26,621 research outputs found
Linear instability of Poiseuille flows with highly non-ideal fluids
The objective of this work is to investigate linear modal and algebraic
instability in Poiseuille flows with fluids close to their vapour-liquid
critical point. Close to this critical point, the ideal gas assumption does not
hold and large non-ideal fluid behaviours occur. As a representative non-ideal
fluid, we consider supercritical carbon dioxide (CO) at pressure of 80 bar,
which is above its critical pressure of 73.9 bar. The Poiseuille flow is
characterized by the Reynolds number
(), the product of Prandtl
() and Eckert number
(), and the wall temperature that in
addition to pressure determines the thermodynamic reference condition. For low
Eckert numbers, the flow is essentially isothermal and no difference with the
well-known stability behaviour of incompressible flows is observed. However, if
the Eckert number increases, the viscous heating causes gradients of
thermodynamic and transport properties, and non-ideal gas effects become
significant. Three regimes of the laminar base flow can be considered,
subcritical (temperature in the channel is entirely below its pseudo-critical
value), transcritical, and supercritical temperature regime. If compared to the
linear stability of an ideal gas Poiseuille flow, we show that the base flow is
more unstable in the subcritical regime, inviscid unstable in the transcritical
regime, while significantly more stable in the supercritical regime. Following
the corresponding states principle, we expect that qualitatively similar
results will be obtained for other fluids at equivalent thermodynamic states.Comment: 34 pages, 22 figure
Topological phases protected by point group symmetry
We consider symmetry protected topological (SPT) phases with crystalline
point group symmetry, dubbed point group SPT (pgSPT) phases. We show that such
phases can be understood in terms of lower-dimensional topological phases with
on-site symmetry, and can be constructed as stacks and arrays of these
lower-dimensional states. This provides the basis for a general framework to
classify and characterize bosonic and fermionic pgSPT phases, that can be
applied for arbitrary crystalline point group symmetry and in arbitrary spatial
dimension. We develop and illustrate this framework by means of a few examples,
focusing on three-dimensional states. We classify bosonic pgSPT phases and
fermionic topological crystalline superconductors with (reflection)
symmetry, electronic topological crystalline insulators (TCIs) with symmetry, and bosonic pgSPT phases with symmetry,
which is generated by two perpendicular mirror reflections. We also study
surface properties, with a focus on gapped, topologically ordered surface
states. For electronic TCIs we find a classification, where
the corresponds to known states obtained from non-interacting electrons,
and the corresponds to a "strongly correlated" TCI that requires strong
interactions in the bulk. Our approach may also point the way toward a general
theory of symmetry enriched topological (SET) phases with crystalline point
group symmetry.Comment: v2: Minor changes/additions to introduction and discussion sections,
references added, published version. 21 pages, 11 figure
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