Open Access. Powered by Scholars. Published by Universities.®
Articles 1 - 10 of 10
Full-Text Articles in Engineering
Nonlinear Effects In Squeeze Film Gas Damping On Microbeams, S Chigullapalli, A Weaver, Alina A. Alexeenko
Nonlinear Effects In Squeeze Film Gas Damping On Microbeams, S Chigullapalli, A Weaver, Alina A. Alexeenko
School of Aeronautics and Astronautics Faculty Publications
We consider squeeze film gas damping during microbeam motion away and toward a substrate as occurs during opening and closing of RF switches and other MEMS devices. The numerical solution of the gas damping problem in two-dimensional geometries is obtained based on the Boltzmann-ESBGK equation. The difference in damping force between downward and upward moving beams is shown to vary from as little from as 5% for low beam velocities of 0.1m/s to more than 200% at 2.4m/s. For a constant velocity magnitude of 0.8m/s, this difference increases from 60% to almost 90% when the pressure is reduced by an …
A Study Of Field Emission Process In Electrostatically Actuated Mems Switches, L Michalas, A Garg, A Venkattraman, M Koutsoureli, Alina A. Alexeenko, Dimitrios Peroulis, G Papaioannou
A Study Of Field Emission Process In Electrostatically Actuated Mems Switches, L Michalas, A Garg, A Venkattraman, M Koutsoureli, Alina A. Alexeenko, Dimitrios Peroulis, G Papaioannou
School of Aeronautics and Astronautics Faculty Publications
A study of field emission process in MEMS-based capacitor/switch-like geometries is presented. High resolution current-voltage characteristics up to breakdown have been obtained across micro-gaps in fixed-fixed Metal-Air- Metal and Metal-Air-Insulator-Metal structures. In metallic devices the I-V dependence reveals Fowler –Nordheim theory effects. In the presence of insulator the process is found to be limited by the film conductivity following Poole –Frenkel dependence. The data analysis reveals the major importance of surface asperities on the onset of the field emission process while it is also presented that charge transfer may occur between metal and insulator surfaces even in the presence of …
Scaling Law For Direct Current Field Emission-Driven Microscale Gas Breakdown, A Venkattraman, Alina A. Alexeenko
Scaling Law For Direct Current Field Emission-Driven Microscale Gas Breakdown, A Venkattraman, Alina A. Alexeenko
School of Aeronautics and Astronautics Faculty Publications
The effects of field emission on direct current breakdown in microscale gaps filled with an ambient neutral gas are studied numerically and analytically. Fundamental numerical experiments using the particle-in-cell/Monte Carlo collisions method are used to systematically quantify microscale ionization and space-charge enhancement of field emission. The numerical experiments are then used to validate a scaling law for the modified Paschen curve that bridges field emission-driven breakdown with the macroscale Paschen law. Analytical expressions are derived for the increase in cathode electric field, total steady state current density, and the ion-enhancement coefficient including a new breakdown criterion. It also includes the …
Assessment Of High-Enthalpy Air Chemistry Models For Hypervelocity Ground-Based Experiments, Marat Kulakhmetov, Yevgeniy A. Bondar, Mikhail S. Ivanov, Alina A. Alexeenko
Assessment Of High-Enthalpy Air Chemistry Models For Hypervelocity Ground-Based Experiments, Marat Kulakhmetov, Yevgeniy A. Bondar, Mikhail S. Ivanov, Alina A. Alexeenko
School of Aeronautics and Astronautics Faculty Publications
Hypersonic vehicles re-entering Earth's atmosphere with orbital velocities generate gas flows with significant thermo-chemical non-equilibrium. Detailed kinetics of chemical reactions at such conditions is still not well understood but it may affect vehicle's thermal loads, aerodynamic moments and thermal radiation. This work assesses the Total Collision Energy (TCE) and the Kuznetsov-based state specific (KSS) chemical reaction models at pressures between 1 and 50 torr and velocities between 4 and 10 km/s in order to identify best test conditions for validating the models. Hypersonic flows at such conditions can be studied at the hypervelocity impact range facilities.
