Turbocharging the internal combustion (IC) engine is a common technique to increase the power density. If turbocharging is used with the downsizing technique, the fuel consumption and pollution of green house gases can be decreased. In the turbocharger, the energy of the engine exhaust gas is extracted by expanding it through the turbine which drives the compressor by a shaft. If a turbocharged IC engine is compared with a natural aspirated engine, the turbocharged engine will be smaller, lighter and will also have a better efficiency, due to less pump losses, lower inertia of the system and less friction losses. To be able to further increase the efficiency of the IC engine, the understanding of the highly unsteady flow in turbochargers must be improved, which then can be used to increase the efficiency of the turbine and the compressor. The main objective with this thesis has been to enhance the understanding of the unsteady flow in turbocharger and to assess the sensitivity of inflow conditions on the turbocharger performance. The performance and the flow field in a radial turbocharger turbine working under both non-pulsatile and pulsatile flow conditions has been assessed by using Large Eddy Simulation (LES). To assess the effects of different operation conditions on the turbine performance, different cases have been considered with different perturbations and unsteadiness of the inflow conditions. Also different rotational speeds of the turbine wheel were considered. The results show that the turbine cannot be treated as being quasi-stationary; for example,the shaft power varies for different frequencies of the pulses for the same amplitude of mass flow. The results also show that perturbations and unsteadiness that are created in the geometry upstream of the turbine have substantial effects on the performance of the turbocharger. All this can be summarized as that perturbations and unsteadiness in the inflow conditions to the turbine affect the performance. The unsteady flow field in ported shroud compressor has also been assessed by using LES for two different operational points. For an operational point near surge, the flow field in the entire compressor stage is unsteady, where the driving mechanism is an unsteadiness created in the volute. For an operational point far away from surge, the flow field in the compressor is relatively much more steady as compared with the former case. Although the stable operational point exhibits back-flow from the ported shroud channels, which implies that the flow into the compressor wheel is disturbed due to the structures that are created in the shear layer between the bulk flow and the back-flow from the ported shroud channels. / QC20100622

This thesis presents a study of the wind load forces and their influence on the response of structures. The study is based on the capacity of the Proper Orthogonal Decomposition method (POD) to identify and extract organized patterns that are hidden or embedded inside a complex field. Technically this complex field is defined as a multi-variate random process, which in wind engineering is represented by unsteady pressure signals recorded on multiple points of the surface of a structure. The POD method thus transforms the multi-variate random pressure field into a sequence of load shapes that are uncorrelated with each other. The effect of each uncorrelated load shape on the structural response is relatively easy to evaluate and the individual contributions can be added linearly afterwards. Additionally, since each uncorrelated load shape is associated with a percentage of the total energy involved in the loading process, it is possible to neglect those load shapes with low energy content. Furthermore, the load shapes obtained with the POD often reveal physical flow structures, like vortex shedding, oscillations of shear layers, etc. This later property can be used in conjunction with classical results in fluid mechanics to theorize about the physical nature of different flow mechanics and their interactions. The POD method is well suited to be used in conjunction with the classical modal analysis, not only to calculate the structural response for a given pressure field but to observe the details of the wind-structure interaction. A detailed and complete application is presented here but the methodology is very general since it can be applied to any recorded pressure field and for any type of structure.

This thesis presents a study of the wind load forces and their influence on the response of structures. The study is based on the capacity of the Proper Orthogonal Decomposition method (POD) to identify and extract organized patterns that are hidden or embedded inside a complex field. Technically this complex field is defined as a multi-variate random process, which in wind engineering is represented by unsteady pressure signals recorded on multiple points of the surface of a structure. The POD method thus transforms the multi-variate random pressure field into a sequence of load shapes that are uncorrelated with each other. The effect of each uncorrelated load shape on the structural response is relatively easy to evaluate and the individual contributions can be added linearly afterwards. Additionally, since each uncorrelated load shape is associated with a percentage of the total energy involved in the loading process, it is possible to neglect those load shapes with low energy content. Furthermore, the load shapes obtained with the POD often reveal physical flow structures, like vortex shedding, oscillations of shear layers, etc. This later property can be used in conjunction with classical results in fluid mechanics to theorize about the physical nature of different flow mechanics and their interactions. The POD method is well suited to be used in conjunction with the classical modal analysis, not only to calculate the structural response for a given pressure field but to observe the details of the wind-structure interaction. A detailed and complete application is presented here but the methodology is very general since it can be applied to any recorded pressure field and for any type of structure.

