In mehrstufigen Axialverdichtern für Flugtriebwerke und stationäre Gasturbinen werden die Statoren entweder mit Deckband ausgeführt oder freistehend mit einem Radialspalt zur Rotornabe. Diese zwei üblichen konstruktiven Varianten zur Trennung von rotierenden und stehenden Bauteilen haben erheblichen Einfluss auf das Strömungsverhalten und die Verluste im nabennahen Bereich der Statorpassage. In den hinteren Stufen von mehrstufigen Axialverdichtern kommt dieser Problematik eine große Bedeutung bei, da mit sinkender Kanalhöhe der Anteil der Randverluste am Gesamtverlust steigt. Weiterhin ist mit relativ großen Spaltweiten im Verhältnis zur Kanalhöhe zu rechnen. Die vorliegende Arbeit vergleicht die Strömung an Statoren mit und ohne Deckband auf Basis experimenteller und numerischer Untersuchungen in einem vierstufigen Niedergeschwindigkeits-Axialverdichter. Für beide Bauarten werden die Auswirkungen einer Variation der Spaltweite auf das Strömungsfeld mit stationären Drucksonden ermittelt und mit stationären CFD-Simulationen verglichen. Für Statoren mit Radialspalt zur Nabe wird eine verbesserte dreidimensionale Gestaltung vorgestellt und experimentell untersucht. Ziel der neuen Auslegung ist die Verringerung der Auswirkung der Spaltweitenvergrößerung auf Verluste, Minderumlenkung und Blockage. Die experimentelle Verifizierung der Neuauslegung wird von numerischen Simulationen zur detaillierten Analyse der Verbesserungen unterstützt.:Symbolverzeichnis Einleitung 1 Grundlagen der Strömung im Seitenwandbereich von Statoren / Stand des Wissens 1.1 Sekundärströmung an Statoren 1.2 Historische Entwicklung der Untersuchung von Statoren mit und ohne Deckband in Axialverdichtern 1.3 Reduzierung von Verlusten durch dreidimensionales Schaufeldesign 1.4 Zielstellung der aktuellen Untersuchung 2 Niedergeschwindigkeitsverdichter und Messtechnik 2.1 Allgemeiner Aufbau des Niedergeschwindigkeitsverdichters 2.2 Messtechnik am Niedergeschwindigkeitsverdichter 2.3 Auswertungsmethoden und Analyse der Messergebnisse 2.3.1 Normierung der Messwerte 2.3.2 Kennfeld 2.3.3 Strömungsfeldmessung an der Stufe 2.3.4 Druckverteilungen 2.3.5 Betrachtung der Messgenauigkeit 2.4 Experimentell untersuchteAufbauten 2.4.1 Referenz-Statoren ohne Deckband - Aufbau 16 2.4.2 Referenz-Statoren mit Deckband - Aufbau 17 2.4.3 Neuauslegung für Statoren mit großen Radialspalten - Aufbau 20 3 Numerische Untersuchungen 3.1 Numerisches Modell 3.2 Simulationsmodell 3.2.1 Vernetzung des Verdichters für den Referenzfall 3.2.2 Modellierung unterschiedlicher Radialspaltweiten 3.2.3 Modellierung der Deckbandkavität 3.2.4 Einfluss der Turbulenzmodellierung auf die Strömung an Statoren mit und ohne Deckband 3.2.5 Simulationen mit Non-Linear-Harmonic-Method 4 Statoren mit und ohne Deckband 4.1 Globales Betriebsverhalten des Verdichters für Referenzmessungen an Statoren mit und ohne Deckband 4.1.1 Drehzahllinie des Verdichters bei Radialspaltvariation über Stator 3 und 4 4.1.2 Drehzahllinie des Verdichters bei Deckbandspaltvariation im Stator 3 4.1.3 Globale Ergebnisse der CFD-Simulation für Statoren mit und ohne Deckband 4.2 Strömungsfeldmessungen für Statoren mit und ohne Deckband 4.2.1 Zuströmung zum dritten Stator 4.2.2 Vergleich der Strömung am dritten Stator mit und ohne Deckband 4.2.3 Auswirkung des Designs des dritten Stators auf Rotor 4 4.3 Bewertung der Ergebnisse für Statoren mit und ohne Deckband 5 Statoren ohne Deckband mit verringerter Sensitivität gegenüber großen Radialspaltweiten 5.1 Auslegung eines Stators ohne Deckband für große Radialspaltweiten 5.2 Globales Betriebsverhalten des Verdichters für neuausgelegte Statoren ohne Deckband 5.3 Strömungsfeldmessungen am neuausgelegten Stator mit großem Radialspalt 5.3.1 Zuströmung zum dritten Stator 5.3.2 Strömung am dritten Stator bei Radialspaltvariation 5.3.3 Auswirkung der Radialspaltvariation am dritten Stator auf Rotor 4 5.4 Bewertung der neuen Auslegung für Statoren mit großem Radialspalt 6 Zusammenfassung A Anhang A.1 Eigenschaften des CFD-Netzes A.2 CFD-Netze f¨ur Stator 3 mit Deckband A.3 Einfluss des Fillets am dritten Stator mit Deckband Literaturverzeichnis

