L'écoulement autour des véhicules produit une couche limite turbulente très proche de la paroi. Le caractère turbulent induit des fluctuations de pression pariétale qui font vibrer les panneaux du véhicule. Ces vibrations sont alors transmises à travers la structure et rayonnent du bruit dans l’habitacle. Les niveaux sonores dus à l'écoulement augmentent avec la vitesse du véhicule. Pour cette raison, cette problématique connaît un intérêt croissant dans le secteur aéronautique.Le but de cette thèse est double : comprendre les mécanismes à l’origine de ces fluctuations de pression à la paroi et prédire l’excitation de la structure avion due à l’écoulement turbulent.Pour ce faire, des calculs Large Eddy Simulation (LES) sont disponibles. Il s’agit de bases de données numériques d’écoulements de couches limites turbulentes en présence de gradients de pression favorable, adverse et nul. Ceci permet de caractériser l’écoulement sur des géométries courbes telles que la pointe avant d’un avion. L’effet du gradient de pression sur des structures cohérentes de type « hairpins » et paquets de « hairpins » a pu être identifié et quantifié à travers des méthodes de visualisation et d’analyse statistique. Une méthode d’estimation stochastique du champ de vitesse a révélé la présence de tourbillons contra-rotatifs au-dessus des paquets de hairpins. Ces tourbillons ont une vorticité opposée à celle des hairpins et un modèle de « hairpins inversé » a été proposé.En s’inspirant du travail de Ahn et al. (2010), un modèle stochastique de spectre de la pression pariétale a été développé. Il s’agit de reconstruire un champ stochastique de vitesse instantanée et d’en déduire le champ de pression à la paroi en résolvant une équation de Poisson sur la pression. Le champ de vitesse est obtenu en soumettant des structures de type paquets de hairpins à un écoulement moyen. Les caractéristiques des paquets de hairpins en fonction du gradient de pression sont basées sur l’analyse des bases LES. Les résultats de ce modèle sont comparés à ceux issues de bases de données numériques et expérimentales. Enfin, ce modèle est utilisé pour caractériser l’écoulement de couche limite turbulente dans une simulation de Statistical Energy Analysis (SEA) afin de prédire les niveaux de vibration des panneaux d’une portion de fuselage d’avion. / The flow around vehicles creates a turbulent boundary layer in the vicinity of the wall. The turbulent behavior induces pressure fluctuations that make the panels vibrate. These vibrations are then transmitted though the structure of the vehicle and radiate noise inside the cabin. The flow-induced noise levels increase with the speed of the vehicle. For this reason, aircraft manufacturers show a great interest in this topic.There are two objectives for this thesis: understand the mechanisms responsible for the wall-pressure fluctuations and predict this source of aircraft panel excitation.A study of available Large Eddy Simulation (LES) computations was performed. The database consists in simulations of turbulent boundary layer flows submitted to favorable, adverse and zero pressure gradients. This is necessary to understand the nature of the flow over curved geometries such as the aircraft flight deck. The effect of pressure gradients on coherent hairpin structures and hairpin packets could be identified and quantified based on visualization and statistical analysis methods. Linear stochastic estimation of the velocity fields revealed a pair of counter-rotating streamwise vortices above hairpin packets. These vortices have a vorticity opposite to that of the hairpins and an “inverse hairpin” model was proposed.Following the work of Ahn et al. (2010), a stochastic model for wall-pressure spectrum was developed. The idea is to build a stochastic turbulent velocity field using hairpin packets which are subjected to a mean flow. The characteristics of the packets depending on the pressure gradient are based on the analyses of the LES database. The pressure field at the wall is obtained by solving a Poisson equation. The results of the hairpin packet model are compared to numerical and experimental data. Finally, the model is used as input for a Statistical Energy Analysis (SEA) simulation in order to predict the levels of vibrations of panels submitted to a turbulent boundary layer flow over a portion of an aircraft cabin.

