Browsing by Subject "Convection"
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Item A study of the relation between surface convergence and convective initiation along the dryline(Texas Tech University, 1998-12) Jones, AndrewThe frequency of shower and thunderstorm development is known to be greater near the dryline than in areas further removed. The dryline is a common feature across New Mexico and West Texas during the springtime and is often associated with severe weather across the region. The exact forcing mechanism that initiates convection along the dryline is not known. Recent studies suggest that localized areas of surface convergence and moisture convergence along the dryline may be the primary initiating mechanism. This thesis is an observational study that compared where convection occurred along the dryline to where areas of localized convergence and moisture convergence existed. Three similar dryline cases were examined. It was found that convection initiated in close proximity to areas of maximum convergence in two out of the three cases.Item Convective heat transfer in rooms with ceiling slot diffusers(2009-12) Goldstein, Kaitlin Ryan; Novoselac, Atila; Siegel, Jeffrey A.Convection at the interior surface of a building represents a significant portion of the heat transfer in office buildings with large glazing areas. While a large number of these office buildings utilize ceiling slot diffusers at the glazed building perimeter, convection correlations specific to these diffusers have not yet been investigated. This paper describes convection correlations developed for ceiling slot diffusers and examines the effect of temperature, various window geometries, and diffuser jet momentum on these correlations. The paper also examines the effect of venetian blinds on the overall correlations at various blind angle configurations: open, partially open, and closed. The results of the examined phenomena are validated in both heating and cooling conditions. All together, this paper represents the effort of over 100 individual experiments. The results show that forced convection is dominant at all air flow rates, and correlations are developed as a function of air volumetric flow rate with supply air temperature utilized as the reference. The correlations are found to rely only on window position, and are independent of temperature difference between surface and supply, diffuser position, and diffuser jet momentum. With respect to the blinds, the only relevant parameter is the angle of the blinds except when the blinds are open. When the blinds are open and at 45º, convective heat transfer is enhanced. Conversely, convection is decreased when the blinds are closed and at -45º. There is also a decrease in the convective heat transfer with a full window in contrast to a half window when the blinds are open. Finally, there is little difference between the convection correlations developed for heated and cooled environments.Item Effects of particle concentration and surfactant use in convective heat transfer of CuO nanofluids in microchannel flow(2011-05) Byrne, Matthew Davidson; da Silva, Alexandre K., 1975-; Hidrovo Chavez, Carlos H.Heat exchange systems used in everything from cars to microelectronics have rapidly advanced in recent years to offer high heat transfer rates in increasingly smaller sizes. However, these systems have become essentially optimized using conventional heat transfer fluids. To test the viability of nanofluids as a new heat transfer fluid, an experimental investigation was designed using a constant pressure drop configuration to drive flow into a heated square microchannel test section. The experimental trials included seven different test fluids tested over varying concentrations and surfactant use. Two identical test sections were used to collect results on heat transfer rates, pressure drop, mass flowrate and pumping power for all fluids. These results show a heat transfer improvement for nanofluids of 8-16% over pure water, with no meaningful increase in pumping power. This result is highly desirable, as it indicates an easily obtainable heat transfer improvement without an associated pumping cost increase. Importantly, the experiment shows the potential viability of nanofluids for heat transfer applications, while acknowledging limitations such as long term nanofluid stability.Item Ensemble sensitivity analysis applied to Southern Plains convection(2013-08) Bednarczyk, Christopher N.; Ancell, Brian A.; Weiss, Christopher C.; Kang, Song-LakThe recent increase in use of ensembles in numerical weather prediction has led to new information being available to forecasters, including uncertainty statistics and probabilistic guidance. Ensemble Sensitivity Analysis (ESA) offers additional information that describes the relationship between a forecast metric known as the response function and initial or early forecast errors, and it is capable of revealing features of the flow that are dynamically relevant to the chosen forecast. The applicability of ESA to a high resolution convection forecast of April 2012 is investigated with an Ensemble Kalman Filter based on the Weather Research and Forecasting model. It