Generation Mechanisms of Rotating Stall and Surge in Centrifugal Compressors.

Flow instabilities such as Rotating Stall and Surge limit the operating range of centrifugal compressors at low mass-flow rates. Employing compressible Large Eddy Simulations (LES), their generation mechanisms are exposed. Toward low mass-flow rate operating conditions, flow reversal over the blade tips (generated by the back pressure) causes an inflection point of the inlet flow profile. There, a shear-layer induces vortical structures circulating at the compressor inlet. Traces of these flow structures are observed until far downstream in the radial diffuser. The tip leakage flow exhibits angular momentum imparted by the impeller, which deteriorates the incidence angles at the blade tips through an over imposed swirling component to the incoming flow. We show that the impeller is incapable to maintain constant efficiency at surge operating conditions due to the extreme alteration of the incidence angle. This induces unsteady flow momentum transfer downstream, which is reflected as compression wave at the compressor outlet traveling toward the impeller. There, the pressure oscillations govern the tip leakage flow and hence, the incidence angles at the impeller. When these individual self-exited processes occurs in-phase, a surge limit-cycle establishes

G-equation modelling of thermo-acoustic oscillations of partially-premixed flames

Numerical simulations aid combustor design to avoid and reduce thermo-acoustic oscillations. Non-linear heat release rate estimation and its modelling are essential for the prediction of saturation amplitudes of limit cycles. The heat release dynamics of flames can be approximated by a Flame Describing Function (FDF). To calculate an FDF, a wide range of forcing amplitudes and frequencies needs to be considered. For this reason, we present a computationally inexpensive level-set approach, which accounts for equivalence ratio perturbations on flames with arbitrarily-complex shapes. The influence of flame parameters and modelling approaches on flame describing functions and time delay coefficient distributions are discussed in detail. The numerically-obtained flame describing functions are compared with experimental data and used in an acoustic network model for limit cycle prediction. A reasonable agreement of the heat release gain and limit cycle frequency is achieved even with a simplistic, analytical velocity fluctuation model. However, the phase decay is over-predicted. For sophisticated flame shapes, only the realistic modelling of large-scale flow structures allows the correct phase decay predictions of the heat release rate response.This work was conducted within the EU 7th Framework Project Joint Technology Initiatives - Clean Sky (AMEL- Advanced Methods for the Prediction of Lean-burn Combustor Unsteady Phenomena), project number: JTI-CS-2013-3-SAGE- 06-009 / 641453. This work was performed using the Darwin Supercomputer of the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk/), provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council

