable stratification, increases storage security. The dissolution rate is determined by convection in the brine driven by the increase of brine density with CO2 saturation. We present a new analogue fluid system that reproduces the convective behaviour of CO2-enriched brine. Laboratory experiments and high-resolution numerical simulations show that the convective flux scales with the Rayleigh number to the 4/5 power, in contrast with a classical linear relationship. A scaling argument for the convective flux incorporating lateral diffusion from downwelling plumes explains this nonlinear relationship for the convective flux, provides a physical picture of high Rayleigh number convection in a porous medium, and predicts the CO2 dissolution rates in CO2 accumulations. These estimates of the dissolution rate show that convective dissolution can play an important role in enhancing storage security.
Coupling between convection and large-scale circulation
NASA Astrophysics Data System (ADS)
Becker, T.; Stevens, B. B.; Hohenegger, C.
2014-12-01
The ultimate drivers of convection - radiation, tropospheric humidity and surface fluxes - are altered both by the large-scale circulation and by convection itself. A quantity to which all drivers of convection contribute is moist static energy, or gross moist stability, respectively. Therefore, a variance analysis of the moist static energy budget in radiative-convective equilibrium helps understanding the interaction of precipitating convection and the large-scale environment. In addition, this method provides insights concerning the impact of convective aggregation on this coupling. As a starting point, the interaction is analyzed with a general circulation model, but a model intercomparison study using a hierarchy of models is planned. Effective coupling parameters will be derived from cloud resolving models and these will in turn be related to assumptions used to parameterize convection in large-scale models.
The influence of embedded convection on orographic precipitation
NASA Astrophysics Data System (ADS)
Cannon, D. J.
An in-depth analysis of the dynamical and microphysical processes by which embedded convection influences orographic precipitation, and the representation of these processes in numerical models, is presented. Benchmark idealised convection-permitting simulations are conducted to quantify the impact of embedded convection on orographic precipitation over a broad range of parameter space. Clouds that form over tall and wide mountains (e.g., the Washington Cascades) are found to be highly efficient at forming precipitations even in the absence of embedded convection. When embedded convection develops in such clouds, it produces competing effects that cancel to yield little change in the area-averaged precipitation rate. In contrast, for short and narrow mountains (e.g., the UK Pennines), stratiform precipitation formation is highly inefficient and so rapid precipitation formation within convective updraughts significantly enhances the area-averaged precipitation rate (by up to 133%). A novel analytical model of convective orographic precipitation is developed which successfully reproduces many of the benchmark simulation trends. Simplified representations of the dynamical and microphysical processes governing mixed-phase stratiform and convective clouds are developed to represent a flow with embedded convection. Finally, the representation of embedded convection in idealised simulations employing a commonly-used convection parameterisation scheme is evaluated relative to the benchmark simulations. Significant over-predictions (of up to 90%) in the area-averaged precipitation accumulations are found. Inexpensive modifications to the parameterisation such as changes to both the treatment of precipitation and the trigger function, as well as the introduction of convective memory (representing convective cell advection), may significantly improve the parameterisation of embedded convection in operational numerical weather prediction models.
Electromagnetically driven dwarf tornados in turbulent convection
NASA Astrophysics Data System (ADS)
Kenjereš, Saša
2011-01-01
Motivated by the concept of interdependency of turbulent flow and electromagnetic fields inside the spiraling galaxies, we explored the possibilities of generating a localized Lorentz force that will produce a three-dimensional swirling flow in weakly conductive fluids. Multiple vortical flow patterns were generated by combining arrays of permanent magnets and electrodes with supplied dc current. This concept was numerically simulated and applied to affect natural convection flow, turbulence, and heat transfer inside a rectangular enclosure heated from below and cooled from above over a range of Rayleigh numbers (104<=Ra<=5×109). The large-eddy simulations revealed that for low- and intermediate-values of Ra, the heat transfer was increased more than five times when an electromagnetic forcing was activated. In contrast to the generally accepted view that electromagnetic forcing will suppress velocity fluctuations and will increase anisotropy of turbulence, we demonstrated that localized forcing can enhance turbulence isotropy of thermal convection compared to its neutral state.
Convective instabilities in SN 1987A
NASA Technical Reports Server (NTRS)
Benz, Willy; Thielemann, Friedrich-Karl
1990-01-01
Following Bandiera (1984), it is shown that the relevant criterion to determine the stability of a blast wave, propagating through the layers of a massive star in a supernova explosion, is the Schwarzschild (or Ledoux) criterion rather than the Rayleigh-Taylor criterion. Both criteria coincide only in the incompressible limit. Results of a linear stability analysis are presented for a one-dimensional (spherical) explosion in a realistic model for the progenitor of SN 1987A. When applying the Schwarzschild criterion, unstable regions get extended considerably. Convection is found to develop behind the shock, with a characteristic growth rate corresponding to a time scale much smaller than the shock traversal time. This ensures that efficient mixing will take place. Since the entire ejected mass is found to be convectively unstable, Ni can be transported outward, even into the hydrogen envelope, while hydrogen can be mixed deep into the helium core.
Energy analysis of convectively induced wind perturbations
NASA Technical Reports Server (NTRS)
Fuelberg, Henry E.; Buechler, Dennis E.
1989-01-01
Budgets of divergent and rotational components of kinetic energy (KD and KR) are examined for four upper level wind speed maxima that develop during the fourth Atmospheric Variability Experiment (AVE IV) and the first AVE-Severe Environmental Storms and Mesoscale Experiment (AVE-SESAME I). A similar budget analysis is performed for a low-level jet stream during AVE-SESAME I. The energetics of the four upper level speed maxima is found to have several similarities. The dominant source of KD is cross-contour flow by the divergent wind, and KD provides a major source of KR via a conversion process. Conversion from available potential energy provides an additional source of KR in three of the cases. Horizontal maps reveal that the conversions involving KD are maximized in regions poleward of the convection. Low-level jet development during AVE-SESAME I appears to be assisted by convective activity to the west.
Magnetospheric equilibrium configurations and slow adiabatic convection
NASA Technical Reports Server (NTRS)
Voigt, Gerd-Hannes
1986-01-01
This review paper demonstrates how the magnetohydrostatic equilibrium (MHE) theory can be used to describe the large-scale magnetic field configuration of the magnetosphere and its time evolution under the influence of magnetospheric convection. The equilibrium problem is reviewed, and levels of B-field modelling are examined for vacuum models, quasi-static equilibrium models, and MHD models. Results from two-dimensional MHE theory as they apply to the Grad-Shafranov equation, linear equilibria, the asymptotic theory, magnetospheric convection and the substorm mechanism, and plasma anisotropies are addressed. Results from three-dimensional MHE theory are considered as they apply to an intermediate analytical magnetospheric model, magnetotail configurations, and magnetopause boundary conditions and the influence of the IMF.
Convection in superposed fluid and porous layers
NASA Technical Reports Server (NTRS)
Chen, Falin; Chen, C. F.
1992-01-01
Thermal convection due to heating from below in a porous layer underlying a fluid layer has been analyzed using the Navier-Stokes equations for the fluid layers and the extended Darcy equation (including Brinkman and Forchheimer terms) for the porous layer. The flow is assumed to be two-dimensional and periodic in the horizontal direction. The numerical scheme used is a combined Galerkin and finite-difference method, and appropriate boundary conditions are applied at the interface. Results have been obtained for depth ratios of 0, 0.1, 0.2, 0.5, and 1.0, where this ratio is defined as the ratio of the thickness of the fluid layer to that of the porous layer. For the depth ratio of 0.1, the convection is dominated by the porous layer, similar to the situation at onset, even though the Rayleigh number for the fluid layer is well into the supercritical regime.
Leo Kadanoff's legacy for turbulent thermal convection
NASA Astrophysics Data System (ADS)
Lohse, Detlef
Rayleigh-Benard (RB) convection -- the buoyancy-driven flow of a fluid heated from below and cooled from above -- is a classical problem in fluid dynamics. It played a crucial role in the development of stability theory in hydrodynamics (Rayleigh, Chandrasekhar) and had been paradigmatic in pattern formation and in the study of spatial-temporal chaos (Ahlers, Libchaber, and many other). It was Leo Kadanoff and his associates in Chicago who, in the 1980s and 1990s, propagated the RB system as paradigmatic for the physics of fully developed turbulence and contributed tremendously to today's understanding of thermally driven turbulence. He and his experimental coworkers (Libchaber et al.) revealed the importance of the thermal plumes and the large-scale wind, and elucidated the interplay between thermal boundary layers and bulk. His scaling analysis laid the basis for our present understanding of turbulent convection, which I will review in this talk, highlighting Leo's trailblazing contributions. Kadanoff session.
Can mantle convection be self-regulated?
Korenaga, Jun
2016-01-01
The notion of self-regulating mantle convection, in which heat loss from the surface is constantly adjusted to follow internal radiogenic heat production, has been popular for the past six decades since Urey first advocated the idea. Thanks to its intuitive appeal, this notion has pervaded the solid earth sciences in various forms, but approach to a self-regulating state critically depends on the relation between the thermal adjustment rate and mantle temperature. I show that, if the effect of mantle melting on viscosity is taken into account, the adjustment rate cannot be sufficiently high to achieve self-regulation, regardless of the style of mantle convection. The evolution of terrestrial planets is thus likely to be far from thermal equilibrium and be sensitive to the peculiarities of their formation histories. Chance factors in planetary formation are suggested to become more important for the evolution of planets that are more massive than Earth. PMID:27551689
Reducing the convective losses of cavity receivers
NASA Astrophysics Data System (ADS)
Flesch, Robert; Grobbel, Johannes; Stadler, Hannes; Uhlig, Ralf; Hoffschmidt, Bernhard
2016-05-01
Convective losses reduce the efficiency of cavity receivers used in solar power towers especially under windy conditions. Therefore, measures should be taken to reduce these losses. In this paper two different measures are analyzed: an air curtain and a partial window which covers one third of the aperture opening. The cavity without modifications and the usage of a partial window were analyzed in a cryogenic wind tunnel at -173°C. The cryogenic environment allows transforming the results from the small model cavity to a large scale receiver with Gr≈3.9.1010. The cavity with the two modifications in the wind tunnel environment was analyzed with a CFD model as well. By comparing the numerical and experimental results the model was validated. Both modifications are capable of reducing the convection losses. In the best case a reduction of about 50 % was achieved.
Natural-convection promoter for geothermal wells
Allis, R.G.; James, R.
1980-09-01
Many geothermal wells stand with relatively cold water overlying hot water. If a pipe is inserted into such a well, natural convection will occur and hot water will flow to the top of the well. The convection-promoting pipe enables domestic wells which would normally require the use of a downhole pump or airlift (with attendant environmental problems of fluid disposal) to be satisfactorily operated with a downhole heat exchanger. In potentially powerful steam-water wells which are difficult to discharge, a pipe positioned beneath the water level should raise wellhead pressure to the point where spontaneous discharge is possible. In both cases, the permeability and temperature of the feed zones are the limiting factors for the heat output of the well.
Interactions Between Convective Storms and Their Environment
NASA Technical Reports Server (NTRS)
Maddox, R. A.; Hoxit, L. R.; Chappell, C. F.
1979-01-01
The ways in which intense convective storms interact with their environment are considered for a number of specific severe storm situations. A physical model of subcloud wind fields and vertical wind profiles was developed to explain the often observed intensification of convective storms that move along or across thermal boundaries. A number of special, unusually dense, data sets were used to substantiate features of the model. GOES imagery was used in conjunction with objectively analyzed surface wind data to develop a nowcast technique that might be used to identify specific storm cells likely to become tornadic. It was shown that circulations associated with organized meso-alpha and meso-beta scale storm complexes may, on occasion, strongly modify tropospheric thermodynamic patterns and flow fields.
Effects of Moist Convection on Hurricane Predictability
NASA Technical Reports Server (NTRS)
Zhang, Fuqing; Sippel, Jason A.
2008-01-01
This study exemplifies inherent uncertainties in deterministic prediction of hurricane formation and intensity. Such uncertainties could ultimately limit the predictability of hurricanes at all time scales. In particular, this study highlights the predictability limit due to the effects on moist convection of initial-condition errors with amplitudes far smaller than those of any observation or analysis system. Not only can small and arguably unobservable differences in the initial conditions result in different routes to tropical cyclogenesis, but they can also determine whether or not a tropical disturbance will significantly develop. The details of how the initial vortex is built can depend on chaotic interactions of mesoscale features, such as cold pools from moist convection, whose timing and placement may significantly vary with minute initial differences. Inherent uncertainties in hurricane forecasts illustrate the need for developing advanced ensemble prediction systems to provide event-dependent probabilistic forecasts and risk assessment.
Overshoot Convective Mixing in Nova Outbursts
NASA Astrophysics Data System (ADS)
Glasner, A. S.; Livne, E.; Truran, J. W.
2014-12-01
We present a 2D study of the overshoot convective mechanism in nova outbursts for a wide range of possible compositions of the layer underlying the accreted envelope. Previous surveys studied this mechanism only for solar composition matter accreted on top of carbon oxygen (CO) white dwarfs. Since, during the runaway, mixing with carbon enhances the hydrogen burning rates dramatically, one should question whether significant enrichment of the ejecta is possible also for other underlying compositions (He, O, Ne, Mg) predicted by stellar evolution models. When needed we upgraded our reaction network and simulated several non-carbon cases. Despite large differences in rates, time scales and energetics, our results show that the convective dredge up mechanism predicts significant enrichment in all cases, including that of helium enrichment in recurrent novae.
Convection in vibrated annular granular beds
NASA Astrophysics Data System (ADS)
Wildman, R. D.; Martin, T. W.; Krouskop, P. E.; Talbot, J.; Huntley, J. M.; Parker, D. J.
2005-06-01
The response to vibration of a granular bed, consisting of a standard cylindrical geometry but with the addition of a dissipative cylindrical inner wall, has been investigated both experimentally (using positron emission particle tracking) and numerically (using hard sphere molecular dynamics simulation). The packing fraction profiles and granular temperature distributions (in both vertical and horizontal directions) were determined as a function of height and distance from the axis. The two sets of results were in reasonable agreement. The molecular dynamics simulations were used to explore the behavior of the granular bed in the inner wall-outer wall coefficient of restitution phase space. It was observed that one could control the direction of the toroidal convection rolls by manipulating the relative dissipation at the inner and outer walls via the coefficients of restitution, and with several layers of grains it was seen that double convection rolls could also be formed, a result that was subsequently confirmed experimentally.
Numerical Simulation of a Convective Turbulence Encounter
NASA Technical Reports Server (NTRS)
Proctor, Fred H.; Hamilton, David W.; Bowles, Roland L.
2002-01-01
A numerical simulation of a convective turbulence event is investigated and compared with observational data. The numerical results show severe turbulence of similar scale and intensity to that encountered during the test flight. This turbulence is associated with buoyant plumes that penetrate the upper-level thunderstorm outflow. The simulated radar reflectivity compares well with that obtained from the aircraft's onboard radar. Resolved scales of motion as small as 50 m are needed in order to accurately diagnose aircraft normal load accelerations. Given this requirement, realistic turbulence fields may be created by merging subgrid-scales of turbulence to a convective-cloud simulation. A hazard algorithm for use with model data sets is demonstrated. The algorithm diagnoses the RMS normal loads from second moments of the vertical velocity field and is independent of aircraft motion.
Convection of a stratified colloidal suspension
Cherepanov, I. N.; Smorodin, B. L.
2013-11-15
The convection of a colloidal suspension, which is a binary mixture of a carrier medium with an admixture of nanoparticles having a large positive thermal diffusion parameter, has been studied for the case of the heating of a horizontal cell from below and periodic conditions at the vertical boundaries corresponding to the experimental situation of ring channels. Bifurcation diagrams have been constructed for vibrational and monotonic regimes of the convection of the colloidal mixture. The time dependences of the maximum stream function and the stream function at a fixed point of the cell, as well as the spatial distributions of the concentration field of the colloid admixture, have been obtained. It has been shown that a stable regime of traveling waves exists in a certain region of the parameters of the problem (Boltzmann and Rayleigh numbers characterizing the gravitational stratification and intensity of the thermal effect, respectively)
Studies of heat source driven natural convection
NASA Technical Reports Server (NTRS)
Kulacki, F. A.; Nagle, M. E.; Cassen, P.
1974-01-01
Natural convection energy transport in a horizontal layer of internally heated fluid with a zero heat flux lower boundary, and an isothermal upper boundary, has been studied. Quantitative information on the time-mean temperature distribution and the fluctuating component of temperature about the mean temperature in steady turbulent convection are obtained from a small thermocouple inserted into the layer through the upper bounding plate. Data are also presented on the development of temperature at several vertical positions when the layer is subject to both a sudden increase and to a sudden decrease in power input. For changes of power input from zero to a value corresponding to a Rayleigh number much greater than the critical linear stability theory value, a slight hysteresis in temperature profiles near the upper boundary is observed between the heat-up and cool-down modes.
Acousto-Convective Drying of Pine Nuts
NASA Astrophysics Data System (ADS)
Zhilin, A. A.; Fedorov, A. V.
2014-07-01
An experimental investigation of the process of drying pine nut grains has been carried out by three methods: acousto-convective, thermoconvective, and thermal. A qualitative and a quantitative comparison of the dynamics of the processes of moisture extraction from the nut grains for the considered drying methods have been made. To elucidate the mechanism of moisture extraction from the pine nut grains, we carried out a separate investigation of the process of drying the nut shell and the kernel. The obtained experimental data on the acousto-convective drying of nuts are well described by the relaxation model, the data on the thermoconvective drying are well described by the bilinear law, and the data on the thermal drying are well described by the combined method consisting of three time steps characterized by different kinetic regimes of drying.
Period doubling in magnetospheric convection cycle
NASA Astrophysics Data System (ADS)
Liu, W. William
2012-04-01
A gedanken investigation is performed on magnetospheric complexity. In an attempt to separate complexities due to external (solar wind) and internal (magnetospheric) dynamics, we hold the solar wind condition constant and investigate how the open flux in the magnetosphere changes from one convection cycle to the next. The change of open flux is related to the time integral of the tail electric field. This field, in turn, is proportional to the product of the normal (x) and tangential (z) components of the tail magnetic field. As the magnetosphere evolves, the magnetic components typically vary in opposite directions. We show that this competition leads to a magnetic flux cycle described by the classical logistic equation xn+1 = r(1 - xn)xn, where x is a linear function of open flux, the much researched route to chaos through period-doubling. The result could provide a possible explanation of the steady magnetospheric convection, sawtooth events, and other observed manifestations of nonlinearity.
Edge convection driven by externally applied potentials
D'Ippolito, D. A.; Myra, J. R.
2000-08-01
A theoretical model of convection in collisional tokamak edge and scrape-off-layer plasmas is described. In the linear theory, any mechanism for poloidal and toroidal symmetry breaking of the equilibrium will drive ExB flows; this result stems from the parallel thermal and pressure forces in Ohm's law. In the nonlinear theory, the quadratic coupling of the perturbations leads to quasilinear-type fluxes in the vorticity, density, and temperature equations. If the convection is strong enough, these fluxes lead to an ambipolarity constraint on the equilibrium electric field and to increased transport of particles and energy. The theory shows qualitative agreement with some tokamak experiments in which potential perturbations are externally driven by radio frequency antennas. (c) 2000 American Institute of Physics.
Convective diffusion in protein crystal growth
NASA Technical Reports Server (NTRS)
Baird, J. K.; Meehan, E. J., Jr.; Xidis, A. L.; Howard, S. B.
1986-01-01
A protein crystal modeled as a flat plate suspended in the parent solution, with the normal to the largest face perpendicular to gravity and the protein concentration in the solution adjacent to the plate taken to be the equilibrium solubility, is studied. The Navier-Stokes equation and the equation for convective diffusion in the boundary layer next to the plate are solved to calculate the flow velocity and the protein mass flux. The local rate of growth of the plate is shown to vary significantly with depth due to the convection. For an aqueous solution of lysozyme at a concentration of 40 mg/ml, the boundary layer at the top of a 1-mm-high crystal has a thickness of 80 microns at 1 g, and 2570 microns at 10 to the -6th g.
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.
Can mantle convection be self-regulated?
Korenaga, Jun
2016-08-01
The notion of self-regulating mantle convection, in which heat loss from the surface is constantly adjusted to follow internal radiogenic heat production, has been popular for the past six decades since Urey first advocated the idea. Thanks to its intuitive appeal, this notion has pervaded the solid earth sciences in various forms, but approach to a self-regulating state critically depends on the relation between the thermal adjustment rate and mantle temperature. I show that, if the effect of mantle melting on viscosity is taken into account, the adjustment rate cannot be sufficiently high to achieve self-regulation, regardless of the style of mantle convection. The evolution of terrestrial planets is thus likely to be far from thermal equilibrium and be sensitive to the peculiarities of their formation histories. Chance factors in planetary formation are suggested to become more important for the evolution of planets that are more massive than Earth. PMID:27551689
Electrodynamic convection in silicon floating zones
NASA Astrophysics Data System (ADS)
Mühlbauer, A.; Erdmann, W.; Keller, W.
1983-12-01
Using a simplified Navier-Stokes equation it has been possible to compute the electrodynamic convection generated by a radio frequency coil field for the modern needle-eye float-zone growth of silicon. The calculated electrodynamic force in such a zone shows maximum values up to 11.7 N/cm 3 and generates flow velocities between 25 and 100 cm/s. As only superficial convection can be brought about by electrodynamic forces, the axial and radial dopant incorporation will not be influenced strongly. A comparison of electrodynamic forces with the other forces possibly causing flow in silicon floating zones shows that the electrodynamic forces exceed all other forces by several orders of magnitude.
Role of elasticity in stagnant lid convection
NASA Astrophysics Data System (ADS)
Patocka, Vojtech; Tackley, Paul; Cadek, Ondrej
2016-04-01
A present limitation of global thermo-chemical convection models is that they assume a purely viscous or visco-plastic flow law for solid rock, i.e. elasticity is ignored. This may not be a good assumption in the cold, outer boundary layer known as the lithosphere, where elastic deformation may be important. Elasticity in the lithosphere plays at least two roles: It changes surface topography, which changes the relationship between topography and gravity, and it alters the stress distribution in the lithosphere, which may affect dynamical behaviour such as the formation of plate boundaries and other tectonics features. In the present work we study these effects in the context of stagnant lid convection. We use StagYY (Tackley, 2008) enhanced to include elasticity through adding advected elastic stresses to the momentum equation and replacing viscosity by the "effective" one (the method described in e.g. Moresi et al., 2002). First, a test example with a cylinder rising below the lithosphere (Crameri et al., 2012) is considered in various geometries and the effect of elasticity on the resulting topography and geoid is evaluated. Both free-slip and free-surface upper boundary condition is considered. Second, comparison of stagnant lid convection models with and without elasticity is performed. It is shown that global characteristics of the convection do not change when a realistic value of shear modulus is employed and that the stress pattern in the lithosphere is very similar. The most important effect is that stresses build up gradually when elasticity is considered and thus the stress picture is more stable in the time domain in the elastic than in the viscous case. Viscoelastic lithosphere thus filters internal dynamics more effectively than a purely viscous one, responding only to features which stay stable for times comparable to its relaxation time. This effect is clearly recognizable only when free-surface upper boundary condition is considered. The role of
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
Nonlinear Convective Models of RR Lyrae Stars
NASA Astrophysics Data System (ADS)
Feuchtinger, M.; Dorfi, E. A.
The nonlinear behavior of RR Lyrae pulsations is investigated using a state-of-the-art numerical technique solving the full time-dependent system of radiation hydrodynamics. Grey radiative transfer is included by a variable Eddington-factor method and we use the time-dependent turbulent convection model according to Kuhfuss (1986, A&A 160, 116) in the version of Wuchterl (1995, Comp. Phys. Comm. 89, 19). OPAL opacities extended by the Alexander molecule opacities at temperatures below 6000 K and an equation of state according to Wuchterl (1990, A&A 238, 83) close the system. The resulting nonlinear system is discretized on an adaptive mesh developed by Dorfi & Drury (1987, J. Comp. Phys. 69, 175), which is important to provide the necessary spatial resolution in critical regions like ionization zones and shock waves. Additionally, we employ a second order advection scheme, a time centered temporal discretizaton and an artificial tensor viscosity in order to treat discontinuities. We compute fundamental as well first overtone models of RR Lyrae stars for a grid of stellar parameters both with and without convective energy transport in order to give a detailed picture of the pulsation-convection interaction. In order to investigate the influence of the different features of the convection model calculations with and without overshooting, turbulent pressure and turbulent viscosity are performed and compared with each other. A standard Fourier decomposition is used to confront the resulting light and radial velocity variations with recent observations and we show that the well known RR Lyrae phase discrepancy problem (Simon 1985, ApJ 299, 723) can be resolved with these stellar pulsation computations.
The interaction of evaporative and convective instabilities
NASA Astrophysics Data System (ADS)
Ozen, O.
Evaporative convection arises in a variety of natural and industrial processes, such as drying of lakebeds, heat pipe technology and dry-eye syndrome. The phenomenon of evaporative convection leads to an interfacial instability where an erstwhile flat surface becomes undulated as a control variable, such as temperature drop, exceeds a critical value. This instability has been investigated by others assuming that the vapor phase is infinitely deep and passive, i.e. vapor fluid dynamics has been ignored. However, when we look at some engineering processes, such as distillation columns, heat pipes and drying technologies where phase change takes place we might imagine that the assumption of an infinitely deep vapor layer or at least that of a passive vapor is inappropriate. Previous work on convection in bilayer systems with no phase-change suggests that active vapor layers play a major role in determining the stability of an interface. Hence, for the case of convection with phase-change, we will address this issue and try to answer the question whether the infinitely deep and passive vapor layer is a valid assumption. We have also investigated, theoretically, the gravity and surface tension gradient-driven instabilities occurring during the evaporation of a liquid into its own vapor taking into account the fluid dynamics of both phases and the finiteness of the domains of each phase, i.e. the liquid and its vapor are assumed to be confined between two horizontal plates, and different heating arrangements are applied. The effects of fluid layer depths, the evaporation rate and the temperature gradient applied across the fluids on the stability of the interface are studied. The modes of the flow pattern are determined for each scenario. The physics of the instability are explained and a comparison is made with the results of similar, yet physically different problems.
Convective heat transport in geothermal systems
Lippmann, M.J.; Bodvarsson, G.S.
1986-08-01
Most geothermal systems under exploitation for direct use or electrical power production are of the hydrothermal type, where heat is transferred essentially by convection in the reservoir, conduction being secondary. In geothermal systems, buoyancy effects are generally important, but often the fluid and heat flow patterns are largely controlled by geologic features (e.g., faults, fractures, continuity of layers) and location of recharge and discharge zones. During exploitation, these flow patterns can drastically change in response to pressure and temperature declines, and changes in recharge/discharge patterns. Convective circulation models of several geothermal systems, before and after start of fluid production, are described, with emphasis on different characteristics of the systems and the effects of exploitation on their evolution. Convective heat transport in geothermal fields is discussed, taking into consideration (1) major geologic features; (2) temperature-dependent rock and fluid properties; (3) fracture- versus porous-medium characteristics; (4) single- versus two-phase reservoir systems; and (5) the presence of noncondensible gases.
Study of convective mechanisms under microgravity conditions
NASA Astrophysics Data System (ADS)
Langbein, D.; Heide, W.
This paper reports on the various contributions of spreading, of capillarity and of Marangoni convection on the mixing and demixing of transparent liquids exhibiting a miscibility gap. Such liquid pairs are model systems for monotectic metallic alloys. The experiences from parabolic flights and from two sounding rocket experiments on the system methanol/cyclohexane (TEXUS 7 and 9) are compared with the results of the Spacelab-D1 experiment FPM-03. The liquid pair chosen in the latter was paraffin oil/benzylbenzoate. The main contributions to mixing and demixing were: Thermal and solutal Marangoni convection at the free liquid surface during heating. It results in a rapid schlieren movement. Marangoni migration of bubbles. They moved to the surface and eventually vanished. The spreading of one component along the supporting disks, as required by theory close to the critical point. Capillary effects like the rupture of the inner of the two liquid columns about 4 min after beginning of heating. Due to spreading and mixing the minimum volume condition for stability has been reached and a Rayleigh instability arose. Since also the liquid recovery into the reservoir worked correctly, a second, unscheduled run with higher heater temperature and shorter heating time was granted. It was intended to observe the resulting faster spreading and Marangoni convection. Further valuable information on the contributions to separation has been obtained.
Convective flows in evaporating sessile droplets.
Barmi, Meysam R; Meinhart, Carl D
2014-03-01
The evaporation rate and internal convective flows of a sessile droplet with a pinned contact line were formulated and investigated numerically. We developed and analyzed a unified numerical model that includes the effects of temperature, droplet volume, and contact angle on evaporation rate and internal flows. The temperature gradient on the air/liquid interface causes an internal flow due to Marangoni stress, which provides good convective mixing within the droplet, depending upon Marangoni number. As the droplet volume decreases, the thermal gradient becomes smaller and the Marangoni flow becomes negligible. Simultaneously, as the droplet height decreases, evaporation-induced flow creates a large jet-like flow radially toward the contact line. For a droplet containing suspended particles, this jet-like convective flow carries particles toward the contact line and deposits them on the surface, forming the so-called "coffee ring stain". In addition, we reported a simple polynomial correlation for dimensionless evaporation time as a function of initial contact angle of the pinned sessile droplet which agrees well with the previous experimental and numerical results. PMID:24512008
Convective Dynamo Simulation with a Grand Minimum
NASA Astrophysics Data System (ADS)
Augustson, Kyle C.; Brun, A. S.; Miesch, Mark; Toomre, Juri
2015-01-01
The global-scale dynamo action achieved in a simulation of a Sun-like star rotating at thrice the solar rate is assessed. The 3-D MHD Anelastic Spherical Harmonic (ASH) code, augmented with a viscosity minimization scheme, is employed to capture convection and dynamo processes in this G-type star. The simulation is carried out in a spherical shell that encompasses 3.8 density scale heights of the solar convection zone. It is found that dynamo action with a high degree of time variation occurs, with many periodic polarity reversals occurring roughly every 6.2 years. The magnetic energy also rises and falls with a regular period. The magnetic energy cycles arise from a Lorentz-force feedback on the differential rotation, whereas the processes leading to polarity reversals are more complex, appearing to arise from the interaction of convection with the mean toroidal fields. Moreover, an equatorial migration of toroidal field is found, which is linked to the changing differential rotation, and potentially to a nonlinear dynamo wave. This simulation also enters a grand minimum lasting roughly 20 years, after which the dynamo recovers its regular polarity cycles.
