Non-maxwellian velocity distributions in the transition region of supersonic expansions of the noble gases He, Ne and Ar

• Published in: Physica B + C 1983-01, Vol.121 (3), p.425-436
• Main Authors: Beijerinck, H.C.W., Kaashoek, G.H., Beijers, J.P.M., Verheijen, M.J.
• Format: Article
• Language: English
• Subjects: Compressible flows; shock and detonation phenomena; Exact sciences and technology; Fluid dynamics; Fundamental areas of phenomenology (including applications); Physics
• ISSN: 0378-4363
• DOI: 10.1016/0378-4363(83)90074-8

Small deviations of a Maxwell-Boltzmann velocity distribution are conveniently described by a function B( v)[1 + ∑ N n1 = 3 γ n H n ( υ)], with B(υ) the undisturbed distribution function and H n ( v) the Hermite polynomials. The upper limit N is determined by the accuracy required. A selection of e.g. only odd or even terms is determined by the symmetry of the problem. Measurements for He and Ne of the velocity distribution perpendicular to the streamlines of the expansion, as described by a virtual source, results in a kurtosis parameter γ ⊥,4 = 1.15 × 10 -2 of the central virtual source, independent of the source conditions. This value is in fair agreement with an a priori estimate γ ⊥,4 = 0.7 × 10 -2 from a simple transition region model and full calculations by Klots within the frame work of the thermal conduction model. The radii R 1 and R 2 and the relative populations c 1 and c 2 of the narrow and the wide virtual source, respectively, are in good agreement with the results obtained with undisturbed free jets in a cryo expansion chamber. This indicates that free jets of the gases He and Ne (with their small Van der Waals constants) are hardly influenced by the finite pumping speed of 1000 1/s in the expansion chamber. For Ar we obtain 33% larger values for R 1, showing the influence of scattering by residual gas.