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At least 793 records · Page 44

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NASA NTRS1988

Dendritic growth in the presence of convection

The motion of the freezing front between a dendritic crystal and a supercooled liquid is studied using an interface evolution equation derived from a boundary integral transformation of the transient convective-diffusion equation. A new steady-state theory is introduced that incorporates the effects of convection in dendritic growth. It is shown that in the absence of capillary effects the shape of the crystal-melt interface is a paraboloid of revolution, similar to that found in situations where diffusion is the sole heat transfer mechanism. A relation between the supercooling, the product of the tip velocity and tip radius, and the strength of the flow is derived which reduces to the well-known Ivantsov theory in the absence of convection. A non-linear interface-tracking algorithm is developed and used to study the temporal and spatial evolution of the dendritic interface. The important role of capillarity and convection on the interface dynamics is established and the response of the interface to finite amplitude disturbances is examined for the first time. Tip splitting is identified as the dominant destabilization mechanism in the limit of zero surface tension. Finite surface tension leads to interface stabilization, irrespective of the magnitude and structure of the external perturbations. Finally, convection significantly decreases the magnitude of the freezing velocity.

Beaghton, Pantelis John↗

NASA NTRS1987

Ionospheric convection signatures and magnetic field topology

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.

Coley, W. R.↗

NASA NTRS1988

Trace gas exchanges and convective transports over the Amazonian rain forest

The NASA Amazon Boundary Layer Experiment (ABLE 2A) based in Manaus, Brazil, in July and August 1985, is used to examine meteorological processes responsible for the vertical and horizontal transport of biogenic and anthropogenic trace gases generated over the Amazon basin. Direct sampling of the surrounding environment of deep convective clouds shows marked changes in the vertical distribution of the lower and midtroposphere concentration of O3 and such surface-derived species as CO, CO2, and NO. Thermodynamic observations, together with two-dimensional cloud model simulations, confirm vertical transports within the convection and provide a basis for an estimation of the magnitude and efficiency of cloud upward and downward exchanges. A distinction is drawn between local changes due to convective updrafts and downdrafts and convective overturning as a net result of the storm processes. Marked variability is seen in trace gas concentrations along horizontal flight paths in the vicinity of the convection. Interpretation of simultaneously measured thermodynamic quantities and trace gas concentrations provide the information to infer the presence and direction of atmospheric transports and/or the presence of anthropogenic influences.

Garstang, Michael↗

NASA NTRS1987

A comparison of the reduction of convection by a magnetic field and a microgravity environment

The relative effectiveness of magnetic damping and the microgravity environment of space for suppressing buoyancy driven convection in semiconductor crystal melts was analyzed using flow estimates based on the convection driven by a single vertical heated plate. The results show that convection can be more than a factor of 10 weaker in space. If a strong magnetic field is applied in space, the convection can be reduced by a factor of 1000 over similarly magnetically damped convection in an earth-bound laboratory. The heated plate estimates were checked by comparing them with exact boundary layer theory for the nonmagnetic problem in the same configuration and they were found to be accurate.

Fowlis, William W.↗

NASA NTRS1987

Properties of deep convective clouds in the ISCCP Pilot Data Set

Statistics on tropical deep convective clouds in the International Satellite Cloud Climatology Project Pilot Data Set are compiled by binning daytime data over areas of 2 X 2.5, 4 X 5, and 8 X 10 degrees. To isolate the convective clouds, only pixels with minimum visible optical thickness of 32 and maximum cloud top pressure of 550 mb are considered. Maps of convective cloud cover and frequency of convective events and frequency histograms of deep convective cloud pixels and cloud top pressures are presented.

Del Genio, Anthony D.↗

NASA NTRS1989

Direct numerical simulations of turbulent convection with a variable gravity and Keplerian rotation

Thermal convection was proposed as a possible mechanism for generation and maintenance of turbulence in the inner accretion disk regime of the primordial solar nebula. It is of fundamental interest to design experiments with the basic physical features of the solar nebula conditions cannot be produced in the laboratory, numerical simulations of hydrodynamic flows, which have been very successful in describing aerodynamic flows, can be suitable modified to provide experimental data for solar nebula modelling. The goals are to modify an extant, proven hydrodynamics code with the most important features of the solar nebula and other thin accretion disks: bouyancy terms to generate convection, internal heating representing the release of gravitational potential energy, a variable gravity linearly proportional the the distance from the vertical midplane due to centrifugal balance, rapid rotation with axis aligned with gravity, and Keplerian rotational shear; to determine the effect that these features have on the turbulent convection by introducing them individually and to determine the cumulative nature of the turbulent convection for accretion disk conditions; and to model the convection and the turbulence. In this manner, prior solar nebula models can be tested and their deficiencies rectified.

Cabot, William H.↗

NASA NTRS1989

Interactions among radiation, convection, and large-scale dynamics in a general circulation model

The effects of radiatively active clouds on the climate simulated by a GCM are studied, focusing on the effects of upper tropospheric stratiform clouds associated with deep cumulus convection and the interactions of these clouds with convection and the large-scale circulation. The isolated effects of atmospheric cloud radiative forcing are analyzed using a model climate of an ocean-covered earth. It is found that the upper tropospheric cloudiness associated with cumulus convection has a powerful influence on both the convection and the large-scale circulation. Consideration is given to the importance of developing a GCM which includes a direct representation of the radiative, thermodynamical, and dynamical couplings between the convection parameterization and the large-scale saturation parameterization.