Advanced ray tracing techniques for simulation of thermal radiation in fluids

Zsfassung in dt. SpracheFür die Modellierung von Wärmetransport sind nicht nur Konvektion und Wärmeleitung von Bedeutung, sondern auch thermische und sichtbare Strahlung. Die Berücksichtigung von Strahlung ist besonders wichtig, wenn große Temperaturdifferenzen auftreten oder äußere Lichtquellen in die Betrachtung einfließen. Die heutzutage geläufig verwendeten Modelle der numerische Strömungsmechanik behandeln die Strahlung als untergeordneten Effekt, der mit vereinfachten Algorithmen behandelt werden kann. Alle Standardmodelle in der numerischen Strömungsmechanik für Strahlungssimulationen, wie zB. das Surface-to-Surface Modell, Discrete Transfer Modell, Pn Modell und das Discrete Ordinate Modell, weisen Nachteile in der Berechnungseffizienz oder der physikalischen Modellierung auf. Als Beispiele können Verbrennungskammern, Asche und Rauch Bildung, Solarenergieerzeugung, UV- Wasserdesinfektion, Kondensation in Autoscheinwerfern, Fusion und Fission Reaktorkerne, Lichtbogenbewegungen als auch schwach emittierende Glasfenster genannt werden.In den Bereichen, in denen die Strahlungsuntersuchung den zentralen Aspekt darstellt, wie zB. in der 3D Animation oder Beleuchtungssimulation von Lampen, werden die genannten Methoden nicht mehr verwendet. In diesen Fällen stellt Raytracing die erste Wahl dar.In dieser Arbeit wurden existierende Raytracing Methoden angepasst und implementiert, mit dem Ziel dieses Strahlungsmodell mit Strömungssimulationen zu koppeln und die existierenden Strahlungsmodelle zu ersetzen.Während für Beleuchtungsberechnungen die Geometrie aus Oberflächen für deren Darstellung besteht, benötig die Strömungsberechnung ein volumetrisches Rechengitter. Daher verwendet die implementierte Methode ein volumetrisches Gitter, um volumetrische Effekte mit kleinem zusätzlichem Aufwand in die Berechnung einfließen zu lassen.In dieser Arbeit wurde spektrale volumetrische Path Tracing Methode mit Importance Sampling ausgeführt. Importance Sampling ist eine spezielle Klasse der Monte Carlo Integration, die gegenüber der einfachen Monte Carlo Integration eine schnellere Konvergenz der Lösung aufweist. Mit der implementierten Raytracing Methode ist es möglich Strahlungsquellen in Form von Punkten, Flächen oder auch Volumen zu definieren. Spektrale Materialabhängigkeiten werden ohne starkem Anstieg des Berechnungsaufwandes mit einem Bandmodell berücksichtigt, während in anderen Modellen die Rechnungszeit linear mit der Anzahl der Bänder skaliert. Als Randbedingungen an Oberflächen kann direkte, diffuse und gemischte Reflexion verwirklicht werden. Es wird einen volumetrischer Brechungsindex in die Berechnung mit einbezogen, womit Lichtbrechung und Totalreflexion simuliert werden können. Fokussierung in Linsen oder Spiegelsystemen kann zufriedenstellend wiedergegeben werden. Dies kann mit keinem anderen Strahlungsmodell erreicht werden. Es wurden flächige und volumetrische Absorption implementiert als auch flächige und volumetrische Streuungseffekte.Strahlungsemission kann von Temperaturverteilungen auf Oberflächen oder Volumina hervorgerufen werden. Diese Verteilungen werden ausgehend von einer externen Software, die die Strömungsgleichungen und Energiegleichungen löst, als Randbedingungen in die Raytracing Implementierung importiert. Welche die Resultate der Strahlungssimulation mit den vorgegeben Strahlungsquellen löst und an die externe Software retourniert, wo diese in die weitere Berechnung einfließen. Diese Kopplung wurde implementiert und getestet, wobei als externe Software Fluent, ein kommerzielles Programm für die numerische Strömungsmechanikberechnung, mit seiner plug-in Schnittstelle verwendet wurde. Die meisten Strahlungsmodelle in Fluent werden nur nach einer bestimmten Anzahl impliziter Strömungsiterationen ausgeführt, was zu keinen weiteren Nachteilen oder Einschränkungen führt. Vollständige implizite Berechnung der Strahlung ist unüblich, da die Stabilität für die meiste Anwendungen ausreicht. Natürlich können auch reine Beleuchtungszenerien ohne jegliche sekundären Heizquellen mit der Raytracing Implementierung simuliert werden.Die Implementierung wurde mit analytischen Testfällen validiert und quantitativ mit anderen Strahlungsmodellen verglichen. Auch Streuungseffekte wurden mit experimentellen Daten und Simulationsergebnissen aus der Literatur überprüft.Bei Geometrien von 150000 volumetischen Zellen ist die beobachtete