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1

Wake vortex technology  

NASA Technical Reports Server (NTRS)

A brief overview of the highlights of NASA's wake vortex minimization program is presented. The significant results of this program are summarized as follows: (1) it is technically feasible to reduce significantly the rolling upset created on a trailing aircraft; (2) the basic principles or methods by which reduction in the vortex strength can be achieved have been identified; and (3) an analytical capability for investigating aircraft vortex wakes has been developed.

Dunham, R. E., Jr.; Barber, M. R.; Croom, D. R.

1978-01-01

2

NASA wake vortex research  

NASA Technical Reports Server (NTRS)

NASA is conducting research that will enable safe improvements in the capacity of the nation's air transportation system. The wake-vortex hazard is a factor in establishing the minimum safe spacing between aircraft during landing and takeoff operations and, thus, impacts airport capacity. The ability to accurately model the wake hazard and determine safe separation distances for a wide range of aircraft and operational scenarios may provide the basis for significant increases in airport capacity. Current and planned NASA research is described which is focused on increasing airport capacity by safely reducing wake-hazard-imposed aircraft separations through advances in a number of technologies including vortex motion and decay prediction, vortex encounter modeling, wake-vortex hazard characterization, and in situ flow sensing.

Stough, H. P., III; Greene, George C.; Stewart, Eric C.; Stuever, Robert A.; Jordan, Frank L., Jr.; Rivers, Robert A.; Vicroy, Dan D.

1993-01-01

3

Wake Vortex Minimization  

NASA Technical Reports Server (NTRS)

A status report is presented on research directed at reducing the vortex disturbances of aircraft wakes. The objective of such a reduction is to minimize the hazard to smaller aircraft that might encounter these wakes. Inviscid modeling was used to study trailing vortices and viscous effects were investigated. Laser velocimeters were utilized in the measurement of aircraft wakes. Flight and wind tunnel tests were performed on scale and full model scale aircraft of various design. Parameters investigated included the effect of wing span, wing flaps, spoilers, splines and engine thrust on vortex attenuation. Results indicate that vortives may be alleviated through aerodynamic means.

1977-01-01

4

Cooperative wake vortex instabilities  

NASA Astrophysics Data System (ADS)

Reported herein are results concerning a class of three-dimensional instabilities occurring between nearly parallel wake vortices. The problem was studied experimentally, theoretically, and numerically. The experimental data were taken of pairs of co-rotating vortices produced behind a wing in a towing tank. Particle imaging velocimetry and flow visualization revealed the growth of a sinuous disturbance along the axis of each of the vortices in a pair, followed soon thereafter by merger of the two. A linear instability analysis explains the basic mechanisms responsible for the initial growth of the disturbance. This theory also explains a related instability for the case of a counter-rotating pair. The analysis reveals that planar disturbances on one vortex tend to grow along the extensional axis of the straining field imposed by the other vortex. However, self- and orbit-induced effects tend to prevent instability by rotating the plane of the perturbation through the straining field. For instability, the rotational effects must act in opposite directions. For displacement perturbations, this condition can only occur for a counter-rotating pair. A co-rotating pair, however, can be de-stabilized by shorter-wavelength "elliptic" modes, in which the core and periphery of the vortex are perturbed in opposite directions. A numerical study of the problem was undertaken using the cylindrical spectral Navier-Stokes solver of Matsushima and Marcus (1997). In addition to confirming the linear analysis for early growth of the instabilities, the code allowed for the computation of their long-term behavior. The growth of the elliptic instability allows the vortices to form bridges between each other, which eventually yields merger of the co-rotating pair. The displacement instability, acting on a counter-rotating pair, yields the ejection of large hoop-like structures, as was first seen experimentally by Ortega (2001). Thus, a picture emerges of a strain-induced instability which leads to merger of a co-rotating pair and the ejection of large-scale structures from a counter-rotating pair. The fact that this behavior occurs both in a simplified numerical geometry and in actual airfoil wakes suggests that it is relatively robust, and may apply to other flows involving nearly parallel vortices.

Bristol, Robert Lindsey

5

Atmospheric-wake vortex interactions  

NASA Technical Reports Server (NTRS)

The interactions of a vortex wake with a turbulent stratified atmosphere are investigated with the computer code WAKE. It is shown that atmospheric shear, turbulence, and stratification can provide the dominant mechanisms by which vortex wakes decay. Computations included the interaction of a vortex wake with a viscous ground plane. The observed phenomenon of vortex bounce is explained in terms of secondary vorticity produced on the ground. This vorticity is swept off the ground and advected about the vortex pair, thereby altering the classic hyperbolic trajectory. The phenomenon of the solitary vortex is explained as an interaction of a vortex with crosswind shear. Here, the vortex having the sign opposite that of the sign of the vorticity in the shear is dispersed by a convective instability. This instability results in the rapid production of turbulence which in turn disperses the smoke marking the vortex.

Bilanin, A. J.; Hirsh, J. E.; Teske, M. E.; Hecht, A. M.

1978-01-01

6

Three-Phased Wake Vortex Decay  

NASA Technical Reports Server (NTRS)

A detailed parametric study is conducted that examines vortex decay within turbulent and stratified atmospheres. The study uses a large eddy simulation model to simulate the out-of-ground effect behavior of wake vortices due to their interaction with atmospheric turbulence and thermal stratification. This paper presents results from a parametric investigation and suggests improvements for existing fast-time wake prediction models. This paper also describes a three-phased decay for wake vortices. The third phase is characterized by a relatively slow rate of circulation decay, and is associated with the ringvortex stage that occurs following vortex linking. The three-phased decay is most prevalent for wakes imbedded within environments having low-turbulence and near-neutral stratification.

Proctor, Fred H.; Ahmad, Nashat N.; Switzer, George S.; LimonDuparcmeur, Fanny M.

2010-01-01

7

Wake Vortex Field Measurement Program at Memphis, Tennessee: Data Guide  

NASA Technical Reports Server (NTRS)

Eliminating or reducing current restrictions in the air traffic control system due to wake vortex considerations would yield increased capacity, decreased delays, and cost savings. Current wake vortex separation standards are widely viewed as very conservative under most conditions. However, scientific uncertainty about wake vortex behavior under different atmospheric conditions remains a barrier to development of an adaptive vortex spacing system. The objective of the wake vortex field measurement efforts during December, 1994 and August, 1995 at Memphis, TN were to record wake vortex behavior for varying atmospheric conditions and types of aircraft. This effort is part of a larger effort by the NASA Langley Research Center to develop an Aircraft Vortex Spacing System (AVOSS) as an element of the Terminal Area Productivity (TAP) program. The TAP program is being performed in concert with the FAA Terminal Air Traffic Control Automation (TATCA) program and ATC Automation. Wake vortex behavior was observed using a mobile continuous-wave (CW) coherent laser Doppler radar (lidar) developed at Lincoln Laboratory. This lidar features a number of improvements over previous systems, including the first-ever demonstration of an automatic wake vortex detection and tracking algorithm.

Campbell, S. D.; Dasey, T. J.; Freehart, R. E.; Heinrichs, R. M.; Mathews, M. P.; Perras, G. H.; Rowe, G. S.

1997-01-01

8

ASRS Reports on Wake Vortex Encounters  

NASA Technical Reports Server (NTRS)

ASRS is conducting a structured callback research project of wake vortex incidents reported to the ASRS at all US airports, as well as wake encounters in the enroute environment. This study has three objectives: (1) Utilize the established ASRS supplemental data collection methodology and provide ongoing analysis of wake vortex encounter reports; (2) Document event dynamics and contributing factors underlying wake vortex encounter events; and (3) Support ongoing FAA efforts to address pre-emptive wake vortex risk reduction by utilizing ASRS reporting contributions.

Connell, Linda J.; Taube, Elisa Ann; Drew, Charles Robert; Barclay, Tommy Earl

2010-01-01

9

Wake Vortex Research in the USA (WakeNet-USA)  

NASA Technical Reports Server (NTRS)

This viewgraph presentation reviews the cooperative work that FAA and NASA are engaged in to safely increase the capacity of the National Airspace System by studying the wake vortex operations. Wake vortex avoidance is a limiting factor in defining separation standards in the airport terminal area and could become a reducing separation standards in en route airspace.

Lang, Steve; Bryant, Wayne

2006-01-01

10

Vortex shedding in the wake of a step cylinder  

NASA Astrophysics Data System (ADS)

Flow past a circular cylinder with a single stepwise discontinuity in diameter was investigated numerically for the diameter ratio D/d=2 and two Reynolds numbers, ReD=150 and 300. The primary focus was on vortex shedding and vortex interactions occurring in the cylinder wake. In agreement with previous experimental findings, three distinct spanwise vortex cells were identified in the step-cylinder wake: a single vortex shedding cell in the wake of the small cylinder (the S-cell) and two vortex shedding cells in the wake of the large cylinder, one in the region downstream of the step (the N-cell) and the other away from the step (the L-cell). Due to the differences in vortex shedding frequencies, complex vortex connections occurred in two vortex interaction regions located between the adjacent cells. However, distinct differences in vortex splitting and vortex dislocations were identified in the two regions. The region at the boundary between the S-cell and the N-cell was relatively narrow and its spanwise extent did not fluctuate significantly. In this region, vortex dislocations manifested as half-loop connections between two S-cell vortices of opposite sign. In contrast, the region at the boundary between the N-cell and the L-cell exhibited transient behavior, with large scale vortex dislocations causing cyclic variation in the extent of N-cell vortices. Spectral analysis of velocity data showed that the presence of the N-cell was continuous through all simulations. For ReD=300, small scale streamwise vortices forming in the wake of the large cylinder weaken the primary spanwise vortices and vortex connections, complicating vortex dynamics in the step-cylinder wake. However, no significant Reynolds number effect on the average spanwise extent of the vortex cells and the two transition regions between neighboring cells was observed. Finally, formation of N-cell vortices was shown to be linked to downwash fluctuations near the step.

Morton, Chris; Yarusevych, Serhiy

2010-08-01

11

Review of Idealized Aircraft Wake Vortex Models  

NASA Technical Reports Server (NTRS)

Properties of three aircraft wake vortex models, Lamb-Oseen, Burnham-Hallock, and Proctor are reviewed. These idealized models are often used to initialize the aircraft wake vortex pair in large eddy simulations and in wake encounter hazard models, as well as to define matched filters for processing lidar observations of aircraft wake vortices. Basic parameters for each vortex model, such as peak tangential velocity and circulation strength as a function of vortex core radius size, are examined. The models are also compared using different vortex characterizations, such as the vorticity magnitude. Results of Euler and large eddy simulations are presented. The application of vortex models in the postprocessing of lidar observations is discussed.

Ahmad, Nashat N.; Proctor, Fred H.; Duparcmeur, Fanny M. Limon; Jacob, Don

2014-01-01

12

Aircraft control in wake vortex wind shear  

NASA Technical Reports Server (NTRS)

In the past, there have been a number of fatal incidents attributable to wake vortex encounters, involving both general aviation and commercial aircraft. In fact, the wake vortex hazard is considered to be the single dominant safety issue determining the aircraft spacing requirements at airports. As the amount of air traffic increases, the number of dangerous encounters is likely only to increase. It is therefore imperative that a means be found to reduce the danger. That is the purpose of this research: to use nonlinear inverse dynamic (NID) control methods in the design of an aircraft control system which can improve the safety margin in a wake vortex encounter.

