Shripad P. Mahulikar

• A. von Humboldt Fellow, DFG-Mercator Fellow, PhD, CEng
• Professor (Full) at Indian Institute of Technology Bombay

Purpose In recent years, increased use of all-aspect infrared (IR)-guided missiles based on the long-wave infrared (LWIR; 8–12 µm) band has lowered the probability of aircraft survival in warfare. The lock-on of these highly sensitive missiles is difficult to break, especially from the front. Aerodynamically heated swept-back leading edges (SBLE),...

High speeds have been an advantage for aircraft to fly out of the adversary’s airspace fast, to escape earlier generations of infrared (IR) guided missiles that could lock-on only from the rear aspect. Advancements in IR seekers in long-wave IR (LWIR, 8 to 12 μm) band with imaging sensors in focal plane arrays made them capable of tracking aircraft...

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-...

Aircraft infrared (IR) signature studies are complex, due to their dependence on several parameters, e.g., line of sight (LOS) and viewing aspect (LOS^). There is no equivalent counterpart of the well-known radar cross section (RCS) that can be easily used for obtaining IR lock-on range, RLO,λ1−λ2. This study introduces the concept of IR cross sect...

It is well-known that Poiseuille number (Po, hitherto viewed mainly as a Fluid Mechanics parameter) decreases along a hydrodynamically developing flow, from infinity at inlet to a fixed value downstream. This study reveals that the dimensionless entropy generation rate per unit length due to fluid friction (Sgen,fr) varies exactly the same way; hen...

Dissipative structures (DS) exist at all scales, systems, and at different levels of complexity. A thermodynamic theory integrating simple and complex DS is introduced, which addresses existence of growing/decaying DS based on their entropy analysis. Two entropy-based dimensionless ratios are introduced, which explain negentropy-debt payment and ex...

Jet engine hot parts (e.g. jet-nozzle) are crucial source of aircraft’s infrared (IR) signature from rear view, in 1.9-2.9 and 3-5 µm bands. Exhaust nozzle design used in a jet aircraft affects its performance and IR signature (which is also affected just by performance) from the engine layout. For supersonic aircraft (typically for, M∞ > 1.5), a c...

Examining non-rarefaction effects in laminar micro-convective low subsonic (with negligible compressibility effects) gas flows is as important as the well-explored rarefaction effects. This study numerically analyzes one of the important non-rarefaction effects arising from steep density variations due to the steep temperature-gradients, in micro-c...

The present study analyzes the rarefaction and non-rarefaction effects on heat transfer characteristics of hydrodynamically and thermally developing air flow through microtubes having a constant wall heat flux boundary condition. The problem is especially simulated with two microtubes having diameters 50 µm and 150 µm. The rarefaction effect is tak...

The heat transfer characteristics of laminar single-phase forced convective water flow through a micro-tube heat exchanger are numerically investigated in this paper. Two-dimensional simulations are performed to find the effects of variable fluid properties on heat transfer for hydrodynamically and thermally developed flow. The effects of variable...

In the original publication of the article, under the heading, “Dimensional analysis of volumetric Properties” the dimensions of five principal parameters were incompletely published.

The accuracy requirement in the micro heat exchanger analysis has the utmost importance due to its extensive application in highly compact devices. The available correlations in the literature are observed to be inadequate for the micro heat exchanger analysis as they are based on a conventional macroscale. The individual microscale effects investi...

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 L...

The effects of temperature-dependent viscosity and thermal conductivity on heat transfer and frictional flow characteristics of water flowing through a microchannel are numerically investigated in this work. The hydrodynamically and thermally developing flow with no-slip, notemperature jump, and constant wall heat flux boundary condition is numeric...

With rapid advancements in Infra-Red (IR) detection techniques, the range from where the IR-guided missiles are able to lock the target aircraft has increased. To avoid the detection and tracking by modern IR-guided missiles, the aircraft and helicopters also demand progress in its stealth techniques. Hence, study of Infra-Red Signature Suppression...

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) wa...