Multiple high-fidelity, time-accurate computational fluid dynamics simulations were performed to investigate the effects of upstream stator loading and rotor shock strength on vortex shedding characteristics in a single stage transonic compressor. Various configurations of a transonic axial compressor stage, including three stator/rotor axial spacings of close, mid, and far in conjunction with three stator loadings of decreased, nominal, and increased were simulated in order to understand the flow physics of transonic blade-row interactions. Low-speed compressors typically have reduced stator/rotor axial spacing in order to decrease engine weight, and also because there is an increase in efficiency with reduced axial spacing. The presence of a rotor bow shock in high-speed compressors causes additional losses as the shock interacts with the upstream stator trailing edge. This research analyzes the strength of shock-induced vortices due to these unsteady blade-row interactions. The time-accurate URANS code, TURBO, was used to generate periodic, quarter annulus simulations of the Blade Row Interaction compressor rig. Both time-averaged and time-accurate results compare well with experimentally-observed trends. It was observed that vortex shedding was synchronized to the passing of a rotor bow shock. Normal and large shock-induced vortices formed on the stator trailing edge immediately after the shock passing, but the large vortices were strengthened at the trailing edge due to a low-velocity region on the suction surface. This low velocity region was generated upstream of mid-chord on the suction surface from a shock-induced thickening of the boundary layer or separation bubble, due to the rotor bow shock reflecting off the stator trailing edge and propagating upstream. The circulation of the shock-induced vortices increased with shock strength (decreased axial spacing) and stator loading. Most design tools do not directly account for unsteady effects such as blade-row interactions, so a model is developed to help designers account for shock-induced vortex strength with varying shock strength and stator loading. An understanding of the unsteady interactions associated with blade loading and rotor shock strength in transonic stages will help compressor designers account for unsteady flow physics early in the design process.

The use of turbocharged Diesel engines is nowadays a widespread practice in the automotive sector: heavy-duty vehicles like trucks or buses, in particular, are often equipped with turbocharged engines. An accurate study of the flow field developing inside both the main components of a turbocharger, i.e. compressor and turbine, is therefore necessary: the synergistic use of CFD simulations and experimental tests allows to fulfill this requirement. The aim of this thesis is to investigate the performance and the flow field that develops inside a centrifugal compressor for automotive turbochargers. The study is carried out by means of numerical simulations, both steady-state and transient, based on RANS models (Reynolds Averaged Navier-Stokes equations). The code utilized for the numerical simulations is Ansys CFX.   The first part of the work is an engineering attempt to develop a CFD method for predicting the performance of a centrifugal compressor which is based solely on steady-state RANS models. The results obtained are then compared with experimental observations. The study continues with an analysis of the sensitivity of the developed CFD method to different parameters: influence of both position and model used for the rotor-stator interfaces and the axial tip-clearance on the global performances is studied and quantified.   In the second part, a design optimization study based on the Design of Experiments (DoE) approach is performed. In detail, transient RANS simulations are used to identify which geometry of the recirculation cavity hollowed inside the compressor shroud (ported shroud design) allows to mitigate the backflow that appears at low mass-flow rates. Backflow can be observed when the operational point of the compressor is suddenly moved from design to surge conditions. On actual heavy-duty vehicles, these conditions may arise when a rapid gear shift is performed. / Användningen av turboladdade dieselmotorer ärr numera utbredd inom bilindustrin: i synnerhet tunga fordon som lastbilar eller bussar ärr ofta utrustade med turbo-laddade motorer. En utförlig förståelse av flödesfältet som utvecklas innuti båda huvudkomponenterna hos en turboladdare, dvs kompressor och turbin, är därför nödvändig: den synergistiska användningen av CFD-simuleringar och experimentel-la tester möjliggör att detta krav uppfylls. Syftet med denna avhandling är att undersöka prestanda och det flödesfält som utvecklas i en centrifugalkompressor för turboladdare. Studien utförs genom nu-meriska simuleringar, både steady state och transient, baserat på RANS-modeller (Reynolds Averaged Navier-Stokes-ekvationer). Koden som används för de numeriska simuleringarna är Ansys CFX.   