Randall, David A.↗

NASA NTRS1989

Thermocapillary convection in zone-melting crystal growth - An open-boat physical simulation

Thermocapillary convection in a molten zone of NaNO3 contained in a boat with a free horizontal surface, that is heated from above by a centered wire heater, was studied to simulate flow in zone-melting crystal growth. Using a laser-light-cut technique and fine SiO powder as a tracer, convection in the melt zone was visualized in two different cases. In the first case, the entire melt surface was free, while in the second the melt surface was free only in the immediate vicinity of one vertical wall and was covered elsewhere, this wall being to simulate the melt/crystal interface during crystal growth. It was observed that thermocapillary convection near this wall prevailed in the first case, but was reduced significantly in the second. Since thermocapillary rather than natural convection dominated in the melt, the effect of the partial covering of the melt surface on thermocapillary convection in the melt observed in this study is expected to be similar under microgravity.

Kim, Y. J.↗

NASA NTRS1989

Numerical analysis of natural convection in liquid droplets by phase change

A numerical analysis is performed on thermocapillary buoyancy convection induced by phase change in a liquid droplet. A finite-difference code is developed using an alternating-direction implicit (ADI) scheme. The intercoupling relation between thermocapillary force, buoyancy force, fluid property, heat transfer, and phase change, along with their effects on the induced flow patterns, are disclosed. The flow is classified into three types: thermocapillary, buoyancy, and combined convection. Among the three mechanisms, the combined convection simulates the experimental observations quite well, and the basic mechanism of the observed convection inside evaporating sessile drops is thus identified. It is disclosed that evaporation initiates unstable convection, while condensation always brings about a stable density distribution which eventually damps out all fluid disturbances. Another numerical model is presented to study the effect of boundary recession due to evaporation, and the 'peeling-off' effect (the removal of the surface layer of fluid by evaporation) is shown to be relevant.

Duh, J. C.↗

NASA NTRS1990

Moist convection and the vertical structure and water abundance of Jupiter's atmosphere

The cumulative effects of an ensemble of moist convective plumes on a conditionally unstable atmosphere are predicted by a model of moist convection on Jupiter in which the heating/cooling and drying/moistening of the environment occur through (1) compensating subsidence, (2) detrainment of updraft air at cloud tops, and (3) the evaporation and melting of falling condensate. Parahydrogen is transported as a passive tracer. Pure moist convective, mixed moist-dry convective, and primarily dry convective regimes are possible, depending on the assumed deep-water abundance, efficiency of condensate evaporation, and initial temperature profile.

Del Genio, Anthony D.↗

NASA NTRS1990

Simultaneous conjugate observations of dynamic variations in high-latitude dayside convection due to changes in IMF By

Data from two conjugate HF radars currently operating at Goose Bay (Labrador) and the Halley Station (Antarctica), obtained for a single 45-min period about local noon on April 22, 1988, were used to study the near-instantaneous conjugate two-dimensional patterns of plasma convection in the vicinity of the cusp. In particular, the response of these plasma convection patterns to changes in the By component of the IMF was examined. Results indicate that, under quasi-stationary IMF conditions, the conjugate convection patterns are quite similar to the synthesized patterns of Heppner and Maynard (1987) and that the patterns respond rapidly to changes in the IMF By component. Results also show that transitions between convection states begin to occur within minutes of the time that an IMF state change is incident on the magnetospheric boundary, and that the convection reconfigurations expand poleward, completely filling the field of view of an HF radar within 6 min of the time of onset.

Greenwald, R. A.↗

NASA NTRS1990

Convective dynamics

In addressing meteorological processes, including convection, it is convenient to separate these processes into their kinematic, dynamic, and microphysical components. Dynamic processes deal with the forces that initiate and sustain or alter motion, while kinematic processes describe the motion field. Microphysical processes address the distribution of water substance. This review of convective dynamics is organized along the following lines. First is an assessment of radar capability, particularly the use of one or more Doppler radars, to portray the wind field. Then various types of precipitating moist convection are reviewed. Each type or mode of convection results from a distinct environment. The principal interest of dynamics here is its role in the convection's evolution, structure, and movement. Thus, the important processes of storm rotation, splitting and propagation, including tornadogenesis, are examined.

Ray, Peter S.↗

NASA NTRS1991

Development of a laser-induced heat flux technique for measurement of convective heat transfer coefficients in a supersonic flowfield

A technique is developed to measure the local convective heat transfer coefficient on a model surface in a supersonic flow field. The technique uses a laser to apply a discrete local heat flux at the model test surface, and an infrared camera system determines the local temperature distribution due to heating. From this temperature distribution and an analysis of the heating process, a local convective heat transfer coefficient is determined. The technique was used to measure the load surface convective heat transfer coefficient distribution on a flat plate at nominal Mach numbers of 2.5, 3.0, 3.5, and 4.0. The flat plate boundary layer initially was laminar and became transitional in the measurement region. The experimental results agreed reasonably well with theoretical predictions of convective heat transfer of flat plate laminar boundary layers. The results indicate that this non-intrusive optical measurement technique has the potential to obtain high quality surface convective heat transfer measurements in high speed flowfields.

