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Collisionless flows expanding into a vacuum from a circular slit are investigated using the gaskinetic theory. Both accurate and approximate expressions to the plume fields along a centerline and a constant radius are obtained, including the density, velocity components, and translational temperature. Special non-dimensional parameters such as the exit semi-opening angle, gas speed ratio at the exit, and positions are included in these expressions. Particle simulations with the direct simulation Monte Carlo method are performed to validate the above results. These expressions are complex and may need computers for evaluations; however, the cost is minor when compared to particle simulations. With the assumptions of a high or low exit speed ratio and small exit angle, simple explicit expressions for properties along the centerline and a constant radius are obtained. The investigations also find out that the cosine law relations can provide fast and crude estimations, but it is not recommended to use the cosine law relation to compute collisionless plume flowfields with a large exit speed. [ABSTRACT FROM AUTHOR]

This paper presents a simple model for slightly charged gas expanding into a vacuum from a planar exit. The number density, bulk velocity, temperature, and potential at the exit are given. The electric field force is assumed weaker than the convection term and is neglected in the analysis. As such, the quasi-neutral condition is naturally adopted and the potential field is computed with the Boltzmann relation. At far field, the exit degenerates as a point source, and simplified analytical formulas for flow and electric fields are obtained. The results are generic and offer insights on many existing models in the literature. They can be used to quickly approximate the flowfield and potential distributions without numerical simulations. They can also be used to initialize a simulation. Based on these results, more advanced models may be further developed. [ABSTRACT FROM AUTHOR]

We investigate the gas transport enhancement through nanotubes, relative to the prediction by the prevailing century-old Knudsen diffusion model. This enhancement is usually attributed to the partly specular molecular reflections at the smooth nanotube surface, which break the model assumption of completely diffusive reflections. However, an oversighted cause of the discrepancy between the measurement and theory that we found is that even for the gas transport with completely diffusive reflections, the Knudsen diffusion model based on Fick's first law is accurate only for long nanotubes. Additionally, for smooth nanotubes with partly specular reflections, the Knudsen diffusion model is also invalid even if the diffusion coefficient is corrected to account for the atomic-scale surface smoothness. On the other hand, the Knudsen diffusion model might be used for interpretations instead of predictions, and then the diffusion coefficient inferred from the measured mass flow rate could be completely different from the actual value. All those discrepancies and confusions stem from the implementation of Fick's first law can be avoided by using the molecular transmission probability obtained by the kinetic theory to quantify the flow rate of the Knudsen diffusion process. This work provides the correction to the Knudsen diffusion model for accurate predictions of gas diffusion through nanotubes and better interpretations of experimental measurements. [ABSTRACT FROM AUTHOR]

This paper presents recent investigation results on free molecular flows over a diffusely or specularly reflective ellipse, by using the gaskinetic theory. A virtual density distribution along the diffusely reflective surface is introduced to aid the investigations. Many local surface properties are obtained, including the surface slip velocity and coefficients for pressure, friction, and heat flux. Global coefficients for aerodynamic forces and moments and mass center–force center distances are also obtained by integrating the local surface distributions. In the end, analytical expressions for the flowfields around a diffusely or specularly reflective ellipse are also obtained. Special non-dimensional parameters, such as temperature and speed ratios, are explicitly embedded in these expressions. Particle simulations with the direct simulation Monte Carlo (DSMC) method are performed to validate the above results. Those expressions need computers for evaluations; however, the cost is very minor when compared with DSMC simulations. The approaches are heuristic to investigate other external collisionless flows, and the load coefficients can be considered as baseline references at the high Knudsen number limit. By using these collisionless flow results, it is feasible to further study less rarefied gas flows over an ellipse, for example, with an asymptotic method. Swift parameter studies based on these solutions can be performed to study their effect. [ABSTRACT FROM AUTHOR]

Quark nuggets are theoretical objects composed of approximately equal numbers of up, down, and strange quarks. They are also called strangelets, nuclearites, AQNs, slets, Macros, and MQNs. Quark nuggets are a candidate for dark matter, which has been a mystery for decades despite constituting ~ 85% of the universe's mass. Most previous models of quark nuggets have assumed no intrinsic magnetic field; however, Tatsumi found that quark nuggets may exist in magnetars as a ferromagnetic liquid with a magnetic field BS = 1012±1 T. We apply that result to quark nuggets, a dark-matter candidate consistent with the Standard Model, and report results of analytic calculations and simulations that show they spin up and emit electromagnetic radiation at ~ 104 to ~ 109 Hz after passage through planetary environments. The results depend strongly on the value of Bo, which is a parameter to guide and interpret observations. A proposed sensor system with three satellites at 51,000 km altitude illustrates the feasibility of using radio-frequency emissions to detect 0.003 to 1,600 MQNs, depending on Bo, during a 5 year mission. [ABSTRACT FROM AUTHOR]

