Ocean mixing beneath Pine Island Glacier ice shelf, West Antarctica

Ice shelves around Antarctica are vulnerable to an increase in ocean-driven melting, with the melt rate depending on ocean temperature and the strength of flow inside the ice-shelf cavities. We present measurements of velocity, temperature, salinity, turbulent kinetic energy dissipation rate, and thermal variance dissipation rate beneath Pine Island Glacier ice shelf, West Antarctica. These measurements were obtained by CTD, ADCP, and turbulence sensors mounted on an Autonomous Underwater Vehicle (AUV). The highest turbulent kinetic energy dissipation rate is found near the grounding line. The thermal variance dissipation rate increases closer to the ice-shelf base, with a maximum value found ∼0.5 m away from the ice. The measurements of turbulent kinetic energy dissipation rate near the ice are used to estimate basal melting of the ice shelf. The dissipation-rate-based melt rate estimates is sensitive to the stability correction parameter in the linear approximation of universal function of the Monin-Obukhov similarity theory for stratified boundary layers. We argue that our estimates of basal melting from dissipation rates are within a range of previous estimates of basal melting

Transitional free convection flows induced by thermal line sources

In the present study the usefullness of a large eddy simulation for transition is examined. Numerical results of such simulations are presented from a study to determine the characteristics of a flow induced by a thermal line source. The first bifurcation to time dependent motion and the route to chaos are considered. Qualitatively these features are in good agreement with theory. The governing equations, the concept of large eddy simulation and the numerical code that was used are described extensively. Also the results from a literature survey are presented. Special attention is paid to analytical solutions for the boundary layer equations for laminar flow and the stability of these solutions. It includes also overall conservation principles for turbulent plumes and results obtained by experiments

Application of Laser Techniques in Combustion Environments of Relevance for Gas Turbine Studies

In the work presented in this thesis, different laser-based techniques were employed for measurements in different combustion devices. Laser-based techniques enable non-intrusive and in-situ measurements to be carried out, in which high spatial and temporal resolution can be obtained. Different parameters related to combustion research can be visualized, such as species concentrations, temperature, velocities and particle sizes. The combustion devices investigated can be related in one way or another to gas turbine combustion, the measurements being performed in devices ranging from laboratory-scale burners to industrial gas turbine burners. The Triple Annular Research Swirler (TARS) is a laboratory burner that simulates the character of a gas turbine, in terms of both fuel injection and flame stabilization. Studies using planar laser-induced fluorescence (PLIF) were carried out here to demonstrate how different swirler configurations affect flame and flow dynamics and how flashback depends on different operating conditions. Simultaneous measurements of OH PLIF and of acetone PLIF were employed to visualize flame position and fuel distribution, respectively, the measurements being carried out simultaneously with velocity measurements involving particle image velocimetry (PIV). Visualization of flame position and of fuel distribution through use of OH PLIF and acetone PLIF was applied to several industrial gas turbine burners to investigate their combustion characteristics. The measurements were performed on-site at Siemens in Finsp\r{a}ng, in burners fueled with natural gas or with a mixture of natural gas and hydrogen. The aim of the experimental investigations was to obtain a better understanding both of the mixing of air and fuel and of the flame dynamics, knowledge of which can hopefully be used to further reduce the emission levels from gas turbines. Part of the experimental results was used for the validation of computational fluid dynamics (CFD) models, used to simulate the flow and the turbulent combustion inside the combustion chamber. A Multi-YAG laser system which can generate a rapid burst of laser pulses, and a high-speed framing camera capable of recording sequences of up to eight images, were used to study fuel vaporization, and its consequent mixing with air, in a Jet A or biojet fueled gas turbine pilot burner under elevated pressure conditions. Fuel PLIF was used to visualize both the liquid and the gas phase of the fuel, Mie scattering being used to visualize only the liquid phase of the fuel. The liquid phase of the fuel was found, as expected, to be close to the burner nozzle and the evaporated fuel to be found a distance downstream from the burner nozzle. High-speed OH PLIF was also employed for visualizing the flame position. Additional work carried out included characterization of partially premixed and diffusion flames in a high-pressure vessel and burner (HPVB) using OH PLIF and PAH PLIF for the visualization of flame position and of polyaromatic hydrocarbon distribution, respectively. Flames using liquid fuels (n-heptane, n-decane and ethanol) as well as gaseous (methane) fuels were studied. The results are intended to be used as data base for kinetic mechanisms and as validation data for CFD models

Heavy Vehicle Aerodynamics: Massively-Separated Turbulent Flow & a Bio-Inspired Device

Dr. MacChesney’s doctoral studies focused on improvement of the state of the art in heavy vehicle aerodynamics through computational fluid dynamics methods. Through his research he has devised a drag reduction device inspired by the shape of a harbor seal whisker that reduces drag by up to 22%. Additionally, through application of a novel decomposition method he uncovered a hidden order within massively-separated turbulent flows with evidence that supports a centrifugal mechanism

Two-Dimensional Flow and NOx Emissions in Deflagrative Internal Combustion Wave Rotor Configurations

A wave rotor is proposed for use as a constant volume combustor. A novel design feature is investigated as a remedy for hot gas leakage, premature ignition, and pollutant emissions that are possible in this class of unsteady machines. The base geometry involves fuel injection partitions that allow stratification of fuel/oxidizer mixtures in the wave rotor channel radially, enabling pilot ignition of overall lean mixture for low NOx combustion. In this study, available turbulent combustion models are applied to simulate approximately constant volume combustion of propane and resulting transient compressible flow. Thermal NO production histories are predicted by simulations of the STAR-CD code. Passage inlet/outlet/wall boundary conditions are time-dependent, enabling the representation of a typical deflagrative internal combustor wave rotor cycle. Some practical design improvements are anticipated from the computational results. For a large number of derivative design configurations, fuel burn rate, two-dimensional flow and emission levels are evaluated. The sensitivity of channel combustion to initial turbulence levels is evaluated

Acoustic Confinement and Characterization of a Microwave Plasma

High amplitude acoustic fields are used to confine, characterize, and manipulate collisional plasmas with temperatures of a few thousand Kelvin. This dissertation describes the theory, experimental techniques, and apparatus necessary both to generate high amplitude sound in a few thousand Kelvin plasma and to use that sound field to manipulate the plasma within a resonant acoustic cavity. The acoustic field in a spherically symmetric oscillating plasma has been measured to have a Mach number of .03, which is sufficient to cause acoustic radiation pressure effects to confine the plasma to the center of its container. This field also generates convection in the conducting gas, which we study by measuring its effect on the acoustic spectrum, watching the convection occur on high speed video, and by measuring the microwave signal reflected off of the convecting plasma. I also discuss how the varying electrical conductivity due to a high amplitude acoustic field in a plasma may enable a new type of 3D thermoacoustic oscillation