Le contrôle d’écoulement permet d’éliminer le phénomène de décollement de couches limites, très néfaste pour les performances des machines interagissant avec un fluide (avions, voitures, turbomachines ...). Ces travaux s’intéressent plus particulièrement au contrôle actif d’écoulement au moyen de jets continus. Une maquette permettant de manipuler l’équilibre de la couche limite a été conçue et installée dans la soufflerie du Laboratoire de Mécanique de Lille. La première partie du travail a consisté en la caractérisation de l’écoulement autour du modèle à l’aide de visualisations par fils de laine et par enduit gras, de mesures de répartition de pression, de mesures par anémométrie à fils chauds et par PIV. Ceci a permis de définir la configuration du modèle la plus appropriée pour les études de contrôle mais aussi de connaître précisément les caractéristiques de l’écoulement sélectionné. La configuration retenue correspond à un écoulement en gradient de pression adverse suivi d’une séparation sur le volet, un peu comme sur l’extrados d’une aile d’avion. L’utilisation de sondes de frottement associées à des visualisations aux fils de laine ont permis d’étudier et d’optimiser des actionneurs passifs, puis des actionneurs à jets continus. Certaines des configurations actives optimales ont ensuite été caractérisées plus en détail par une mesure par PIV englobant toute la zone de séparation. Il apparaît que les jets continus ne suppriment pas complètement les mécanismes de la séparation mais réduisent leur intensité et les concentrent plus ou moins près de la paroi / Flow control allows to suppress boundary layers separation, which largely deteriorates the performances of machineries which interact with fluid (aircraft, cars, turbomachineries, etc.). This study concentrates more particularly on active flow control with continuous jets. A ramp model which allows to manipulate the boundary layer equilibrium was realized and set in Laboratoire de Mécanique de Lille wind tunnel. The first part of the work was to characterize the flow over the model with wool-tufts and oil-film visualisations, pressure distribution, hot-wire anemometry and PIV measurements. The aim was to define a ramp configuration for the flow control study and to know precisely the characteristics of the retained flow. The selected configuration corresponds to an adverse pressure gradient flow followed by a separation on the flap, which mimics the flow on the suction side of a wing. With friction probes coupled with wool-tufts visualisations, passive actuators and active continuous jets were studied and optimised. Finally, some of the optimum active configurations found were characterized in more details with PIV measurements over the entire separated region. It appears that continuous jets do not suppress the separation mechanisms, but only reduce their intensity and squeeze them more or less against the wall

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work we present a computational study of the properties of water transport across membranes formed of carbon nanotubes (CNT) armchair with different diameters, when subjected to pressure gradients 80-400 MPa. This study was made through the implementation of the method of molecular dynamics code NAMD. The model adopted for our simulation consists of a unit cell containing water molecules that will cross through the reverse osmosis process, 9 carbon nanotubes 20Å in length, using the periodic boundary conditions. The results show a quadratic dependence of the flow on the diameter of the nanotubes. CNT in smaller diameters, the dependence of the flow on the pressure applied is small. This behavior differs for larger diameter nanotubes, where the flow increases linearly with pressure. When analyzing the mean square displacement (MSD) as a function of the diameter of the CNT, we found the occurrence of a regime of ballistic diffusion for all cases. The effect of CNT on the diameter of MSD for the same pressure gradient indicates differences between the coefficients of diffusion of the bulk water and CNT. We found evidence of the occurrence of a non-Fickian transport for this condition of confinement. The analysis flow of water molecules suggests that the CNT membranes can apparently be competitive systems with existing techniques and used on the market for water purification. / Neste trabalho apresentamos um estudo computacional das propriedades de transporte da água através de membranas formadas por nanotubos de carbono (NC) armchair de diferentes diâmetros, quando submetidos a gradientes de pressão de 80 a 400 MPa. A realização deste estudo se deu através da implementação do método de Dinâmica Molecular ao código NAMD. O modelo adotado para a nossa simulação consta de uma célula unitária contendo moléculas de água que irão atravessar, através do processo de osmose reversa, 9 nanotubos de carbono de 20Å de comprimento, valendo-se das condições periódicas de contorno. Os resultados obtidos demonstram uma dependência quadrática do fluxo com o diâmetro dos nanotubos. Em NC de diâmetros menores, a dependência do fluxo com a pressão aplicada é pequena. Este comportamento difere para nanotubos de diâmetros maiores, onde o fluxo aumenta linearmente com a pressão. Quando analisado o deslocamento quadrático médio (MSD) como função do diâmetro dos NC, verificamos a ocorrência de um regime de difusão balístico para todos os casos. O efeito do diâmetro dos NC sobre o MSD, para um mesmo gradiente de pressão, indica diferenças entre os coeficientes de difusão da água bulk e NC. Foram encontradas evidências de ocorrência de um transporte não Fickiano para esta condição de confinamento. A análise da vazão das moléculas de água sugere que as membranas de NC podem, aparentemente, ser sistemas competitivos com as técnicas já existentes e utilizadas no mercado para purificação de água.

The object of the diploma thesis is to optimize vacuum function of the ejector using computional fluid dynamics, or CFD. The ejector is inserted to the intake system of the six - cylinder diesel engine to increase the pressure gradient of the exhaust gas recirculation system. The analysis of the current design solution is performed and then the ejector with the new shape affording higher vacuum effect is proposed. The introductory part is devoted to the questions of diesel engines emissions and technology to reduce emissions in the exhaust system focused on the EGR.

Couette, pipe, channel, and zero-pressure gradient (ZPG) turbulent boundary layer (TBL) flows have classically been considered as canonical wall-bounded turbulent flows since their near-wall behavior is generally considered to be universal, i.e. invariant of the flow case and the Reynolds number. Nevertheless, the idea that large-scale motions, being dominant in regions further away from the wall, might interact with and influence small-scale fluctuations close to the wall has not been disregarded. This view was mainly motivated due to the observed failure of collapse of the Reynolds normal stresses in viscous scaling. While this top-down influence has been studied extensively over the last decade, the idea of a bottom-up influence (backward energy transfer) is less examined. One exception was the recent experimental work on a Couette flow by Kawata, T. & Alfredsson, P. H. (Phys. Rev. Lett. 120, 244501, 2018). In the present work, a spectral representation of the Reynolds Stress transport equation is used to perform a scale-by-scale analysis of the terms in the equation. Two flow cases were studied: first, a Direct Numerical Simulation (DNS) of a Couette flow at a similar Reynolds number as Kawata and Alfredsson. The Reynolds number was ReT = 120, viscosity v. Second, a Large Eddy Simulation (LES) of a ZPG TBL at ReT = 730, 1270, and 2400. For both cases the classic interscale transport or turbulent kinetic energy was observed. However, also an inverse interscale transport of Reynolds shear stress was observed for both cases.

No description available.

