Continuum models in problems of the hypersonic flow of a rarefied gas around blunt bodiest

All possible continuum (hydrodynamic) models in the case of two-dimensional problems of supersonic and hypersonic flows around blunt bodies in the two-layer model (a viscous shock layer and shock-wave structure) over the whole range of Reynolds numbers, Re, from low values (free molecular and transitional flow conditions) up to high values (flow conditions with a thin leading shock wave, a boundary layer and an external inviscid flow in the shock layer) are obtained from the Navier-Stokes equations using an asymptotic analysis. In the case of low Reynolds numbers, the shock layer is considered but the structure of the shock wave is ignored. Together with the well-known models (a boundary layer, a viscous shock layer, a thin viscous shock layer, parabolized Navier-Stokes equations (the single-layer model) for high, moderate and low Re numbers, respectively), a new hydrodynamic model, which follows from the Navier-Stokes equations and reduces to the solution of the simplified (“local”) Stokes equations in a shock layer with vanishing inertial and pressure forces and boundary conditions on the unspecified free boundary (the shock wave) is found at Reynolds numbers, and a density ratio, k, up to and immediately after the leading shock wave, which tend to zero subject to the condition that (k/Re)^1/2 → 0. Unlike in all the models which have been mentioned above, the solution of the problem of the flow around a body in this model gives the free molecular limit for the coefficients of friction, heat transfer and pressure. In particular, the Newtonian limit for the drag is thereby rigorously obtained from the Navier-Stokes equations. At the same time, the Knudsen number, which is governed by the thickness of the shock layer, which vanishes in this model, tends to zero, that is, the conditions for a continuum treatment are satisfied. The structure of the shock wave can be determined both using continuum as well as kinetic models after obtaining the solution in the viscous shock layer for the weak physicochemical processes in the shock wave structure itself. Otherwise, the problem of the shock wave structure and the equations of the viscous shock layer must be jointly solved. The equations for all the continuum models are written in Dorodnitsyn--Lees boundary layer variables, which enables one, prior to solving the problem, to obtain an approximate estimate of second-order effects in boundary-layer theory as a function of Re and the parameter k and to represent all the aerodynamic and thermal characteristic; in the form of a single dependence on Re over the whole range of its variation from zero to infinity.

An efficient numerical method of global iterations, previously developed for solving viscous shock-layer equations, can be used to solve problems of supersonic and hypersonic flows around the windward side of blunt bodies using a single hydrodynamic model of a viscous shock layer for all Re numbers, subject to the condition that the limit (k/Re)^1/2 → 0 is satisfied in the case of small Re numbers. An aerodynamic and thermal calculation using different hydrodynamic models, corresponding to different ranges of variation Re (different types of flow) can thereby, in fact, be replaced by a single calculation using one model for the whole of the trajectory for the descent (entry) of space vehicles and natural cosmic bodies (meteoroids) into the atmosphere.     

An efficient computation of near-continuum rarefied gas flows

A numerical scheme for solving integral equations of rarefied gas dynamics is modified to provide more accurate results in the near continuum flows. The modified scheme is applied to the Poiseuille flow of a rarefied gas between parallel plates and in a cylindrical tube. Flow rate results obtained for both geometries are good over the complete range of Knudsen numbers. In particular, the results are found in excellent agreement with the analytic asymptotic formulas in the near continuum regime even when fairly low order of quadrature formulas are used.

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Zusammenfassung Eine numerische Methode zur Lösung der Integralgleichung für die Dynamik verdünnter Gase wird modifiziert, um genauere Resultate nahe zur Kontinuumsgrenze zu liefern; sie wird für die Poiseuille-Strömung von verdünnten Gasen zwischen parallelen Platten und im zylindrischen Rohr angewendet. Die Resultate für die Durchlaßmenge sind über den ganzen Bereich der Knudsen-Zahlen gut. Die Ergebnisse sind in besonders guter Übereinstimmung mit den asymptotischen Formeln nahe zum Kontinuum-Regime, selbst wenn Quadraturformeln von eher niedrigem Grade verwendet werden.

     

Computation of rarefied diatomic gas flows through a plane microchannel

A numerical method based on a model kinetic equation was developed for computing diatomic rarefied gas flows in two dimensions. Nitrogen flows through a plane microchannel were computed, and the gas flow rate was constructed as a function of the Knudsen number for various channel lengths.     