Den första delen av arbetet ¨ar ett försöka att utveckla en CFD-metod för att förutsäga prestanda för en centrifugalkompressor med hjälp av steady-state RANS-modeller. De erhållna resultaten jämförs sedan med experimentella observationer. Studien fortsätter med en analys av känsligheten hos den utvecklade CFD-metoden till olika parametrar: Inflytande av både position och modell som används för rotor-statorgränssnitt samt axiellt spel mellan rotor och hus på de globala prestationerna studeras och kvantifieras.   I andra delen utförs en designoptimeringsstudie baserad på Design of Experiments (DoE). I detalj används tidsupplösta RANS-simuleringar för att identifiera vilken utformning av ported shroud som minskar backflöde i kompressorn under en snabb minskning av massflöde och varvtal och därmed ger bättre prestanda i transient surge. På tunga fordon kan dessa förhållanden uppstå under växling.

This dissertation presents results from a thorough study of the tip-gap turbulent flow structure in a low-speed linear compressor cascade wind tunnel at Virginia Tech that includes a moving belt system to simulate the relative motion between the tip and the casing. The endwall pressure measurements and the surface oil flow visualizations were made on a stationary endwall to obtain the flow features and to determine the measurement profiles of interest. A custom-made miniature 3-orthogonal-velocity-component fiber-optic laser-Doppler velocimetry (LDV) system was used to measure all three components of velocity within a 50 mm spherical measurement volume within the gap between the endwall and the blade tip, mainly for the stationary wall with 1.65% and 3.30% tip gaps as well as some initial experiments for the moving wall. Since all of the vorticity in a flow originates from the surfaces under the action of strong pressure gradient, it was very important to measure the nearest-wall flow on the endwall and around the blade tip. The surface skin friction velocity was measured by using viscous sublayer velocity profiles, which verified the presence of an intense lateral shear layer that was observed from surface oil flow visualizations. All second- and third-order turbulence quantities were measured to provide detailed data for any parallel CFD efforts. The most complete data sets were acquired for 1.65% and 3.30% tip gap/chord ratios in a low-speed linear compressor cascade. This study found that tip gap flows are complex pressure-driven, unsteady three-dimensional turbulent flows. The crossflow velocity normal to the blade chord is nearly uniform in the mid tip-gap and changes substantially from the pressure to suction side. The crossflow velocity relies on the local tip pressure loading that is different from the mid-span pressure loading because of tip leakage vortex influence. The tip gap flow is highly skewed three-dimensional flow throughout the full gap. Normalized circulation within the tip gap is independent of the gap size. The tip gap flow interacts with the primary flow, separates from the endwall, and rolls up on the suction side to form the tip leakage vortex. The tip leakage vortex is unsteady from the observation of the TKE transport vector and oil flow visualizations. The reattachment of tip separation vortex on the pressure side strongly depends on the blade thickness-to-gap height ratio after the origin of tip leakage vortex but is weakly related to it before the origin of tip leakage vortex for a moderate tip gap. Other than the nearest endwall and blade tip regions, the TKE does not vary much in tip gap. The tip leakage vortex produces high turbulence intensities. The tip gap flow correlations of streamwise and wall normal velocity fluctuations decrease significantly from the leading edge to the trailing edge of the blade due to flow skewing. The tip gap flow is a strongly anisotropic turbulent flow. Rapid distortion ideas can not apply to it. A turbulence model based on stress transport equations and experimental data is necessary to reflect the tip gap flow physics. For the moving endwall, relative motion skews the inner region flow and is decorrelated with the outer layer flow. Hence, the TKE and correlations of streamwise and wall normal velocity fluctuations decrease. / Ph. D.

