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An air data sensor (ADS) system is one of the key components in space transports. It can provide precious information on the pressure and the Mach number data that cannot be obtained from internal motion sensors. When a spaceship or a reentry vehicle is cruising at a very high altitude, the ADS will suffer problem of the time-delay in its pressure signals that propagates through a thin pressure tube. In this study, the time delay of the pressure propagation inside a thin tube under a low-pressure environment is evaluated based on the direct-simulation Monte-Carlo (DSMC) method. A CFD analysis based on axisymmetric Navier-Stokes equations is also conducted to evaluate the contribution of rarefied gas effects to the pressure propagation speed. These results are compared with experimental results. Based on the DSMC and CFD analyses, the time constant for the pressure change is determined to be around 0.013 seconds whereas the experimental results yielded 0.12 seconds when the pressure at the exit of the tube increases for about 20 to 40 Pa starting from 30 Pa. These results suggest that the propagation delay observed in the flight test of the atmospheric-entry capsule descending at an altitude of 79 km with Mach-4.5 flight velocity was partly caused by the rarefied gas effect. [ABSTRACT FROM AUTHOR]

Preliminary studies are made to determine the aerodynamic and accommodation characteristics of the wind tunnel model under hypersonic rarefied flow. In the method, the wind tunnel models are exposed to the test flow in the hypersonic rarefied wind tunnel in JAXA. The displacements of the models due to the aerodynamic forces are measured during the testing. The flowfield around the wind tunnel model in the test section is also analyzed by using the numerical calculation. In the calculation, the accommodation coefficient of wind tunnel model surface is parametrically changed. The accommodation parameter of the wind tunnel model surface is then deduced from the comparison between the calculation and the measurement. For the sphere wind tunnel model, agreement between measurement and calculation could be obtained by present method with the accommodation coefficient from 0.8 to 0.9. It is also found that the determination of accommodation parameter becomes more achievable by using the displacement along the flow direction rather than that along the horizontal direction. On the other hand, further detailed modeling is required in the numerical analysis for the flat plate model. The uncertainties in the force measurement in the HRWT are discussed in this paper. [ABSTRACT FROM AUTHOR]

A method of calculating rapidly forces and torques caused by rarefied gas flow acting on a space-craft is presented. The spacecraft surface is described mathematically in detail. Our results are in good agreement with results obtained by other methods. [ABSTRACT FROM AUTHOR]

Abstract: We present the results of an experimental study concerning the gas flow through a packed bed of spheres located between two concentric cylinders, in non isothermal conditions. This problem is prototypal for more complex geometries which can be found in Micro Electro-Mechanical Systems (MEMS) where the heat transfer is a crucial issue. In these applications the characteristic lengths are such that the gas flow regime could fall in almost rarefied regime where the features of heat and mass flows can be markedly different with respect to the continuum regime. We investigated a wide range of regimes, spanning from the compressible continuum flow to rarefied conditions with a variation of the peculiar Knudsen number of about three orders of magnitude, i.e. from Kn =10−3, which corresponds to almost continuum regime, to Kn ≈1 which falls in the transitional region. The heat transfer through the porous medium have been studied by the analysis of the temperature field and the heat flux as a function of the flow rate, the boundary conditions and the porous materials. [Copyright &y& Elsevier]

The kinetic S-model is used to study the steady rarefied gas flow through a long pipe of variable cross section joining two tanks with arbitrary differences in pressure and temperature. The kinetic equation is solved numerically by applying a second-order accurate conservative method on an unstructured mesh. The basic quantity to be computed is the gas flow rate through the pipe. The possibility of finding a solution based on the assumption of the plane cross sectional flow is also explored. The resulting solutions are compared with previously known results. [ABSTRACT FROM AUTHOR]

The direct simulation Monte Carlo method is used to study a plane-parallel supersonic gas flow through a grid formed by a series of parallel infinite cylinders. Characteristic features of the shock disturbance formation during the interaction of a supersonic flow with a permeable grid and the effect of this disturbance on the flow parameters behind the grid are revealed. The boundaries of the domain of supersonic flow breakup ahead of the grid and the laws of the total momentum loss on the grid are obtained. Kinetic and energetic characteristics of the flow behind the grid are determined. [ABSTRACT FROM AUTHOR]

The gas flow between two concentric rotating cylinders is considered in order to investigate non-equilibrium effects associated with the Knudsen layers over curved surfaces. We investigate the nonlinear flow physics in the near-wall regions using a new power-law (PL) wall-scaling approach. This PL model incorporates Knudsen layer effects in near-wall regions by taking into account the boundary limiting effects on the molecular free paths. We also report new direct simulation Monte Carlo results covering a wide range of Knudsen numbers and accommodation coefficients, and for various outer-to-inner cylinder radius ratios. Our simulation data are compared with both the classical slip flow theory and the PL model, and we find that non-equilibrium effects are not only dependent on Knudsen number and accommodation coefficient but are also significantly affected by the surface curvature. The relative merits and limitations of both theoretical models are explored with respect to rarefaction and curvature effects. The PL model is able to capture some of the nonlinear trends associated with Knudsen layers up to the early transition flow regime. The present study also illuminates the limitations of classical slip flow theory even in the early slip flow regime for higher curvature test cases, although the model does exhibit good agreement throughout the slip flow regime for lower curvature cases. Torque and velocity profile comparisons also convey that a good prediction of integral flow properties does not necessarily guarantee the accuracy of the theoretical model used, and it is important to demonstrate that field variables are also predicted satisfactorily. [ABSTRACT FROM AUTHOR]

