The strand similarity and inclusion removal capability are two critical parameters to measure the performance of multi-strand tundish in clean steel production. In this work, the effect of two flow regulators, i.e., Flow Control Devices (FCD) and Electromagnetic Stirring (EMS) on eight-strand tundish performance have been investigated by establishing a water model and conducting numerical simulations of water model. The water model was focused on revealing the effect of stirring while the simulation was employed to investigate the effect of two FCDs, namely baffle wall and turbo-stopper. The analysis of strand similarity and inclusion removal were conducted by analyzing flow characteristics derived from Combined Model of Residence Time Distribution (RTD) curve and observing the flow movement in the tundish model. In addition, the tundish capability to remove inclusions was also studied by injecting inclusion particles using Discrete Phase Model (DPM) in ANSYS Fluent. Experiment results cause the Combined Model needs to be modified. This modification was employed when analyzing tundish configuration involving stirring. By using the modified Combined Model, the stirring can significantly increase the well-mix volume to almost 100% as it annihilates dead zone. The stirring also increases the similarity between strands and makes the RTD curve more similar to ideal mixing curve.  However, the problem of short-circuiting flow need to be solved and care should be taken into consideration regarding the selection of stirring direction as well as bath surface condition when implementing EMS in reality. The simulation results show that the addition of baffle wall and turbo-stopper are beneficial to improve mixing as well as to avoid the short-circuiting flow. Furthermore, compared to individual FCD, the combination of baffle wall and turbo-stopper results in the best performance to remove inclusions by providing surface-directed flow and generating a higher plug flow.

As one of the key components of aero engines, compressor is required to endure higher pressure,  possess  higher  efficiency  and  wider  operating  range. Intensive studies have been made on tip leakage flow and researchers find that by reasonably organizing tip leakage flow, aero engines are  more likely to achieve better performance and reliability. Conventional flow controlling methods  like casing treatment and micro jet could substantially modify tip leakage flow, unfortunately with  a price of additional loss, not to mention the difficulty in manufacturing such structure. Whereas  plasma actuation  flow control method  uses  plasma actuators,  such equipment is easy  to  build,  responses  fast  and  has  a  wide  excitation  bandwidth.  This  method  has  become  a  new  trend  in  internal flow active control field.  In this research, a phenomenological model is adopted to simulate DBD plasma actuation in the  flow field inside a compressor cascade. The aim is to find out how plasma actuation will influence  tip  leakage  flow.  Meanwhile  possible  means  to  improve  plasma  actuation  performance  are  discussed.  First of all, numerical simulation of flow inside a compressor cascade without plasma actuation is  conducted to validate accuracy of the numerical methodology adopted and then determine one  numerical  approach  that  satisfies  specific  needs  sufficiently.  Meanwhile,  influence  of  casing  movement on tip leakage flow as well as possible mechanism of tip leakage vortex core generation  is investigated in detail. The results indicate:    1. Generating position of tip leakage vortex moves towards leading edge with increasing moving  speed of shroud.  2. As shroud moving speed increases, trajectory of tip leakage vortex moves away from suction  side of blade and closely towards shroud.  3. Casing movement  tends  to  transform  tip leakage vortex  from  circular  to  oval  shape  due  to  circumferential shearing.  4. Casing  movement  has  little  influence  on  total  pressure  field  concerning  absolute  pressure  value. While total pressure loss does reduce slightly with increasing moving speed of shroud.  5.Vorticity  transport  from  tip  clearance  into  passage  may  be  contributing  significantly  to  generation of tip leakage vortex inner core.  