Browsing Department of Aerospace and Ocean Engineering by Issue Date
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- Simple Model of Convection-Diffusion Coupling Near FlamesInger, G. R. (AIP Publishing, 1976)A Couette-flame sheet flow model is analyzed to study combined indraft convection_thermal diffusion effects on oxygen distribution near intense fires. These effects enhance the local oxygen concentration, thermal diffusion causing local overshoots relative to ambient conditions.
- Final Report: Second Forum on Space Structures(Virginia Tech, 1984-06)This document consists of summaries of presentations and discussions from the Forum, which was a meeting for investigators of structural dynamics and control issues in large space structures technology held 11-13 June 1984. The major issues considered are modeling of spacecraft structures, passive and active control techniques, integrated design of structure and control, and experiment and implementation (hardware related) topics, such as sensors, actuators, test techniques, etc. The current status of the technology is reviewed, deficiencies are identified, and recommendations for future research are made.
- Closed-form approximations and series representations for partially saturated ocean acoustic processesPerakis, Anastassios N.; Nikolaidis, Efstratios; Katzouros, Emmanuel (Acoustical Society of America, 1988-04-01)An approximate, closed-form expression for the value of the integral encountered in the calculation of the probability density function (PDF) of the envelope of a partially saturated ocean acoustic process is obtained. Furthermore, an expression of this PDF as a series of modified Bessel functions is presented. The results may also be directly applied to the evaluation of the PDF encountered in the structural reliability analysis of rotating machinery components. Numerical applications show that the closed-form expression is always within 1-2% of the exact result. The required computational effort is substantially lower than that required by direct numerical integration. Copyright 1988 Acoustical Society of America
- Turbulence Measurements in Trailing Vortices for B.W.I. Noise PredictionDevenport, William J.; Glegg, Stewart A. L.; Sharma, Gautam (Virginia Polytechnic Institute and State University, 1992-06-01)Blade wake interaction (BWI) noise is the broadband noise generated by the ingestion of turbulent tip vortices into helicopter rotors. Prediction of BWI noise requires knowledge of the turbulence structure of the tip vortex. This report describes a joint investigation to measure that structure and incorporate the results into a noise prediction scheme. Measurements were performed on the tip vortex shed by a single rectangular NACA 0012 half wing placed in a wind tunnel test section. The properties of the vortex were studied for a range of angles of attack (2.5° to 7.5°) and chord Reynolds numbers (130000 to 530000). Initially the vortex was examined for stability and probe interference effects through flow visualization. Then detailed three-component velocity, turbulence and spectral measurements were made using multiple hot wires between 20 and 30 chordlengths downstream of the wing. These measurements show the flow to consist of a small axisymmetric core surrounded by a large non-axisymmetric region in which the wing wake forms a spiral around the core. In contrast to the results of previous workers, most of whom studied vortices generated by split wing configurations, there appeared to be little axisymmetric region of merged turbulent flow. Turbulence levels in the spiral wake decay with downstream distance. They also fall as the core is approached, presumably because of the effects of the increased straining and curvature on the turbulent structures here. Turbulence spectra measured in the wake are very similar in form, regardless of conditions and exact location, and bear a strong resemblance to a von Karman spectrum. At most conditions true turbulence levels in and immediately adjacent to the core are very low. Velocity fluctuations, however, are intense as a consequence of coherent lateral motions of the core and possible wave motions and instabilities travelling along it. Velocity autospectra in the core show the lateral motions to be anisotropic at very low frequencies and isotropic and mid frequencies. A large part of these motions may well be self induced. Circulation profiles in this region show Hoffman and Joubert's semi-logarithmic region, and in one case reveal the core to be fully developed. From the point of view of BWI noise prediction the flow measurements identify three sources of velocity fluctuations; low-frequency anisotropic core motions, mid-frequency isotropic core motions, and turbulence in the surrounding spiral wake. Estimates of the noise produced by these different frequency regimes show that it is the mid-frequency isotropic motions which is the most important mechanism for noise production, but the spectral shape and the predicted directionality for this mechanism are not in agreement with the measurements of BWI noise. The original noise-prediction scheme, based on the concept of a more turbulent vortex, has been shown to be the wrong basis for the correct description of the flow, but in spite of this the results are significantly better than those presented here. This suggests that the flow measured in the wind tunnel does not have the same turbulence upwash spectrum as that encountered by the helicopter rotor. One of the features of a real helicopter rotor which was not considered is the effect of a downstream blade upon the tip vortex. This may cause vortex bursting or alter the flow structure in the rotor disc plane in other ways. Measurements to evaluate this concept are planned for the future.