In microducts deviation from continuum flow behaviour of a gas increases with rarefaction. When using Navier–Stokes equations to calculate a flow under slightly and moderately rarefied conditions, slip boundary conditions are used which in turn refer to the tangential momentum accommodation coefficient (TMAC). Here we demonstrate that, in the so-called slip and transition regime, the flow in microducts can be reliably described by a consistently non-empirical model without considering the TMAC. We obtain this equation by superposition of convective transport and Fickian diffusion using two-dimensional solutions of Navier–Stokes equations and a description for the Knudsen diffusion coefficient as derived from kinetic theory respectively. For a wide variety of measurement series found in the literature the calculation predicts the data accurately. Surprisingly only size of the duct, temperature, gas properties and inlet and outlet pressure are necessary to calculate the resulting mass flow by means of a single algebraic equation. From this, and taking the discrepancies of the TMAC concerning surface roughness and nature of the gases into account, we could conclude that neither the diffusive proportions nor the total mass flow rates are influenced by surface topology and chemistry at Knudsen numbers below unity. Compared to the tube geometry, the model slightly underestimates the flow rate in rectangular channels when rarefaction increases. Likewise, the dimensionless mass flow rate and the diffusive proportion of the total flow are distinctly higher in a tube. Thus the cross-sectional geometry has a significant influence on the transport mechanisms under rarefied conditions. [ABSTRACT FROM PUBLISHER]

The author opines on a research related to data on internal rarefied gas flows from hydrodynamic to free-molecular flow regime by F. Sharipov and V. Seleznev that was published in a 1998 issue of the periodical and mentions that entire system cannot be considered isolated.

We present a comparative study of gaseous microflow systems using the recently introduced Fokker-Planck approach and other methods such as: direct simulation Monte Carlo, lattice Boltzmann, and variational solution of Boltzmann-BGK. We show that this Fokker-Plank approach performs efficiently at intermediate values of Knudsen number, a region where direct simulation Monte Carlo becomes expensive and lattice Boltzmann becomes inaccurate. We also investigate the effectiveness of a recently proposed Fokker-Planck model in simulations of heat transfer, as a function of relevant parameters such as the Prandtl, Knudsen numbers. Furthermore, we present simulation of shock wave as a function of Mach number in transonic regime. Our results suggest that the performance of the Fokker-Planck approach is superior to that of the other methods in transition regime for rarefied gas flow and transonic regime for shock wave. [ABSTRACT FROM AUTHOR]

A response from the authors of an research related to data on internal rarefied gas flows from hydrodynamic to free-molecular flow regime, that was published in a 1998 issue is presented.

Computations of rarefied gas dynamical flows governed by the semiclassical Boltzmann ellipsoidal-statistical (ES) kinetic model equation using an accurate numerical method are presented. The semiclassical ES model was derived through the maximum entropy principle and conserves not only the mass, momentum and energy, but also contains additional higher order moments that differ from the standard quantum distributions. A different decoding procedure to obtain the necessary parameters for determining the ES distribution is also devised. The numerical method in phase space combines the discrete-ordinate method in momentum space and the high-resolution shock capturing method in physical space. Numerical solutions of two-dimensional Riemann problems for two configurations covering various degrees of rarefaction are presented and various contours of the quantities unique to this new model are illustrated. When the relaxation time becomes very small, the main flow features a display similar to that of ideal quantum gas dynamics, and the present solutions are found to be consistent with existing calculations for classical gas. The effect of a parameter that permits an adjustable Prandtl number in the flow is also studied. [ABSTRACT FROM AUTHOR]

Rarefied gas flows generated by resonating nanomechanical structures pose a significant challenge to theoretical analysis and physical interpretation. The inherent noncontinuum nature of such flows obviates the use of classical theories, such as the Navier-Stokes equations, requiring more sophisticated physical treatments for their characterization. In this Letter, we present a universal dynamic similarity theorem: The quality factor of a nanoscale mechanical resonator at gas pressure P0 is α times that of a scaled-up microscale resonator at a reduced pressure αP0, where α is the ratio of nanoscale and microscale resonator sizes. This holds rigorously for any nanomechanical structure at all degrees of rarefaction, from continuum through to transition and free molecular flows. The theorem is demonstrated for a series of nanomechanical cantilever devices of different size, for which precise universal behavior is observed. This result is of significance for research aimed at probing the fundamental nature of rarefied gas flows and gas-structure interactions at nanometer length scales. [ABSTRACT FROM AUTHOR]