Frank Y. Wang

Aircraft Wake Vortex Measurements at Denver International Airport

Airport capacity is constrained, in part, by spacing requirements associated with the wake vortex hazard. The National Aeronautics Space Administrations (NASA’s) Wake Vortex Avoidance Project has a goal to establish the feasibility of reducing this spacing while maintaining safety. Passive acoustic phased array sensors, if shown to have operational potential, may aid in this effort by detecting and tracking the vortices. During August/September 2003, NASA and the United States Department of Transportation (USDOT) sponsored a wake acoustics test at the Denver International Airport. The central instrument of the test was a large microphone phased array. This paper describes the test in general terms and gives an overview of the array hardware. It outlines one of the analysis techniques that is being applied to the data and gives sample results. The technique is able to clearly resolve the wake vortices of landing aircraft and measure their separation, height, and sinking rate. These observations permit an indirect estimate of the vortex circulation. The array also provides visualization of the vortex evolution, including the Crow instability.

Acoustic Imaging of Aircraft Wake Vortex Dynamics

The experience in utilizing a phased microphone array to passively image aircraft wake vortices is highlighted. It is demonstrated that the array can provide visualization of wake dynamics similar to smoke release or natural condensation of vortices. Examples on how the technique has been integrated with other data to address some of the research issues in wake vortices are documented. These topics include the initial vortex spacing characterization, collaboration with simultaneous pulsed Lidar data to provide a more complete understanding of vortices at a late stage in their evolution, and correlation between atmospheric turbulence with the time-scale involved for vortices developing into a very contorted state. In addition, the paper provides a comparison between the vortex tracks as inferred from their acoustics characteristics vs. those detected from the velocity field (i.e., pulsed Lidar). A preliminary attempt is also made to examine the global link between wake vortex sound and vortex circulation, suggesting that there could be a power law relationship between the two datasets.

Acoustic Characterization of Wake Vortices in Ground Effect

The experience and findings of an exploratory effort to characterize the sound emitted by aircraft wake vortices near the ground are presented. A line array of four directional microphones was deployed and recorded the wakes of several commercial aircraft in the approach phase with particular attention being paid to the characterization of background noise. It is found that vortices near the ground emit broadband sound with frequency contents ranging from below 100 Hz to near 2000 Hz and possibly beyond. However, addressing the degree of consistency of the vortex sound would require deployment of a larger array. The mechanism of vortex sound generation and suggestions on future studies are discussed.

Atmospheric Effects on Microphone Array Analysis of Aircraft Vortex Sound

*This paper provides the basis of a comprehensive analysis of vortex sound propagation through the atmosphere in order to assess real atmospheric effects on acoustic array processing. Such effects may impact vortex localization accuracy and detection probability over longer distances. The paper shows that propagation through the atmosphere can significantly alter the propagation time of the rays from the source to the receiver from that which would be experienced in an ambient atmosphere. More importantly, it can also change the delay time between microphones on which the array processing is based. Implications of these results on the design and resolution of microphone arrays are discussed.

Sound Generation by Aircraft Wake Vortices Interacting with the Ground Plane

This paper examines the acoustic radiation produced by the interaction of aircraft wake vortices with the ground plane. The wake vortices are modeled as vortex filaments of finite length and the theory of vortex sound is applied to calculate the resulting noise generation. A new expression for evaluating the retarded time integrals in this geometry is developed. An example is presented which illustrates that the sound generation by this source for commercial aircraft is generally infrasonic. In addition, a comparison of the sound pressure time histories with and without taking retarded time differences into account is presented. It is determined that, for the case of wake vortex-ground interaction, retarded time differences can be ignored, thereby allowing more efficient computation.