- Diode array velocimeter(United States Patent and Trademark Office, 1996-05-14)A velocimeter includes a light source for emitting a light beam through a flow, the flow including particles for scattering light as the light beam passes through the flow, a focussing device for focussing the scattered light into a particle image, a detecting device for detecting the particle image, and a device for timing passage of the particles through a predetermined section of the light beam. The detecting device includes at least one array of photodiodes for receiving light scattered by the particles. The array has a predetermined shape and is positioned so as to control a measurement volume geometry from which light scattered by the particles is received by the photodiodes.
- The Last Western Flyer: The Western Auto CenturyMarchman, James F. III (2004)A treasure trove of pictures and history, The Last Western Flyer chronicles one of the most successful small town franchise operations in 20th Century America, Western Auto. It grew from humble beginnings as a penny mailer sent from the kitchen of a Kansas City home to over 5,000 retail outlets under its Circle Arrow signs. Western Auto brought everything from tires, spark plugs, socks, vitamin pills, fishing lures, radios and more, including its famous Western Flyer bikes to America’s major cities and small towns.
- A Brief History of Aerospace Engineering at the Virginia Polytechnic Institute and State UniversityWalters, Robert W.; Johnston, Jane Echols (American Institute of Aeronautics and Astronautics, 2004)This book chapter provides an historical of Aerospace Engineering at Virginia Tech, covering 1913-2004.
- Octant analysis based structural relations for three-dimensional turbulent boundary layersOlcmen, S. M.; Simpson, Roger L.; Newby, J. W. (American Institute of Physics, 2006-02-01)A flow structure based triple-product correlation model developed by Nagano and Tagawa [J. Fluid Mech. 215, 639 (1990)] has been expanded to three-dimensional turbulent flows. Three-dimensional turbulent boundary layer data obtained away from the vortex in a wing-body junction flow are analyzed to calculate the contributions from eight velocity octants to the stresses and higher-order products. The analysis showed that the sweep and ejection modes dominate the flow physics of some shear stresses and some triple products, while the interaction modes are negligible away from the wall. These experimental observations are used together with the extended Nagano-Tagawa mathematical model to obtain relations among the triple products in three-dimensional turbulent boundary layers that can simplify the turbulent diffusion modeling used in Reynolds-averaged Navier-Stokes equations. Results show that (u) over cap (3), (u) over cap (2)(v) over cap, and (v) over cap (3) triple product correlations can be modeled if an appropriate turbulence model is described for the (u) over cap(v) over cap (2) triple product correlation, and that (u) over cap (2)(w) over cap(v) over cap (2)(w) over cap triple products correlations can be modeled if an appropriate turbulence model is described for the uvw triple product correlation.
- Electric Propulsion Plume Simulations Using Parallel ComputerWang, Joseph C.; Cao, Yong; Kafafy, Raed; Decyk, Viktor (Hindawi, 2007-01-01)A parallel, three-dimensional electrostatic PIC code is developed for large-scale electric propulsion simulations using parallel supercomputers. This code uses a newly developed immersed-finite-element particle-in-cell (IFE-PIC) algorithm designed to handle complex boundary conditions accurately while maintaining the computational speed of the standard PIC code. Domain decomposition is used in both field solve and particle push to divide the computation among processors. Two simulations studies are presented to demonstrate the capability of the code. The first is a full particle simulation of near-thruster plume using real ion to electron mass ratio. The second is a high-resolution simulation of multiple ion thruster plume interactions for a realistic spacecraft using a domain enclosing the entire solar array panel. Performance benchmarks show that the IFE-PIC achieves a high parallel efficiency of ≥ 90%
- Detailed flow physics of the supersonic jet interaction flow fieldViti, Valerio; Neel, Reece; Schetz, Joseph A. (American Institute of Physics, 2009-04-01)The supersonic jet interaction flow field generated by a sonic circular jet with a pressure ratio of 532 exhausting into a turbulent MACH 4.0 cross flow over a flat plate was investigated using numerical simulations. The simulations made use of the three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations coupled with Wilcox's 1998 k-omega turbulence model. The numerical solution was validated with experimental data that include the pressure distribution on the flat plate, with an empirical formula for the height of the barrel shock, and with the Schlieren pictures showing the location and shape of the main shock formations. The simulations correctly captured the location and shape of the main flow features and compared favorably with the experimental pressure distribution on the flat plate. The validated numerical simulation was used to investigate in detail the flow physics. The flow field was found to be dominated by the shock formations and their coupling with the strong vortical structures. Three primary shock formations were observed: a barrel shock, a bow shock, and a separation-induced shock wave. While the general structure of the barrel shock was found to be similar to that of the underexpanded jet exhausting into a quiescent medium, two unique features distinguished the flow field: the concave indentation in the leeside of the recompression (barrel) shock and the folding of the windward side of the barrel shock due to an inner reflection line. The presence of the steep pressure gradients associated with the shocks creates strong vortical motions in the fluid. Six primary vortices were identified: (i) the well-known horseshoe vortex, (ii) an upper trailing vortex, (iii) two trailing vortices formed in the separation region and, aft of the bow shock wave, (iv) two more trailing vortices that eventually merge together into one single rotational motion. The low-pressure region aft of the injector was found to be generated by the combined effect of the concave indentation in the leeside of the barrel shock and the lower trailing vortices. The trailing vortices were found to be the main mechanism responsible for the mixing of the injectant with the freestream fluid.