Wold, Gregory R.

1995-01-01

13

Vortex interactions in multiple vortex wakes behind aircraft  

NASA Technical Reports Server (NTRS)

A flow visualization technique has been developed which allows the nature of lift-generated wakes behind aircraft models to be investigated. Several different configurations of a 0.61-m span model of a Boeing 747-type transport aircraft were tested to allow observation of typical vortex interactions and merging in multiple vortex wakes. The vortices were identified by emitting tracer dyes from selected locations on the model. Wing span loading and model attitude were found to effect both vortex motions within the wake and resulting far-field wake velocity. Landing gear deployment caused a far-field reformation of vorticity behind a model configuration which dissipated concentrated vorticity in the near-field wake. A modified landing configuration was developed which appeared to significantly alleviate the concentrated wake vorticity.

Ciffone, D. L.

1976-01-01

14

An Operational Wake Vortex Sensor Using Pulsed Coherent Lidar  

NASA Technical Reports Server (NTRS)

NASA and FAA initiated a program in 1994 to develop methods of setting spacings for landing aircraft by incorporating information on the real-time behavior of aircraft wake vortices. The current wake separation standards were developed in the 1970's when there was relatively light airport traffic and a logical break point by which to categorize aircraft. Today's continuum of aircraft sizes and increased airport packing densities have created a need for re-evaluation of wake separation standards. The goals of this effort are to ensure that separation standards are adequate for safety and to reduce aircraft spacing for higher airport capacity. Of particular interest are the different requirements for landing under visual flight conditions and instrument flight conditions. Over the years, greater spacings have been established for instrument flight than are allowed for visual flight conditions. Preliminary studies indicate that the airline industry would save considerable money and incur fewer passenger delays if a dynamic spacing system could reduce separations at major hubs during inclement weather to the levels routinely achieved under visual flight conditions. The sensor described herein may become part of this dynamic spacing system known as the "Aircraft VOrtex Spacing System" (AVOSS) that will interface with a future air traffic control system. AVOSS will use vortex behavioral models and short-term weather prediction models in order to predict vortex behavior sufficiently into the future to allow dynamic separation standards to be generated. The wake vortex sensor will periodically provide data to validate AVOSS predictions. Feasibility of measuring wake vortices using a lidar was first demonstrated using a continuous wave (CW) system from NASA Marshall Space Flight Sensor and tested at the Volpe National Transportation Systems Center's wake vortex test site at JFK International Airport. Other applications of CW lidar for wake vortex measurement have been made more recently, including a system developed by the MIT Lincoln Laboratory. This lidar has been used for detailed measurements of wake vortex velocities in support of wake vortex model validation. The first measurements of wake vortices using a pulsed, lidar were made by Coherent Technologies, Inc. (CTI) using a 2 micron solid-state, flashlamp-pumped system operating at 5 Hz. This system was first deployed at Denver's Stapleton Airport. Pulsed lidar has been selected as the baseline technology for an operational sensor due to its longer range capability.

Barker, Ben C., Jr.; Koch, Grady J.; Nguyen, D. Chi

1998-01-01

15

Vortex shedding in compressor blade wakes  

NASA Technical Reports Server (NTRS)

The wakes of highly loaded axial compressor blades were often considered to be turbulent, unstructured flows. Recent work has suggested that the blade wakes are in fact dominated by a vortex street-like structure. The work on the wake structure at MIT is reviewed, the results of a viscous numerical simulation are presented, the blade wake vortices are compared to those shed from a cylinder, and the implications of the wake structure on compressor performance are discussed. In particular, a two-dimensional, time accurate, viscous calculation shows both a periodic wake structure and time variations in the passage shock strength. The numerical calculations are compared to laser anemometer and high frequency response probe data. The effect of the wake structure on the entropy production and apparent adiabatic efficiency of the compressor rotor is discussed.

Epstein, A. H.; Gertz, J. B.; Owen, P. R.; Giles, M. B.

1987-01-01

16

Wake Vortex Study at Wallops Island  

NSDL National Science Digital Library

This page from the NASA Langley Research Center contains an image and description of a wake vortex created by the wing of a small propeller plane. The page explains how the image was made and how this research helps determine the spacing between airplanes approaching an airport.

2007-07-30

17

A Candidate Wake Vortex Strength Definition for Application to the NASA Aircraft Vortex Spacing System (AVOSS)  

NASA Technical Reports Server (NTRS)

A significant effort is underway at NASA Langley to develop a system to provide dynamical aircraft wake vortex spacing criteria to Air Traffic Control (ATC). The system under development, the Aircraft Vortex Spacing System (AVOSS), combines the inputs of multiple subsystems to provide separation matrices with sufficient stability for use by ATC and sufficient monitoring to ensure safety. The subsystems include a meteorological subsystem, a wake behavior prediction subsystem, a wake sensor subsystem, and system integration and ATC interfaces. The proposed AVOSS is capable of using two factors, singly or in combination, for reducing in-trail spacing. These factors are wake vortex motion out of a predefined approach corridor and wake decay below a strength that is acceptable for encounter. Although basic research into the wake phenomena has historically used wake total circulation as a strength parameter, there is a requirement for a more specific strength definition that may be applied across multiple disciplines and teams to produce a real-time, automated system. This paper presents some of the limitations of previous applications of circulation to aircraft wake observations and describes the results of a preliminary effort to bound a spacing system strength definition.

Hinton, David A.; Tatnall, Chris R.

1997-01-01

18

Viscous effects on a vortex wake in ground effect  

NASA Technical Reports Server (NTRS)

Wake vortex trajectories and strengths are altered radically by interactions with the ground plane. Prediction of vortex strength and location is especially important in the vicinity of airports. Simple potential flow methods have been found to yield reasonable estimates of vortex descent rates in an otherwise quiescent ambient background, but those techniques cannot be adjusted for more realistic ambient conditions and they fail to provide satisfactory estimates of ground-coupled behavior. The authors have been involved in a systematic study concerned with including viscous effects in a wake-vortex system which is near the ground plane. The study has employed numerical solutions to the Navier-Stokes equations, as well as perturbation techniques to study ground coupling with a descending vortex pair. Results of a two-dimensional, unsteady numerical-theoretical study are presented in this paper. A time-based perturbation procedure has been developed which permits the use of analytical solutions to an inner and outer flow domain for the initial flow field. Predictions have been compared with previously reported laminar experimental results. In addition, the influence of stratification and turbulence on vortex behavior near the ground plane has been studied.

Zheng, Z.; Ash, Robert L.

1992-01-01

19

NOWVIV - Nowcasting wake vortex impact variables  

NASA Astrophysics Data System (ADS)

A central task of the ongoing DLR project "Wirbelschleppe" (Wake Vortex) is to forecast meteorological quantities which influence the behaviour of wake vortices of landing aircraft. In the first place these are wind, temperature and turbulence, resp. the vertical shear thereof, which impact the lateral drift and turbulent decay of wake vortices. For this purpose the nowcasting system NOWVIV has been developed at DLR. It combines operational forecasts of the Lokal Modell (LM; Doms and Schaettler 1999) of the German weather service DWD with a high-resolution forecasting system. For the latter, the NOAA/FSL version of the mesoscale model MM5 (Grell et al. 2000) has been adapted to particular sites. Orography, land use, and soil type have been generated from available data sources for a 80 km square domain centered on a particular airport with a horizontal resolution of 2.1 km. As a good representation of the boundary layer is of particular importance for predicting wake vortex impact variables, the vertical spacing of model layers has been selected rather small throughout the lower model atmosphere, starting with 20 m at the ground and increasing to about 60 m at 2 km height. NOWVIV delivers vertical profiles of vortex impact variables, which are used by the wake prediction model ``P2P'' developed at DLR (Holzaepfel 2002) to predict wake vortex behaviour. During the two field campaigns ``WakeOP'' and ``WakeTOUL'' in April/May 2001 and May/June 2002 which aimed at measuring (by lidar) and predicting wake vortex behaviour of landing aircraft, NOWVIV has been run in an operational mode for the airports of Oberpfaffenhofen (Germany) and Tarbes (France). A statistical evaluation of the NOWVIV forecasting performance during these campaigns achieved satisfactory results as compared to local measurements of wind and temperature from radio acoustic sounding instruments (Frech et al. 2002). However, there are uncertainties in the daily variation of the boundary layer. Also, the dependency of the forecast variables on the quality of the LM forecast is rather obvious. Therefore, NOWVIV will be extended with a data assimilation cycle where local data from bistatic wind radar and aircraft (AMDAR) are taken into account. This will force the model atmosphere more closely to reality and reduce the dependency on LM data. References Doms, G., and U. Schaettler, 1999: The nonhydrostatic Limited Area Model LM (Lokal Modell) of DWD. Part I: Scientific Documentation. Deutscher Wetterdienst, Offenbach Frech, M., and A. Tafferner, 2002: The Performance of the Model System NOWVIV during the field campaign WakeOP. Confer. on Aviation, Range and Aerospace Meteorology, 12-16 May 2002, Portland, Oregon, AMS, Boston, Proc. Vol., 216-219 Grell, G. A., S. Emeis, W. R. Stockwell, T. Schoenemeyer, R. Forkel, J. Michalakes, R. Knoche, and W. Seidl, 2000: Application of a multiscale, coupled MM5/chemistry model to the complex terrain of the VOTALP valley campaign. Atmos. Environm. , 34, 1435-1453 Holzaepfel F., 2002: A Probabilistic Two-Phase Wake Vortex Decay and Transport Model. Journal of Aircraft, in press.

Tafferner, A.; Birke, L.; Frech, M.

2003-04-01

20

Wake Vortex Algorithm Scoring Results  

NASA Technical Reports Server (NTRS)

This report compares the performance of two models of trailing vortex evolution for which interaction with the ground is not a significant factor. One model uses eddy dissipation rate (EDR) and the other uses the kinetic energy of turbulence fluctuations (TKE) to represent the effect of turbulence. In other respects, the models are nearly identical. The models are evaluated by comparing their predictions of circulation decay, vertical descent, and lateral transport to observations for over four hundred cases from Memphis and Dallas/Fort Worth International Airports. These observations were obtained during deployments in support of NASA's Aircraft Vortex Spacing System (AVOSS). The results of the comparisons show that the EDR model usually performs slightly better than the TKE model.