Berechnungsleistung ähnlich oder sogar besser als die, der Standardstrahlungsmodelle, wobei auch die physikalische Modellierung genauer ist. Für größere Geometrien können sich diese Vorteile noch stärker auswirken.For modeling thermal heat transfer, not only the effects of convection and conduction are relevant, but also thermal and visible radiation. Radiation is especially important for setups with large temperature differences, as well as for interaction with external light sources.Common computational fluid dynamic models usually treat radiation transport as a minor effect, that can be handled by simplified algorithms. All these normal models, e.g. surface to surface model, discrete transfer model, Pn method, discrete ordinates model, exhibit disadvantages in the computing performance and the physical modeling. Hence, there are many technical applications, where the fluid simulation are limited both in accuracy and calculation time by the available radiation model. As exemplary cases combustion chambers, smoke and soot creation, solar power generation, UV water disinfection, condensation in car headlights, fusion and fission reactor chambers, electric arc movement, as well as low-emissivity glass windows can be named.In the fields investigating radiation as main effect, e.g. cinematic 3d animation or illumination simulation for lamps and workspaces, the mentioned methods are not in use anymore as ray tracing is the first choice.In this work, the existing methods for ray tracing were adapted and implemented with the goal to interact with fluid flow simulations and replace existing radiation modeling. This can be regarded as innovative, interdisciplinary method for the interaction of fluids and solids with radiation, incorporating physical effects that could not be included in previous simulations.While in usual light calculations, the geometry exists solely in the form of surfaces and their triangulation, fluid flow requires volumetric calculation grids. Hence, methods are implemented that actually use the volumetric grid, and incorporate volumetric effects with little additional effort.Spectral volumetric path tracing with Monte Carlo integrated, importance sampled emission was hence the method of choice for this work.The implemented ray tracer is able to emit radiation from point sources, geometric surfaces, as well as from volumetric sources. Spectral dependence of material values is treated using radiation bands with hardly no increase of calculation time, whereas in all other models, the calculation time scales linearly with the amount of bands. Direct, diffuse and mixed surface reflection is modeled. The volumetric refraction index is implemented, so refraction is modeled, even including partial and total reflexion. The focusing of lenses or mirror systems can hence be simulated satisfactory, which cannot be treated sufficiently by any other radiation model. Surface and volumetric absorption are implemented, as well as surface and volumetric scattering effects.The radiation emission can be caused by a temperature field at surfaces and volumes. These fields are imported from software calculating the fluid and the thermal system. Ray tracing results in volumetric and surface heat sources that can be returned to the original code, and their effect further calculations.This coupling was implemented and tested with the commercial computational fluid dynamics code Fluent, using its plug-in interface.As most of Fluent's radiation models are only performed after a fixed number of implicit flow and turbulence iterations, no further disadvantages or limitations occur, that are not as well existing for the existing radiation simulations. A fully implicit treatment of radiation is unlikely to be performed, as stability is already sufficient for most applications. Of course, systems containing only heat sources caused by light and no secondary heat radiation can be treated by the implemented ray tracer with high performance.The implemented ray tracer is validated with analytically solved systems, and compared to quantitative simulation results of other simulation methods. Also, the scattering effects are validated against experimental and simulation results from literature.The observed calculation performance is similar or faster then for standard models with geometries of approximately 150000 volume elements, while the modeling is done more accurately. For larger models, even larger advantages can be expected.9

