Experimental findings show that transition from laminar toturbulent ow may occur also if the exponentially growingperturbations, eigensolutions to the linearised disturbanceequations, are damped. An alternative non-modal growthmechanism has been recently identi fied, also based on thelinear approximation. This consists of the transient growth ofstreamwise elongated disturbances, with regions of positive andnegative streamwise velocity alternating in the spanwisedirection, called streaks. These perturbation are seen toappear in boundary layers exposed to signi ficant levels offree-stream turbulence. The effect of the streaks on thestability and transition of the Blasius boundary layer isinvestigated in this thesis. The analysis considers the steadyspanwise-periodic streaks arising from the nonlinear evolutionof the initial disturbances leading to the maximum transientenergy growth. In the absence of streaks, the Blasius pro filesupports the viscous exponential growth of theTollmien-Schlichting waves. It is found that increasing thestreak amplitude these two-dimensional unstable waves evolveinto three-dimensional spanwiseperiodic waves which are lessunstable. The latter can be completely stabilised above athreshold amplitude. Further increasing the streak amplitude,the boundary layer is again unstable. The new instability is ofdifferent character, being driven by the inectional pro filesassociated with the spanwise modulated ow. In particular, it isshown that, for the particular class of steady streaksconsidered, the most ampli fied modes are antisymmetric andlead to spanwise oscillations of the low-speed streak (sinuousscenario). The transition of the streak is then characterisedby the appearance of quasi-streamwise vorticesfollowing themeandering of the streak. Simulations of a boundary layer subjected to high levels offree-stream turbulence have been performed. The receptivity ofthe boundary layer to the external perturbation is studied indetail. It is shown that two mechanisms are active, a linearand a nonlinear one, and their relative importance isdiscussed. The breakdown of the unsteady asymmetric streaksforming in the boundary layer under free-stream turbulence isshown to be characterised by structures similar to thoseobserved both in the sinuous breakdown of steady streaks and inthe varicose scenario, with the former being the mostfrequently observed. <b>Keywords:</b>Fluid mechanics, laminar-turbulent transition,boundary layer ow, transient growth, streamwise streaks,lift-up effect, receptivity, free-stream turbulence, secondaryinstability, Direct Numerical Simulation.

<p>Experimental findings show that transition from laminar toturbulent ow may occur also if the exponentially growingperturbations, eigensolutions to the linearised disturbanceequations, are damped. An alternative non-modal growthmechanism has been recently identi fied, also based on thelinear approximation. This consists of the transient growth ofstreamwise elongated disturbances, with regions of positive andnegative streamwise velocity alternating in the spanwisedirection, called streaks. These perturbation are seen toappear in boundary layers exposed to signi ficant levels offree-stream turbulence. The effect of the streaks on thestability and transition of the Blasius boundary layer isinvestigated in this thesis. The analysis considers the steadyspanwise-periodic streaks arising from the nonlinear evolutionof the initial disturbances leading to the maximum transientenergy growth. In the absence of streaks, the Blasius pro filesupports the viscous exponential growth of theTollmien-Schlichting waves. It is found that increasing thestreak amplitude these two-dimensional unstable waves evolveinto three-dimensional spanwiseperiodic waves which are lessunstable. The latter can be completely stabilised above athreshold amplitude. Further increasing the streak amplitude,the boundary layer is again unstable. The new instability is ofdifferent character, being driven by the inectional pro filesassociated with the spanwise modulated ow. In particular, it isshown that, for the particular class of steady streaksconsidered, the most ampli fied modes are antisymmetric andlead to spanwise oscillations of the low-speed streak (sinuousscenario). The transition of the streak is then characterisedby the appearance of quasi-streamwise vorticesfollowing themeandering of the streak.</p><p>Simulations of a boundary layer subjected to high levels offree-stream turbulence have been performed. The receptivity ofthe boundary layer to the external perturbation is studied indetail. It is shown that two mechanisms are active, a linearand a nonlinear one, and their relative importance isdiscussed. The breakdown of the unsteady asymmetric streaksforming in the boundary layer under free-stream turbulence isshown to be characterised by structures similar to thoseobserved both in the sinuous breakdown of steady streaks and inthe varicose scenario, with the former being the mostfrequently observed.</p><p><b>Keywords:</b>Fluid mechanics, laminar-turbulent transition,boundary layer ow, transient growth, streamwise streaks,lift-up effect, receptivity, free-stream turbulence, secondaryinstability, Direct Numerical Simulation.</p>

No description available.

