James I. Hileman

Large-scale structure evolution and sound emission in high-speed jets : real-time visualization with simultaneous acoustic measurements

This investigation presents a unique and elaborate set of experiments relating the generation of noise to the evolution of large-scale turbulence structures within an ideally expanded, Mach 1.28, high-Reynolds-number

Environmental Performance of Algal Biofuel Technology Options

(1.03\,{\times}\,10^{6})

Airframe Design for "Silent Aircraft"

jet. The results appear to indicate many similarities between the noise generation processes of high-speed low-Reynolds-number and high-speed high-Reynolds-number jets. Similar to the rapid changes observed in theregion of noise generation in low-Reynolds-number jets in previous experimental and computational work, a series of robust flow features formed approximately one convective time scale before noise emission and then rapidly disintegrated shortly before the estimated moment of noise emission. Coincident with the disintegration, a positive image intensity fluctuation formed at the jet centreline in a region that is immediately past the endof the potential core. This indicates mixed fluid had reached the jet core. These results are consistent with the formation of large-scale structures within the shear layer, which entrain ambient air into the jet, and their eventual interaction and disintegration apparently result in noise generation. These results are quite different from the evolution of the jet during prolonged periods that lacked significant sound emission. The observations presented in this work were made through the use of well-established technique that were brought together in an unconventional fashion. The sources of large-amplitude sound waves were estimated in time and three-dimensional space using a novel microphone array/beamforming algorithm while the noise-generation region of the mixing layer was simultaneously visualized on two orthogonal planes (one of which was temporally resolved). The flow images were conditionally sampled based on whether or not a sound wave wascreated within the region of the flow while it was being imaged and a series of images was compiled that was roughly phase-locked onto the moment of sound emission. Another set of images was gathered based on a lack of sound waves reaching the microphone array over several convective time scales. Proper orthogonal decomposition (POD) was then used tocreate a basis for the flow images and this basis was used to reconstruct the evolution of the jet.

Energy Content and Alternative Jet Fuel Viability

Considerable research and development is underway to produce fuels from microalgae, one of several options being explored for increasing transportation fuel supplies and mitigating greenhouse gas emissions (GHG). This work models life-cycle GHG and on-site freshwater consumption for algal biofuels over a wide technology space, spanning both near- and long-term options. The environmental performance of algal biofuel production can vary considerably and is influenced by engineering, biological, siting, and land-use considerations. We have examined these considerations for open pond systems, to identify variables that have a strong influence on GHG and freshwater consumption. We conclude that algal biofuels can yield GHG reductions relative to fossil and other biobased fuels with the use of appropriate technology options. Further, freshwater consumption for algal biofuels produced using saline pond systems can be comparable to that of petroleum-derived fuels.

Turbulence Structures and the Acoustic Far Field of a Mach 1.3 Jet

The noise goal of the Silent Aircraft Initiative, a collaborative effort between industry, academia and government agencies led by Cambridge University and MIT, demands an airframe design with noise as a prime design variable. This poses a number of design challenges and the necessary design philosophy inherently cuts across multiple disciplines involving aerodynamics, structures, acoustics, mission analysis and operations, and dynamics and control. This paper discusses a novel design methodology synthesizing first principles analysis and high-fidelity simulations, and presents the conceptual design of an aircraft with a calculated noise level of 62 dBA at the airport perimeter. This is near the background noise in a well populated area, making the aircraft imperceptible to the human ear on takeoff and landing. The all-lifting airframe of the conceptual aircraft design also has the potential for a reduced fuel burn of 124 passenger-miles per gallon, a 25% improvement compared to existing commercial aircraft. A key enabling technology in this conceptual design is the aerodynamic shaping of the airframe centerbody which is the main focus of this paper. Design requirements and challenges are identified and the resulting aerodynamic design is discussed in depth. The paper concludes with suggestions for continued research on enabling technologies for quiet commercial aircraft.

Quantifying variability in life cycle greenhouse gas inventories of alternative middle distillate transportation fuels.

This paper examines the chemical composition and energy content of several fuel options that could hypothetically be used with the existing fleet of aircraft. Fuel specific energy (energy per unit mass) is an important consideration in determining alternative-fuel viability, because aircraft must travel fixed distances before refueling. Since most aircraft fly with excess tank capacity, fuel energy density (energy per unit volume) is of secondary concern relative to specific energy. A first-order approach using the Breguet-range equation shows that the fleet-wide use of pure synthetic paraffinic kerosene fuels, such as those created from Fischer-Tropsch synthesis or hydroprocessing of renewable oil sources, could reduce aircraft energy consumption by 0.3%. Conversely, fuels with reduced specific energy, such as fatty acid methyl esters (biodiesel and biokerosene) and alcohols, will result in increased fuel volume usage and also a decrease in fleet-wide energy efficiency. No penalty in energy efficiency would occur were these fuels used in ground transportation; thus, fatty acid methyl esters and alcohols are better suited to use in ground-based applications.