Secondly, a simplified model of DBD plasma actuation based on literature [1] is derived and applied  through  UDF  function  of  commercial  software  Fluent  into  the  flow  field.  Different  actuation  positions,  voltages  and  frequencies  are  applied  in  simulation  and  compared.  After  that  casing  movement is included. Main conclusions are as follow:    6. Plasma  actuation  shows  significant  suppressing  effect  on  tip  leakage  vortex  on  both  size,  trajectory and strength.  7. The suppressing effect on tip leakage vortex grows stronger as actuator moves towards leading  edge.  8. Increasing actuation voltage results in stronger suppressing effect on tip leakage vortex.  9. Plasma actuation can effectively improve total pressure loss situation near shroud region with  increasing actuation power.  10. Increasing actuation frequency results in stronger suppressing effect on tip leakage vortex as  well. Additionally, frequency performs slightly better than voltage. 11. Casing  movement  tends  to  weaken  suppressing  effect  of  tip  leakage  vortex  by  plasma  actuation. More  actuation  power  is  needed  to  achieve  sufficient  suppressing  effect  in  real  compressors. / Som en av de viktigaste komponenterna i flygmotorer krävs det att kompressorn utsätts för högre  tryck, har högre effektivitet och  större driftsintervall. Intensiva  studier har gjorts om  skovlarnas  toppspel  läckageflöde  och  man  anser  att  det  är  mer  sannolikt  att  flygmotorer  uppnår  bättre  prestanda  och  tillförlitlighet  genom  att  på  ett  rimligt  sätt  reglera  läckageflödet  i  toppspelet.  Konventionella metoder  för reglering av flödet, som behandling av “casing” och mikrojet, skulle  kunna  ändra  läckageflödet  avsevärt,  men  medför  tyvärr  ytterligare  förlust,  för  att  inte  tala  om  svårigheten att tillverka en sådan struktur. Samtidig flödeskontroll med hjälp av plasma aktuatorer  som är relativt lätta att bygga, reagerar snabbt och har en bred excitationsbandvid. Denna metod  har blivit en ny trend inom det interna flödesaktiva kontrollområdet.  I  denna  forskning  antas  en  modell  för  att  simulera  plasmaaktivering  av  DBD  i  flödesfältet  i  en  kompressorskaskad. Man försöker ta reda på hur plasmaaktivering påverkar läckageflödet. Möjliga  sätt att förbättra effekten av plasmaaktivering diskuteras.  För  det  första  genomförs  numerisk  simulering  av  flödet  i  en  kompressorskaskad  utan  plasmaaktivering för att validera noggrannheten i den numeriska metoden. Därefter undersöks i  detalj vilken inverkan den relativa rörelsen av ”casing” har på läckageflödet genom toppspelet och  mekanismen för toppspelsvirvel analyseras. Resultaten visar:    1. Startposition för läckagevirveln rör sig mot skovelns framkant när man introducerar och ökar  den relativa hastigheten för ”casing”.    2. I takt med att den relativa hastigheten ökar, kretsbanan för    läckage virveln rör sig bort från  skovelns sugsida och närmare mot ”casing”.  3. Den  relativa  rörelsen  tenderar  att  omvandla  virveln  från  cirkulär  till  oval  form  på  grund  av  skjuvkrafter.  4. Den relativa rörelsen av ”casing” påverkar inte det totala tryckfältet när det gäller det absoluta  tryckvärdet. Samtidigt som den totala tryckförlusten minskar något med ökad hastighet.  5. Virveltransport från toppspelet till huvudkanalen kan på ett betydande sätt bidra till att skapa  virvelns inre kärna.  I senare delen av arbetet utvecklas och tillämpas en förenklad modell för plasmaaktivering av DBD  baserad  på  litteratur  [1],  genom  att  använda  UDF‐funktionen  i  kommersiell  CFD  programvara  Fluent.  Olika  aktuatorläge,  spänningar  och  frekvenser  prövas  i  simuleringen  och  jämförs.  De  viktigaste slutsatserna är följande:    6. Aktuering av plasma visar en betydande dämpningseffekt på läckagevirveln i toppspelet både  va det gäller dess storlek, bana och styrka.  7. Den  dämpande  effekten  på  läckagevirveln  blir  starkare  när  aktuator  monteras  närmare  skovelns framkant.  8. Ökad aktuatorspänning leder till en starkare dämpande effekt på läckagevirveln.  9. Ökad aktuatorfrekvens leder till starkare dämpningseffekt på läckagevortex också.mDessutom  fungerar frekvensen något bättre än spänningen.  10. Den  relativa  rörelsen  av  ”casing”  försvagar  effekten  av  plasmaaktuering.  För  att  uppnå  tillräcklig dämpningseffekt i riktiga kompressorer krävs mer effekt till aktuatorn.