Numerical study of unsteady rarefied diatomic gas flows in a plane microchannel

The numerical solution of a kinetic equation for a diatomic gas (nitrogen) is used to study two-dimensional unsteady gas flows in a plane microchannel caused by discontinuous in the initial distributions of macroscopic gas parameters. The plane discontinuity fronts are perpendicular to the walls of the channel. The arising flows are model ones for gas flows in a shock tube and a microchannel. The reflection of an incident shock wave from a flat end face is studied. It is found that the gas piles up at the cold wall, which slows down the shock wave detachment. The numerical results are in qualitative agreement with experimental data.     

An abstract approach to evaporation models in rarefied gas dynamics

An efficient algorithm is presented for determining the unique solvability of certain one-dimensional stationary transport problems. The non-existence of stationary evaporation states with supersonic drift velocities for one and three dimensional BGK model is recovered.

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Sommario È presentato un algoritmo efficiente per determinare la solubilità univoca di alcuni probiemi unidimensionali di trasporto stazionario. È ripresa la non esistenza di stati stazionari di evaporazione con velocità di deriva supersoniche per modelli BGK a una e tre dimensioni.

     

The H matrix for time-dependent problems in rarefied gas dynamics

A system of singular integral equations and a set of integral constraints are shown to be uniquely solvable to yield the H matrix useful for half-space applications in time-dependent studies of the theory of rarefied gas dynamics. In addition some useful relationships concerning the H matrix are established.

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Zusammenfassung Es wird gezeigt, dass ein System von singulären Integralgleichungen unter gegebenen Bedingungen eindeutig gelöst werden kann, und eine H-Matrix liefert die für Halbraum-Anwendungen in zeitabhängigen Untersuchungen in der Theorie der verdünnten Gase brauchbar ist. Daneben werden noch einige nützliche Relationen, die die H-Matrix betreffen, abgeleitet.

     

Numerical method for computing two-dimensional unsteady rarefied gas flows in arbitrarily shaped domains

A high-order accurate method for analyzing two-dimensional rarefied gas flows is proposed on the basis of a nonstationary kinetic equation in arbitrarily shaped regions. The basic idea behind the method is the use of hybrid unstructured meshes in physical space. Special attention is given to the performance of the method in a wide range of Knudsen numbers and to accurate approximations of boundary conditions. Examples calculations are provided.     

Implicit numerical method for computing three-dimensional rarefied gas flows on unstructured meshes

The method based on the numerical solution of a model kinetic equation is proposed for analyzing three-dimensional rarefied gas flows. The basic idea behind the method is the use of a second-order accurate TVD scheme on hybrid unstructured meshes in physical space and a fast implicit time discretization method without iterations at the upper level. The performance of the method is illustrated by computing test examples of three-dimensional rarefied gas flows in variously shaped channels in a wide range of Knudsen numbers.     

Application of a momentum method to some rarefied gas flow problems

Zusammenfassung Eine Kombination der Lees-Methode und des Mott-Smith Ansatzes wird auf zwei Probleme der Couette-Strömung zwischen zwei Zylindern angewandt. Es werden Ausdrücke hergeleitet für das Moment im Fall, dass ein Zylinder rotiert, und für den Wärmefluss im Fall, dass beide Zylinder stationär sind, aber verschiedene Temperatur haben; diese Ausdrücke sind für alle Werte der Knudsenzahl gültig.     

Semiclassical lattice hydrodynamics of rarefied channel flows

The two-dimensional channel flows of gas of arbitrary statistics in the slip and transition regimes as characterized by the Knudsen number are studied using a newly developed semiclassical lattice Boltzmann method. The method is directly derived by projecting the Uehling-Uhlenbeck Boltzmann-BGK equations onto the tensor Hermite polynomials using moment expansion method. The intrinsic discrete nodes of the Gauss-Hermite quadrature provide the natural lattice velocities for the semiclassical lattice Boltzmann method. The mass flow rates and the velocity profiles are calculated for the three particle statistics over wide range of Knudsen numbers and the Knudsen minimum can be captured. The results indicate distinct characteristics of the effects of quantum statistics.     