Magneto-convective instabilities in horizontal cavities
NASA Astrophysics Data System (ADS)
Mistrangelo, Chiara; Bühler, Leo
2016-02-01
A linear stability analysis is performed to investigate the onset of convective motions in a flat cavity filled with liquid metal. A volumetric heat source is uniformly distributed in the fluid and a horizontal magnetic field is imposed. Walls perpendicular to the magnetic field are thermally insulating, and the top wall is isothermal and the bottom adiabatic. When a magnetic field is applied, electromagnetic forces tend to transform 3D convective flow structures into quasi-2D rolls aligned to the magnetic field. By integrating 3D equations along magnetic field lines, a quasi-2D mathematical model has been derived. A dissipation term in the 2D equations accounts for 3D viscous effects in boundary layers at Hartmann walls perpendicular to the magnetic field. The influence of various parameters on flow stability is investigated. The flow is stabilized by increasing the magnetic field intensity or the electric conductance of Hartmann walls and by reducing the aspect ratio of the cavity. Numerical simulations are performed to verify the analytical results and to describe the main convective flow patterns in the non-linear regime.
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).
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.
Convective boundary layer and modeling of dispersion
NASA Astrophysics Data System (ADS)
Ahmed, Nizam Uddin
Looping, bifurcation, and meandering of a plume are generally observed in a convective field. The blobby or puffy concentration patterns associated with these plumes are marked deviations from what is expected from either conventional K-theory or Gaussian distribution formulae. A numerical model was developed for material dispersion in a convective boundary layer from both elevated and ground sources. Mechanistic formulation, rather than parameterization, or statistical behavior of planetary boundary layer (PBL) phenomena, was used as a basis. The dispersion mechanism is considered to be due to mixing between the updraft and the downdraft. This model uses two universal constants, (turbulent entrainment constant, a, and decay constant A) and a mixing scheme directly supported by observations. Researchers examined the dispersion pattern from the elevated and ground sources. For elevated sources, the maximum concentration descends first to the ground level at some distance downwind, and then rises, depending on the inversion height, the mean wind and height at which material is released. The updrafts have a higher velocity than the downdrafts and consequently the downdrafts occupy a larger horizontal area. In some cases the updrafts and downdrafts are comparable and materials are caught equally in the updrafts and downdrafts. The concentration of materials is split into two parts, one moving downward and the other upward. It is shown using the same mechanistic principles, that different convective situations cause different concentration patterns (for example, looping, bifurcating of a plume, and ascending of center line).
Layered convection in double diffusive fluids
NASA Astrophysics Data System (ADS)
Zaussinger, F.; Kupka, F.; Muthsam, H. J.; Happenhofer, N.; Grimm-Strele, H.
2012-04-01
Double diffusive convection plays an important role in astrophysics and oceanography where under certain conditions a thermally unstable temperature gradient is counteracted by a stable solute gradient. This configuration is well known from salt lakes, where the salt concentration stabilizes convective motions and a layered structure emerges. Similar conditions are found in stellar interiors, where helium as the stabilizing component inhibits the development of convection and the occurrence of double-diffusive staircases is assumed. We investigate mixing timescales and stability conditions using theoretical estimates and numerical simulations covering a broad range of parameter sets by varying Prandtl-, Lewis- and Rayleigh numbers. To shed light on the numerically inaccessible astrophysical case we extrapolate to the relevant parameter range. We investigate the initial layer formation process as well as the stability of evolved layers by performing direct numerical simulations in 2D and 3D using the Boussinesq approximation. A fitting formula for the Nusselt numbers and the effective mixing rates is given. Finally, we present a semi-implicit method to solve the compressible counterpart of the governing equations which has the advantage to cover the entire relevant Mach number range.
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. PMID:23696657
Solar Wind Drivers for Steady Magnetospheric Convection
NASA Technical Reports Server (NTRS)
McPherron, Robert L.; O'Brien, T. Paul; Thompson, Scott; Lui, A. T. Y. (Editor)
2005-01-01
Steady magnetospheric convection (SMC) also known as convection bays, is a particular mode of response of the magnetosphere to solar wind coupling. It is characterized by convection lasting for times longer than a typical substorm recovery during which no substorms expansions can be identified. It is generally believed that the solar wind must be unusually steady for the magnetosphere to enter this state. However, most previous studies have assumed this is true and have used such conditions to identify events. In a preliminary investigation using only the AE and AL indices to select events we have shown that these expectations are generally correct. SMC events seem to be associated with slow speed solar wind and moderate, stable IMF Bz. In this report we extend our previous study including additional parameters and the time variations in various statistical quantities. For the intervals identified as SMCs we perform a detailed statistical analysis of the properties of different solar wind variables. We compare these statistics to those determined from all data, and from intervals in which substorms but not SMCs are present. We also consider the question of whether substorms are required to initiate and terminate an SMC. We conclude that the intervals we have identified as SMC are likely to be examples of the original Dungey concept of balanced reconnection at a pair of x-lines on the day and night side of the Earth.
Convective instability in protein crystal growth
NASA Astrophysics Data System (ADS)
Lima, D.; de Wit, A.
2004-08-01
The conditions for the onset of convection during protein crystalization from a solution are studied theoretically on the basis of diffusion-convection evolution equations for the concentrations coupled to the Navier-Stokes equation describing the flow velocity. We consider that the density of the solution depends on the concentration of two species, namely, a protein and a precipitating agent, a salt. While the protein is crystallized at the crystal/solution interface, the salt is rejected, and these mechanisms are described by means of boundary conditions for the interface. We find the base profiles for both protein and salt concentrations and perform a linear stability analysis of this basic state with regard to buoyancy induced perturbations. This gives information on the critical diameter of capillaries above which convection may be observed, as well as on the influence of the speed of growth V of the crystal interface on the stability of the system. Numerical integration of the model shows good agreement with the predictions of the linear stability analysis.
Torsional oscillations in the solar convection zone
NASA Astrophysics Data System (ADS)
Covas, E.; Tavakol, R.; Moss, D.; Tworkowski, A.
2000-08-01
Recent analysis of the helioseismic observations indicate that the previously observed surface torsional oscillations extend significantly downwards into the solar convection zone. In an attempt to understand these oscillations, we study the nonlinear coupling between the magnetic field and the solar differential rotation in the context of a mean field dynamo model, in which the nonlinearity is due to the action of the azimuthal component of the Lorentz force of the dynamo generated magnetic field on the solar angular velocity. The underlying zero order angular velocity is chosen to be consistent with the most recent helioseismic data. The model produces butterfly diagrams which are in qualitative agreement with the observations. It displays torsional oscillations that penetrate into the convection zone, and which with time migrate towards the equator. The period of these oscillations is found to be half that of the period of the global magnetic fields. This is compatible with the observed period of the surface torsional oscillations. Inside the convection zone, this is a testable prediction that is not ruled out by the observations so far available.
On natural solutal convection in magnetic fluids
NASA Astrophysics Data System (ADS)
Ivanov, A. S.; Pshenichnikov, A. F.
2015-09-01
An experiment was carried out to investigate natural solutal convection in a magnetic fluid caused by non-homogeneous initial distribution of colloidal particles in a vertical Hele-Shaw cell. For experiment, we used a dilute magnetic fluid of the "magnetite-kerosene-oleic acid" type. The initial distribution of particles was formed by magnetophoresis of the drop-like aggregates and their sedimentation on the surface of the diamagnetic disk located in the center of the cell. Application of the magnetic field on the system led to the onset of the Rayleigh-Taylor instability and formation of descending convective jets. The velocity of the flow at the front of descending jets was measured for different values of cell thickness (up to 0.18 mm) and strength of the magnetic field generating the drop-like aggregates (up to 21 kA/m). The solutal Rayleigh numbers varied in the range Ra = 50-105. It was shown that the intensity of the convective flow characterized by the Reynolds number Re, increases with the Rayleigh number according to the power law: Re = 1.16 × 10-5Ra0.86.
On the convective overstability in protoplanetary discs
NASA Astrophysics Data System (ADS)
Latter, Henrik N.
2016-01-01
This paper explores the driving of low-level hydrodynamical activity in protoplanetary-disc dead zones. A small adverse radial entropy gradient, ordinarily stabilized by rotation, excites oscillatory convection (`convective overstability') when thermal diffusion, or cooling, is neither too strong nor too weak. I revisit the linear theory of the instability, discuss its prevalence in protoplanetary discs, and show that unstable modes are exact non-linear solutions in the local Boussinesq limit. Overstable modes cannot grow indefinitely, however, as they are subject to a secondary parametric instability that limits their amplitudes to relatively low levels. If parasites set the saturation level of the ensuing turbulence then the convective overstability is probably too weak to drive significant angular momentum transport or to generate vortices. But I also discuss an alternative, and far more vigorous, saturation route that generates radial `layers' or `zonal flows' (witnessed in semiconvection). Numerical simulations are required to determine which outcome is favoured in realistic discs, and consequently how important the instability is for disc dynamics.
Convective flows around sunspot-like objects
NASA Technical Reports Server (NTRS)
Fox, Peter A.; Sofia, Sabatino; Chan, Kwing L.
1991-01-01
Calculations of convective flows around objects in the outer layers of the sun that have characteristics similar to those of sunspots are presented. It is assumed that these objects are allowed to radiatively exchange heat with their surroundings, but convective motions and exchange are absent. The flow structure around the object, and the question of the overall balance or redistribution of the emerging heat flux as suggested by earlier empirical models are discussed. In all the cases studied, there was a significant increase in the kinetic energy flux and thus the lateral transport of the energy flux surrounding the object. This mainly occurred below the object to allow the heat to appear at the surface, with some time delay. The percentage reduction in the specific intensity agrees well with the observed deficits for pores or small sunspot-type regions. The diverted heat flow was not totally blocked by the object. Because the sunspot-like object is magnetic and the convective flow closely surrounds it, some conversion between internal, kinetic, and magnetic energy is likely.
Convection in deformed bodies: The effect of equatorial ellipticity on convective behavior
NASA Astrophysics Data System (ADS)
Evonuk, M.
2015-11-01
Tidal interactions between bodies such as hot jupiter planets and their host stars are likely to result in non-spherical geometries. These elliptical instabilities may have interesting effects on interior fluid convective patterns, which in turn could influence the nature of the magnetic dynamo within these planets. Simulations of thermal convection in the 2D rotating equatorial plane are conducted to determine to first order the effect of equatorial eccentricity on convection for varying density contrasts with differing convective vigor and rotation rates. This survey is conducted in two dimensions in order to simulate a broad range of eccentricities and to maximize the parameter space explored. The location of the three regimes documented in previous work (Evonuk and Samuel, 2012), dipolar flow, transitional flow, and differential flow, are found to be offset by the introduction of equatorial eccentricity to the system. The introduction of equatorial eccentricity changes the fluid behavior such that bodies with high amounts of deformation are likely to have weaker differential flows shifting their behavior towards transitional and dipolar flow structures. A scaling law based on the convective Rossby number, density contrast, and the eccentricity of the equatorial plane can therefore provide a way to estimate which regime a given body lies in.
A convective forecast experiment of global tectonics
NASA Astrophysics Data System (ADS)
Coltice, Nicolas; Giering, Ralf
2016-04-01
Modeling jointly the deep convective motions in the mantle and the deformation of the lithosphere in a self-consistent way is a long-standing quest, for which significant advances have been made in the late 1990's. The complexities used in lithospheric models are making their way into the models of mantle convection (density variations, pseudo-plasticity, elasticity, free surface), hence global models of mantle motions can now display tectonics at their surface, evolving self-consistantly and showing some of the most important properties of plate tectonics on Earth (boundaries, types of boundaries, plate sizes, seafloor spreading properties, continental drift). The goal of this work is to experiment the forecasting power of such convection models with plate-like behavior, being here StagYY (Tackley, 2008). We generate initial conditions for a 3D spherical model in the past (50Ma and younger), using models with imposed plate velocities from 200Ma. By doing this, we introduce errors in the initial conditions that propagate afterwards. From these initial conditions, we run the convection models free, without imposing any sort of motion, letting the self-organization take place. We compare the forecast to the present-day plate velocities and plate boundaries. To investigate the optimal parameterization, and also have a flavor of the sensitivity of the results to rheological parameters, we compute the derivatives of the misfit of the surface velocities relative to the yield stress, the magnitude of the viscosity jump at 660km and the properties of a weak crust. These derivates are computed thanks to the tangent linear model of StagYY, that is built through the automatic differentiation software TAF (Giering and Kaminski, 2003). References Tackley, P. J., Modeling compressible mantle convection with large viscosity contrasts in a three-dimensional spherical shell using the yin-yang grid, Phys. Earth Planet. Inter. 171, 7-18 (2008). Giering, R., Kaminski, T., Applying TAF
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.
Multiple convection patterns and thermohaline flow in an idealized OGCM
Rahmstorf, S.
1995-12-01
This paper investigates how multiple steady states arise in an ocean general circulation model, caused by the fact that many different convection patterns can be stable under the same surface boundary conditions. Two alternative boundary conditions are used in the experiments: classical mixed boundary conditions and a diffusive atmospheric heat balance combined with fixed salt fluxes. In both cases, transitions between different quasi-steady convection patterns can be triggered by briefly adding fresh water at convection sites. Either a large-scale freshwater anomaly is used to completely erase the previous convection pattern or a {open_quotes}surgical{close_quotes} anomaly is added to single grid points to turn off convection there. Under classical mixed-boundary conditions, different convection sites can lead to different overturning rates of deep water. The dynamics of the convection-driven flow is analyzed in some detail. With an energy balance atmosphere, in contrast, the overturning rate is very robust, apparently regulated by a negative thermal feedback. In spite of this, different convection patterns are associated with very different climatic states, since the heat transport of the deep circulation depends strongly on where convection takes place. It is suggested that considerable climate variability in the North Atlantic could be caused by changes in high-latitude convection.
Modeling for Convective Heat Transport Based on Mixing Length Theory
NASA Astrophysics Data System (ADS)
Yamagishi, Y.; Yanagisawa, T.
2002-12-01
Convection is the most important mechanism for the Earth's internal dynamics, and plays a substantial role on its evolution. On investigating the thermal history of the Earth, convective heat transport should be taken into account. However, it is difficult to treat full convective flow throughout the Earth's entire history. Therefore, the parameterized convection has been developed and widely used. Convection occurring in the Earth's interior has some complicated aspects. It has large variation of viscosity, internal heating, phase boundaries, etc. Especially, the viscosity contrast has significant effect on the efficiency of the heat transport of the convection. The parameterized convection treats viscosity variation artificially, so it has many limitations. We developed an alternative method based on the concept of "mixing length theory". We can relate local thermal gradient with local convective velocity of fluid parcel. Convective heat transport is identified with effective thermal diffusivity, and we can calculate horizontally averaged temperature profile and heat flux by solving a thermal conduction problem. On estimating the parcel's velocity, we can include such as the effect of variable viscosity. In this study, we confirm that the temperature profile can be calculated correctly by this method, on comparing the experimental and 2D calculation results. We further show the effect of the viscosity contrast on the thermal structure of the convective fluid, and calculate the relationship between Nusselt number and modified Rayleigh number.
Theory of Stellar Convection: Removing the Mixing-Length parameter
NASA Astrophysics Data System (ADS)
Pasetto, Stefano; Chiosi, Cesare; Cropper, Mark; Grebel, Eva K.
2015-08-01
Stellar convection is customarily described by the mixing-length theory, which makes use of the mixing-length scale to express the convective flux, velocity, and temperature gradients of the convective elements and stellar medium. The mixing-length scale is taken to be proportional to the local pressure scale height, and the proportionality factor (the mixing-length parameter) must be determined by comparing the stellar models to some calibrator, usually the Sun.No strong arguments exist to claim that the mixing-length parameter is the same in all stars and all evolutionary phases. Because of this, all stellar models in literature are hampered by this basic uncertainty.In a recent paper (Pasetto et al 2014) we presented a new theory of stellar convection that does not require the mixing length parameter. Our self-consistent analytical formulation of stellar convection determines all the properties of stellar convection as a function of the physical behaviour of the convective elements themselves and the surrounding medium. The new theory of stellar convection is formulated starting from a conventional solution of the Navier-Stokes/Euler equations, i.e. the Bernoulli equation for a perfect fluid, but expressed in a non-inertial reference frame co-moving with the convective elements. In our formalism, the motion of stellar convective cells inside convective-unstable layers is fully determined by a new system of equations for convection in a non-local and time dependent formalism.We obtained an analytical, non-local, time-dependent solution for the convective energy transport that does not depend on any free parameter. The predictions of the new theory are compared with those from the standard mixing-length paradigm with exceptional results for atmosphere models of the Sun and all the stars in the Hertzsprung-Russell diagram.
Theory of stellar convection: removing the mixing-length parameter
NASA Astrophysics Data System (ADS)
Pasetto, S.; Chiosi, C.; Cropper, M.; Grebel, E. K.
2014-12-01
Stellar convection is customarily described by Mixing-Length Theory, which makes use of the mixing length-scale to express the convective flux, velocity, and temperature gradients of the convective elements and stellar medium. The mixing length-scale is taken to be proportional to the local pressure scaleheight, and the proportionality factor (the mixing-length parameter) must be determined by comparing the stellar models to some calibrator, usually the Sun. No strong arguments exist to suggest that the mixing-length parameter is the same in all stars and at all evolutionary phases. The aim of this study is to present a new theory of stellar convection that does not require the mixing-length parameter. We present a self-consistent analytical formulation of stellar convection that determines the properties of stellar convection as a function of the physical behaviour of the convective elements themselves and of the surrounding medium. This new theory is formulated starting from a conventional solution of the Navier-Stokes/Euler equations, i.e. the Bernoulli equation for a perfect fluid, but expressed in a non-inertial reference frame comoving with the convective elements. In our formalism, the motion of stellar convective cells inside convectively unstable layers is fully determined by a new system of equations for convection in a non-local and time-dependent formalism. We obtain an analytical, non-local, time-dependent subsonic solution for the convective energy transport that does not depend on any free parameter. The theory is suitable for the outer convective zones of solar type stars and stars of all mass on the main-sequence band. The predictions of the new theory are compared with those from the standard mixing-length paradigm for the most accurate calibrator, the Sun, with very satisfactory results.
Convective Weather Avoidance with Uncertain Weather Forecasts
NASA Technical Reports Server (NTRS)
Karahan, Sinan; Windhorst, Robert D.
2009-01-01
Convective weather events have a disruptive impact on air traffic both in terminal area and in en-route airspaces. In order to make sure that the national air transportation system is safe and efficient, it is essential to respond to convective weather events effectively. Traffic flow control initiatives in response to convective weather include ground delay, airborne delay, miles-in-trail restrictions as well as tactical and strategic rerouting. The rerouting initiatives can potentially increase traffic density and complexity in regions neighboring the convective weather activity. There is a need to perform rerouting in an intelligent and efficient way such that the disruptive effects of rerouting are minimized. An important area of research is to study the interaction of in-flight rerouting with traffic congestion or complexity and developing methods that quantitatively measure this interaction. Furthermore, it is necessary to find rerouting solutions that account for uncertainties in weather forecasts. These are important steps toward managing complexity during rerouting operations, and the paper is motivated by these research questions. An automated system is developed for rerouting air traffic in order to avoid convective weather regions during the 20- minute - 2-hour time horizon. Such a system is envisioned to work in concert with separation assurance (0 - 20-minute time horizon), and longer term air traffic management (2-hours and beyond) to provide a more comprehensive solution to complexity and safety management. In this study, weather is dynamic and uncertain; it is represented as regions of airspace that pilots are likely to avoid. Algorithms are implemented in an air traffic simulation environment to support the research study. The algorithms used are deterministic but periodically revise reroutes to account for weather forecast updates. In contrast to previous studies, in this study convective weather is represented as regions of airspace that pilots
Stellar convection 2: A multi-mode numerical solution for convection in spheres
NASA Technical Reports Server (NTRS)
Marcus, P. S.
1979-01-01
The convective flow of a self gravitating sphere of Boussinesq fluid for small Reynolds and Peclet numbers is numerically determined. The decomposition of the equations of motion into modes is reviewed and a relaxation method is developed and presented to compute the solutions to these equations. The stable equilibrium flow for a Rayleigh number of 10 to the 4th power and a Prandtl number of 10 is determined. The 2 and 3 dimensional spectra of the kinetic and thermal energies and the convective flux as a function of wavelengths are calculated in terms of modes. The anisotropy of the flow as a function of wavelength is defined.
Observations of Florida Convective Storms Using Dual Wavelength Airborne Radar
NASA Technical Reports Server (NTRS)
Heymsfield, G. M.; Heymsfield, A. J.; Belcher, L.
2004-01-01
NASA conducted the Cirrus Regional Study of Tropical Anvils and Cirrus Layers (CRYSTAL) Florida Area Cirrus Experiment (FACE) during July 2002 for improved understanding of tropical cirrus. One of the goals was to improve the understanding of cirrus generation by convective updrafts. The reasons why some convective storms produce extensive cirrus anvils is only partially related to convective instability and the vertical transport ice mass by updrafts. Convective microphysics must also have an important role on cirrus generation, for example, there are hypotheses that homogeneous nucleation in convective updrafts is a major source of anvil ice particles. In this paper, we report on one intense CRYSTAL- FACE convective case on 16 July 2002 that produced extensive anvil.
Vorticity imbalance and stability in relation to convection
NASA Technical Reports Server (NTRS)
Read, W. L.; Scoggins, J. R.
1977-01-01
A complete synoptic-scale vorticity budget was related to convection storm development in the eastern two-thirds of the United States. The 3-h sounding interval permitted a study of time changes of the vorticity budget in areas of convective storms. Results of analyses revealed significant changes in values of terms in the vorticity equation at different stages of squall line development. Average budgets for all areas of convection indicate systematic imbalance in the terms in the vorticity equation. This imbalance resulted primarily from sub-grid scale processes. Potential instability in the lower troposphere was analyzed in relation to the development of convective activity. Instability was related to areas of convection; however, instability alone was inadequate for forecast purposes. Combinations of stability and terms in the vorticity equation in the form of indices succeeded in depicting areas of convection better than any one item separately.
Contribution of a mesoscale analysis to convection nowcasting
NASA Astrophysics Data System (ADS)
Calas, C.; Ducrocq, V.; Sénési, S.
We study the contribution of surface data to convection nowcasting outside mountainous areas and under weak synoptic forcing. The CANARI optimal interpolation mesoscale analysis scheme is used, which combines guess-fields from the fine-mesh (10 km) ALADIN model with hourly routine observations arising from a mesonet of automated ground stations. A tuning of this scheme is realized in order to fit convective systems scales (meso-β scale) as well as the mesonet scale. Then, these grid point analyses allow the computation of diagnostic fields such as Convective Available Potential Energy (CAPE) and MOisture CONvergence (MOCON), which are relevant in convection nowcasting. These diagnostic fields are compared with radar reflectivities observed from 1 to 5 hours later, so as to estimate their skill in predicting convection triggering. First results on six case studies show that convection generally occurs over areas where high CAPE values and persisting convergence were analyzed 4 to 1 hour before.
THE MASS MIXING LENGTH IN CONVECTIVE STELLAR ENVELOPES
Trampedach, Regner; Stein, Robert F. E-mail: stein@pa.msu.edu
2011-04-20
The scale length over which convection mixes mass in a star can be calculated as the inverse of the vertical derivative of the unidirectional (up or down) mass flux. This is related to the mixing length in the mixing length theory of stellar convection. We give the ratio of mass mixing length to pressure scale height for a grid of three-dimensional surface convection simulations, covering from 4300 K to 6900 K on the main sequence, and up to giants at log g = 2.2, all for solar composition. These simulations also confirm what is already known from solar simulations that convection does not proceed by discrete convective elements, but rather as a continuous, slow, smooth, warm upflow and turbulent, entropy deficient, fast down drafts. This convective topology also results in mixing on a scale comparable to the classic mixing length formulation, and is simply a consequence of mass conservation on flows in a stratified atmosphere.
Magnetic flux concentrations from turbulent stratified convection
NASA Astrophysics Data System (ADS)
Käpylä, P. J.; Brandenburg, A.; Kleeorin, N.; Käpylä, M. J.; Rogachevskii, I.
2016-04-01
Context. The formation of magnetic flux concentrations within the solar convection zone leading to sunspot formation is unexplained. Aims: We study the self-organization of initially uniform sub-equipartition magnetic fields by highly stratified turbulent convection. Methods: We perform simulations of magnetoconvection in Cartesian domains representing the uppermost 8.5-24 Mm of the solar convection zone with the horizontal size of the domain varying between 34 and 96 Mm. The density contrast in the 24 Mm deep models is more than 3 × 103 or eight density scale heights, corresponding to a little over 12 pressure scale heights. We impose either a vertical or a horizontal uniform magnetic field in a convection-driven turbulent flow in set-ups where no small-scale dynamos are present. In the most highly stratified cases we employ the reduced sound speed method to relax the time step constraint arising from the high sound speed in the deep layers. We model radiation via the diffusion approximation and neglect detailed radiative transfer in order to concentrate on purely magnetohydrodynamic effects. Results: We find that super-equipartition magnetic flux concentrations are formed near the surface in cases with moderate and high density stratification, corresponding to domain depths of 12.5 and 24 Mm. The size of the concentrations increases as the box size increases and the largest structures (20 Mm horizontally near the surface) are obtained in the models that are 24 Mm deep. The field strength in the concentrations is in the range of 3-5 kG, almost independent of the magnitude of the imposed field. The amplitude of the concentrations grows approximately linearly in time. The effective magnetic pressure measured in the simulations is positive near the surface and negative in the bulk of the convection zone. Its derivative with respect to the mean magnetic field, however, is positive in most of the domain, which is unfavourable for the operation of the negative
Absolute and Convective Instability of a Liquid Jet in Microgravity
NASA Technical Reports Server (NTRS)
Lin, Sung P.; Vihinen, I.; Honohan, A.; Hudman, Michael D.
1996-01-01
The transition from convective to absolute instability is observed in the 2.2 second drop tower of the NASA Lewis Research Center. In convective instability the disturbance grows spatially as it is convected downstream. In absolute instability the disturbance propagates both downstream and upstream, and manifests itself as an expanding sphere. The transition Reynolds numbers are determined for two different Weber numbers by use of Glycerin and a Silicone oil. Preliminary comparisons with theory are made.
Parameterized rotating convection for core and planetary atmosphere dynamics
NASA Astrophysics Data System (ADS)
Zhang, K.
1991-04-01
New types of convective instability and associated nonlinear phenomena in rapidly rotating spherical systems have been discovered through numerical simulations. The Prandtl number, defined as the ratio of the viscous and thermal diffusivities of a fluid, Pr = nu/kappa, plays a crucial role in determining the fundamental features of both the instabilities and the corresponding nonlinear convection. The results shed new light on regimes of convection in the earth's core and the atmospheres of the major planets.
On the Influence of Surface Heterogeneities onto Roll Convection
NASA Astrophysics Data System (ADS)
Gryschka, M.; Drüe, C.; Raasch, S.; Etling, D.
2009-04-01
Roll convection is a common phenomenon in atmospheric convective boundary layers (CBL) with background wind. Roll convection is observed both over land and over sea for different synoptic situations. There is still some debate about the different types of roll convection and their causes or rather the necessary conditions for their appearance. The stability parameter Î¶ = -ziL (zi: boundary layer height, L: Monin-Obukhov stability length) is widely used as a predictor for roll convection, since numerous studies suggest that convective rolls only appear when 0 < Î¶ < 20. In other words, roll development becomes unlikely for strong surface heating and weak vertical wind shear. In contrast to those studies the presence of roll convection in almost any polar cold air outbreak (as can be seen in numerous satellite images as cloud streets) reveals that even for large Î¶ roll convection can develop. Some studies report roll convection in cold air outbreaks for Î¶ = 250. Our large eddy simulations (LES) on roll convection suggests that the contrasting results concerning the dependency of roll convection on Î¶ are due to two different types of roll convection: One type which develops purely by self organization if Î¶ < 20 ("free rolls") and another type which is triggered by heterogeneities in surface temperature and develops also for large Î¶ ("forced rolls"). We think that most of the cloud streets observed in polar cold air outbreaks over open water are due to rolls of forced type which are tied to upstream located heterogeneities in the sea-ice distribution. The results of this study suggests that the omission of surface inhomogeneities in previous LES is the reason for the absence of rolls in all LES with strong surface heating and weak vertical wind shear so far. In this contribution we will present a large eddy simulation which successfully represents forced rolls under such conditions.
Convective mixing in formations with horizontal barriers
NASA Astrophysics Data System (ADS)
Elenius, Maria T.; Gasda, Sarah E.
2013-12-01
It has been shown that convective mixing in porous media flow is important for applications such as saltwater intrusion and geological storage of carbon dioxide. In the latter case, dissolution from the injected phase to the resident brine is assisted by convective mixing, which leads to enhanced storage security through reduced buoyancy. Here, we focus on the effect of horizontal barriers on the efficiency of convective mixing. Previous investigations of the effect of heterogeneity on mixing efficiency have focused on random permeability fields or barriers of small extent compared to the intrinsic finger wavelength. The effect of horizontal barriers of larger extent, such as mudstone inclusions or thin shale deposits, has not been given sufficient attention. We perform detailed numerical investigations to represent the continuous solution of this problem in semi-infinite domains with barriers arranged in a periodic manner. The results show that mass flux into the domain, which is a measure of the efficiency of redistribution of the solute, is inversely proportional to the barrier length and proportional to the horizontal and vertical aperture between the barriers, for the cases studied. The flow structure is complex, and it depends not only on the total area of barriers but also largely on the distribution of barriers. Therefore, neither simple analytical models nor simple upscaling methods that lack information about the flow paths, can be used to predict the behavior. However, we compute the effective vertical permeability by flow-based upscaling and show that it can be used to directly obtain a first-order approximation to the mass flux into the domain.
The Convective Signature of the Solar Supergranulation
NASA Astrophysics Data System (ADS)
Goldbaum, Nathan Jonathan; Rast, M. P.
