Open Access. Powered by Scholars. Published by Universities.®
Articles 1 - 7 of 7
Full-Text Articles in Engineering
Flowfield Uncertainty Analysis For Hypersonic Cfd Simulations, A B. Weaver, Alina A. Alexeenko, R B. Greendyke, J A. Camberos
Flowfield Uncertainty Analysis For Hypersonic Cfd Simulations, A B. Weaver, Alina A. Alexeenko, R B. Greendyke, J A. Camberos
School of Aeronautics and Astronautics Faculty Publications
Uncertainty quantification (UQ) in the hypersonic flow regime offers valuable information to determine physical models in need of improvement and to assist in design of vehicles and flight experiments. Here we present results of UQ analysis based on polynomial chaos method to determine flowfield and surface heat flux uncertainty under typical blunt-body re-entry conditions. The NASA Langley code, LAURA, was used for axisymmetric CFD calculations of chemically reacting hypersonic flow over FIRE-II configuration. A third order polynomial chaos (PC) method using the Gauss-Hermite quadrature was applied for determining probability density functions and moments of output quantities. Input parameters such as …
Knudsen Force Modeling In Application To Microsystems, Jeremy S. Nabeth, Sruti Chigullapalli, Alina A. Alexeenko
Knudsen Force Modeling In Application To Microsystems, Jeremy S. Nabeth, Sruti Chigullapalli, Alina A. Alexeenko
School of Aeronautics and Astronautics Faculty Publications
At the microscale, even moderate temperature differences can result in significant Knudsen forces generated by the energy exchange between gas molecules and solids immersed in a gas. Creating, controlling and measuring Knudsen forces in microsystems can be an arduous task since only limited theory exists at present. This present study investigates the mechanism of Knudsen forces in detail based on numerical solution of the Boltzmann kinetic equation. The Knudsen force is shown, in general, to be a result of thermal nonequilibrium between gas and solid. The simulations are validated by comparison with experimental measurements that have been reported by Passian …
Experimental Determination Of The Key Heat Transfer Mechanisms In Pharmaceutical Freeze Drying, Arnab Ganguly, Steven L. Nail, Alina A. Alexeenko
Experimental Determination Of The Key Heat Transfer Mechanisms In Pharmaceutical Freeze Drying, Arnab Ganguly, Steven L. Nail, Alina A. Alexeenko
School of Aeronautics and Astronautics Faculty Publications
Freeze-drying is often used in manufacture of pharmaceuticals to remove a solvent in such a way that the sensitive molecular structure of the active substance of a drug is least disturbed, and to provide a sterile powder that can be quickly and completely rehydrated. In this work heat transfer rates in a laboratory-scale freeze-dryer have been measured to investigate the contribution of different heat transfer modes. Pure water was partially dried under low-pressure conditions and sublimation rates were determined gravimetrically. The heat transfer rates were observed to be independent of the separation distance between a product vial and a dryer …
What Determines Knudsen Force At The Microscale, Jeremy S. Nabeth, Sruti Chigullapalli, Alina A. Alexeenko
What Determines Knudsen Force At The Microscale, Jeremy S. Nabeth, Sruti Chigullapalli, Alina A. Alexeenko
School of Aeronautics and Astronautics Faculty Publications
Knudsen forces arise in microscale systems when there is a thermal gradient with a characteristic length scale comparable to the molecular mean free path of the ambient gas. These forces are sometimes referred as radiometric or thermo‐molecular forces [1] and have been recently measured experimentally in a microscale configuration using heated atomic force microscopy (AFM) probes [2]. The Knudsen force on microstructures with thermal gradients can provide a novel actuation mechanism for mass detection, thermogravimetry, and very high‐resolution heat flux measurements. While measuring such forces precisely at microscale can be an arduous task especially since only limited analytical results exist, …
Implications Of Rarefied Gas Damping For Rf Mems Reliability, Alina A. Alexeenko, Sruti Chigullapalli
Implications Of Rarefied Gas Damping For Rf Mems Reliability, Alina A. Alexeenko, Sruti Chigullapalli
School of Aeronautics and Astronautics Faculty Publications
Capacitive and ohmic RF MEMS switches are based on micron‐sized structures moving under electrostatic force in a gaseous environment. Recent experimental measurements [4, 5] point to a critical role of gas‐phase effects on the lifetime of RF MEMS switches. In this paper, we analyze rarefied flow effects on the gas‐damping behavior of typical capacitive switches. Several damping models based on Reynolds equation [7, 8] and on Boltzmann kinetic equation [9, 6] are applied to quantify the effects of uncertainties in fabrication and operating conditions on the impact velocity of switch contact surfaces for various switch configurations. Implications of rarefied flow …
Molecular Models For Dsmc Simulations Of Metal Vapor Deposition, A Venkattraman, Alina A. Alexeenko
Molecular Models For Dsmc Simulations Of Metal Vapor Deposition, A Venkattraman, Alina A. Alexeenko
School of Aeronautics and Astronautics Faculty Publications
The direct simulation Monte Carlo (DSMC) method is applied here to model the electron‐beam (e‐beam) physical vapor deposition of copper thin films. A suitable molecular model for copper‐copper interactions have been determined based on comparisons with experiments for a 2D slit source. The model for atomic copper vapor is then used in axi‐symmetric DSMC simulations for analysis of a typical e‐beam metal deposition system with a cup crucible. The dimensional and non‐dimensional mass fluxes obtained are compared for two different deposition configurations with non‐uniformity as high as 40% predicted from the simulations.
Direct Simulation Monte Carlo Modeling Of E-Beam Metal Deposition, A Venkattraman, Alina A. Alexeenko
Direct Simulation Monte Carlo Modeling Of E-Beam Metal Deposition, A Venkattraman, Alina A. Alexeenko
School of Aeronautics and Astronautics Faculty Publications
Three-dimensional direct simulation Monte Carlo (DSMC) method is applied here to model the electron-beam physical vapor deposition of copperthin films. Various molecular models for copper-copper interactions have been considered and a suitable molecular model has been determined based on comparisons of dimensional mass fluxes obtained from simulations and previous experiments. The variable hard sphere model that is determined for atomic copper vapor can be used in DSMC simulations for design and analysis of vacuum deposition systems, allowing for accurate prediction of growth rates, uniformity, and microstructure.