Numerical simulations have been done with the CHEMKINsoftware to study different aspects of wall effects in thecombustion of lean, laminar and premixed flames in anaxisymmetric boundary-layer flow. The importance of the chemical wall effects compared to thethermal wall effects caused by the development of the thermaland velocity boundary layer has been investigated in thereaction zone by using different wall boundary conditions, walltemperatures and fuel/air ratios. Surface mechanisms include acatalytic surface (Platinum), a surface that promotesrecombination of active intermediates and a completely inertwall with no species and reactions as the simplest possibleboundary condition. When hydrogen is the model fuel, the analysis of the resultsshow that for atmospheric pressure and a wall temperature of600 K, the surface chemistry gives significant wall effects atthe richer combustion case (f=0.5), while the thermal andvelocity boundary layer gives rather small effects. For theleaner combustion case (f=0.1) the thermal and velocityboundary layer gives more significant wall effects, whilesurface chemistry gives less significant wall effects comparedto the other case. For methane as model fuel, the thermal and velocity boundarylayer gives significant wall effects at the lower walltemperature (600 K), while surface chemistry gives rather smalleffects. The wall can then be modelled as chemically inert forthe lean mixtures used (f=0.2 and 0.4). For the higher walltemperature (1200 K) the surface chemistry gives significantwall effects. For both model fuels, the catalytic wall unexpectedlyretards homogeneous combustion of the fuel more than the wallthat acts like a sink for active intermediates. This is due toproduct inhibition by catalytic combustion. For hydrogen thisoccurs at atmospheric pressure, but for methane only at thehigher wall temperature (1200 K) and the higher pressure (10atm). As expected, the overall wall effects (i.e. a lowerconversion) were more pronounced for the leaner fuel-air ratiosand at the lower wall temperatures. To estimate a possible discrepancy in flame position as aresult of neglecting the axial diffusion in the boundary layerassumption, calculations have been performed with PREMIX, alsoa part of the CHEMKIN software. With PREMIX, where axialdiffusion is considered, steady, laminar, one-dimensionalpremixed flames can be modelled. Results obtained with the sameinitial conditions as in the boundary layer calculations showthat for the richer mixtures at atmospheric pressure the axialdiffusion generally has a strong impact on the flame position,but in the other cases the axial diffusion may beneglected. Keywords:wall effects, laminar premixed flames,platinum surfaces, boundary layer flow / QC 20100504

This dissertation presents an experimentally supported multi-physics model of a dielectric barrier discharge boundary layer flow control actuator. The model is independent of empirical data about the specific behavior of the system. This model contributes to the understanding of the specific mechanisms that enable the actuator to induce flow control. The multi-physics numerical model couples a fluid model, a chemistry model, and an electrostatics model. The chemistry model has been experimentally validated against known spectroscopic techniques, and the fluid model has been experimentally validated against the time-resolved shadowgraphy. The model demonstrates the capability to replicate emergent flow structures near a wall. These structures contribute to momentum transport that enhance the boundary layer’s wall attachment and provide for better flow control. An experiment was designed to validate the model predictions. The spectroscopic results confirmed the model predictions of an electron temperature of 0.282eV and an electron number density of 65.5 × 10⁻¹²kmol/m³ matching to within a relative error of 12.4% and 14.8%, respectively. The shadowgraphic results also confirmed the model predicted velocities of flow structures of 3.75m/s with a relative error of 10.9%. The distribution of results from both experimental and model velocity calculations strongly overlap each other. This validated model provides new and useful information on the effect of Dielectric Barrier Discharge actuators on flow control and performance. This work was supported in part by NSF grant CNS-0960081 and the HokieSpeed supercomputer at Virginia Tech. / Ph. D.

