No abstract

Aircraft low observables’ features are crucial in the long-wave infrared (LW-IR) band, due to imaging sensors used in IR search and track and in the latest generation of IR-guided missiles. Earthshine irradiance on the aircraft bottom surface is an important source; hence, it is derived using data for atmospheric transmission. Emission due to skin-friction heating (important at high ε b o t ) and earthshine reflection (important at low ε b o t ) are compared by a dimensionless ratio for different bottom surface emissivities ( ε b o t ). The infrared cross section of aircraft in direct view from below is obtained in the LW-IR band, which shows that aircraft is seen also due to negative contrast.

In this theoretical study, a fully developed laminar convective water flow in a circular tube is “convectively overloaded” toward the microscale, by decreasing the tube diameter below 1 mm. The entropy generation rate (S˙gen) is obtained (with and without the viscous dissipation term) for a given rate of heat removal using a fixed rate of coolant (water) flow. The uniform wall heat flux and mass flux in a tube increase toward the micro-scale, which is “thermal and flow overloading,” respectively. The variations of—S˙gen due to fluid friction, fluid conduction heat transfer, and their total (S˙gen,tot), toward the micro-scale, are analyzed. Since S˙gen,tot remains more or less the same toward the microscale, it is worth overloading a tube for miniaturization up to the laminar-flow limit.

In this theoretical study, fully developed forced convective laminar water flow is considered in circular micro-tubes, for the constant wall heat flux boundary condition. The change in entropy generation rate (\Delta {\dot{S}_{\mathrm{gen}}}) for N micro-tubes (each of diameter {D_{\mathrm{N}}}) relative to a reference tube (of 1 mm diameter) was investigated towards the micro-scale, for different tube length (l). A given total heat flow rate is to be removed using a fixed total mass flow rate through N tubes. Hence, the wall heat flux for one of the N tubes decreases towards the micro-scale, which is “thermal under-loading”. For given l, \Delta {\dot{S}_{\mathrm{gen}}} due to fluid conduction decreases and \Delta {\dot{S}_{\mathrm{gen}}} due to fluid friction increases towards the micro-scale. There exists an optimum {D_{\mathrm{N}}} (={D_{\mathrm{N},\mathrm{opt}}}) at which the change in sum-total {\dot{S}_{\mathrm{gen}}} (\Delta {\dot{S}_{\mathrm{gen},\mathrm{tot}}}) is minimum; where {D_{\mathrm{N},\mathrm{opt}}} decreases with decreasing l. For given l, cooling capacity of the heat sink increases towards the micro-scale. A general criterion for minimization of \Delta {\dot{S}_{\mathrm{gen},\mathrm{tot}}} is obtained in terms of Reynolds number, Brinkman number, and dimensionless l.

Entropy fluctuations with time occur in finite-sized time-evolving dissipative systems. There is a need to comprehend the role of these fluctuations on the fluctuations-averaged entropy generation rate, over a large enough observation time interval. In this non-equilibrium thermodynamic investigation, the Fluctuation Theorem (FT) and Principle of Least Action are re-visited to articulate their implications for dissipative systems. The Principle of Maximum Entropy Production (MaxEP: the entropy generation rate of a dissipative system is maximized by paths of least action) is conceptually identified as the Principle of Least Action for dissipative systems. A Thermodynamic Fusion Theorem that merges the FT and the MaxEP is introduced for addressing the role of fluctuations in entropy production. It identifies “entropy fluctuations” as the “least-action path” for maximizing the time-averaged entropy production in a dissipative system. The validity of this introduced theorem is demonstrated for the case of entropy fluctuations in Rayleigh–Taylor flow instability.