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ESTIMATION OF TANGENTIAL MOMENTUM ACCOMMODATION COEFFICIENT USING MOLECULAR DYNAMICS SIMULATION
Title
ESTIMATION OF TANGENTIAL MOMENTUM ACCOMMODATION COEFFICIENT USING MOLECULAR DYNAMICS SIMULATION.
Creator
Finger, George, Kapat, Jayanta, University of Central Florida
Abstract / Description
The Tangential Momentum Accommodation Coefficient (TMAC) is used to improve the accuracy of fluid flow calculations in the slip flow regime. Under such conditions (indicated by Knudsen number greater than 0.001), the continuum assumption that a fluid velocity at a solid surface is equal to the surface velocity is inaccurate because relatively significant fluid "slip" occurs at the surface. Prior work has not led to a method to quickly estimate a value for TMAC - it is frequently assumed. In...
Show moreThe Tangential Momentum Accommodation Coefficient (TMAC) is used to improve the accuracy of fluid flow calculations in the slip flow regime. Under such conditions (indicated by Knudsen number greater than 0.001), the continuum assumption that a fluid velocity at a solid surface is equal to the surface velocity is inaccurate because relatively significant fluid "slip" occurs at the surface. Prior work has not led to a method to quickly estimate a value for TMAC - it is frequently assumed. In this work, Molecular Dynamics techniques are used to study the impacts of individual gas atoms upon solid surfaces to understand how approach velocity, crystal geometry and interatomic forces affect the scattering of the gas atoms, specifically from the perspective of tangential momentum. It is a logical step in the development of a comprehensive technique to estimate total coefficient values to be used by those investigating flows in micro- and nano-channels or on orbit spacecraft where slip flow occurs. TMAC can also help analysis in transitional or free molecular regimes of flow. The gas – solid impacts were modeled using Lennard Jones potentials. Solid surfaces were modeled with approximately 3 atoms wide by 3 atoms deep by 40 or more atoms long. The crystal surface was modeled as a Face Centered Cubic (100). The gas was modeled as individual free gas atoms. Gas approach angles were varied from 10° to 70° from normal. Gas speed was either specified directly or by way of a ratio relationship with the Lennard-Jones energy potential (Energy Ratio). In order to adequately model the trajectories and maintain conservation of energy, very small time steps (on the order of 0.0005 ô , where ô is the natural time unit) were used. For each impact the initial and final tangential momenta were determined and after a series of many impacts, a value of TMAC was calculated for those conditions. The modeling was validated with available experimental data for He gas atoms at 1770 m/s impacting Cu over angles ranging from 10° to 70°. The model agreed within 3% of the experimental values and correctly predicted that the coefficient changes with angle of approach. Molecular Dynamics results estimate TMAC values from a high of 1.2 to a low of 0.25, generally estimating a higher coefficient at the smaller angles. TMAC values above 1.0 indicate backscattering, which has been experimentally observed in numerous instances. The ratio of final to initial momenta, when plotted for a given sequence of gas atoms spaced across a lattice cycle typically follows a discontinuous curve, with continuous portions indicating forward and back scattering and discontinuous portions indicating multiple bounces. Increasing the Energy Ratio above a value of 5 tends to decrease the coefficient at all angles. Adsorbed layers atop a surface influence the coefficient similar to their Energy Ratio. The results provide encouragement to develop the model further, so as to be able in the future to evaluate TMAC for gas flows with Maxwell temperature distributions involving numerous impact angles simultaneously.
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Date Issued
2005
Identifier
CFE0000760, ucf:46567
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0000760
Characterization of Acoustic Modes in Aeroengines
Title
Characterization of Acoustic Modes in Aeroengines.
Creator
Otero, Michelle, Ahmed, Kareem, Kapat, Jayanta, Bhattacharya, Samik, University of Central Florida
Abstract / Description
Acoustic instabilities remain a key design concern faced in the development of liquid rocket engines. The interaction between the acoustic modes and the occurring combustion reactions can be detrimental to the engine. The fluctuating pressure waves resulting from the flame oscillations in the system can potentially lead to engine failure. For this reason, research in acoustic instabilities and methods to minimize the influences on the engine, has maintain interest in the aerospace community....
Show moreAcoustic instabilities remain a key design concern faced in the development of liquid rocket engines. The interaction between the acoustic modes and the occurring combustion reactions can be detrimental to the engine. The fluctuating pressure waves resulting from the flame oscillations in the system can potentially lead to engine failure. For this reason, research in acoustic instabilities and methods to minimize the influences on the engine, has maintain interest in the aerospace community. The scope of this study was to design, optimize and characterize acoustic behaviors of a scaled rocket combustion chamber simulating acoustic pressure waves. Tangential and longitudinal acoustic waves of the system were extracted and validated through analytical and computational fluids dynamics models. The results of this study will assist with the process of extracting dominant oscillation frequencies of a system essential in the design of acoustic suppression devices for attenuation of critical frequencies.
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Date Issued
2018
Identifier
CFE0007354, ucf:52081
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0007354