Robins, R. E.; Delisi, D. P.; Hinton, David (Technical Monitor)

2002-01-01

21

Hazard criteria for wake vortex encounters  

NASA Technical Reports Server (NTRS)

A piloted, motion-base simulation was conducted to evaluate the ability of simulators to produce realistic vortex encounters and to develop criteria to define hazardous encounters. Evaluation of the simulation by pilots experienced in vortex encounters confirmed the capability of the simulator to realistically reproduce wake vortex encounters. A boundary for encounter hazard based on subjective pilot opinion was identified in terms of maximum bank angle. For encounter altitudes from 200 to 500 ft (61.0 to 152.4 m), tentative hazard criteria established for visual flight conditions indicated that the acceptable upset magnitude increased nearly linearly with increasing altitude. The data suggest that the allowable upsets under instrument conditions no greater than 50 percent of that allowable under visual conditions.

Sammonds, R. I.; Stinnett, G. W., Jr.

1975-01-01

22

Wake Vortex Detection: Phased Microphone vs. Linear Infrasonic Array  

NASA Technical Reports Server (NTRS)

Sensor technologies can make a significant impact on the detection of aircraft-generated vortices in an air space of interest, typically in the approach or departure corridor. Current state-of-the art sensor technologies do not provide three-dimensional measurements needed for an operational system or even for wake vortex modeling to advance the understanding of vortex behavior. Most wake vortex sensor systems used today have been developed only for research applications and lack the reliability needed for continuous operation. The main challenges for the development of an operational sensor system are reliability, all-weather operation, and spatial coverage. Such a sensor has been sought for a period of last forty years. Acoustic sensors were first proposed and tested by National Oceanic and Atmospheric Administration (NOAA) early in 1970s for tracking wake vortices but these acoustic sensors suffered from high levels of ambient noise. Over a period of the last fifteen years, there has been renewed interest in studying noise generated by aircraft wake vortices, both numerically and experimentally. The German Aerospace Center (DLR) was the first to propose the application of a phased microphone array for the investigation of the noise sources of wake vortices. The concept was first demonstrated at Berlins Airport Schoenefeld in 2000. A second test was conducted in Tarbes, France, in 2002, where phased microphone arrays were applied to study the wake vortex noise of an Airbus 340. Similarly, microphone phased arrays and other opto-acoustic microphones were evaluated in a field test at the Denver International Airport in 2003. For the Tarbes and Denver tests, the wake trajectories of phased microphone arrays and lidar were compared as these were installed side by side. Due to a built-in pressure equalization vent these microphones were not suitable for capturing acoustic noise below 20 Hz. Our group at NASA Langley Research Center developed and installed an infrasonic array at the Newport News-Williamsburg International Airport early in the year 2013. A pattern of pressure burst, high-coherence intervals, and diminishing-coherence intervals was observed for all takeoff and landing events without exception. The results of a phased microphone vs. linear infrasonic array comparison will be presented.

Shams, Qamar A.; Zuckerwar, Allan J.; Sullivan, Nicholas T.; Knight, Howard K.

2014-01-01

23

Coherent Pulsed Lidar Sensing of Wake Vortex Position and Strength, Winds and Turbulence in the Terminal Area  

NASA Technical Reports Server (NTRS)

NASA Langley Research Center (LaRC) has field tested a 2.0 gm, 100 Hertz, pulsed coherent lidar to detect and characterize wake vortices and to measure atmospheric winds and turbulence. The quantification of aircraft wake-vortex hazards is being addressed by the Wake Vortex Lidar (WVL) Project as part of Aircraft Vortex Spacing System (AVOSS), which is under the Reduced Spacing Operations Element of the Terminal Area Productivity (TAP) Program. These hazards currently set the minimum, fixed separation distance between two aircraft and affect the number of takeoff and landing operations on a single runway under Instrument Meteorological Conditions (IMC). The AVOSS concept seeks to safely reduce aircraft separation distances, when weather conditions permit, to increase the operational capacity of major airports. The current NASA wake-vortex research efforts focus on developing and validating wake vortex encounter models, wake decay and advection models, and wake sensing technologies. These technologies will be incorporated into an automated AVOSS that can properly select safe separation distances for different weather conditions, based on the aircraft pair and predicted/measured vortex behavior. The sensor subsystem efforts focus on developing and validating wake sensing technologies. The lidar system has been field-tested to provide real-time wake vortex trajectory and strength data to AVOSS for wake prediction verification. Wake vortices, atmospheric winds, and turbulence products have been generated from processing the lidar data collected during deployments to Norfolk (ORF), John F. Kennedy (JFK), and Dallas/Fort Worth (DFW) International Airports.

Brockman, Philip; Barker, Ben C., Jr.; Koch, Grady J.; Nguyen, Dung Phu Chi; Britt, Charles L., Jr.; Petros, Mulugeta

1999-01-01

24

Prediction and Control of Vortex Dominated and Vortex-wake Flows  

NASA Technical Reports Server (NTRS)

This report describes the activities and accomplishments under this research grant, including a list of publications and dissertations, produced in the field of prediction and control of vortex dominated and vortex wake flows.

Kandil, Osama

1996-01-01

25

Vortex wake and exhaust plume interaction, including ground effect  

Microsoft Academic Search

Computational modeling and studies of the near-field wake-vortex turbulent flows, far-field turbulent wake- vortex\\/exhaust-plume interaction for subsonic and High Speed Civil Transport (HSCT) airplane, and wake- vortex\\/exhaust-plume interaction with the ground are carried out. The three-dimensional, compressible Reynolds-Averaged Navier-Stokes (RANS) equations are solved using the implicit, upwind, Roe-flux-differencing, finite-volume scheme. The turbulence models of Baldwin and Lomax, one-equation model of

Ihab Gaber Adam

1998-01-01

26

An Improved Wake Vortex Tracking Algorithm for Multiple Aircraft  

NASA Technical Reports Server (NTRS)

The accurate tracking of vortex evolution from Large Eddy Simulation (LES) data is a complex and computationally intensive problem. The vortex tracking requires the analysis of very large three-dimensional and time-varying datasets. The complexity of the problem is further compounded by the fact that these vortices are embedded in a background turbulence field, and they may interact with the ground surface. Another level of complication can arise, if vortices from multiple aircrafts are simulated. This paper presents a new technique for post-processing LES data to obtain wake vortex tracks and wake intensities. The new approach isolates vortices by defining "regions of interest" (ROI) around each vortex and has the ability to identify vortex pairs from multiple aircraft. The paper describes the new methodology for tracking wake vortices and presents application of the technique for single and multiple aircraft.

Switzer, George F.; Proctor, Fred H.; Ahmad, Nashat N.; LimonDuparcmeur, Fanny M.

2010-01-01

27

Turbulence Climatology at Dallas/Ft.Worth (DFW) Airport: Implications for a Departure Wake Vortex Spacing System  

NASA Technical Reports Server (NTRS)

Potential adaptive wake vortex spacing systems may need to rely on wake vortex decay rather than wake vortex transport in reducing wake separations. A wake vortex takeoff-spacing system in particular will need to rely on wake decay. Ambient turbulence is the primary influence on wake decay away from the ground. This study evaluated 18 months of ambient turbulence measurements at Dallas/Ft. Worth (DFW) Airport. The measurements show minor variation in the turbulence levels at various times of the year or times of the day for time periods when a departure system could be used. Arrival system operation was also examined, and a slightly lower overall turbulence level was found as compared to departure system benefit periods. The Sarpkaya model, a validated model of wake vortex behavior, was applied to various turbulence levels and compared to the DFW turbulence statistics. The results show that wake vortices from heavy aircraft on takeoff should dissipate within one minute for the majority of the time and will rarely last two minutes. These results will need to be verified by wake vortex measurements on departure.

Perras, G. H.; Dasey, T. J.

2000-01-01

28

Feasibility of wake vortex monitoring systems for air terminals  

NASA Technical Reports Server (NTRS)

Wake vortex monitoring systems, especially those using laser Doppler sensors, were investigated. The initial phases of the effort involved talking with potential users (air traffic controllers, pilots, etc.) of a wake vortex monitoring system to determine system requirements from the user's viewpoint. These discussions involved the volumes of airspace to be monitored for vortices, and potential methods of using the monitored vortex data once the data are available. A subsequent task led to determining a suitable mathematical model of the vortex phenomena and developing a mathematical model of the laser Doppler sensor for monitoring the vortex flow field. The mathematical models were used in combination to help evaluate the capability of laser Doppler instrumentation in monitoring vortex flow fields both in the near vicinity of the sensor (within 1 kilometer and at long ranges(10 kilometers).

Wilson, D. J.; Shrider, K. R.; Lawrence, T. R.

1972-01-01

29

Progress Towards the Investigation of Technical Issues Relevant to the Design of an Aircraft Wake Vortex Advisory System (WakeVAS)  

NASA Technical Reports Server (NTRS)

Wake vortex separations applied to aircraft during instrument operations have been shown to potentially introduce inefficiencies in air traffic operations during certain weather conditions conducive to short duration wake hazards between pairs of landing aircraft. NASA Langley Research Center (LaRC) demonstrated an integration of technologies that provided real-time observations and predictions of aircraft wake behavior, from which reduced wake spacing from the current criteria was derived. In order to take this proof of concept to an operational prototype system, NASA has been working in cooperation with the FAA and other government and industry members to design operational concepts for a Wake Vortex Advisory System (WakeVAS). In addition to concept development, open research issues are being addressed and activities to quantify system requirements and specifications are currently underway. This paper describes the technological issues relevant to WakeVAS development and current NASA efforts to address these issues.

Rutishauser, David K.

2003-01-01

30

Vortex wake and exhaust plume interaction, including ground effect  

NASA Astrophysics Data System (ADS)

Computational modeling and studies of the near-field wake-vortex turbulent flows, far-field turbulent wake- vortex/exhaust-plume interaction for subsonic and High Speed Civil Transport (HSCT) airplane, and wake- vortex/exhaust-plume interaction with the ground are carried out. The three-dimensional, compressible Reynolds-Averaged Navier-Stokes (RANS) equations are solved using the implicit, upwind, Roe-flux-differencing, finite-volume scheme. The turbulence models of Baldwin and Lomax, one-equation model of Spalart and Allmaras and two-equation shear stress transport model of Menter are implemented with the RANS solver for turbulent-flow modeling. For the near-field study, computations are carried out on a fine grid for a rectangular wing with a NACA-0012 airfoil section and a rounded tip. The focus of study is the tip-vortex development, the near-wake-vortex roll-up, and validation of the results with the available experimental data. For the far-field study, the computations of wake-vortex interaction with the exhaust-plume of a single engine of a medium-size subsonic aircraft in a holding condition and two engines of a HSCT in a cruise condition are carried out using an overlapping zonal method for several miles downstream. The overlapping zonal method has been carefully developed and investigated for accurate and efficient calculations of the far-field wake-vortex flow. The results of the subsonic flow are compared with those of a Parabolized Navier-Stokes (PNS) solver known as the UNIWAKE code. Next, the problem of wake-vortex/ground interaction is investigated. For the simulation of this problem, typical velocity profiles of a tip vortex with and without the exhaust-plume temperature profiles are used for inflow boundary conditions and the computations are carried out using the overlapping zonal method for long distances downstream. The effects of the exhaust-plume temperature on the vortex descent, ground boundary-layer separation, vortex rebound and vortex decay are studied and validated with the available experimental data. A parametric study, which covers the effects of atmospheric conditions such as axial wind, crosswind, wind shear, turbulence and, Reynolds number on vortex motion and dynamics near the ground, is also carried out.

