The research was motivated by the desire to use Large Eddy Simulations (LES) to calculate liner heat transfer in industrial scale gas turbine combustors, which operate at high Reynolds numbers and high Swirl numbers. LES has several challenges which need to be surmounted for general application to complex high Reynolds number turbulent flows. The primary challenge in wall bounded flows is the need for very fine grids in the vicinity of walls, which makes LES impractical at high Reynolds numbers. An additional challenge is the accurate representation of inlet turbulent conditions for developing flows such that the computational domain size is limited to the immediate region of interest. The generalization of solutions to surmount these issues in complex geometries and grids is yet another challenge. To meet these challenges, a novel formulation, implementation, and validation of a two layer velocity and temperature zonal wall model along with the implementation of the synthetic eddy method in a generalized coordinate system LES framework is presented in this thesis. The wall model greatly alleviates the grid requirements, whereas the synthetic eddy method provides accurate turbulent inlet boundary conditions. The methods are validated in turbulent channel flow up to a Reynolds number of 2x106, a backward facing step at Re=40,000, before application to a model swirl combustor at Re=20,000 with a Swirl number of 0.43 and flow and heat transfer in an industrial scale can combustor at Re=80,000 and Swirl number of 0.7. The integrated zonal near wall approach for velocity and temperature is then successfully used to investigate flow and heat transfer in a statistically three-dimensional flow of a ribbed duct passage used for the internal cooling of turbine blades. The zonal wall model is further modified to take in to account the effects of surface roughness and successfully used to investigate flow in a rod roughened channel at high Reynolds numbers up to 60,000. In all cases it is shown that the zonal wall model used with the synthetic eddy method for inlet turbulence generation can result in large savings in computational cost without any significant loss in accuracy when compared to wall resolved LES and experiments. In a turbulent channel flow at Re=45,000, computational complexity was reduced by a factor of 285 using wall modeled LES, whereas in a statistically three-dimensional flow and heat transfer in a ribbed duct, at Re=20,000, the computational complexity was reduced by a factor between 60 and 140. In a swirl dominated can combustor at Re=20,000, the reduction was more modest at a factor of 9. / Ph. D.

Although spectral methods have been in use for decades, there is still room for innovation, refinement and improvement of the methods in terms of efficiency and accuracy, for generalized homogeneous turbulent flows, and especially for specialized applications like the computation of atmospheric flows and numerical weather prediction. In this thesis, two such innovations are presented. First, inspired by the adaptive mesh refinement (AMR) technique, which was developed for the computation of fluid flows in physical space, an algorithm is presented for accelerating direct numerical simulation (DNS) of isotropic homogeneous turbulence in spectral space. In the adaptive spectral resolution (ASR) technique developed here the spectral resolution in spectral space is dynamically refined based on refinement criteria suited to the special features of isotropic homogeneous turbulence in two, and three dimensions. Applying ASR to computations of two- and three-dimensional turbulence allows significant savings in the computational time with little to no compromise in the accuracy of the solutions. In the second part of this thesis the effect of explicit filtering on large eddy simulation (LES) of atmospheric flows in spectral space is studied. Apply an explicit filter in addition to the implicit filter due to the computational grid and discretization schemes in LES of turbulent flows allows for better control of the numerical error and improvement in the accuracy of the results. Explicit filtering has been extensively applied in LES of turbulent flows in physical space while few studies have been done on explicitly filtered LES of turbulent flows in spectral space because of perceived limitations of the approach, which are shown here to be incorrect. Here, explicit filtering in LES of the turbulent barotropic vorticity equation (BVE) as a first model of the Earth's atmosphere in spectral space is studied. It is shown that explicit filtering increases the accuracy of the results over implicit filtering, particularly where the location of coherent structures is concerned. / Ph. D.

