Nonlinear Control and Modeling of Rotating Stall in an Axial Flow Compressor

This thesis focuses on understanding the use of air injection as a means of controlling rotating stall in an axial flow compressor, involving modeling, dynamical systems analysis, and experimental investigations. The first step towards this understanding was the development of a low order model for air injection control, the starting point of which was the Moore and Greitzer model for axial flow compressors. The Moore and Greitzer model was extended to include the effects of air injection and bifurcation analysis was performed to determine how the closed loop system dynamics are different from those of the open loop system. This low order model was then used to determine the optimal placement of the air injection actuators. Experimental work focused on verifying that the low order model, developed for air injection actuation, qualitatively captured the behavior of the Caltech compressor rig. Open loop tests were performed to determine how the placement of the air injectors on the rig affected the performance of the compressor. The positioning of the air injectors that provided the greatest control authority were used in the development of air injection controllers for rotating stall. The controllers resulted in complete elimination of the hysteresis associated with rotating stall. The use of a throttle actuator for the control of the surge dynamics was investigated, and then combined with an air injection controller for rotating stall; the resulting controller performed quite well in throttle disturbance rejection tests. A higher order model was developed to qualitatively match the experimental results with a simulation. The results of this modeling effort compared quite well with the experimental results for the open loop behavior of the Caltech rig. The details of how the air injection actuators affect the compressor flow were included in this model, and the simulation predicted the same optimal controller that was developed through experimentation. The development of the higher order model also included the investigation of systematic methods for determining the simulation parameters. Based on experimental measurements of compression system transients, the open loop simulation parameters were identified, including values for the compressor performance characteristic in regions where direct measurements were not possible. These methods also provided information on parameters used in the modeling of the pressure rise delivered by the compressor under unsteady flow conditions

Vibration control of rotating piezo-composite blade beam with CUS configuration based on optimal LQG controller

Vibration control of rotating composite blade beam with piezoelectric patch embedded is investigated. Stall flutter of piezo-composite blade driven by nonlinear aerodynamic forces is analyzed based on anisotropic circumferentially uniform stiffness (CUS) configuration. The blade is modeled as single-cell thin-walled beam structure, exhibiting the couplings among three displacements of vertical bending, lateral bending and transverse shear deformation, with structural tailoring implemented. The transversely piezoelectric actuating element is embedded in a manner such that its surface is parallel to the mid-surface of the blade beam. Piezoelectric damping ratios of rotating piezo-composite blade are described, with influences of different ply angles and rotating speeds illustrated. The flutter suppression for stall aeroelastic behavior based on an optimal LQG controller (OLC) with a dynamic regulator is highlighted with obvious effects demonstrated. In contrast with conventional LQG controller, the superiority of OLC controller is apparently demonstrated by time response and piezoelectric feedback voltage. Analytical proof of the structural modeling and feasibility analysis of the physical realization of the OLC algorithm are also investigated by comparisons of different modeling theories, and demonstrated by experimental platform

Review of rotating wing dynamic stall: Experiments and flow control

Dynamic stall has been a technical challenge and a fluid dynamical subject of interest for more than fifty years; but in the last decade significant advances have been made in the understanding, prediction, modeling, and control of dynamic stall on rotors. This paper provides a summary of the state of the art of dynamic stall experiments and future directions in the understanding of dynamic stall on rotors. Experimental data sets are discussed, as well the direction of future research for control of dynamic stall. Coordinated testing between airfoils and rotating blades, as well as close integration between computational and experimental studies were found to be productive approaches. Advanced analysis methods, including statistical methods, modal representations, and artificial intelligence methods have led to significant advances in the understanding of dynamic stall. Investigations of dynamic stall control devices have allowed many useful targeted investigations of the transition to separated flow, but have not yet resulted in a commercially implemented device

Model reduction, centering, and the Karhunen-Loeve expansion

We propose a new computationally efficient modeling method that captures a given translation symmetry in a system. To obtain a low order approximate system of ODEs, prior to performing a Karhunen Loeve expansion, we process the available data set using a “centering” procedure. This approach has been shown to be efficient in nonlinear scalar wave equations

A survey of instabilities within centrifugal pumps and concepts for improving the flow range of pumps in rocket engines

Design features and concepts that have primary influence on the stable operating flow range of propellant-feed centrifugal turbopumps in a rocket engine are discussed. One of the throttling limitations of a pump-fed rocket engine is the stable operating range of the pump. Several varieties of pump hydraulic instabilities are mentioned. Some pump design criteria are summarized and a qualitative correlation of key parameters to pump stall and surge are referenced. Some of the design criteria were taken from the literature on high pressure ratio centrifugal compressors. Therefore, these have yet to be validated for extending the stable operating flow range of high-head pumps. Casing treatment devices, dynamic fluid-damping plenums, backflow-stabilizing vanes and flow-reinjection techniques are summarized. A planned program was undertaken at LeRC to validate these concepts. Technologies developed by this program will be available for the design of turbopumps for advanced space rocket engines for use by NASA in future space missions where throttling is essential