<p>Highly loaded airfoils in low-pressure turbines (LPTs) suffer from laminar flow separation from the suction side of the airfoils aft of the throat of the passages. This separation harms the performance of the engine by reducing the power extraction from the turning air and ultimately reduces the overall turbine efficiency. Flow control techniques have been investigated to eliminate flow separation in aerodynamic surfaces to abate the losses associated with it. This Master of Science Thesis investigates the design, implementation and testing of pulsated injection actuation in a low-Reynolds flow over a wall-mounted hump.</p> <p>Furthermore, this Thesis expands on the existing expertise in the infrared (IR) thermography measurement technique at the Purdue Experimental Turbine Aerothermal Lab. This is done through an investigation of the factors affecting the IR measurement technique and the development of an optical instrument (borescope) to implement in an annular cascade wind tunnel. IR thermography is used on the wall-mounted hump blowdown tests to detect the separation point in the boundary layer using two techniques: by an investigation of the surface temperature distribution and an investigation of the heat transfer behavior at the surface. Finally, the borescope is commissioned through the first testing campaign of the LPT airfoils, and are processed to thermally investigate the passage.</p> <p>This thesis succeeds in expanding the IR capabilities within PETAL, and at demonstrating pulsated injection as an effective method to eliminate flow separation. Furthermore, IR successfully detects flow separation on the wall-mounted hump through the two methods presented, as well as detecting the boundary layer reattachment caused by the flow control technique. The limitations of the thermal methodology, as well as those of the optical probe are addressed, and the uncertainties in the measurements are quantified. Finally, steps to continue the studies are suggested at the end of each methodology chapter, including the potential redesign of the IR borescope to improve the quality of measurements. </p>

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The main aim of this thesis was to understand the mechanism behind the classical transition scenario inside the boundary layer over an airfoil and eventually attempting to control this transition utilizing passive devices for transition delay. The initial objective of analyzing the transition phenomenon based on TS wave disturbance growth was conducted at 90 Hz using LDV and CTA measurement techniques at two different angles of attack. This was combined with the studies performed on two other frequencies of 100 and 110 Hz, in order to witness its impact on the neutral stability curve behavior. The challenges faced in the next phase of the thesis while trying to control the transition location, was to understand and encompass the effect of adverse pressure gradient before setting up the passive control devices, which in this case was miniature vortex generators. Consequently, several attempts were made to optimize the parameters of the miniature vortex generators depending upon the streak strength and stability. Finally, for 90 Hz a configuration of miniature vortex generators have been found to successfully stabilize the TS wave disturbances below a certain forcing amplitude, which also led to transition delay.

The combustor-diffuser system remains one of the most studied sections of the turbomachine. Most of these investigations are due to the fact that quite a bit of flow diffusion is required in this section as the high speed flow exits the compressor and must be slowed down to enter the combustor. Like any diffusion process there is the chance for the development of an unfavorable adverse pressure gradient that can lead to flow separation; a cause of drastic losses within a turbine. There are two diffusion processes in the combustor-diffuser system: The flow first exits the compressor into a pre-diffuser, or compressor discharge diffuser. This diffuser is responsible for a majority of the pressure recovery. The flow then exits the pre-diffuser by a sudden expansion into the dump diffuser. The dump diffuser comprises the majority of the losses, but is necessary to reduce the fluid velocity within acceptable limits for combustion. The topic of active flow control is gaining interest in the industry because such a technique may be able to alleviate some of the requirements of the dump diffuser. If a wider angle pre-diffuser with separation control were used the fluid velocity would be slowed more within that region without significant losses. Experiments were performed on two annular diffusers to characterize the flow separation to create a foundation for future active flow control techniques. Both diffusers had the same fully developed inlet flow condition, however, the expansion of the two diffusers differed such that one diffuser replicated a typical compressor discharge diffuser found in a real machine while the other would create a naturally separated flow along the outer wall. Both diffusers were tested at two Reynolds numbers, 5x104 and 1x105, with and without a vertical wall downstream of the exit to replicate the dump diffuser that re-directs the flow from the pre-diffuser outlet to the combustor. Static pressure measurements were obtained along the OD and ID wall of the diffusers to determine the recovered pressure throughout the diffuser. In addition to these measurements, tufts were used to visualize the flow. A turbulent CFD model was also created to compare against experimental results. In the end, the results were validated against empirical data as well as the CFD model. It was shown that the location of the vertical wall was directly related to the amount of separation as well as the separation characteristics. These findings support previous work and help guide future work for active flow control in a separated annular diffuser both computationally and experimentally.