This paper reports theoretical and numerical investigations on free molecular gas flows through microchannels. Both diffusely and specularly reflective channel surfaces are considered. Gas kinetic methods are adopted to develop the analytical solutions for surface and flowfield properties. The crucial steps include constructing the velocity distribution functions (VDFs) for points at the plate surfaces and inside flowfield and then completing the integration over the related velocity phases. For diffusely reflective surfaces, the VDFs are related to the densities and temperatures at the two exits and the plate, respectively. For surfaces with specular reflections, the VDFs at the plate surface and inside the flowfield are identical and are independent of the surface temperature ratio and the geometric aspect ratio. Based on the VDFs and velocity phases, surface property coefficients (e.g., Cp, Cf, and Cq) and flowfield properties (e.g., density, velocity components, and temperature) are obtained. For the diffusely reflective surface scenario, the mass flow rate can be approximated and the results include four non-dimensional parameters: the aspect ratio, the density ratio, and two temperature ratios. For the specularly reflective surface scenario, the surface and flowfield properties are uniform everywhere, and the channel aspect ratio and plate temperatures do not have any influence. Particle simulations with the direct simulation Monte Carlo method are performed, and essentially identical results validate the theoretical work. This work is heuristic and can be used to investigate less rarefied microchannel gaseous flows, for example, aid experimental measurement design for thermal transpiration flows. [ABSTRACT FROM AUTHOR]

This paper reports the recent development of a full-scale particle-in-cell (PIC) simulation package highlighting an efficient electro-statics (ES)-PIC implementation for spacecraft charging problems. Numerical simulations are crucial in studying plasma flows because analytical solutions are rare, and experiments are expensive. There are many types of plasma flow that need various numerical methods for efficient and accurate simulations; as such, how to implement and organize those different methods into one comprehensive simulation package is challenging. This work adopted several modern software design patterns and developed a versatile package that includes various PIC schemes. This package has an open architecture, clean interfaces with both serial and parallel simulation capabilities. Two benchmark test cases are included to demonstrate the capabilities of this package. Then two plasma flows around a positively charged probe are simulated, and the results are discussed. The simulation results are consistent with past simulation results, and new insights are obtained. This work can lead to the development and organization of more sophisticated plasma simulation solvers in the future. [ABSTRACT FROM AUTHOR]

The problem of a compressible gas flows over a flat plate with the velocity-slip and temperature-jump boundary conditions are being studied. The standard single- shooting method is applied to obtain the exact solutions for velocity and temperature profiles when the momentum and energy equations are weakly coupled. A double-shooting method is applied if these two equations are closely coupled. If the temperature affects the velocity directly, more significant velocity slip happens at locations closer to the plate's leading edge, and inflections on the velocity profiles appear, indicating flows may become unstable. As a consequence, the temperature-jump and velocity-slip boundary conditions may trigger earlier flow transitions from a laminar to a turbulent flow state. [ABSTRACT FROM AUTHOR]

This report presents improvements of investigations on the Tethered Satellite System (TSS)-1R electron current enhancement due to magnetic limited collections. New analytical expressions are obtained for the potential and temperature changes across the pre-sheath. The mathematical treatments in this work are more rigorous than one past approach. More experimental measurements collected in the ionosphere during the TSS-1R mission are adopted for validations. The relations developed in this work offer two bounding curves for these data points quite successfully; the average of these two curves is close to the curve-fitting results for the measurements; and an average of 2.95 times larger than the Parker-Murphy theory is revealed. The results indicate that including the pre-sheath analysis is important to compute the electron current enhancement due to magnetic limitations. [ABSTRACT FROM AUTHOR]

In this short note, we present some work on investigating electron temperatures and potentials in steady dilute plasma flows. The analysis is based on the detailed fluid model for electrons. Ionizations, normalized electron number density gradients, and magnetic fields are neglected. The transport properties are assumed as local constants. With these treatments, the partial differential equation for electron temperature degenerates as an ordinary differential equation. Along an electron streamline, two simple formulas for electron temperature and plasma potential are obtained. These formulas offer some insights, e.g., the electron temperature and plasma potential distributions along an electron streamline include two exponential functions, and the one for plasma potential includes an extra linear distribution function. [ABSTRACT FROM AUTHOR]

We analyze the near continuum flow created by a rotating disk facing a stagnant gas. The flow-field properties change from the traditional continuum solutions, due to the introductions of new velocity-slip and temperature-jump boundary conditions. To compute the velocity profiles, a self-similar transformation simplifies the Navier-Stokes equations into a system of ordinary differential equations. The introduction of new boundary conditions generates new parameters which can be adjusted at different degrees of rarefaction. Shooting methods are adopted to solve the differential equations with the new boundary conditions. Based on the solved velocity profiles, exact solutions for the temperature distribution are obtained. The gas temperature at the disk surface shifts towards the free stream temperature, while the heat flux between the gas and surface is reduced. Stream function solutions for the flow at the disk surface are presented to demonstrate the effects of the slip boundary conditions. The torque generated by the disk is obtained with different disk rotating speed, and the gas at the disk surface has different slip velocities. [ABSTRACT FROM AUTHOR]

