The current state of those aspects of rarefied gas dynamics research that appear to be most important to research planned over the next decade is evaluated. These aspects encompass assessments of computational rarefied-gas dynamics (CRGD) that will allows their use as surrogates for experiments, the development of hybris-flowfield computational techniques matching continuum computations with particle computations, and the validation of CRGD through the results of experimental studies of Knudsen layers in simple flows. The design of surfaces for the achievement of stable, low-momentum and thermal accommodation coefficients will be a major priority, together with theorization and experimentation on evaporation and condensation effects close to surfaces.

Rarefied Gas Dynamics

https://www.pure.ed.ac.uk/ws/files/21924394/PalhariniEtAlCompFluids2015.pdf

Benchmark numerical simulations of rarefied non-reacting gas flows using an open-source DSMC code

The impact of the length-to-depth ratio on aerodynamic surface quantities of a rarefied hypersonic cavity flow

This work is a computational study of a rarefied non-reacting hypersonic flow past a forward-facing step at zero-degree angle of attack in thermal non-equilibrium. Effects on the flow field structure and on the aerodynamic surface quantities due to changes in step frontal-face height are investigated by employing the direct simulation Monte Carlo method. The work focuses the attention of designers of hypersonic configurations on the fundamental parameter of surface discontinuity, which can have an important impact on even initial design. The results presented highlight the sensitivity of the primary flow field properties, velocity, density, pressure and temperature, to changes in the step frontal-face height. In addition, the behaviour of heat transfer, pressure and skin friction coefficients with variation of the step frontal-face height is detailed. The analysis shows that hypersonic flow past a forward-facing step in the transition flow regime is characterized by a strong compression ahead of the frontal face, which influences the aerodynamic surface properties upstream and adjacent to the frontal face. The analysis also shows that the extension of the upstream disturbance depends on the step frontal-face height. It was found that the recirculation region ahead of the step is also a function of the frontal-face height. A sequence of Moffatt eddies of decreasing size and intensity is observed in the concave step corner. Locally high heating and pressure loads were observed at three locations along the surface, i.e. on the lower surface, on the frontal face and on the upper surface. The results showed that both loads rely on the frontal-face height. The peak values for the heat transfer coefficient on the frontal-face surface were at least one order of magnitude larger than the maximum value observed for a smooth surface, i.e. a flat plate without a step. A comparison of the present simulation results with numerical and experimental data showed close agreement concerning the wall pressure acting on the step surface.

Computational analysis of the flow field structure of a non-reacting hypersonic flow over forward-facing steps

Numerical simulations of two-dimensional steady-state rarefied hypersonic flow in a gap at different length-to-depth (L/H) ratio are performed by using the direct simulation Monte Carlo (DSMC) meth...

Length-to-Depth Ratio Effects on Aerodynamic Surface Quantities of a Hypersonic Gap Flow

The direct simulation Monte Carlo technique is applied to a diverging flowfield along a flat plate. The model accounts for a plate of finite thickness with a nonzero leading edge bevel angle, and permits the specification of surface accommodation coefficients ranging from diffuse to specular reflection. The computed density, velocity, and temperature profiles at several axial stations along the plate are compared with experimental data. These comparisons indicate good agreement when the leading edge bevel is included in the calculation, and when the surface accommodation coefficients are each assigned a value of 0.8. Results are also presented that indicate the sensitivity of the computed skin friction to the ratio of the computational cell size to molecular mean free path. The results indicate that the skin friction is insensitive to the cell size if the local Knudsen number based on the cell size is maintained larger than unity everywhere in the flowfield. The results also indicate that the skin friction is underpredicted toward the plate leading edge and overpredicted at downstream axial positions when the cell size exceeds the molecular mean free path.

W. L. Hermina

Papers

Monte Carlo Simulation of Rarefied Flow Along a Flat Plate