Rarefied Aerodynamics Research Articles

A statistical boundary for 3D rarefied flows through meshes: implementation to a new version of dsmcFoam+ and wind tunnel validation

AbstractThe Direct Simulation Monte Carlo (DSMC) method has become a standard tool for rarefied aerodynamics and microchannel flows. However, the performance benefits of DSMC, such as adaptive grid sizes and number of particles, are constrained by the need to resolve small geometric details of mesh applications within relatively large simulation volumes. The requirement for a sufficient number of particles in even the smallest cells imposes a significant computational burden. A novel set of cyclic statistical boundary conditions is proposed to address the computational bottleneck associated with simulating micrometre-scale structures prevalent in atmospheric and space research under rarefied flow conditions. These conditions account for the geometric parameters of a geometric mesh and the angular dependency of impacting particles, aiming to alleviate the computational challenges posed by conventional approaches. Validation against wind tunnel measurements demonstrates excellent agreement for one of the implemented boundaries, able to simulate fine meshes for conditions of rocket soundings in the Mesosphere. The newly developed boundary conditions are implemented within the advanced DSMC solver, dsmcFoam+ framework. For this study, the solver is ported from OpenFOAM® version 2.4.0 to the OpenFOAM® version v2306 to leverage recent code developments, particularly in dynamic meshes, load balancing, and barycentric particle tracking. This advancement enhances the capabilities of DSMC simulations, offering improved fidelity and accuracy in capturing rarefied flow phenomena.

Computer analysis of rarefied aerodynamics around a winged space-plane for Mars entry

The forthcoming use of Orion for Mars landing stimulated Zuppardi to compute global aerodynamic coefficients in rarefied flow along an entry path. Zuppardi and Mongelluzzo also studied Aerodynamics of a blunt cylinder, provided with flapped fins, as a possible alternative to Orion for Mars Entry, Descent and Landing. Computer tests were carried out, in the altitude interval 60-100 km, by three codes: i) home made code computing the entry trajectory, ii) Direct Simulation Monte Carlo code (DS2V), solving 2D/axisymmetric flow field and computing local quantities, iii) Direct Simulation Monte Carlo code (DS3V) solving 3D flow field and computing global aerodynamic coefficients. The comparison of the aerodynamic behaviour of the two capsules in axisymmetric flow field verified that heat flux and wall temperature for the finned-cylinder are higher than those of Orion. The DS3V results verified that Orion is better than the finned-cylinder to produce an aerodynamic force for slowing down the capsule. On the contrary, the results indicated that the finned-cylinder is better in terms of attitude control capability. The purpose of the present paper is to compare Aerodynamics of: Orion, finned-cylinder, a hypothetical, winged space-plane in high altitude Mars entry path. Computations were carried out by means of the two above mentioned DSMC codes, along both orbit and direct entry trajectories. While the global aerodynamic coefficients of the space-plane are comparable with those of the finned cylinder, the aerodynamic and thermal stresses (or pressure, temperature and heat flux) at the nose stagnation point are higher for the space-plane. Therefore, the finned-cylinder seems to be a valid alternative to Orion.

A novel lightweight aerodynamic design for the wings of hypersonic vehicles cruising in the upper atmosphere

Base on the concept of molecular mean free paths in high-altitude rarefied aerodynamics, a novel conceptual lightweight design is proposed and applied to the aerodynamic design of wings of hypersonic vehicles cruising in the upper atmosphere between 120 km to 300 km. The lightweight scheme proposed here is promising in hypersonic vehicles cruising in the upper atmosphere in the sense that it is able to not only reduce the weight of a wing by about 14%, but also save the material by the same amount. A comprehensive aerodynamic analysis indicates that the lightweight aerodynamic design can apply to a wide range of cruising altitudes and Mach numbers. In comparison with the conventional flat-plate wing, the lightweight wing always has smaller drags and larger effective lift-to-drag ratios, proving the feasibility of lightweight design for hypersonic vehicles at high altitudes. Moreover, as the cruising altitude is increased, the lightweight wing has even better lift and lift-to-drag-ratio performances, further demonstrating the excellent promise of lightweight wings for hypersonic vehicles cruising in the upper atmosphere. This lightweight design concept is of great value because the region of the upper atmosphere is relatively unexplored, and hypersonic vehicles cruising in this region can be used for high-resolution earth surveillance, accurate gravity/magnetic field mapping, and global ocean climate exploring.

Patterns of Carbon Nanotubes by Flow-Directed Deposition on Substrates with Architectured Topographies.

We develop and perform continuum mechanics simulations of carbon nanotube (CNT) deployment directed by a combination of surface topography and rarefied gas flow. We employ the discrete elastic rods method to model the deposition of CNT as a slender elastic rod that evolves in time under two external forces, namely, van der Waals (vdW) and aerodynamic drag. Our results confirm that this self-assembly process is analogous to a previously studied macroscopic system, the "elastic sewing machine", where an elastic rod deployed onto a moving substrate forms nonlinear patterns. In the case of CNTs, the complex patterns observed on the substrate, such as coils and serpentines, result from an intricate interplay between van der Waals attraction, rarefied aerodynamics, and elastic bending. We systematically sweep through the multidimensional parameter space to quantify the pattern morphology as a function of the relevant material, flow, and geometric parameters. Our findings are in good agreement with available experimental data. Scaling analysis involving the relevant forces helps rationalize our observations.