2009-05-01
The solar supergranulation is an elusive, yet well-observed, surface-filling network of roughly polygonal cells made up of horizontally diverging material. Cells have diameters of 30 Mm, flow speeds of 500 m s-1, and lifetimes of 1 day. Theoretical models for the supergranulation abound but can be separated into two classes: convective (Simon and Leighton 1964; van der Borght 1979) and non-convective (Rieutord et al. 2000; Rast 2003b; Rieutord et al. 2008). If supergranulation is convective, then cells should be warmer at their centers than at their borders, on average. However, the sign and magnitude of the supergranular temperature gradient is poorly constrained. The Precision Solar Photometric Telescope (PSPT), operated by the High Altitude Observatory at the Mauna Loa Solar Observatory, off ers 0.1% relative photometric accuracy, good enough to resolve the expected low-amplitude thermal intensity modulation. For this work we have used a library of 3174 PSPT images to measure the mean azimuthally averaged thermal intensity profile in supergranules. Using a morphological algorithm (Berrilli et al. 1998; Rast 2003a), we have produced maps of the chromospheric network present in Ca II K images. After carefully aligning concurrent continuum images with these maps, we find that cell borders are on average 0.30 - 0.25% brighter. This difference, due to the presence of the magnetic network on supergranule borders, is consistent with previous measurements (Lin and Kuhn 1992). Once the magnetic contribution is removed from the intensity signal, we find that cell borders are on average 0.10% dimmer than cell centers. This corresponds to a temperature drop of 1.0K at the borders of supergranules. This measurement is in good agreement with the only other values for this quantity available in the literature (Rast 2003a; Meunier et al. 2007b, 2008).
Melt migration through Io's convecting mantle
NASA Astrophysics Data System (ADS)
Elder, C. M.; Showman, A. P.
2013-12-01
The extensive volcanism occurring on the surface of Io suggests that its interior must contain at least some partial melt. Unlike Earth, Io cannot lose its internal heat through convection alone [1]. Instead, melt moving through the solid mantle helps remove heat from Io's interior by carrying its latent heat towards the surface as it buoyantly ascends through the mantle. We investigate this process by considering melt migration in a column of rock rising through the mantle between downwelling plumes. Convective scaling laws provide the upwelling velocity and the temperature of the rising mantle. Properties of melt migration in this rising mantle are calculated using porous flow equations and an equation for the conservation of energy which includes latent heat consumption, heat advection and heat conduction [2]. This combination of convective scaling laws and porous flow laws allows us to self-consistently determine the radial melt fraction profile in Io's interior, the average melt fraction in Io's interior and the heat flux due to advection of melt. The average melt fraction can be compared to the melt fraction constraints calculated by [3] from Galileo magnetometer measurements. The surface heat flux calculations can be compared to the value of Io's observed surface heat flux which ranges with observation from 1.5-4 W m-2 [4]. [1] Moore W. B. (2003) J. Geophys. Res., 108, E8, 15-1. [2] Hewitt I. J. and Fowler A. C. (2008) Proc. R. Soc. A., 464, 2467-2491. [3] Khurana K. K. et al. (2011) Science, 332, 1186-1189. [4] Moore, W. B. et al. (2007) In: Io After Galileo, Springer-Praxis, 89-108.
Approximate convective heating equations for hypersonic flows
NASA Technical Reports Server (NTRS)
Zoby, E. V.; Moss, J. N.; Sutton, K.
1979-01-01
Laminar and turbulent heating-rate equations appropriate for engineering predictions of the convective heating rates about blunt reentry spacecraft at hypersonic conditions are developed. The approximate methods are applicable to both nonreacting and reacting gas mixtures for either constant or variable-entropy edge conditions. A procedure which accounts for variable-entropy effects and is not based on mass balancing is presented. Results of the approximate heating methods are in good agreement with existing experimental results as well as boundary-layer and viscous-shock-layer solutions.
Surface Tension Driven Convection Experiment Completed
NASA Technical Reports Server (NTRS)
Jacobson, Thomas P.; Sedlak, Deborah A.
1997-01-01
The Surface Tension Driven Convection Experiment (STDCE) was designed to study basic fluid mechanics and heat transfer on thermocapillary flows generated by temperature variations along the free surfaces of liquids in microgravity. STDCE first flew on the USML-1 mission in July 1992 and was rebuilt for the USML-2 mission that was launched in October 1995. This was a collaborative project with principal investigators from Case Western Reserve University (CWRU), Professors Simon Ostrach and Yasuhiro Kamotani, along with a team from the NASA Lewis Research Center composed of civil servants and contractors from Aerospace Design & Fabrication, Inc. (ADF), Analex, and NYMA, Inc.
Forced convection modulates gas exchange in cnidarians
Patterson, Mark R.; Sebens, Kenneth P.
1989-01-01
Boundary layer thickness is a potentially important component of the diffusive pathway for gas exchange in aquatic organisms. The soft coral Alcyonium siderium (Octocorallia) and sea anemone Metridium senile (Actiniaria) exhibit significant increases in respiration with water flow over a range of Reynolds numbers encountered subtidally. A nondimensional mass transfer analysis of the effect of forced convection demonstrates the importance of the state of the organism's boundary layer in regulating metabolism in these invertebrates. Flow-modulated gas exchange may limit secondary productivity in subtidal environments. PMID:16594087
Hamiltonian Description of Convective-cell Generation
J.A. Krommes and R.A. Kolesnikov
2004-03-11
The nonlinear statistical growth rate eq for convective cells driven by drift-wave (DW) interactions is studied with the aid of a covariant Hamiltonian formalism for the gyrofluid nonlinearities. A statistical energy theorem is proven that relates eq to a second functional tensor derivative of the DW energy. This generalizes to a wide class of systems of coupled partial differential equations a previous result for scalar dynamics. Applications to (i) electrostatic ion-temperature-gradient-driven modes at small ion temperature, and (ii) weakly electromagnetic collisional DW's are noted.
Rapid PCR thermocycling using microscale thermal convection.
Muddu, Radha; Hassan, Yassin A; Ugaz, Victor M
2011-01-01
Many molecular biology assays depend in some way on the polymerase chain reaction (PCR) to amplify an initially dilute target DNA sample to a detectable concentration level. But the design of conventional PCR thermocycling hardware, predominantly based on massive metal heating blocks whose temperature is regulated by thermoelectric heaters, severely limits the achievable reaction speed(1). Considerable electrical power is also required to repeatedly heat and cool the reagent mixture, limiting the ability to deploy these instruments in a portable format. Thermal convection has emerged as a promising alternative thermocycling approach that has the potential to overcome these limitations(2-9). Convective flows are an everyday occurrence in a diverse array of settings ranging from the Earth's atmosphere, oceans, and interior, to decorative and colorful lava lamps. Fluid motion is initiated in the same way in each case: a buoyancy driven instability arises when a confined volume of fluid is subjected to a spatial temperature gradient. These same phenomena offer an attractive way to perform PCR thermocycling. By applying a static temperature gradient across an appropriately designed reactor geometry, a continuous circulatory flow can be established that will repeatedly transport PCR reagents through temperature zones associated with the denaturing, annealing, and extension stages of the reaction (Figure 1). Thermocycling can therefore be actuated in a pseudo-isothermal manner by simply holding two opposing surfaces at fixed temperatures, completely eliminating the need to repeatedly heat and cool the instrument. One of the main challenges facing design of convective thermocyclers is the need to precisely control the spatial velocity and temperature distributions within the reactor to ensure that the reagents sequentially occupy the correct temperature zones for a sufficient period of time(10,11). Here we describe results of our efforts to probe the full 3-D velocity and
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.
Mixed convection opposing flow in porous annulus
NASA Astrophysics Data System (ADS)
Salman, Ahmed N. J.; Kamangar, Sarfaraz; Al-Rashed, Abdullah A. A. A.; Khan, T. M. Yunus; Khaleed, H. M. T.
2016-06-01
The current work investigates the mixed convection flow in a vertical porous annulus embedded with fluid saturated porous medium. The annulus is isothermally heated discretely at 20%, 35% and 50% of the height of cylinder at the center of annulus. Darcy law with thermal non-equilibrium approach is considered. The governing partial differential equations are solved using Finite Element Method (FEM). The effects of Peclet number Pe and conductivity ratio Kr on heat transfer and fluid flow is discussed It is found that the applied velocity in the downward direction, in case of an opposing flow, does not allow the thermal energy to reach from a hot to a cold surface.
Sunspots and Giant-Cell Convection
NASA Technical Reports Server (NTRS)
Moore, Ron L.; Hathaway, David H.; Reichmann, Ed J.
2000-01-01
From analysis of Doppler velocity images from SOHO/MDI, Hathaway et al (2000, Solar Phys., in press) have found clear evidence for giant convection cells that fill the solar surface, have diameters 3 - 10 times that typical of supergranules, and have lifetimes approx. greater than 10 days. Analogous to the superposition of the granular convection on the supergranular convection, the approx. 30,000 km diameter supergranules are superposed on these still larger giant cells. Because the giant cells make up the large-scale end of a continuous power spectrum that peaks at the size scale of supergranules, it appears that the giant cells are made by the same mode of convection as the supergranules. This suggests that the giant cells are similar to supergranules, just longer-lived, larger in diameter, and deeper. Here we point out that the range of lengths of large bipolar sunspot groups is similar to the size range of giant cells. This, along with the long lives (weeks) of large sunspots, suggests that large sunspots sit in long-lived, deep downflows at the corners of giant cells, and that the distance from leader to follower sunspots in large bipolar groups is the distance from one giant-cell corner to the next. By this line of reasoning, an unusually large and strong downdraft might pull in both legs of a rising spot-group magnetic flux loop, resulting in the formation of a delta sunspot. This leads us to suggest that a large, strong giant-cell corner downdraft should be present at the birthplaces of large delta sunspots for some time (days to weeks) before the birth. Thus, early detection of such downdrafts by local helioscismology might provide an early warning for the formation of those active regions (large delta sunspot groups) that produce the Sun's most violent flares and coronal mass ejections. This work is supported by NASA's Office of Space Science through the Solar Physics Branch of its Sun-Earth Connection Program.
Convective flow effects on protein crystal growth
NASA Technical Reports Server (NTRS)
Rosenberger, Franz; Monaco, Lisa A.
1994-01-01
A high-resolution microscopic interferometric setup for the monitoring of protein morphologies has been developed. Growth or dissolution of a crystal can be resolved with a long-term depth resolution of 200 A and a lateral resolution of 2 microns. This capability of simultaneously monitoring the interfacial displacement with high local depth resolution has yielded several novel results. We have found with lysozyme that (1) the normal growth rate is oscillatory, and (2) depending on the impurity content of the solution, the growth step density is either greater or lower at the periphery of a facet than in its center. The repartitioning of Na plus and Cl minus ions between lysozyme solutions and crystals was studied for a wide range of crystallization conditions. A nucleation-growth-repartitioning model was developed, to interpret the large body of data in unified way. The results strongly suggest that (1) the ion to lysozyne ratio in the crystal depends mostly on kinetic rather than crystallographic parameters, and (2) lysozyme crystals possess a salt-rich core with a diameter electron microscopy results appear to confirm this finding, which could have far-reaching consequences for x-ray diffraction studies. A computational model for diffusive-convective transport in protein crystallization has been applied to a realistic growth cell geometry, taking into account the findings of the above repartitioning studies and our kinetics data for the growth of lysozyme. The results show that even in the small cell employed, protein concentration nonuniformities and gravity-driven solutal convection can be significant. The calculated convection velocities are of the same order to magnitude as those found in earlier experiments. As expected, convective transport, i.e., at Og, lysozyme crystal growth remains kinetically limited. The salt distribution in the crystal is predicted to be non-uniform at both 1g and 0g, as a consequence of protein depletion in the solution. Static and
Turbulence originating from convectively stable internal waves
NASA Technical Reports Server (NTRS)
Lindzen, R. S.; Forbes, J.
1983-01-01
The Lindzen (1981) theory for the generation of turbulence by unstable tides and gravity omits consideration of the possibility that turbulence may be generated by gravity waves which are not convectively unstable, as suggested by the McComas and Bretherton (1977) demonstration that internal gravity waves are unstable to other gravity waves with shorter wavelengths. Attention is presently given to the estimation of the maximum turbulence that might be generated by such a process, and it is shown that even such turbulence would not significantly alter earlier results.
Multistability in rotating spherical shell convection.
Feudel, F; Seehafer, N; Tuckerman, L S; Gellert, M
2013-02-01
The multiplicity of stable convection patterns in a rotating spherical fluid shell heated from the inner boundary and driven by a central gravity field is presented. These solution branches that arise as rotating waves (RWs) are traced for varying Rayleigh number while their symmetry, stability, and bifurcations are studied. At increased Rayleigh numbers all the RWs undergo transitions to modulated rotating waves (MRWs) which are classified by their spatiotemporal symmetry. The generation of a third frequency for some of the MRWs is accompanied by a further loss of symmetry. Eventually a variety of MRWs, three-frequency solutions, and chaotic saddles and attractors control the dynamics for higher Rayleigh numbers. PMID:23496624
Intermittent flow regimes near the convection threshold in ferromagnetic nanofluids
NASA Astrophysics Data System (ADS)
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.
The wavelength of supercritical surface tension driven Benard convection
NASA Technical Reports Server (NTRS)
Koschmieder, E. L.
1991-01-01
The size or the wavelength of moderately supercritical surface tension driven Benard convection has been investigated experimentally in a thin fluid layer of large aspect ratio. It has been found that the number of the hexagonal convection cells increases with increased temperature differences, up to 1.3 times the critical temperature difference. That means that the wavelength of surface tension driven convection decreases after onset of the instability for moderately nonlinear conditions. This result is in striking contrast to the well-known increase of the wavelength of buoyancy driven Rayleigh-Benard convection.
Self-consistent modeling of inner magnetospheric convection
NASA Technical Reports Server (NTRS)
Toffoletto, F. R.; Spiro, R. W.; Wolf, R. A.; Hesse, M.; Birn, J.
1996-01-01
The initial results of a model of inner magnetospheric convection are presented. The model employs the Rice convection model with a magnetic field computed with the constraint of magnetostatic equilibrium. The approach computes equilibria from a magnetofriction code which is a modified version of the Hesse-Birn equilibrium code adopted for use in the inner magnetosphere. The code uses the pressure distribution computed from the Rice convection model to update the magnetic field. The algorithm used to compute the inner magnetospheric equilibria is outlined, and the coupling of the equilibrium code with the convection model is described.
Convective and radiative heating of a Saturn entry probe
NASA Technical Reports Server (NTRS)
Tiwari, S. N.; Szema, K. Y.; Moss, J. N.; Subramanian, S. V.
1984-01-01
The extent of convective and radiative heating for a Saturn entry probe is investigated in the absence and presence of ablation mass injection. The flow in the shock layer is assumed to be axisymmetric, viscous and in local thermodynamic equilibrium. The importance of chemical nonequilibrium effects for both the radiative and convective nonblowing surface heating rates is demonstrated for prescribed entry conditions. Results indicate that the nonequilibrium chemistry can significantly influence the rate of radiative heating to the entry probes. With coupled carbon-phenolic ablation injection, the convective heating rates are reduced substantially. Turbulence has little effect on radiative heating but it increases the convective heating considerably.
Granular convection and its application to asteroidal resurfacing timescale
NASA Astrophysics Data System (ADS)
Yamada, Tomoya; Ando, Kosuke; Morota, Tomokatsu; Katsuragi, Hiroaki
2016-04-01
A model for the asteroid resurfacing resulting from regolith convection is built to estimate its timescale. The regolith convection by impact-induced global seismic shaking could be a possible reason for regolith migration and resultant segregated terrain which were found on the asteroids Itokawa [1]. Some recent studies [2, 3] experimentally investigated the convective velocity of the vibrated granular bed to discuss the feasibility of regolith convection under the microgravity condition such as small asteroids. These studies found that the granular convective velocity is almost proportional to the gravitational acceleration [2, 3]. Namely, the granular (regolith) convective velocity would be very low under the microgravity condition. Therefore, the timescale of resurfacing by regolith convection would become very long. In order to examine the feasibility of the resurfacing by regolith convection on asteroids, its timescale have to be compared with the surface age or the lifetime of asteroids. In this study, we aim at developing a model of asteroid resurfacing process induced by regolith convection. The model allows us to estimate the resurfacing timescale for various-sized asteroids covered with regolith. In the model, regolith convection is driven by the impact-induced global seismic shaking. The model consists of three phases, (i) Impact phase: An impactor intermittently collides with a target asteroid [4], (ii) Vibration phase: The collision results in a global seismic shaking [5], (iii) Convection phase: The global seismic shaking induces the regolith convection on the asteroid [3]. For the feasibility assessment of the resurfacing process driven by regolith convection, we estimate the regolith-convection-based resurfacing timescale T as a function of the size of a target asteroid Da. According to the estimated result, the resurfacing time scale is 40 Myr for the Itokawa-sized asteroid, and this value is shorter than the mean collisional lifetime of Itokawa
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.
A heat engine based moist convection parametrization for Jupiter
NASA Astrophysics Data System (ADS)
Zuchowski, L. C.; Read, P. L.; Yamazaki, Y. H.; Renno, N. O.
2009-11-01
We have developed a parametrization of Jovian moist convection based on a heat engine model of moist convection. In comparison to other moist convection schemes, this framework allows the computation of the total available convective energy TCAPE and the corresponding mass flux M as dynamic variables from the mean atmospheric state. The effects of this parametrization have been investigated both analytically and numerically. In agreement with previous numerical experiments and observations, the inclusion of moist convection leads to heat and water vapor transport from the water condensation level into higher altitudes. The time development of the modeled convective events was found to be strongly influenced by a rapid reduction of kinetic energy and a subsequent lowering of the cumulus tower's top in response to convective heating. We have tested the sensitivity of the scheme to different variations in the fractional cloud coverage and under the inclusion of external radiative forcing towards a stable/unstable temperature profile. While the time development of convective events differs in response to these variations, the general moist convective heating and moistening of the upper troposphere was a robust feature observed in all experiments.
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
NASA Technical Reports Server (NTRS)
Parker, E. N.
1979-01-01
The effect of negative aerodynamic drag in an ideal fluid subject to convective instability is considered. It is shown that a cylinder moving in such a fluid is propelled forward in its motion by the convective forces and that the characteristic acceleration time is comparable to the onset time of convective motions in the fluid. It is suggested that convective propulsion plays an important role in the dynamics of flux tubes extending through the surface of the sun. The suppression of the upward heat flow in a Boussinesq convective cell with free upper and lower boundaries by a downdraft is then analyzed. Application to the solar convection zone indicates that downdrafts of 1 to 2 km/s at depths of 1000 to 4000 km beneath the visible surface of the sun are sufficient to reduce the upward heat flux to a small fraction of the ambient value.
NASA Astrophysics Data System (ADS)
Stothers, Richard B.; Chin, Chao-wen
1997-04-01
A calibration of two different stellar convection theories is made by theoretically reproducing with stellar models the effective temperatures of very luminous red supergiants. This calibration is then compared with previous calibrations using less luminous objects. The corresponding range of stellar masses is 1-20 Msolar. For consistency with previous work, the calibrating parameter is taken to be the convective mixing length l. If l is assumed to be proportional to the local pressure scale height Hp, the constant of proportionality αP must vary significantly with stellar mass. If, however, l is assumed to be proportional to the distance z below the outer boundary of the convection zone, the constant of proportionality αz emerges as a universal constant, within the uncertainties due to possible errors of the observed effective temperatures and of the theoretically calculated low-temperature opacities. In particular, standard mixing-length theory yields a constant αz = 2, whereas the new full-spectrum-of-turbulence theory of Canuto & Mazzitelli yields a constant αz = 1. Physical constraints, laboratory experiments, and observations of turbulent convection in the Earth's atmosphere indicate αz <= 1. This basic consistency of results over such an enormous range of physical dimensions suggests a very great degree of generality.
Hayat, Tasawar; Ashraf, Muhammad Bilal; Alsulami, Hamed H; Alhuthali, Muhammad Shahab
2014-01-01
The objective of present research is to examine the thermal radiation effect in three-dimensional mixed convection flow of viscoelastic fluid. The boundary layer analysis has been discussed for flow by an exponentially stretching surface with convective conditions. The resulting partial differential equations are reduced into a system of nonlinear ordinary differential equations using appropriate transformations. The series solutions are developed through a modern technique known as the homotopy analysis method. The convergent expressions of velocity components and temperature are derived. The solutions obtained are dependent on seven sundry parameters including the viscoelastic parameter, mixed convection parameter, ratio parameter, temperature exponent, Prandtl number, Biot number and radiation parameter. A systematic study is performed to analyze the impacts of these influential parameters on the velocity and temperature, the skin friction coefficients and the local Nusselt number. It is observed that mixed convection parameter in momentum and thermal boundary layers has opposite role. Thermal boundary layer is found to decrease when ratio parameter, Prandtl number and temperature exponent are increased. Local Nusselt number is increasing function of viscoelastic parameter and Biot number. Radiation parameter on the Nusselt number has opposite effects when compared with viscoelastic parameter. PMID:24608594
Hayat, Tasawar; Ashraf, Muhammad Bilal; Alsulami, Hamed H.; Alhuthali, Muhammad Shahab
2014-01-01
The objective of present research is to examine the thermal radiation effect in three-dimensional mixed convection flow of viscoelastic fluid. The boundary layer analysis has been discussed for flow by an exponentially stretching surface with convective conditions. The resulting partial differential equations are reduced into a system of nonlinear ordinary differential equations using appropriate transformations. The series solutions are developed through a modern technique known as the homotopy analysis method. The convergent expressions of velocity components and temperature are derived. The solutions obtained are dependent on seven sundry parameters including the viscoelastic parameter, mixed convection parameter, ratio parameter, temperature exponent, Prandtl number, Biot number and radiation parameter. A systematic study is performed to analyze the impacts of these influential parameters on the velocity and temperature, the skin friction coefficients and the local Nusselt number. It is observed that mixed convection parameter in momentum and thermal boundary layers has opposite role. Thermal boundary layer is found to decrease when ratio parameter, Prandtl number and temperature exponent are increased. Local Nusselt number is increasing function of viscoelastic parameter and Biot number. Radiation parameter on the Nusselt number has opposite effects when compared with viscoelastic parameter. PMID:24608594
From convection rolls to finger convection in double-diffusive turbulence.
Yang, Yantao; Verzicco, Roberto; Lohse, Detlef
2016-01-01
Double-diffusive convection (DDC), which is the buoyancy-driven flow with fluid density depending on two scalar components, is ubiquitous in many natural and engineering environments. Of great interests are scalars' transfer rate and flow structures. Here we systematically investigate DDC flow between two horizontal plates, driven by an unstable salinity gradient and stabilized by a temperature gradient. Counterintuitively, when increasing the stabilizing temperature gradient, the salinity flux first increases, even though the velocity monotonically decreases, before it finally breaks down to the purely diffusive value. The enhanced salinity transport is traced back to a transition in the overall flow pattern, namely from large-scale convection rolls to well-organized vertically oriented salt fingers. We also show and explain that the unifying theory of thermal convection originally developed by Grossmann and Lohse for Rayleigh-Bénard convection can be directly applied to DDC flow for a wide range of control parameters (Lewis number and density ratio), including those which cover the common values relevant for ocean flows. PMID:26699474
From convection rolls to finger convection in double-diffusive turbulence
Verzicco, Roberto; Lohse, Detlef
2016-01-01
Double-diffusive convection (DDC), which is the buoyancy-driven flow with fluid density depending on two scalar components, is ubiquitous in many natural and engineering environments. Of great interests are scalars' transfer rate and flow structures. Here we systematically investigate DDC flow between two horizontal plates, driven by an unstable salinity gradient and stabilized by a temperature gradient. Counterintuitively, when increasing the stabilizing temperature gradient, the salinity flux first increases, even though the velocity monotonically decreases, before it finally breaks down to the purely diffusive value. The enhanced salinity transport is traced back to a transition in the overall flow pattern, namely from large-scale convection rolls to well-organized vertically oriented salt fingers. We also show and explain that the unifying theory of thermal convection originally developed by Grossmann and Lohse for Rayleigh–Bénard convection can be directly applied to DDC flow for a wide range of control parameters (Lewis number and density ratio), including those which cover the common values relevant for ocean flows. PMID:26699474
NASA Technical Reports Server (NTRS)
Gao, Shou-Ting; Ping, Fan; Li, Xiao-Fan; Tao, Wei-Kuo
2004-01-01
Although dry/moist potential vorticity is a useful physical quantity for meteorological analysis, it cannot be applied to the analysis of 2D simulations. A convective vorticity vector (CVV) is introduced in this study to analyze 2D cloud-resolving simulation data associated with 2D tropical convection. The cloud model is forced by the vertical velocity, zonal wind, horizontal advection, and sea surface temperature obtained from the TOGA COARE, and is integrated for a selected 10-day period. The CVV has zonal and vertical components in the 2D x-z frame. Analysis of zonally-averaged and mass-integrated quantities shows that the correlation coefficient between the vertical component of the CVV and the sum of the cloud hydrometeor mixing ratios is 0.81, whereas the correlation coefficient between the zonal component and the sum of the mixing ratios is only 0.18. This indicates that the vertical component of the CVV is closely associated with tropical convection. The tendency equation for the vertical component of the CVV is derived and the zonally-averaged and mass-integrated tendency budgets are analyzed. The tendency of the vertical component of the CVV is determined by the interaction between the vorticity and the zonal gradient of cloud heating. The results demonstrate that the vertical component of the CVV is a cloud-linked parameter and can be used to study tropical convection.
Comparison of auroral latitude convection to central polar cap convection. (Invited)
NASA Astrophysics Data System (ADS)
Bristow, W. A.; Amata, E.
2013-12-01
The SuperDARN radar at McMurdo station has been providing convection observations in the central polar cap since January 2010. The Antarctic magnetic pole lies in the center of the radar field of view at about 1000 km range, which is optimum for convection observations. A new pair of SuperDARN radars was constructed in the Antarctic summer of 2012/2013, which add highly complimentary fields of view. The radars, one located at the Italian station at Dome-C, and one located at the US South Pole Station, are directed into a region directly equatorward of the McMurdo field of view. The radars came on line in late January 2013 and are producing excellent convection observations. This paper presents initial results combining the three radar's convection observations. Intervals when the IMF clock angle was between 135 and 225 for periods of more than an hour were selected for study. Just under 50 hours of observations met this criteria since the radars began operation. Convection vectors were formed using the standard SuperDARN algorithm [Ruohoniemi and Baker, 1998] and the auroral-zone flows were compared to those in the central polar cap. Central polar cap flows are typically spatially uniform though highly variable in time, even though the lower latitude observations were spatially structured. The central polar cap average flow velocity is less than 500 m/s, though it often exceeds 1000 m/s. Conditions that lead to the high-speed flow are presented. In addition, correlation with the IMF and solar wind are presented. At times the correlation exceeds 80% while at others it is near zero.
Dynamics of Turbulent Convection and Convective Overshoot in a Moderate-mass Star
NASA Astrophysics Data System (ADS)
Kitiashvili, I. N.; Kosovichev, A. G.; Mansour, N. N.; Wray, A. A.
2016-04-01
We present results of realistic three-dimensional (3D) radiative hydrodynamic simulations of the outer layers of a moderate-mass star (1.47 M ⊙), including the full convection zone, the overshoot region, and the top layers of the radiative zone. The simulation results show that the surface granulation has a broad range of scales, from 2 to 12 Mm, and that large granules are organized in well-defined clusters, consisting of several granules. Comparison of the mean structure profiles from 3D simulations with the corresponding one-dimensional (1D) standard stellar model shows an increase of the stellar radius by ∼800 km, as well as significant changes in the thermodynamic structure and turbulent properties of the ionization zones. Convective downdrafts in the intergranular lanes between granulation clusters reach speeds of more than 20 km s‑1, penetrate through the whole convection zone, hit the radiative zone, and form an 8 Mm thick overshoot layer. Contrary to semi-empirical overshooting models, our results show that the 3D dynamic overshoot region consists of two layers: a nearly adiabatic extension of the convection zone and a deeper layer of enhanced subadiabatic stratification. This layer is formed because of heating caused by the braking of the overshooting convective plumes. This effect has to be taken into account in stellar modeling and the interpretation of asteroseismology data. In particular, we demonstrate that the deviations of the mean structure of the 3D model from the 1D standard model of the same mass and composition are qualitatively similar to the deviations for the Sun found by helioseismology.
Rayleigh-Bénard convection in a vertical annular container near the convection threshold.
Wang, Bo-Fu; Wan, Zhen-Hua; Ma, Dong-Jun; Sun, De-Jun
2014-04-01
The instabilities and transitions of flow in an annular container with a heated bottom, a cooled top, and insulated sidewalls are studied numerically. The instabilities of the static diffusive state and of axisymmetric flows are investigated by linear stability analysis. The onset of convection is independent of the Prandtl number but determined by the geometry of the annulus, i.e., the aspect ratio Γ (outer radius to height) and radius ratio δ (inner radius to outer radius). The stability curves for onset of convection are presented for 0.001≤δ≤0.8 at six fixed aspect ratios: Γ=1, 1.2, 1.6, 1.75, 2.5, and 3.2. The instability of convective flow (secondary instability), which depends on both the annular geometry and the Prandtl number, is studied for axisymmetric convection. Two pairs of geometric control parameters are chosen to perform the secondary instability analysis-Γ=1.2, δ=0.08 and Γ=1.6, δ=0.2-and the Prandtl number ranges from 0.02 to 6.7. The secondary instability exhibits some similarities to that for convection in a cylinder. A hysteresis stability loop is found for Γ=1.2, δ=0.08 and frequent changes of critical mode with Prandtl number are found for Γ=1.6, δ=0.2. The three-dimensional flows beyond the axisymmetry-breaking bifurcations are obtained by direct numerical simulation for Γ=1.2, δ=0.08. PMID:24827339
NASA Astrophysics Data System (ADS)
Gao, Shouting; Li, Xiaofan; Tao, Wei-Kuo; Shie, Chung-Lin; Lang, Steve
2007-01-01
The relationships between cloud hydrometeors and convective/moist vorticity vectors are investigated using hourly data from a three-dimensional, 5-day cloud-resolving model (CRM) simulation during the Tropical Rainfall Measuring Mission (TRMM) Kwajalein Experiment (KWAJEX). Vertical components of convective and moist vorticity vectors are highly correlated with cloud hydrometeors. The vertical components represent the interaction between horizontal vorticity and horizontal moist potential temperature/specific humidity gradient. The vertical components of convective and moist vorticity vectors can be used to study tropical oceanic convection in both two-dimensional and three-dimensional frameworks.