La dynamique d'instabilité d'un écoulement laminaire décollé est étudiée expérimentalement et son contrôle par le biais de la simulation numérique. La configuration étudiée est une couche limite laminaire décollée au dessus d'une géométrie de type bosse.Pour une certaine gamme de paramètres, l'écoulement de recirculation en aval de la bosse est caractérisé par un battement basse fréquence. L'étude expérimentale de cette dynamique a permis de retrouver les différents régimes d'instabilité mis a jour par voie numérique. Ces résultats prouvent notamment que les instabilités basse fréquence, dont l'existence a été surtout mise en évidence dans des configurations d'écoulements compressibles, sont un phénomène générique pour des bulles de recirculations allongées. Le contrôle du battement basse fréquence est ensuite étudié par voie numérique suivant deux approches complémentaires. Un asservissement en boucle fermée de la dynamique de perturbation linéaire est tout d'abord proposé. Les modes d'instabilité linéaires sont utilisés afin de construire des modèles réduits de la dynamique de perturbation. Cette réduction de modèle donne lieu à des estimateurs de faible dimension capables d'estimer la dynamique et de la contrôler. Ainsi la dynamique d'instabilité linéaire peut être supprimée en couplant le système de Navier-Stokes linéarisé avec le contrôleur.Le contrôle de la dynamique non linéaire est ensuite étudié en utilisant une méthode d'optimisation Lagrangienne. Cette méthode permet de calculer les lois de contrôle à partir de la dynamique non linéaire des équations de Navier-Stokes. / The dynamics and control of a separated boundary-layer flow have been investigated. Separation is triggered by a bump mounted on a flat plate and the transition dynamics has been investigated experimentally. For a certain parameter range, the recirculation region is subject to self-sustained low-frequency oscillations, and results from the numerical simulation for the same geometry are recovered. These results show that low frequency oscillations, observed mainly in compressible flow regimes, are inherent to elongated recirculation bubbles.The control of this low-frequency instability has been investigated using modern control theory based on two complementary approaches. Feedback control of the linear perturbation dynamics is first considered. Global instability modes are used to build reduced-order estimators. This model reduction gives rise to low-dimensional compensators capable of controlling the unstable dynamics. Once coupled to the unstable linearised Navier-Stokes system, the compensator is seen to succesfully control the unstable dynamics. The control of the nonlinear dynamics is then investigated using adjoint-based optimisation procedures. This method is used to compute control laws based on a complete knowledge of the nonlinear dynamics. Although the low-frequency instability is clearly attenuated, it seems difficult to control the flow towards its steady state, using only a few blowing/suction actuators localized on the wall.