Properties of simple model problems for reacting gas flows

The compressible Navier–Stokes equations for reacting gases are extremely complex. Simpler models have been considered, and for these completely non-physical propagation speeds have been observed. These model problems are stiff, meaning that several different scales are present in the solution. Numerical solution of non-reacting flows almost always involves addition of extra dissipation. It will be shown that this action will render a totally wrong propagation speed for a simple model equation of reacting flows. This problem will be accentuated by increasing stiffness of the problem. Existence and uniqueness of a solution to this model equation is proved. The dependence of the propagation speed on the viscosity and a term governing the stiffness (comparable to the reaction rate for a more complete model) is investigated. A remedy for the wrong propagation speed for this simple model equation is proposed such that the speed is correct although the front is smeared out.     

Poiseuille flow of a rarefied gas mixture

The flow of a rarefied binary gas mixture is governed by transport equations which correspond to a coupled set of Fredholm integral equations of the second kind.By means of the Fourier transform technique and by expanding the transformed kernels in terms or orthogonal functions the problem is reduced to the solution of a system of linear algebraic equations.As a significant application, the case of a plane Poiseuille flow of a binary mixture following Sirovich-Hamel's model is considered.The convergence of the method is excellent.

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Riassunto Il flusso di una miscela binaria di gas rarefatti è retto da equazioni del trasporto che corrispondono ad un sistema di equazioni integrali di Fredholm di seconda specie. Mediante la tecnica della trasformata di Fourier e sviluppando i kernel trasformati in termini di funzioni ortogonali, il problema si riduce alla soluzione di un sistema di equazioni algebriche lineari. Quale applicazione significative si considera il flusso piano di Poiseuille di una miscela binaria che segue il modello di Sirovich-Hamel. La convergenza del metodo e'eccellente.

     

Kinetic model for chemical reactions in hypersonic non-equilibrium flows

The interaction between vibrational relaxation and chemical processes is significant in many gas dynamic environments such as high-enthalpy air flows. Extending a work of Marrone and Treanor [1] the influence of the vibrational excitation is examined not only on dissociation reactions, but also on exchange reactions. A synthetic cross section is chosen which allows to obtain a relatively simple form of a coupling factor between experimentally measured rates, valid only in thermal equilibrium, and theoretically derived rates, valid also in strong thermal-nonequilibrium. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Unified solutions to some classical problems in rarefied gas dynamics based on the one-dimensional linearized S-model equations

An analytical version of the discrete-ordinates method is used to derive solutions for a class of problems defined in terms of the S-model kinetic equations. In addition to a general derivation, which is common to all the problems, specific analytical and computational aspects for each one of the problems are presented. In particular, numerical results for velocity profile, heat-flow profile and flow rates are obtained with high accuracy for the plane channel problems. In the case of half-space problems, the thermal and viscous-slip coefficients are also listed. Received: September 20, 2004; revised: March 31, 2005     

Simulation of rarefied gas flows on the basis of the Boltzmann kinetic equation solved by applying a conservative projection method

Flows of a simple rarefied gas and gas mixtures are computed on the basis of the Boltzmann kinetic equation, which is solved by applying various versions of the conservative projection method, namely, a two-point method for a simple gas and gas mixtures with a small difference between the molecular masses and a multipoint method in the case of a large mass difference. Examples of steady and unsteady flows are computed in a wide range of Mach and Knudsen numbers.     

Continuum eigenmodes in some linear stellar models

We apply parallel approaches in the study of continuous spectra to adiabatic stellar models. We seek continuum eigenmodes for the LAWE formulated as both finite difference and linear differential equations. In particular, we apply methods of Jacobi matrices and methods of subordinancy theory in these respective formulations. We find certain pressure-density conditions which admit positive-measured sets of continuous oscillation spectra under plausible conditions on density and pressure. We arrive at results of unbounded oscillations and computational or, perhaps, dynamic instability.     

An analytical approach to the unified solution of kinetic equations in rarefied gas dynamics. I. Flow problems

The ADO method, an analytical version of the discrete-ordinates method, is used to solve several classical problems in the rarefied gas dynamics field. The complete development of the solution, which is analytical in terms of the spatial variable, is presented in a way, such that, a wide class of kinetic models are considered, in an unified approach. A series of numerical results are showed and different simulations are used in order to establish a general comparative analysis based on this consistent set of results provided by the same methodology.