Computable model on the collision integral of Boltzmann equation and application to rarefied aerodynamics

Due to its complexity in dealing with the collisional integral term of the Boltzmann equation and computational costs associated with multi-dimensional problems, deterministic methods are still restricted to simple flow such as one-dimensional linear flow. However, the recently emerged fast spectrum method has achieved breakthroughs in computational efficiency and accuracy, which can enable simulations for more realistic three-dimensional non-linear flows. In comparison with the dominant direct simulation Monte Carlo method, the deterministic method has advantages especially in simulating low-speed flows where statistical variations prevail. Here, we review the development of fast spectrum method and discuss its applications for practical flow simulations. In particular, extended Boltzmann model is required for polyatomic and dense gases where the Boltzmann equation may not be valid. We present the applications of extended Boltzmann model for polyatomic gases in predicting spectra of both spontaneous and coherent Rayleigh-Brillouin Scattering, and in simulating space vehicle reentries with a broad range of $Kn$. Finally, we discuss the gas-kinetic unified algorithm (GKUA) of computable model Boltzmann equation and applications to the hypersonic aerodynamics of space reentry covering various flow regimes.

Aerodynamic Measurements and Computational Analyses in Hypersonic Rarefied Flows

The Japan Aerospace Exploration Agency (JAXA) recently proposed a landing mission to Mars. To improve the feasibility of the mission, rarefied aerodynamics need to be accurately estimated for a precise determination of its flight path and landing site. This work has investigated rarefied hypersonic aerodynamics experimentally and numerically. A rarefied aerodynamic measurement scheme has been developed by using pendulous models in a hypersonic rarefied wind tunnel at JAXA, and the results have been compared with direct simulation Monte Carlo computations. Consequently, numerically predicted aerodynamic coefficients are consistent with the measured values.

Development of an aerodynamic measurement system for hypersonic rarefied flows

A hypersonic rarefied wind tunnel (HRWT) has lately been developed at Japan Aerospace Exploration Agency in order to improve the prediction of rarefied aerodynamics. Flow characteristics of hypersonic rarefied flows have been investigated experimentally and numerically. By conducting dynamic pressure measurements with pendulous models and pitot pressure measurements, we have probed flow characteristics in the test section. We have also improved understandings of hypersonic rarefied flows by integrating a numerical approach with the HRWT measurement. The development of the integration scheme between HRWT and numerical approach enables us to estimate the hypersonic rarefied flow characteristics as well as the direct measurement of rarefied aerodynamics. Consequently, this wind tunnel is capable of generating 25 mm-core flows with the free stream Mach number greater than 10 and Knudsen number greater than 0.1.

Suppression of unimolecular decay of laser desorbed peptide and protein ions by entrainment in rarefied supersonic gas jets under weak electric fields

Unimolecular decay of sample ions imposes a limit on the usable laser fluence in matrix-assisted laser desorption/ionization (MALDI) ion sources. Traditionally, some modest degree of collisional sample ion cooling has been achieved by connecting MALDI ion sources directly to gas-filled radio frequency (RF) multipoles. It was also discovered in the early 1990s that gas-filled RF multipoles exhibit increased ion transmission efficiency due to collisional ion focusing effects. This unexpected experimental finding was later supported by elementary Monte Carlo simulations. Both experiments and simulations assumed a resting background gas with typical pressures of the order of 1 Pa. However, considerable additional improvements can be achieved if laser desorbed sample ions are introduced immediately after desorption, still within the ion source, in an axisymmetric rarefied supersonic gas jet with peak pressure of the order of 100 Pa and flow velocities >300 m/s, and under weak electric fields. We describe here the design principle and report performance data of an ion source coined "MALDI-2," which incorporates elements of both rarefied aerodynamics and particle optics. Such a design allows superb suppression of metastable fragmentation due to rapid collisional cooling in <10 μs and nearly perfect injection efficiency into the attached RF ion guide, as numerous experiments have confirmed.

Rigid-Body Dynamics in Free-Molecular and Transition Flow

Detailed rarefied aerodynamics are usually not coupled with the rigid-body dynamics of spacecraft. In part because of computational constraints, simpler models based upon the ballistic and drag coefficients are employed. Presented is an approach that capitalizes on the availability of ever-increasing computational methods and resources. The feasibility of increasing the fidelity of modeling spacecraft dynamics is explored by coupling rarefied aerodynamics with a rigid-body dynamics model similar to that traditionally used for aircraft in atmospheric flight. Aerodynamic force inputs for an airfoil section in a rarefied flow are provided by analytical equations in the free-molecular regime, and the direct simulation Monte Carlo method in the transition regime. Examples of nonlinear dynamics that cannot be predicted using simpler approaches are presented. The results of this straightforward approach to the coupled-field problem highlight the possibilities for future improvements in drag prediction, control system design, and atmospheric science.

Simulated rarefied aerodynamics of the Magellan spacecraft during aerobraking

Covers advancements in spacecraft and tactical and strategic missile systems, including subsystem design and application, mission design and analysis, materials and structures, developments in space sciences, space processing and manufacturing, space operations, and applications of space technologies to other fields.