Adam, Ihab Gaber

31

Updated Results for the Wake Vortex Inverse Model  

NASA Technical Reports Server (NTRS)

NorthWest Research Associates (NWRA) has developed an Inverse Model for inverting aircraft wake vortex data. The objective of the inverse modeling is to obtain estimates of the vortex circulation decay and crosswind vertical profiles, using time history measurements of the lateral and vertical position of aircraft vortices. The Inverse Model performs iterative forward model runs using estimates of vortex parameters, vertical crosswind profiles, and vortex circulation as a function of wake age. Iterations are performed until a user-defined criterion is satisfied. Outputs from an Inverse Model run are the best estimates of the time history of the vortex circulation derived from the observed data, the vertical crosswind profile, and several vortex parameters. The forward model, named SHRAPA, used in this inverse modeling is a modified version of the Shear-APA model, and it is described in Section 2 of this document. Details of the Inverse Model are presented in Section 3. The Inverse Model was applied to lidar-observed vortex data at three airports: FAA acquired data from San Francisco International Airport (SFO) and Denver International Airport (DEN), and NASA acquired data from Memphis International Airport (MEM). The results are compared with observed data. This Inverse Model validation is documented in Section 4. A summary is given in Section 5. A user's guide for the inverse wake vortex model is presented in a separate NorthWest Research Associates technical report (Lai and Delisi, 2007a).

Robins, Robert E.; Lai, David Y.; Delisi, Donald P.; Mellman, George R.

2008-01-01

32

Analog Processing Assembly for the Wake Vortex Lidar Experiment  

NASA Technical Reports Server (NTRS)

The Federal Aviation Administration (FAA) and NASA have initiated a joint study in the development of reliable means of tracking, detecting, measuring, and predicting trailing wake-vortices of commercial aircraft. Being sought is an accurate model of the wake-vortex hazard, sufficient to increase airport capacity by reducing minimum safe spacings between planes. Several means of measurement are being evaluated for application to wake-vortex detection and tracking, including Doppler RADAR (Radio Detection and Ranging) systems, 2-micron Doppler LIDAR (Light Detection And Ranging) systems, and SODAR (Sound Detection And Ranging) systems. Of specific interest there is the lidar system, which has demonstrated numerous valuable capabilities as a vortex sensor Aerosols entrained in the vortex flow make the wake velocity signature visible to the lidar, (the observable lidar signal is essentially a measurement of the line-of-sight velocity of the aerosols). Measurement of the occurrence of a wake vortex requires effective reception and monitoring of the beat signal which results from the frequency-offset between the transmitted pulse and the backscattered radiation. This paper discusses the mounting, analysis, troubleshooting, and possible use of an analog processing assembly designed for such an application.

Stowe, Edwood G.

1995-01-01

33

Hazard Criteria for Wake Vortex Encounters.  

National Technical Information Service (NTIS)

A piloted, motion-base simulation was conducted to evaluate the ability of simulators to produce realistic vortex encounters and to develop criteria to define hazardous encounters. Evaluation of the simulation by pilots experienced in vortex encounters co...

R. I. Sammonds G. W. Stinnett

1975-01-01

34

Computational hydrodynamics of animal swimming: boundary element method and three-dimensional vortex wake structure.  

PubMed

The slender body theory, lifting surface theories, and more recently panel methods and Navier-Stokes solvers have been used to study the hydrodynamics of fish swimming. This paper presents progress on swimming hydrodynamics using a boundary integral equation method (or boundary element method) based on potential flow model. The unsteady three-dimensional BEM code 3DynaFS that we developed and used is able to model realistic body geometries, arbitrary movements, and resulting wake evolution. Pressure distribution over the body surface, vorticity in the wake, and the velocity field around the body can be computed. The structure and dynamic behavior of the vortex wakes generated by the swimming body are responsible for the underlying fluid dynamic mechanisms to realize the high-efficiency propulsion and high-agility maneuvering. Three-dimensional vortex wake structures are not well known, although two-dimensional structures termed 'reverse Karman Vortex Street' have been observed and studied. In this paper, simulations about a swimming saithe (Pollachius virens) using our BEM code have demonstrated that undulatory swimming reduces three-dimensional effects due to substantially weakened tail tip vortex, resulting in a reverse Karman Vortex Street as the major flow pattern in the three-dimensional wake of an undulating swimming fish. PMID:11733166

Cheng, J Y; Chahine, G L

2001-12-01

35

Tip Vortex and Wake Characteristics of a Counterrotating Open Rotor  

NASA Technical Reports Server (NTRS)

One of the primary noise sources for Open Rotor systems is the interaction of the forward rotor tip vortex and blade wake with the aft rotor. NASA has collaborated with General Electric on the testing of a new generation of low noise, counterrotating Open Rotor systems. Three-dimensional particle image velocimetry measurements were acquired in the intra-rotor gap of the Historical Baseline blade set. The velocity measurements are of sufficient resolution to characterize the tip vortex size and trajectory as well as the rotor wake decay and turbulence character. The tip clearance vortex trajectory is compared to results from previously developed models. Forward rotor wake velocity profiles are shown. Results are presented in a form as to assist numerical modeling of Open Rotor system aerodynamics and acoustics.

VanZante, Dale E.; Wernet, Mark P.

2012-01-01

36

Feasibility of an onboard wake vortex avoidance system  

NASA Technical Reports Server (NTRS)

It was determined that an onboard vortex wake detection system using existing, proven instrumentation is technically feasible. This system might be incorporated into existing onboard systems such as a wind shear detection system, and might provide the pilot with the location of a vortex wake, as well as an evasive maneuver so that the landing separations may be reduced. It is suggested that this system might be introduced into our nation's commuter aircraft fleet and major air carrier fleet and permit a reduction of current landing separation standards, thereby reducing takeoff and departure delays.

Bilanin, Alan J.; Teske, Milton E.; Curtiss, Howard C., Jr.

1987-01-01

37

Wake Vortex Transport in Proximity to the Ground  

NASA Technical Reports Server (NTRS)

A sensitivity study for aircraft wake vortex transport has been conducted using a validated large eddy simulation (LES) model. The study assumes neutrally stratified and nonturbulent environments and includes the consequences of the ground. The numerical results show that the nondimensional lateral transport is primarily influenced by the magnitude of the ambient crosswind and is insensitive to aircraft type. In most of the simulations, the ground effect extends the lateral position of the downwind vortex about one initial vortex spacing (b(sub o)) in the downstream direction. Further extension by as much as one b(sub o) occurs when the downwind vortex remains 'in ground effect' (IGE) for relatively long periods of time. Results also show that a layer-averaged ambient wind velocity can be used to bound the time for lateral transport of wake vortices to insure safe operations on a parallel runway.

Hamilton, David W.; Proctor, Fred H.

2000-01-01

38

Rotor Wake Vortex Definition Using 3C-PIV Measurements: Corrected for Vortex Orientation  

NASA Technical Reports Server (NTRS)

Three-component (3-C) particle image velocimetry (PIV) measurements, within the wake across a rotor disk plane, are used to determine wake vortex definitions important for BVI (Blade Vortex Interaction) and broadband noise prediction. This study is part of the HART II test program conducted using a 40 percent scale BO-105 helicopter main rotor in the German-Dutch Wind Tunnel (DNW). In this paper, measurements are presented of the wake vortex field over the advancing side of the rotor operating at a typical descent landing condition. The orientations of the vortex (tube) axes are found to have non-zero tilt angles with respect to the chosen PIV measurement cut planes, often on the order of 45 degrees. Methods for determining the orientation of the vortex axis and reorienting the measured PIV velocity maps (by rotation/projection) are presented. One method utilizes the vortex core axial velocity component, the other utilizes the swirl velocity components. Key vortex parameters such as vortex core size, strength, and core velocity distribution characteristics are determined from the reoriented PIV velocity maps. The results are compared with those determined from velocity maps that are not corrected for orientation. Knowledge of magnitudes and directions of the vortex axial and swirl velocity components as a function of streamwise location provide a basis for insight into the vortex evolution.

Burley, Casey L.; Brooks, Thomas F.; vanderWall, Berend; Richard, Hughues Richard; Raffel, Markus; Beaumier, Philippe; Delrieux, Yves; Lim, Joon W.; Yu, Yung H.; Tung, Chee

2003-01-01

39

Assessment of a wake vortex flight test program  

NASA Technical Reports Server (NTRS)

A proposed flight test program to measure the characteristics of wake vortices behind a T-33 aircraft was investigated. A number of facets of the flight tests were examined to define the parameters to be measured, the anticipated vortex characteristics, the mutual interference between the probe aircraft and the wake, the response of certain instruments to be used in obtaining measurements, the effect of condensation on the wake vortices, and methods of data reduction. Recommendations made as a result of the investigation are presented.

Spangler, S. B.; Dillenius, M. F. E.; Schwind, R. G.; Nielsen, J. N.

1974-01-01

40

Wake evolution and trailing vortex instabilities  

NASA Astrophysics Data System (ADS)

The production losses and inhomogeneous loads of wind power turbines placed in the wake of another turbine is a well-known problem when building new wind power farms, and a subject of intensive research. The present work aims at developing an increased understanding of the behaviour of turbine wakes, with special regard to wake evolution and the stability of the trailing vortices. Single point velocity measurements with hot-wire anemometry were performed in the wake of a small-scale model turbine. The model was placed in the middle of the wind tunnel test section, outside the boundary layers from the wind tunnel walls. In order to study the stability of the wake and the trailing vortices, a disturbance was introduced at the end of the nacelle. This was accomplished through two orifices perpendicular to the main flow, which were connected to a high-pressure tank and two fast-switching valves. Both varicose and sinusoidal modes of different frequencies could be triggered. By also triggering the measurements on the blade passage, the meandering of the wake and the disturbance frequency, phase averaged results could be computed. The results for different frequencies as well as studies of wake evolution will be presented.

Odemark, Ylva; Fransson, Jens H. M.

2011-11-01

41

Vortex age as a wake turbulence scaling parameter  

NASA Technical Reports Server (NTRS)

Tests were conducted in the Virginia Tech Stability Wind Tunnel to determine the significance of vortex age as a scaling parameter in wake turbulence development and dissipation. Vortex structure was measured over a range of three angles of attack, three free-stream speeds, and seven downstream positions from 2 to 30 chordlengths using an NACA 0012 wing and a five hole yawhead probe. The resulting data indicates that vortex age is not a self-sufficient scaling parameter but a free-stream velocity influence also exists at higher angles of attack which cannot be explained in terms of Re or M.

Marchman, J. F., III; Marshall, J. R.