In this thesis study, Detached Eddy Simulation turbulence model is studied in two dimension mainly for flow over single element airfoils in high Reynolds numbers to gain experience with model before applying it to a three dimensional simulations. For this aim, Spalart-Allmaras and standard DES ,DES97, turbulence models are implemented to parallel, viscous, hybrid grid flow solver. The flow solver ,Set2d, is written in FORTRAN language. The Navier-Stokes equations are discretized by first order accurately cell centered finite volume method and solved explicitly by using Runge-Kutta dual time integration technique. Inviscid fluxes are computed using Roe flux difference splitting method. The numerical simulations are performed in parallel environment using domain decomposition and PVM library routines for inter-process communications. To take into account the effect of unsteadyness after the convergence is ensured by local time stepping technique for four order magnitude drop in density residual, global time stepping is applied for 20000 iterations. The solution algorithm is validated aganist the numerical and experimental studies for single element airfoils in subsonic and transonic flows. It is seen that Spalart-Allmaras and DES97 turbulence models give the same results in the non-seperated flows. Grey area is investigated by changing $C_{DES}$ coefficient. Modeled Stress Depletion which cause reduction of eddy viscosity is observed.

A cause de sa température élevée, le jet propulsif d'un avion de combat est une source de rayonnement infrarouge qui rend l'appareil très vulnérable. Ce rayonnement peut toutefois être réduit en accélérant le mélange du jet avec l'atmosphère. <br />Cette thèse est consacrée à la simulation numérique d'un jet propulsif réaliste contrôlé par des injections radiales et à l'analyse physique des mécanismes d'augmentation de son mélange. <br />Deux types de simulations, RANS et ZDES, ont été réalisés sur la base du modèle de Spalart-Allmaras. Dans le modèle ZDES, une nouvelle longueur caractéristique de maille est formulée et améliore sensiblement la prévision de la région initiale du jet. Globalement, les simulations ZDES restituent fidèlement le champ moyen du jet supersonique sans et avec contrôle, aussi bien les cellules de détente/compression que la diffusion turbulente. <br />L'analyse physique est dédiée à la compréhension d'une part des mécanismes compressibles concentrées au coeur du jet et d'autre part des mécanismes tourbillonnaires périphériques ainsi qu'à l'évaluation de leurs rôles respectifs dans l'augmentation du mélange. Il en ressort que l'augmentation du mélange est exclusivement due aux mécanismes tourbillonnaires. Une étude paramétrique fournit des indications pour concevoir un mélangeur efficace. L'analyse des tourbillons focalisée sur le régime lointain quasi-bidimensionnel souligne leur dynamique moyenne et fait apparaître l'action des fluctuations turbulentes sur leur taux de dégénérescence. Enfin, deux régimes de contrôle sont identifiés et associés aux pénétrations respectivement quasi-stationnaire et intermittente des jets secondaires.

Large Eddy Simulation (LES) is a relatively more accurate and reliable alternative to solution of Reynolds Averaged Navier Stokes (RANS) equations in simulating complex turbulent flows at a lesser computational cost than a direct numerical simulation (DNS). However, LES of wall-bounded flows still requires a very high grid resolution in the inner wall layer making its widespread use difficult. Different attempts have been made in the past time to overcome this problem by modeling the near wall turbulence instead of resolving it. One such approach is a two-layer wall model that solves for a reduced one-dimensional equation in the inner wall layer, while solving for the filtered Navier-Stokes equations in the outer layer. The use of such a model allows for a coarser grid resolution than a wall resolved LES. This work validates the performance of a two-layer wall model developed for an arbitrary body fitted non-orthogonal grid in the flow over a wall mounted hump at Reynolds number 9.36x105. The wall modeled large eddy simulation (WMLES) relaxes the grid requirement compared to a wall resolved LES (WRLES) by allowing the first off-wall grid point to be placed at a y+ of approximately 20-40. It is found that the WMLES results are general good agreement with WRLES and experiments. Surface pressure coefficient, skin friction, mean velocity profiles, and the reattachment location compare very well with experiment. The WMLES and WRLES exhibit some under prediction of the peak values in the turbulent quantities close to the reattachment location, with better agreement with the experiment in the separated region. In contrast, a simulation that did not employ the wall model on the grid used for WMLES failed to predict flow separation and showed large discrepancies with the experimental data. In addition to the relaxation of the grid requirement in the wall normal direction, it was also observed that the wall model allowed a reduction in the number of computational cells in the span-wise direction by half. However an LES calculation on a grid with reduced number of cells in span-wise direction turned unstable almost immediately, thereby highlighting the effectiveness of the wall model. Besides reducing the number of grid points in the spatial domain, the relaxed grid resolution for the WMLES also permitted the use of a larger time step. This resulted in an order of magnitude reduction in the total CPU time relative to WRLES. / Master of Science