O escoamento líquido-líquido, em especial o escoamento óleo-água, vem atraindo a atenção de pesquisadores devido à alta demanda pelo combustível fóssil no atual cenário petrolífero mundial e nacional. Os desafios tecnológicos colocados pelas descobertas de reservas de óleos pesados e altamente viscosos consideram, em especial, a preocupação por minimizar as perdas energéticas nas linhas. Emulsões inversas ou dispersões óleo-em-água, na qual o óleo se encontra disperso de maneira uniforme em água, caracteriza-se pela baixa viscosidade aparente, tornando-se um tipo de emulsão desejável em algumas etapas do transporte de petróleo. Esses fatos tornam essencial o estudo deste tipo de padrão para o dimensionamento e operação ótima de dutos de produção de petróleo. Contudo, não existe ainda um número abrangente de trabalhos sobre padrão disperso líquido-líquido, ao comparar com escoamento em fases separadas. Trabalhos sobre dispersões têm reportado redução de atrito sem a adição de substâncias químicas em regime turbulento. No entanto, não há ainda um entendimento satisfatório do fenômeno. Na maioria dos trabalhos, sendo quase todos realizados com óleos leves e pouco viscosos, a redução é reportada em dispersões água-em-óleo, com escassos trabalhos reportando o fenômeno em dispersões óleo-em-água. A pesquisa realizada tratou do estudo experimental e teórico de dispersões óleo-em-água em tubulações. O escoamento foi caracterizado a partir da obtenção de dados de holdup, gradiente de pressão por fricção, distribuição das fases e padrão de escoamento. Uma teoria foi proposta para explicar a redução de atrito detectada neste trabalho, baseada na existência de um filme fino de água que escoa em contato com a parede do tubo, a baixos números de Reynolds, evitando o contato direto do núcleo turbulento (mistura bifásica) com a parede do tubo. O referido filme líquido foi detectado e quantificado utilizando-se técnica visual. Além disso, um modelo dinâmico baseado na teoria de lubrificação hidrodinâmica foi desenvolvido como tentativa de explicar a formação do filme líquido parietal no escoamento turbulento de dispersões óleo-água. / Liquid-liquid flow, especially oil-water flow, has attracted the attention of researchers due to the high demand for petroleum in the current global scenario. The discovery of reserves of heavy and highly viscous oils creates new challenges which are mainly concerned with reducing the significant pressure drop in pipes. Inverse emulsion or oil-in-water dispersions in which the oil is dispersed in water is characterized by its low effective viscosity, making it a desirable type of emulsion in some steps of oil production. These facts make the study of dispersed liquid-liquid flow essential for the design and optimal operation of oil pipelines. However, the studies on such flow pattern are scanty in comparison to those on separate flows, as stratified and annular flow patterns. Drag reduction in oil-water turbulent flow without the addition of any chemical substance has been reported in some studies. This phenomenon has received increasing attention in recent years, because there is not a satisfactory understanding of its dynamics yet. Most studies, almost all using light oils, report drag reduction in dispersion of water-in-oil, with few studies reporting the phenomenon in oil-in-water dispersions. This research comprises an experimental and theoretical study on oil-in-water dispersions in pipes. Pressure gradient, holdup, phase distribution and flow patterns data were obtained to characterize the two-phase flow. A theory was proposed to explain the drag reduction detected in this work, based on the existence of a thin water film flowing in contact with the pipe wall at low Reynolds numbers, avoiding contact between the turbulent core (mixture) and the pipe wall. The liquid film was detected and quantified using visual technique. In addition, a dynamic model based on the hydrodynamic lubrication theory was developed as an attempt to explain the formation of the liquid film.

The work reported in this thesis is an experimental and theoretical investigation of the so-called sink flow boundary layers. These are two-dimensional (in the mean), favourable-pressure-gradient (FPG) boundary layer flows where the boundary layers experience stream-wise acceleration inside a two-dimensional convergent channel with smooth and plane walls. The boundary layers studied are mainly turbulent with few cases that may be identified as reverse-transitional. The sink flow turbulent boundary layers (TBLs) are the only smooth-walled layers that are in ‘perfect equilibrium’ or ‘exact self-preservation’ in the sense of Townsend (1976) and Rotta (1962). The present boundary layer experiments were conducted in an open-return low-speed wind tunnel. The sink flow conditions were established on the test-plate by using a contoured test-section ceiling for creating a convergent channel with smooth and plane walls. The strength of the streamwise FPG was varied by changing the freestream speed in the test-section. Few zero-pressure-gradient (ZPG) turbulent boundary layers were also measured in the same tunnel for which the contoured ceiling was replaced by a straight one. The velocity measurement techniques used include conventional Pitot-tubes for mean flow measurements and hotwire/crosswire probes for turbulence measurements. For measurement of skin friction in ZPG flows, Preston-tube was used while for the sink flows the so-called surface hotwire method was employed. Static pressures were measured on the test-surface using an alcohol-based projection manometer. Boundary layers were tripped at the beginning of the test-plate to ensure quick transition to turbulence. The mean velocity scaling in sink flow TBLs in the presence of strong FPG has been studied systematically, especially in view of the apparent pressure-gradient-dependence of the logarithmic laws reported in the literature (Spalart & Leonard, 1986; Nickels, 2004; Chauhan et al., 2007). The experimental study of sink flow TBLs carried out over a wide range of streamwise FPGs has shown that the mean velocity profiles (in inner coordinates) exhibit systematic departures from the universal logarithmic law as the pressure gradient parameter ∆p is varied. Even so, each of these profiles exhibits a logarithmic region, albeit non-universal, whose constants are functions of the pressure gradient. Systematic dependence of these constants on the pressure gradient parameter ∆p is observed. Moreover, the wake region is uniformly absent in all these profiles. In other words, each profile looks like a ‘pure wall-flow’, in the sense of Coles (1957), only if it is viewed in relation to