1974-01-01

42

Perspective: Numerical simulation of wakes and blade-vortex interaction  

SciTech Connect

A method for simulating incompressible flows past airfoils and their wakes is described. Vorticity panels are used to represent the body, and vortex blobs (vortex points with their singularities removed) are used to represent the wake. The procedure can be applied to the simulation of completely attached flow past an oscillating airfoil. The rate at which vorticity is shed from the trailing edge of the airfoil into the wake is determined by simultaneously requiring the pressure along the upper and lower surface streamlines to approach the same value at the trailing edge and the circulation around both the airfoil and its wake to remain constant. The motion of the airfoils is discretized, and a vortex is shed from the trailing edge at each time step. The vortices are convected at the local velocity of fluid particles, a procedure that renders the pressure continuous in an inviscid fluid. When the vortices in the wake begin to separate they are split into more vortices, and when they begin to collect they are combined. The numerical simulation reveals that the wake, which is originally smooth, eventually coils, or wraps, around itself, primarily under the influence of the velocity it induces on itself, and forms regions of relatively concentrated vorticity. Although discrete vortices are used to represent the wake, the spatial density of the vortices is so high that the computed velocity profiles across a typical region of concentrated vorticity are quite smooth. Although the computed wake evolves in an entirely inviscid model of the flowfield, these profiles appear to have a viscous core. As an application, a simulation of the interaction between vorticity in the oncoming stream and a stationary airfoil is also discussed.

Dong, B. (Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States). Dept. of Engineering Science and Mechanics); Mook, D.T.

1994-03-01

43

Aircraft Wake Vortex Measurements at Denver International Airport  

NASA Technical Reports Server (NTRS)

Airport capacity is constrained, in part, by spacing requirements associated with the wake vortex hazard. 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 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.

Dougherty, Robert P.; Wang, Frank Y.; Booth, Earl R.; Watts, Michael E.; Fenichel, Neil; D'Errico, Robert E.

2004-01-01

44

Unsteady Free-Wake Vortex Particle Model for HAWT  

NASA Astrophysics Data System (ADS)

In the design of horizontal axis wind turbines (HAWT) one problem is to determine the aeroelastic behaviour of the rotor blades for the various wind inflow conditions. A step in this process is to predict with accuracy the aerodynamic loads on the blades. The Vortex Lattice Method (VLM) provides a transparent investigation concerning the role of various physical parameters which influence the aerodynamic problem. In this paper we present a method for the calculation of the non-uniform induced downwash of a HAWT rotor using the vortex ring model for the lifting surface coupled with an unsteady free-wake vortex particle model. Comparative studies between results obtained with different models of wake for a generic HAWT are presented.

Bogateanu, R.; Frunzulic?, F.; Cardos, V.

2010-09-01

45

Near wake vortex dynamics of a hovering hawkmoth  

NASA Astrophysics Data System (ADS)

Numerical investigation of vortex dynamics in near wake of a hovering hawkmoth and hovering aerodynamics is conducted to support the development of a biology-inspired dynamic flight simulator for flapping wing-based micro air vehicles. Realistic wing-body morphologies and kinematics are adopted in the numerical simulations. The computed results show 3D mechanisms of vortical flow structures in hawkmoth-like hovering. A horseshoe-shaped primary vortex is observed to wrap around each wing during the early down- and upstroke; the horseshoe-shaped vortex subsequently grows into a doughnut-shaped vortex ring with an intense jet-flow present in its core, forming a downwash. The doughnut-shaped vortex rings of the wing pair eventually break up into two circular vortex rings as they propagate downstream in the wake. The aerodynamic yawing and rolling torques are canceled out due to the symmetric wing kinematics even though the aerodynamic pitching torque shows significant variation with time. On the other hand, the time-varying the aerodynamics pitching torque could make the body a longitudinal oscillation over one flapping cycle.

Aono, Hikaru; Shyy, Wei; Liu, Hao

2009-02-01

46

On the Development of Turbulent Wakes from Vortex Streets  

NASA Technical Reports Server (NTRS)

Wake development behind circular cylinders at Reynolds numbers from 40 to 10,000 was investigated in a low-speed wind tunnel. Standard hotwire techniques were used to study the velocity fluctuations. The Reynolds number range of periodic vortex shedding is divided into two distinct subranges. At R = 40 to 150, called the stable range, regular vortex streets are formed and no turbulent motion is developed. The range R = 150 to 300 is a transition range to a regime called the irregular range, in which turbulent velocity fluctuations accompany the periodic formation of vortices. The turbulence is initiated by laminar-turbulent transition in the free layers which spring from the separation points on the cylinder. This transition first occurs in the range R = 150 to 300. Spectrum and statistical measurements were made to study the velocity fluctuations. In the stable range the vortices decay by viscous diffusion. In the irregular range the diffusion is turbulent and the wake becomes fully turbulent in 40 to 50 diameters downstream. It was found that in the stable range the vortex street has a periodic spanwise structure. The dependence of shedding frequency on velocity was successfully used to measure flow velocity. Measurements in the wake of a ring showed that an annular vortex street is developed.

Roshko, Anatol

1953-01-01

47

Use of Individual Flight Corridors to Avoid Vortex Wakes  

NASA Technical Reports Server (NTRS)

Vortex wakes of aircraft pose a hazard to following aircraft until the energetic parts of their flow fields have decayed to a harmless level. It is suggested here that in-trail spacings between aircraft can be significantly and safely reduced by designing an individual, vortex-free flight corridor for each aircraft. Because each aircraft will then have its own flight corridor, which is free of vortex wakes while in use by the assigned aircraft, the time intervals between aircraft operations can be safely reduced to the order of seconds. The productivity of airports can then be substantially increased. How large the offset distances between operational corridors need to be to have them vortex free, and how airports need to be changed to accommodate an individual flight-corridor process for landing and takeoff operations, are explored. Estimates are then made of the productivity of an individual flight-corridor system as a function of the in-trail time interval between operations for various values of wake decay time, runway width, and the velocity of a sidewind. The results confirm the need for short time intervals between aircraft operations if smaller offset distances and increased productivity are to be achieved.

Rossow, Vernon J.

2001-01-01

48

The Wake Vortex Prediction and Monitoring System WSVBS  

NASA Astrophysics Data System (ADS)

Design and performance of the Wake Vortex Prediction and Monitoring System WSVBS are described. The WSVBS has been developed to tactically increase airport capacity for approach and landing on closely-spaced parallel runways. It is thought to dynamically adjust aircraft separations dependent on weather conditions and the resulting wake vortex behaviour without compromising safety. The WSVBS consists of components that consider meteorological conditions, aircraft glide path adherence, aircraft parameter combinations representing aircraft weight categories, the resulting wake-vortex behaviour, the surrounding safety areas, wake vortex monitoring, and the integration of the predictions into the arrival manager. The WSVBS has been designed and applied to Frankfurt Airport. However, its components are generic and can well be adjusted to any runway system and or airport location. The prediction horizon is larger than 45 min (as required by air traffic control) and updated every 10 minutes. It predicts the concepts of operations and procedures established by DFS and it further predicts additional temporal separations for in-trail traffic. A specific feature of the WSVBS is the usage of both measured and predicted meteorological quantities as input to wake vortex prediction. In ground proximity where the probability to encounter wake vortices is highest, the wake predictor employs measured environmental parameters that yield superior prediction results. For the less critical part aloft, which can not be monitored completely by instrumentation, the meteorological parameters are taken from dedicated numerical terminal weather predictions. The wake vortex model predicts envelopes for vortex position and strength which implicitly consider the quality of the meteorological input data. This feature is achieved by a training procedure which employs statistics of measured and predicted meteorological parameters and the resulting wake vortex behaviour. The WSVBS combines various conservative elements that presumably lead to a very high overall safety level of the WSVBS. The combination of these conservative measures certainly leads to a very high but currently unknown overall safety. Once the methodology of a comprehensive risk analysis will be established, it is planned to adjust all components to appropriate and consistent confidence levels. The WSVBS has demonstrated its functionality at Frankfurt airport during 66 days in the period from 18/12/06 until 28/02/07. The performance test indicates that (i) the system ran stable - no forecast breakdowns occurred, (ii) aircraft separations could have been reduced in 75% of the time compared to ICAO standards, (iii) reduced separation procedures could have been continuously applied for at least several tens of minutes and up to several hours occasionally, (iv) the predictions were correct as for about 1100 landings observed during 16 days no warnings occurred from the LIDAR. Fast-time simulations reveal that adapted concepts of operation yield significant reductions in delay and/or an increase in capacity to 3% taking into account the real traffic mix and operational constraints in the period of one month. Before the WSVBS can be handed over for final adaptations to become a customized fully operational system some further steps are planned. A risk analysis needs to be pursued to convince all stakeholders of the usefulness and capabilities of the system.

Gerz, T.; Holzäpfel, F.

2009-09-01

49

The vortex wake of a 'hovering' model hawkmoth  

PubMed Central

Visualization experiments with Manduca sexta have revealed the presence of a leading-edge vortex and a highly three-dimensional flow pattern. To further investigate this important discovery, a scaled-up robotic insect was built (the 'flapper') which could mimic the complex movements of the wings of a hovering hawkmoth. Smoke released from the leading edge of the flapper wing revealed a small but strong leading-edge vortex on the downstroke. This vortex had a high axial flow velocity and was stable, separating from the wing at approximately 75 per cent of the wing length. It connected to a large, tangled tip vortex, extending back to a combining stopping and starting vortex from pronation. At the end of the downstroke, the wake could be approximated as one vortex ring per wing. Based on the size and velocity of the vortex rings, the mean lift force during the downstroke was estimated to be about 1.5 times the body weight of a hawkmoth, confirming that the downstroke is the main provider of lift force.

Berg, C. van den; Ellington, C. P.

1997-01-01

50

Numerical Modeling Studies of Wake Vortex Transport and Evolution Within the Planetary Boundary Layer  

NASA Technical Reports Server (NTRS)

The fundamental objective of this research is study behavior of aircraft wake vortices within atmospheric boundary layer (ABL) in support of developing the system, Aircraft VOrtex Spacing System (AVOSS), under NASA's Terminal Area Productivity (TAR) program that will control aircraft spacing within the narrow approach corridors of airports. The purpose of the AVOSS system is to increase airport capacity by providing a safe reduction in separation of aircraft compared to the now-existing flight rules. In our first funding period (7 January 19994 - 6 April 1997), we have accomplished extensive model development and validation of ABL simulations. Using the validated model, in our second funding period (7 April 1997 - 6 April 2000) we have investigated the effects of ambient atmospheric turbulence on vortex decay and descent, Crow instability, and wake vortex interaction with the ground. Recognizing the crucial influence of ABL turbulence on wake vortex behavior, we have also developed a software generating vertical profiles of turbulent kinetic energy (TKE) or energy dissipation rate (EDR), which are, in turn, used as input data in the AVOSS prediction algorithms.

Lin, Yuh-Lang; Arya, S. Pal; Kaplan, Michael L.; Han, Jongil

2000-01-01

51

Characterizing the Hazard of a Wake Vortex Encounter  

NASA Technical Reports Server (NTRS)

The National Aeronautics and Space Administration (NASA) is conducting research with the goal of enabling safe improvements in the capacity of the nation's air transportation system. The wake vortex upset hazard is an important factor in establishing the minimum safe spacing between aircraft during landing and take-off operations, thus impacting airport capacity. Static and free-flight wind tunnel tests and flight tests have provided an extensive data set for improved understanding of vortex encounter dynamics and simulation. Piloted and batch simulation studies are also ongoing to establish a first-order hazard metric and determine the limits of an operationally acceptable wake induced upset. This paper outlines NASA's research in these areas.