In this paper, we investigate highly rarefied gaseous jet flows out of a planar exit and impinging at a normally set flat plate. Especially, we concentrate on the plate center stagnation point pressure and heat flux coefficients. For a specular reflective plate, the stagnation point pressure coefficient can be represented using two non-dimensional factors: the characteristic gas exit speed ratio S0 and the geometry ratio of H/L, where H is the planar exit semi-height and L is the center-to-center distance from the exit to the plate. For a diffuse reflective plate, the stagnation point pressure and heat flux coefficients involve an extra factor of T0/Tw, i.e., the ratio of exit gas temperature to the plate wall temperature. These results allow us to develop four diagrams, from which we can conveniently obtain the pressure and heat flux coefficients for the stagnation impingement point, at the collisionless flow limit. After normalization with these maximum coefficients, the pressure and heat flux coefficient distributions along the surface essentially degenerate to almost identical curves. As a result, with known plate surface pressure coefficient distributions and these diagrams, we can conveniently construct the heat flux coefficient distributions along the plate surface, and vice versa. [ABSTRACT FROM AUTHOR]

In this paper, we investigate rarefied jet gas flows out of a circular exit impinging on a vertical flat plate. We employ a constraint relation about the velocity components of gas particles leaving a nozzle exit point and arriving at a given spatial point outside the nozzle. This relation leads to several analytical expressions for collisionless flow property distributions on the plate surface, including density, slip-velocity, temperature, pressure, shear stress, and heat flux. Numerical simulation results obtained with the direct simulation Monte Carlo method validate the analytical collisionless flow solutions. The impingement properties on the plate surface are accurate when the Knudsen number is large. [ABSTRACT FROM AUTHOR]

In this paper, we investigate highly rarefied jet gas flows out of a two-dimensional slit that impinges on a flat plate which is set vertically to the plume flow direction. The plate is assumed to be completely diffusive and the gaseous plume flow out of the slit is modeled with a Maxwellian distribution function which is characterized with known number density, nonzero exit velocity, and temperature. We present some analytical collisionless flow properties for a free plume and the impingement on the plate. The outcomes include simple results of the largest shear stress on the plate and the corresponding location. Numerical simulation results obtained with the direct simulation Monte Carlo method validate the analytical collisionless flow solutions. [ABSTRACT FROM AUTHOR]

This article concerns collisionless gas flows over a flat cryogenic pump plate with a specific sticking probability. It presents exact density, velocity, temperature, and pressure solutions which are useful in determining the plate sticking probability. At any point off the plate, the local velocity distribution function (VDF) consists of several pieces of Maxwellian VDFs, one is characterized by free stream flow properties and the others by plate surface properties. Integrating the VDFs with different moments leads to these exact solutions, which are further validated by simulations with the direct simulation Monte Carlo (DSMC) method. We use these solutions to derive several useful expressions, based on special measured flow field properties at specific locations off the plate, to evaluate the pump sticking probability. Also, the exact solutions complement past studies of aerodynamic coefficients and heat transfer rate for collisionless gas flowing over a flat plate. They explicitly express the physical and geometrical factors. Evaluations of those solutions are much faster than DSMC simulations. This study can find other useful applications, e.g., cold gas spray for thin film depositions inside a vacuum chamber, and spacecraft plume impingement estimations. [ABSTRACT FROM AUTHOR]

This study analyzes heat transfer effects inside vacuum packaged microelectromechanical system (MEMS) devices. A packaged device is simplified as four plates forming a square cavity, the bottom plate represents a hot chip, while the other three plates are maintained at room temperature. For a highly rarefied free molecular internal gas flow scenario, the corresponding detailed density and temperature fields are analytically determined with a proposed speculation. This speculation indicates that for a steady free molecular gas flow inside a convex closure domain formed by walls maintained at different temperatures: (1) the velocity distribution functions for those molecules diffusely reflected at different walls and traveling away from them are Maxwellian with different number densities; (2) for each distribution, [formula] is a constant, where ni is the number density for the group of reflected molecules, and Ti is the temperature for the ith plate. For a near continuum flow scenario, the governing energy equation degenerates to Laplace’s equation with several temperature-jump wall boundary conditions. This study also includes discussions and comparisons among analytical results, simulation results from the direct simulation Monte Carlo method, and results by solving the Navier–Stokes equations with proper wall boundary conditions. The approach used in this study is generally applicable to study internal flows and heat transfer effects in other vacuum packaged MEMS devices with different shapes. [ABSTRACT FROM AUTHOR]

This short note presents concise semi-analytical expressions for electron temperature and potential in unsteady dilute cold plasma flows. The analysis is based on the detailed fluid model for electrons. Ionizations and normalized electron number density gradients are neglected, and the transport properties are assumed as local constants. Flow is unsteady and Maxwell's equations are adopted in the analysis. With these treatments, the partial differential equations for unsteady electron temperature and potential degenerate as ordinary differential equations. Along an electron streamline, two simple formulas for unsteady electron temperature and plasma potential are obtained. These formulas offer some insights, for example, the electron temperature and plasma potential distributions along an electron streamline include two exponential functions: one for spatial distance along a streamline and the other for time. [ABSTRACT FROM AUTHOR]