The scope for the heat transfer enhancement in the tubular heat exchanger is high due to its unique property of having two separate convective heat transfer coefficients. The variation of diameter and annular space has a direct effect on the value of convective heat transfer coefficients due to their inverse relation. Thus, the strong emphasis must...

Low flight altitude missions are a threat due to the combat aircraft's Infrared (IR) Signature Level (IRSL) as seen by ground-based IR-seeker. The bands in which, the aircraft is susceptible to a typical IR-guided Surface-to-Air Missile are 3-5 microns and 8-12 microns. The rear fuselage skin contributes substantially to the overall aircraft IRSL e...

The energy degraded into heat (qdeg) in laminar microconvective flow is of fundamental interest for minimizing the irreversibilities. The change in qdeg for N—number of microtubes each of diameter (DN) is studied relative to the reference tube (N = 1) of 1 mm diameter, towards the microscale. The total mass flow rate and total heat flow rate to be...

The entropy generation (S˙gen) due to fluid friction and convective heat transfer is studied en-route the microscale. The S˙gen for water-flow through a circular tube for the constant wall heat flux boundary condition is estimated. The number of tubes (N) en-route the microscale is increased by correspondingly decreasing each tube diameter, for fix...

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 (...

Micro-scale cooling is an efficient and effective cooling technique to achieve the goal of higher heat removal capabilities. The present research focuses to find the physical effects of fluid property variations on flow and thermal development in micro-channel. The effects of temperature-dependent density, viscosity, and thermal conductivity variat...

The influence of reducing dimensions to microscale on the performance parameters of parallel flow tubular micro heat exchanger is numerically analysed considering the thermophysical property variation effect. Also, the improvement in convective heat transfer coefficient, at a given temperature, with the decrease in dimensions to microscale is inves...

The gas micro flows are deployed in heat-sinks of many practical micro devices, and cooling systems for gas turbine blades etc. In present study, fluid friction characteristics of laminar micro flow, operating at low Reynolds number, are studied. This flow model is representative of fluid flows in many micro device applications. Fanning friction fa...

In this study, an inverse heat conduction analysis based on the Levenberg-Marquardt method is applied to reconstruct the aerothermal heating and thermal protection material response of a reusable hypersonic vehicle (RHV) from the knowledge of temperature measurements taken within the thermal protection system (TPS). In addition to reconstruction of...

The physical effects of variable fluid properties on heat transfer and frictional flow characteristics of laminar gas microconvective flow are investigated in this research. The fully developed flow through a microtube is studied numerically by using 2D continuum-based governing equations. The physical effects induced due to variations in gas densi...

The area of the event horizon of a black hole (Aeh) is so far linked only with its entropy (SBH). In this theoretical investigation, it is shown that relating Aeh only to SBH is inadequate, because Aeh is linked to the black hole’s negentropy, which encompasses its entropy. Increasing Aeh of black holes that grow now follows from the negentropy the...

Incorporation of an infrared suppressor is generally accompanied by a compromise in engine performance, which indirectly reduces the effectiveness of infrared signature suppression. This investigation illustrates the percentage increase in the infrared signature level in the 1.9–2.9 μm and 3–5 μm bands resulting from an increase in engine backpress...

The aero-thermal analysis of a hypersonic vehicle is of fundamental interest for designing its thermal protection system. The aero-thermal environment predictions over several critical regions of the hypothesized lifting body vehicle, including the stagnation region of the nose-cap, cylindrically swept leading edges, fuselage-upper, and fuselage-lo...

In general, jet engines operate with choked nozzle during take-off, climb and cruise, whereas unchoking occurs while landing and taxiing (when engine is not running at full power). Appropriate thrust in an aircraft in all stages of the flight, i.e., take-off, climb, cruise, descent and landing is achieved through variation in the nozzle exit area....

A methodology has been established aiming to design a lightweight thermal protection system (TPS), using advanced lightweight ablative materials developed at the NASA Ames Research Center. An explicit finite-difference scheme is presented for the analysis of one-dimensional transient heat transfer in a multilayer TPS. This problem is solved in two...