its own non-universal logarithmic law. To support the experimental observation of the pressure-gradient-dependence of logarithmic laws in sink flow TBLs, a theory based on the method of matched asymptotic expansions has been applied to sink flow TBLs and this theory reveals a systematic dependence of inner and outer logarithmic laws on the pressure gradient parameter ∆p. This dependence is essentially a higher-order effect and therefore becomes significant only in the presence of relatively strong pressure gradients. Comparison of the theory with the experimental data demonstrates that the disappearance of the universal logarithmic law in strong FPG situations does not necessarily imply the absence of classical inner-outer overlap region. The overlap may still manifest itself as a logarithmic functional form with constants that are strongly influenced by the magnitude of the FPG. An immediate use of the non-universal log laws is towards the estimation skin friction in strong-pressure-gradient equilibrium and near-equilibrium TBL flows and this issue has been studied in some detail. It is shown that the conventional Clauser-chart method for estimation of skin friction (which gives fairly accurate results for ZPG or mild-pressure-gradient flows), originally proposed by Clauser (1954), can be modified to deal with the situations involving strong streamwise pressure gradients, provided that the equilibrium or near-equilibrium TBL under consideration is not very close to relaminarization or separation. In such cases, the overlap layer manifests itself in the form of non-universal logarithmic laws that are dependent on the local strength of the pressure gradient. Using these non-universal log laws in conjunction with the measured pressure distribution (necessary for obtaining the acceleration parameter K) and a measured mean velocity profile, it is possible to obtain the local skin friction coefficient to an accuracy which is typical of skin friction measurements. This modified Clauser-chart method (MCCM) employs a two-fold iterative procedure (one iteration on Cf and the other on ∆p) in contrast to the conventional method that involves only one iteration (on Cf alone). As a by-product of this MCCM, one obtains the local pressure gradient parameter ∆p and the slope 1/κ and intercept C of the non-universal log law for that profile. It is also demonstrated that the arm´MCCM is quite robust to the changes in the universal values of K´arman constant κ0 and intercept C0 for the ZPG turbulent boundary layer. Various aspects of the large-scale structure in turbulent and reverse-transitional sink flow boundary layers subjected to streamwise FPGs have also been investigated. The use of sink flow configuration allows systematic characterization of the large-scale structure with the strength of the FPG as a parameter where the characterization is not contaminated by the upstream history effects. The large-scale structure is identified by cross-correlating the wall-shear stress fluctuation with the streamwise velocity fluctuation. The structure orientation is found to be linear over a large wall-normal extent typically extending from y/δ of 0.1 to 0.6. Beyond y/δ =0.6, the correlation under consideration becomes very weak to allow any conclusive results. The average structure inclination angle αavg is found to decrease systematically with increase in the streamwise FPG. This result is important and has implications towards modeling of the near-wall region. Further it is found that the structure gets elongated considerably as the FPG is increased, i.e. the streamwise spatial extent of the structure increases. Taken together, it is observed that the structure becomes flatter and longer with the increase in FPG. Structural models are proposed for sink flow TBLs in the form of either the shape of individual hairpin vortices or the possible structural self-organization. These models are then discussed in the light of present experimental results. It is also shown that the process of relaminarization of a TBL by strong FPG may be better appreciated by appealing to these structural models. The validity of Taylor’s hypothesis for structure angle measurements in the present study has been established experimentally. This exercise is important since the flows under consideration are highly accelerated and sometimes even reverse-transitional. In most of the previous work on the validity of Taylor’s hypothesis, at least for the measurements similar to the present work, the emphasis has been on ZPG turbulent boundary layers. The present exercise is therefore crucial for accelerating flows. Possible reasons for the observed validity of Taylor’s hypothesis have also been identified − specifically it is seen that the condition ∆xp/L << 1 needs to be met for Taylor’s hypothesis to be valid in pressure gradient flows. Investigation of the structure convection velocity from the space-time correlations has revealed that the convection velocity of a typical structure in the present sink flow boundary layers is almost equal to the local mean velocity (more than 90%). This implies that the structure gets convected downstream almost along with the mean flow. Near-wall ‘active’ and ‘inactive’ motions in sink flow TBLs have been studied, discussed and compared with the corresponding results for ZPG turbulent boundary layers from five different aspects: (i) turbulent diffusion of TKE, (ii) quadrant statistics, (iii) profiles of the streamwise turbulence intensity, (iv) event correlation length scales obtained from conditional sampling on the instantaneous flux signal and (v) profiles of the Townsend parameter Tp =(−uv) /u2. Near-wall inactive motion is seen to be related to the strength of the large-eddy structure in the outer region of TBL flow. For APG flows the near-wall inactive motion is known to be more intense (Bradshaw, 1967b) than the ZPG flows, say at the same K´arman number δ+. This observation is consistent with a stronger large-eddy structure that may be perceived from the stronger wake component in the mean velocity variation and the larger mean entrainment in an APG turbulent boundary layer as compared to the ZPG flow at same δ+. In sink flow TBLs, the large-eddy structure is much weaker in comparison to the ZPG flow at same δ+ which is consistent with the absence of wake component in the mean velocity profile as well as the zero mean entrainment into the layer. A sink flow TBL represents, a state of weakest large-eddy structure and hence minimum intensity of inactive motion compared to any other equilibrium or near-equilibrium TBL flow having the same K´arman number δ+. All the analysis of the relevant experimental data seems to support this.