Vicroy, Dan D.; Brandon, Jay; Greene, George; Rivers, Robert; Shah, Gautam; Stewart, Eric; Stuever, Robert

1998-01-01

52

Analysis of the Radar Reflectivity of Aircraft Vortex Wakes  

NASA Technical Reports Server (NTRS)

Radar has been proposed as a way to track wake vortices to reduce aircraft spacing and tests have revealed radar echoes from aircraft wakes in clear air. The results are always interpreted qualitatively using Tatarski's theory of weak scattering by isotropic atmospheric turbulence. The goal of the present work was to predict the value of the radar cross-section (RCS) using simpler models. This is accomplished in two steps. First, the refractive index is obtained. Since the structure of the aircraft wakes is different from atmospheric turbulence, three simple mechanisms specific to vortex wakes are considered: (1) Radial density gradient in a two-dimensional vortex, (2) three-dimensional fluctuations in the vortex cores, and (3) Adiabatic transport of the atmospheric fluid in a two-dimensional oval surrounding the pair of vortices. The index of refraction is obtained more precisely for the two-dimensional mechanisms than for the three-dimensional ones. In the second step, knowing the index of refraction, a scattering analysis is performed. Tatarski's weak scattering approximation is kept but the usual assumptions of a far-field and a uniform incident wave are dropped. Neither assumption is generally valid for a wake that is coherent across the radar beam. For analytical insight, a simpler approximation that invokes, in addition to weak scattering, the far-field and wide cylindrical beam assumptions, is also developed and compared with the more general analysis. The predicted RCS values for the oval surround the vortices (mechanism C) agree with the experiments of Bilson conducted over a wide range of frequencies. However, the predictions have a cut-off away from normal incidence which is not present in the measurements. Estimates suggest that this is due to turbulence in the baroclinic vorticity generated at the boundary of the oval. The reflectivity of a vortex itself (mechanism A) is comparable to that of the oval (mechanism C) but cuts-off at frequencies lower than those considered in all the experiments to date. The RCS of a vortex happens to peak at the frequency (about 49 MHz) where atmospheric radars (known as ST radars) operate and so the present prediction could be verified in the future. Finally , we suggest that hot engine exhaust could increase RCE by 40 db and reveal vortex circulation, provided its mixing with the surroundings is prevented in the laminarising flow of the vortices.

Shariff, Karim; Wray, Alan; Yan, Jerry (Technical Monitor)

2000-01-01

53

Numerical simulation of vortex structures in a near wake  

NASA Astrophysics Data System (ADS)

Numerical simulation of steady and unsteady regimes for subsonic and transonic flows is realized for the classical problem concerned with the flow near a cylinder. The conservative numerical schemes are used. The large-scale vortex structures in a near-wake bluff body observed experimentally in real streams are investigated within the inviscid gas framework. For the small and moderate Reynolds number, the numerical simulation of nonseparated and separated stationary and unsteady regimes is based on the Navier-Stokes model.

Babakov, A. V.

54

Flow visualization of vortex interactions in multiple vortex wakes behind aircraft  

NASA Technical Reports Server (NTRS)

A flow visualization technique was developed which allows the nature of lift-generated wakes behind aircraft models to be investigated. The technique was applied to models being towed underwater in a ship model basin. Seven different configurations of a small-scale model of a 747 transport aircraft were used to allow observation of typical vortex interactions and merging in multiple vortex wakes. It was established that the motion of the wake vortices is often sensitive to small changes in either wing span loading or model attitude. Landing gear deployement was found to cause a far-field reformation of vorticity behind a model configuration which dissipated concentrated vorticity in the near-field wake. Alleviation of wake vorticity is achievable by configuring the wing span loading to cause the wake vortices to move in paths that result in their interactions and merging. The vortices shed from the horizontal stabilizer always moved down rapidly into the wake and merged with the other vortices, primarily the inboard flap vortices.

Ciffone, D. L.; Lonzo, C., Jr.

1975-01-01

55

Vortex age as a wake turbulence scaling parameter  

NASA Technical Reports Server (NTRS)

Research which was conducted to determine the significance of vortex age as a scaling parameter in wake turbulence development and dissipation is reported. Tests were conducted at three angles of attack, three free stream speeds, and seven downstream positions from 2 to 30 chordlengths using an NACA 0012 wing and a five hole yawhead pitot probe. The end surface of the wing tip was flat. Speeds were selected to give a predetermined range of vortex ages. The complete velocity structure of the vortex was measured at each station and speed. The resulting plots of maximum tangential velocity and vortex core diameter versus downstream distance and vortex age indicate that vortex age is not a self sufficient scaling parameter. In addition to the expected effect of lift coefficient there is also a definite free stream speed influence at high wing angles of attack. The exact cause and nature of this effect is not fully understood, but it does not appear to be explainable in terms of Mach number or Reynolds number; however, the influence of tip edge shape on spanwise flow separation appears to be an important factor.

Marshall, J. R.; Marchman, J. F., III

1973-01-01

56

Wake Vortex Tracking Using a 35 GHz Pulsed Doppler Radar  

NASA Technical Reports Server (NTRS)

A 35 GHz, pulsed-Doppler radar system has been designed and assembled for wake vortex detection and tracking in low visibility conditions. Aircraft wake vortices continue to be an important factor in determining safe following distances or spacings for aircraft in the terminal area. Currently, under instrument meteorological conditions (IMC), aircraft adhere to conservative, fixed following-distance guidelines based primarily on aircraft weight classifications. When ambient conditions are such that vortices will either drift or dissipate, leaving the flight corridor clear, the prescribed spacings are unnecessarily long and result in decreased airport throughput. There is a potential for significant airport efficiency improvement, if a system can be employed to aid regulators and pilots in setting safe and efficient following distances based on airport conditions. The National Aeronautics and Space Administration (NASA), the Federal Aviation Agency, and Volpe National Transportation Systems Center have promoted and worked to develop systems that would increase airport capacity and provide for safe reductions in aircraft separation. The NASA Aircraft Vortex Spacing System (AVOSS), a wake vortex spacing system that can provide dynamic adjustment of spacings based on real-time airport weather conditions, has demonstrated that Lidar systems can be successfully used to detect and track vortices in clear air conditions. To fill the need for detection capability in low-visibility conditions, a 35 GHz, pulsed-Doppler radar system is being investigated for use as a complimentary, low-visibility sensor for wake vortices. The radar sensor provides spatial and temporal information similar to that provided by Lidar, but under weather conditions that a Lidar cannot penetrate. Currently, we are analyzing the radar design based upon the data and experience gained during the wake vortex Lidar deployment with AVOSS at Dallas/Fort Worth International Airport. As part of this study, two numerical models were utilized in system simulations. The results of this study improve our understanding of the method of detection, resolution requirements for range and azimuth, pulse compression, and performance prediction. Simulations applying pulse compression techniques show that detection is good in heavy fog to greater than 2000 m. Both compressed and uncompressed short pulses show the vortex structure. To explore operational challenges, siting and scanning strategies were also analyzed. Simulation results indicate that excellent wake vortex detection, tracking and classification is possible in drizzle (+15 dBZ) and heavy fog (- 13 dBZ) using short pulse techniques (<99ns) at ranges on the order of 900 m, with a modest power of 500 W output. At 1600 m, detection can be expected at reflectivities as low as -13 dBZ (heavy fog). The radar system, as designed and built, has the potential to support field studies of a wake vortex spacing system in low-visibility conditions ranging from heavy fog to rain, when sited within 2000m of the flight path.

Neece, Robert T.; Britt, Charles L.; White, Joseph H.; Mudukutore, Ashok; Nguyen, Chi; Hooper, Bill

2005-01-01

57

An Aircraft Vortex Spacing System (AVOSS) for Dynamical Wake Vortex Spacing Criteria  

NASA Technical Reports Server (NTRS)

A concept is presented for the development and implementation of a prototype Aircraft Vortex Spacing System (AVOSS). The purpose of the AVOSS is to use current and short-term predictions of the atmospheric state in approach and departure corridors to provide, to ATC facilities, dynamical weather dependent separation criteria with adequate stability and lead time for use in establishing arrival scheduling. The AVOSS will accomplish this task through a combination of wake vortex transport and decay predictions, weather state knowledge, defined aircraft operational procedures and corridors, and wake vortex safety sensors. Work is currently underway to address the critical disciplines and knowledge needs so as to implement and demonstrate a prototype AVOSS in the 1999/2000 time frame.

Hinton, D. A.

1996-01-01

58

History of Wake Vortex Research: Problems and Accomplishments  

NASA Technical Reports Server (NTRS)

Significant progress has been made in understanding vortex behavior but much remains to be done. The primary challenge is to bring "science" into operational use. Success will require cooperation from a diverse group of organizations.

Greene, George C.

1997-01-01

59

Modeling of Wake-vortex Aircraft Encounters. Appendix B  

NASA Technical Reports Server (NTRS)

There are more people passing through the world's airports today than at any other time in history. With this increase in civil transport, airports are becoming capacity limited. In order to increase capacity and thus meet the demands of the flying public, the number of runways and number of flights per runway must be increased. In response to the demand, the National Aeronautics and Space Administration (NASA), in conjunction with the Federal Aviation Administration (FAA), airport operators, and the airline industry are taking steps to increase airport capacity without jeopardizing safety. Increasing the production per runway increases the likelihood that an aircraft will encounter the trailing wake-vortex of another aircraft. The hazard of a wake-vortex encounter is that heavy load aircraft can produce high intensity wake turbulence, through the development of its wing-tip vortices. A smaller aircraft following in the wake of the heavy load aircraft will experience redistribution of its aerodynamic load. This creates a safety hazard for the smaller aircraft. Understanding this load redistribution is of great importance, particularly during landing and take-off. In this research wake-vortex effects on an encountering 10% scale model of the B737-100 aircraft are modeled using both strip theory and vortex-lattice modeling methods. The models are then compared to wind tunnel data that was taken in the 30ft x 60ft wind tunnel at NASA Langley Research Center (LaRC). Comparisons are made to determine if the models will have acceptable accuracy when parts of the geometry are removed, such as the horizontal stabilizer and the vertical tail. A sensitivity analysis was also performed to observe how accurately the models could match the experimental data if there was a 10% error in the circulation strength. It was determined that both models show accurate results when the wing, horizontal stabilizer, and vertical tail were a part of the geometry. When the horizontal stabilizer and vertical tail were removed there were difficulties modeling the sideforce coefficient and pitching moment. With the removal of only the vertical tail unacceptable errors occurred when modeling the sideforce coefficient and yawing moment. Lift could not be modeled with either the full geometry or the reduced geometry attempts.

Smith, Sonya T.