Porro, A. Robert↗

NASA NTRS1991

Analysis and modeling of summertime convective cloud and precipitation structure over the Southeastern United States

A summary of an investigation of deep convective cloud systems that typify the summertime subtropical environment of northern Alabama is presented. The major portion of the research effort included analysis of data acquired during the 1986 Cooperative Huntsville Meteorological Experiment (COHMEX), which consisted of the joint programs Satellite Precipitation and Cloud Experiment (SPACE) under NASA direction, the Microburst and Service Thunderstorm (MIST) Program under NSF sponsorship, and the FAA-Lincoln Laboratory Weather Study (FLOWS). This work relates closely to the SPACE component of COHMEX, one of the general goals of which was to further the understanding of kinematic and precipitation structure of convective cloud systems. The special observational plateforms that were available under the SPACE/COHMEX Program are shown. The original objectives included studies of both isolated deep convection and of (small) mesoscale convection systems that are observed in the Southeast environment. In addition, it was proposed to include both observational and comparative numerical modeling studies of these characteristic cloud systems. Changes in scope were made during the course of this investigation to better accommodate both the manpower available and the data that was acquired. A greater emphasis was placed on determination of the internal structure of small mesoscale convective systems, and the relationship of internal dynamical and microphysical processes to the observed cloud top behavior as inferred from GOES IR (30 min) data. The major accomplishments of this investigation are presented.

Knupp, Kevin R.↗

NASA NTRS1991

A laser-induced heat flux technique for convective heat transfer measurements in high speed flows

A technique is developed to measure the local convective heat transfer coefficient on a model surface in a supersonic flow field. The technique uses a laser to apply a discrete local heat flux at the model test surface, and an infrared camera system determines the local temperature distribution due to the heating. From this temperature distribution and an analysis of the heating process, a local convective heat transfer coefficient is determined. The technique was used to measure the local surface convective heat transfer coefficient distribution on a flat plate at nominal Mach numbers of 2.5, 3.0, 3.5, and 4.0. The flat plate boundary layer initially was laminar and became transitional in the measurement region. The experimentally determined convective heat transfer coefficients were generally higher than the theoretical predictions for flat plate laminar boundary layers. However, the results indicate that this nonintrusive optical measurement technique has the potential to measure surface convective heat transfer coefficients in high speed flow fields.

Porro, A. R.↗

NASA NTRS1990

Behavior of deep convective clouds in the tropical Pacific deduced from ISCCP radiances

The characteristic features, the diurnal cycle, and the spatial distribution of deep convection over the equatorial Pacific and the relationship of deep convection to SST and surface-wind convergence were examined using a combined visible-IR (VS-IR) threshold method and an IR-only threshold method for diagnosing deep convection clouds (DCCs). Results suggest that deep convection is latitudinally confined to a much smaller spatial scale than that suggested by maps of outgoing long-wave radiation. The results suggested that there are two types of relationships between deep convection, SST, and surface-wind convergence: the west Pacific type and the east Pacific type. The latter relationship is observed in the east Pacific only when SST is not abnormally warm.

Fu, Rong↗

NASA NTRS1989

A divided meso-beta scale convectively explicit simulation

A convective event over the Gulf of Mexico is used to address the question of how well nonhydrostratic convectively explicit models initialized by imposing a 'bubble', typically a temperature perturbation, on a horizontally homogeneous stratification background field, respond to usually subtle variations on meso-beta scale thermodynamic structure. Whether a meso-beta scale convective precipitation pattern can be produced numerically by performing a group of convectively explicit simulations is studied. Storm-scale low-level forcing is found to be important for producing desired convective precipitation patterns. The additional numerical approaches that must be incorporated to understand and forecast meso-beta scale weather events are considered.

Song, J. Aaron↗

NASA NTRS1990

Average patterns of precipitation and plasma flow in the plasma sheet flux tubes during steady magnetospheric convection

Average patterns of plasma drifts and auroral precipitation in the nightside auroral zone were constructed during a steady magnetospheric convection (SMC) event on February 19, 1978. By comparing these patterns with the measurements in the midtail plasma sheet made by ISEE-1, and using the corresponding magnetic field model, the following features are inferred: (1) the concentration of the earthward convection in the midnight portion of the plasma sheet (convection jet); (2) the depleted plasma energy content of the flux tubes in the convection jet region; and (3) the Region-1 field-aligned currents generated in the midtail plasma sheet. It is argued that these three elements are mutually consistent features appearing in the process of ionosphere-magnetosphere interaction during SMC periods. These configurational characteristics resemble the corresponding features of substorm expansions (enhanced convection and 'dipolarized' magnetic field within the substorm current wedge) and appear to play the same role in regulating the plasma flow in the flux tubes connected to the plasma sheet.

Sergeev, V. A.↗