Multiphase flows are a common phenomenon. Rains, sediment transport in rivers, snow and dust storms, mud slides and avalanches are examples of multiphase flows occurring in nature. Blood flow is an example of multiphase flow in the human body, which is of vital importance for survival. Multiphase flows occur widely in industrial applications from hydrocarbon extrac-tion to fuel combustion in engines, from spray painting to spray drying, evaporators, pumps and pneumatic conveying. Predicting multiphase flows is of vital importance to understand natural phenomenon and to design and improve industrial processes. Separated flows and dispersed flows are two types of multiphase flows, which occur together in many industrial applications. Physical features of these two classes are different and the transition from one to another involves complex flow physics. Experimental studies of multiphase flows are not easy, as most real world phenomenon cannot be scaled down to laboratory models. Even for those phenomenon that can be demonstrated at lab-oratory scale, rescaling to real world applications requires mathematical models. There are many challenges in experimental measurements of multiphase flows as well. Measurement techniques well suited for single phase flows have constraints when measuring multiphase phenomenon. Un-certainty in experimental measurements poses considerable difficulties in validating numerical models developed for predicting these flows. Owing to the computational effort required, direct simulation of multiphase flows, even for small scale real world applications is out of present scope. Numerical methods have been developed for dealing with each class of flow separately, that in-volves use of models for phenomenon that is computationally demanding. Reynolds Averaged Navier-Stokes (RANS) methods for predicting multiphase flows place strong requirements on turbulence models, as information about fluctuating quantities in the field, that have significant effects on dispersed phase, is not available. Large Eddy Simulation (LES) gives better predictions than RANS as the instantaneous field data is available and large scale unsteadiness that effects the dispersed phase can be captured. Recent LES studies of multiphase flows showed that the sub-grid-scale (SGS) model used for the continuous phase has an effect on the evolution of the dispersed phase. In this work, LES of multiphase flows is performed using Explicit Filtering Large Eddy Sim-ulation method. In this method, spatial derivatives are computed using higher order compact schemes that have spectral-like resolution. SGS modeling is provided by the use of a filter with smoothly falling transfer function. This method is mathematically consistent and converges to a DNS as the grid is refined. It has been successfully applied to combustion and aero-acoustics and this work is the first application of the method to multiphase flows. Study of dispersed multiphase flows was carried out in this work. Modeling of the dispersed phase is kept simple since the in-tention was to evaluate the capability of explicit filtering LES method in predicting multiphase flows. Continuous phase is solved using a compressible formulation with explicit filtering method. Spatial derivatives are computed using fourth and sixth order compact schemes that use derivative splitting method proposed by Hixon & Turkel (2000a) and second order Runge-Kutta (RK2) time stepping. The grid is stretched as needed. Non-reflecting boundary conditions due to Poinsot & Lele (1992) are used to avoid acoustic reflections from boundaries. Buffer zones (Bogey & Bailly (2002)) are employed at outflow and lateral boundaries to damp vortical structures. The code developed for continuous phase is evaluated by studying round jets at Re =36,000 and comparing with experimental measurements of Hussein et al. (1994) and Panchapakesan & Lumley (1993). Simulations showed excellent agreement with experimental results. Rate of decay of axial velocity and the evolution of turbulence intensities on the centerline matched very well with measurements. Radial profiles of mean and fluctuating