This report documents the experimental study for four different three-dimensional turbulent flows. The investigation focuses on near wall measurements in these flows. Several experimental techniques are used in the studies; however, the bulk of the investigation focuses on a three-orthogonal-velocity-component fiber-optic laser Doppler anemometer (3D-LDA) system. The control volume of the 3D-LDA is on the order of 50 micro-meter in size, or a y⁺ distance of around 2.3 units (using average values of U_&#964 and ν from the experiment). An auxiliary small boundary layer wind tunnel (auxiliary tunnel) and a low speed linear compressor cascade wind tunnel (cascade tunnel) are utilized in this study. One of four flow experiments is done in the auxiliary tunnel the other three are in the cascade tunnel. The first three-dimensional turbulent flow is a vortical flow created by two half-delta wing vortex generators. Near wall secondary flow features are found. The second flow is an investigation of the first quarter chord tip gap flow in the cascade tunnel. Strong three-dimensional phenomena are found. The third flow investigated is the inflow to the compressor cascade with the moving wall. The experiment records shear layer interaction between the upstream flow and moving wall. Finally the fourth flow investigated is the inflow to the compressor cascade with the moving wall with half-delta wing vortex generators attached. Phase-averaged data reveal asymmetrical vortex structures just downstream of the vortex generators. This is the first time any near wall data has been taken on any of these flows. / Master of Science

There are many useful guidelines and theories to be found on mixing and mastering. One of these statements is that the monitoring level affects how well one can detect and evaluate the Dynamic Range Compression (DRC) when mixing. However, some of the sources that support this idea are contradictory as to whether a low or high monitoring level is better for this purpose. No previous research on this subject has been found, even though monitoring levels are presumed to be essential for the mixing and mastering engineer. The aim of the study was to investigate if the monitoring level will affect the listeners’ detection threshold for DRC when applied to a vocal track. For this, one higher and one lower monitoring level was chosen, and a simple up-down method was used to obtain the X50 in each monitoring condition, in which these were compared. The results indicate that the monitoring level does not affect the listeners’ threshold of detection for DRC. Moreover, it can be stated that the monitoring level in which the participants had the most habit of working in did not affect the result either. This research can be used to take more informed decisions regarding monitoring levels and to revisit some previous observations.

In axial compressor, corner separation phenomenon can occur between the blade surface and the hub. This leads to high total pressure losses, blockage and may worsen to surge. Various studies on NACA65-009 blade were previously performed experimentally and numerically to predict the corner separation. The LMFA showed that RANS simulations tend to overestimate it while the Wall-Resolved LES (WRLES) was able to well capture it. The conclusions drawn on RANS are validated here with another solver software. An extensive parametric study is performed on RANS which highlights the good performance of two non-linear turbulence models k − ω Wilcox QCR and EARSM k − kl for for predicting the topology and the intensity of corner separation. They are however very dependent on the mesh and the numerics. A Wall-Modeled LES (WMLES) is then computed. It reproduces well the topology of the separation given by the experiments and predicts similar anisotropy to the WRLES. Nevertheless it shows high sensitivity to the level of turbulence close to the endwall and the boundary layer profile of the upstream flow. Finally, this confirms that the WMLES is a promising alternative to the WRLES in order to study the corner separation on more costly geometries (several blades for instance). / I axiell kompressor kan hörnseparationsfenomen uppstå mellan bladytan och navet. Konsekvenserna är stora totala tryckförluster och kompressor blockering. Olika studier på NACA65-009 bladet utfördes tidigare experimentellt och numeriskt för att förutsäga hörnseparationen. LMFA visade att RANS simuleringar tenderar att överskatta den hörnseparationen medan Vägg-Löst LES (WRLES på engelska) kunde fånga bra den. Slutsatserna som dras om RANS valideras här med en annan lösningsprogramvara. En omfattande parametrisk studie utförs på RANS som belyserde goda prestandan för två icke-linjära turbulensmodeller k − ω Wilcox QCRoch EARSM k − kl för att förutsäga topologin och intensiteten för hörnseparation. Dock är de mycket beroende av nät och numerik. En Vägg-Modell LES (WMLES på engelska) beräknas sedan. Det reproducerar väl topologin för separationen som ges av experimenten och förutsäger liknande anisotropi som WRLES. Dock visar det hög känslighet för turbulensnivån nära ändväggen och gränsskiktsprofilen för uppströmsflödet. Slutligen bekräftar detta att WMLES är ett lovande alternativ till WRLES för att studera hörnseparationen på dyrare geometrier (till exempelflera blad).