Convective Stirring in Liquid Lithium (LTX)
NASA Astrophysics Data System (ADS)
Cassin, Margaret; Kearns, Eugene; Majeski, Richard
2011-10-01
LTX is a spherical tokamak with R=0.4 m, a=0.26 m, and elongation=1.5. LTX has a heated (300 - 400 C) liner, designed to be coated with lithium. During experiments in 2010, oxidation of the lithium surface was observed when the liner was heated to 300 C, above the melting point of lithium (182 C). A pumping system is being installed to absorb and pump background gasses which react with lithium, similar to a getter pump, using liquid and solid lithium. Lithium will be loaded into a yttria crucible heated from below by a small, HeatWave model TB175 300W cartridge heater to produce convective currents in order to maintain a clean lithium surface and decrease the time for oxide formation. This system was tested in an argon glove box using a copper heat concentrator - instead of the HeatWave vacuum-compatible unit. Infrared thermometry and thermocouples were used to monitor the surface temperature of the molten lithium, and convective flow patterns. A 200 FPS high speed camera was also employed to monitor flows, using the motion of residual oxide patches. Results from the measurements will be presented. Supported by US DOE contract DE-AC02-09CH11466.
Thermal Convection on an Ablating Target
NASA Astrophysics Data System (ADS)
Mehmedagic, Igbal; Thangam, Siva
2015-11-01
Modeling and analysis of thermal convection of a metallic targets subject to radiative flux is of relevance to various manufacturing processes as well as for the development of protective shields. The present work involves the computational modeling of metallic targets subject to high heat fluxes that are both steady and pulsed. Modeling of the ablation and associated fluid dynamics when metallic surfaces are exposed to high intensity pulsed laser fluence at normal atmospheric conditions is considered. The incident energy from the laser is partly absorbed and partly reflected by the surface during ablation and subsequent vaporization of the convecting melt also participates in the radiative exchange. The energy distribution during the process between the bulk and vapor phase strongly depends on optical and thermodynamic properties of the irradiated material, radiation wavelength, and laser pulse intensity and duration. Computational findings based on effective representation and prediction of the heat transfer, melting and vaporization of the targeting material as well as plume formation and expansion are presented and discussed in the context of various ablation mechanisms, variable thermo-physical and optical properties, plume expansion and surface geometry. Funded in part by U. S. Army ARDEC, Picatinny Arsenal, NJ.
Magnetospheric Convection as a Global Force Phenomenon
NASA Astrophysics Data System (ADS)
Siscoe, G.
2007-12-01
Since 1959 when Thomas Gold showed that motions in the magnetosphere were possible despite plasma being frozen to the magnetic field, magnetospheric convection as a subject of study has gone through several stages (to be reviewed) leading to a recent one that integrates convection into a global system of balance of forces. This area of research has opened by focusing on the region 1 current system as a carrier of force between the solar wind and the ionosphere/thermosphere fluid. An important result to emerge from it is the realization that the force that the solar wind delivers to the magnetosphere in being transferred by the region 1 current system to the ionosphere/thermosphere fluid is amplified by about an order of magnitude. (Vasyliunas refers to this as "leveraging.") The apparent violation of Newton's Third Law results from the main participants in the force balance being not the solar wind force but the JxB force on the ionosphere/thermosphere fluid and the mu-dot-grad-B force on the Earth's dipole. This talk extends the study by considering the global force-balance problem separately for the Pedersen current (a completion of the region 1 problem), the Hall current (thus introducing the region 2 current system), and the Cowling current (bringing in the substorm current wedge). The approach is through representing the ionosphere/thermosphere fluid by the shallow water equations. Novelties that result include force balance by means of tidal bulges and tidal bores.
Venusian Applications of 3D Convection Modeling
NASA Technical Reports Server (NTRS)
Bonaccorso, Timary Annie
2011-01-01
This study models mantle convection on Venus using the 'cubed sphere' code OEDIPUS, which models one-sixth of the planet in spherical geometry. We are attempting to balance internal heating, bottom mantle viscosity, and temperature difference across Venus' mantle, in order to create a realistic model that matches with current planetary observations. We also have begun to run both lower and upper mantle simulations to determine whether layered (as opposed to whole-mantle) convection might produce more efficient heat transfer, as well as to model coronae formation in the upper mantle. Upper mantle simulations are completed using OEDIPUS' Cartesian counterpart, JOCASTA. This summer's central question has been how to define a mantle plume. Traditionally, we have defined a hot plume the region with temperature at or above 40% of the difference between the maximum and horizontally averaged temperature, and a cold plume as the region with 40% of the difference between the minimum and average temperature. For less viscous cases (1020 Pa?s), the plumes generated by that definition lacked vigor, displaying buoyancies 1/100th of those found in previous, higher viscosity simulations (1021 Pa?s). As the mantle plumes with large buoyancy flux are most likely to produce topographic uplift and volcanism, the low viscosity cases' plumes may not produce observable deformation. In an effort to eliminate the smallest plumes, we experimented with different lower bound parameters and temperature percentages.
Control of Oscillatory Thermocapillary Convection in Microgravity
NASA Technical Reports Server (NTRS)
Skarda, Ray
1998-01-01
This project focused on the generation and suppression of oscillatory thermocapillary convection in a thin liquid layer. The bulk of the research was experimental in nature, some theoretical work was also done. ne first phase of this research generated, for the first time, the hydrothermal-wave instability predicted by Smith and Davis in 1983. In addition, the behavior of the fluid layer under a number of conditions was investigated and catalogued. A transition map for the instability of buoyancy-thermocapillary convection was prepared which presented results in terms of apparatus-dependent and apparatus-independent parameters, for ease of comparison with theoretical results. The second phase of this research demonstrated the suppression of these hydrothermal waves through an active, feed-forward control strategy employing a CO2 laser to selectively heat lines of negative disturbance temperature on the free surface of the liquid layer. An initial attempt at this control was only partially successful, employing a thermocouple inserted slightly below the free surface of the liquid to generate the control scheme. Subsequent efforts, however, were completely successful in suppressing oscillations in a portion of the layer by utilizing data from an infrared image of the free surface to compute hydrothermal-wave phase speeds and, using these, to tailor the control scheme to each passing wave.
Vertical Slot Convection: A linear study
McAllister, A.; Steinolfson, R.; Tajima, T.
1992-11-01
The linear stability properties of fluid convection in a vertical slot were studied. We use a Fourier-Chebychev decomposition was used to set up the linear eigenvalue problems for the Vertical Slot Convection and Benard problems. The eigenvalues, neutral stability curves, and critical point values of the Grashof number, G, and the wavenumber were determined. Plots of the real and imaginary parts of the eigenvalues as functions of G and {alpha} are given for a wide range of the Prandtl number, Pr, and special note is made of the complex mode that becomes linearly unstable above Pr {approximately} 12.5. A discussion comparing different special cases facilitates the physical understanding of the VSC equations, especially the interaction of the shear-flow and buoyancy induced physics. Making use of the real and imaginary eigenvalues and the phase properties of the eigenmodes, the eigenmodes were characterized. One finds that the mode structure becomes progressively simpler with increasing Pr, with the greatest complexity in the mid ranges where the terms in the heat equation are of roughly the same size.
Vertical Slot Convection: A linear study
McAllister, A. ); Steinolfson, R. ); Tajima, T. . Inst. for Fusion Studies)
1992-11-01
The linear stability properties of fluid convection in a vertical slot were studied. We use a Fourier-Chebychev decomposition was used to set up the linear eigenvalue problems for the Vertical Slot Convection and Benard problems. The eigenvalues, neutral stability curves, and critical point values of the Grashof number, G, and the wavenumber were determined. Plots of the real and imaginary parts of the eigenvalues as functions of G and [alpha] are given for a wide range of the Prandtl number, Pr, and special note is made of the complex mode that becomes linearly unstable above Pr [approximately] 12.5. A discussion comparing different special cases facilitates the physical understanding of the VSC equations, especially the interaction of the shear-flow and buoyancy induced physics. Making use of the real and imaginary eigenvalues and the phase properties of the eigenmodes, the eigenmodes were characterized. One finds that the mode structure becomes progressively simpler with increasing Pr, with the greatest complexity in the mid ranges where the terms in the heat equation are of roughly the same size.
Magnetic Helicity in a Cyclic Convective Dynamo
NASA Astrophysics Data System (ADS)
Miesch, Mark S.; Zhang, Mei; Augustson, Kyle C.
2016-05-01
Magnetic helicity is a fundamental agent for magnetic self-organization in magnetohydrodynamic (MHD) dynamos. As a conserved quantity in ideal MHD, it establishes a strict topological coupling between large and small-scale magnetic fields. The generation of magnetic fields on scales larger than the velocity field is linked to an upscale transfer of magnetic helicity, either locally in spectral space as in the inverse cascade of magnetic helicity in MHD turbulence or non-locally, as in the turbulent alpha-effect of mean-field dynamo theory. Thus, understanding the generation, transport, and dissipation of magnetic helicity is an essential prerequisite to understanding manifestations of magnetic self-organization in the solar dynamo, including sunspots, the prominent dipole and quadrupole moments, and the 22-year magnetic activity cycle. We investigate the role of magnetic helicity in a convective dynamo model that exhibits regular magnetic cycles. The cycle is marked by coherent bands of toroidal field that persist within the convection zone and that are antisymmetric about the equator. When these toriodal bands interact across the equator, it initiates a global restructuring of the magnetic topology that contributes to the reversal of the dipole moment. Thus, the polar field reversals are preceeded by a brief reversal of the subsurface magnetic helicity. There is some evidence that the Sun may exhibit a similar magnetic helicity reversal prior to its polar field reversals.
Performance of thermal adhesives in forced convection
NASA Technical Reports Server (NTRS)
Kundu, Nikhil K.
1993-01-01
Cooling is critical for the life and performance of electronic equipment. In most cases cooling may be achieved by natural convection but forced convection may be necessary for high wattage applications. Use of conventional type heat sinks may not be feasible from the viewpoint of specific applications and the costs involved. In a heat sink, fins can be attached to the well by ultrasonic welding, by soldering, or with a number of industrially available thermal adhesives. In this paper, the author investigates the heat transfer characteristics of several adhesives and compares them with ultrasonic welding and theoretically calculated values. This experiment was conducted in an air flow chamber. Heat was generated by using heaters mounted on the well. Thermstrate foil, Uniset A401, and Aremco 571 adhesives were tested along with an ultrasonically welded sample. Ultrasonic welding performed far better than the adhesives and Thermstrate foil. This type of experiment can be adapted for a laboratory exercise in an upper level heat transfer course. It gives students an exposure to industrial applications that help them appreciate the importance of the course material.
Convective Polymer Depletion on Pair Particle Interactions
NASA Astrophysics Data System (ADS)
Fan, Tai-Hsi; Taniguchi, Takashi; Tuinier, Remco
2011-11-01
Understanding transport, reaction, aggregation, and viscoelastic properties of colloid-polymer mixture is of great importance in food, biomedical, and pharmaceutical sciences. In non-adsorbing polymer solutions, colloidal particles tend to aggregate due to the depletion-induced osmotic or entropic force. Our early development for the relative mobility of pair particles assumed that polymer reorganization around the particles is much faster than particle's diffusive time, so that the coupling of diffusive and convective effects can be neglected. Here we present a nonequilibrium two-fluid (polymer and solvent) model to resolve the convective depletion effect. The theoretical framework is based on ground state approximation and accounts for the coupling of fluid flow and polymer transport to better describe pair particle interactions. The momentum and polymer transport, chemical potential, and local viscosity and osmotic pressure are simultaneously solved by numerical approximation. This investigation is essential for predicting the demixing kinetics in the pairwise regime for colloid-polymer mixtures. This work is supported by NSF CMMI 0952646.
Convection effects in protein crystal growth
NASA Technical Reports Server (NTRS)
Roberts, Glyn O.
1988-01-01
Protein crystals for X-ray diffraction study are usually grown resting on the bottom of a hanging drop of a saturated protein solution, with slow evaporation to the air in a small enclosed cell. The evaporation rate is controlled by hanging the drop above a reservoir of water, with its saturation vapor pressure decreased by a low concentration of a passive solute. The drop has a lower solute concentration, and its volume shrinks by evaporation until the molecular concentrations match. Protein crystals can also be grown from a seed crystal suspended or supported in the interior of a supersaturated solution. The main analysis of this report concerns this case because it is less complicated than hanging-drop growth. Convection effects have been suggested as the reason for the apparent cessation of growth at a certain rather small crystal size. It seeems that as the crystal grows, the number of dislocations increases to a point where further growth is hindered. Growth in the microgravity environment of an orbiting space vehicle has been proposed as a method for obtaining larger crystals. Experimental observations of convection effects during the growth of protein crystals have been reported.
Mesospheric heating due to intense tropospheric convection
NASA Technical Reports Server (NTRS)
Taylor, L. L.
1979-01-01
A series of rocket measurements made twice daily at Wallops Island, Va., revealed a rapid heating of the mesosphere on the order of 10 K on days when thunderstorms or squall lines were in the area. This heating is explained as the result of frictional dissipation of vertically propagating internal gravity waves generated by intense tropospheric convection. Ray-tracing theory is used to determine the spectrum of gravity wave groups that actually reach mesospheric heights. This knowledge is used in an equation describing the spectral energy density of a penetrative convective element to calculate the fraction of the total energy initially available to excite those waves that do reach the level of heating. This value, converted into a vertical velocity, is used as the lower boundary condition for a multilayer model used to determine the detailed structure of the vertically propagating waves. The amount of frictional dissipation produced by the waves is calculated from the solutions of the frictionless model by use of a vertically varying eddy viscosity coefficient. The heating produced by the dissipation is then calculated from the thermodynamic equation.
Convection-Driven Resurfacing on Icy Satellites
NASA Astrophysics Data System (ADS)
Barr, Amy
2015-04-01
Ridge and trough terrain, characterized by kilometer-scale sub-parallel ridges and troughs, is found in a variety of settings on the icy satellites of the solar system. Examples include Ganymede's grooved terrain [1], Europa's bands [2,3], Miranda's coronae [4,5], and swaths of ridges and troughs in the northern plains of Enceladus [6]. The fault spacing implies a shallow brittle/ductile depth and thus, a high thermal gradient at the time of formation [e.g., 7]. I will show that similar rheological parameters can give rise to the heat flows and deformation rates inferred for the formation of many examples of ridge and trough terrain. These results suggest that convection in ice mantles with weak surfaces can explain the formation of these terrains, just as convection in Earth's mantle, beneath a weakened crust, can drive surface deformation. References: [1] Pappalardo, R. T. et al., 2004. in Jupiter, Cambridge Univ. Press, pp.363. [2] Prockter, L. M. et al., 2002. JGR 107, 5028. [3] Stempel, M. M. et al., 2005. Icarus 177, 297. [4] Pappalardo, R. T. et al., 1997. JGR 102, 13369. [5] Hammond, N. P. and A. C. Barr, 2014. Geology 42, 931-934. [6] Bland, M. et al., 2007. Icarus 192, 92. [7] Nimmo, F. et al., 2002. GRL 29 62-1.
Kinetic energy budgets in areas of convection
NASA Technical Reports Server (NTRS)
Fuelberg, H. E.
1979-01-01
Synoptic scale budgets of kinetic energy are computed using 3 and 6 h data from three of NASA's Atmospheric Variability Experiments (AVE's). Numerous areas of intense convection occurred during the three experiments. Large kinetic energy variability, with periods as short as 6 h, is observed in budgets computed over each entire experiment area and over limited volumes that barely enclose the convection and move with it. Kinetic energy generation and transport processes in the smaller volumes are often a maximum when the enclosed storms are near peak intensity, but the nature of the various energy processes differs between storm cases and seems closely related to the synoptic conditions. A commonly observed energy budget for peak storm intensity indicates that generation of kinetic energy by cross-contour flow is the major energy source while dissipation to subgrid scales is the major sink. Synoptic scale vertical motion transports kinetic energy from lower to upper levels of the atmosphere while low-level horizontal flux convergence and upper-level horizontal divergence also occur. Spatial fields of the energy budget terms show that the storm environment is a major center of energy activity for the entire area.
Convected element method for simulation of angiogenesis.
Pindera, Maciej Z; Ding, Hui; Chen, Zhijian
2008-10-01
We describe a novel Convected Element Method (CEM) for simulation of formation of functional blood vessels induced by tumor-generated growth factors in a process called angiogenesis. Angiogenesis is typically modeled by a convection-diffusion-reaction equation defined on a continuous domain. A difficulty arises when a continuum approach is used to represent the formation of discrete blood vessel structures. CEM solves this difficulty by using a hybrid continuous/discrete solution method allowing lattice-free tracking of blood vessel tips that trace out paths that subsequently are used to define compact vessel elements. In contrast to more conventional angiogenesis modeling, the new branches form evolving grids that are capable of simulating transport of biological and chemical factors such as nutrition and anti-angiogenic agents. The method is demonstrated on expository vessel growth and tumor response simulations for a selected set of conditions, and include effects of nutrient delivery and inhibition of vessel branching. Initial results show that CEM can predict qualitatively the development of biologically reasonable and fully functional vascular structures. Research is being carried out to generalize the approach which will allow quantitative predictions. PMID:18365201
Solutocapillary Convection Effects on Polymeric Membrane Morphology
NASA Technical Reports Server (NTRS)
Krantz, William B.; Todd, Paul W.; Kinagurthu, Sanjay
1996-01-01
Macro voids are undesirable large pores in membranes used for purification. They form when membranes are cast as thin films on a smooth surface by evaporating solvent (acetone) from a polymer solution. There are two un-tested hypotheses explaining the growth of macro voids. One states that diffusion of the non-solvent (water) is solely responsible, while the other states that solutocapillary convection is the primary cause of macro void growth. Solutocapillary convection is flow-caused by a concentration induced surface-tension gradient. Macrovoid growth in the former hypothesis is gravity independent, while in the latter it is opposed by gravity. To distinguish between these two hypotheses, experiments were designed to cast membranes in zero-gravity. A semi-automated apparatus was designed and built for casting membranes during the 20 secs of zero-g time available in parabolic aircraft flight such as NASA's KC-135. The phase changes were monitored optically, and membrane morphology was evaluated by scanning electron microscopy (SEM). These studies appear to be the first quantitative studies of membrane casting in micro-gravity which incorporate real-time data acquisition. Morphological studies of membranes cast at 0, 1, and 1.8 g revealed the presence of numerous, sparse and no macrovoids respectively. These results are consistent with the predictions of the solutocapillary hypothesis of macrovoid growth.
Turbulent convection at high Rayleigh numbers
NASA Astrophysics Data System (ADS)
Ahlers, Guenter
2015-11-01
Russ Donnelly had a vision of building a ten-meter tall Rayleigh-Bénard convection cell for use at helium temperatures at one of the high-energy physics facilities with very large helium liquefaction capacity. It would have reached Rayleigh numbers in the 1020 range and had the promise of yielding detailed information about the so-called ultimate state of turbulent convection which is highly relevant to many geophysical and astrophysical problems as well as to oceanography and climate physics. Although this was not to happen for reasons beyond his control, a laboratory-sized precursor of this venture yielded data for Ra up to 1017. The results were interpreted to yield no definitive indication of a transition to the ultimate state. This talk will review some of these data and compare them with more recent measurements using SF6 at ambient temperatures and high pressure. This comparison suggests that the Donnelly group actually entered a transition range to the ultimate state near Ra1* ~= 6 ×1012 , but re-entered the classical state at larger Ra because with increasing Ra the Prandtl number (which affects Ra1*) also increased in those experiments. In view of the above, one can estimate that, for the same parameter values, the originally envisioned ten-meter cell could have yielded a range of a couple of decades of Ra in the ultimate state. Supported by NSF Grant DMR11-58514.
Free surface calculations in mantle convection
NASA Astrophysics Data System (ADS)
Rose, I.; Buffett, B. A.; Heister, T.
2015-12-01
Geodynamic simulations increasingly rely on simulations with a true free surface to investigate questions of dynamic topography, tectonic deformation, gravity perturbations, and global mantle convection. However, implementations of free surface boundary conditions have proven challenging from a standpoint of accuracy, robustness, and stability. In particular, free surfaces tend to suffer from sloshing instabilities, also known as the "drunken sailor" instability, which severely limit time step sizes. Several schemes have been proposed in the literature to deal with these instabilities. Here we analyze the problem of creeping viscous flow with a free surface and discuss the origin of these instabilities. We demonstrate their cause and how existing stabilization schemes work to damp them out. Our analysis is based on formulating a generalized eigenvalue problem for the relaxation spectra of the linearized free surface problem. We also propose a new scheme for removing instabilites from free surface calculations. It does not require modifications to the system matrix, nor additional variables, but is instead an explicit scheme based on nonstandard finite differences. It relies on a single stabilization parameter which may be identified with the smallest relaxation timescale of the free surface. We analyze the stability and accuracy of the nonstandard finite difference scheme, and describe its implementation in the open source mantle convection software Aspect. We also provide comparisons between the nonstandard finite difference scheme and the quasi-implicit scheme proposed by Kaus, Muhlhaus, and May (2010).
Lobe cell convection and polar cap precipitation
NASA Astrophysics Data System (ADS)
Eriksson, S.; Peria, W. J.; Bonnell, J. W.; Su, Y.-J.; Ergun, R. E.; Tung, Y.-K.; Parks, G. K.; Carlson, C. W.
2003-05-01
The characteristic electric and magnetic field signature of lobe cells as observed by the low-altitude FAST satellite in 55 dawn-dusk passes are compared with Polar ultraviolet images of polar cap auroral activity. Initial results from 34 events of UV image coverage suggest that there is an intimate coupling between the sunward convection flow of the lobe cell and transpolar auroral arcs or diffuse polar cap precipitation in ˜62% of these cases. However, in some cases where the field signatures are suggestive of lobe cell convection, there is no detectable particle precipitation either in Polar UVI or the FAST data sets. Moreover, the presence of lobe cells coincide with UV data intensifications in the premidnight 2000-2400 MLT sector and/or the postnoon 1500 MLT region in ˜59% of all cases with UVI coverage. The magnetic local time dependence of the lobe cells and polar cap precipitation on the interplanetary magnetic field (IMF) are examined using the upstream Wind monitor. The relative importance of the IMF By and Bz components are investigated and compared with the predictions of the antiparallel merging model and strongly suggests a connection with the magnetospheric sash, as is further implied by the mapping of magnetic field lines using the [2002] (T01) model. It was also noted that a majority of lobe cell events occurred during enhanced AE index substorm-like conditions and that generally stronger AE indices are measured for stronger IMF By magnitudes during these events.
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
Convective initiation in the vicinity of the subtropical Andes
NASA Astrophysics Data System (ADS)
Rasmussen, K. L.; Houze, R.
2014-12-01
Extreme convection tends to form in the vicinity of mountain ranges, and the Andes in subtropical South America help spawn some of the most intense convection in the world. An investigation of the most intense storms for 11 years of TRMM Precipitation Radar (PR) data shows a tendency for squall lines to initiate and develop in this region with the canonical leading convective line/trailing stratiform structure. The synoptic environment and structures of the extreme convection and MCSs in subtropical South America are similar to those found in other regions of the world, especially the United States. In subtropical South America, however, the topographical influence on the convective initiation and maintenance of the MCSs is unique. A capping inversion in the lee of the Andes is important in preventing premature triggering. The Andes and other mountainous terrain of Argentina focus deep convective initiation in a narrow region. Subsequent to initiation, the convection often evolves into propagating mesoscale convective systems similar to those seen over the Great Plains of the U. S. and produces damaging tornadoes, hail, and floods across a wide agricultural region. Numerical simulations conducted with the NCAR Weather Research and Forecasting (WRF) Model extend the observational analysis and provide an objective evaluation of storm initiation, terrain effects, and development mechanisms. The simulated mesoscale systems closely resemble the storm structures seen by the TRMM Precipitation Radar as well as the overall shape and character of the storms shown in GOES satellite data. A sensitivity experiment with different configurations of topography, including both decreasing and increasing the height of the Andes Mountains, provides insight into the significant influence of orography in focusing convective initiation in this region. Lee cyclogenesis and a strong low-level jet are modulated by the height of the Andes Mountains and directly affect the character
Convection and plate tectonics on extrasolar planets
NASA Astrophysics Data System (ADS)
Sotin, C.; Grasset, O.; Schubert, G.
2012-04-01
The number of potential Earth-like exoplanets is still very limited compared to the overall number of detected exoplanets. But the different methods keep improving, giving hope for this number to increase significantly in the coming years. Based on the relationship between mass and radius, two of the easiest parameters that can be known for exoplanets, four categories of planets have been identified: (i) the gas giants including hot Jupiters, (ii) the icy giants that can be like their solar system cousins Uranus and Neptune or that can have lost their H2-He atmosphere and have become the so-called ocean planets, (iii) the Earth-like planets with a fraction of silicates and iron similar to that of the Earth, and (iv) the Mercury like planet that have a much larger fraction of iron. The hunt for exoplanets is very much focused on Earth-like planets because of the desire to find alien forms of life and the science goal to understand how life started and developed on Earth. One science question is whether heat transfer by subsolidus convection can lead to plate tectonics, a process that allows material to be recycled in the interior on timescales of hundreds of millions of years. Earth-like exoplanets may have conditions quite different from Earth. For example, COROT-7b is so close to its star that it is likely locked in synchronous orbit with one very hot hemisphere and one very cold hemisphere. It is also worth noting that among the three Earth-like planets of the solar system (Earth, Venus and Mars), only Earth is subject to plate tectonics at present time. Venus may have experienced plate tectonics before the resurfacing event that erased any clue that such a process existed. This study investigates some of the parameters that can influence the transition from stagnant-lid convection to mobile-lid convection. Numerical simulations of convective heat transfer have been performed in 3D spherical geometry in order to determine the stress field generated by convection
Plains Elevated Convection at Night (PECAN) Experiment Science Plan
Turner, D; Parsons, D; Geerts, B
2015-03-01
The Plains Elevated Convection at Night (PECAN) experiment is a large field campaign that is being supported by the National Science Foundation (NSF) with contributions from the National Oceanic and Atmospheric Administration (NOAA), the National Atmospheric and Space Administration (NASA), and the U.S. Department of Energy (DOE). The overarching goal of the PECAN experiment is to improve the understanding and simulation of the processes that initiate and maintain convection and convective precipitation at night over the central portion of the Great Plains region of the United States (Parsons et al. 2013). These goals are important because (1) a large fraction of the yearly precipitation in the Great Plains comes from nocturnal convection, (2) nocturnal convection in the Great Plains is most often decoupled from the ground and, thus, is forced by other phenomena aloft (e.g., propagating bores, frontal boundaries, low-level jets [LLJ], etc.), (3) there is a relative lack of understanding how these disturbances initiate and maintain nocturnal convection, and (4) this lack of understanding greatly hampers the ability of numerical weather and climate models to simulate nocturnal convection well. This leads to significant uncertainties in predicting the onset, location, frequency, and intensity of convective cloud systems and associated weather hazards over the Great Plains.
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…
Mantle convection and the state of the earth's interior
NASA Technical Reports Server (NTRS)
Hager, Bradford H.; Gurnis, Michael
1987-01-01
During 1983 to 1986 emphasis in the study of mantle convection shifted away from fluid mechanical analysis of simple systems with uniform material properties and simple geometries, toward analysis of the effects of more complicated, presumably more realistic models. The important processes related to mantle convection are considered. The developments in seismology are discussed.
Mantle convection and the state of the Earth's interior
NASA Technical Reports Server (NTRS)
Hager, Bradford H.
1987-01-01
During 1983 to 1986 emphasis in the study of mantle convection shifted away from fluid mechanical analysis of simple systems with uniform material properties and simple geometries, toward analysis of the effects of more complicated, presumably more realistic models. The important processes related to mantle convection are considered. The developments in seismology are discussed.
Long-wave interactions in morphological and convective instabilities
NASA Technical Reports Server (NTRS)
Riley, D. S.; Davis, S. H.
1990-01-01
A binary liquid that undergoes directional solidification is susceptible to morphological and solutal-convective instabilities that cause the solid/liquid interface to change from a planar to a cellular state. This paper gives derivations for those long-wave evolution equations that describe the weak couplings between convection and interface morphology and gives some analytical results obtainable from these.
CONVECTIVE DIFFUSION FIELD MEASUREMENTS COMPARED WITH LABORATORY AND NUMERICAL EXPERIMENTS
Some of the more fundamental diffusion parameters measured in the CONDORS convective diffusion field experiment are compared with laboratory experiment and numerical modeling results by means of nondimensionalizations using convective scaling (i.e., mixing depth, z sub i, for len...
Deep convection in the Sahel : a focus on gust fronts
NASA Astrophysics Data System (ADS)
Dione, Cheikh; Lothon, Marie; Campistron, Bernard; Sall, Saidou M.; Guichard, Françoise; Badiane, Daouda; Couvreux, Fleur
2014-05-01
Convection in the Sahel presents a diurnal variability that is influenced by deep convection systems like the Mesoscale Convective Systems (MCSs) and isolated storms or smaller convective systems. These smaller systems have drew less attention than the MCSs, even though they also play a role in the water cycle of this region, contribute to the monsoon dynamics, and to set the scales of the surface heterogeneities. During the African Monsoon Multidisciplinary Analysis (AMMA) intensive observation period in 2006, many of these smaller systems have been observed with the Massachusetts Institute of Technology (MIT) radar that was installed in Niamey, Niger. A systematic study of daytime convection observed during the month of July 2006 is carried out based on the MIT radar data and on the complementary observations given by the ARM mobile facility, in order to analyse the processes and mechanisms involved of deep convection initiation. The results highlight a large frequency of occurrence of the density currents, and their importance in the initiation of new convective cells. Based on this observational analysis and on results from Large Eddy Simulation (LES), density currents are specifically studied, with the aim at : (1) evaluating their frequency of occurrence, (2) assessing their ability to trigger new convective cells, (3) analysing the sensitivity of density currents velocity to land surface contrasts, (4) testing a simple model for their parametrization, (5) evaluating the ability of the LES to represent density currents.
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.
Dielectrophoretic Rayleigh-Bénard convection under microgravity conditions
NASA Astrophysics Data System (ADS)
Yoshikawa, H. N.; Tadie Fogaing, M.; Crumeyrolle, O.; Mutabazi, I.
2013-04-01
Thermal convection in a dielectric fluid layer between two parallel plates subjected to an alternating electric field and a temperature gradient is investigated under microgravity conditions. A thermoelectric coupling resulting from the thermal variation of the electric permittivity of the fluid produces the dielectrophoretic (DEP) body force, which can be regarded as thermal buoyancy due to an effective gravity. This electric gravity can destabilize a stationary conductive state of the fluid to develop convection. The similarity of the DEP thermal convection with the Rayleigh-Bénard (RB) convection is examined by considering its behavior in detail by a linear stability theory and a two-dimensional direct numerical simulation. The results are analyzed from an energetic viewpoint and in the framework of the Ginzburg-Landau (GL) equation. The stabilizing effects of a thermoelectric feedback make the critical parameters different from those in the RB instability. The nonuniformity of the electric gravity arising from the finite variation of permittivity also affects the critical parameters. The characteristic constants of the GL equation are comparable with those for the RB convection. The heat transfer in the DEP convection is weaker than in the RB convection as a consequence of the feedback that impedes the convection.
