Aero-Astro Magazine Highlight

Passion = vibrations and waves in turbomachinery

The world of air-breathing propulsion is full of challenging problems where well-defined modeling goals can help tackle real and complex situations.

Zoltan Spakovszky's research and passion center on challenges in energy, power, and propulsion. (William Litant photograph)

Since I was a child, the smell of unburned hydrocarbons from jet aircraft on the tarmac has sent a chill down my spine with the feeling that something exciting and powerful was going to happen.

After all these years, this passion has not faded. The first big step toward my childhood dreams of becoming an aeronautical engineer came true when finishing my Master’s degree in Mechanical Engineering at ETH Zurich and working at Swissair Technics in the aircraft engine overhaul and maintenance services. This was the first time I got involved in solving a severe flow instability problem jeopardizing the safe operation of jet engines in service. This also brought me to the United States. After receiving my Ph.D. from the MIT Aeronautics and Astronautics Department in 2000, I joined GE Aircraft Engines as a lead engineer in the preliminary design and performance department. A short year later, I returned to MIT, and today, I still find myself attracted to research involving jet engines and turbomachinery. Undeniably, the field has evolved, but there will always be challenging problems in the energy, power, and propulsion fields that remain to be solved. G. F. Carrier, applied mathematician and National Medal of Science recipient, said of the future of engineering science, “There is still a multitude of worthwhile, unanswered questions in the world of natural sciences whose answers will ultimately be obtained with the help of mathematical tools and mathematical reasoning. Many of these will require a variety of heuristic arguments, the ‘find it before you prove it exists’ attitude, and the determination to understand the real phenomenon.”

Computed acoustic resonance that can lead to engine surge (left) in agreement with acoustic pressure signals measured on wing in large commercial jet engines (right). -enlarge-

Characteristics of my research projects are that they are rich in concepts and ideas, entail first principles modeling for new insight of complex technological problems, focus on challenging problems important to society and supported by industry and government agencies, and involve team work and industry contacts.
Gas turbine jet engines can encounter a large variety of unsteady turbomachinery flow problems. Although unsteadiness in fluid motion is necessary for the work exchange in such machines (if the Beach Boys were aerodynamicists, they’d call these oscillations “the good vibrations”), the unsteady nature of fluid flow can be a real nuisance and can be manifested in detrimental vibrations and waves. These phenomena can limit the operating range (aerodynamic oscillations, and instabilities), induce structural failure and malfunction (mechanical vibrations), and generate noise (acoustics of fluid flow and structure interaction). The overarching theme that ties these diverse areas together is dynamic behavior and unsteadiness in fluids, and in fluid-structure interaction.

Following is a selection of turbomachinery problems where vibrations and waves are essential and where a dynamical system point of view proved successful in the modeling, analysis and the solution of the issues at hand.

Unsteady compressor aerodynamics and stability

Gas turbine engine performance is limited by flow instabilities known as surge and rotating stall, the mature forms of the natural oscillations of the fluid flow in the compression system. Surge is basically a circumferentially uniform pulsation of mass flow through the machine, while rotating stall appears as a reduced flow region in part of the circumference, which travels around the compressor annulus at a fraction of rotor speed. In-flight aero engine shut downs due to compressor instability happen on average in one out of 300,000 aircraft operations.

Together with Pratt & Whitney, I investigated and explained the mechanisms of observed unsteady flow phenomena in large civil aircraft engines. The work focused on the rigorous modeling of compressor stability loss due to engine deterioration and wear. Our analysis of stability during engine accelerations confirmed the mechanism of small-scale acoustic resonances seen in more than 360 aircraft engines in service. The results were useful for the development and improvement of an engine diagnostic and health monitoring test currently employed by airlines for fleet management purposes and now required by an FAA airworthiness directive.

Aerodynamically induced turbomachinery rotor whirl

Non-uniform engine tip-clearance distributions, due, for example, to a compressor shaft offset from its casing centerline or whirling in its bearing journal, can induce destabilizing rotordynamic forces. These forces stem from the strong influence of the blade tip-clearance on the local performance of the compressor and can lead to destructive rotordynamic instabilities, potentially increasing engine fuel consumption by as much as 1 percent (for a major airline, this could amount to about $45 million of loss per year, given that fuel cost is about one-quarter of direct operating cost). Despite a large number of investigations of this topic over the past 40 years there has been disparity in the findings on the magnitude and direction of the whirl inducing force. To resolve this issue, in collaboration with GE Aircraft Engines experimenters I developed a theoretical model to explain observed rotordynamic-aerodynamic interaction mechanisms. The analysis showed that the direction of the whirl tendency is governed by the phase angle between the tip-clearance asymmetry and the blade loading distribution set by the flow turning and flow coefficient. Together with the new theory, the experiments resolved the long-standing whirl instability issues in compressors and turbines, and established a firm scientific basis for the observed phenomena. The findings are important for the development of next generation high-bypass ratio engines where lightweight structures and advanced engine architectures introduce new rotordynamic challenges.

Quiet aircraft turbomachinery air-brake concept

Airports in key locations are operating at full capacity and the noise in the vicinity of airports is so intrusive that local communities object to any expansion. One example is a partially-constructed runway at Boston’s Logan Airport where construction was stopped by a court order and remains unfinished after 30 years. Aircraft on approach in high-drag and high-lift configuration create unsteady flow structures, which inherently generate noise. For devices such as flaps, spoilers, and the undercarriage, there is a strong correlation between overall noise and drag. In the quest for quieter aircraft, one challenge is to generate drag at low noise levels. We developed a novel drag concept, a so-called “swirl tube.” The idea is that a swirling exhaust flow (for example from an engine exhaust) can yield a streamwise vortex which is supported by a radial pressure gradient responsible for pressure drag and which will be quieter than conventional drag devices.

In collaboration with NASA Langley, we carried out detailed quantitative acoustic measurements, using a directional microphone array and a previously developed, state-of-the-art post-processing technique, to map acoustic sources. A scale model jet engine nacelle with stationary swirl vanes was designed and tested in the NASA Langley Quiet Flow Facility at full-scale approach Mach numbers. The analysis showed that the acoustic signature is comprised of quadrupole-type turbulent mixing noise from the swirling core flow scattering noise from vane boundary layers, and turbulent eddies of the burst vortex structure near sharp edges. The theory and the experiments, which were in agreement, demonstrated that low noise levels at high drag coefficients, comparable to that of bluff body drag, are, in fact, achievable.

The technological problems discussed above are different in nature and in disciplines but “vibrations and waves” form a common thread. In all three cases, the modeling focused on determining the dynamic behavior of the problem at hand. The approach is to simplify complicated problems to lay bare the underlying mechanisms.

The world of air-breathing propulsion is full of challenging problems where well-defined modeling goals can help tackle real and complex situations. As long as there are vibrations and waves at play, long shall live the eigenvalue — both with its real and imaginary parts.

Zoltan Spakovszky is the H. N. Slater Associate Professor of Aeronautics and Astronautics at the Massachusetts Institute of Technology and the director of the Gas Turbine Laboratory. He obtained his Dipl. Ing. degree in Mechanical Engineering from the Swiss Federal Institute of Technology (ETH) Zürich and his M.S. and Ph.D. degrees in Aeronautics and Astronautics from MIT. His principal fields of interest include internal flows in turbomachinery, compressor aerodynamics and stability, dynamic system modeling of aircraft gas turbine engines, micro-scale gas bearing dynamics, and aero-acoustics. Spakovszky may be reached at zolti@mit.edu.