Large Eddy Simulation of Turbulent Compressible Jets

Acoustic noise pollution is an environmental aggressor in everyday life. Aero- dynamically generated noise annoys and was linked with health issues. It may be caused by high-speed turbulent free flows (e.g. aircraft jet exhausts), by airflow interacting with solid surfaces (e.g. fan noise, wind turbine noise), or it may arise within a confined flow environment (e.g. air ventilation systems, refrigeration systems). Hence, reducing the acoustic noise levels would result in a better life quality, where a systematic approach to decrease the acoustic noise radiation is required to guarantee optimal results. Computational predic- tion methods able to provide all the required flow quantities with the desired temporal and spatial resolutions are perfectly suited in such application areas, when supplementing restricted experimental investigations. This thesis focuses on the use of numerical methodologies in compressible flow applications to understand aerodynamically noise generation mechanisms and to assess technologies used to suppress it. Robust and fast steady-state Reynolds Averaged Navier-Stokes (RANS) based formulations are employed for the optimal design process, while the high fidelity Large Eddy Simulation (LES) approach is utilized to reveal the detailed flow physics and to investigate the acoustic noise production mechanisms. The employment of fast methods on a wide range of cases represents a brute-force strategy used to scrutinize the optimization parameter space and to provide general behavioral trends. This in combination with accurate simulations performed for particular condi- tions of interest becomes a very powerful approach. Advance post-processing techniques (i.e. Proper Orthogonal Decomposition and Dynamic Mode Decomposition) have been employed to analyze the intricate, highly turbulent flows. The impact of using fluidic injection inside a convergent-divergent nozzle for acoustic noise suppression is analyzed, first using steady-state RANS simulations. More than 250 cases are investigated for the optimal injection location and angle, amount of injected flow and operating conditions. Based on a-priori established criteria, a few optimal candidate solutions are detected from which one geometrical configuration is selected for being thoroughly investigated by using detailed LES calculations. This allows analyzing the unsteady shock pattern movement and the flow structures resulting with fluidic injec- tion. When investigating external fluidic injection configurations, some lead to a high amplitude shock associated noise, so-called screech tones. Such unsteady phenomena can be captured and explained only by using unsteady simulations. Another complex flow scenario demonstrated using LES is that of a high ve- locity jet ejected into a confined convergent-divergent ejector (i.e. a jet pump). The standing wave pattern developed in the confined channel and captured by LES, significantly alters the acoustic noise production. Steady-state methods failed to predict such events. The unsteady highly resolved simulations proved to be essential for analyzing flow and acoustics phenomena in complex problems. This becomes a very powerful approach when is used together with steady-state, low time-consuming formulations and when complemented with experimental measurements. QC 20141202</p

Shape Optimisation of Turbomachinery Components

Low-order models are the first choice to find the initial design of turbomachinery components screening many configurations. The final optimisation of the three-dimensional geometry is crucial for the best performance. Because of the ability to accurately predict the performance of turbomachinery, fluid dynamic simulations became a powerful tool [10]. However, parameter studies for shape optimisation relying on fluid dynamic simulations are computationally expensive and might fail to reveal the optimal geometry. Gradient-based optimisation approaches allow a significant reduction of simulations and hence, determine the optimum efficiency. The adjoint method finds the optimisation gradient by calculating the derivatives of the state variables with respect to the design objective without the need for finite differences [6]. Thus, the adjoint optimisation is especially efficient for problems with many degrees of freedom and few design objectives, e.g. increasing efficiency. The application of the adjoint method for shape optimisation is demonstrated on the example of a centrifugal compressor impeller. The shape of the rotor blades is optimised, and the impact of different objection functions, i.e. reducing the required moment or increasing the achieved pressure ratio, and optimisation constraints, i.e. retaining the operating point or keeping an area ratio, is analysed. The results demonstrate that the compressor performance can be significantly improved using the adjoint method. However, the challenge is to obtain not only an optimised shape for operating points but also for the entire operating map. The final shapes, obtained for different operating points, are compared. QC 20230117</p

Flow Structure Generation by Multiple Jets in Supersonic Cross-Flow

The flow structure generation by multiple jets impinging a supersonic crossflow in the divergent section of a Convergent-Divergent (C-D) duct is investigated using compressible Large Eddy Simulations (LES). The supersonic flow-field in the C-D duct is mainly characterized by the evolving shock-structure. The effect of increasing the compressible jet to crossflow velocity ratio R to the generation of flow structures and the ability to modify the shock-pattern in the duct was studied. Traversing R, the shock-pattern can be significantly altered. This paper demonstrates that for close located jets in crossflow the vortical structures generated by the jets can interact and give rise to vortical structures in the interspace plane between the jets. The spectra for different probes are shown illustrating the characteristic flow frequencies. For all simulated cases the spectra show peaks for a defined Strouhal-number of 0.5. The jets choke in the crossflow above an R of about 0.65, which results in a faster disruption of the coherent flow structures induced by the jets. The flow field is analyzed using Proper Orthogonal Decomposition (POD).QC 20140819</p

