3 research outputs found
Simultaneous Extrema in the Entropy Production for Steady-State Fluid Flow in Parallel Pipes
Steady-state flow of an incompressible fluid in parallel pipes can
simultaneously satisfy two contradictory extremum principles in the entropy
production, depending on the flow conditions. For a constant total flow rate,
the flow can satisfy (i) a pipe network minimum entropy production (MinEP)
principle with respect to the flow rates, and (ii) the maximum entropy
production (MaxEP) principle of Ziegler and Paltridge with respect to the
choice of flow regime. The first principle - different to but allied to that of
Prigogine - arises from the stability of the steady state compared to
non-steady-state flows; it is proven for isothermal laminar and turbulent flows
in parallel pipes with a constant power law exponent, but is otherwise invalid.
The second principle appears to be more fundamental, driving the formation of
turbulent flow in single and parallel pipes at higher Reynolds numbers. For
constant head conditions, the flow can satisfy (i) a modified maximum entropy
production (MaxEPMod) principle of \v{Z}upanovi\'c and co-workers with respect
to the flow rates, and (ii) an inversion of the Ziegler-Paltridge MaxEP
principle with respect to the flow regime. The interplay between these
principles is demonstrated by examples.Comment: Revised version 2; 5 figure
THE TIGHT-BINDING APPROACH TO THE DIELECTRIC RESPONSE IN THE MULTIBAND SYSTEMS
Starting from the random phase approximation for the weakly coupled multiband
tightly-bounded electron systems, we calculate the dielectric matrix in terms
of intraband and interband transitions. The advantages of this representation
with respect to the usual plane-wave decomposition are pointed out. The
analysis becomes particularly transparent in the long wavelength limit, after
performing the multipole expansion of bare Coulomb matrix elements. For
illustration, the collective modes and the macroscopic dielectric function for
a general cubic lattice are derived. It is shown that the dielectric
instability in conducting narrow band systems proceeds by a common softening of
one transverse and one longitudinal mode. Furthermore, the self-polarization
corrections which appear in the macroscopic dielectric function for finite band
systems, are identified as a combined effect of intra-atomic exchange
interactions between electrons sitting in different orbitals and a finite
inter-atomic tunneling.Comment: 20 pages, LaTeX, no figure