Experimental studies on compressible leading-edge vortices

Abstract A review emphasizing the quantitative studies of leading-edge vortices at supersonic speed is presented. While quantitative investigations of vortical flow over delta wings are extensive in the incompressible regime, and to a degree in transonic range, their measurements in supersonic freestream are very scarce. It is illustrated that the existing knowledge of leading-edge vortices in the supersonic regime is mainly qualitative, compiled from large amount of flow visualization experiments. A brief account of the flow visualization studies is first presented, followed by a comprehensive survey of the various measurement attempts to quantify these vortices. On the qualitative side, this survey reveals that in spite of the past efforts, the literature still lacks a unified topological description of the compressible leeward vortical flows. In quantitative investigations, the experience with pressure probes and seed based optical measurement techniques is highlighted, and the associated results summarized. Amongst them, although there exists a topological similarity in the delta wing leeward flow at low- and high-speeds, available measurements suggest that leading-edge vortices in supersonic flow have a very different axial flow character. Additional salient features of leading-edge vortices in supersonic freestreams are also provided in the paper.

A Quantitative Comparison of Leading-edge Vortices in Incompressible and Supersonic Flows

When requiring quantitative data on delta-wing vortices for design purposes, low-speed results have often been extrapolated to configurations intended for supersonic operation. This practice stems from a lack of database owing to difficulties that plague measurement techniques in high-speed flows. In the present paper an attempt is made to examine this practice by comparing quantitative data on the nearwake properties of such vortices in incompressible and supersonic flows. The incompressible flow data are obtained in experiments conducted in a low-speed wind tunnel. Detailed flow-field properties, including vorticity and turbulence characteristics, obtained by hot-wire and pressure probe surveys are documented. These data are compared, wherever possible, with available data from a past work for a Mach 2.49 flow for the same wing geometry and angles-of-attack. The results indicate that quantitative similarities exist in the distributions of total pressure and swirl velocity. However, the streamwise velocity of the core exhibits different trends. The axial flow characteristics of the vortices in the two regimes are examined, and a candidate theory is discussed.

A Quantitative Comparison of Delta Wing Vortices in the Near-Wake For Incompressible and Supersonic Free Streams

When requiring quantitative data on delta wing vortices for design purposes, low-speed results have often been extrapolated to configurations intended for supersonic operation. This practice stems from a lack of database in high-speed flows due to measurement difficulties. An attempt is made to examine this practice by comparing data from an incompressible flow experiment designed specifically to correspond to an earlier experiment in supersonic flows. The comparison is made for a 75° sweptback delta wing at angles of attack of 7° and 12°. For the incompressible flow, detailed flow-field properties including vorticity and turbulence characteristics are obtained by hot-wire and pressure probe surveys

A Soot Precursor Formation Embedded Reaction Mechanism of Diesel Surrogate Fuel

A new chemical kinetics model of diesel surrogate fuel including polycyclic aromatic hydrocarbon formation and growth is presented, which consists of 153 species and 697 reactions. Comparisons with various experimental data available in the literature, including shock tube, n-heptane premixed flames, jet stirred reactor and homogeneous charge compression ignition engine, show good performance of this new chemical kinetics model. The presented mechanism can be used as the basis for further reduction and is applied to the combustion and emission simulation by coupling chemical reaction kinetics model with computational fluid dynamics model in internal combustion engines.

Duality of Circulation Decay Statistics and Survival Probability

Survival probability and circulation decay history have both been used for setting wake turbulence separation standards. Conceptually a strong correlation should exist between these two characterizations of the vortex behavior, however, the literature lacks a rigorous development in establishing this correlation. The objective of this current paper is to establish the clear linkage between survival probability and circulation decay, and illustrate how they can be mapped to one another depending on need. The impact of the study is twofold. Firstly, it provides a mean to infer circulation decay probabilistic bounds from survival probability curves, which is particularly useful in providing insight on circulation statistics at old wake ages where direct circulation measurements are often challenging. This is referred to as the reverse process. Second, in analyses that require survival probability such as encounter probability analysis, this survival probability can be inferred from known statistical decay curves. This will be referred to as the forward process.