The present work investigates the frictional flow characteristics of water flowing through a circular microchannel with variable fluid properties. The computational analysis reveals the importance of physical mechanisms due to variations in thermophysical fluid properties such as viscosity μ(T), thermal conductivity k(T) and density ρ(T) and also t...

Optimization of a parallel flow gas-to-gas tubular micro heat exchanger with hot core and cold annulus fluid is numerically analyzed, considering the beneficial role of surface radiation. Operating and geometric parameters are varied for fixed overall mass flow rate and temperature of cold core fluid, to study the effects on the following performan...

Dynamically Ordered self-organized dissipative structure exists in various forms and at different scales. This investigation first introduces the concept of an isolated embedding system, which embeds an open system, e.g., dissipative structure and its mass and/or energy exchange with its surroundings. Thereafter, scale-invariant theoretical analysi...

The effect of missile turn rate on aircraft susceptibility to an infrared (IR)-guided missile was studied. The maximum lethal range increases with an increase in the missile turn rate. It is observed that, for m - 0.5 and missile turn rate restricted to 16 deg/s, there is 35% reduction in the area of lethal envelope compared to the no-restriction c...

The analysis of black holes fed by the omnipresent Cosmic Microwave Background Radiation (CMBR) constitutes benchmark cases. The rate of energy and entropy variation of a Schwarzschild black hole fed by CMBR is analytically obtained. The entropy analysis revealed that there is a higher value of black hole’s critical mass than that obtained from an...

The exact closed-form mathematical solution for Schwarzschild black-hole fed by the Cosmic Micro-wave Background Radiance (CMBR) is obtained analytically. This unique solution holds for both growth (evolution) and decay, which are bifurcated by the critical initial mass, which depends on the temperature of CMBR. Results are presented to illustrate...

Pressure drop (Δp) in microconvective flows is of basic importance in several applications in which fluid flows through micropassages for heat removal. This investigation studies the effects of variations in air properties due to convective heat transfer on basic laminar microflow parameters: wall shear stress, Fanning friction factor, pressure dro...

The lock-on envelope of target aircraft is important in determining its susceptibility against a heat-seeking missile and depends on the aircraft's infrared (IR) signature level (IRSL). In this investigation, the lock-on envelope is estimated in 3-5 μm and 8-12 μm bands, considering internal and external sources of IR signature from surfaces of air...

The lock-on envelope of target aircraft is important in determining its susceptibility against a heat-seeking missile and depends on the aircraft's infrared (IR) signature level (IRSL). In this investigation, the lock-on envelope is estimated in 3-5 μm and 8-12 μm bands, considering internal and external sources of IR signature from surfaces of air...

This work deals with the analysis of forced convection in single-phase laminar flow of liquid through microsized circular geometry with a diameter of 100 × 10−6 m. The problem with hydrodynamically and thermally developing flow in the entrance region with no-slip, no-temperature jump and constant wall heat flux boundary condition is numerically stu...

Infrared (IR) guided missiles have become major cause of aircraft destructions since their inception. Owing to their passive tracking, fire and forget capabilities and ease of operation, they emerged as an important anti-aircraft arsenal. MAN Portable Air Defense Systems (MANPADS) have become major threats to military aircraft and helicopters. The...

A numerical program is developed to solve two-dimensional continnum-based governing differential equations for liquid flow in axisymmetric circular microchannel geometry. The effects of variable thermal properties in single-phase laminar forced convection with constant wall heat flux boundary conditions are studied. The numerical analysis of fully...

The influence of surface radiative transfer in parallel flow microheat exchanger is numerically studied for its importance at high temperatures and for small flow dimensions. For these heat exchangers, the role of radiation is beneficial when the convective heat transfer to the annulus flow exceeds the convective heat transfer from the core flow. F...

Analyses of laminar fully-developed forced convection with viscous dissipation in annulus has revealed additional theoretical concepts and mathematical handling techniques, even for limiting cases with simple solutions. Because Nusselt and Brinkman numbers by themselves are incomplete to handle convection with viscous dissipation, another dimension...