<p>Transition from laminar to turbulent flow has beentraditionally studied in terms of exponentially growingeigensolutions to the linearized disturbance equations.However, experimental findings show that transition may occuralso for parameters combinations such that these eigensolutionsare damped. An alternative non-modal growth mechanism has beenrecently identified, also based on the linear approximation.This consists of the transient growth of streamwise elongateddisturbances, mainly in the streamwise velocity component,called streaks. If the streak amplitude reaches a thresholdvalue, secondary instabilities can take place and provoketransition. This scenario is most likely to occur in boundarylayer flows subject to high levels of free-stream turbulenceand is the object of this thesis. Different stages of theprocess are isolated and studied with different approaches,considering the boundary layer flow over a flat plate. Thereceptivity to free-stream disturbances has been studiedthrough a weakly non-linear model which allows to disentanglethe features involved in the generation of streaks. It is shownthat the non-linear interaction of oblique waves in thefree-stream is able to induce strong streamwise vortices insidethe boundary layer, which, in turn, generate streaks by thelift-up effect. The growth of steady streaks is followed bymeans of Direct Numerical Simulation. After the streaks havereached a finite amplitude, they saturate and a new laminarflow, characterized by a strong spanwise modulation isestablished. Using Floquet theory, the instability of thesestreaks is studied to determine the features of theirbreakdown. The streak critical amplitude, beyond which unstablewaves are excited, is 26% of the free-stream velocity. Theinstability appears as spanwise (sinuous-type) oscillations ofthe streak. The late stages of the transition, originating fromthis type of secondary instability, are also studied. We foundthat the main structures observed during the transition processconsist of elongated quasi-streamwise vortices located on theflanks of the low speed streak. Vortices of alternating signare overlapping in the streamwise direction in a staggeredpattern.</p><p><strong>Descriptors:</strong>Fluid mechanics, laminar-turbulenttransition, boundary layer flow, transient growth, streamwisestreaks, lift-up effect, receptivity, free-stream turbulence,nonlinear mechanism, streak instability, secondary instability,Direct Numerical Simulation.</p> / QC 20100518

No description available.

No description available.