Ένα από τα σπουδαιότερα προβλήµατα της σύγχρονης αεροδυναµικής και διαστηµικής τεχνολογίας, αν όχι το σπουδαιότερο, είναι αυτό του ελέγχου (control) του οριακού στρώµατος (boundary layer) που αναπτύσσεται (περιβάλλει) ένα στερεό σώµα που κινείται µέσα σ’ ένα ρευστό. Παρ’ όλο που στην αρχή του αιώνα που διανύουµε συµπληρώνονται εκατό περίπου χρόνια από την διατύπωση της έννοιας του οριακού στρώµατος από τον L. Prandtl (1904), η έρευνα στο πρόβληµα αυτό εξακολουθεί να παραµένει επιτακτική και αναγκαία όσο και κατά τα πρώτα χρόνια της ανάπτυξης της αεροπορικής και διαστηµικής τεχνολογίας. Με τον όρο έλεγχο του οριακού στρώµατος εννοούµε την ανάπτυξη µεθόδων - τεχνικών η εφαρµογή των οποίων πάνω στην ροή θα της µεταβάλλει την δοµή και θα της προσδώσει επιθυµητά χαρακτηριστικά. Από τις αρχές του εικοστού αιώνα (1904) ο Prandtl περιέγραψε αρκετές πειραµατικές διατάξεις µέσω των οποίων πραγµατοποιούσε έλεγχο του οριακού στρώµατος. Με την ανάπτυξη της αεροπορικής τεχνολογίας κατά και µετά τον ∆εύτερο Παγκόσµιο Πόλεµο, και αργότερα της διαστηµικής, το πρόβληµα του ελέγχου του οριακού στρώµατος απέκτησε τεράστια σηµασία, ειδικά για την αποφυγή του διαχωρισµού ή της αποκόλλησης (separation) αυτού, της ελάττωσης της αντίστασης (drag) και την αύξηση της άντωσης (lift). Μεταξύ των σπουδαιότερων και πιο αποτελεσµατικών µεθόδων – τεχνικών που αναπτύχθηκαν για τον σκοπό αυτό µπορεί να αναφερθούν: 1. Η κίνηση του στερεού τοιχώµατος (motion of the solid wall) 2. Η επιτάχυνση του οριακού στρώµατος (blowing) 3. Η απορρόφηση (suction) 4. Η έγχυση ίδιου ή διαφορετικού ρευστού (injection, binary boundary layers) 5. Πρόληψη της µετάπτωσης της ροής από στρωτή σε τυρβώδη µε διαµόρφωση κατάλληλων σχηµάτων των στερεών τοιχωµάτων (laminar airfoils) 6. Ψύξη των τοιχωµάτων (cooling) Η προσπάθεια υπολογισµού του σηµείου αποκόλλησης και των συνθηκών που οδηγούν σ’ αυτήν οδήγησε στην επινόηση διαφόρων µεθόδων για την τεχνική της παρεµπόδιση. Σε µια ροή η αποκόλληση µπορεί να εµποδιστεί ή να καθυστερήσει, όπως αναφέρθηκε, µε την εφαρµογή ενεργητικών ή παθητικών µεθόδων ελέγχου, όπως απορρόφηση, έγχυση, παθητικές διατάξεις, ψύξη ή θέρµανση, κλπ. Τέτοιες τεχνικές ελέγχου χρησιµοποιούνται στις άκρες των πτερύγων των αεροσκαφών της Boeing (γεννήτριες στροβίλων), στα αεροσκάφη παλαιότερης γενιάς στις πίσω επιφάνειες καµπυλότητας (flaps) ή στην οδηγούσα ακµή της πτέρυγας στις νεώτερες γενιές, µε τις εµπρόσθιες επιφάνειες καµπυλότητας (slats). Η πιο αποδεκτή τεχνική ελέγχου του οριακού στρώµατος είναι η τεχνική της έγχυσης/απορρόφησης. Σαν τεχνική ελέγχου χρησιµοποιείται από παλιά. Κατά την δεκαετία του ’60 δοκιµαστικές πτήσεις του πειραµατικού αεροσκάφους X-21 έδειξαν ότι η στρωτή ροή διατηρείται πάνω από την πτέρυγα µε την χρήση απορρόφησης µέσα από πολλές σχισµές πάνω σ’ αυτήν. Πρόσφατες δοκιµαστικές πτήσεις ενός µετασκευασµένου αεροσκάφους F-16XL, που χρησιµοποιεί την τεχνική της απορρόφησης πάνω σε ειδικές διατάξεις LERX (LEading Root eXtensions), έδειξαν διατήρηση της στρωτής ροής και µείωση της αντίστασης. Πρόσφατα πειράµατα εφαρµογής απορρόφησης κατά µήκος της οδηγούσας ακµής πτέρυγας έδειξαν ότι, κάτω από κατάλληλες συνθήκες, καθυστερεί η “µόλυνση” (contamination) της ακµής που οφείλεται στις γειτονικές µ’ αυτήν δοµές (κινητήρας, άτρακτος, λοιπές αεροδυναµικές διατάξεις) που συµµετέχουν στην ροή. Πολλοί είναι αυτοί που έχουν προτείνει διάφορες διατάξεις έγχυσης/απορρόφησης. Μεταξύ αυτών συγκαταλέγεται η υβριδική επιφάνεια απορρόφησης (hybrid suction surface) που αποτελείται από µια συστοιχία σχισµών κοντά η µια στην άλλη προς την διεύθυνση της µέσης ροής και η επιλεκτική απορρόφηση (selective suction), στην οποία µικρής έντασης απορρόφηση εφαρµόζεται σε σχισµές τοποθετηµένες σε κατάλληλες θέσεις. Τέλος, και η τοπική απορρόφηση (localized suction) που εφαρµόζεται σ’ ένα µικρό τµήµα της επιφάνειας. Επίσης, µε την ανάπτυξη της µαγνητοϋδροδυναµικής (MHD), της επιστήµης δηλαδή που µελετά τα ροϊκά φαινόµενα όταν το ηλεκτρικά αγώγιµο ρευστό υπόκειται στην επίδραση ενός ηλεκτρικού ή και µαγνητικού πεδίου, προστέθηκε στα µέσα ελέγχου του οριακού στρώµατος ένα επιπλέον. Από την δεκαετία του ’60 το µαγνητικό πεδίο χρησιµοποιείται επίσης σαν τεχνική