Dielectrophoretic Rayleigh-Bénard convection under microgravity conditions.
Yoshikawa, H N; Tadie Fogaing, M; Crumeyrolle, O; Mutabazi, I
2013-04-01
Thermal convection in a dielectric fluid layer between two parallel plates subjected to an alternating electric field and a temperature gradient is investigated under microgravity conditions. A thermoelectric coupling resulting from the thermal variation of the electric permittivity of the fluid produces the dielectrophoretic (DEP) body force, which can be regarded as thermal buoyancy due to an effective gravity. This electric gravity can destabilize a stationary conductive state of the fluid to develop convection. The similarity of the DEP thermal convection with the Rayleigh-Bénard (RB) convection is examined by considering its behavior in detail by a linear stability theory and a two-dimensional direct numerical simulation. The results are analyzed from an energetic viewpoint and in the framework of the Ginzburg-Landau (GL) equation. The stabilizing effects of a thermoelectric feedback make the critical parameters different from those in the RB instability. The nonuniformity of the electric gravity arising from the finite variation of permittivity also affects the critical parameters. The characteristic constants of the GL equation are comparable with those for the RB convection. The heat transfer in the DEP convection is weaker than in the RB convection as a consequence of the feedback that impedes the convection. PMID:23679509
Observations and Modelling of Convective Rolls Over Low Hills
NASA Astrophysics Data System (ADS)
Tian, W.; Parker, D. J.; Kilburn, C. A. D.
Radar and satellite images provide observations of convective rolls and other struc- tures in the convective boundary layer (CBL), but numerical modelling is a neces- sary complement to the observations, to investigate the temporal and spatial evolu- tion of convective rolls. Numerical simulations have been performed to investigate observed convective rolls over the south of England, using BLASIUS, a relatively simple boundary layer code for flow over topography. The principal features of the convective structures can be successfully reproduced by the model, notably the roll orientation and spacing and the basic features of the cloud field. These features are in good agreement for two case studies, one with distinct rolls and the other with more dispersed convective structures and a time-dependent basic state. The presence of low topography (with maximum height of order 30% of the CBL depth) does not significantly change the orientation and spacing, nor the time of initial occurrence of modelled rolls, but local flow anomalies can be related to the hills. These anomalies are related to coherent patterns in the diagnosed cloud fields, with a tendency for more cloud cover upstream and over hills, and cloud clearing in the lee as a result of descent suppressing convective eddies. This kind of control of the shallow convection by the topography is evident in the satellite imagery.
Temperature-driven groundwater convection in cold climates
NASA Astrophysics Data System (ADS)
Engström, Maria; Nordell, Bo
2016-05-01
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.
Multidimensional hydrodynamic convection in full amplitude RR Lyrae models
NASA Astrophysics Data System (ADS)
Deupree, R.; Geroux, C.
2016-05-01
Multidimensional (both 2D and 3D) hydrodynamic calculations have been performed to compute full amplitude RR Lyrae models. The multi- dimensional nature allows convection to be treated in a more realistic way than simple 1D formulations such as the local mixing length theory. We focus on some aspects of multidimensional calculations and on the model for treating convection.
Experimental investigation on thermo-magnetic convection inside cavities.
Gontijo, R G; Cunha, F R
2012-12-01
This paper presents experimental results on thermo-magnetic convection inside cavities. We examine the flow induced by convective currents inside a cavity with aspect ratio near the unity and the heat transfer rates measurements inside a thin cavity with aspect ratio equal to twelve. The convective unstable currents are formed when a magnetic suspension is subjected to a temperature gradient combined with a gradient of an externally imposed magnetic field. Under these conditions, stratifications in the suspension density and susceptibility are both important effects to the convective motion. We show a comparison between flow patterns of magnetic and gravitational convections. The impact of the presence of a magnetic field on the amount of heat extracted from the system when magnetic and gravitational effects are combined inside the test cell is evaluated. The convection state is largely affected by new instability modes produced by stratification in susceptibility. The experiments reveal that magnetic field enhances the instability in the convective flow leading to a more effective mixing and consequently to a more statistically homogenous temperature distribution inside the test cell. The experimental results allow the validation of the scaling law proposed in a previous theoretical work that has predicted that the Nusselt number scales with the magnetic Rayleigh number to the power of 1/3, in the limit in which magnetic force balances viscous force in the convective flow. PMID:23447978
Flux Emergence in a Magnetized Convection Zone
NASA Astrophysics Data System (ADS)
Pinto, R. F.; Brun, A. S.
2013-07-01
We study the influence of a dynamo magnetic field on the buoyant rise and emergence of twisted magnetic flux ropes and their influence on the global external magnetic field. We ran three-dimensional MHD numerical simulations using the ASH code (anelastic spherical harmonics) and analyzed the dynamical evolution of such buoyant flux ropes from the bottom of the convection zone until the post-emergence phases. The global nature of this model can only very crudely and inaccurately represent the local dynamics of the buoyant rise of the implanted magnetic structure, but nonetheless allows us to study the influence of global effects, such as self-consistently generated differential rotation and meridional circulation, and of Coriolis forces. Although motivated by the solar context, this model cannot be thought of as a realistic model of the rise of magnetic structures and their emergence in the Sun, where the local dynamics are completely different. The properties of initial phases of the buoyant rise are determined essentially by the flux-rope's properties and the convective flows and consequently are in good agreement with previous studies. However, the effects of the interaction of the background dynamo field become increasingly strong as the flux ropes evolve. During the buoyant rise across the convection zone, the flux-rope's magnetic field strength scales as Bvpropρα, with α <~ 1. An increase of radial velocity, density, and current density is observed to precede flux emergence at all longitudes. The geometry, latitude, and relative orientation of the flux ropes with respect to the background magnetic field influences the resulting rise speeds, zonal flow amplitudes (which develop within the flux ropes), and the corresponding surface signatures. This influences the morphology, duration and amplitude of the surface shearing, and the Poynting flux associated with magnetic flux-rope emergence. The emerged magnetic flux influences the system's global polarity
FLUX EMERGENCE IN A MAGNETIZED CONVECTION ZONE
Pinto, R. F.; Brun, A. S.
2013-07-20
We study the influence of a dynamo magnetic field on the buoyant rise and emergence of twisted magnetic flux ropes and their influence on the global external magnetic field. We ran three-dimensional MHD numerical simulations using the ASH code (anelastic spherical harmonics) and analyzed the dynamical evolution of such buoyant flux ropes from the bottom of the convection zone until the post-emergence phases. The global nature of this model can only very crudely and inaccurately represent the local dynamics of the buoyant rise of the implanted magnetic structure, but nonetheless allows us to study the influence of global effects, such as self-consistently generated differential rotation and meridional circulation, and of Coriolis forces. Although motivated by the solar context, this model cannot be thought of as a realistic model of the rise of magnetic structures and their emergence in the Sun, where the local dynamics are completely different. The properties of initial phases of the buoyant rise are determined essentially by the flux-rope's properties and the convective flows and consequently are in good agreement with previous studies. However, the effects of the interaction of the background dynamo field become increasingly strong as the flux ropes evolve. During the buoyant rise across the convection zone, the flux-rope's magnetic field strength scales as B{proportional_to}{rho}{sup {alpha}}, with {alpha} {approx}< 1. An increase of radial velocity, density, and current density is observed to precede flux emergence at all longitudes. The geometry, latitude, and relative orientation of the flux ropes with respect to the background magnetic field influences the resulting rise speeds, zonal flow amplitudes (which develop within the flux ropes), and the corresponding surface signatures. This influences the morphology, duration and amplitude of the surface shearing, and the Poynting flux associated with magnetic flux-rope emergence. The emerged magnetic flux
Deep convective clouds at the tropopause
NASA Astrophysics Data System (ADS)
Aumann, H. H.; Desouza-Machado, S. G.
2010-07-01
Data from the Advanced Infrared Sounder (AIRS) on the EOS Aqua spacecraft identify thousands of cloud tops colder than 225 K, loosely referred to as Deep Convective Clouds (DCC). Many of these cloud tops have "inverted" spectra, i.e. areas of strong water vapor, CO2 and ozone opacity, normally seen in absorption, are now seen in emission. We refer to these inverted spectra as DCCi. They are found in about 0.4% of all spectra from the tropical oceans excluding the Western Tropical Pacific (WTP), 1.1% in the WTP. The cold clouds are the anvils capping thunderstorms and consist of optically thick cirrus ice clouds. The precipitation rate associated with DCCi suggests that imbedded in these clouds, protruding above them, and not spatially resolved by the AIRS 15 km FOV, are even colder bubbles, where strong convection pushes clouds to within 5 hPa of the pressure level of the tropopause cold point. Associated with DCCi is a local upward displacement of the tropopause, a cold "bulge", which can be seen directly in the brightness temperatures of AIRS and AMSU channels with weighting function peaking between 40 and 2 hPa, without the need for a formal temperature retrieval. The bulge is not resolved by the analysis in numerical weather prediction models. The locally cold cloud tops relative to the analysis give the appearance (in the sense of an "illusion") of clouds overshooting the tropopause and penetrating into the stratosphere. Based on a simple model of optically thick cirrus clouds, the spectral inversions seen in the AIRS data do not require these clouds to penetrate into the stratosphere. However, the contents of the cold bulge may be left in the lower stratosphere as soon as the strong convection subsides. The heavy precipitation and the distortion of the temperature structure near the tropopause indicate that DCCi are associated with intense storms. Significant long-term trends in the statistical properties of DCCi could be interesting indicators of climate
Convective dominated flows in open capillary channels
NASA Astrophysics Data System (ADS)
Rosendahl, Uwe; Grah, Aleksander; Dreyer, Michael E.
2010-05-01
This paper is concerned with convective dominated liquid flows in open capillary channels. The channels consist of two parallel plates bounded by free liquid surfaces along the open sides. In the case of steady flow the capillary pressure of the free surface balances the differential pressure between the liquid and the surrounding constant pressure gas phase. A maximum flow rate is achieved when the adjusted volumetric flow rate exceeds a certain limit leading to a collapse of the free surfaces. The convective dominated flow regime is a special case of open capillary flow, since the viscous forces are negligibly small compared with the convective forces. Flows of this type are of peculiar interest since the free surfaces possess a quasisymmetry in the flow direction. This quasisymmetry enables the application of a new effective method for evaluation of the flow limit. The flow limit is caused by a choking effect. This effect is indicated by the speed index, S, which is defined by the ratio of the flow velocity and the longitudinal capillary wave speed. The speed index is defined analogously to Mach number and tends toward unity in the case of flow limitation, i.e., when the maximum flow rate is reached. Utilizing the quasisymmetry, a new approach for a very precise determination of the speed index is presented. This approach uses a new approximation for the curvature of the surfaces by means of the empirical surface profiles. On the basis of empirical and theoretical data, the paper discusses the typical features of the stable flow. The experiments were performed under microgravity aboard the sounding rockets TEXUS 41 and TEXUS 42. The experiment setup enables the approach to the flow limit through either increase in flow rate or channel length. The theoretical data have been gained from numerical solutions of a one-dimensional flow model. The empirical and theoretical results are in good agreement and both confirm the choking effect as cause of the flow limitation
Feedbacks of Rock Hydration on Hydrothermal Convection
NASA Astrophysics Data System (ADS)
Iyer, K.; Ruepke, L.
2009-04-01
Hydration of the oceanic lithosphere is an important process which alters both the chemical and physical properties of the affected lithologies. Although hydrothermal convection has been extensively researched, little work has been done on the effects of hydration reactions occurring during convection. One of the most important reactions occurring in the oceanic lithosphere is serpentinization of ultramafic rocks. We present a numerical solution for hydrothermal circulation which explores the feedbacks generated during serpentinization of mantle rocks. The model is two dimensional and uses the FEM approach. Three coupled, time-dependent equations are solved: the first equation is mass conserving and is based on Darcy flow. The second equation describes heat transport and accounts for advective and diffusive heat transfer as well as latent heat effects. The final equation describes the serpentinization rate of olivine in ultramafic rocks (Emmanuel and Berkowitz, 2006) and is derived from experimental results (Martin and Fyfe, 1970). Serpentinization is a fluid-consuming process and manifests itself as a sink term in the Darcy flow equation. The exothermic heat of reaction is added as a source term in the heat transport equation. Moreover, serpentinization is associated with a large positive volume change. This large volume change may decrease the porosity of the rock but can also increase permeability by deformation. The rate of serpentinization used in the model is, therefore, also coupled to the porosity and permeability. We investigate the role of hydration in a box model using thermodynamically constrained fluid properties where the lower part is composed of reactive mantle rocks. The effects of serpentinization on the temperatures of the venting fluids and variations in flow pathways are explored. Furthermore, the model is also used in a mid-ocean ridge setting and the amount and depth of serpentinization, in addition to the above mentioned effects, is also
Characterizing convection in geophysical dynamo systems
NASA Astrophysics Data System (ADS)
Cheng, Jonathan Shuo
The Earth's magnetic field is produced by a fluid dynamo in the molten iron outer core. This geodynamo is driven by fluid motions induced by thermal and chemical convection and strongly influenced by rotational and magnetic field effects. While frequent observations are made of the morphology and time-dependent field behavior, flow dynamics in the core are all but inaccessible to direct measurement. Thus, forward models are essential for exploring the relationship between the geomagnetic field and its underlying fluid physics. The goal of my PhD is to further our understanding of the fluid physics driving the geodynamo. In order to do this, I have performed a suite of nonrotating and rotating convection laboratory experiments and developed a new experimental device that reaches more extreme values of the governing parameters than previously possible. In addition, I conduct a theoretical analysis of well-established results from a suite of dynamo simulations by Christensen and Aubert (2006). These studies are conducted at moderate values of the Ekman number (ratio between viscosity and Coriolis forces, ˜ 10-4), as opposed to the the extremely small Ekman numbers in planetary cores (˜ 10 -15). At such moderate Ekman values, flows tend to take the form of large-scale, quasi-laminar axial columns. These columnar structures give the induced magnetic field a dipolar morphology, similar to what is seen on planets. However, I find that some results derived from these simulations are fully dependent on the fluid viscosity, and therefore are unlikely to reflect the fluid physics driving dynamo action in the core. My findings reinforce the need to understand the turbulent processes that arise as the governing parameters approach planetary values. Indeed, my rotating convection experiments show that, as the Ekman number is decreased beyond ranges currently accessible to dynamo simulations, the regime characterized by laminar columns is found to dwindle. We instead find a
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.
Preventing Blow up by Convective Terms in Dissipative PDE's
NASA Astrophysics Data System (ADS)
Bilgin, Bilgesu; Kalantarov, Varga; Zelik, Sergey
2016-06-01
We study the impact of the convective terms on the global solvability or finite time blow up of solutions of dissipative PDEs. We consider the model examples of 1D Burger's type equations, convective Cahn-Hilliard equation, generalized Kuramoto-Sivashinsky equation and KdV type equations. The following common scenario is established: adding sufficiently strong (in comparison with the destabilizing nonlinearity) convective terms to equation prevents the solutions from blowing up in a finite time and makes the considered system globally well-posed and dissipative and for weak enough convective terms the finite time blow up may occur similar to the case, when the equation does not involve convective term. This kind of result has been previously known for the case of Burger's type equations and has been strongly based on maximum principle. In contrast to this, our results are based on the weighted energy estimates which do not require the maximum principle for the considered problem.
Convection and segregation in a flat rotating sandbox
NASA Astrophysics Data System (ADS)
Rietz, Frank; Stannarius, Ralf
2012-01-01
A flat box, almost completely filled with a mixture of granulate, is rotated slowly about its horizontal central axis. In this experiment, a regular vortex flow of the granular material is observed in the cell plane. These vortex structures have a superficial analogy to convection rolls in dissipative structures of ordinary liquids. Whereas in the latter, the origin of the convection can often be attributed to gradients e.g. of densities or surface tensions, there is no trivial explanation at present for the convection of the granulate in the rotating container. Despite the simplicity of the experiment, the underlying mechanisms for convection and segregation are difficult to extract. Here, we present a comprehensive experimental study of the patterns under various experimental conditions and propose a mechanism for the convection.
Modeling the interaction between convection and nonthermal ion outflows
NASA Astrophysics Data System (ADS)
Varney, R. H.; Wiltberger, M.; Lotko, W.
2015-03-01
Initial demonstrations of an ionosphere/polar wind model including a phenomenological treatment of transverse heating by wave particle interactions (WPIs) are presented. Tests with fixed WPI parameters in a designated heating region on the dayside with time-varying convection show that the parameters of the resulting nonthermal ion outflow are strongly coupled to the convection. The hemispheric outflow rate is positively correlated with the convection speed with a time delay related to the travel time to the upper boundary. Increases in convection increase the thermal plasma access to the heating region, both by increasing the upflow associated with frictional heating and by increasing the horizontal transport. The average parallel velocities and energies of the escaping nonthermal ions are anticorrelated with the convection speed due to the finite dwell time in the heating region. The computationally efficient model can be readily coupled into global geospace modeling frameworks in the future.
A groundwater convection model for Rio Grande rift geothermal resources
NASA Technical Reports Server (NTRS)
Morgan, P.; Harder, V.; Daggett, P. H.; Swanberg, C. A.
1981-01-01
It has been proposed that forced convection, driven by normal groundwater flow through the interconnected basins of the Rio Grande rift is the primary source mechanism for the numerous geothermal anomalies along the rift. A test of this concept using an analytical model indicates that significant forced convection must occur in the basins even if permeabilities are as low as 50-200 millidarcies at a depth of 2 km. Where groundwater flow is constricted at the discharge areas of the basins forced convection can locally increase the gradient to a level where free convection also occurs, generating surface heat flow anomalies 5-15 times background. A compilation of groundwater data for the rift basins shows a strong correlation between constrictions in groundwater flow and hot springs and geothermal anomalies, giving strong circumstantial support to the convection model.
Subsolidus convection in the mantles of terrestrial planets
NASA Technical Reports Server (NTRS)
Schubert, G.
1979-01-01
The role of heat transport by solid state mantle convection in determining the past and present thermal states of terrestrial planets is examined. Mantle convection models have relied on two-dimensional and axisymmetric three-dimensional numerical calculations incorporating the temperature and pressure dependence of mantle rheology and its non-Newtonian nature. Convection at high Rayleigh numbers has been investigated through theoretical scaling arguments and boundary layer theories; nevertheless, computational limits prevent modeling of the fully three-dimensional, time-dependent, very high Rayleigh number convection which probably prevails in terrestrial planets. Radar measurements of Venus, as well as Voyager exploration of the Galilean satellites, should also provide information on mantle convection.
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.
Influence of In-Well Convection on Well Sampling
Vroblesky, Don A.; Casey, Clifton C.; Lowery, Mark A.
2006-01-01
Convective transport of dissolved oxygen (DO) from shallow to deeper parts of wells was observed as the shallow water in wells in South Carolina became cooler than the deeper water in the wells due to seasonal changes. Wells having a relatively small depth to water were more susceptible to thermally induced convection than wells where the depth to water was greater because the shallower water levels were more influenced by air temperature. The potential for convective transport of DO to maintain oxygenated conditions in a well was diminished as ground-water exchange through the well screen increased and as oxygen demand increased. Convective flow did not transport oxygen to the screened interval when the screened interval was deeper than the range of the convective cell. The convective movement of water in wells has potential implications for passive, or no-purge, and low-flow sampling approaches. Transport of DO to the screened interval can adversely affect the ability of passive samplers to produce accurate concentrations of oxygen-sensitive solutes, such as iron. Other potential consequences include mixing the screened-interval water with casing water and potentially allowing volatilization loss at the water surface. A field test of diffusion samplers in a convecting well during the winter, however, showed good agreement of chlorinated solvent concentrations with pumped samples, indicating that there was no negative impact of the convection on the utility of the samplers to collect volatile organic compound concentrations in that well. In the cases of low-flow sampling, convective circulation can cause the pumped sample to be a mixture of casing water and aquifer water. This can substantially increase the equilibration time of oxygen as an indicator parameter and can give false indications of the redox state. Data from this investigation show that simple in-well devices can effectively mitigate convective transport of oxygen. The devices can range from
Convection in Icy Satellites: Implications for Habitability and Planetary Protection
NASA Technical Reports Server (NTRS)
Barr, A. C.; Pappalardo, R. T.
2004-01-01
Solid-state convection and endogenic resurfacing in the outer ice shells of the icy Galilean satellites (especially Europa) may contribute to the habitability of their internal oceans and to the detectability of any biospheres by spacecraft. If convection occurs in an ice I layer, fluid motions are confined beneath a thick stagnant lid of cold, immobile ice that is too stiff to participate in convection. The thickness of the stagnant lid varies from 30 to 50% of the total thickness of the ice shell, depending on the grain size of ice. Upward convective motions deliver approximately 10(exp 9) to 10(exp 13) kg yr(sup -1) of ice to the base of the stagnant lid, where resurfacing events driven by compositional or tidal effects (such as the formation of domes or ridges on Europa, or formation of grooved terrain on Ganymede) may deliver materials from the stagnant lid onto the surface. Conversely, downward convective motions deliver the same mass of ice from the base of the stagnant lid to the bottom of the satellites ice shells. Materials from the satellites surfaces may be delivered to their oceans by downward convective motions if material from the surface can reach the base of the stagnant lid during resurfacing events. Triggering convection from an initially conductive ice shell requires modest amplitude (a few to tens of kelvins) temperature anomalies to soften the ice to permit convection, which may require tidal heating. Therefore, tidal heating, compositional buoyancy, and solid-state convection in combination may be required to permit mass transport between the surfaces and oceans of icy satellites. Callisto and probably Ganymede have thick stagnant lids with geologically inactive surfaces today, so forward contamination of their surfaces is not a significant issue. Active convection and breaching of the stagnant lid is a possibility on Europa today, so is of relevance to planetary protection policy.
What Does the Mantle Remember of Its Convection History?
NASA Astrophysics Data System (ADS)
Atkins, S.; Tackley, P. J.; Valentine, A. P.; Trampert, J.
2014-12-01
Mantle convection is a non-linear system. Convection patterns are therefore not directly proportional to initialization parameters. We investigate which parameters affect convection evolution, and the timescales over which these effects are observable. To do this, we run thousands of mantle convection simulations, each model having different, randomly selected, thermal, rheological, and compositional starting parameters. Our convection program is StagYY, run in 2D spherical-annulus geometry with self-consistent mineral physics properties calculated in Perple_X. StagYY allows melting and basaltic crust production and includes decaying radiogenic heat production in both core and mantle. At various timesteps after initiation, the temperature, density, and composition fields can be extracted. We train neural networks to find the most likely initial parameters responsible for any convection pattern. The networks produce probability density functions, so that the relative likelihood of any value of the inputs parameters can be calculated, allowing inversion of the non-linear convection systems. This means that the uncertainty of the result is also provided. After two billion years, we can find initial mantle potential temperature, initial CMB temperature, approximated initial mantle composition, reference viscosity, depth dependent viscosity, yield stress and the existence and thickness of any initial layer of primordial material. Our results suggest that the signal from these initialisation parameters is retained in the pattern of convection for much longer than 2 Gy. Our ultimate goal is to use tomographic models of the real mantle to investigate the 'initialisation' parameters of the Earth. We can also apply the same methodology to back-step convection, allowing us to build up a history of Earth's mantle convection.
Balanced dynamics and convection in the tropical troposphere
NASA Astrophysics Data System (ADS)
Raymond, David; Fuchs, Željka; Gjorgjievska, Saška; Sessions, Sharon
2015-09-01
This paper presents a conceptual picture of balanced tropical tropospheric dynamics inspired by recent observations. The most important factor differentiating the tropics from middle and higher latitudes is the absence of baroclinic instability; upward motion occurs primarily via deep convective processes. Thus, convection forms an integral part of large-scale tropical motions. Since convection itself is small-scale and chaotic in detail, predictability lies in uncovering the hidden hands that guide the average behavior of convection. Two appear, balanced dynamics and thermodynamic constraints. Contrary to conventional expectations, balanced dynamics plays a crucial role in the tropical atmosphere. However, due to the smallness of the Coriolis parameter there, nonlinear balance is more important in the tropics than at higher latitudes. Three thermodynamic constraints appear to play an important role in governing the average behavior of convection outside of the cores of tropical storms. First, convection is subject to control via a lower tropospheric buoyancy quasi-equilibrium process, wherein destabilization of the lower troposphere by nonconvective processes is balanced by convective stabilization. Second, the production of precipitation is extraordinarily sensitive to the saturation fraction of the troposphere. Third, "moisture quasi-equilibrium" governs the saturation fraction, with moister atmospheres being associated with smaller moist convective instability. The moist convective instability is governed by the balanced thermodynamic response to the pattern of potential vorticity, which in turn is slowly modified by convective and radiative heating. The intricate dance between these dynamic and thermodynamic processes leads to complex behavior of the tropical atmosphere in ways that we are just beginning to understand.
NASA Astrophysics Data System (ADS)
Barkiđija, Sanda; Fuchs, Željka
2013-04-01
Saturation fraction (SF), convective inhibition (CIN) and convective available potential energy (CAPE) are discussed to see with which of these parameters' precipitation rate is better correlated in the middle latitudes. The study is based on measurements from 20 European stations for the period of 1972-2009. We also use the results of the Global Forecasting System (GFS) model to see how mentioned parameters behave in numerical models. Our research results indicate that CAPE is not a good measure of precipitation rate for all latitudes, although, in model results, CAPE and precipitation rate are found to be better correlated for middle latitudes then in higher latitudes and tropical regions. The best correlation with precipitation rate in middle latitudes is one with SF. Our results suggest that moisture is underestimated in numerical models for middle latitudes and encourage further work in including SF or similar parameter into precipitation parameterization in addition to the current one.
Hayat, Tasawar; Nawaz, Sadaf; Alsaedi, Ahmed; Rafiq, Maimona
2016-01-01
Main objective of present study is to analyze the mixed convective peristaltic transport of water based nanofluids using five different nanoparticles i.e. (Al2O3, CuO, Cu, Ag and TiO2). Two thermal conductivity models namely the Maxwell's and Hamilton-Crosser's are used in this study. Hall and Joule heating effects are also given consideration. Convection boundary conditions are employed. Furthermore, viscous dissipation and heat generation/absorption are used to model the energy equation. Problem is simplified by employing lubrication approach. System of equations are solved numerically. Influence of pertinent parameters on the velocity and temperature are discussed. Also the heat transfer rate at the wall is observed for considered five nanofluids using the two phase models via graphs. PMID:27104596
Hayat, Tasawar; Nawaz, Sadaf; Alsaedi, Ahmed; Rafiq, Maimona
2016-01-01
Main objective of present study is to analyze the mixed convective peristaltic transport of water based nanofluids using five different nanoparticles i.e. (Al2O3, CuO, Cu, Ag and TiO2). Two thermal conductivity models namely the Maxwell's and Hamilton-Crosser's are used in this study. Hall and Joule heating effects are also given consideration. Convection boundary conditions are employed. Furthermore, viscous dissipation and heat generation/absorption are used to model the energy equation. Problem is simplified by employing lubrication approach. System of equations are solved numerically. Influence of pertinent parameters on the velocity and temperature are discussed. Also the heat transfer rate at the wall is observed for considered five nanofluids using the two phase models via graphs. PMID:27104596
NASA Astrophysics Data System (ADS)
Li, Tingting; Li, Xiaofan
2016-06-01
The budget of perturbation kinetic energy is calculated to study barotropic and baroclinic processes associated with convective development in the tropical deep convective regime. The barotropic kinetic-energy conversion is further examined through the derivation and analysis of budget of vertical transport of zonal momentum. The two-dimensional cloud-resolving model simulation data during TOGA COARE are analyzed in this budget study. The analysis of the energy budget shows that while baroclinic conversion from perturbation available potential energy is a major source for the growth of perturbation kinetic energy, barotropic conversion from the mean kinetic energy has significant modification on the variation of perturbation kinetic energy. The general evolution of barotropic conversion is associated with that of the vertical wind shear of the mean circulations, but the significant change in barotropic conversion corresponds to the change in the vertical transport of zonal momentum. Zonal transport of cloud hydrometeor is responsible for the variation of vertical transport of zonal momentum.
Convection, stability, and low dimensional dynamics
Doering, C.R.
1997-05-01
Recent developments concerning the connection between notions of hydrodynamic stability{emdash}usually associated with stationary laminar flows{emdash}and dynamics, most notably turbulent fluid flows, are reviewed. Based on a technical device originally introduced by Hopf in 1941, a rigorous mathematical relationship between criteria for nonlinear energy stability and bounds on global transport by steady, unsteady, or even turbulent flows, has been established. The optimal {open_quotes}marginal stability{close_quotes} criteria for the best bound leads to a novel variational problem, and the differential operator associated with the stability condition generates an adapted basis in which turbulent flow fields may naturally be decomposed. The application and implications of Galerkin truncations in these bases to produce low dimensional dynamical systems models is discussed in the context of thermal convection in a saturated porous layer. {copyright} {ital 1997 American Institute of Physics.}
Translation and convection of Earth's inner core
NASA Astrophysics Data System (ADS)
Monnereau, M.; Calvet, M.; Margerin, L.; Mizzon, H.; Souriau, A.
2012-12-01
The image of the inner core growing slowly at the center of the Earth by gradual cooling and solidification of the surrounding liquid outer core is being replaced by the more vigorous image of a ``deep foundry'', where melting and crystallization rates exceed by many times the net growth rate. Recently, a particular mode of convection, called translation, has been put forward as an important mode of inner core dynamics because this mechanism is able to explain the observed East-West asymmetry of P-wave velocity and attenuation (Monnereau et al. 2010). Translation is a pure solid displacement of the inner core material (solid iron) within its envelop, implying crystallization of entering iron on one side of the inner core and melting on the opposite side. Translation is consistent with multiple scattering models of wave propagation. If they do not experience deformation, iron crystals grow as they transit from one hemisphere to the other. Larger crystals constituting a faster and more attenuating medium, a translation velocity of some cm/yr (about ten times the growth rate) is enough to account for the superficial asymmetry observed for P-wave velocity and attenuation, with grains of a few hundred meters on the crystallizing side (West) growing up to a few kilometers before melting on the East side, and a drift direction located in the equatorial plane. Among all hypotheses that have been proposed to account for the seismic asymmetry, translation is the only one based on a demonstrated link between the seismic data and the proposed dynamics, notably through a model of seismic wave propagation. This mechanism was also proposed to be responsible for the formation of a dense layer at the bottom of the outer core, since the high rate of melting and crystallization would release a liquid depleted in light elements at the surface of the inner core (Alboussiere et al 2010). This would explain the anomalously low gradient of P wave velocity in the lowermost 200 km of the
Vortices in simulations of solar surface convection
NASA Astrophysics Data System (ADS)
Moll, R.; Cameron, R. H.; Schüssler, M.