is shown that forecasts of convection are primarily sensitive to positional differences in the synoptic-scale flow. The selection of the response function is also explored to determine how to choose a convective forecast metric. Sensitivity does vary with the choice of response, but the same features tend to be highlighted in all cases. Sensitivity is also compared with a standardized form in which the raw value is weighted by the ensemble spread in order to determine the merit of each type. The standardized sensitivity provides information on expected forecast error, and it reveals features that are not highlighted in the raw sensitivity. In addition, a cross-grid approach to sensitivity is studied in order to determine if it shows similar results as the same-grid method. Results show them to have differences, but the cross-grid method still reveals realistic features in the context of the event.Item Experimental study of convective dissolution of carbon dioxide in porous media(2014-12) Liang, Yu, active 21st century; DiCarlo, David Anthony, 1969-Geological carbon dioxide (CO₂) capture and storage in geological formations has the potential to reduce anthropogenic emissions. The viability of technology depends on the long-term security of the geological CO₂ storage. Dissolution of CO₂ into the brine, resulting in stable stratification, has been identified as the key to long-term storage security. The dissolution rate determined by convection in the brine is driven by the increase of brine density with CO₂ saturation. Here we present a new analog laboratory experiment system to characterize convective dissolution in homogeneous porous medium. By understanding the relationship between dissolution and the Rayleigh number in homogeneous porous media, we can evaluate if convective dissolution occurs in the field and, in turn, to estimate the security of geological CO₂ storage fields. The large experimental assembly will allow us to quantify the relationship between convective dynamics and the Rayleigh number of the system, which could be essential to trapping process at Bravo Dome. A series of pictures with high resolution are taken to show the existence and movement of fingers of analog fluid. Also, these pictures are processed, clearly showed the concentration of analog fluid, which is essential to analyze the convective dissolution in detail. We measured the reduction in the convective flux due to hydraulic dispersion effect compared to that in homogeneous media, to determine if convective dissolution is an important trapping process at Bravo Dome.Item Overshooting Convection from High-resolution NEXRAD Observations(2014-01-09) Solomon, DavidConvection can rapidly and irreversibly transport tropospheric air into the upper troposphere and, in some cases, through the tropopause into the lower stratosphere. Previous studies have shown that stratosphere-troposphere exchange of any kind can have a significant impact on the composition of the upper troposphere and lower stratosphere. This will in turn affect the climate, chemistry, and radiation budget of the atmosphere. Large scale transport events such as Rossby wave breaking, the Brewer-Dobson circulation, and tropopause folding have been well studied, but smaller-scale processes are less well understood. In order to understand the importance of convective transport on the composition of the lower stratosphere, it is necessary to know the frequency, magnitude, and location of overshooting convection events. A new method that combines radar reflectivities from individual radars into a three-dimensional composite with high vertical resolution is used to obtain storm top altitudes. These altitudes are compared to the lapse-rate tropopause height calculated using the ERA-Interim reanalysis to determine if a storm is overshooting. We compute a 1-year analysis of overshooting convection at three-hour intervals for 2004 for the continental U.S. east of the Rocky Mountains. Overshooting convection is most common over the high plains, and there is a pronounced seasonal and diurnal cycle present. The majority of overshooting systems occur during the warm season, and a diurnal maximum of overshooting occurs at 00 UTC. Analyzing the total volume of tropopause penetrating storms reveals that the largest amount of overshooting penetrates less than 1 km above the tropopause, and that the volume of overshooting drops off rapidly with increasing height above the tropopause.Item Quasi-Stationary Convective Systems Forming Perpendicular to, Above the Cold Pool of, Strong Bow Echoes(2012-10-19) Keene, Kelly M.The accurate prediction of warm-season convective systems, and the heavy rainfall and severe weather associated with them, remains a challenge for numerical weather prediction models. This study looks at one such circumstance in which back-building convection forms perpendicular to, and above the cold pool region behind strong bow echoes. We refer to this phenomenon as a "bow and arrow" because, on radar imagery, the two convective lines resemble an archer's bow and arrow. The "arrow" can extend over hundreds of kilometers and can cause damage from high winds, hail and flooding. Events of this nature