components of velocities exhibit self-similarity. A set of studies were then performed using this code to assess the effect of numerical scheme, grid refinement & stretching and simulation times on the predictions. Results from these simulations showed good agreements with experiments and established the code for use in multiphase flows under various simulation conditions. To assess the prediction of multiphase flows using this LES method, an evaporating spray ex-periment by Chen et al. (2006) was simulated. The experiment uses a nebuliser for generating a finely atomized spray of acetone, which avoids complex breakdown phenomenon associated with air blast atomizers and provides well defined boundary conditions for model evaluation. The neb-uliser sits upstream in a pipe carrying air and droplets travel along with air for a distance of 10 diameters before exiting into a wind tunnel with co-flowing air. Droplet breakdown, if any, takes place inside the pipe and the spray is finely atomized by the time it reaches pipe exit. One of the experimental cases at Re =31,600, with a mass loading of 1.1% and a jet velocity of 56 m/s is simulated. Particle size has a χsquared distribution with a Sauter mean diameter of 18µm. In the self-similar region, decay of centerline velocity and turbulence intensities matched well with ex-perimental results. Continuous phase exhibits self-similar behavior. A series of simulations were then performed to match the initial region of the spray by altering the inflow conditions in the sim-ulation. Simulation that matched the breakdown location of the experiment revealed the presence of a relaxation zone with a higher initial spreading rate, followed by a lower asymptotic spreading rate. Studies were performed to understand the effect of various phenomenon like evaporation and droplet size on this behavior. A study of breakdown region of particle-laden jets was performed to understand the presence of relaxation zone post breakdown. Flow conditions were similar to evaporating spray experiment except that particles do not evaporate, mass loading is 2% and jet Reynolds number Re =2000. A series of grid refinements were performed and on the largest grid, gird spacing Δy =7.5η, where ηis an estimate of the Kolmogorov length scale based on flow conditions. Decay of axial velocity on the centerline showed variations with grid refinement, tending to the experimentally measured value as the grid is refined. Variation of turbulence intensities along the centerline revealed a jump in axial velocity fluctuations at the breakdown location, while radial and azimuthal velocities showed a smooth increase to their asymptotic value. This jump was resolved on grid refinement and on fine grids axial velocity fluctuations followed the other two quantities closely in their rise to asymptotic state. Comparison of these quantities with a jet without particles revealed that the flow features are same for a jet with and without particles, and at the mass loading studied, particles have negligible effect on jet breakdown. Another study performed at a higher Reynolds number of Re =11,000, under similar flow conditions showed similar behavior. To assess the ability of predicting dispersed phase, simulations of particle-laden flows at low Stokes number were performed and compared against an experiment by Lau & Nathan (2014). The experiment studies variation of velocity and particle concentration along the centerline, and half widths of a jet velocity and concentration. Particles are injected into a pipe along with air, and the two phase flow is fully developed by the time it exits the pipe into a wind tunnel along with a co-flow. Particles are mono-disperse with a density of 1200 kg/m3. Mass loading is 40% so that particles have a significant effect on the continuous phase. Two cases at particle Stokes number of 1.4, one with Re =10,000, bulk velocity of 12 m/s and particle diameter of 20µm and another with Re =22,500, bulk velocity of 36 m/s and particle diameter of 10µm were simulated. Simulations of both the cases showed good match with experimental measurements of centerline decay for the continuous phase. For the dispersed case, simulations with larger particles showed good match with experimental results, while smaller particles showed differences. This was understood to be the effect of lateral migration which is prominent in case of smaller particles, the models for which have not been used in the present simulation study.