ελέγχου στην σύγχρονη αεροδυναµική, λόγω της ικανότητας του να σταθεροποιεί την ροή και να εµποδίζει την µετάπτωση της. Χρησιµοποιήθηκε σαν τεχνική ελέγχου στα διαστηµικά οχήµατα που επανέρχονται στην ατµόσφαιρα από το διάστηµα και σε αεροσκάφη που πετούν σε µεγάλα ύψη µε µεγάλες ταχύτητες. Βρίσκει όµως εφαρµογές και στις MHD ροές µέσα σε σήραγγες όπου κι εκεί οι ροές είναι συµπιεστές (γεννήτριες πλάσµατος, MHD επιταχυντές, συσκευές πυρηνικής σύντηξης). Εφαρµογές της MHD υπάρχουν επίσης στα αέρια των νεφελωµάτων που συνθέτουν τα άστρα, στην κίνηση του υδρογόνου του Ήλιου ή ακόµα και στον ηλιακό άνεµο που µεταφέρει τα ιονισµένα σωµατίδια στην επιφάνεια της Γης. Η παρούσα διατριβή αναφέρεται στην µελέτη της χρονοανεξάρτητης διδιάστατης µαγνητοϋδροδυναµικής (MHD) συµπιεστής ροής οριακού στρώµατος πάνω από επίπεδη επιφάνεια µε αντίξοη βαθµίδα πίεσης και µεταφορά θερµότητας και µάζας. Το ερευνητικό µέρος της εργασίας αυτής µπορεί να χωριστεί σε δύο κύρια µέρη (Κεφάλαια ΙΙ και ΙΙΙ). Στο πρώτο µέρος (Κεφάλαιο ΙΙ) γίνεται µελέτη της MHD συµπιεστής ροής στρωτού οριακού στρώµατος µε αντίξοη βαθµίδα πίεσης και µεταφορά θερµότητας και µάζας πάνω από επίπεδη πλάκα. Στο δεύτερο µέρος (Κεφάλαιο ΙΙΙ) µελετάται η MHD συµπιεστή ροή τυρβώδους οριακού στρώµατος µε αντίξοη βαθµίδα πίεσης και µεταφορά θερµότητας και µάζας πάνω από επίπεδη πλάκα. Αρχικά, σε ένα εισαγωγικό Κεφάλαιο (Κεφάλαιο Ι), παρουσιάζονται, πολύ περιληπτικά, οι βασικές έννοιες που είναι απαραίτητες για την κατανόηση της διατριβής καθώς και οι θεµελιώδεις εξισώσεις της µαγνητοϋδροδυναµικής που διέπουν την κίνηση ηλεκτρικά αγώγιµου ρευστού που κινείται υπό την επίδραση µαγνητικού πεδίου. Στο πρώτο µέρος της διατριβής (Κεφάλαιο ΙΙ), όπως αναφέρθηκε, µελετάται αριθµητικά η MHD συµπιεστή ροή στρωτού οριακού στρώµατος µε αντίξοη βαθµίδα πίεσης και µεταφορά θερµότητας και µάζας. Το ρευστό (αέρας) θεωρείται ιδανικό, νευτώνειο, ηλεκτρικά αγώγιµο και το µαγνητικό πεδίο είναι σταθερό και κάθετα εφαρµοζόµενο ως προς την πλάκα και συνεπώς ως προς την κατεύθυνση της ροής. Η αντίξοη βαθµίδα πίεσης, που επιβάλλεται στην ροή, γνωστή ως ροή τύπου Howarth, προκύπτει από µια γραµµικά ελαττούµενη ταχύτητα. Το σύστηµα των µερικών διαφορικών εξισώσεων που περιγράφουν το πρόβληµα έχει αδιαστατοποιηθεί µε τον µετασχηµατισµό των Falkner-Skan, για συµπιεστή ροή, και επιλύεται αριθµητικά χρησιµοποιώντας την µέθοδο του Keller. ix Τα αποτελέσµατα του Κεφαλαίου αυτού αναφέρονται σε τρία είδη ροής: (i) αδιαβατική ροή ρευστού πάνω από την πλάκα, (ii) σε ροή πάνω από θερµαινόµενη πλάκα και (iii) σε ροή πάνω από ψυχόµενη πλάκα. Γίνονται αριθµητικοί υπολογισµοί για κάθε µια από τις παραπάνω περιπτώσεις εφαρµόζοντας συνεχή ή τοπική έγχυση/απορρόφηση, για διάφορες τιµές της έντασης του µαγνητικού πεδίου και για διάφορες τιµές αριθµού Mach του ελεύθερου ρεύµατος πάνω από την επίπεδη επιφάνεια. Εξετάζεται η επίδραση των ανωτέρω µεγεθών σε αυτόν τον τύπο της ροής. Αναλυτικότερα, δείχθηκε µετά τους αριθµητικούς υπολογισµούς, ότι η τεχνική της απορρόφησης διατηρεί την ροή για περισσότερο διάστηµα πάνω από την πλάκα µετατοπίζοντας το σηµείο αποκόλλησης προς το χείλος εκφυγής. Τα αντίθετα αποτελέσµατα δίνει η εφαρµογή έγχυσης. Το µαγνητικό πεδίο που εφαρµόζεται στην πλάκα βοηθά την ροή και την διατηρεί στρωτή πάνω από αυτήν για µεγαλύτερο διάστηµα κατά µήκος της πλάκας. Τα αποτελέσµατα αυτά επιβεβαιώθηκαν για τις τρεις περιπτώσεις της στρωτής ροής (αδιαβατική ροή, θερµαινόµενη και ψυχόµενη πλάκα) και για διάφορους αριθµούς Mach. Στο δεύτερο µέρος (Κεφάλαιο ΙΙΙ) µελετάται αριθµητικά η MHD συµπιεστή ροή τυρβώδους οριακού στρώµατος µε αντίξοη βαθµίδα πίεσης και µεταφορά θερµότητας και µάζας. Για το ρευστό (αέρας) και το µαγνητικό πεδίο ακολουθούνται οι ίδιες παραδοχές µε την περίπτωση της στρωτής ροής. Οι εξισώσεις που περιγράφουν το πρόβληµα προκύπτουν από τις εξισώσεις που έχει προτείνει ο Reynolds για την τυρβώδη ροή οριακού στρώµατος, κατάλληλα τροποποιηµένες για την περίπτωση MHD ροής. Οι εξισώσεις αυτές αδιαστατοποιούνται µε τον µετασχηµατισµό των Falkner-Skan για συµπιεστή ροή και επιλύονται µε την ίδια µέθοδο µε την στρωτή MHD ροή (µέθοδος Keller). Για το τυρβώδες κινηµατικό ιξώδες χρησιµοποιούνται