The effects of property variations in single-phase laminar forced micro-convection with constant wall heat flux boundary condition are investigated in this work. The fully-developed flow through micro-sized circular (axisymmetric) geometry is numerically studied using two-dimensional continuum-based conservation equations. The non-dimensional gover...

The negentropy proposed first by Schroedinger is re-examined, and its conceptual and mathematical definitions are introduced. This re-definition of negentropy integrates Schroedinger’s intention of its introduction, and the subsequent diverse notions in literature. This negentropy is further corroborated by its ability to state the two exact thermo...

The present work investigates the influence of property variations of air in laminar forced convection with entrance effect. Two-dimensional micro-sized geometry (with axisymmetry) with constant wall heat flux boundary condition is considered to predict flow behaviour and thermal development. The continnum-based conservation equations are numerical...

Investigations are reported on infrared (IR) signature studies from internal sources of aircraft and helicopters, e.g. engine heated surfaces and exhaust plume. Due to the increasing sensitivities of IR sensors, irradiances from external sources are investigated here, namely sunshine, skyshine, and earthshine. Their role in determining the IR contr...

The infrared (IR) seekers have exploited techniques to passively acquire and inter-cept airborne targets, by detecting their IR energy [1]. The basic principle of IR detection is the discrimination of target's IR radiance in the detector's wavelength band with the background IR radiance (atmospheric emission/solar radiation) [2]. In an aircraft, th...

The temperature field history of passive Thermal Protection System (TPS) material at the nose-cap (forward stagnation region) of a Reusable Hypersonic Vehicle (RHV) is generated. The 3-D unsteady heat transfer model couples conduction in the solid with external convection and radiation that are modeled as time-varying boundary conditions on the sur...

1) An infrared signature suppression system is conceptualized for a helicopter engine exhaust duct, based on low observables principles of conceal and camouflage. 2) The exposed surfaces of the IRSS system are camouflaged with the background by thermal design, which considers multimode heat transfer, including surface radiation interchange. 3) The...

The Reynolds' analogy between the Stanton number (St) and the skin friction coefficient (cf) is popularly believed to hold when St increases with increasing cf, for simple situations. In this investigation, the validity of Reynolds' analogy between St and cf for micro-convection of liquids with variations in fluid properties is re-examined. It is f...

Gas microconvection is a strategic research area in transport phenomena since it is the basis for a wide range of miniaturized high-performance applications like microelectromechanical systems. The literature reveals that in the single-phase continuum regime, microconvection characteristics significantly differ from those in the conventionally size...

Infrared (IR) emissions from aircraft are used to detect, track, and lock-on to the target. MAN Portable Air Defence Systems (MANPADS) have emerged as a major cause of aircraft and helicopter loss. Therefore, IR signature studies are important to counter this threat for survivability enhancement, and are an important aspect of stealth technology. T...

The solid angle (Ω) subtended by the hot power-plant surfaces of a typical fighter aircraft, on the detector of an infrared (IR) guided missile, is analytically obtained. The use of the parallel rays projection method simplifies the incorporation of the effect of the optical blocking by engine surfaces, on Ω-subtended. This methodology enables the...

We report numerical simulation of pure continuum-based laminar gas micro-flow convection with steep density gradients, which cause, for the case of heated air, flattening and rate of flattening of axial velocity profile. This flattening is similar to the characteristics in constant-properties slip flow, and high rate of flattening can cause hydrody...

In a systematic approach we address the question of how important variable property effects are for flows in micro-sized channels. Due to heat transfer, the temperature dependence of fluid properties like viscosity and thermal conductivity results in deviations in a solution which accounts for that dependence compared to a solution only considering...

It is of engineering importance to accurately predict the surface cooling characteristics in bath quenching of metals and alloys. In this investigation, the surface cooling characteristics in quenching of Wolfson probe are estimated with reasonable accuracy by solving an inverse heat conduction problem. Regularization method is used for smoothening...

The governing continuum-based conservation equations for laminar 2D (with axisymmetry) convective micro-tube-flow of gas are numerically solved. These equations comprehensively incorporate the physical effects of variations of gas density with pressure and temperature, and gas viscosity and thermal conductivity with temperature. Their non-dimension...