pose a particular challenge to forecast because they require an accurate forecast of the earlier convection and the effects of that convection on the environment. In this study, radar and surface observations of four events are presented to identify common environmental conditions prior to the development of the back-building convection. Additionally, simulations of three cases using the Weather Research and Forecasting (WRF) model are analyzed in an attempt to understand the mechanisms responsible for initiating and maintaining the convective line. Due to coarse resolution, observational analyses are only useful for inspection of the synoptic-scale. Model output from numerical simulations is utilized to examine the mesoscale in the vicinity of the convective arrow. Several environmental characteristics are evident in each of the studied cases. Strong southwesterly flow (inducing warm air advection and gradual isentropic lifting), in addition to directional and speed convergence into the convective arrow region possibly contribute to convection initiation. Horizontal wind speed shear and increased wind speed in the area surrounding the arrow may be associated with the linear orientation of the arrow. It seems as though when these ingredients are combined with thermodynamic instability, there is a greater possibility of formation and maintenance of a convective arrow behind a bow echo.Item Semi-empirical model of convection heat transfer at windows and blinds near floor diffusers for use in building energy modeling(2010-08) Clark, Jordan Douglas; Novoselac, Atila; Siegel, JeffreyAccurate modeling of energy flows in buildings is necessary for optimization of mechanical systems, and architectural designs and components. One specific process which has been studied little is that of forced convection on the interior surfaces of window assemblies, which is present in the majority of newly constructed commercial buildings. To this end, energy flows associated with a specific Heating Ventilation and Air-Conditioning (HVAC) configuration- a floor register near a glass curtain wall with or without Venetian blinds- are analyzed experimentally and partially described with accepted theory. Natural convection at the same surface is analyzed as well, both to establish a baseline and to experimentally validate the experimental setup. A 60 cubic meter environmental chamber with precisely controlled interior conditions and electrical resistance heating panels is employed to study heat transfer at the interior surfaces of a building’s envelope. Convection heat transfer processes for various blind angles, HVAC regimes, surface temperatures, and window sizes are examined. Results show that convection at window and blind surfaces is highly dependent on blind angle, supply temperature and flow rate, moderately dependent on room-supply air temperature difference and HVAC regime, and weakly dependent on surface-supply air temperature difference. A simplified model of convection heat transfer in this particular situation is proposed for easy implementation in energy modeling software.Item The QBO's influence on lightning production and deep convection in the tropics(2009-05-15) Hernandez, Celina AnneVariations in characteristics of tropical deep convection are examined for an association with the stratospheric quasi-biennial oscillation (QBO). Eight years (1998-2005) of Tropical Rainfall Measuring Mission (TRMM) Lightning Imaging Sensor (LIS) flash densities and ten years (1998-2007) of TRMM Precipitation Radar (PR) deep convective and stratiform rainfall and convective echo top heights are analyzed. The QBO can be linked to deep convection through two hypothesized mechanisms: 1) modulation of tropopause height, which may affect the altitude that convection can penetrate; and 2) modulation of cross-tropopause shear, which may affect the vertical development of convection via shearing of cloud tops. Tropopause height and cross-tropopause shear is measured by National Centers for Environmental Prediction (NCEP) reanalysis 100 hPa temperatures and 50-200 hPa zonal wind shear, respectively. When partitioned by QBO east and west phases, zonal monthly mean anomalies and anomalous monthly mean difference maps illustrate a QBO signal in lightning flash rates, convective and stratiform rain amounts, and the number of convective echo tops > 12 km. QBO modulation of cross-tropopause shear causes 50-200 hPa shear east (west) phase anomalies to decrease (increase) about the equator and increase (decrease) in off-equator regions. QBO modulation of tropopause height induces a higher/colder (lower/warmer) tropopause near the equator during the east (west) phase. While the expectation was that decreases in cross-tropopause shear and tropopause temperatures at monthly time scales during the QBO would result in an increase of deep convective properties near the equator, observations suggest that deep convective properties may increase or decrease depending on the location and season. Similar to the QBO results, the increase or decrease of deep convective properties with general variations in cross-tropopause shear and tropopause temperatures depends on the location and season.