Current Research at the Gas Turbine Laboratory
Acoustic Shielding Prediction of a Hybrid Wing-Body Aircraft for NASA N+2 Goals
Leo Ng, Phil Weed
Advisor: Prof. Spakovszky
Reducing the environmental impact of air travel is a major focus in aeronautical research today. The hybrid wing-body aircraft, in which the lifting fuselage is blended with the wings, has the potential to burn less fuel, produce fewer emissions, and generate less noise. Building upon previous work from the Silent Aircraft Initiative, this project aims to develop a set of advanced predictive methods that will enable the design of a hybrid wing-body aircraft to meet NASA’s N+2 goals: (i) 25% less fuel burn, (ii) 80% less emissions, and (iii) 52 dB less noise compared to current aircrafts in service.
MIT, in collaboration with Boeing, NASA, and UC Irvine, is defining the aircraft configuration, assessing its noise impact, identifying noise reduction strategies, and developing the tools to evaluate these strategies. One key approach to lower noise is to mount the engines above the airframe, utilizing the large planform area to shield the propulsion noise. Unfortunately, current shielding prediction methods are either incompatible with a hybrid wing-body configuration or are too computationally expensive. Effort is underway to implement a fast medium-fidelity method that can capture the shielding characteristics of a hybrid wing-body aircraft. A wind tunnel experiment is planned for the final phase of this project to validate the shielding method and other analysis tools used throughout the aircraft development process.
Carbon Nanotube Bearing
Eugene Cook, Draper & MIT
Project lead at Draper: David J Carter (PI),
Marc Weinberg,
Peter Miraglia
Advisor: Zoltan Spakovszky
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Carbon nanotube rotor [not to scale] |
Rotating Micro Electro-Mechanical Systems (MEMS) require rotary bearings, but current MEMS bearing technologies have drawbacks. Silicon rubbing on silicon wears out quickly. Gas bearings require a gas source, and are relatively low stiffness. A promising alternative, proposed and being pursued by the Charles Stark Draper Laboratory in collaboration with the GTL, is to use Carbon Nanotubes (CNTs). Multi-walled CNTs have a concentric-tube structure that lends itself to bearings. Each tube is strong, but there is little or no bonding between tubes, allowing them to slide relative to each other. However, the friction characteristics of these bearings are not precisely quantified. This project’s goal is to construct a simple CNT bearing rotary device, demonstrating MEMS and CNT compatible fabrication techniques, and allowing some data on the friction characteristics to be gathered. Applications of such a bearing technology could include microscale turbomachinery, as well as gyroscopes, pumps, and other rotating devices.
Assessment of Inlet Distortion Noise in Highly Integrated Propulsion Systems
Jeff Defoe, Alex Narkaj
Advisor: Prof. Spakovszky
Among the Silent Aircraft Initiative SAX40’s many unique characteristics are engines which are highly integrated into the airframe. S-shaped inlet ducts deliver air to the engines, but these ducts ingest the boundary layer over the wing and thus the flow through the duct is highly non-uniform. Predicting noise propagation through non-uniform flow fields is a complex task, as most classical acoustics formulations deal with uniform flow fields.
This research project deals with the numerical simulation of the noise source caused by the engine fan in the duct, in particular multiple-pure-tone (MPT) noise caused by blade-to-blade variations, and with its propagation upstream through the duct and to far-field receivers. The acoustic waves involved are nonlinear and the propagation is through a highly non-uniform flow field.
Predictions of noise propagation for these types of conditions are challenging due to the inherent coupling between the aerodynamics and acoustics in the problem. Therefore a simultaneous simulation of the noise generation and propagation is required. The calculation is rendered tractable via the use of a body force representation of the fan in an Euler calculation. The body forces impart the same pressure rise and turning on the flow as the real fan. The force field responds to the local flow conditions and therefore can be used for the nonuniform flows generated by the boundary layer ingestion and flow through the S-duct. The body force field is perturbed with a rotating disturbance representative of the variations in blade force caused by the blade-to-blade variations which result in MPT noise. The perturbed body force field generates unsteady pressure variations which propagate out of the duct to the far field. Integral acoustic methods are used to obtain the sound pressure levels (SPL) at the far-field receivers. The intention is to examine the noise propagation characteristics of several duct designs with the ultimate goal of developing a semi-empirical model for the noise propagation characteristics of S-shaped ducts as a function of major flow and geometric parameters.
Single Crystal Silicon as a Macro-World Structural Material: Design of Compact,
Lightweight High Pressure Vessels
Tanya Cruz Garza
Advisor, Prof. Epstein
This research explores the use of micromachined single crystal silicon as a macro-world structural material, understanding its advantages and limitations. Single crystal silicon is a material with theoretical strengths higher than steel and with a lower density than aluminum. This high strength, light weight nature of silicon make it an ideal structural material. Silicon has shown favorable performance as a structural material in micro-scale applications but the brittle nature of this material makes it difficult to reliably achieve high usable strengths on a macro-scale. This research explores how microfabrication techniques affect silicon strength and how high strength macro-scale silicon structures can be made.
This research has an engineering objective which is to find better ways of building compact, lightweight high pressure vessels for demanding applications such as spacecraft. The advantages of a silicon based pressure vessel is the potential to integrate the vessel regulator and control circuit on chip, the large inherent strength of silicon, the low density and thus lightweight characteristic of silicon and the ability to machine silicon into unique, compact geometries. This work is part of a joint effort of MIT and Ventions, LLC, to design a silicon-based microlaunch vehicle system for small satellite applications. This launch system will be complete with chamber and nozzle, valve, regulator, power supply and tanks. The motivation of this endeavor is to expand the definition of low cost access to space with a cost per mission versus cost per payload pound.