A Comparative Study Of No-Time-Counter And Majorant Collision Frequency Numerical Schemes In Dsmc, A Venkattraman, Alina A. Alexeenko, M A. Gallis, M S. Ivanov
A Comparative Study Of No-Time-Counter And Majorant Collision Frequency Numerical Schemes In Dsmc, A Venkattraman, Alina A. Alexeenko, M A. Gallis, M S. Ivanov
School of Aeronautics and Astronautics Faculty Publications
The direct simulation Monte Carlo (DSMC) method is a stochastic approach to solve the Boltzmann equation and is built on various numerical schemes for transport, collision and sampling. This work aims to compare and contrast two popular O(N) DSMC collision schemes - no-time-counter (NTC) and majorant collision frequency (MCF) - with the goal of identifying the fundamental differences. MCF and NTC schemes are used in DSMC simulations of a spatially homogeneous equilibrium gas to study convergence with respect to various collision parameters. While the MCF scheme forces the reproduction of the exponential distribution of time between collisions, …
Climatic Effects Of The Chicxulub Impact Ejecta, Devon Parkos, Marat Kulakhmetov, Brandon Johnson, Henry J. Melosh, Alina A. Alexeenko
Climatic Effects Of The Chicxulub Impact Ejecta, Devon Parkos, Marat Kulakhmetov, Brandon Johnson, Henry J. Melosh, Alina A. Alexeenko
School of Aeronautics and Astronautics Faculty Publications
Examining the short and long term effects of the Chicxulub impact is critical for understanding how life developed on Earth. While the aftermath of the initial impact would have produced harmful levels of radiation sufficient for eradicating a large portion of terrestrial life, this process does not explain the concurrent marine extinction. Following the primary impact, a large quantity of smaller spherules would de-orbit and re-enter the earths atmosphere, dispersed nearly uniformly across the planet. This secondary wave of debris would re-enter at high velocities, altering the chemical composition of the atmosphere. Furthermore, the combined surface area for the spherules …
Binary Scattering Model For Lennard-Jones Potential: Transport Coefficients And Collision Integrals For Non-Equilibrium Gas Flow Simulations, Ayyaswamy Venkattraman, Alina A. Alexeenko
Binary Scattering Model For Lennard-Jones Potential: Transport Coefficients And Collision Integrals For Non-Equilibrium Gas Flow Simulations, Ayyaswamy Venkattraman, Alina A. Alexeenko
School of Aeronautics and Astronautics Faculty Publications
A Lennard-Jones (LJ) binary interaction model for dilute gases is obtained by representing the exact scattering angle as a polynomial expansion in non-dimensional collision variables. Rigorous theoretical verification of the model is performed by comparison with exact values of diffusion and viscosity cross sections and related collision integrals. The collision quantities given by the polynomial approximation model agree within 3.5% with those of the exact LJ scattering. The proposed model is compared in detail with the generalized soft sphere (GSS) model which is the closest in terms of fidelity among existing direct simulation Monte Carlo collision models. The GSS model's …
A Runge-Kutta Discontinuous Galerkin Solver For 2d Boltzmann Model Equations: Verification And Analysis Of Computational Performance, Wei Su, Alina A. Alexeenko, Guobiao Cai
A Runge-Kutta Discontinuous Galerkin Solver For 2d Boltzmann Model Equations: Verification And Analysis Of Computational Performance, Wei Su, Alina A. Alexeenko, Guobiao Cai
School of Aeronautics and Astronautics Faculty Publications
The high-order Runge-Kutta discontinuous Galerkin (DG) method is extended to the 2D kinetic model equations describing rarefied gas flows. A DG-type discretization of the equilibrium velocity distributions is formulated for the Bhatnagar-Gross-Krook and ellipsoidal statistical models which enforce a weak conservation of mass, momentum and energy in the collision relaxation term. The RKDG solutions have up to 3rd-order spatial accuracy and up to 4th-order time accuracy. Verification is carried out for a steady 1D Couette flow and a 2D thermal conduction problem by comparison with DSMC and analytical solutions. The computational performance of the RKDG method …
Direct Measurements And Numerical Simulations Of Gas Charging In Microelectromechanical System Capacitive Switches, A Venkattraman, A Garg, D Peroulis, Alina A. Alexeenko
Direct Measurements And Numerical Simulations Of Gas Charging In Microelectromechanical System Capacitive Switches, A Venkattraman, A Garg, D Peroulis, Alina A. Alexeenko
School of Aeronautics and Astronautics Faculty Publications
Gas breakdown in microelectromechanical system capacitive switches is demonstrated using high resolution current measurements and by particle-in-cell/Monte Carlo collision (PIC/MCC) simulations. Measurements show an electric current through a 3 μm air gap increasing exponentially with voltage, starting at 60 V. PIC/MCC simulations with Fowler-Nordheim [Proc. R. Soc. London, Ser. A 119, 173 (1928)] field emission reveal self-sustained discharges with significant ion enhancement and a positive space charge. The effective ion-enhanced field emission coefficient increases with voltage up to about 0.3 with an electron avalanche occurring at 159 V. The measurements and simulations demonstrate a charging mechanism for microswitches …
Immersed Boundary Method For Boltzmann Model Kinetic Equations, Cem Pekardan, Sruti Chigullapalli, Alina A. Alexeenko
Immersed Boundary Method For Boltzmann Model Kinetic Equations, Cem Pekardan, Sruti Chigullapalli, Alina A. Alexeenko
School of Aeronautics and Astronautics Faculty Publications
Three different immersed boundary method formulations are presented for Boltzmann model kinetic equations such as Bhatnagar-Gross-Krook (BGK) and Ellipsoidal statistical Bhatnagar-Gross-Krook (ESBGK) model equations. 1D unsteady IBM solution for a moving piston is compared with the DSMC results and 2D quasi-steady microscale gas damping solutions are verified by a conformal finite volume method solver. Transient analysis for a sinusoidally moving beam is also carried out for the different pressure conditions (1 atm, 0.1 atm and 0.01 atm) corresponding to Kn=0.05,0.5 and 5. Interrelaxation method (Method 2) is shown to provide a faster convergence as compared to the traditional interpolation scheme …