1999-01-01

60

Numerical Simulation of the Aircraft Wake Vortex Flowfield  

NASA Technical Reports Server (NTRS)

The near wake vortex flowfield from a NACA0012 half-wing was simulated using a fully unstructured Navier-Stokes flow solver in three dimensions at a chord Reynolds number of 4.6 million and a Mach number of approximately 0.15. Several simulations were performed to examine the effect of boundary conditions, mesh resolution and turbulence scheme on the formation of wingtip vortex and its downstream propagation. The standard Spalart-Allmaras turbulence model was compared with the Dacles-Mariani and Spalart-Shur corrections for rotation and curvature effects. The simulation results were evaluated using the data from experiment performed at NASA Ames' 32in x 48in low speed wind tunnel.

Ahmad, Nashat N.; Proctor, Fred H.; Perry, R. Brad

2013-01-01

61

Wake Vortex Advisory System (WakeVAS) Evaluation of Impacts on the National Airspace System  

NASA Technical Reports Server (NTRS)

This report is one of a series that describes an ongoing effort in high-fidelity modeling/simulation, evaluation and analysis of the benefits and performance metrics of the Wake Vortex Advisory System (WakeVAS) Concept of Operations being developed as part of the Virtual Airspace Modeling and Simulation (VAMS) project. A previous study, determined the overall increases in runway arrival rates that could be achieved at 12 selected airports due to WakeVAS reduced aircraft spacing under Instrument Meteorological Conditions. This study builds on the previous work to evaluate the NAS wide impacts of equipping various numbers of airports with WakeVAS. A queuing network model of the National Airspace System, built by the Logistics Management Institute, Mclean, VA, for NASA (LMINET) was used to estimate the reduction in delay that could be achieved by using WakeVAS under non-visual meteorological conditions for the projected air traffic demand in 2010. The results from LMINET were used to estimate the total annual delay reduction that could be achieved and from this, an estimate of the air carrier variable operating cost saving was made.

Smith, Jeremy C.; Dollyhigh, Samuel M.

2005-01-01

62

Model Validation of Wake-Vortex/Aircraft Encounters  

NASA Technical Reports Server (NTRS)

Wake-vortex effects on an 10% scale model of the B737-100 aircraft are calculated using both strip theory and vortex-lattice methods. The results are then compared to data taken in the 30ft x 60ft wind tunnel at NASA Langley Research Center (LaRC). The accuracy of the models for a reduced geometry, such with the horizontal stabilizer and the vertical tail removed, is also investigated. Using a 10% error in the circulation strength and comparing the model's results with the experiment illustrates the sensitivity of the models to the vortex circulation strength. It was determined that both strip theory and the vortex lattice method give accurate results when all the geometrical information is used. When the horizontal stabilizer and vertical tail were removed there were difficulties modeling the sideforce coefficient and pitching moment. With the removal of only the vertical tail unacceptable errors occurred when modeling the sideforce coefficient and yawing moment. Lift could not be accurately modeled with either the full geometry or the reduced geometry.

Pete, Kimberly R.; Vicroy, Dan D.; Smith, Sonya T.

2000-01-01

63

Wake Vortex Transport and Decay in Ground Effect: Vortex Linking with the Ground  

NASA Technical Reports Server (NTRS)

Numerical simulations are carried out with a three-dimensional Large-Eddy Simulation (LES) model to explore the sensitivity of vortex decay and transport in ground effect (IGE). The vortex decay rates are found to be strongly enhanced following maximum descent into ground effect. The nondimensional decay rate is found to be insensitive to the initial values of circulation, height, and vortex separation. The information gained from these simulations is used to construct a simple decay relationship. This relationship compares well with observed data from an IGE case study. Similarly, a relationship for lateral drift due to ground effect is constructed from the LES data. In the second part of this paper, vortex linking with the ground is investigated. Our numerical simulations of wake vortices for IGE show that a vortex may link with its image beneath the ground, if the intensity of the ambient turbulence is moderate to high. This linking with the ground (which is observed in real cases)gives the appearance of a vortex tube that bends to become vertically oriented and which terminates at the ground. From the simulations conducted, the linking time for vortices in the free atmosphere; i.e., a function of ambient turbulence intensity.

Proctor, Fred H.; Hamilton, David W.; Han, Jongil

2000-01-01

64

A dynamic prescribed vortex wake model for the FAST\\/AeroDyn wind energy conversion simulation code  

Microsoft Academic Search

A Dynamic Prescribed Vortex Wake model for analysis of horizontal axis wind turbines has been developed. This model extends the HAWTDAWG steady state prescribed wake code to dynamic flows. This extension assumes wake vortices follow prescription functions valid at the time each vortex is generated. This allows modeling of dynamic wake effects known to exist. This assumption is supported through

Hugh D. Currin

2007-01-01

65

The LaRC Wake Vortex Modelling Effort  

NASA Technical Reports Server (NTRS)

The purpose of the modelling effort at NASA Langley, including goals, is outlined in this presentation. Included, is a description of the numerical model that is used for the NASA wake vortex modeling effort and the approach that is taken in order to achieve the stated goals. Also shown are: 1) a demonstration of using the model in a fog environment; 2) preliminary results from a 3-D simulation in a nonturbulent and thermally-stable environment with comparison to a comparable 2-D simulation of the same event; and 3) several validation cases from the Idaho-Falls and Memphis field studies where results from the 2-D version of the model are compared with Lidar and tower data.

Proctor, Fred

1997-01-01

66

Dynamic response of a turbulent cylinder wake to sinusoidal inflow perturbations across the vortex lock-on range  

NASA Astrophysics Data System (ADS)

Large-eddy simulations are employed to investigate the dynamic response of the turbulent wake of a circular cylinder to sinusoidal perturbations in the inflow velocity superposed on a mean component. The perturbation frequency is varied across the vortex lock-on range at a constant amplitude of 5% of the mean velocity corresponding to a Reynolds number of 2580. The effect on the instantaneous, time-averaged and phase-averaged characteristics of the near-wake flow and fluid forces on the cylinder is reported. Comparisons of the present simulations to experimental realizations show that the physics of the unsteady three-dimensional separated flow are well reproduced. The simulations capture the modification of the wake structure including the shrinking of the recirculation bubble and vortex-formation region and the enhancement of the wake fluctuations and vortex strength in the lock-on regime. These wake effects are accompanied by an increase in the steady and unsteady drag and the unsteady lift acting on the cylinder. An empirical formula for the amplification of the mean drag coefficient due to inflow perturbations and equivalent oscillations of the cylinder in a steady flow is provided from compilation of available data. Particular attention is given to the change in the timing of vortex shedding with respect to the imposed perturbation across the lock-on range in order to reveal the link between the vortex dynamics and the fluid-induced forces on the cylinder. It is shown that the phase at which vortices are shed from the cylinder shifts monotonically as a function of the perturbation frequency resulting in corresponding changes in the phase of the unsteady forces. It is further shown that the phase of the lift is directly linked to that of vortex shedding but the phase of the drag is biased by inertial forces due to added mass and induced pressure waves. Decomposition of the total in-line force to inviscid ``potential-flow'' and viscous ``vortex-drag'' components indicates that the latter exhibits a behavior which is not physically consistent. The stochastic character of vortex synchronization in turbulent wakes and the implications of the present findings for vortex-induced free in-line vibrations are also discussed.

Konstantinidis, Efstathios; Liang, Chunlei

2011-07-01

67

Effect of Velocity Ratio on the Streamwise Vortex Structures in the Wake of a Stack  

NASA Astrophysics Data System (ADS)

The time-averaged velocity and streamwise vorticity fields within the wake of a short stack were investigated in a low-speed wind tunnel using a seven-hole pressure probe. The stack was mounted normal to a ground plane and was partially immersed in a flat-plate turbulent boundary layer. The jet-to-cross-flow velocity ratio was varied from R = 0 to 3, which covered the downwash, cross-wind-dominated and jet-dominated flow regimes. In the downwash and cross-wind-dominated flow regimes, two pairs of counter-rotating streamwise vortex structures were identified within the stack wake. The tip-vortex pair and base-vortex pair were similar to those found in the wake of a finite circular cylinder, located close to the free end and the base of the stack, respectively. In the jet-dominated flow regime, a third pair of streamwise vortex structures was observed, referred to as the jet-wake vortex pair, which occurred within the jet-wake region above the free end of the stack. The jet-wake vortex pair has the same orientation as the base vortex pair and is associated with the jet rise.

Adaramola, M. S.; Sumner, D.; Bergstrom, D. J.

68

An experimental investigation of bending wave instability modes in a generic four-vortex wake  

SciTech Connect

An experimental study of a planar wake consisting of four vortices that simulate the trailing vortex wakes generated by transport airplanes in either takeoff or landing configurations is presented. The objective of this study was to examine naturally occurring wake instabilities. Specifically, the focus of the study was centered on bending wave instabilities of which the Crow instability represents a particular case. A unique method of generating a four-vortex wake was developed for this study. The four-vortex wake generating device permitted direct variation of the spacing between vortices as well as control over the vortex circulation strength. Two quantitative flow visualization experiments were instrumental in identifying wake configurations that were conducive to the rapid growth of bending wave modes and in the identification of the long-wavelength mode. Detailed experiments were also conducted to examine the flow structure in the near-field or roll-up region using a four sensor, hot-wire probe that could measure all three velocity components in the wake simultaneously. The results of both the flow visualization and hot-wire experiments indicate that the long-wavelength mode and the first short-wavelength mode likely dominate the far-field wake physics and may potentially be utilized in a wake control strategy.

Babie, Brian M.; Nelson, Robert C. [Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, Indiana 46556 (United States)

2010-07-15

69

Cellular vortex shedding in the wake of a tapered plate at low Reynolds number  

NASA Astrophysics Data System (ADS)

The unsteady near wake behind a linearly tapered plate has been investigated numerically. The tapering made the Reynolds number based on the inflow velocity and the local width of the plate vary from 25 to 100. The wake flow comprised three different flow regimes coexisting side by side. The wake flow was steady behind the narrow end of the plate. Periodic vortex shedding occurred downstream from where the local Reynolds number exceeded 32. Vortex dislocations enabled a cellular shedding pattern with shedding frequency decreasing toward the wide end of the plate. The regular oblique vortex shedding near midspan was subjected to three-dimensional scrambling toward the wide end of the plate which gave rise to streamwise-oriented vortex structures. The Strouhal number was distinctly lower than in the wake of a uniform plate whereas the base pressure coefficient was substantially higher.

Khaledi, Hatef A.; Narasimhamurthy, Vagesh D.; Andersson, Helge I.