2011-09-01
We report on the occurrence of small-scale vortices in simulations of the convective solar surface. Using an eigenanalysis of the velocity gradient tensor, we find the subset of high-vorticity regions in which the plasma is swirling. The swirling regions form an unsteady, tangled network of filaments in the turbulent downflow lanes. Near-surface vertical vortices are underdense and cause a local depression of the optical surface. They are potentially observable as bright points in the dark intergranular lanes. Vortex features typically exist for a few minutes, during which they are moved and twisted by the motion of the ambient plasma. The bigger vortices found in the simulations are possibly, but not necessarily, related to observations of granular-scale spiraling pathlines in "cork animations" or feature tracking. Three movies are available in electronic form at http://www.aanda.org
Radiative convection with a fixed heat flux
NASA Astrophysics Data System (ADS)
Aumaı̂tre, S.
2001-10-01
We have determined the marginal stability curve of convective instability in the usual Rayleigh-Bénard configuration with radiative transfer and a fixed total heat flux at the boundaries instead of a fixed temperature. In the Milne-Eddington approximation, radiative transfer introduces a new length scale and breaks the invariance of the Boussinesq equations under an arbitrary temperature shift, which occurs when the heat flux is fixed at the boundaries. The convergence to the limits where the non-radiative cases are expected is studied in this approximation. Then, using a second-order perturbative calculation, we show that the presence of radiation can change qualitatively the instability pattern: there is a range of optical parameters where the Cahn-Hillard equation is not anymore the one appropriate to describe the instability near the threshold.
Magnetogenesis through convection in barotropic fluids
Miller, E. Rogers, B.
2015-04-15
It is shown that an unmagnetized plasma with non-uniform bulk velocity can generate magnetic fields through consideration of the non-relativistic isentropic two-fluid equations, even when the initial conditions contain with no fields or currents, uniform densities and pressures, and a divergence-free bulk velocity. This effect does not depend on the baroclinicity of the plasma and is therefore relevant even in barotropic flows, where the Biermann battery is absent. It also does not rely on kinetic effects or shear discontinuities. Instead, our magnetogenesis effect arises from convection terms proportional to the electron mass in the generalized Ohm's law. The resulting magnetic fields are typically weak but may still serve as seed fields for dynamo mechanisms.
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.
Vertically Velocity Characteristics of Oceanic Convection.
NASA Astrophysics Data System (ADS)
Jorgensen, David P.; Lemone, Margaret A.
1989-03-01
Oceanic cumulonimbus updraft and downdraft events observed in the Western Pacific during the TAMEX program by NOAA P-3 research aircraft are analyzed and discussed. The basic dataset consists of flight-level data from 10 missions in the Taiwan region during May and June 1987. The 1 Hz time series of vertical velocity is used to define convective updrafts using the criteria that the velocity must be continuously positive for at least 0.5 km and exceed 0.5 m s1 for 1 s. A subset of the strongest drafts, termed cores, are defined as events that exceed 1 m s1 for 0.5 km. Downdrafts and downdraft cores are defined analogously. The statistics are from a total of 12 841 km of flight legs and consist of 359 updrafts and 466 downdrafts at altitudes from 150 m to 6.8 km MSL. The populations of average vertical velocity, maximum vertical velocity, diameter, and mass transport for both drafts and cores are approximately log-normally distributed, consistent with the results of previous studies of convective characteristics in other locations. TAMEX drafts and cores are comparable in size and strength with those measured in GATE and hurricanes but much weaker than those measured in continental thunderstorms.The median core updraft was less than 3 m s1, implying a time scale for ascent from cloud base to the freezing level of about 35 min. The microphysical implications of the low updraft rates are illustrated by comparing vertical profiles of radar reflectivity for TAMEX with those in other regions. The data are consistent with the hypothesis that the oceanic convection that was studied in GATE, hurricanes, and TAMEX is dominated by warm rain coalescence processes and that a large fractional rainout occurs below the freezing level. The rapid reduction of cloud water and radar reflectivity above the freezing level, as well as observations of abundant ice particles in all but the strongest updraft cores at temperatures just below 0°C, implies a rapid conversion of cloud water and
Role of viscoelasticity in mantle convection models
NASA Astrophysics Data System (ADS)
Patocka, Vojtech; Cadek, Ondrej; Tackley, Paul
2015-04-01
A present limitation of global thermo-chemical convection models is that they assume a purely viscous or visco-plastic flow law for solid rock, i.e. elasticity is ignored. This may not be a good assumption in the cold, outer boundary layer known as the lithosphere, where elastic deformation may be important. Elasticity in the lithosphere plays at least two roles: It changes surface topography, which changes the relationship between topography and gravity, and it alters the stress distribution in the lithosphere, which may affect dynamical behaviour such as the formation of plate boundaries and other tectonics features. A method for adding elasticity to a viscous flow solver to make a visco-elastic flow solver, which involves adding advected elastic stress to the momentum equation and introducing an "effective" viscosity has been proposed (e.g. Moresi, 2002). The proposed method is designed primarily for a regional-scale numerical model which employs tracers for advection and co-rotation of the stress field. In this study we test a grid-based version of the method in context of thermal convection in the Boussinesq approximation. A simple finite difference/volume model with staggered grid is used, with the aim to later use the same method to implement viscoelasticity into StagYY (Tackley, 2008). The main obstacle is that Maxwell viscoelastic rheology produces instantaneous deformation if instantaneous change of the driving forces occurs. It is not possible to model such deformation in a velocity formulated convection model, as velocity undergoes a singularity for an instantaneous deformation. For a given Rayleigh number there exists a certain critical value of the Deborah number above which it is necessary to use a thermal time step different from the one used in viscoelastic constitutive equation to avoid this numerical instability from happening. Critical Deborah numbers for various Rayleigh numbers are computed. We then propose a method to decouple the thermal and
Convection zone origins of solar atmospheric heating
NASA Technical Reports Server (NTRS)
Schatten, Kenneth H.; Mayr, Hans G.
1986-01-01
Spicules are examined as a means for supplying the corona with mass, energy, and magnetic field. It is suggested that spicules form from the supersonic upward expansion of material on nearly evacuated network flux tubes embedded within the sun's convection zone. This allows supersonic but subescape velocities to be attained by the material as it flows outward through the photosphere. Although supersonic, the kinetic energy (subescape) of the spicule material, as observed, is insufficient for coronal heating. It is suggested that, through buoyancy changes on evacuated flux tubes, the magnetic field first 'wicks' material flow into the solar atmosphere. Subsequently, the magnetic field energizes the gaseous material to form the conventional hot, dynamically expanding, solar corona. This occurs through momentum and energy transport by Alfven waves and associated Maxwell stresses concurrently flowing upward on these 'geysers' (spicules). The vertical momentum equation governing fluid flow is examined, and a particular equipartition solution is presented for the flow velocity along a simple field geometry.
Magnetogenesis through convection in barotropic fluids
NASA Astrophysics Data System (ADS)
Miller, E.; Rogers, B.
2015-04-01
It is shown that an unmagnetized plasma with non-uniform bulk velocity can generate magnetic fields through consideration of the non-relativistic isentropic two-fluid equations, even when the initial conditions contain with no fields or currents, uniform densities and pressures, and a divergence-free bulk velocity. This effect does not depend on the baroclinicity of the plasma and is therefore relevant even in barotropic flows, where the Biermann battery is absent. It also does not rely on kinetic effects or shear discontinuities. Instead, our magnetogenesis effect arises from convection terms proportional to the electron mass in the generalized Ohm's law. The resulting magnetic fields are typically weak but may still serve as seed fields for dynamo mechanisms.
Thermocapillary Convection in Bubbles and Drops
NASA Technical Reports Server (NTRS)
Balassubramaniam; Subramanian, R. Shankar
2003-01-01
When bubbles or drops are present in an immiscible liquid in reduced gravity and the temperature of the liquid is non-uniform, a thermocapillary stress is generated at the interface due to the variation of interfacial tension with temperature. The resulting flow propels the drop freely suspended in the liquid towards warmer regions, so as to minimize the interfacial energy. In this presentation, we will focus on the effect of convective transport of momentum and energy, that are characterized by the Reynolds number and the Marangoni number, respectively. The results of asymptotic analyses for the speed of the drop for low and large values of these parameters will be discussed. These predictions as well as those from numerical simulations will be compared with reduced gravity experimental results obtained from experiments performed aboard the space shuttle.
Convectively Driven Vortex Flows in the Sun
NASA Astrophysics Data System (ADS)
Bonet, J. A.; Márquez, I.; Sánchez Almeida, J.; Cabello, I.; Domingo, V.
2008-11-01
We have discovered small whirlpools in the Sun, with a size similar to terrestrial hurricanes (lesssim0.5 Mm). The theory of solar convection predicts them, but they had remained elusive so far. The vortex flows are created at the downdrafts where the plasma returns to the solar interior after cooling down, and we detect them because some magnetic bright points (BPs) follow a logarithmic spiral on their way to being engulfed by a downdraft. Our disk-center observations show 0.9 × 10-2 vortexes per Mm2, with a lifetime of the order of 5 minutes, and with no preferred sense of rotation. They are not evenly spread out over the surface, but they seem to trace the supergranulation and the mesogranulation. These observed properties are strongly biased by our type of measurement, unable to detect vortexes except when they are engulfing magnetic BPs.
Mathematical modeling of near-critical convection
Cox, B.L.; Pruess, K.; McKibbin, R.
1988-01-01
Fluid and heat flow at temperatures approaching or exceeding that at the critical point (374ºC for pure water, higher for saline fluids) may be encountered in deep zones of geothermal systems and above cooling intrusives. Laboratory experiments have demonstrated strong enhancements in heat transfer at near-critical conditions (Dunn and Hardee, 1981). We have developed special numerical techniques for modeling porous flow at near-critical conditions, which can handle the extreme non-linearities in water properties near the critical point. Our numerical experiments show strong enhancements of convective heat transfer at near-critical conditions; however, the heat transfer rates obtained in the numerical simulations are considerably smaller than those seen in the laboratory experiments by Dunn and Hardee. We discuss possible reasons for this discrepancy and develop suggestions for additional laboratory experiments.
Global Variability of Mesoscale Convective System
NASA Technical Reports Server (NTRS)
Yuan, J.; Houze, R. A., Jr.
2010-01-01
Mesoscale convective systems (MCSs) in the tropics produce extensive anvil clouds, which significantly affect the transfer of radiation. This study develops an objective method to identify MCSs and their anvils by combining data from three A-train satellite instruments: Moderate Resolution Imaging Spectroradiometer (MODIS) for cloud-top size and coldness, Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) for rain area size and intensity, and CloudSat for horizontal and vertical dimensions of anvils. The authors distinguish three types of MCSs: small and large separated MCSs and connected MCSs. The latter are MCSs sharing a contiguous rain area. Mapping of the objectively identified MCSs shows patterns of MCSs that are consistent with previous studies of tropical convection, with separated MCSs dominant over Africa and the Amazon regions and connected MCSs favored over the warm pool of the Indian and west Pacific Oceans. By separating the anvil from the raining regions of MCSs, this study leads to quantitative global maps of anvil coverage. These maps are consistent with the MCS analysis, and they lay the foundation for estimating the global radiative effects of anvil clouds. CloudSat radar data show that the modal thickness of MCS anvils is about 4--5 km. Anvils are mostly confined to within 1.5--2 times the equivalent radii of the primary rain areas of the MCSs. Over the warm pool, they may extend out to about 5 times the rain area radii. The warm ocean MCSs tend to have thicker non-raining and lightly raining anvils near the edges of their actively raining regions, indicating that anvils are generated in and spread out from the primary raining regions of the MCSs. Thicker anvils are nearly absent over continental regions.
Stochastic microhertz gravitational radiation from stellar convection
Bennett, M. F.; Melatos, A.
2014-09-01
High Reynolds-number turbulence driven by stellar convection in main-sequence stars generates stochastic gravitational radiation. We calculate the wave-strain power spectral density as a function of the zero-age main-sequence mass for an individual star and for an isotropic, universal stellar population described by the Salpeter initial mass function and redshift-dependent Hopkins-Beacom star formation rate. The spectrum is a broken power law, which peaks near the turnover frequency of the largest turbulent eddies. The signal from the Sun dominates the universal background. For the Sun, the far-zone power spectral density peaks at S(f {sub peak}) = 5.2 × 10{sup –52} Hz{sup –1} at frequency f {sub peak} = 2.3 × 10{sup –7} Hz. However, at low observing frequencies f < 3 × 10{sup –4} Hz, the Earth lies inside the Sun's near zone and the signal is amplified to S {sub near}(f {sub peak}) = 4.1 × 10{sup –27} Hz{sup –1} because the wave strain scales more steeply with distance (∝d {sup –5}) in the near zone than in the far zone (∝d {sup –1}). Hence the Solar signal may prove relevant for pulsar timing arrays. Other individual sources and the universal background fall well below the projected sensitivities of the Laser Interferometer Space Antenna and next-generation pulsar timing arrays. Stellar convection sets a fundamental noise floor for more sensitive stochastic gravitational-wave experiments in the more distant future.
Engineering photochemical smog through convection towers
Elliott, S.; Prueitt, M.L.; Bossert, J.E.; Mroz, E.J.; Krakowski, R.A.; Miller, R.L.; Jacobson, M.Z.; Turco, R.P. |
1995-02-01
Reverse convection towers have attracted attention as a medium for cleansing modern cities. Evaporation of an aqueous mist injected at the tower opening could generate electrical power by creating descent, and simultaneously scavenge unsightly and unhealthful particulates. The study offered here assesses the influence to tower water droplets on the photochemical component of Los Angeles type smog. The primary radical chain initiator OH is likely removed into aqueous phases well within the residence time of air in the tower, and then reacts away rapidly. Organics do not dissolve, but nighttime hydrolysis of N{sub 2}O{sub 5} depletes the nitrogen oxides. A lack of HOx would slow hydrocarbon oxidation and so also ozone production. Lowering of NOx would also alter ozone production rates, but the direction is uncertain. SO{sub 2} is available in sufficient quantities in some urban areas to react with stable oxidants, and if seawater were the source of the mist, the high pH would lead to fast sulfur oxidation kinetics. With an accommodation coefficient of 10{sup {minus}3}, however, ozone may not enter the aqueous phase efficiently. Even if ozone is destroyed or its production suppressed, photochemical recovery times are on the order of hours, so that tower processing must be centered on a narrow midday time window. The cost of building the number of structures necessary for this brief turnover could be prohibitive. The increase in humidity accompanying mist evaporation could be controlled with condensers, but might otherwise counteract visibility enhancements by recreating aqueous aerosols. Quantification of the divergent forcings convection towers must exert upon the cityscape would call for coupled three dimensional modeling of transport, microphysics, and photochemistry. 112 refs.
Thermal and chemical convection in planetary mantles
NASA Technical Reports Server (NTRS)
Dupeyrat, L.; Sotin, C.; Parmentier, E. M.
1995-01-01
Melting of the upper mantle and extraction of melt result in the formation of a less dense depleted mantle. This paper describes series of two-dimensional models that investigate the effects of chemical buoyancy induced by these density variations. A tracer particles method has been set up to follow as closely as possible the chemical state of the mantle and to model the chemical buoyant force at each grid point. Each series of models provides the evolution with time of magma production, crustal thickness, surface heat flux, and thermal and chemical state of the mantle. First, models that do not take into account the displacement of plates at the surface of Earth demonstrate that chemical buoyancy has an important effect on the geometry of convection. Then models include horizontal motion of plates 5000 km wide. Recycling of crust is taken into account. For a sufficiently high plate velocity which depends on the thermal Rayleigh number, the cell's size is strongly coupled with the plate's size. Plate motion forces chemically buoyant material to sink into the mantle. Then the positive chemical buoyancy yields upwelling as depleted mantle reaches the interface between the upper and the lower mantle. This process is very efficient in mixing the depleted and undepleted mantle at the scale of the grid spacing since these zones of upwelling disrupt the large convective flow. At low spreading rates, zones of upwelling develop quickly, melting occurs, and the model predicts intraplate volcanism by melting of subducted crust. At fast spreading rates, depleted mantle also favors the formation of these zones of upwelling, but they are not strong enough to yield partial melting. Their rapid displacement toward the ridge contributes to faster large-scale homogenization.
Microwave Brightness Temperatures of Tilted Convective Systems
NASA Technical Reports Server (NTRS)
Hong, Ye; Haferman, Jeffrey L.; Olson, William S.; Kummerow, Christian D.
1998-01-01
Aircraft and ground-based radar data from the Tropical Ocean and Global Atmosphere Coupled-Ocean Atmosphere Response Experiment (TOGA COARE) show that convective systems are not always vertical. Instead, many are tilted from vertical. Satellite passive microwave radiometers observe the atmosphere at a viewing angle. For example, the Special Sensor Microwave/Imager (SSM/I) on Defense Meteorological Satellite Program (DMSP) satellites and the Tropical Rainfall Measurement Mission (TRMM) Microwave Imager (TMI) on the TRMM satellite have an incident angle of about 50deg. Thus, the brightness temperature measured from one direction of tilt may be different than that viewed from the opposite direction due to the different optical depth. This paper presents the investigation of passive microwave brightness temperatures of tilted convective systems. To account for the effect of tilt, a 3-D backward Monte Carlo radiative transfer model has been applied to a simple tilted cloud model and a dynamically evolving cloud model to derive the brightness temperature. The radiative transfer results indicate that brightness temperature varies when the viewing angle changes because of the different optical depth. The tilt increases the displacements between high 19 GHz brightness temperature (Tb(sub 19)) due to liquid emission from lower level of cloud and the low 85 GHz brightness temperature (Tb(sub 85)) due to ice scattering from upper level of cloud. As the resolution degrades, the difference of brightness temperature due to the change of viewing angle decreases dramatically. The dislocation between Tb(sub 19) and Tb(sub 85), however, remains prominent.
Natural and forced convection during solidification
NASA Astrophysics Data System (ADS)
Neufeld, Jerome A.
The following work marries theoretical and experimental approaches to study the interaction of an external shear flow with a solidifying porous medium. The porous medium, a dendritic 'mushy layer', is created when a super-eutectic binary alloy is cooled leading to solid crystals bathed in an interstitial fluid which is compositionally enriched. This compositional enrichment leads to natural buoyant instabilities in the solidifying porous medium coupled with instabilities in the adjoining liquid layer. Theoretically, the effect of an external shear flow on the convective instabilities inherent to this mushy layer is investigated using a linear stability analysis. The external flow is coupled to advective perturbations in the liquid and to flow in the mush through a perturbed mush-liquid interface. A complete numerical solution of the stability of the system is performed and a critical porous medium Rayleigh number is found which is a function of both the external flow speed and the wavenumber of the interfacial perturbations. By neglecting the effects of buoyancy in the liquid and solving only for the pressure perturbations on the corrugated mush-liquid interface induced by the external flow, a reduced model is constructed and solved analytically. These theoretical results are compared with experimental observations obtained in a laboratory flume in which an ammonium-chloride solution is solidified from below at a constant rate. The experimental results reveal that at flow speeds above critical, convection is forced within the mush leading to a series of zero solid fraction tesselations aligned perpendicular to the applied shear flow. The results of the experiments compare favorably to the linear stability analysis.
NASA Astrophysics Data System (ADS)
Holloway, C. E.; Woolnough, S. J.
2016-03-01
Idealized explicit convection simulations of the Met Office Unified Model exhibit spontaneous self-aggregation in radiative-convective equilibrium, as seen in other models in previous studies. This self-aggregation is linked to feedbacks between radiation, surface fluxes, and convection, and the organization is intimately related to the evolution of the column water vapor field. Analysis of the budget of the spatial variance of column-integrated frozen moist static energy (MSE), following Wing and Emanuel (2014), reveals that the direct radiative feedback (including significant cloud longwave effects) is dominant in both the initial development of self-aggregation and the maintenance of an aggregated state. A low-level circulation at intermediate stages of aggregation does appear to transport MSE from drier to moister regions, but this circulation is mostly balanced by other advective effects of opposite sign and is forced by horizontal anomalies of convective heating (not radiation). Sensitivity studies with either fixed prescribed radiative cooling, fixed prescribed surface fluxes, or both do not show full self-aggregation from homogeneous initial conditions, though fixed surface fluxes do not disaggregate an initialized aggregated state. A sensitivity study in which rain evaporation is turned off shows more rapid self-aggregation, while a run with this change plus fixed radiative cooling still shows strong self-aggregation, supporting a "moisture-memory" effect found in Muller and Bony (2015). Interestingly, self-aggregation occurs even in simulations with sea surface temperatures (SSTs) of 295 and 290 K, with direct radiative feedbacks dominating the budget of MSE variance, in contrast to results in some previous studies.
Bhattacharya, Mrittika; Srivastav, Prem Prakash; Mishra, Hari Niwas
2015-04-01
Oyster mushroom samples were dried under selected convective, microwave-convective drying conditions in a recirculatory hot-air dryer and microwave assisted hot-air dryer (2.45 GHz, 1.5 kW) respectively. Only falling rate period and no constant rate period, was exhibited in both the drying technique. The experimental moisture loss data were fitted to selected semi-theoretical thin-layer drying equations. The mathematical models were compared according to three statistical parameters, i.e. correlation coefficient, reduced chi-square and residual mean sum of squares. Among all the models, Midilli et al. model was found to have the best fit as suggested by 0.99 of square correlation coefficient, 0.000043 of reduced-chi square and 0.0023 of residual sum of square. The highest effective moisture diffusivity varying from 10.16 × 10(-8) to 16.18 × 10(-8) m(2)/s over the temperature range was observed in microwave-convective drying at an air velocity of 1.5 m/s and the activation energy was calculated to be 16.95 kJ/mol. The above findings can aid to select the most suitable operating conditions, so as to design drying equipment accordingly. PMID:25829581
Evaluation of T-111 forced-convection loop tested with lithium at 1370 C. [free convection
NASA Technical Reports Server (NTRS)
Devan, J. H.; Long, E. L., Jr.
1975-01-01
A T-111 alloy (Ta-8% W-2% Hf) forced-convection loop containing molten lithium was operated 3000 hr at a maximum temperature of 1370 C. Flow velocities up to 6.3 m/sec were used, and the results of this forced-convection loop are very similar to those observed in lower velocity thermal-convection loops of T-111 containing lithium. Weight changes were determined at 93 positions around the loop. The maximum dissolution rate occurred at the maximum wall temperature of the loop and was less than 1.3 microns/year. Mass transfer of hafnium, nitrogen, and, to a lesser extent, carbon occurred from the hotter to cooler regions. Exposed surfaces in the highest temperature region were found to be depleted in hafnium to a depth of 60 microns with no detectable change in tungsten content. There was some loss in room-temperature tensile strength for specimens exposed to lithium at 1370 C, attributable to depletion of hafnium and nitrogen and to attendant grain growth.
Convective Effects During the Physical Vapor Transport Process. II - Thermosolutal Convection
NASA Technical Reports Server (NTRS)
Duval, Walter M. B.
1993-01-01
The effect of an inert gas on the diffusive-convective physical vapor transport process is investigated for the case when the temperature gradient is stabilizing, and the concentration gradient destabilizing, for a wide parametric range. When an inert gas is present, the thermal and solutal convection oppose each other. The solutal field is destabilizing while the thermal field and the advective-diffusive flux stabilize the flow field. When the pressure of the inert component is increased, the stabilizing effect of the advective-diffusive flux is decreased; thus, convection becomes more vigorous. The nonlinear dynamics of the flow field here show a transition from quasi-periodic to chaotic state. When both stabilizing mechanisms are present, the flow field shows a transition to a steady state. Toward steady state, growth and amalgamation of rolls occur, which result in an overturning motion. This leads to a superposed flow consisting of one roll and a unidirectional flow. However, when the pressure is increased, the advective-diffusive stability mechanism is decreased. and oscillations of the flow field occur. The low gravity environment is effective at eliminating oscillatory behavior of the flow field and results in uniform temperature and concentration gradients.
From convection rolls to finger convection in double-diffusive turbulence
NASA Astrophysics Data System (ADS)
Yang, Yantao; Verzicco, Roberto; Lohse, Detlef
2015-11-01
The double diffusive convection (DDC), where the fluid density depends on two scalar components with very different molecular diffusivities, is frequently encountered in oceanography, astrophysics, and electrochemistry. In this talk we report a systematic study of vertically bounded DDC for various control parameters. The flow is driven by an unstable salinity difference between two plates and stabilized by a temperature difference. As the relative strength of temperature difference becomes stronger, the flow transits from a state with large-scale convection rolls, which is similar to the Rayleigh-Bénard (RB) flow, to a state with well-organised salt fingers. When the temperature difference increases further, the flow breaks down to a purely conductive state. During this transit the velocity decreases monotonically. Counterintuitively, the salinity transfer can be enhanced when a stabilising temperature field is applied to the system. This happens when convection rolls are replaced by salt fingers. In addition, we show that the Grossmann-Lohse theory originally developed for RB flow can be directly applied to the current problem and accurately predicts the salinity transfer rate for a wide range of control parameters. Supported by Stichting FOM and the National Computing Facilities (NCF), both sponsored by NWO. The simulations were conducted on the Dutch supercomputer Cartesius at SURFsara.
Hexaroll Convection and Heteroclinic Chaos in Convection Driven by Centrifugal Buoyancy
NASA Astrophysics Data System (ADS)
Busse, Friedrich
1998-11-01
An experimental study is reported in which convection in a rotating vertical cylindrical annulus cooled from within and heated from the outside is observed. Both, thermochromatic liquid crystals and suspended flat particles, are used for visualisation. While the centrifugal force by far exceeds laboratory gravity the limit of vanishing Coriolis force is approached through the use of rather viscous liquids. In particular, glycerol-water mixtures and 10cSt silicone oil have been used. Hexaroll convection patterns and knot convection are observed as predicted by the theory of Auer et al. (J. Fluid Mech. 301, 371, 1995). At Rayleigh numbers R of the order 3000 oblique rolls appear to be preferred. Their angle \\chi with respect to the axis of rotation increases with R and axisymmetric rolls are found for R in the range of 10^4 as long as the Coriolis force can be regarded as a perturbation. Spatio-temporal chaos is found in a transition regime where hexarolls and oblique rolls tend to replace each other in a heteroclinic fashion. These findings will be related to computational simulations presently performed by O. Brausch and W. Pesch.
Characteristics of Extreme Summer Convection over equatorial America and Africa
NASA Astrophysics Data System (ADS)
Zuluaga, M. D.; Houze, R.
2013-12-01
Fourteen years of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) version 7 data for June-August show the temporal and spatial characteristics of extreme convection over equatorial regions of the American and African continents. We identify three types of extreme systems: storms with deep convective cores (contiguous convective 40 dBZ echoes extending ≥10 km in height), storms with wide convective cores (contiguous convective 40 dBZ echoes with areas >1,000 km2) and storms with broad stratiform regions (stratiform echo >50,000 km2). European Centre for Medium-Range Weather Forecast (ECMWF) reanalysis is used to describe the environmental conditions around these forms of extreme convection. Storms with deep convective cores occur mainly over land: in the equatorial Americas, maximum occurrence is in western Mexico, Northern Colombia and Venezuela; in Africa, the region of maximum occurrence is a broad zone enclosing the central and west Sudanian Savanna, south of the Sahel region. Storms with wide convective radar echoes occur in these same general locations. In the American sector, storms with broad stratiform precipitation regions (typifying robust mesoscale convective systems) occur mainly over the eastern tropical Pacific Ocean and the Colombia-Panama bight. In the African sector, storms with broad stratiform precipitation areas occur primarily over the eastern tropical Atlantic Ocean near the coast of West Africa. ECMWF reanalyses show how the regions of extreme deep convection associated with both continents are located mainly in regions affected by diurnal heating and influenced by atmospheric jets in regions with strong humidity gradients. Composite analysis of the synoptic conditions leading to the three forms of extreme convection provides insights into the forcing mechanisms in which these systems occur. These analyses show how the monsoonal flow directed towards the Andes slopes is mainly what concentrates the occurrence of extreme
NASA Astrophysics Data System (ADS)
Kipling, Zak; Stier, Philip; Wagner, Till
2014-05-01
Convection plays an important role in the climate system through its effects on radiation, precipitation, large-scale dynamics and vertical transport of aerosols and trace gases. The effects of aerosols on the development of convective cloud and precipitation are a source of considerable uncertainty in current climate modelling. Most current global climate models use 'mass-flux' convection schemes, which represent the ensemble of convective clouds in a GCM column by a single 'mean' updraught. In addition to over-simplifying the representation of such clouds, this presents particular problems in the context of aerosol-convection interactions: firstly because the relationship between aerosol and the droplet size distribution depends on the vertical velocity distribution, about which little or no information is available, and secondly because the effects of convective transport and scavenging may vary nonlinearly over the ensemble (e.g. between precipitating and non-precipitating clouds and due to different loadings). The Convective Cloud Field Model (CCFM) addresses these limitations by simulating a spectrum of updraughts with different cross-sectional areas within each GCM column, based on the quasi-equilibrium approach of Arakawa and Schubert. For each cloud type, an entraining Lagrangian parcel model is initiated by perturbations at the surface, allowing a realistic vertical velocity to develop by cloud base so that detailed size-resolved microphysics can be represented within the cloud above. These different cloud types interact via competition for resolved-scale convective available potential energy (CAPE). Transport of water, aerosol and other tracers is calculated separately for each cloud type, allowing for different entrainment and scavenging behaviours. By using CCFM embedded within the ECHAM6-HAM aerosol-climate model, we show how this approach can both improve the distribution of convective precipitation events compared to a typical mass-flux scheme, and
NASA Astrophysics Data System (ADS)
Dong, X.; Logan, T.; Xi, B.