This research presents a detailed study of non-synchronous vibration (NSV) and the development of an efficient design method for NSV. NSV occurs as a result of the complex interaction of an aerodynamic instability with blade vibrations. Two NSV design methods are considered and applied to three test cases: 2-D circular cylinder, 2-D airfoil cascade tip section of a modern compressor, and 3-D high pressure compressor cascade that encountered NSV in rig testing. The current industry analysis method is to search directly for the frequency of the instability using CFD analysis and then compare it with a fundamental blade mode frequency computed from a structural analysis code. The main disadvantage of this method is that the blades' motion is not considered and therefore, the maximum response is assumed to be when the blade natural frequency and fluid frequency are coincident. An alternate approach, the enforced motion method, is also presented. In this case, enforced blade motion is used to promote lock-in of the blade frequency to the fluid natural frequency at a specified critical amplitude for a range of interblade phase angles (IBPAs). For the IBPAs that are locked-on, the unsteady modal forces are determined. This mode is acceptable if the equivalent damping is greater than zero for all IBPAs. A method for blade re-design is also proposed to determine the maximum blade response by finding the limit cycle oscillation (LCO) amplitude. It is assumed that outside of the lock-in region is an off-resonant, low amplitude condition. A significant result of this research is that for all cases studied herein, the maximum blade response is not at the natural fluid frequency as is assumed by the direct frequency search approach. This has significant implications for NSV design analysis because it demonstrates the requirement to include blade motion. Hence, an enforced motion design method is recommended for industry and the current approach is of little value. / Dissertation

Wave Number Selection and Defect Dynamics in Patterns with Hexagonal Symmetry Denis B. Semwogerere 108 Pages Directed by Dr. Michael F. Schatz We report quantitative measurements of wave number selection, secondary instability and defect dynamics in hexagonal patterns. A novel optical technique ("thermal laser writing") is used to imprint initial patterns with selected characteristics in a B뮡rd-Marangoni convection experiment. Initial patterns of ideal hexagons are imposed to determine the band of stable-pattern wave numbers. For small values of control parameter epsilon the measured stable band is found to agree quantitatively with theoretical predictions at the low-wave-number side of the band, and qualitatively at the high-wave-number side. Long-wavelength perturbations of ideal hexagonal patterns suggested by theory are imposed for epsilon=0.46 and their growth rates are measured to investigate the mechanisms of secondary instability. Our results suggest a transverse-phase instability limits stable hexagons at low wave number while a longitudinal-phase instability limits high-wave-number hexagons. Initial patterns containing an isolated penta-hepta defect are imprinted to study defect propagation directions and velocities. The experimental results agree well with theoretical predictions. The experimental investigations are discussed in the context of patterns with hexagonal symmetry formed under nonequilibrium external driving conditions.

This work introduces the method of Distributed Volumetric Sources (DVS) to solve the transient and pseudosteady-state flow of fluids in a rectilinear reservoir with closed boundaries. The development and validation of the DVS solution for simple well/fracture configurations and its extension to predict the pressure and productivity behavior of complex well/fracture systems are the primary objectives of this research. In its simplest form, the DVS method is based on the calculation of the response for a closed rectilinear system to an instantaneous change in a rectilinear, uniform volumetric source inside the reservoir. Integration of this response over the time provides us with the solution to a continuous change (constantrate pressure response). Using the traditional material balance equations and the DVS pressure response of the system, we can calculate the productivity index of the system in both transient and pseudosteadystate flow periods, which enables us to predict the production behavior over the life of the well/reservoir. Solutions for more complex situations, such as sources with infinite or finite-conductivity (i.e., a fracture), are provided using discretization of the source. This work considers the case of a complex system with a horizontal well intersecting multiple transverse fractures as an example to show the ability (and flexibility) of the new method. The DVS solution method provides accurate solutions for complex well/fracture configurations — which will help engineers to design and implement optimum well completions. The DVS solutions has been validated by comparing to existing analytical solutions (where applicable), as well as to numerical (simulation) solutions. In all cases the DVS solution was successfully validated — at least in a practical sense — specifically in terms of the accuracy and precision of the DVS solution. As the DVS method is approximate (at early times), there are small discrepancies which are of little or no practical consequence. In terms of computation times, because of its analytic nature, the DVS method is not always optimal in terms of speed for certain problems, but the DVS approach is similar in computation speed with commercial reservoir simulation programs.