In this investigation we report the identification of laminar microconvection physical effects due to the variation of viscosity and thermal-conductivity of liquid. Viscosity variation significantly distorts the axial velocity profile and varies this distortion along the microflow, thereby inducing radial flow due to flow continuity. The resulting...

Creation and existence of order are of philosophical significance and practical utility. This paper spells clear, succinct, yet comprehensive directives for order creation, existence and total destruction, in the form of nine exclusive directives established from the broken symmetry in the inter-convertibility of order and disorder stated by the En...

This paper analyzes the main contributors of infrared (IR) signature in a typical aircraft on a low-altitude mission. Various computational models are used to predict IR radiation from the aircraft. The bands within IR spectrum in which aircraft are susceptible to a typical IR-guided surface-to-air and air-to-air missile, for typical cases of tacti...

Convection coefficients and heat fluxes due to aerodynamic heating on critical surfaces of hypersonic vehicle are obtained analytically. The applicability of recovery temperature for stagnation regions is discussed. Convection coefficient for the bicurvature forward stagnation region is obtained directly from 2-D stagnation region correlation, usin...

This paper assesses aircraft susceptibility from the first principles, with respect to the threat posed by passively guided infrared homing missiles; with an objective of gaining insight into the comprehensiveness of the relationship between aircraft susceptibility and aircraft infrared signature level. The conventional criterion of aircraft suscep...

This investigation analyzes the calibration nonlinearity of the ball-in-vortex flow-meter designed to work on the principle of a rotating sphere in and due to a vortex flow. The comparison of this flow-meter reading with the standard flow-meter indicates the existence of different calibration regimes, bifurcated by a sharp change in slope of the ca...

This paper elaborates the role of atmosphere in determining infrared signatures of aircraft as perceived by a ground-based infrared detector, similar to the case of an infrared guided heat-seeking surface-to-air missile. The main objectives are to assess the effect of atmosphere on aircraft infrared signature and to evaluate infrared bands in which...

A methodology to evaluate IR radiation emitted by aircraft plume and received by ground-based IR detectors is discussed. The plume IR signatures were compared with other sources such as rear fuselage and tail pipe role of the atmosphere in dictating plume IR signature levels produced by the rear fuselage and engine tailpipe. It was found that plume...

This paper elaborates the role of atmosphere in determining infrared signatures of aircraft as perceived by aground-based infrared detector, similar to the case of an infrared guided heat-seeking surface-to-air missile. The main objectives are to assess the effect of atmosphere on aircraft infrared signature and to evaluate infrared bands in which...

We re-examine the governing continuum-based conservation equations for single-phase laminar forced convection of liquids, incorporating temperature dependence of fluid viscosity and thermal conductivity. The effects of property variations become highly significant from macro-to-microscale convection. Further, the effects of property variations alon...

Infrared radiation emitted by aircraft makes it susceptible to infrared guided threats. This paper analyses the two main sources of infrared signatures in aircraft, namely the powerplant and the exhaust plume. Models are discussed that can be used to evaluate infrared signature from aircraft powerplant (rear fuselage surface and tailpipe) and exhau...

The temperature distribution of the exposed rear fuselage of fighter aircraft, which is necessary for estimating passively emitted infrared signature levels, is predicted. Hot combustion gases flowing through the gas-turbine engine and aerodynamic heating of the rear fuselage at high freestream Mach numbers are the two heat sources. Turbulent force...

The negentropy proposed first by Schroedinger is re-examined, and its conceptual and mathematical definition are proposed; which is shown to integrate Schroedinger s intention of its introduction, and the subsequent diverse notions in literature. This definition of negentropy is corroborated by its unique ability to state direct thermodynamic princ...

This paper investigates the qualitative behaviour of single-phase laminar convection for microchannels and conventionallysized channels formed between two parallel plates, captured by a numerical simulation on water flow. The convection parameters are obtained by separate numerical calculations on a series of parallel plates at constant temperature...

This paper discusses the evolution of airpower and the ever-increasing component of stealth technology in dictating warfare, based on the need felt in the literature to integrate and place them in the right perspective. Hence, the role of stealth aircraft and the dominance of all aspects of stealth technology, especially in recent conflicts, are re...