In dieser Dissertation wird das kompressible, nicht-hydrostatische und dreidimensionale Modell All Scale Atmospheric Model (ASAM) für Grobstruktur- bzw. Large-Eddy-Simulationen (LES) angewendet, um lokale Inseleffekte in der karibischen Passatwindzone zu untersuchen. Da das Modell bis dato noch keine Anwendung im Bereich von LES feuchter atmosphärischer Grenzschichten und heterogener Oberflächen fand, wurden einige Bestandteile zum Modellcode hinzugefügt oder überarbeitet. Ein Hauptaugenmerk liegt dabei auf das Einbeziehen orographischer Strukturen mittels angeschnittener Zellen (engl. cut cells). Sowohl die räumliche und zeitliche Diskretisierung der Modellgleichungen als auch die nötigen physikalischen Parameterisierungen werden in einer umfassenden Modellbeschreibung zusammengefasst. Die Robustheit und Stabilität der Modellformulierung wird durch eine Reihe von Simulationen idealisierter Testfälle bestätigt. Large-Eddy-Simulationen werden für das Gebiet der Karibikinsel Barbados zur Untersuchung von Inseleffekten bezüglich Grenzschichtmodifikation, Wolkenbildung und vertikaler Durchmischung von Aerosolen durchgeführt. Durch das Vorhandensein einer topographisch strukturierten Inseloberfläche in der Mitte des Modellgebietes muss das Modellsetup offene seitliche Randbedingungen beinhalten. Damit das einströmende Windfeld konsistent mit der Dynamik einer turbulenten, marinen Grenzschicht ist, wird eine neue Methode implementiert und angewendet, welche auf Störungen des potentiellen Temperaturfeldes mittels finiter Amplituden basiert. Beobachtungen aus der SALTRACE-Messkampagne werden benutzt, um die Modellläufe anzutreiben. Die Ergebnisse einiger Sensitivitätstests zeigen Probleme der Modellierung im Bereich der \"Terra incognita\" auf. Dabei handelt es sich um die Modellierung auf räumlichen Skalen, welche zwischen denen von LES und wolkenauflösenden Modellen liegen. Außerdem werden Auswirkungen von entweder turbulent oder laminar anströmenden Windfeldern auf die Simulationsergebnisse untersucht. Besonders die Wolkeneigenschaften im Lee von Barbados werden in diesen Simulationen merklich beeinflusst. Ergebnisse einer weiteren Simulation mit einer sehr starken Passatinversion bringt deren Einfluss auf die Dicke und Höhe der simulierten Wolkenschichten zum Vorschein. Die Veränderung von Saharastaubschichten, welche Barbados über weiträumigen Transport über den Atlantik erreichen, wird analysiert. Die Auswirkungen beinhalten sowohl eine Ausdünnung und ein Absinken dieser Schichten als auch turbulenter Transport in Richtung Erdoberfläche. Die genaue Position der beeinflussten Schichten und die Stärke des turbulenten Mischens werden hauptsächlich von der atmosphärischen Schichtung, der Inversionsstärke und Windscherung gesteuert. Vergleiche zwischen den LES-Modellergebnissen und Daten aus Doppler-Windlidarmessungen zeigen gute Übereinstimmungen in der Formierung der konvektiven Strukturen tagsüber und des Vertikalwindfeldes. / In this thesis, the fully compressible, three-dimensional, nonhydrostatic atmospheric model called All Scale Atmospheric Model (ASAM) is utilized for large eddy simulations (LES) to investigate local island effects at the Caribbean. Since the model has not been applied to LES for moist boundary layers and heterogeneous surfaces so far, several parts are added to the model code or reworked. A special focus lies on the inclusion of orographical structures via the cut cell method. Spatial and temporal discretization as well as necessary physical parameterizations are summarized in a thorough model description. The robustness of the model formulation is confirmed by a set of idealized test case simulations. Large eddy simulations are performed for the area of the Caribbean island Barbados to investigate island effects on boundary layer modification, cloud generation and vertical mixing of aerosols. Due to the presence of a topographically structured island surface in the domain center, the model setup has to be designed with open lateral boundaries. In order to generate inflow turbulence consistent with the upstream marine boundary layer forcing, the newly developed cell perturbation method based on finite amplitude perturbations is applied. Observations from the SALTRACE field campaign are used to initialize the model runs. Several numerical sensitivity tests are carried out to demonstrate the problems related to \"gray zone modeling\" beyond LES scales or when the turbulent marine boundary layer flow is replaced by laminar winds. Especially cloud properties west of Barbados (downwind) are markedly affected in these simulations. Results of an additional simulation with a strong trade-wind inversion reveal its effect on cloud layer depth and height. The modification of Saharan dust layers reaching Barbados via long-range transport over the North Atlantic is analyzed. Effects of layer thinning, subsidence and turbulent downward transport near the layer bottom become apparent. The position of these layers and strength of downward mixing is found to be mainly controlled atmospheric stability, inversion strength and wind shear. Comparisons of LES model output with wind lidar data show similarities in the formation of the daytime convective plume and the vertical wind structure.