2015-12-01
Three deep convective cloud cases were selected during the 2011 Mid-Latitude Continental Convective Clouds Experiment (MC3E). Although biomass burning smoke advected from Mexico and Central America was the dominant source of cloud condensation nuclei (CCN) for deep convective cloud formation, the 11 May, 20 May, and 23 May cases exhibited different convective characteristics. The convection in the 11 May and 23 May cases formed in smoke laden environments in the presence of convective available potential energy (CAPE) values exceeding 1000 m2 s-2 and 3000 m2 s-2 along with low-level (0-1 km) shear of 10.3 m s-1 and 5.1 m s-1, respectively. The 11 May case had linear convection while the 23 May case featured discrete supercells. The 20 May case featured elevated linear convection that formed in a more moist environment with cleaner aerosol conditions, weak CAPE (<50 m2 s-2), and stronger low-level shear (25.6 m s-1). Though the 20 May case had the highest precipitation amount and duration, the 23 May case had the highest ice water content (IWC) in the upper levels of the convection (>9 km) suggesting a warm rain suppression mechanism caused by a combination of strong aerosol loading, large CAPE, and weak low-level wind shear. The observed results for the 20 May and 23 May cases agree well with recent modeling studies that simulated the convection and precipitation in these cases. Furthermore, the modeling of the 11 May case is suggested since the abundant amount of smoke CCN did not greatly enhance the overall precipitation amount and could be a possible aerosol-induced precipitation suppression case.
Convective and stratiform precipitation trends in the Spanish Mediterranean coast
NASA Astrophysics Data System (ADS)
Ruiz Leo, Ana M.; Hernández Martín, Emiliano; Queralt, Sara; Cony, Marco Marco
2010-05-01
Eastern Iberian Peninsula is characterized by the large occurrence of convective precipitation events, which entail important economic and social damages. It is necessary to achieve a good knowledge and understanding of the meteorological processes involved. In this regard, an algorithm for classifying convective and stratiform precipitation components has been applied to a decadal precipitation record. Dataset were provided by National Spanish Meteorological Agency (AEMET) for the period 1998-2008. Hourly precipitation records have been analyzed. The goals of this study are: a) classifying total precipitation into its stratiform and convective components in Levante region (located in the Eastern Spanish coast) and b) analyzing annual and seasonal trends of such components. In order to determine both convective and stratiform precipitation components, a suitable exponential function has been used. After iterative computation process critical precipitation intensity (so-called Rc) is obtained for each year and season of the study period. Every precipitation episode in Levante region is classified into prevailing convective or stratiform regime according to the threshold value defined by Rc. First results show an annual and seasonal significant positive trend in total precipitation and stratiform component for 1998-2008 decade. Further analysis reveals that convective precipitation exhibits no significant trend. Therefore, preliminary conclusions state that the total precipitation amounts in Levante Region strongly depends on the stratiform component evolution. Current analyses are focused on evaluating the importance of convective precipitation component and assessing the main triggering factors involved in the severe weather episodes registered.
Convective intitiation over a heated mountain: mechanisms and predictability
NASA Astrophysics Data System (ADS)
Kirshbaum, D.
2010-09-01
In conditionally unstable flows over orography, the strong horizontal convergence generated by elevated heating locally weakens convective inhibition and increases the likelihood of convective initiation. This generally serves to enhance the predictability of deep convection, except when the associated uplift lies just at the margin of the forcing needed for convective initiation. In such marginal cases, airflows with very small initial differences may experience substantially different evolutions. To investigate the processes that govern cloud development in such cases, this study analyzes ensembles of idealized, high-resolution 2d simulations of the diurnal cycle in conditionally unstable flow over a mountain ridge. The case considered is based on a well-observed event from the Convective and Orographic Precipitation Study (COPS) that has proven highly difficult to predict in NWP models. This event was characterized by strong conditional instability but also large convective inhibition and a very dry mid-troposphere that presented a hostile environment for ascending clouds. Within each ensemble, the members differ only in their random seeds of low-amplitude, white-noise thermal perturbations added to the initial flow (0600 local time). The members of each ensemble experience similar mesoscale evolution, with convective inhibition (CIN) eroding completely and large CAPE developing over the high terrain by noon. Shallow orographic cumuli form predictably in response, but only in some cases do these transition to deep cumulonimbi. The dynamical and microphysical mechanisms that determine the cloud evolution in these simulations are examined through parcel trajectory analysis and an entraining thermal model.
Where is the level of neutral buoyancy for deep convection?
NASA Astrophysics Data System (ADS)
Takahashi, Hanii; Luo, Zhengzhao
2012-08-01
This study revisits an old concept in meteorology - level of neutral buoyancy (LNB). The classic definition of LNB is derived from the parcel theory and can be estimated from the ambient sounding (LNB_sounding) without having to observe any actual convective cloud development. In reality, however, convection interacts with the environment in complicated ways; it will eventually manage to find its own effective LNB and manifests it through detraining masses and developing anvils (LNB_observation). This study conducts a near-global survey of LNB_observation for tropical deep convection using CloudSat data and makes comparison with the corresponding LNB_sounding. The principal findings are as follows: First, although LNB_sounding provides a reasonable upper bound for convective development, correlation between LNB_sounding and LNB_observation is low suggesting that ambient sounding contains limited information for accurately predicting the actual LNB. Second, maximum mass outflow is located more than 3 km lower than LNB_sounding. Hence, from convective transport perspective, LNB_sounding is a significant overestimate of the “destination” height level of the detrained mass. Third, LNB_observation is consistently higher over land than over ocean, although LNB_sounding is similar between land and ocean. This difference is likely related to the contrasts in convective strength and environment between land and ocean. Finally, we estimate the bulk entrainment rates associated with the observed deep convection, which can serve as an observational basis for adjusting GCM cumulus parameterization.
SST dependence of convective aggregation in three General Circulation Models
NASA Astrophysics Data System (ADS)
Bony, Sandrine; Becker, Tobias; Coppin, David; Medeiros, Brian; Reed, Kevin; Stevens, Bjorn; Voigt, Aiko
2015-04-01
Studies using cloud-resolving models or simple models have shown that under certain conditions, the radiative-convective equilibrium state becomes unstable to large-scale overturning circulations, and leads to the phenomenon of self-aggregation of moist convection. Modeling and observational studies suggest that the degree of aggregation of moist convection can influence the large-scale atmospheric state (e.g. humidity, clouds) and its energy budget. The question thus arises as to what extent the aggregation of convection may rectify the Earth's climate, including the large-scale atmospheric circulation, hydrological sensitivity and climate feedbacks. We explore these issues by running three General Circulation Models (IPSL-CM5A-LR, ECHAM6, CAM5) in radiative-convective equilibrium, i.e. a non-rotating aqua-planet configuration forced by a globally-uniform insolation and sea surface temperature (SST). We show that in these conditions, all three models can predict the spontaneous emergence of a large-scale convective organization and overturning circulation, and that the equilibrium aggregation state depends on SST and cloud-radiative effects. We will explore the reasons why the equilibrium aggregation state depends on temperature, and the impact of convective aggregation on the global mean state. Robust behaviors will be highlighted, as well as inter-model differences. The implications of these results will be discussed.
Thermodynamic Environments Supporting Extreme Convection in Subtropical South America
NASA Astrophysics Data System (ADS)
Rasmussen, K. L.; Trier, S. B.
2015-12-01
Extreme convection tends to form in the vicinity of mountain ranges, and the Andes in subtropical South America help spawn some of the most intense convection in the world. Subsequent to initiation, the convection often evolves into propagating mesoscale convective systems (MCSs) similar to those seen over the U.S. Great Plains and produces damaging tornadoes, hail, and floods across a wide agricultural region. In recent years, studies on the nature of convection in subtropical South America using spaceborne radar data have elucidated key processes responsible for their extreme characteristics, including a strong relationship between the Andes topography and convective initiation. Building on previous work, an investigation of the thermodynamic environment supporting some of the deepest convection in the world will be presented. In particular, an analysis of the thermodynamic destabilization in subtropical South America, which considers the parcel buoyancy minimum for conditionally unstable air parcels, will be presented. Additional comparisons between the nocturnal nature and related diurnal cycle of MCSs in subtropical South America the U.S. Great Plains will provide insights into the processes controlling MCS initiation and upscale growth.
Towards global constraints of aerosol-convection interactions from space
NASA Astrophysics Data System (ADS)
Stier, Philip
2015-04-01
Aerosol-cloud interactions arguably remain the single greatest uncertainty among anthropogenic perturbations of the climate system. In particular interactions between aerosols and convection remain highly uncertain. In this presentation, I will critically review some of the achievements made towards constraining aerosol-convection interactions from satellite remote sensing and their role in the evaluation of global aerosol-climate models. This will particularly focus on model-data synergies in the assessment of observational constraints, from the suitability of satellite retrieved aerosol properties as proxy for cloud condensation nuclei, all the way to the difficulty to constrain meteorological co-variability in observational studies of aerosol-cloud interactions. Most of the satellite-based work on aerosol-convection interactions has been based on data from sun-synchronous polar orbiting satellites. While these instruments often offer superior data quality, they capture only a certain stage of the convective life-cycle. A life-cycle based approach to aerosol-convection interactions, combining results from polar orbiting instruments with non sun-synchronous or geostationary data provides entirely novel insights into the convective lifecycle and aerosol-convection interactions, which we apply to constrain aerosol-precipitation interactions.
SHARP simulation of discontinuities in highly convective steady flow
NASA Technical Reports Server (NTRS)
Leonard, B. P.
1987-01-01
For steady multidimesional convection, the Quadratic Upstream Interpolation for Convective Kinematics (QUICK) scheme has several attractive properties. However, for highly convective simulation of step profiles, QUICK produces unphysical overshoots and a few oscillations, and this may cause serious problems in nonlinear flows. Fortunately, it is possible to modify the convective flux by writing the normalized convected control-volume face value as a function of the normalized adjacent upstream node value, developing criteria for monotonic resolution without sacrificing formal accuracy. This results in a nonlinear functional relationship between the normalized variables, whereas standard methods are all linear in this sense. The resulting Simple High Accuracy Resolution Program (SHARP) can be applied to steady multidimensional flows containing thin shear or mixing layers, shock waves, and other frontal phenomena. This represents a significant advance in modeling highly convective flows of engineering and geophysical importance. SHARP is based on an explicit, conservative, control-volume flux formation, equally applicable to one, two, or three dimensional elliptic, parabolic, hyperbolic, or mixed-flow regimes. Results are given for the bench-mark purely convective first-order results and the nonmonotonic predictions of second- and third-order upwinding.
Particle simulation of supersonic convection in the protostellar nebula
NASA Astrophysics Data System (ADS)
Riegger, J. M.
1995-02-01
For the simulation of compressible convection and a possible description of inner processes in meteorites a new algorithm for particle-in-cell methods in particle simulation is developed, which allows a direct description of the inner pressure by use of individual particle temperatures and therefore a description of gas dynamics without the approximations of perturbation theory. The simulation of nonadiabatic processes in superadiabatic stratified atmospheres leads to the self-organization of convection cells and to supersonic convection and instationary shock front systems for high Rayleigh numbers Ra > 5 × 10 5 as they are received by other numerical methods. The transport of material through shock fronts yields much faster temperature and pressure changes than ordinary convective transport in the subsonic range. Tracing the values of state along the pathlines shows that fast entropy increases occur either within shock fronts or due to local dissipation in turbulences. Transport through shock front systems results in multiple rapid temperature changes per cycle. Investigations on the local convective structure of the protostellar nebula with a simple radiative transfer and standard opacities and accretion rates indicate supersonic convection and multiple shock front systems in the outer layers of the solar nebula due to radiative cooling. Supersonic convection provides a very effective mechanism of dissipation for the protostellar nebula and makes a contribution to the discussion on the turbulent structure of the protostellar nebula and to the formation of chrondrules.
Do tropical wetland plants possess convective gas flow mechanisms?
Konnerup, Dennis; Sorrell, Brian K; Brix, Hans
2011-04-01
• Internal pressurization and convective gas flow, which can aerate wetland plants more efficiently than diffusion, are common in temperate species. Here, we present the first survey of convective flow in a range of tropical plants. • The occurrence of pressurization and convective flow was determined in 20 common wetland plants from the Mekong Delta in Vietnam. The diel variation in pressurization in culms and the convective flow and gas composition from stubbles were examined for Eleocharis dulcis, Phragmites vallatoria and Hymenachne acutigluma, and related to light, humidity and air temperature. • Nine of the 20 species studied were able to build up a static pressure of > 50 Pa, and eight species had convective flow rates higher than 1 ml min(-1). There was a clear diel variation, with higher pressures and flows during the day than during the night, when pressures and flows were close to zero. • It is concluded that convective flow through shoots and rhizomes is a common mechanism for below-ground aeration of tropical wetland plants and that plants with convective flow might have a competitive advantage for growth in deep water. PMID:21175639
The influence of convective activity on the vorticity budget
NASA Technical Reports Server (NTRS)
Townsend, T. L.; Scoggins, J. R.
1983-01-01
The influence of convective activity on the vorticity budget was determined during the AVE VII and AVE-SESAME I periods. This was accomplished by evaluating each term in the expanded vorticity equation with twisting and tilting and friction representing the residual of all other terms. Convective areas were delineated by use of radar summary charts. The influence of convective activity was established by analyzing contoured fields of each term as well as numerical values and profiles of the various terms in convective and nonconvective areas. Vertical motion was computed by the kinematic method, and all computations were performed over the central United States using a grid spacing of 158 km. The results show that, in convective areas in particular, the residual is of comparable magnitude to the horizontal advection and divergence terms, and therefore, cannot be neglected. In convective areas, the residual term represents a sink of vorticity below 500 mb and a strong source near 300 mb. In nonconvective areas, the residual was small in magnitude at all levels, but tended to be negative (vorticity sink) at 300 mb. The local change term, over convective areas, tended to be balanced by the residual term, and appeared to be a good indicator of development (vorticity becoming more cyclonic). Finally, the shape of the vertical profiles of the term in the budget equation agreed with those found by other investigators for easterly waves, but the terms were one order of magnitude larger than those for easterly waves.
Observations of Florida Convective Storms using Dual Wavelength Airborne Radar
NASA Technical Reports Server (NTRS)
Heymsfield, G. M.; Heymsfield, A. J.; Belcher, L.
2004-01-01
NASA conducted the Cirrus Regional Study of Tropical Anvils and Cirrus Layers (CRYSTAL) Florida Area Cirrus Experiment (FACE) during July 2002 for improved understanding of tropical cirrus. One of the goals was to improve the understanding of cirrus generation by convective updrafts. The reasons why some convective storms produce extensive cirrus anvils is only partially related to convective instability and the vertical transport ice mass by updrafts. Convective microphysics must also have an important role on cirrus generation, for example, there are hypotheses that homogeneous nucleation in convective updrafts is a major source of anvil ice particles. In this paper, we report on one intense CRYSTAL-FACE convective case on 16 July 2002 that produced extensive anvil. During CRYSTAL-FACE, up to 5 aircraft flying from low- to high-altitudes, were coordinated for the study of thunderstorm-generated cirrus. The NASA high-altitude (20 km) ER-2 aircraft with remote sensing objectives flew above the convection, and other aircraft such as the WB-57 performing in situ measurements flew below the ER-2. The ER-2 remote sensing instruments included two nadir viewing airborne radars. The CRS 94 GHz radar and the EDOP 9.6 GHz radar were flown together for the first time during CRYSTAL-FACE and they provided a unique opportunity to examine the structure of 16 July case from a dual-wavelength perspective. EDOP and CRS are complementary for studying convection and cirrus since CRS is more sensitive than EDOP for cirrus, and EDOP is considerably less attenuating in convective regions. In addition to the aircraft, coordinated ground-based radar measurements were taken with the NPOL S-Band (3 GHz) multiparameter radar. One of the initial goals was to determine whether dual-wavelength airborne measurements could identify supercooled water regions.
The Deep Convective Clouds and Chemistry (DC3) Field Experiment
NASA Astrophysics Data System (ADS)
Barth, M. C.; Brune, W. H.; Cantrell, C. A.; Rutledge, S. A.; Crawford, J. H.; Huntrieser, H.; Homeyer, C. R.; Nault, B.; Cohen, R. C.; Pan, L.; Ziemba, L. D.
2014-12-01
The Deep Convective Clouds and Chemistry (DC3) field experiment took place in the central U.S. in May and June 2012 and had the objectives of characterizing the effect of thunderstorms on the chemical composition of the lower atmosphere and determining the chemical aging of upper troposphere (UT) convective outflow plumes. DC3 employed ground-based radars, lightning mapping arrays, and weather balloon soundings in conjunction with aircraft measurements sampling the composition of the inflow and outflow of a variety of thunderstorms in northeast Colorado, West Texas to central Oklahoma, and northern Alabama. A unique aspect of the DC3 strategy was to locate and sample the convective outflow a day after active convection in order to measure the chemical transformations within the UT convective plume. The DC3 data are being analyzed to investigate transport and dynamics of the storms, scavenging of soluble trace gases and aerosols, production of nitrogen oxides by lightning, relationships between lightning flash rates and storm parameters, and chemistry in the UT that is affected by the convection. In this presentation, we give an overview of the DC3 field campaign and highlight results from the campaign that are relevant to the upper troposphere and lower stratosphere region. These highlights include stratosphere-troposphere exchange in connection with thunderstorms, the 0-12 hour chemical aging and new particle formation in the UT outflow of a dissipating mesoscale convective system observed on June 21, 2012, and UT chemical aging in convective outflow as sampled the day after convection occurred and modeled in the Weather Research and Forecasting coupled with Chemistry model.
The Tropical Convective Spectrum. Part 1; Archetypal Vertical Structures
NASA Technical Reports Server (NTRS)
Boccippio, Dennis J.; Petersen, Walter A.; Cecil, Daniel J.
2005-01-01
A taxonomy of tropical convective and stratiform vertical structures is constructed through cluster analysis of 3 yr of Tropical Rainfall Measuring Mission (TRMM) "warm-season" (surface temperature greater than 10 C) precipitation radar (PR) vertical profiles, their surface rainfall, and associated radar-based classifiers (convective/ stratiform and brightband existence). Twenty-five archetypal profile types are identified, including nine convective types, eight stratiform types, two mixed types, and six anvil/fragment types (nonprecipitating anvils and sheared deep convective profiles). These profile types are then hierarchically clustered into 10 similar families, which can be further combined, providing an objective and physical reduction of the highly multivariate PR data space that retains vertical structure information. The taxonomy allows for description of any storm or local convective spectrum by the profile types or families. The analysis provides a quasi-independent corroboration of the TRMM 2A23 convective/ stratiform classification. The global frequency of occurrence and contribution to rainfall for the profile types are presented, demonstrating primary rainfall contribution by midlevel glaciated convection (27%) and similar depth decaying/stratiform stages (28%-31%). Profiles of these types exhibit similar 37- and 85-GHz passive microwave brightness temperatures but differ greatly in their frequency of occurrence and mean rain rates, underscoring the importance to passive microwave rain retrieval of convective/stratiform discrimination by other means, such as polarization or texture techniques, or incorporation of lightning observations. Close correspondence is found between deep convective profile frequency and annualized lightning production, and pixel-level lightning occurrence likelihood directly tracks the estimated mean ice water path within profile types.
Enhancement of free tropospheric ozone production by deep convection
NASA Technical Reports Server (NTRS)
Pickering, Kenneth E.; Thompson, Anne M.; Scala, John R.; Tao, Wei-Kuo; Simpson, Joanne
1994-01-01
It is found from model simulations of trace gas and meteorological data from aircraft campaigns that deep convection may enhance the potential for photochemical ozone production in the middle and upper troposphere by up to a factor of 60. Examination of half a dozen individual convective episodes show that the degree of enhancement is highly variable. Factors affecting enhancement include boundary layer NO(x) mixing ratios, differences in the strength and structure of convective cells, as well as variation in the amount of background pollution already in the free troposphere.
Ozone production potential following convective redistribution of biomass burning emissions
NASA Technical Reports Server (NTRS)
Pickering, Kenneth E.; Thompson, Anne M.; Scala, John R.; Tao, Wei-Kuo; Simpson, Joanne
1992-01-01
The effects of deep convection on the potential for forming ozone in the free troposphere have been simulated for regions where the trace gas composition is influenced by biomass burning. Cloud photochemical and dynamic simulations based on observations in the 1980 and 1985 Brazilian campaigns form the basis of a sensitivity study of the ozone production potential under differing conditions. It is seen that there is considerably more ozone formed in the middle and upper troposphere when convection has redistributed hydrocarbons, NO(x), and CO compared to the example of no convection.
Laminar free convection from a sphere with blowing and suction
Huang, Mingjer; Chen, Gahokuang )
1987-05-01
The effect of mass transfer on free convection from a vertical plate has been studied by Eichhorn (1960), Sparrow and Cess (1961), Merkin (1972), and Parikh (1974). Recently, Merkin (1975) gave an asymptotic series solution for two-dimensional bodies. Minkowycz and Sparrow (1979) studied a vertical cylinder in a natural convective flow. According to their conclusions, the heat transfer rate increases with suction and decreases with blowing. The present note is concerned with the study of the influence of Prandtl number and surface mass transfer on a steady, laminar, free convective flow over a sphere with nonuniform surface temperature or heat flux.
Budget study of a mesoscale convective system - Model simulation
NASA Technical Reports Server (NTRS)
Tao, Wei-Kuo; Simpson, Joanne; Mccumber, Michael
1988-01-01
A tropical squall-type cloud cluster is examined as an example of a mesoscale convective complex. The precipitation growth processes and air circulations that develop in the convective and stratiform regions are studied using a data set generated from a time-dependent numerical cloud model. The relationship of the stratiform region of the mesoscale convective complex is discussed. The vertical profiles of heating and drying are calculated. Comparisons are made between simulations with and without ice-phase microphysical processes and a simulation with forcing by weaker lifting at middle and upper levels.
Rotating convective turbulence in Earth and planetary cores
NASA Astrophysics Data System (ADS)
Aurnou, J. M.; Calkins, M. A.; Cheng, J. S.; Julien, K.; King, E. M.; Nieves, D.; Soderlund, K. M.; Stellmach, S.
2015-09-01
An accurate description of turbulent core convection is necessary in order to build robust models of planetary core processes. Towards this end, we focus here on the physics of rapidly rotating convection. In particular, we present a closely coupled suite of advanced asymptotically-reduced theoretical models, efficient Cartesian direct numerical simulations (DNS) and laboratory experiments. Good convergence is demonstrated between these three approaches, showing that a comprehensive understanding of the dynamics appears to be within reach in our simplified rotating convection system. The goal of this paper is to review these findings, and to discuss their possible implications for planetary cores dynamics.
Convection in a two-layer fluid system
NASA Technical Reports Server (NTRS)
Prakash, A.; Peltier, L. J.; Fujita, D.; Koster, J.; Biringen, S.
1991-01-01
Experimental results are presented, and preliminary computations are performed on a system of two immiscible liquid layers with a temperature gradient applied parallel to the interface. The experiments reflect the combined contribution of buoyancy and surface-tension-induced (Marangoni) convection. It is concluded that buoyancy effects appear to be dominant and mask any surface-tension-induced convection present. Numerical computations show significant modification of pure buoyant convection by surface-tension gradients. The results are of interest in connection with the liquid encapsulation of GaAs melts in a microgravity environment.
Extensive chaos in Rayleigh-Bénard convection.
Paul, M R; Einarsson, M I; Fischer, P F; Cross, M C
2007-04-01
Using large-scale numerical calculations we explore spatiotemporal chaos in Rayleigh-Bénard convection for experimentally relevant conditions. We calculate the spectrum of Lyapunov exponents and the Lyapunov dimension describing the chaotic dynamics of the convective fluid layer at constant thermal driving over a range of finite system sizes. Our results reveal that the dynamics of fluid convection is truly chaotic for experimental conditions as illustrated by a positive leading-order Lyapunov exponent. We also find the chaos to be extensive over the range of finite-sized systems investigated as indicated by a linear scaling between the Lyapunov dimension of the chaotic attractor and the system size. PMID:17500952
Extensive chaos in Rayleigh-Bénard convection
NASA Astrophysics Data System (ADS)
Paul, M. R.; Einarsson, M. I.; Fischer, P. F.; Cross, M. C.
2007-04-01
Using large-scale numerical calculations we explore spatiotemporal chaos in Rayleigh-Bénard convection for experimentally relevant conditions. We calculate the spectrum of Lyapunov exponents and the Lyapunov dimension describing the chaotic dynamics of the convective fluid layer at constant thermal driving over a range of finite system sizes. Our results reveal that the dynamics of fluid convection is truly chaotic for experimental conditions as illustrated by a positive leading-order Lyapunov exponent. We also find the chaos to be extensive over the range of finite-sized systems investigated as indicated by a linear scaling between the Lyapunov dimension of the chaotic attractor and the system size.
Ionospheric convection associated with discrete levels of particle precipitation
Foster, J.C.; Holt, J.M.; Musgrove, R.G.; Evans, D.S.
1986-07-01
A precipitation index is described which quantifies the intensity and spatial extent of high-latitude particle precipitation based on observations made along individual satellite passes. By sorting plasma-convection data according to this index, average patterns of the ionospheric convection electric field were derived from a data set consisting of five years' observations by the Millstone Hill radar. Reference to the instantaneous precipitation index, and the average patterns keyed to it, provides a means of characterizing the global precipitation and convection patterns throughout an event.
A variable mixing-length ratio for convection theory
NASA Technical Reports Server (NTRS)
Chan, K. L.; Wolff, C. L.; Sofia, S.
1981-01-01
It is argued that a natural choice for the local mixing length in the mixing-length theory of convection has a value proportional to the local density scale height of the convective bubbles. The resultant variable mixing-length ratio (the ratio between the mixing length and the pressure scale height) of this theory is enhanced in the superadiabatic region and approaches a constant in deeper layers. Numerical tests comparing the new mixing length successfully eliminate most of the density inversion that typically plagues conventional results. The new approach also seems to indicate the existence of granular motion at the top of the convection zone.
Boiling incipience and convective boiling of neon and nitrogen
NASA Technical Reports Server (NTRS)
Papell, S. S.; Hendricks, R. C.
1977-01-01
Forced convection and subcooled boiling heat transfer data for liquid nitrogen and liquid neon were obtained in support of a design study for a 30 tesla cryomagnet cooled by forced convection of liquid neon. The cryogen data obtained over a range of system pressures, fluid flow rates, and applied heat fluxes were used to develop correlations for predicting boiling incipience and convective boiling heat transfer coefficients in uniformly heated flow channels. The accuracy of the correlating equations was then evaluated. A technique was also developed to calculate the position of boiling incipience in a uniformly heated flow channel. Comparisons made with the experimental data showed a prediction accuracy of + or - 15 percent.
Analytical model of ionospheric convection at subauroral latitudes
NASA Astrophysics Data System (ADS)
Deminov, M. G.; Kim, V. P.; Shubin, V. N.; Khegai, V. V.
An analytical model of plasma convection in the subauroral ionosphere is developed, assuming that the electric shielding of the inner magnetosphere is controlled by polarization of the hot ion zone situated behind the inner boundary of the plasma sheet. It is shown that, at subauroral latitudes at night, the plasma drifts eastward, while during the day it shifts westward. Thus, in the predmidnight sector, the direction of convection in the subauroral ionosphere is opposite to that of auroral convection. In general, the electric field, with a strength of 10 mV/m, has a meridional direction.
Some Consequences of Thermosolutal Convection: The Grain Structure of Castings
NASA Technical Reports Server (NTRS)
Hansen, G.; Hellawell, A.; Lu, S. Z.; Steube, R. S.
1996-01-01
The essential principles of thermosolutal convection are outlined, and how convection provides a transport mechanism between the mushy region of a casting and the open bulk liquid is illustrated. The convective flow patterns which develop assist in heat exchange and macroscopic solute segregation during solidification; they also provide a mechanism for the transport of dendritic fragments from the mushy region into the bulk liquid. Surviving fragments become nuclei for equiaxed grains and so lead to blocking of the parental columnar, dendritic growth front from which they originated. The physical steps in such a sequence are considered and some experimental data are provided to support the argument.
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.
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
X-ray spectra from convective photospheres of neutron stars
Zavlin, V.E.; Pavlov, G.G. |; Shibanov, Yu.A.; Rogers, F.J.; Iglesias, C.A.
1996-01-17
We present first results of modeling convective photospheres of neutron stars. We show that in photospheres composed of the light elements convection arises only at relatively low effective temperatures ({le}3 - 5 x 10{sup 4} K), whereas in the case of iron composition it arises at T{sub eff}{le} 3 x 10{sup 5}K. Convection changes the depth dependence of the photosphere temperature and the shapes of the emergent spectra. Thus, it should be taken into account for the proper interpretation of EUV/soft-X-ray observations of the thermal radiation from neutron stars.
Modeling of Thermal Convection of Liquid TNT for Cookoff
McCallen, R; Dunn, T; Nichols, A; Reaugh, J; McClelland, M
2003-02-27
The objective is to computationally model thermal convection of liquid TNT in a heated cylindrical container for what are called 'cookoff' experiments. Our goal is to capture the thermal convection coupled to the heat transfer in the surrounding container. We will present computational results that validate the functionality of the model, numerical strategy, and computer code for a model problem with Rayleigh number of O(10{sup 6}). We solve the problem of thermal convection between two parallel plates in this turbulent flow regime and show that the three-dimensional computations are in excellent agreement with experiment.
2-D traveling-wave patterns in binary fluid convection
Surko, C.M.; Porta, A.L.
1996-12-31
An overview is presented of recent experiments designed to study two-dimensional traveling-wave convection in binary fluid convection in a large aspect ratio container. Disordered patterns are observed when convection is initiated. As time proceeds, they evolve to more ordered patterns, consisting of several domains of traveling-waves separated by well-defined domain boundaries. The detailed character of the patterns depends sensitively on the Rayleigh number. Numerical techniques are described which were developed to provide a quantitative characterization of the traveling-wave patterns. Applications of complex demodulation techniques are also described, which make a detailed study of the structure and dynamics of the domain boundaries possible.
Convection of colloidal suspensions stratified by thermodiffusion and gravity.
Smorodin, B L; Cherepanov, I N
2014-11-01
The convective stability thresholds and nonlinear evolution of convective rolls are numerically investigated in a plane horizontal layer of a colloidal suspension with positive separation ratio in the case of no-slip, impermeable horizontal boundaries. The characteristics of the steady and oscillatory patterns are analyzed under heating and gravity stratification. The standing and traveling waves are found as stable solutions within certain domains of parameters (on the plane of the Rayleigh and the Boltzmann numbers). Complex bifurcation and spatiotemporal properties are caused by the interaction of gravity sedimentation, Soret-induced gradients, and convective mixing of the fluid. PMID:25416242
Helium enrichment during convective carbon dioxide dissolution
NASA Astrophysics Data System (ADS)
Larson, T.; Hesse, M. A.