In this thesis, the seminal work of Stokes concerning the ow of a Navier-Stokesuid due to a suddenly accelerated or oscillating plate and the ow due to torsionaloscillations of an innitely long rod in a Navier-Stokes uid is extended to a uid withpressure dependent viscosity. The viscosity of many uids varies signicantly withpressure, a fact recognized by Stokes; and Barus, in fact, conducted experiments thatshowed that the variation of the viscosity with pressure was exponential. Given sucha tremendous variation, we study how this change in viscosity aects the nature of thesolution to these problems. We nd that the velocity eld, and hence the structureof the vorticity and the shear stress at the walls for uids with pressure dependentviscosity, are markedly dierent from those for the classical Navier-Stokes uid.

This work presents a method for building a three-dimensional mesh from two- dimensional topologically identical layers, for use in aeroelastic simulations. The method allows modeling of large deformations of the wing in both the span direction and deformations in the cord of the wing. In addition, the method allows for the modeling of wings attached to fuselages. The mesh created is a hybrid mesh, which allows cell clustering in the viscous region. The generated mesh is high quality and allows capturing of nonlinear uid structure interactions in the form of limit cycle oscillation.

The aerodynamics of two-dimensional and three-dimensional flapping motion in hover is analyzed in incompressible, laminar flow at low Reynolds number regime. The aim of this study is to understand the physics and the underlying mechanisms of the flapping motion using both numerical tools (Direct Numerical Simulation) and experimental tools (Particle Image Velocimetry PIV technique). Numerical analyses cover both two-dimensional and three-dimensional configurations for different parameters using two different flow solvers. The obtained results are then analyzed in terms of aerodynamic force coefficients and vortex dynamics. Both symmetric and cambered airfoil sections are investigated at different starting angle of attacks. Both numerical and experimental simulations are carried out at Reynolds number 1000. The experimental analysis is carried out using Particle Image Velocimetry (PIV) technique in parallel with the numerical tools. Experimental measurements are taken for both two-dimensional and three-dimensional wing configurations using stereoscopic PIV technique.

ABSTRACT EXPERIMENTAL AND NUMERICAL INVESTIGATION OF FLOW FIELD AROUND FLAPPI G AIRFOILS MAKING FIGURE-OF-EIGHT IN HOVER BASKAN, &Ouml / zge M.Sc., Department of Aerospace Engineering Supervisor: Prof. Dr. H. Nafiz Alemdaroglu September 2009, 94 pages The aim of this study is to investigate the flow field around a flapping airfoil making figure-of-eight motion in hover and to compare these results with those of linear flapping motion. Aerodynamic characteristics of these two-dimensional flapping motions are analyzed in incompressible, laminar flow at very low Reynolds numbers regime using both the numerical (Computational Fluid Dynamics, CFD) and the experimental (Particle Image Velocimetry, PIV) tools. Numerical analyses are performed to investigate the effect of different parameters such as the amplitude of motion in y-direction, angle of attack, Reynolds number and camber on the aerodynamic force coefficients and vortex formation mechanisms. Both symmetric and cambered airfoil sections are investigated at three different starting angles of attack for five different amplitudes of motion in y-direction including linear flapping motion. Experimental simulations are performed in order to verify the numerical results only for linear motion at Reynolds number of 1000 for symmetric and cambered airfoils at three different angles of attack. Computed vortical structures are then compared to vorticity contours obtained from the experiments and advantages of figure-of&ndash / eight motion over linear motion are discussed.