δύο διαφορετικά αλγεβρικά µοντέλα τύρβης, αυτά των Cebeci-Smith και Baldwin-Lomax. Τα µοντέλα αυτά τροποποιήθηκαν ώστε να περιγράφουν το τυρβώδες κινηµατικό ιξώδες και στην περίπτωση της έγχυσης/απορρόφησης. Για τον τυρβώδη αριθµό Prandtl χρησιµοποιήθηκε µια τροποποίηση του µοντέλου των Kays και Crawford. Αριθµητικοί υπολογισµοί έγιναν για τον αέρα, για την περίπτωση που η ροή πάνω από την οριακή επιφάνεια ήταν αδιαβατική ή η επιφάνεια θερµαινόταν ή ψυχόταν. Για κάθε µια από τις παραπάνω περιπτώσεις εξετάζεται η επίδραση του µαγνητικού πεδίου, της τοπικής ή συνεχούς έγχυσης/απορρόφησης και του αριθµού Mach του ελευθέρου ρεύµατος πάνω στο τυρβώδες οριακό στρώµα. Μετά τους αριθµητικούς υπολογισµούς, τα συµπεράσµατα που προκύπτουν για την τυρβώδη ροή είναι παρόµοια µε την στρωτή. Η τεχνική της απορρόφησης βοηθάει στην διατήρηση του τυρβώδους οριακού στρώµατος πάνω από την πλάκα σε αντίθεση µε την έγχυση. Ο συνδυασµός αρχικά έγχυσης και έπειτα απορρόφησης βοηθά στην διατήρηση της ροής για µεγαλύτερο διάστηµα πάνω από την πλάκα, δηλαδή στην µετατόπιση του σηµείου αποκόλλησης προς το χείλος εκφυγής ελαττώνοντας ταυτόχρονα την συνολική αντίσταση σε αυτήν. Αυτό το αποτέλεσµα ισχύει και στην στρωτή ροή. Το µαγνητικό πεδίο βοηθάει την τυρβώδη ροή µετατοπίζοντας το σηµείο αποκόλλησης προς το χείλος εκφυγής. Το αποτέλεσµα αυτό είναι λιγότερο έντονο στην τυρβώδη ροή από ότι στην στρωτή. Τα παραπάνω αποτελέσµατα παρουσιάζονται για τις τρεις περιπτώσεις της τυρβώδης ροής (αδιαβατική ροή, θερµαινόµενη και ψυχόµενη πλάκα), για διάφορους αριθµούς Mach () και για τα δύο µοντέλα τύρβης (C-S και B-L). Στο τέλος του Κεφαλαίου γίνεται σύγκριση των δύο τύπων ροών, στρωτής και τυρβώδους. Λόγω της απουσίας ερευνητικών αποτελεσµάτων πάνω στο συγκεκριµένο αυτό πρόβληµα, τα παραπάνω αποτελέσµατα εκτιµάται ότι είναι πολύ ενδιαφέροντα για την περιγραφή του µηχανισµού ελέγχου του στρωτού και τυρβώδους οριακού στρώµατος για συµπιεστές ροές. / In this thesis the steady two-dimensional magnetohydrodynamic (MHD), compressible boundary layer flow, over a flat plate is numerically studied. The flow is subjected to an adverse pressure gradient, due to a linearly retarded velocity, that is known as Howarth’s flow. The plate is electrically non-conducting and it is subjected to a suction/injection velocity, continuous or localized, normal to it. The case of an impermeable plate is also studied. The plate is parallel to the free stream of a heat-conducting perfect gas (air) flowing with velocity u∞ along the plate. The flow field is subjected to the action of a constant magnetic field which acts normal to the plate. The fluid (air) is considered Newtonian, compressible and electrically conducting. The fundamental equations of MHD flow are presented in Chapter I as well as the characteristic quantities of the boundary layer which are used in this study. The laminar flow is studied in Chapter II where as the turbulent flow is studied in Chapter III. For both cases (laminar and turbulent) the partial differential equations and their boundary conditions, describing the problem under consideration, are transformed using the compressible Falkner-Skan transformation and the numerical solution of the problem is obtained by using a modification of the well known Keller’s box method. The obtained numerical results for the velocity and temperature field, as well as for the associated boundary layer parameters, are shown in figures for different free-stream Mach numbers M∞ and for the case (i) of an adiabatic flow (0wS′=), (ii) heating of the wall () and (iii) cooling of the wall (1wS>1wS<), followed by an extensive discussion. For turbulent flow, in Chapter III, the Reynolds-averaged boundary layer equations are used. Two different turbulent models, namely the model of Cebeci-Smith and Baldwin-Lomax, are used to represent eddy kinematic viscosity and eddy diffusivity of heat. These models are the most simple with acceptable generality and their accuracy has been explored for a wide range of flows for which there are experimental data. It has also been found that they give results sufficiently accurate for most engineering problems. For the turbulent Prandtl