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Boundary-layer stability of supercritical fluids in the vicinity of the Widom line

Ren, Jie, Marxen, Olaf and Pecnik, Rene (2019) Boundary-layer stability of supercritical fluids in the vicinity of the Widom line Journal of Fluid Mechanics, 871. pp. 831-864.

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Abstract

We investigate the hydrodynamic stability of compressible boundary layers over adiabatic walls with fluids at supercritical pressure in the proximity of the Widom line (also known as the pseudo-critical line). Depending on the free-stream temperature and the Eckert number that determines the viscous heating, the boundary-layer temperature profile can be either sub-, trans- or supercritical with respect to the pseudo-critical temperature, Tpc. When transitioning from sub- to supercritical temperatures, a seemingly continuous phase change from a compressible liquid to a dense vapour occurs, accompanied by highly non-ideal changes in thermophysical properties. Using linear stability theory (LST) and direct numerical simulations (DNS), several key features are observed. In the sub- and supercritical temperature regimes, the boundary layer is substantially stabilized the closer the free-stream temperature is to Tpc and the higher the Eckert number. In the transcritical case, when the temperature profile crosses Tpc, the flow is significantly destabilized and a co-existence of dual unstable modes (Mode II in addition to Mode I) is found. For high Eckert numbers, the growth rate of Mode II is one order of magnitude larger than Mode I. An inviscid analysis shows that the newly observed Mode II cannot be attributed to Mack’s second mode (trapped acoustic waves), which is characteristic in high-speed boundary-layer flows with ideal gases. Furthermore, the generalized Rayleigh criterion (also applicable for non-ideal gases) unveils that, in contrast to the trans- and supercritical regimes, the subcritical regime does not contain an inviscid instability mechanism.

Item Type: Article
Divisions : Faculty of Engineering and Physical Sciences > Mechanical Engineering Sciences
Authors :
NameEmailORCID
Ren, Jie
Marxen, Olafo.marxen@surrey.ac.uk
Pecnik, Rene
Date : 25 July 2019
DOI : 10.1017/jfm.2019.348
Copyright Disclaimer : © 2019 Cambridge University Press. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
Uncontrolled Keywords : Boundary layer stability; Compressible boundary layers
Depositing User : Clive Harris
Date Deposited : 14 Aug 2019 07:37
Last Modified : 14 Aug 2019 07:37
URI: http://epubs.surrey.ac.uk/id/eprint/852404

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