This thesis provides a comprehensive investigation of flow-sound-structure coupling in spring-loaded valves subjected to air flow. While they are commonly used in a multitude of applications, these types of valves have been found to experience severe vibrations when interaction is present among the structure, the hydrodynamic field, and the acoustic field for a range of operational valve structural characteristics, flow parameters, and connected piping length. The first part of this investigation was aimed at characterizing experimentally the valve’s dynamic behaviour and the parameters affecting the onset of self-excited instability. The occurrence of instability was mainly driven by the presence of acoustic feedback: the connected length of piping had to be sufficiently long, with a longer pipe correlating to more severe vibrations. In addition, it was found that the valve’s oscillation frequency depends on the modal characteristics of the combined valve piping system, rather than the structural natural frequency alone. Furthermore, an increase in the valve’s spring stiffness caused the vibrations to become more severe. Meanwhile, other parameters such as initial spring preload force and valve plate area only had moderate effects on the stability behaviour of the valve. The second part of the investigation sought to develop a theoretical model that could simulate the valve’s response when subjected to air flow while considering the effects of acoustic feedback and impact on the seat and limiter. Thus, a structural model of the valve was developed based on a single-degree-of-freedom model of the system with impact computed based on a pseudo-force method. The hydrodynamic field relied on a one dimensional unsteady Bernoulli description of the flow. Finally, the acoustic interaction was accounted for using the one-dimensional wave equation resolved using a finite difference scheme. The model has demonstrated remarkable agreement with the experimental results. It has shown an ability to predict the modal characteristics of the system as well as correctly predict the effect of increased stiffness or increased piping length on vibration amplitude. The final part of the investigation consisted in designing countermeasures to mitigate the effects of this self-excited instability mechanism. A concentric Helmholtz-type cavity resonator, an orifice plate, and an anechoic termination are placed at the downstream side of a model valve which were seen to be unstable in the experimental and modelling phases of the investigation. All tested devices were able to eliminate the self excited instability mechanism. The applicability and robustness of each of these methods were discussed. / Thesis / Doctor of Philosophy (PhD)

<p dir="ltr">Current industrial processes are energy and carbon emission intensive. Amidst the growing demand for decarbonization, it is critical to utilize alternate sources of energy and innovative technologies that could improve efficiency and reduce power consumption. In this context, twin-screw compressors are used extensively in commercial and industrial applications. Profile optimization and capacity modulation solutions (e.g., slide valves, variable-speed, etc.) are continuously investigated to improve the performance and operation of the compressors. This study focuses on an exploratory investigation of an additively manufactured twin-screw compressor with internal cooling channels to achieve a near isothermal compression process by evaluating both the potential compressor performance improvement and the structural integrity by means of rotordynamics and fatigue analyses.</p><p dir="ltr">To predict the compressor performance, complex coupling between compression process and heat transfer during the operation of the compressor must be investigated. The interactions between solid (i.e., rotors) and fluid phases (i.e., air and coolant) were modeled using a transient 3D CFD model with conjugated heat transfer (CHT). The CFD model predicted compressor performance parameters such as isentropic efficiency, heat transfer rate, work input and compression forces on the rotors. The performance of the twin-screw compressor with internal cooling channels has been compared with a conventional twin-screw compressor for which experimental data was available. Further investigations have been conducted at different operating conditions, including various pressure ratios, rotational speeds, and mass flow rates to improve the compressor efficiency. The results of the CFD model were used to quantify compression loads, assess the characteristics of the heat transfer processes, and optimize the internal flow through the cooling channels. As the rotors can be affected by stress accumulation and deformations due to their hollowness and reduced wall thickness over time, this study also established a detailed rotordynamic simulation model and a fatigue model using the actual compression forces obtained from previous CFD studies. Both hollow and solid rotors have been analyzed and compared. The bearing loads have been verified against Campbell diagrams whereas the fatigue results have been compared with experimental testing. With the validated model, the hollow rotor compressor durability was analyzed and compared with the conventional rotors. Lastly, a general mechanistic model to better understand bearing loads and frictional losses in a twin-screw compressor is also established and studied.