Most flows in nature and engineering are turbulent, and many are wall-bounded. Further, in turbulent flows, the turbulence generally has a large impact on the behavior of the flow. It is therefore important to be able to predict the effects of turbulence in such flows. The Navier-Stokes equations are known to be an excellent model of the turbulence phenomenon. In simple geometries and low Reynolds numbers, very accurate numerical solutions of the Navier-Stokes equations (direct numerical simulation, or DNS) have been used to study the details of turbulent flows. However, DNS of high Reynolds number turbulent flows in complex geometries is impractical because of the escalation of computational cost with Reynolds number, due to the increasing range of spatial and temporal scales. In Large Eddy Simulation (LES), only the large-scale turbulence is simulated, while the effects of the small scales are modeled (subgrid models). LES therefore reduces computational expense, allowing flows of higher Reynolds number and more complexity to be simulated. However, this is at the cost of the subgrid modeling problem. The goal of the current research is then to develop new subgrid models consistent with the statistical properties of turbulence. The modeling approach pursued here is that of "Optimal LES". Optimal LES is a framework for constructing models with minimum error relative to an ideal LES model. The multi-point statistics used as input to the optimal LES procedure can be gathered from DNS of the same flow. However, for an optimal LES to be truly predictive, we must free ourselves from dependence on existing DNS data. We have done this by obtaining the required statistics from theoretical models which we have developed. We derived a theoretical model for the three-point third-order velocity correlation for homogeneous, isotropic turbulence in the inertial range. This model is shown be a good representation of DNS data, and it is used to construct optimal quadratic subgrid models for LES of forced isotropic turbulence with results which agree well with theory and DNS. The model can also be filtered to determine the filtered two-point third-order correlation, which describes energy transfer among filtered (large) scales in LES. LES of wall-bounded flows with unresolved wall layers commonly exhibit good prediction of mean velocities and significant over-prediction of streamwise component energies in the near-wall region. We developed improved models for the nonlinear term in the filtered Navier-Stokes equation which result in better predicted streamwise component energies. These models involve (1) Reynolds decomposition of the nonlinear term and (2) evaluation of the pressure term, which removes the divergent part of the nonlinear models. These considerations significantly improved the performance of our optimal models, and we expect them to apply to other subgrid models as well. / text

Thermische Energiespeicher tragen u. a. zur Erhöhung der Versorgungsicherheit in der Fernwärmeversorgung und zur Effizienzsteigerung des Fernwärmesystems (z. B. Flexibilisierung der Erzeuger, Speicherung überschüssiger Wärme, besserer hydraulischer Betrieb) bei. Dafür eignen sich Druckbehälter, sog. schlanke Heißwasserspeicher (Speichertyp b1). Die oben genannten Vorteile setzen einen effizienten Speicherbetrieb (niedrige interne und externe Speicherverluste) voraus. Dieser Beitrag beschäftigt sich mit der Minimierung der internen Verluste durch die Verbesserung des thermischen Schichtungsverhaltens. Eine thermische Schichtung mit einem möglichst schmalen Übergangsbereich zwischen heißer und kalter Zone ist ein Indikator für geringe Mischvorgänge während der Beladung. Die Minimierung dieser Mischungsvorgänge bei der Beladung nimmt eine Schlüsselrolle bei der Minimierung der internen Speicherverluste ein. Lohse und Brähmer untersuchten die Beladung mit herkömmlichen radialen Diffusor in schlanken Heißwasserspeicher mit numerischer Strömungssimulation. Die Arbeiten identifizieren aufgrund der schlanken Speicherform nachteilige Strömungseffekte wie z. B. einen ausgeprägten Wandstrahl. Dieser Wandstrahl regt Mischvorgänge an und damit steigen die internen Speicherverluste. Zur Überwindung dieser Strömungsproblematik schlägt die Beladung mit Drall vor. Die Untersuchungen von Oestreich zeigten das Strömungsverhalten im Diffusor und im Speicher, die Auswirkungen auf die thermische Schichtung sowie die Vorteilhaftigkeit. Dieser Beitrag soll eine detailliertere Beschreibung der Strömungsvorgänge liefern. Dieses Wissen ist unbedingt notwendig, um die Ursachen und Wirkungen bei der Beladung mit Drall und beim Aufbau der thermischen Schichtung besser zu verstehen. Die Modellierung und Simulation des Diffusors bzw. des Speichers erfolgen mit ANSYS CFX. Zur Auflösung turbulenter Strukturen findet die Large Eddy Simulation Anwendung. Dieser Artikel präsentiert erstmalig die Wirbelstrukturen im Diffusor mit Leitelementen zur Drallerzeugung. Die Speicherströmung weist ein ähnliches Verhalten zu bekannten Dichteströmungen (z. B. Lappen-Kluft-Struktur, Instabilitäten in den freien