Applying rotorcraft modelling technology to renewable energy research

The perceived need to reduce mankind's impact on the global climate motivates towards a future society in which a significant proportion of its energy needs will be extracted from the winds and the tides of the planet. This paper shows several examples of the application of Brown's Vorticity Transport Model, originally developed to perform simulations of helicopter aeromechanics and wake dynamics, to the analysis of the performance of renewable energy devices and their possible impact on the environment. Prediction of the loading on wind turbines introduces significant additional challenges to such a model, including the need to account fully for the effects of radial flow on blade stall. The wake-mediated aerodynamic interactions that occur within a wind farm can reduce its power output significantly, but this problem is very similar to that where the aerodynamic unsteadiness of the coupled wake of the main and tail rotors of a helicopter can result in significantly increased pilot workload. The helicopter-related problem of brownout, encountered during operations in desert conditions, has its analogue in the entrainment of sediment into the wakes of tidal turbines. In both cases it may be possible to ameliorate the influence of the rotor on its environment by careful and well-informed design. Finally, calculations of the distortion and dispersal of the exhaust plumes of a helicopter by the wake of its rotor allow insight into how wind turbines might interfere with the dispersal of pollutants from nearby industrial sites. These examples show how cross-disciplinary information transfer between the rotorcraft field and the renewable energy community is helping to develop the technologies that will be required by our future society, as well as helping to understand the environmental issues that might need to be faced as these technologies become more prevalent

Analytical modeling of rotating stall and surge

The life and performance of axial compressors are limited by the occurrence of instabilities such as rotating stall and surge. Indeed, in the course of the design phase a great effort is usually devoted to guarantee an adequate safety margin from the region of instabilities’ onset. On the other hand, during its operating life, an axial compressor can be subjected to several conditions that can lead to the inception of stall and its dynamics. A few examples of possible stall causes, for the specific case of an axial compressor embedded in an aircraft engine, are inlet flow distortion, engine wear or shaft failure. The shaft failure case can be seen as an exception, as a matter of fact, after this event surge is a desirable outcome since it can potentially decelerate the over-speeding turbine by reducing the mass flow passing through the engine. The possible occurrence of surge and stall should be predicted and controlled in order to avoid severe damage to the compressor and its surroundings. A lot of research has been carried out in the past years to understand the inception and development of stall to achieve the capability for predicting and controlling this severe phenomenon. Nonetheless, this problem is still not well understood and unpredictable outcomes are still a great concern for many axial compressor’s applications. The lack of knowledge in what concerns inception and development of stall and surge reflects in a lack of tools to investigate, predict and control these unstable phenomena. The tools available to study stall and surge events are still not highly reliable or they are very time consuming as 3D CFD simulations. The doctoral research described herein, aimed at the investigation of the rotating stall phenomenon and the derivation of the compressor characteristic during this unstable condition. Following a detailed analysis of the tools and techniques available in the public domain and the identification of their limitations, the development of a FORTRAN through-flow tool was the methodology chosen. A distinctive feature of the developed tool is the independency from steady state characteristics which is a limitation for the majority of the available tools and its computational efficiency. Particular attention was paid to capture various viscous flow features occuring during rotating stall through the selection and implementation of appropriate semiempirical models and correlations. Different models for pressure loss, stall inceptions and stall cell growth/ speed were implemented and verified along with different triggering techniques to achieve a very close to reality simulation of the overall phenomenon, from stall inception to full development. lel compressors’ technique that allows the correct modeling of asymmetric phenomena. The methodology implemented has proved promising since several simulations were run to test the tool adopting different compressor geometries. Verifications were performed in terms of overall compressor performance, with simulations in all the three possible operating regions (forward, stall and reverse flow), in order to verify the tool’s capability in predicting the compressor characteristics. In terms of flow field, the ability to capture the right circumferential trends of the flow properties was checked through a comparison against 3D CFD simulations. The results obtained have demonstrated the ability of the tool to capture the real behavior of the flow across a compressor subjected to several different unstable conditions that can lead to the onset of phenomena such as rotating stall, classic and deep surge. Indeed, the tool has shown ability to tackle steady and transient phenomena characterized by asymmetric and axis-symmetric flow fields. This document provides several examples of investigations emphasizing the flexibility of the developed methodology. As a matter of fact, within this dissertation, many examples can be found on the effect of the plenum size, on the different transient phenomena experienced by the compressor when subjected to multiple regions of inlet distortion instead of a localized region of low or high flow, on the differences between temporary and stationary inlet disturbances and so on. This document describes in detail the methodology, the implementation of the tool, its verification and possible applications and the recommended future work. The work was funded by Rolls-Royce plc and was carried out within the Rolls-Royce UTC in Performance Engineering at Cranfield as three-year Ph.D. program that started in October 2010

Rotorcraft aeroelastic stability

Theoretical and experimental developments in the aeroelastic and aeromechanical stability of helicopters and tilt-rotor aircraft are addressed. Included are the underlying nonlinear structural mechanics of slender rotating beams, necessary for accurate modeling of elastic cantilever rotor blades, and the development of dynamic inflow, an unsteady aerodynamic theory for low-frequency aeroelastic stability applications. Analytical treatment of isolated rotor stability in hover and forward flight, coupled rotor-fuselage stability in hover and forward flight, and analysis of tilt-rotor dynamic stability are considered. Results of parametric investigations of system behavior are presented, and correlation between theoretical results and experimental data from small and large scale wind tunnel and flight testing are discussed

Survey of Army/NASA rotorcraft aeroelastic stability research

Theoretical and experimental developments in the aeroelastic and aeromechanical stability of helicopters and tilt-rotor aircraft are addressed. Included are the underlying nonlinear structural mechanics of slender rotating beams, necessary for accurate modeling of elastic cantilever rotor blades, and the development of dynamic inflow, an unsteady aerodynamic theory for low frequency aeroelastic stability applications. Analytical treatment of isolated rotor stability in hover and forward flight, coupled rotor-fuselage stability are considered. Results of parametric investigations of system behavior are presented, and correlations between theoretical results and experimental data from small- and large-scale wind tunnel and flight testing are discussed