number model a modification of the extended Kays and Crawford’s model is also used. In the case of laminar flow (Chapter II) the numerical calculations showed that the application of suction moves separation point downstream, whereas injection moves the separation point towards the leading edge of the plate. The presence of the magnetic field always increases frictional drag on the wall but moves the separation point downstream for every value of free-stream Mach number. Τhis displacement is greater for small values of M∞. The combined influence of the magnetic field, localized injection and localized suction moves separation point downstream reducing frictional drag. These results confirmed for the three cases (adiabatic flow, heating of the wall, cooling of the wall) of the laminar flow and for various free-stream Mach numbers. Since most flows, which occur in practical applications, are turbulent the results in this case (Chapter III) are more important and are similar with those in laminar flow. 162 Precisely, application of suction moves separation point downstream but injection moves separation point towards the leading edge of the plate reducing drag. Application of localized injection and localized suction moves the separation point downstream reducing total drag. The presence of the magnetic field moves separation point downstream increasing frictional drag. The combined influence of magnetic field, localized injection and localized suction moves separation point further downstream as regards the other cases. These results confirmed for the three cases (adiabatic flow, heating of the wall, cooling of the wall) of turbulent flow, for various free-stream numbers and for two turbulent models (C-S and B-L). It is hoped that, in the absence of detailed investigations of this problem, the obtained results, are very interesting and give a clearer insight into the mechanism of controlling a laminar or turbulent boundary layer compressible flow.

This thesis is an experimental study of turbulent flows over smooth and rough walls in a channel that consists of an upstream parallel section to produce a fully developed channel flow and a diverging section to produce an adverse pressure gradient (APG) flow. The roughness elements used were two-dimensional square ribs of nominal height k = 3 mm. The ribs were secured to the lower wall of the channel and spaced to produce the following three pitches: 2k, 4k and 8k, corresponding to d-type, intermediate and k-type rough walls, respectively. For each rough wall type, the ribs were inclined at 90°, 45° and 30° to the approach flow. The velocity measurements were performed using a particle image velocimetry technique. The results showed that rib roughness enhanced the drag characteristics, and the degree of enhancement increased with increasing pitch. The level of turbulence production and Reynolds stresses were significantly increased by roughness beyond the roughness sublayer. It was observed that the population, sizes and the level of organization of hairpin vortices varied with roughness and more intense quadrant events were found over the smooth wall than the rough walls. APG reinforced wall roughness in augmenting the equivalent sand grain roughness height, turbulence production and Reynolds stresses. APG also reduced the sizes of the hairpin packets but strengthened the quadrant events in comparison to the results obtained in the parallel section. The secondary flow induced by inclined ribs significantly altered the distributions of the flow characteristics across the span of the channel. Generally, the mean flow was less uniform close to the trailing edge of the ribs compared to the flows at the mid-span and close to the leading edge of the ribs. The Reynolds stresses and hairpin packets were distinctly larger close to the trailing edge of the ribs. Rib inclination also decreased the drag characteristics and significantly modified the distributions of the Reynolds stresses and quadrant events. In the parallel section, the physical sizes of the hairpin packets were larger over 45° ribs whereas in the diverging section, the sizes were larger over perpendicular ribs.