Scherschichten) auf, was bisher nicht bekannt war. Hohe Peclet-Zahlen (hohe Advektionsströme) im Speichermodell führen zu numerischer Instabilität der Simulation und erfordern deshalb erhöhten Diskretisierungsaufwand. / Thermal energy storage systems contribute, among other things, to increasing the security of supply in the district heating system and to improving the efficiency of the district heating system (e.g., making the generators more flexible, storing waste heat, better hydraulic operation). Pressure vessels, so-called slim hot water storage tanks (storage type b1) are suitable for this purpose. The above mentioned advantages require efficient storage operation (low internal and external storage losses). This paper deals with the minimization of internal losses by improving the thermal stratification behavior. Thermal stratification with a thermocline between hot and cold zone as narrow as possible is an indicator of low mixing processes during loading. Minimizing these mixing processes during loading takes a key role in minimizing internal storage losses. Lohse and Brähmer investigated loading with conventional radial diffuser in slim hot water storage tanks with numerical flow simulation. The work identifies adverse flow effects due to the slim tank shape, such as a wall jet. This wall jet stimulates mixing processes and thus increases the internal storage losses. To overcome this flow problem, Findeisen et al. proposes swirl loading. The investigations of Oestreich et al. showed the flow behavior in the diffuser and in the storage, the effects on the thermal stratification as well as the advantageousness. This paper aims to provide a more detailed description of the flow processes. This knowledge is essential to better understanding the causes and effects of swirl loading and the structure of thermal stratification. Modeling and simulation of the diffuser and storage, respectively, are performed using Ansys CFX. Large eddy simulation (LES) is applied to resolve turbulent structures. This paper presents for the first time the vortex structures in the diffuser with internal elements for swirl generation. The storage flow exhibits similar behavior to known density flows (e.g., head and nose formation, instabilities in the free shear layers), which was previously unknown. High Peclet numbers (high advection currents) in the storage model lead to numerical instability of the simulation and therefore require increased discretization efforts.

This thesis investigates the impact of upstream duct convolution on the plume development for high speed jets. In particular, investigations are carried out into an unconventional aero-engine exhaust systems comprised of a modified convergent-divergent rectangular nozzle where the converging section of the nozzle includes an S-bend in the duct. The motivation for this work comes from both the military and civilian sectors of the aerospace industry. The growing interest into highly efficient engines in the civilian sector and increasing complexities involved in stealth technologies for military applications has led to new design constraints on aero-engine exhaust systems that require further research into flows through more complex duct geometries. Due to a lack of experimental data into this area in the open literature validation studies are undertaken into flows through an S-bend duct and exhaust plume development from a rectangular convergent-divergent nozzle. The validation work is simulated using RANS CFD with common industrial turbulence models as well as LES with artificial inlet conditions. Subsequently, a CFD investigation into three unconventional aero-engine exhaust systems, with over-expanded conditions, with differing angles of curvature across the converging S-bend is undertaken using both RANS and LES methodologies governed by the validation work. As the curvature of the S-bend was increased it was found that the thrust and effective NPR both decrease. Whilst these changes were within acceptable levels (with some optimisation) for a circumferential extent of up to 53.1 the losses became prohibitive large at extents. For the ducts with a greater circumferential extents separation was seen to occur at the throat of the nozzle; this changes the design parameters of the nozzle leading to a higher Mach number and could potentially be harnessed to improve performance of the engine creating a `variable throat' nozzle. The impact of using different numerical solvers to simulate the flow through an unconventional aero-engine exhaust system has also been considered. The use of LES has shown that the octagonal, hexahedral and trapezoidal shapes initially observed in the development of the plumes of the RANS cases are likely to be an artifact caused by the RANS solver, as would the transverse total pressure gradients observed in the RANS cases at the nozzle exit as they are both absent from all of the LES results. Likewise the implementation of realistic inlet conditions has a significant impact on the development of the plume, particularly in the length of the potential core and the number of shock cells.