- Prevention of Pressure Oscillations in Modeling a Cavitating Acoustic FluidKlenow, Bradley A.; Brown, Alan J. (Hindawi, 2010-01-01)Cavitation effects play an important role in the UNDEX loading of a structure. For far-field UNDEX, the structural loading is affected by the formation of local and bulk cavitation regions, and the pressure pulses resulting from the closure of the cavitation regions. A common approach to numerically modeling cavitation in far-field underwater explosions is Cavitating Acoustic Finite Elements (CAFE) and more recently Cavitating Acoustic Spectral Elements (CASE). Treatment of cavitation in this manner causes spurious pressure oscillations which must be treated by a numerical damping scheme. The focus of this paper is to investigate the severity of these oscillations on the structural response and a possible improvement to CAFE, based on the original Boris and Book Flux-Corrected Transport algorithm on structured meshes , to limit oscillations without the energy loss associated with the current damping schemes.
- A closed-form method for calculating the angular distribution of multiply scattered photons through isotropic turbid slabsSun, Xueqiang; Li, Xuesong; Ma, Lin (Optical Society of America, 2011-11-01)This paper develops a method for calculating the angular distribution (AD) of multiply scattered photons through isotropic turbid slabs. Extension to anisotropic scattering is also discussed. Previous studies have recognized that the AD of multiply scattered photons is critical for many applications, such as the design of imaging optics and estimation of image quality. This paper therefore develops a closed-from method that can accurately calculate the AD over a wide range of conditions. Other virtues of the method include its simplicity in implementation and its prospective for extension to anisotropic scattering. (C) 2011 Optical Society of America
- Simultaneous measurements of multiple flow parameters for scramjet characterization using tunable diode-laser sensorsLi, Fei; Yu, XiLong; Gu, Hongbin; Li, Zhi; Zhao, Yan; Ma, Lin; Chen, Lihong; Chang, Xinyu (Optical Society of America, 2011-12-01)This paper reports the simultaneous measurements of multiple flow parameters in a scramjet facility operating at a nominal Mach number of 2.5 using a sensing system based on tunable diode-laser absorption spectroscopy (TDLAS). The TDLAS system measures velocity, temperature, and water vapor partial pressure at three different locations of the scramjet: the inlet, the combustion region near the flame stabilization cavity, and the exit of the combustor. These measurements enable the determination of the variation of the Mach number and the combustion mode in the scramjet engine, which are critical for evaluating the combustion efficiency and optimizing engine performance. The results obtained in this work clearly demonstrated the applicability of TDLAS sensors in harsh and high-speed environments. The TDLAS system, due to its unique virtues, is expected to play an important role in the development of scramjet engines. (C) 2011 Optical Society of America
- Scaling Law for Photon Transmission through Optically Turbid Slabs Based on Random Walk TheoryLi, Xuesong; Ma, Lin (MDPI, 2012-03-01)Past work has demonstrated the value of a random walk theory (RWT) to solve multiple-scattering problems arising in numerous contexts. This paper's goal is to investigate the application range of the RWT using Monte Carlo simulations and extending it to anisotropic media using scaling laws. Meanwhile, this paper also reiterates rules for converting RWT formulas to real physical dimensions, and corrects some errors which appear in an earlier publication. The RWT theory, validated by the Monte Carlo simulations and combined with the scaling law, is expected to be useful to study multiple scattering and to greatly reduce the computation cost.