In boundary-layer flows it is possible to reduce the friction drag by breaking the path from laminar to turbulent state. In low turbulence environments, the laminar-to-turbulent transition is dominated by local flow instabilities – Tollmien-Schlichting (TS) waves – that exponentially grows while being con- vected by the flow and, eventually, lead to transition. Hence, by attenuating these disturbances via localised forcing in the flow it is possible to delay farther downstream the onset of turbulence and reduce the friction drag. Reactive control techniques are widely investigated to this end. The aim of this work is to compare model-based and adaptive control techniques and show how the adaptivity is crucial to control TS-waves in real applications. The control design consists in (i) choosing sensors and actuators and (ii) designing the system responsible to process on-line the measurement signals in order to compute an appropriate forcing by the actuators. This system, called compen- sator, can be static or adaptive, depending on the possibility of self-adjusting its response to unmodelled flow dynamics. A Linear Quadratic Gaussian (LQG) regulator is chosen as representative of static controllers. Direct numerical simulations of the flow are performed to provide a model for the compensator design and test its performance. An adaptive Filtered-X Least-Mean-Squares (FXLMS) compensator is also designed for the same flow case and its per- formance is compared to the model-based compensator via simulations and experiments. Although the LQG regulator behaves better at design conditions, it lacks robustness to small flow variations. On the other hand, the FXLMS compensator proved to be able to adapt its response to overcome the varied conditions and perform an adequate control action. It is thus found that an adaptive control technique is more suitable to delay the laminar-to-turbulent transition in situations where an accurate model of the flow is not available. / I det tunna gränsskikt som uppstår en yta, kan friktionen minskas genom att förhindra omslag från ett laminärt till ett turbulent flöde. När turbulensnivån är låg  i omgivningen, domineras till en början omslaget av lokala instabiliteter (Tollmien-Schlichting (TS) v ågor) som växer i en exponentiell takt samtidigt som de propagerar nedströms. Därför, kan man förskjuta omslaget genom att dämpa TS vågors tillväxt i ett gränsskikt och därmed minska friktionen.Med detta mål i sikte, tillämpas och jämförs två reglertekniska metoder, nämligen en adaptiv signalbaserad metod och en statiskt modellbaserad metod. Vi visar att adaptivitet är av avgörande betydelse för att kunna dämpa TS vågor i en verklig miljö. Den reglertekniska konstruktionen består av val av givare och aktuatorer samt att bestämma det system som behandlar mätsignaler (on- line) för beräkning av en lämplig signal till aktuatorer. Detta system, som kallas för en kompensator, kan vara antingen statisk eller adaptiv, beroende på om det har möjlighet till att anpassa sig till omgivningen. En så kallad linjär regulator (LQG), som representerar den statiska kompensator, har tagits fram med hjälp av numeriska simuleringar of strömningsfältet. Denna kompensator jämförs med en adaptiv regulator som kallas för Filtered-X Least-Mean-Squares (FXLMS) både experimentellt och numeriskt. Det visar sig att LQG regulatorn har en bättre prestanda än FXLMS för de parametrar som den var framtagen för, men brister i robusthet. FXLMS å andra sidan, anpassar sig till icke- modellerade störningar och variationer, och kan därmed hålla en god och jämn prestanda.Man kan därmed dra slutsaten att adaptiva regulatorer är mer lämpliga för att förhala omslaget fr ån laminär till turbulent strömning i situationer då en exakt modell av fysiken saknas. / <p>QC 20141020</p>

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