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The phenomena of stall and surge in axial-centrifugal compressors is investigated through high-response measurements of both the pressure field and the flowfield throughout the surge cycle. A unique high-response forward-facing and aft-facing probe provides flow information. Several axial-centrifugal compressors are examined, both in compressor rigs and engines. Extensive discussion is presented on the differences in axial and centrifugal rotors and their effect on the system response characteristics. The loading parameters of both are examined and data is presented that shows the increased tolerance of the centrifugal stage to instability. The dynamics of the compressor blade response are shown to be related to the transport time of a fluid particle moving through a blade passage. The data presented provides new insight into the dynamic interactions that occur prior to and during stall and surge. In addition, the inception of rotating stall and the inception of surge are shown to be the same phenomena . An analytical dynamic model (DYNTECC) is applied to one of the compression systems and the results are compared to data. The results show that the model can capture the global effects of rotating stall and surge. The data presented, along with the analytical results, provide useful information for the design of active and passive stall control systems. / Ph. D.

Forces and moments have been measured on slender bodies in both static angle conditions as well as rapid time-dependent large amplitude maneuvers with the Dynamic Pitch Plunge Roll (DyPPiR) apparatus. <br /> <br /> Lateral and transversal forces as well as all three moments have been measured at static angles of attack and sideslip and unsteady pitch ramp maneuvers at a fixed point of rotation at the quarter length of the body. The two bodies are the DARPA Suboff generic submarine shape and a non-Body-of-Revolution scalene ellipsoid with a constant cross-section midbody. An analytical two-mode equation has been shown to accurately describe the normal force and pitch moment as well as side force and yaw moment for the ellipsoid body. It is based on the observation that the center of pressure for the cross-flow contribution is at a fixed location. For the Suboff body, this assumption is invalid. Unsteady forces and moments can be measured to a very small magnitude of uncertainty and were found to differ from steady forces and moments at the time-instantaneous flow angle during the motion. / Ph. D.

Fundamental intuition of aerodynamics begins with understanding steady flow, a time- independent flow state. A fluid region undergoing steady flow consists of constant properties such as pressure and velocity at different positions in the flow field. This time-independent principle is crucial for beginning a foundation of understanding aerodynamics; however, analyzing this state of flow was beyond the limit at my university's Fundamentals of Aerodynamics course. There was minimal education on time-dependent unsteady flow, which created a vacuum on my understanding of how flow can be analyzed with time. The purpose of writing this thesis is to create a framework for aspiring learners of aerodynamics to better comprehend unsteady flow, including myself. The basis for developing an understanding of unsteady flow is accomplished by analyzing the aerodynamics of a simple two-dimensional zero-thickness flat plate, using a numerical method called Discrete Vortex Method under steady and unsteady conditions. Constructing a numerical method for steady and unsteady flow requires a software to compute enormous quantities of linear equations, therefore a combination of numerous arguments, functions, and loops were developed on MATLAB written in the C/C++ languages. Results from the numerical methods will be compared with the experimental and theoretical results from Katz & Plotkin (2001). The Steady Discrete Vortex Method was a basis for calculating the circulation of the flat plate at varying angles of attack and freestream velocities. The Unsteady Discrete Vortex Method derived much of the self-induced calculations in the body-fixed coordinate system. At the same time, a time-stepping method was developed to calculate the coordinates as the flat plate and shed vortices translated from the origin of an additional frame of reference called the inertial coordinate system. A wake vortex is shed from the trailing-edge of the flat plate at each time step iv to model vorticity shed from a body in motion. The flat plate undergoes sudden acceleration and plunging maneuvers to demonstrate further effects of unsteady aerodynamic conditions. The results from the flat plate undergoing sudden acceleration with a Reynolds number of 68,435.8 was an increasing proportionality between the lift and circulation of the steady and unsteady case until reaching a constant trend as time increases, demonstrating the nature of low-speed flow reaching a steady state after a given period. The results from the flat plate undergoing plunging with a Reynolds number of 106,759.8 demonstrate a sinusoidal trend in the normal force experienced as the flat plate traverses in its sinusoidal plunging translation like that observed in the theoretical results. This thesis intends to expand on the understanding of unsteady aerodynamics by developing a numerical method that can alter its dependent factors to visualize the effects of changing specific parameters on pressure and force acting on the two-dimensional body.

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This study seeks to reduce the cost of numerically solving non-linear partial differential equations by reducing the number of computations without compromising accuracy. This was done by using accurate local time stepping. This algorithm uses local time stepping but compensates for the inconsistencies in the temporal dimension by interpolations and/or extrapolations. Reduction in computations are obtained by time-stepping only a particular region with small time steps. A shock tube problem and a detonation wave were the two test cases considered. The performance of the solution using this algorithm was compared with an algorithm that does not use accurate local time stepping.

The laminar flow in the small bronchial tubes is quite complex due to the presence of vortex-dominated, secondary flows. Contributing to this complexity are the geometrical characteristics of the bronchial tubes that include asymmetric and nonplanar branching. These secondary flow fields play a crucial role in particle deposition; however, the actual mechanisms that determine the particle distributions are not fully understood. The research reported here increases understanding of this phenomenon by studying flow structure/ particle interaction in the small bronchial tubes for steady and unsteady respiratory conditions. Specifically, the effects of simultaneous nonplanar and asymmetric branching were investigated. The nonplanar model was generated by applying a 90◦ out-of-plane rotation to the third-generation branches. Steady-state inspiratory flows for a Reynolds number of 1,000 and unsteady periodic flows with a 30-respiration-per-minute breathing frequency were simulated in three-generation, asymmetric, planar and nonplanar models. The asymmetry and nonplanarity produced asymmetric secondary flow patterns and unequal mass flow partitioning in the third-generation branches. Ten micron water droplet deposition in the nonplanar model was found to be significantly different from the planar model, demonstrating the impact of simultaneous nonplanar and asymmetric branching. The unsteady nature of the flow also affected particle deposition. Particles released at the same instantaneous inflow conditions during off-peak inhalation conditions, generated significantly different particle deposition patterns. The differences were attributed to the high temporal variations of the fluid velocities at these off-peak times and history effects in the flows. It was also observed that the initial particle velocities had a significant impact on particle deposition. The study of flow structure and particle interaction was facilitated by the development of a novel visualization technique that employs finite-time Lyapunov exponents (FTLE). This research provides a better understanding of the fluid dynamics driving the particle deposition in the bronchial tubes.

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No description available.