2013-12-01
, in particular the He/ CO2-ratio, can be used to infer the amount of dissolved CO2 in the field where pressure evolution is not available. Our experiments show that the rate of dissolution is determined by convective mass transfer in the brine. Convective transport is driven by the increase of water density with increasing CO2 saturation. However, unlike previous experiments with analog systems we do not observe a constant dissolution rate. This is due to the continued drop in gas pressure that continuously reduces the equilibrium aqueous CO2 concentration and with it the driving force for convection. This feed back may significantly reduce the magnitude of solubility trapping that can be expected during geological CO2 storage.
Paluszkiewicz, T.; Hibler, L.F.; Romea, R.D.
1995-01-01
The current generation of ocean general circulation models (OGCMS) uses a convective adjustment scheme to remove static instabilities and to parameterize shallow and deep convection. In simulations used to examine climate-related scenarios, investigators found that in the Arctic regions, the OGCM simulations did not produce a realistic vertical density structure, did not create the correct quantity of deep water, and did not use a time-scale of adjustment that is in agreement with tracer ages or observations. A possible weakness of the models is that the convective adjustment scheme does not represent the process of deep convection adequately. Consequently, a penetrative plume mixing scheme has been developed to parameterize the process of deep open-ocean convection in OGCMS. This new deep convection parameterization was incorporated into the Semtner and Chervin (1988) OGCM. The modified model (with the new parameterization) was run in a simplified Nordic Seas test basin: under a cyclonic wind stress and cooling, stratification of the basin-scale gyre is eroded and deep mixing occurs in the center of the gyre. In contrast, in the OGCM experiment that uses the standard convective adjustment algorithm, mixing is delayed and is wide-spread over the gyre.
Lagrangian analysis of a convective mixed layer
NASA Astrophysics Data System (ADS)
D'Asaro, Eric A.; Winters, Kraig B.; Lien, Ren-Chieh
2002-05-01
We characterize and quantify the transport of heat (Boussinesq density) in a highly idealized entraining convective mixed layer based on simulations of Lagrangian measurements in a two-dimensional model. The primary objectives are to assess and explore the merits and difficulties in estimating the heat budget from perfect and imperfect Lagrangian floats. A significant advantage of Lagrangian measurements is that the time derivative of temperature along these trajectories gives a direct measure of the diffusive heat flux. Using simulated perfect Lagrangian floats, estimates of the surface buoyancy flux, the depth of the mixed layer, vertical profiles of advective and diffusive heat flux, and the overall rate of cooling are shown to agree accurately with the known results extracted from the Eulerian simulations. The Lagrangian nature of the data is exploited to reveal the structure of the flow within the convective layer and to quantify the heat fluxes associated with the different types of eddies. Phase plots of Lagrangian trajectories in density-depth space reveal three distinct classes of motions: (1) plumes, which develop in the cold, heavy near-surface thermal boundary layer and plunge into the mixed layer interior carrying heavy water downward; (2) interior turbulence, comprising random motions between the base of the thermal boundary layer and the base of the surface mixed layer; and (3) entrainment of interior water into plumes below the thermal boundary layer, i.e., a transition from class 2 to class 1. Plumes dominate the heat transport. Simulations were also made using slightly buoyant floats; these are not perfectly Lagrangian. Buoyancy concentrates the floats near the surface resulting in an oversampling of the stronger plumes. Making the same heat budget calculations as with the perfect floats results in a nonzero estimated Lagrangian heating rate in the interior and a curved profile of vertical heat flux that is up to 3 times too large. The local time
Deep Convective Event extraction procedure: first results
NASA Astrophysics Data System (ADS)
Berges, Jean Claude; Beltrando, Gérard; Cacault, Philippe
2015-04-01
The tropopause temperature is one of the rare non ambiguous threshold which can be applied to geostationary infra-red measurements. Therefore a straightforward procedure to delineate in space and time Deep Convective Event (DCE) is first to binarize according to this threshold and then to apply a 3D connectivity on successive 10.8 µm satellite images. This process circumnavigate the issue of splitting or merging systems as extracted DCE are only 3D volumes. A key point of this method is that it does not require any local tuning parameter and thus is well suited for building long term climatology. A specific algorithm has been developed to optimize long series data processing and then applied on 18 months database of MSG, MET7 and MTSAT. This presentation will not address the issue of actual rainfall estimation but will focus on behavior of DCE seen as a proxy for intense precipitation event. Basic morphological parameters, such as duration or maximum area, are extracted and related with ground and upper air parameters. Some well known features appear clearly: continent/sea difference, diurnal cycle on land. A more innovative result is the relationship with elevation which appears strong but irregular. The relief triggering effect on DCE appears clearly. In strong contrast with orography, the vegetation cover does not show any clear dependence beyond DCE frequency. Upper air data are supplied by NCEP and ECMWF. Although total precipitable water content is related both with DCE frequency and mean size, such a relationship does not appear for low level winds. This observation does not match with theoretical results which emphasize on wind shear in low troposphere. A possible explanation should rely in limits of deep convection representation in weather reanalysis models. These first results suggest that DCE extraction procedure on one hand is stable enough to run on an operational basis and on another hand can bring significant information. As it relies on the 10
NASA Astrophysics Data System (ADS)
Gill, T. E.; Rivera Rivera, N. I.; Novlan, D. J.
2014-12-01
El Paso, Texas (USA) and Ciudad Juarez, Chihuahua (Mexico) form the Paso del Norte, the largest metropolitan area in North America's Chihuahuan Desert. The cities are subject to frequent dust storms presenting a hazard to local infrastructure and health, including synoptic-scale dust events during winter and spring, and dusty outflows from convective storms (haboobs) primarily during the summer. We evaluate particulate matter (PM2.5 and PM10) concentrations over a decade of convective and non-convective dust events, based on hourly aerosol data collected by Texas Commission on Environmental Quality (TCEQ) continuous air monitors in El Paso cross-referenced to weather observations from the USA National Weather Service. A total of 219 dust events (95 convective and 124 non-convective) events occurred between 2001 and 2010. The PM2.5/PM10 ratio was significantly higher (proportionally greater concentration of fine aerosols) in convective episodes and during summertime events than during non-convective dust events and dust episodes in other seasons, although overall concentrations of both PM2.5 and PM10 were higher in the non-convective events, which were also longer-lasting. These differences in fine/coarse aerosol ratios are likely related to different atmospheric stability conditions, and/or different mechanisms of dust particle entrainment and transport in haboobs versus non-convective dust events. Since visibility degradation and adverse human health effects are known to be exacerbated by to fine aerosol concentrations, thunderstorm-related dust events may present a proportionally greater hazard.
NASA Astrophysics Data System (ADS)
Liu, X.; Zhong, S.
2013-12-01
Classic mantle dynamic models for the Earth's geoid are mostly based on whole mantle convection and constrain that the upper mantle is significantly weaker than the lower mantle. Whole mantle convection models with such mantle viscosity structure have successfully explained the long-wavelength structure in the mantle. However, with increasing consensus on the existence of chemically distinct piles above the core mantle boundary (CMB) (also known as large low shear velocity provinces or LLSVPs), questions arise as to what extent the chemical piles influence the Earth's geoid and long-wavelength mantle convection. Some recent studies suggested that the chemical piles have a controlling effect on the Earth's degree two mantle structure, geoid, and true polar wander, although the chemical piles are estimated to be of small volume (~2% of the whole mantle) by seismic studies. We have formulated dynamically consistent 3D mantle convection models using CitcomS and studied how the chemical piles above CMB influence the long-wavelength convective structure and geoid. The models have free slip boundary conditions and temperature dependent viscosity. By comparing with purely thermal convection models, we found that the long wavelength convective structure is not sensitive to the presence of the chemical piles. By determining the geoid from the buoyance of a certain layer of the mantle, we found that for both purely thermal and thermochemical convection, the geoid is mostly contributed by the upper part of the mantle, with ~80% geoid explained by the buoyancy in the upper half of the mantle. In purely thermal convection, the contribution to the geoid from the bottom layer of the mantle always has the same sign with the total geoid (a bottom ~ 600 km thick layer gives ~3.5% of the total geoid). However, in the thermochemical convection, the bottom layer with overall negatively buoyant chemical piles gives rise to the geoid that has opposite sign with the total geoid and has a
NASA Astrophysics Data System (ADS)
Pu, Z.; Lin, C.; Wei, L.
2012-12-01
Our previous studies demonstrate that assimilation of surface mesonet observations and atmospheric profiles results in improved numerical simulations and predictability of mesoscale convective systems and their related atmospheric boundary layer conditions. In this study, numerical simulations of mesoscale convective systems observed during the Midlatitude Continental Convective Clouds Experiment (MC3E) field campaign are conducted with the mesoscale community Weather Research and Forecasting (WRF) model at a convective permitting scale. First, five time periods, with convective systems evolved are identified over the Oklahoma-Kansas area. High-resolution numerical simulations are conducted with the WRF model for those five periods. A multiple-level nested domain technique is used to achieve the cloud-permitting scale simulation. Specifically, the model domains are set up for 3 nested domains with horizontal resolutions at 12 km, 4 km and 1.33 km, respectively. The model initial and boundary conditions are derived from the NCEP Northern American Mesoscale model (NAM) analysis. Results show that the WRF model reproduces the convective initiations and relatively reasonable evolution of those convective systems during most of the simulation periods, although errors are present in all numerical simulations when compared the model simulated convective initiation time, location and quantitative precipitation with observations. As an early investigation, sensitivity of numerical simulations of mesoscale convective systems to WRF model physical parameterization schemes is examined. For the cases during 12 UTC 19 May to 12 UTC 21 May 2011, it is found that the numerical simulation results are very sensitive to the choice of microphysical schemes in the WRF model. However, even with the best numerical simulation, errors are still found in terms of the prediction of convective initiation time, location and quantitative precipitation, indicating the need for data assimilation
Long- range transport of Xe-133 emissions under convective and non-convective conditions.
NASA Astrophysics Data System (ADS)
Kusmierczyk-Michulec, Jolanta; Gheddou, Abdelhakim
2015-04-01
The International Monitoring System (IMS) developed by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) is a global system of monitoring stations, using four complementary technologies: seismic, hydroacoustic, infrasound and radionuclide. Data from all stations, belonging to IMS, are collected and transmitted to the International Data Centre (IDC) in Vienna, Austria. The radionuclide network comprises 80 stations, of which more than 60 are certified. The aim of radionuclide stations is a global monitoring of radioactive aerosols and radioactive noble gases, in particular xenon isotopes, supported by the atmospheric transport modeling (ATM). The aim of this study is to investigate the long-range transport of Xe-133 emissions under convective and non-convective conditions. For that purpose a series of 14 days forward simulations was conducted using the Lagrangian Particle Diffusion Model FLEXPART, designed for calculating the long-range and mesoscale dispersion of air pollution from point sources. The release point was at the ANSTO facility in Australia. The geographical localization to some extent justifies the assumption that the only source of Xe-133 observed at the neighbouring stations, comes from the ANSTO facility. In the simulations the analysed wind data provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) were used with the spatial resolution of 0.5 degree. Studies have been performed to link Xe-133 emissions with detections at the IMS stations supported by the ATM, and to assess the impact of atmospheric convection on non-detections at the IMS stations. The results of quantitative and qualitative comparison will be presented.
Performance of a convective, infrared and combined infrared- convective heated conveyor-belt dryer.
El-Mesery, Hany S; Mwithiga, Gikuru
2015-05-01
A conveyor-belt dryer was developed using a combined infrared and hot air heating system that can be used in the drying of fruits and vegetables. The drying system having two chambers was fitted with infrared radiation heaters and through-flow hot air was provided from a convective heating system. The system was designed to operate under either infrared radiation and cold air (IR-CA) settings of 2000 W/m(2) with forced ambient air at 30 °C and air flow of 0.6 m/s or combined infrared and hot air convection (IR-HA) dryer setting with infrared intensity set at 2000 W/m(2) and hot at 60 °C being blown through the dryer at a velocity of 0.6 m/s or hot air convection (HA) at an air temperature of 60 °C and air flow velocity 0.6 m/s but without infrared heating. Apple slices dried under the different dryer settings were evaluated for quality and energy requirements. It was found that drying of apple (Golden Delicious) slices took place in the falling rate drying period and no constant rate period of drying was observed under any of the test conditions. The IR-HA setting was 57.5 and 39.1 % faster than IR-CA and HA setting, respectively. Specific energy consumption was lower and thermal efficiency was higher for the IR-HA setting when compared to both IR-CA and HA settings. The rehydration ratio, shrinkage and colour properties of apples dried under IR-HA conditions were better than for either IR-CA or HA. PMID:25892769
Dynamically Evolving Sectors for Convective Weather Impact
NASA Technical Reports Server (NTRS)
Drew, Michael C.
2010-01-01
A new strategy for altering existing sector boundaries in response to blocking convective weather is presented. This method seeks to improve the reduced capacity of sectors directly affected by weather by moving boundaries in a direction that offers the greatest capacity improvement. The boundary deformations are shared by neighboring sectors within the region in a manner that preserves their shapes and sizes as much as possible. This reduces the controller workload involved with learning new sector designs. The algorithm that produces the altered sectors is based on a force-deflection mesh model that needs only nominal traffic patterns and the shape of the blocking weather for input. It does not require weather-affected traffic patterns that would have to be predicted by simulation. When compared to an existing optimal sector design method, the sectors produced by the new algorithm are more similar to the original sector shapes, resulting in sectors that may be more suitable for operational use because the change is not as drastic. Also, preliminary results show that this method produces sectors that can equitably distribute the workload of rerouted weather-affected traffic throughout the region where inclement weather is present. This is demonstrated by sector aircraft count distributions of simulated traffic in weather-affected regions.
On Unsteady Natural Convection Between Spherical Shells
NASA Astrophysics Data System (ADS)
Feldman, Yuri; Colonius, Tim
2011-11-01
Natural convection between two concentric spheres is investigated with three-dimensional numerical simulations. Buoyancy is achieved by preserving a temperature difference between the internal hotter and the external colder boundaries of the spherical shell. The numerical simulations were performed for the two basic configurations characterized by external to internal radius ratios of 1.2 and 1.5. Slightly supercritical laminar regimes characterized by the Rayleigh numbers of order Ra ~ O(104-105) were simulated by utilizing a Direct Numerical Simulation (DNS) approach while a Large Eddy Simulation (LES) was used for investigation of turbulent regimes for Ra ~ O (108-109) . We discuss the topological characteristics of the both laminar and turbulent flows. One of the possible scenarios of steady-unsteady transition is proposed as well. Implications of the results for the design of a double-walled Montgolfiere aerobot for the exploration of Titan's atmosphere are discussed. Research supported by Jet Propulsion Laboratory with Dr. Jeffrey Hall as monitor.
Striation and convection in penumbral filaments
NASA Astrophysics Data System (ADS)
Spruit, H. C.; Scharmer, G. B.; Löfdahl, M. G.
2010-10-01
Observations with the 1-m Swedish Solar Telescope of the flows seen in penumbral filaments are presented. Time sequences of bright filaments show overturning motions strikingly similar to those seen along the walls of small isolated structures in the active regions. The filaments show outward propagating striations with inclination angles suggesting that they are aligned with the local magnetic field. We interpret it as the equivalent of the striations seen in the walls of small isolated magnetic structures. Their origin is then a corrugation of the boundary between an overturning convective flow inside the filament and the magnetic field wrapping around it. The outward propagation is a combination of a pattern motion due to the downflow observed along the sides of bright filaments, and the Evershed flow. The observed short wavelength of the striation argues against the existence of a dynamically significant horizontal field inside the bright filaments. Its intensity contrast is explained by the same physical effect that causes the dark cores of filaments, light bridges and “canals”. In this way striation represents an important clue to the physics of penumbral structure and its relation with other magnetic structures on the solar surface. We put this in perspective with results from the recent 3-D radiative hydrodynamic simulations. 4 movies are only available in electronic form at http://www.aanda.org
Surface Tension Driven Convection Experiment (STDCE)
NASA Technical Reports Server (NTRS)
Ostrach, Simon; Kamotani, Y.; Pline, A.
1994-01-01
Results are reported of the Surface Tension Driven Convection Experiment (STDCE) aboard the USML-1 (first United States Microgravity Laboratory) Spacelab which was launched on June 25, 1992. In the experiment 10 cSt silicone oil was placed in an open circular container which was 10 cm wide by 5 cm deep. The fluid was heated either by a cylindrical heater (1.11 cm dia.) located along the container centerline or by a CO2 laser beam to induce thermocapillary flow. The flow field was studied by flow visualization. Several thermistor probes were placed in the fluid to measure the temperature distribution. The temperature distribution along the liquid free surface was measured by an infrared imager. Tests were conducted over a range of heating powers, laser beam diameters, and free surface shapes. In conjunction with the experiments an extensive numerical modeling of the flow was conducted. In this paper some results of the velocity and temperature measurements with flat and curved free surfaces are presented and they are shown to agree well with the numerical predictions.
Influence of convection on eutectic microstructure
NASA Technical Reports Server (NTRS)
Baskaran, V.; Eisa, G. F.; Wilcox, W. R.
1985-01-01
When the MnBi-Bi eutectic is directionally solidified, it forms fibers of MnBi in a matrix of bismuth. When the material solidified in space at rates of 30 and 50 cm/hr, the average fiber spacing lambda was about one half of the value obtained in cases in which the same material solidified on earth. Neither an altered temperature gradient nor a fluctuating freezing rate are apparently responsible for the change in lambda, and the possibility is studied that natural convection increases lambda on earth by perturbing the compositional field in the melt ahead of the growing solid. A theoretical analysis is conducted along with some experiments. On the basis of the theoretical results for lamellar growth, it is concluded that the spacing lambda increases with increasing stirring, especially at small freezing rates. The experiments indicate that at low growth rates the cross-sectional area of the MnBi blades increases with increased stirring and with decreased growth rate.
Solar Convective Furnace for Metals Processing
NASA Astrophysics Data System (ADS)
Patidar, Deepesh; Tiwari, Sheetanshu; Sharma, Piyush; Pardeshi, Ravindra; Chandra, Laltu; Shekhar, Rajiv
2015-11-01
Metals processing operations, primarily soaking, heat treatment, and melting of metals are energy-intensive processes using fossil fuels, either directly or indirectly as electricity, to operate furnaces at high temperatures. Use of concentrated solar energy as a source of heat could be a viable "green" option for industrial heat treatment furnaces. This paper introduces the concept of a solar convective furnace which utilizes hot air generated by an open volumetric air receiver (OVAR)-based solar tower technology. The potential for heating air above 1000°C exists. Air temperatures of 700°C have already been achieved in a 1.5-MWe volumetric air receiver demonstration plant. Efforts to retrofit an industrial aluminium soaking furnace for integration with a solar tower system are briefly described. The design and performance of an OVAR has been discussed. A strategy for designing a 1/15th-scale model of an industrial aluminium soaking furnace has been presented. Preliminary flow and thermal simulation results suggest the presence of recirculating flow in existing furnaces that could possibly result in non-uniform heating of the slabs. The multifarious uses of concentrated solar energy, for example in smelting, metals processing, and even fuel production, should enable it to overcome its cost disadvantage with respect to solar photovoltaics.
Restricted meniscus convective self-assembly.
Chen, Kai; Stoianov, Stefan V; Bangerter, Justin; Robinson, Hans D
2010-04-15
Convective (or evaporation-induced) self-assembly is a standard technique for depositing uniform, poly-crystalline coatings of nanospheres across multiple square centimeters on the timescale of minutes. In this paper, we present a variation of this technique, where the drying meniscus is restricted by a straight-edge located approximately 100 microm above the substrate adjacent to the drying zone. Surprisingly, we find this technique to yield films at roughly twice the growth rate compared to the standard technique. We attribute this to differing rates of diffusion of vapor from the drying crystal in the two cases. We also investigate the crystal growth rate dependence on ambient relative humidity and find, contrary to some previous reports, that the growth rate depends strongly on the humidity. We introduce a model which indicates that while the length of the drying zone may increase with humidity, this alone cannot compensate for the simultaneous reduction in evaporation rate, so a lower humidity must always lead to a higher growth speed. Comparing the model to our experimental results, we find that the length of the drying zone is constant and mostly independent of parameters such as humidity and surface tension. PMID:20132947
Convective flow effects on protein crystal growth
NASA Technical Reports Server (NTRS)
Rosenberger, Franz; Monaco, Lisa A.
1993-01-01
The experimental setup for the in-situ high resolution optical monitoring of protein crystal growth/dissolution morphologies was substantially improved. By augmenting the observation system with a temperature-controlled enclosure, laser illumination for the interferometric microscope, and software for pixel by pixel light intensity recording, a height resolution of about two unit cells for lysozyme can now be obtained. The repartitioning of Na(+) and Cl(-) ions between lysozyme solutions and crystals was studied. Quite unexpectedly, it was found that the longer crystals were in contact with their solution, the lower was their ion content. The development of a model for diffusive-convective transport and resulting distribution of the growth rate on facets was completed. Results obtained for a realistic growth cell geometry show interesting differences between 'growth runs' at 1g and 0g. The kinematic viscosity of lysozyme solutions of various supersaturations and salt concentrations was monitored over time. In contrast to the preliminary finding of other authors, no changes in viscosity were found over four days. The experimental setup for light scattering investigations of aggregation and nucleation in protein solutions was completed, and a computer program for the evaluation of multi-angle light scattering data was acquired.
Entropy Convective Flux for Tropical Cyclone Haiyan
NASA Astrophysics Data System (ADS)
Pegahfar, Nafiseh; Gharaylou, Maryam; Ghafarian, Parvin
2016-07-01
It is well-known that the environmental factors control tropical cyclones (TCs). one of the most considered thermodynamical parameters is entropy that its significant role on tropical cyclogenesis and TC intensification has been professionally focused in some recent research studies. In the current work, two data sets including satellite data and NCEP-GFS data have been used to investigate the entropy parameter and its convective flux, during tropical cyclone Haiyan (TCH) occurred on 3-11 November 2013 and nominated as the strongest TC over Pacific Ocean before 2014. This purpose has been proceeded for three domain areas with different size. These domains cover inner, eyewall and rainbands, and environmental regions of TCH at various pressure levels. Also three terms of entropy vertical flux including dissipative heating, surface entropy flux and difference between entropy values over inner and outer regions have been analyzed. Our obtained results showed relatively similar behavior of averaged entropy over all selected domain, but with a delay and decrease in maximum values for the smaller domains. In addition our findings revealed different considerable contributions for three terms of entropy vertical flux.
Free convective condensation in a vertical enclosure
Fox, R.J.; Peterson, P.F.; Corradini, M.L.; Pernsteiner, A.P.
1995-09-01
Free convective condensation in a vertical enclosure was studied numerically and the results were compared with experiments. In both the numerical and experimental investigations, mist formation was observed to occur near the cooling wall, with significant droplet concentrations in the bulk. Large recirculation cells near the end of the condensing section were generated as the heavy noncondensing gas collecting near the cooling wall was accelerated downward. Near the top of the enclosure the recirculation cells became weaker and smaller than those below, ultimately disappearing near the top of the condenser. In the experiment the mist density was seen to be highest near the wall and at the bottom of the condensing section, whereas the numerical model predicted a much more uniform distribution. The model used to describe the formation of mist was based on a Modified Critical Saturation Model (MCSM), which allows mist to be generated once the vapor pressure exceeds a critical value. Equilibrium, nonequilibrium, and MCSM calculations were preformed, showing the experimental results to lie somewhere in between the equilibrium and nonequilibrium predictions of the numerical model. A single adjustable constant (indicating the degree to which equilibrium is achieved) is used in the model in order to match the experimental results.
Surface Tension Driven Convection Experiment (STDCE)
NASA Technical Reports Server (NTRS)
Ostrach, S.; Kamotani, Y.
1996-01-01
This document reports the results obtained from the Surface Tension Driven Convection Experiment (STDCE) conducted aboard the USML-1 Spacelab in 1992. The experiments used 10 cSt silicone oil placed in an open circular container that was 10 cm wide and 5 cm deep. Thermocapillary flow was induced by using either a cylindrical heater placed along the container centerline or by a CO2 laser. The tests were conducted under various power settings, laser beam diameters, and free surface shapes. Thermistors located at various positions in the test section recorded the temperature of the fluid, heater, walls, and air. An infrared imager was used to measure the free surface temperature. The flow field was studied by flow visualization and the data was analyzed by a PTV technique. The results from the flow visualization and the temperature measurements are compared with the numerical analysis that was conducted in conjunction with the experiment. The compared results include the experimental and numerical velocity vector plots, the streamline plots, the fluid temperature, and the surface temperature distribution.
Lobe Cell Convection and Polar cap Precipitation
NASA Astrophysics Data System (ADS)
Eriksson, S.; Peria, W. J.; Su, Y.; Ergun, R. E.; Tung, Y.; Parks, G.; Carlson, C. W.
2002-12-01
The characteristic electric and magnetic field signature of lobe cells as observed by the low-altitude FAST satellite are compared with Polar ultraviolet images of polar cap auroral activity. Initial results from 55 events suggest that there is an intimate coupling between the sunward convection flow of the lobe cell and transpolar auroral arcs or diffuse polar cap precipitation. Moreover, the presence of lobe cells coincide with UV data intensifications in the premidnight 2100-2400 MLT sector and/or the postnoon 1500 MLT region in ~54% of all cases with UVI coverage. The magnetic local time dependence of the lobe cells and polar cap precipitation on the interplanetary magnetic field (IMF) are examined using the upstream Wind monitor. The relative importance of the IMF By and Bz components are investigated and compared with the predictions of the antiparallel merging model and strongly suggests a connection with the magnetospheric sash, as is further implied by the mapping of magnetic field lines using the Tsyganenko [2002] (T01) model. It was also noted that a majority of events occurred during enhanced AE index substorm-like conditions and that generally stronger AE indices are measured for stronger IMF By magnitudes.
Convective polymerase chain reaction around micro immersion heater
NASA Astrophysics Data System (ADS)
Hennig, Martin; Braun, Dieter
2005-10-01
Polymerase chain reaction (PCR) is performed in the thermal convection created by a micro immersion heater. Instead of repetitive heating and cooling, the temperature gradient induces thermal convection which drives the reaction liquid between hot and cold parts of the chamber. The convection triggers DNA amplification as the DNA melts into two single strands in the hot region and replicates with the use of proteins into twice the amount in the cold region. The constant heater is simply dipped into the reaction solution. Compared to previous experiments, we demonstrate that convective PCR is possible in a robotically accessible open vessel. Our approach compares well with fast PCR cyclers and replicates DNA 500 000 fold within 20minutes. We reduce the necessary components for PCR to cheap, single-use components and therefore increasing the prospects of bringing PCR to point of care applications—even in third world countries.
Test of a new theory for stellar convection using helioseismology
NASA Technical Reports Server (NTRS)
Paterno, L.; Ventura, R.; Canuto, V. M.; Mazzitelli, I.
1993-01-01
Two evolutionary models of the sun have been tested using helioseismological data. The two models use the same input microphysics (nuclear reaction rates, opacity, equation of state) and the same numerical evolutionary code, but differ in the treatment of turbulent convection. The first model employs the standard mixing - length theory of convection, while the second one employs a new turbulent convection model which overcomes some basic inconsistencies of the standard theory of convection. The test rests on the calculation of p-mode eigenfrequencies and on the comparison with the helioseismological data. The comparison shows an overall improvement of the eigenfrequencies calculated with the new model with respect to those calculated with the standard model, although it appears that both models still suffer from inaccuracies especially in the treatment of the surface layers.
Injun 5 observations of magnetospheric electric fields and plasma convection
NASA Technical Reports Server (NTRS)
Gurnett, D. A.
1971-01-01
Recent measurements of magnetospheric electric fields with the satellite Injun 5 have provided a comprehensive global survey of plasma convection at low altitudes in the magnetosphere. A persistent feature of these electric field observations is the occurrence of an abrupt reversal in the convection electric field at auroral zone latitudes. The plasma convection velocities associated with these reversals are generally directed east-west, away from the sun on the poleward side of the reversal, and toward the sun on the equatorward side of the reversal. Convection velocities over the polar cap region are normally less than those observed near the reversal region. The electric field reversal is observed to be coincident with the trapping boundary for electrons with energies E greater than 45 keV.
VERTICAL REDISTRIBUTION OF A POLLUTANT TRACER DUE TO CUMULUS CONVECTION
Mathematical formalisms that incorporate the physical processes responsible for the vertical redistribution of a conservative pollutant tracer due to a convective cloud field are presented. Two modeling approaches are presented differing in the manner in which the cloud fields ar...
Vertical transport by convective clouds: Comparisons of three modeling approaches
NASA Technical Reports Server (NTRS)
Pickering, Kenneth E.; Thompson, Anne M.; Tao, Wei-Kuo; Rood, Richard B.; Mcnamara, Donna P.; Molod, Andrea M.
1995-01-01
A preliminary comparison of the GEOS-1 (Goddard Earth Observing System) data assimilation system convective cloud mass fluxes with fluxes from a cloud-resolving model (the Goddard Cumulus Ensemble Model, GCE) is reported. A squall line case study (10-11 June 1985 Oklahoma PRESTORM episode) is the basis of the comparison. Regional (central U. S.) monthly total convective mass flux for June 1985 from GEOS-1 compares favorably with estimates from a statistical/dynamical approach using GCE simulations and satellite-derived cloud observations. The GEOS-1 convective mass fluxes produce reasonable estimates of monthly-averaged regional convective venting of CO from the boundary layer at least in an urban-influenced continental region, suggesting that they can be used in tracer transport simulations.
Modeling lava lake heat loss, rheology, and convection
NASA Astrophysics Data System (ADS)
Harris, Andrew J. L.
2008-04-01
Measurements at Erta Ale's lava lake and theoretical equations for lake rheology, density driven convection and thermally-driven plume ascent allow the constraint of lake dynamics. Cooling and crystallization expected from surface heat losses imply a viscosity increase from 150 Pa s to 300-1800 Pa s for cooled surface layers. Convection is expected to proceed vigorously under low viscosity conditions driving rapid (0.1-0.4 m s-1) surface motions and sluggishly under moderate-to-high viscosity conditions to drive slower motions (<0.08 m s-1). Convection is likely driven by small (~6 kg m-3) density differences, where surface cooling can influence lake rheology and explain variable rates of surface convective motion.