- Method to correct the distortion caused by amplified stimulated emission as motivated by LIF-based flow diagnosticsLi, Xuesong; Zhao, Yan; Ma, Lin (Optical Society of America, 2012-04-01)Amplified stimulated emission (ASE) represents a significant issue in two-photon laser-induced fluorescence (TPLIF). The ASE effects are nonlinear and nonlocal, i.e., the ASE effects distort the LIF signal nonlinearly, and the distortion at one location depends on conditions at other locations. In this sense, the ASE effects pose a greater challenge to quantitative TPLIF than quenching and ionization. This work therefore seeks a method to correct such distortion. The method uses two LIF measurements, one with low signal-to-noise ratio (SNR) and negligible ASE distortion and another with high SNR but significant distortion, to generate a faithful measurement with high SNR. Extensive simulations were performed to evaluate the performance of this method for practical applications. c 2012 Optical Society of America OCIS codes: 300.2530, 300.6420, 120.1740.
- Uncertainty in velocity measurement based on diode-laser absorption in nonuniform flowsLi, Fei; Yu, XiLong; Cai, Weiwei; Ma, Lin (Optical Society of America, 2012-07-01)This work investigates the error caused by nonuniformities along the line-of-sight in velocity measurement using tunable diode-laser absorption spectroscopy (TDLAS). Past work has demonstrated TDLAS as an attractive diagnostic technique for measuring velocity, which is inferred from the Doppler shift of two absorption features using two crossing laser beams. However, because TDLAS is line-of-sight in nature, the obtained velocity is a spatially averaged value along the probing laser beams. As a result, nonuniformities in the flow can cause uncertainty in the velocity measurement. Therefore, it is the goal of this work to quantify the uncertainty caused by various nonuniformities typically encountered in practice, including boundary layer effects, the divergence/convergence of the flow, and the methods used to fit the Doppler shift. Systematic analyses are performed to quantify the uncertainty under various conditions, and case studies are reported to illustrate the usefulness of such analysis in interpreting experimental data obtained from a scramjet facility. We expect this work to be valuable for the design and optimization of TDLAS-based velocimetry, and also for the quantitative interpretation of the measurements. (C) 2012 Optical Society of America
- 50-kHz-rate 2D imaging of temperature and H2O concentration at the exhaust plane of a J85 engine using hyperspectral tomographyMa, Lin; Li, Xuesong; Sanders, Scott T.; Caswell, Andrew W.; Roy, Sukesh; Plemmons, David H.; Gord, James R. (Optical Society of America, 2013-01-01)This paper describes a novel laser diagnostic and its demonstration in a practical aero-propulsion engine (General Electric J85). The diagnostic technique, named hyperspectral tomography (HT), enables simultaneous 2-dimensional (2D) imaging of temperature and water-vapor concentration at 225 spatial grid points with a temporal response up to 50 kHz. To our knowledge, this is the first time that such sensing capabilities have been reported. This paper introduces the principles of the HT techniques, reports its operation and application in a J85 engine, and discusses its perspective for the study of high-speed reactive flows. (C) 2013 Optical Society of America
- Numerical and experimental validation of a three-dimensional combustion diagnostic based on tomographic chemiluminescenceCai, Weiwei; Li, Xuesong; Li, Fei; Ma, Lin (Optical Society of America, 2013-03-25)Three-dimensional (3D) measurements are highly desirable both for fundamental combustion research and practical monitoring and control of combustion systems. This work discusses a 3D diagnostic based on tomographic chemiluminescence (TC) to address this measurement need. The major contributions of this work are threefold. First, a hybrid algorithm is developed to solve the 3D TC problem. The algorithm was demonstrated in extensive tests, both numerical and experimental, to yield 3D reconstruction with high fidelity. Second, an experimental approach was designed to enable quantifiable metrics for examining key aspects of the 3D TC technique, including its spatial resolution and reconstruction accuracy. Third, based on the reconstruction algorithm and experimental results, we investigated the effects of the view orientations. The results suggested that for an unknown flame, it is better to use projections measured from random orientations than restricted orientations (e.g., coplanar orientations). These findings are expected to provide insights to the fundamental capabilities of the TC technique, and also to facilitate its practical application.