Flow Structure Generation by Multiple Jets in Supersonic Cross-Flow

The flow structure generation by multiple jets impinging a supersonic crossflow in the divergent section of a Convergent-Divergent (C-D) duct is investigated using compressible Large Eddy Simulations (LES). The supersonic flow-field in the C-D duct is mainly characterized by the evolving shock-structure. The effect of increasing the compressible jet to crossflow velocity ratio R to the generation of flow structures and the ability to modify the shock-pattern in the duct was studied. Traversing R, the shock-pattern can be significantly altered. This paper demonstrates that for close located jets in crossflow the vortical structures generated by the jets can interact and give rise to vortical structures in the interspace plane between the jets. The spectra for different probes are shown illustrating the characteristic flow frequencies. For all simulated cases the spectra show peaks for a defined Strouhal-number of 0.5. The jets choke in the crossflow above an R of about 0.65, which results in a faster disruption of the coherent flow structures induced by the jets. The flow field is analyzed using Proper Orthogonal Decomposition (POD).QC 20140819</p

Flow Structure Generation by Multiple Jets in Supersonic Cross-Flow

The flow structure generation by multiple jets impinging a supersonic crossflow in the divergent section of a Convergent-Divergent (C-D) duct is investigated using compressible Large Eddy Simulations (LES). The supersonic flow-field in the C-D duct is mainly characterized by the evolving shock-structure. The effect of increasing the compressible jet to crossflow velocity ratio R to the generation of flow structures and the ability to modify the shock-pattern in the duct was studied. Traversing R, the shock-pattern can be significantly altered. This paper demonstrates that for close located jets in crossflow the vortical structures generated by the jets can interact and give rise to vortical structures in the interspace plane between the jets. The spectra for different probes are shown illustrating the characteristic flow frequencies. For all simulated cases the spectra show peaks for a defined Strouhal-number of 0.5. The jets choke in the crossflow above an R of about 0.65, which results in a faster disruption of the coherent flow structures induced by the jets. The flow field is analyzed using Proper Orthogonal Decomposition (POD)

Flow-structures Generated by Valve and Piston Motion in an Exhaust Port of a Truck Engine

The exhaust port of a truck internal combustion engine forms the interface between the combustion engine and the turbocharger. Approximately 30-40% of the energy potential is lost in the exhaust gasses after combustion, which can be partially recuperated in a turbocharger. Hence, energy losses in the connection are highly undesired. However, due to the high occurring velocities and the complex geometry, flow separation, flowstructure formation, and secondary flow motion are the major sources of energy losses. Within the exhaust process, the valves open while the piston continues moving in the combustion camber. This process is often analyzed by modeling the piston and valves at fixed locations, but conserving the total mass flow. Using advanced methods, this process can be simulated numerically in a more accurate manner. This study compares Large Eddy Simulation based data, assessing the implied differences due to the choise of method for simulating the exhaust process from an engine cylinder. A simple case using fixed positions for valve and piston is contrasted with the cases where static valve and moving piston, and moving valve and moving piston are considered, respectively. The generated flow phenomena are  compared within the cases.QC 20131204</p

The generation mechanism of higher screech tone harmonics in supersonic jets

The generation mechanism of screech harmonics in supersonic exhausts is revealed using shadowgraph imaging and acoustic far-field measurements for a rectangular, high aspect-ratio nozzle. The coherent information associated with screech and its harmonics, i.e. flow structures and acoustic radiation pattern, is extracted from the time-resolved shadowgraph images. We show that, for large lateral distortions of the jet plume, the passage of screech associated flow structures triggers the screech-cyclic formation of shocks, which travel downstream and merge with the original shocks. The interaction of the shock waves with the flow structures associated with screech alters the appearance of the perturbations in the mixing layer, which constitute the higher harmonics of screech. Visualisations of the acoustic radiation pattern expose that the third and higher screech tone harmonics originate from these interaction locations. Further, the occurrence of mode resonance between the screech and its harmonics is demonstrated, where the mode resonance location coincides with the screech tone originQC 20200427</p