By re-examining the general criterion for thermal development of fluid flow in a circular tube under laminar forced convection, it is possible to define a thermal development ratio, which leads to the identification of four different conditions of flow within the laminar regime. These different flow conditions subsequently lead to 12 possible ways...

This paper begins with an outline of the procedure for predicting the infrared signature emissions from the airframe, engine casing, and the plume, and their attenuation by the intervening atmosphere. These emissions are contrasted against the background, to obtain the infrared signature levels. The infrared detector's - noise equivalent flux densi...

Experiments were performed using water flow in microchannel specimens, with thermocouples mounted axially along the flow for local wall temperature measurement. The processed experimental data obtained locally in the laminar regime exhibited the unusual behaviour of the local Nusselt number decreasing with increasing local Reynolds number along the...

Surface radiation interchange in an annular enclosure is numerically modeled together with evaporating meniscus-driven convection, for investigating the application of the concept for cooling in microelectronic devices. The geometry is axially discretised into ring elements, where the wall and fluid temperatures within each element are unknowns. Th...

In this study, the view factors for three cases of shell interior surface segments are obtained analytically. The analytical results agree well with the available reported numerical results. The analytical solutions are used to generate view factors for various radius ratios.

The experimental data in the literature have indicated that there are unexplained unusual behaviours in heat transfer and flow in microchannels. In particular, the geometry of the microchannels and Reynolds number alone do not determine the flow regime boundaries and the transition range. The reported experimental data on convection in microchannel...

Previously reported and present experimental data are processed to give the Nusselt number (Nu) along the flow in microchannels, which is segregated based on the conventional thermal entry length into thermally-developing and thermally-developed modes. The Nu is also found to reduce in the thermally-developed regime, thereby confirming that the unu...

This paper studies meniscus-driven convection and surface radiation interchange in a 2-D microchannel, formed between two parallel plates, with specified axial wall heat flux, thereby simulating a 2-D array of multichips. The configuration is discretized along the flow, and the governing equations for each discrete element for meniscus-driven conve...

From a survey on studies on convective heat transfer in microchannels, the Brinkman number is proposed as a parameter for correlating the convective heat transfer parameters in microchannels. The proposal emerges from a dimensional analysis of the variables influencing the laminar forced convection in microchannels and it can explain the unusual be...

This paper presents a mathematical model for the simulation of evaporating meniscus-driven dielectric coolant flow in a channel between two parallel flat plates, one of which is heated and simulates the I.C. chip surface to be cooled, and the other is assumed to be insulated for convenience of modelling. The model comprises of the energy and moment...

This paper outlines a quick method of estimating the transmissivity of the intervening atmosphere (between the infrared detector and the object to be detected - aircraft/missile), for vertical and slant beams of infrared radiation. The data needed is the transmissivity for horizontal beams of infrared radiation, of a given length, at various altitu...

Prediction of engine casing temperature of combat aircraft for various operating conditions needs modeling of convective and radiative heat flow from the jet pipe to the casing. Radiation modeling which implies modeling of multiple reflections and scattering is done here by the radiance-irradiance approach. The engine layout is approximated by thre...

Extended Abstract Creation and existence of self-organisation or dynamic order has puzzled even pioneering scientists, because it "appears" to work against the Entropy Principle (EP). Known thermodynamic principles were conceptually analysed for deducing nine statements based on the broken symmetry stated by EP and the Law of Maximum Entropy Produc...

Micro-convection is a strategic area in transport phenomena, since it is the basis for a wide range of miniaturised high-performance applications. Microchannel heat-sinks are appealing solutions to the increasing heat dissipation demands, especially in electronic devices. Literature reveals that in the single-phase continuum regime, the behaviour o...

Quenching characterization involves the determination of the cooling properties of a quenchant when cooling a piece of metal or alloy. In this investigation, the surface cooling characteristics in quenching of Wolfson test probe are estimated with reasonable accuracy, by solving an inverse heat conduction problem. Regularization method is used for...