Cooling of electronic components is one of the most important issues concerned in the electronic industry for design of equipment. Maintaining the temperature of an electronic device within its safe operating temperature limits is essential to operate the equipment safely with proper functionality. According to the Arrhenious law of failure rate, for a device with activation energy 0.65eV, every 10°C increase in temperature doubles the failure rate. Recent miniaturisation of components and high device heat dissipation rates lead to high heat fluxes, which cause temperature rise. Hence, there is an increasing need for research to achieve high heat removal rates and optimal design. Several cooling techniques are used for cooling of electronics based on the application and cooling rate requirements. Air-cooling of electronics has a wide range of applications due to its greater reliability, simplicity, easy maintenance, low cost, easy availability of coolant (air), and light weight. Air-cooling is also free from boiling and dripping problems. Air-cooling is used in applications such as avionics, cooling of personal computers, cooling of data centers, and in automobile electronics. In a typical electronic cooling application, cooling fluid is driven by the combination of external pressure forces and buoyancy forces. Based on the relative contribution of these forces towards the total driving force, the cooling techniques can be categorized as forced, natural or mixed convection cooling. However, in many of the electronic cooling situations, such as in the applications with very high heat fluxes, tall Printed Circuits Boards (PCBs) with low forced convection velocity, and in large scale applications such as data centers, the contributions of the buoyancy forces and external pressure forces for the total driving force are comparable, which results in a mixed convection situation. In the present study, mixed convection in vertical channels heated with five heating configurations, which represent typical electronic cooling applications, is studied numerically. The five different heating configurations are channels with flush-mounted continuous heater, flush-mounted strip heaters, flush-mounted square block heaters, protruding rib heaters and protruding square heaters. The first three configurations are categorised as flush-mounted heating configurations and the latter two configurations are categorised as protruded heating configurations. One of the channel walls represents the substrate on which the heaters are mounted and the heat sources represent the heat generating electronic components. Heat transfer under steady state conditions is considered in the study. The study includes laminar as well as turbulent heat transfer. For a systematic study of mixed convection, an analytical or semi-analytical formulation is desirable for a simplified model, as it can highlight the effect of relevant non-dimensional parameters on the heat transfer characteristics of a system. The results of a simplified model can be used for benchmarking the results of practical situations. Hence, before numerically solving the governing equations for mixed convection in channels, mixed convection boundary layer flows over a heated vertical plate is considered for study. Perturbation technique is used to solve the boundary layer equations with non-isothermal boundary conditions. The perturbation analysis is carried out for an arbitrarily variation of wall temperature or heat flux. Subsequently, the results are extended to find heat transfer rates in the cases of power-law variation of temperature and heat flux, as special cases. It is always required to design a cooling system to remove maximum possible amount of heat, keeping the device temperature within its safe operating limits. Hence, optimization of heat transfer in boundary layers is attempted, whose results can be used as guidelines to achieve optimal heat transfer in practical situations of channels with continuous as well as discrete heating. Similarity analysis is used for the optimization of heat distribution in boundary layer flows. In the similarity analysis, in the search of optimal heat transfer from the plate, the boundary layer equations are solved for various power-law heat flux variations and the appropriate power-law variation of optimal heat transfer is found. Similarly, the heat flux variation for optimal heat transfer is found for the cases of natural and forced convection, as they are the limiting cases of mixed convection. In the numerical part of the study, the generalised three-dimensional governing equations for the five heating configurations considered for the study are solved numerically with appropriate boundary conditions. Separation of natural, forced and mixed convection regimes is carried out in all the heating configurations using a criterion based on individual contributions of pressure force and buoyancy force towards the total driving force for the fluid movement. Heat transfer characteristics are studied in laminar as well as turbulent regimes in terms of parameters such as Grashof number, Reynolds number, Nusselt number, maximum temperature of heaters, pressure drop across the channel, and so on. The influence of conjugate effects on the heat transfer characteristics is studied by varying the substrate thermal conductivity. A systematic comparison of various effects such as the effect of discrete heating in plain channels, effect of discrete heating in channels with heated ribs, and the effect of three-dimensional protrusions on heat transfer, is achieved. The parameters in the individual configurations, which affect heat transfer, are explored for better cooling solutions. Optimal heat distribution among the heaters to minimise the temperature of the hottest heater for a given total amount of heat generation in the channel is found for all the channel configurations, which are heated either continuously or discretely. In the process of finding the optimal heat distribution among heaters, guidelines are taken from the optimal heat distribution in boundary layer flows. Compared to usual optimization approaches such as genetic algorithm, the present physics based optimisation procedure requires fewer runs to arrive at the optimal distribution. The fluid flow characteristics in all the three configurations with flush-mounted heaters are found to be similar. However, heat transfer characteristics in channels with flush-mounted square heaters differ from those in the other two flush-mounted channel configurations. Hot spots with higher temperatures are found at heater locations in channels with flush-mounted square heaters. The effect of substrate follows the same trend in all the flush-mounted configurations. At lower thermal conductivities, the maximum temperature decreases sharply with increasing thermal conductivity. However, at higher conductivities, the influence reduces. In all the flush-mounted configurations, heat transfer will not be influenced by substrate thermal conductivity increment at conductivities more than 150 times the fluid thermal conductivity. The fluid flow and heat transfer characteristics in channels with protruded heaters differ significantly from those in channels with flush-mounted heaters. The protrusions in the channels interact with the fluid flow and make it different from that of smooth channels. In turn, the protrusions affect heat transfer characteristics in the channels. The influence of the protrusions on the heat transfer and locations of hot spots in the domain is examined. Effect of thermal conductivity in channels with protruded square heaters is similar to that in channels with flush-mounted heaters. However, conductivity in channels with protruded rib heaters affects the heat transfer in a wider range of conductivities than in the other heating configurations. Unlike in the other configurations, at low thermal conductivities, maximum temperature does not drop sharply with increase of conductivity. In channels with protruded square heaters, staggering arrangement of heaters results in higher heat transfer rates than those with in-line heater arrangement. In all the configurations, pressure drop is found to be independent of Grashof number in the range of heat dissipation rates considered in the study. Heat transfer rates in turbulent region are much higher than the heat transfer rates in laminar regime. However, the pressure drops encountered are also high in the turbulent regime. Turbulent heat transfer results in a more uniform temperature distribution in channels. The cooling performances of the individual configurations are compared. For a given pressure drop the cooling performances decreases in the order of flush-mounted strip heating, protruded square heating, flush-mounted square heating, protruded rib heating. For a given inlet fluid flow rate, the cooling performances decreases in the order of protruded rib heating, protruded square heating, flush-mounted square heating, flush-mounted strip heating. However, for a given inlet fluid flow rate, the pressure drop increases in the order of increasing cooling performance.