Airframe Design for Silent Fuel-Efficient Aircraft

The temporal characteristics of the acoustic far field of a Mach 1.3, high-Reynolds-number, ideally expanded axisymmetric jet and their potential correlation with large-scale turbulence structures within the jet were explored. A dual microphone array, placed approximately 30 deg from the jet axis in the acoustic far field, was used to determine the temporal variations of the acoustic field and the approximate locations of intense noise sources within the jet, as well as the time of noise emission with respect to the acquired planar flow images. Simultaneous double-pulse flow visualizations were used to identify turbulence structures, as well as their development and interaction in the region of intense noise generation. The time history of the acoustic data showed individual large-amplitude noise events, periodic large-amplitude noise events, and long periods of relative quiet without any large-amplitude noise production

Comparison of Noise Mechanisms in High and Low Reynolds Number High-Speed Jets

The presence of variability in life cycle analysis (LCA) is inherent due to both inexact LCA procedures and variation of numerical inputs. Variability in LCA needs to be clearly distinguished from uncertainty. This paper uses specific examples from the production of diesel and jet fuels from 14 different feedstocks to demonstrate general trends in the types and magnitudes of variability present in life cycle greenhouse gas (LC-GHG) inventories of middle distillate fuels. Sources of variability have been categorized as pathway specific, coproduct usage and allocation, and land use change. The results of this research demonstrate that subjective choices such as coproduct usage and allocation methodology can be more important sources of variability in the LC-GHG inventory of a fuel option than the process and energy use of fuel production. Through the application of a consistent analysis methodology across all fuel options, the influence of these subjective biases is minimized, and the LC-GHG inventories for each feedstock-to-fuel option can be effectively compared and discussed. By considering the types and magnitudes of variability across multiple fuel pathways, it is evident that LCA results should be presented as a range instead of a point value. The policy implications of this are discussed.

A technique for real-time visualization of flow structure in high-speed flows

The noise goal of the Silent Aircraft Initiative, a collaborative effort between the University of Cambridge and Massachusetts Institute of Technology, demanded an airframe design with noise as a prime design variable and a design philosophy that cut across multiple disciplines. This paper discusses a novel design methodology synthesizing first-principles analysis and high-fidelity simulations, and it presents the conceptual design of an aircraft with a calculated noise level of 62 dBA at the airport perimeter. This is near the background noise in a well-populated area, making the aircraft imperceptible to the human ear on takeoff and landing. The all-lifting airframe of the conceptual aircraft design also has the potential for improved fuel efficiency, as compared with existing commercial aircraft. A key enabling technology in this conceptual design is the aerodynamic shaping of the airframe centerbody. Design requirements and challenges are identified, and the resulting aerodynamic design is discussed in depth. The paper concludes with suggestions for continued research on enabling technologies for quiet commercial aircraft.

Exploring Noise Sources Using Simultaneous Acoustic Measurements and Real-Time Flow Visualizations in Jets

Direct numerical simulation results from a low Reynolds number jet and experimental results from a high Reynolds number jet were analyzed and compared to investigate the influence of the Reynolds number on the mechanisms generating jet noise. The direct numerical simulation results are for a Mach 0.9, low Reynolds number (Re D ∼ 3600) axisynunetric jet, and the experimental results are for an ideally expanded, Mach 1.3, high Reynolds number (Re D ∼ 1.06 x 10 6 ) axisymmetricjet. Previous experimental work on the high Reynolds numberjet usinga a three-dimensional far-field microphone array, located at 30 deg with respect to the downstream jet axis, estimated the source location and the time of generation of each large amplitude sound wave. Simultaneously with the far-field sound measurements, the source region of the flowfield was visualized using a MHz rate imaging system. In the current work, this technique is used with the direct numerical simulation data and the results are compared with the experimental data. There are many similarities between the two results including the far-field acoustic spectrum, coherence, and average waveform as well as the mean noise source location. The few differences can be attributed to the limited range of turbulence scales and the laminar initial shear layer of the low Reynolds number jet. The main conclusion is that the rapid breakdown of the large-scale structures appears to be important, perhaps the main mechanism of jet noise, independent of the Reynolds number. Right before the breakdown, the structures seem to contract in size, tilt, and eventually disintegrate. To offer a possible explanation for the observed noise mechanism, a simple one-dimensional wave packet model is shown to create more noise in the far field when truncated to simulate a breakdown.