arXiv subjects

Chunpei Cai

Publications and source records attributed to Chunpei Cai.

6 recordsLinked to original sources

arXiv2024

Numerical Investigations on Dilute Cold Plasma Potential and Electron Temperature

Simulation results are presented to demonstrate electron temperature and electrical potential development in dilute and cold plasma development. The simulation method is a hybrid method which adopted fluid model for electrons due to their high mobility, while heavy ions and neutrals are modelled with the direct simulation Monte Carlo and Particle-In-Cell methods. The flows include steady, starting-up and shutting-down scenarios. The goal is to illustrate the exponential behaviors which were predicted in several recently developed formulas. Those formulas include many coefficients related with local properties, and they are difficult to determine. Hence, those trends can only efficiently demonstrate by numerical simulations which are more convenient than experimental measurements. The results confirm several facts. For steady plasma flows, the electron temperature and potential profiles are smooth, very likely, they can be approximated with exponential functions. For unsteady flows, the property developing trends in the shutting down or starting-up processes change monotonically. Further, at locations with large gradients, the property change trends are less ideal than those formulas. This is consistent with the assumptions with which those formulas were developed.

physics.plasm-ph↗

arXiv2023

Fundamental Electron and Potential Relations in Dilute Plasma Flows

In this short note, we present some work on investigating electron temperatures and potentials in steady or unsteady dilute plasma flows. The analysis is based on the detailed fluid model for electrons. Ionization, 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, fundamental formulas for electron temperature and plasma potential are obtained. These formulas offer significant insights, 1). for steady flow, 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; 2). for unsteady flows, both the temporal and spatical parts include potential functions.

physics.plasm-ph↗

arXiv2020

Radio frequency emissions from dark-matter-candidate magnetized quark nuggets interacting with matter

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 ${\sim}85\%$ of matter. 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 between $10^{ 11}$ T and $10^{ 13}$ 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 ${\sim}10^{ 4}$ Hz to ${\sim}10^{ 9}$ Hz after passage through planetary environments. The results depend strongly on the value of $B_{o}$, 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 1600 MQNs, depending on Bo, during a five year mission.

astro-ph.HE↗

arXiv2020

Numerical study on thermal transpiration flows through a rectangular channel

Gaseous thermal transpiration flows through a rectangular micro-channel are simulated by the direct simulation BGK (DSBGK) method. These flows are rarefied, within the slip and transitional flow regimes, which are beyond many traditional computational fluid dynamic simulation schemes, such as those based on the continuum flow assumption. The flows are very slow and thus many traditional particle simulation methods suffer large statistical noises. The adopted method is a combination of particle and gas kinetic methods and it can simulate micro-flows properly. The simulation results of mass flow rates have excellent agreement with experimental measurements. In another case of 2D channel, the DSBGK comparisons with the DSMC result and the solution of Shakhov equation are also in very good agreement. Another finding from this study is that numerical simulations by including two reservoirs at the channel ends lead to appreciable differences in simulation results of velocity and pressure distributions within the micro-channel. This is due to the inhaling and exhaling effects of reservoirs at the channel ends. Even though excluding those reservoirs may accelerate the simulations significantly by using a single channel in simulations, special attentions are needed because this treatment may over-simplify the problem, and some procedures and results may be questionable. One example is to determine the surface momentum accommodation coefficient by using analytical solution of the mass flow rate obtained in a single-channel problem without the confinement effect of reservoirs at the two ends.

physics.flu-dyn↗

arXiv2020

Highly dilute gas flows over an ellipse

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 a diffusely reflective surface is introduced to aid the investigations. Many local surface properties are obtained, including surface slip velocity, coefficients for pressure, friction, and heat flux. Global coefficients for aerodynamic forces and moments, 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 a specularly reflective ellipse are also obtained. Special non-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. It is feasible to further study less rarefied gas flows over an ellipse. Swift parameter studies based on these solutions are feasible to study their influences.

physics.flu-dyn↗

arXiv2020

Highly Dilute Gas Flows Through A Non-Isothermal Planar Micro-Channel

This paper reports theoretical and numerical investigations on free molecular gas flows through micro-channels. Both diffusely and specularly reflective channel surfaces are considered. Gaskinetic 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 temperatures. For surfaces with specular reflections, the VDFs at the plate surface and inside the flowfield are identical, and independent of the surface temperature ratio and the geometric aspect ratio. Based on the VDFs and velocity phases, surface property coefficients (e.g., C_p, C_f, and C_q) 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 specularly reflective surface scenario, the surface and flowfield properties are uniform everywhere, the channel aspect ratio and plate temperatures do not have any influence. Particle simulations with the direct simulation Monte Carlo (DSMC) method are performed, and essentially identical results validate the theoretical work. This work is heuristic and can be used to investigate less rarefied micro-channel gaseous flows, for example, aid experimental measurement design for thermal transpiration flows.

physics.flu-dyn↗