Assessment of Propfan Propulsion Systems for Reduced Environmental Impact
Andreas Peters
Advisor: Prof. Spakovszky
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Open counter-rotating propfan engine [from Flug Revue, July 2007] |
Current aircraft engine design studies tend towards higher bypass ratio, low-speed fan configurations in order to attain reductions in fuel consumption, emissions and noise. Propfan (advanced turboprop) engine concepts investigated in the past by American, European and Russian aircraft manufacturers have demonstrated significant benefits in these areas. However, considerable concern remains about the potential noise generated by propfan engines, including both inflight cabin noise and community noise during takeoff and approach. In addition, performance limitations at high flight Mach number as well as engine configuration and airframe integration present challenges to be addressed in the course of assessing a propfan-powered next generation commercial aircraft.
Taking into account the stringent noise certification requirements, the level of potential propfan overall noise reduction using advanced technologies in conjunction with a design strategy for both low noise and reduced fuel burn / emissions is investigated within the scope of this research effort. The goal of the project is to define the optimum propfan engine configuration and airframe integration concept with respect to improved performance and reduced noise and fuel burn.
A Methodology for Centrifugal Compressor Stability Prediction
Björn Benneke, Jonathan Everitt
Advisor: Prof. Spakovszky
Although centrifugal compressors exhibit the same type of instabilities as axial compressors, rotating stall and surge are characterized by a much broader spectrum of unstable behavior. The wide variety of instability behavior, along with the inherently complicated flow in such a machine, are primary reasons that rotating stall and surge in centrifugal compressors are less well understood than similar phenomena in axial compressors. As a consequence, a general theory or a criterion for the onset of instability in centrifugal compressors does not exist.
Instead, correlations are used to describe the surge point for a certain class of centrifugal compressors and to estimate the stability limit based on a priori knowledge of blade row characteristics. The major limitation of these methods is that these characteristics are only available after experimental measurements and thus the method is not of predictive nature. This research project is different from past efforts in that the prediction is purely based on centrifugal compressor geometry and does not rely on correlations or a priori knowledge of compressor characteristics.
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Experimental evidence of spike and modal stall inception in a high pressure ratio centrifugal compressor of advanced design - diffuser static pressure rise characteristic with and without air bleed) |
The approach is two-pronged. Previous research indicates that for certain classes of centrifugal compressors the inception of instability is in the diffuser; however the underlying fluid mechanics is not well understood. To gain insight, unsteady 3-D RANS calculations are being carried out on the isolated diffuser using an inlet flow field derived from full stage calculations. The inlet conditions are perturbed with a short wavelength (spike shaped) total pressure disturbance. If the disturbance grows in time, the machine is deemed dynamically unstable, and the operating point is determined to be the surge point. The inlet conditions can be varied to determine what flow features need to be present in order to trigger instability.
The second prong to the approach borrows ideas from previous work on axial compressors and consists of 3-D steady RANS calculations to determine the body force distributions representing the effects of discrete blades on the flow field. The body forces are then coupled to a 3-D unsteady Euler solver. The compressor model is then forced with either a short wavelength radial body force impulse in the vaneless space or a long wavelength modal wave-like body force impulse at the same location. The goal is to demonstrate that the method can accurately predict both the stall point and the type of stall inception pattern (spike or modal waves) in centrifugal compressors.
Small-scale Gas Turbine Engines
Shinji Tanaka, Daanish Maqbool
Advisor: Prof. Zoltan Spakovszky
Small scale gas turbine engines can provide much higher power densities than conventional batteries, and show promise as a portable and enduring power source. Such a small-scale turboshaft engine is being developed for this purpose, but some challenges remain before it can be considered practical for ground-based, portable use. The first challenge is noise, which must be kept within comfortable limits. An understanding and assessment of the noise sources is essential for devising noise reduction strategies. However, the dominant phenomena might be different from those in aircraft engines because of the unique operating condition, which makes the problem more challenging. Secondly, the engine generates heat which must be effectively removed in order to keep the surface temperatures within safe limits. Therefore, another goal is to devise an efficient cooling scheme, which will allow safe hand-held operation.
The "Swirl Tube" - an Aircraft Drag Management Device
to Reduce Noise and Fuel Burn
Hiten Mulchandani (former students: Darius Mobed, Parthiv Shah)
Advisor: Prof. Zoltan Spakovszky
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 such that, in the quest for quieter aircraft, one challenge is to generate drag at low noise levels.
The invention is a novel aircraft drag management concept to reduce aircraft noise during approach and to improve fuel burn in cruise. The idea is based on a swirling exhaust flow emanating, for example, from a jet engine nacelle (see figure) or a wing-tip mounted duct. A novel application is to exploit the low pressure in the vortex core of the swirling exhaust flow to generate drag. The idea is that in a steady streamwise vortex the centripetal acceleration of fluid particles is balanced by a radial pressure gradient. The very low pressure near the vortex core at the exit of the duct generates pressure drag. This streamwise vortex is in essence steady, yielding low noise levels and a quiet acoustic signature. To see a Quicktime movie of the swirl tube in action, click here (this is a large file so please be patient while it loads).