</p><p dir="ltr">The CHT study concluded that the hollow rotor with single-phase internal cooling yielded to an increase in isentropic efficiency of 1% for the higher pressure ratio and 2% for lower pressure ratio at 19,000 RPM. More importantly, the hollow rotors also showed a decrease of 40 K and 20 K in discharge temperatures for the two operating conditions respectively, thereby arriving closer to isothermal conditions and reducing the thermal stresses on the rotors. The rotordynamic study revealed that the male rotor would endure highest amount of von Misses stress reaching up to 338 MPa for the pressure ratio of 3.29 bar and 19,000 RPM. Because of this, a maximum fatigue factor of safety of 5 occurs on the male rotor. From the analyses, the rotors were deemed to be safe and optimized for the designed operating conditions and proof of concept rotors were additively manufacturers with an Inconel alloy through Direct Metal Laser Sintering.</p>

<p dir="ltr">Increased scrutiny from customers and regulators to design aeroengines that are more efficient and environmentally friendly has pushed the need to investigate new engine architectures, manufacturing techniques, and computational fluid dynamic methods. This has led to the development of the CSTAR Gen. 2.5 centrifugal compressor, which uses an additively manufactured diffusion system and investigates the aerodynamic performance of an axi-centrifugal aeroengine. Additionally, an isolated unsteady diffuser computational model was previously developed that seeks to significantly reduce the computational cost of unsteady CFD in the diffuser.</p><p dir="ltr">The research presented in this paper is part of an ongoing attempt to utilize the capabilities of isolated unsteady diffuser modeling and rapid prototyping enabled through additive manufacturing in CSTAR Gen. 2.5 to develop a design framework that allows for quick computational and experimental analysis of diffusion systems in centrifugal compressors. Specifically, the scope of isolated unsteady diffuser modeling, which was previously only implemented in CSTAR Gen. 1 and at a single loading condition, is expanded by analyzing computational instabilities when applying the methodology to CSTAR Gen. 2.5 and analyzing results from four loading conditions (high loading, design point, low loading, and near choke) along a speedline.</p><p dir="ltr">Computational instabilities in the CSTAR Gen. 2.5 isolated diffuser models were determined to be caused by the decreased vaneless space compared to Gen. 1, which led to less “mixed” flow at the impeller outlet and a stronger diffuser potential field affecting the inlet profile. A boundary profile correction approach was developed which slightly increased very low total pressure near the diffuser shroud and negative radial velocity regions near the shroud and pitchwise locations of the diffuser vane leading edges while minimizing the overall affected area. The correction was successfully validated using 3D flow structure and minimum, average, and maximum total pressure, absolute velocity magnitude, and pressure comparisons at the diffuser inlet between an isolated and full-stage model.</p><p dir="ltr">Prediction capabilities of 3D flow structures and 1D performance parameters by isolated unsteady diffuser models were validated with results from full-stage unsteady models at each loading condition. The analysis indicated consistent performance by the isolated unsteady diffuser model at all loading conditions. An overall agreement in 3D flow structures was found, and trends in the full-stage unsteady models along the speedline were tracked well by the isolated unsteady model. At all loading conditions, there was a consistent over-representation of the separation region along the diffuser vane pressure side in the diffuser passage, overprediction of total pressure magnitude at the core of the flow at the diffuser outlet, and over- or underprediction of total pressure loss and static pressure recovery respectively. The similarity in the results between full-stage and isolated unsteady models, tracking of trends along the speedline, and consistent differences in 3D flow structure predictions and 1D performance parameters validates the isolated unsteady diffuser methodology for use at loading conditions from surge to choke.</p>