2009-01-01

70

Coupling of structure and wake oscillators in vortex-induced vibrations  

Microsoft Academic Search

A class of low-order models for vortex-induced vibrations is analyzed. A classical van der Pol equation models the near wake dynamics describing the fluctuating nature of vortex shedding. This wake oscillator interacts with the equation of motion of a one degree-of-freedom structure oscillator and several types of linear coupling terms modeling the fluid-structure interaction are considered. The model dynamics is

MATTEO LUCA FACCHINETTI; EMMANUEL DE LANGRE; FRANCIS BIOLLEY

2003-01-01

71

Coupling of structure and wake oscillators in vortex-induced vibrations  

Microsoft Academic Search

A class of low-order models for vortex-induced vibrations is analyzed. A classical van der Pol equation models the near wake dynamics describing the fluctuating nature of vortex shedding. This wake oscillator interacts with the equation of motion of a one degree-of-freedom structural oscillator and several types of linear coupling terms modelling the fluid–structure interaction are considered. The model dynamics is

M. L. Facchinetti; E. de Langre; F. Biolley

2004-01-01

72

A comparison of airborne wake vortex detection measurements with values predicted from potential theory  

NASA Technical Reports Server (NTRS)

An analysis of flight measurements made near a wake vortex was conducted to explore the feasibility of providing a pilot with useful wake avoidance information. The measurements were made with relatively low cost flow and motion sensors on a light airplane flying near the wake vortex of a turboprop airplane weighing approximately 90000 lbs. Algorithms were developed which removed the response of the airplane to control inputs from the total airplane response and produced parameters which were due solely to the flow field of the vortex. These parameters were compared with values predicted by potential theory. The results indicated that the presence of the vortex could be detected by a combination of parameters derived from the simple sensors. However, the location and strength of the vortex cannot be determined without additional and more accurate sensors.

Stewart, Eric C.

1991-01-01

73

Effect of velocity ratio on the streamwise vortex structures in the wake of a stack  

NASA Astrophysics Data System (ADS)

The time-averaged velocity and streamwise vorticity fields within the wake of a stack were investigated in a low-speed wind tunnel using a seven-hole pressure probe. The experiments were conducted at a Reynolds number, based on the stack external diameter, of ReD=2.3×104. The stack, of aspect ratio AR=9, was mounted normal to a ground plane and was partially immersed in a flat-plate turbulent boundary layer, where the ratio of the boundary layer thickness to the stack height was ?/H?0.5. The jet-to-cross-flow velocity ratio was varied from R=0 to 3, which covered the downwash, crosswind-dominated and jet-dominated flow regimes. In the downwash and crosswind-dominated flow regimes, two pairs of counter-rotating streamwise vortex structures were identified within the stack wake. The tip vortex pair located close to the free end of the stack, and the base vortex pair located close to the ground plane within the flat-plate boundary layer, were similar to those found in the wake of a finite circular cylinder, and were associated with the upwash and downwash flow fields within the stack wake, respectively. In the jet-dominated flow regime, a third pair of streamwise vortex structures was observed, referred to as the jet-wake vortex pair, which occurred within the jet-wake region above the free end of the stack. The jet-wake vortex pair had the same orientation as the base vortex pair and was associated with the jet rise. The peak vorticity and strength of the streamwise vortex structures were functions of the jet-to-cross-flow velocity ratio. For the tip vortex structures, their peak vorticity and strength reduced as the jet-to-cross-flow velocity ratio increased.

Adaramola, M. S.; Sumner, D.; Bergstrom, D. J.

2010-01-01

74

Wake vortex modeling for airborne and ground-based measurements using a coherent lidar  

NASA Astrophysics Data System (ADS)

Simulated signatures of aircraft wakes as detected by a scanning coherent short wavelength lidar are presented and characterized for a number of candidate surveillance scenarios. Ground based and airborne scanning configurations that emulate candidate operational detection and warning systems are compared for spatial coverage and detection capability as a function of system design parameters and atmospheric conditions. Examples of trailing geometries characteristic of onboard wake detection and warning systems are presented. The dependence of predicted detection capability on hydrodynamic parameters, such as vortex circulation, axial motion in the wake vortices, and ambient turbulence is discussed. Simulated wake signatures are compared to observations for wakes observed at Denver's Stapleton International Airport in 1993.

Thomson, J. Alex L.; Hannon, Stephen M.

1995-06-01

75

The NASA-Langley Wake Vortex Modelling Effort in Support of an Operational Aircraft Spacing System  

NASA Technical Reports Server (NTRS)

Two numerical modelling efforts, one using a large eddy simulation model and the other a numerical weather prediction model, are underway in support of NASA's Terminal Area Productivity program. The large-eddy simulation model (LES) has a meteorological framework and permits the interaction of wake vortices with environments characterized by crosswind shear, stratification, humidity, and atmospheric turbulence. Results from the numerical simulations are being used to assist in the development of algorithms for an operational wake-vortex aircraft spacing system. A mesoscale weather forecast model is being adapted for providing operational forecast of winds, temperature, and turbulence parameters to be used in the terminal area. This paper describes the goals and modelling approach, as well as achievements obtained to date. Simulation results will be presented from the LES model for both two and three dimensions. The 2-D model is found to be generally valid for studying wake vortex transport, while the 3-D approach is necessary for realistic treatment of decay via interaction of wake vortices and atmospheric boundary layer turbulence. Meteorology is shown to have an important affect on vortex transport and decay. Presented are results showing that wake vortex transport is unaffected by uniform fog or rain, but wake vortex transport can be strongly affected by nonlinear vertical change in the ambient crosswind. Both simulation and observations show that atmospheric vortices decay from the outside with minimal expansion of the core. Vortex decay and the onset three-dimensional instabilities are found to be enhanced by the presence of ambient turbulence.

Proctor, Fred H.

1998-01-01

76

Development of a time-accurate viscous Lagrangian vortex wake model for wind turbine applications  

Microsoft Academic Search

A second-order accurate model has been developed and validated for modeling the unsteady aerodynamics of a wind turbine. The free-vortex wake method consists of the Lagrangian description of the rotor flow field and viscous effects were incorporated using a viscous splitting approach. The wake geometry solution was then integrated with the rotor aerodynamics model in a consistent manner. The analysis

Sandeep Gupta

2006-01-01

77

Separation of Lift-Generated Vortex Wakes Into Two Diverging Parts  

NASA Technical Reports Server (NTRS)

As part of an ongoing study of the spreading rate of lift-generated vortex wakes, the present investigation considers possible reasons as to why segments of lift-generated wakes sometimes depart from the main part of the wake to move rapidly in either an upward or downward direction. It is assumed that deficiencies or enhancements of the lift carry over across the fuselage-shrouded wing are the driving mechanism for departures of wake-segments. The computations presented first indicate that upwardly departing wake segments that were observed and photographed could have been produced by a deficiency in lift carryover across the fuselage-shrouded part of the wing. Computations made of idealized vortex wakes indicate that upward departure of a wake segment requires a centerline reduction in the span loading of 70% or more, whether the engines are at idle or robust thrust. Similarly, it was found that downward departure of wake segments is produced when the lift over the center part of the wing is enhanced. However, it was also found that downward departures do not occur without the presence of robust engine-exhaust streams (i.e., engines must NOT be at idle). In those cases, downward departures of a wake segment occurs when the centerline value of the loading is enhanced by any amount between about 10% to 100%. Observations of condensation trails indicate that downward departure of wake segments is rare. Upward departures of wake segments appears to be more common but still rare. A study to determine the part of the aircraft that causes wake departures has not been carried out. However, even though departures of wake segments rarely occur, some aircraft do regularly shed these wake structures. If aircraft safety is to be assured to a high degree of reliability, and a solution for eliminating them is not implemented, existing guidelines for the avoidance of vortex wakes [1,3] may need to be broadened to include possible increases in wake sizes caused by vertical departures of wake segments. Further study may indicate that it is not possible to modify existing aircraft enough to prevent wake departures. Wake-avoidance guidelines must then be adjusted to provide the desired degree of safety. It appears that steps to avoid upwardly moving wake segments have already been incorporated into the avoidance procedures used for aircraft on approach to runways at the Frankfurt Airport [6,7]. The uncertainty in the prospects for compromises in flight safety caused by rapidly upwardly or downwardly moving wake segments suggest that it be specified that aircraft do not fly above or below each other during operations in the airport vicinity where aircraft are likely to be closely spaced [20].

Rossow, Vernon J.; Brown, Anthony P.

2010-01-01

78

Numerical Study of Wake Vortex Interaction with the Ground Using the Terminal Area Simulation System  

NASA Technical Reports Server (NTRS)

A sensitivity study for the in-ground effect on aircraft wake vortices has been conducted using a validated large eddy simulation model. The numerical results are compared with observed data and show good agreement for vortex decay and lateral vortex transport. The vortex decay rate is strongly influenced by the ground, but appears somewhat insensitive to ambient turbulence. In addition, the results show that the ground can affect the trajectory and descent-rate of a wake vortex pair at elevations up to about 3 b(sub o) (where b(sub o) is the initial vortex separation). However, the ground does not influence the average circulation of the vortices until the cores descend to within about 0.6 b(sub o), after which time the ground greatly enhances their rate of demise. Vortex rebound occurs in the simulations, but is more subtle than shown in previous numerical studies.

Proctor, Fred H.; Han, Jongil

1999-01-01

79

Wake vortex detection at Denver Stapleton Airport with a pulsed 2-micron coherent lidar  

NASA Technical Reports Server (NTRS)

This report describes the effort undertaken to relate aircraft wake history to the local environment. This involved the monitoring of the embedded windfield, monitoring of local meteorological parameters, a high-resolution velocity field analysis in vertical scan planes and measurement of the axial velocity signature. A flashlight pumped 2.09 micron solid state coherent laser radar system was used to detect and track wake vortices. Strong wake vortex signatures were measured for moderate to large aircraft at Denver's Stapleton airport and a large vortex database was compiled.

Hannon, Stephen M.; Thomson, J. Alex

1994-01-01

80

Development of a time-accurate viscous Lagrangian vortex wake model for wind turbine applications  

NASA Astrophysics Data System (ADS)

A second-order accurate model has been developed and validated for modeling the unsteady aerodynamics of a wind turbine. The free-vortex wake method consists of the Lagrangian description of the rotor flow field and viscous effects were incorporated using a viscous splitting approach. The wake geometry solution was then integrated with the rotor aerodynamics model in a consistent manner. The analysis was then used to predict the performance and airloads on a wind turbine in the upwind configuration under unyawed and yawed flow conditions. The present work has demonstrated the versatility and robustness of the free-vortex wake method for wind turbine applications. The understanding of the accuracy and the stability of the numerical method is very important in developing robust wake methodology. The accuracy of the straight-line segmentation method has been examined for a vortex ring and helical vortex, and it has been shown to be second-order accurate. However, a minimum discretization of ten degrees is shown to be required to obtain second-order accuracy and also keep the maximum error in the induced velocity field less than 10%. Linear and nonlinear numerical stability of various time-marching schemes were also examined, and a two-step backward differencing scheme was chosen. The overall numerical solution was demonstrated to converge with a second-order accuracy. The nonlinear unsteady aerodynamics of the blade section was modeled using the Leishman-Beddoes dynamic stall model modified for wind turbine applications. The numerical simulations captured the dynamics of the unsteady flow over the airfoil surface for both attached and stalled flow conditions. Validation of the numerical predictions of the aerodynamic force coefficients against measurements obtained for the S809 airfoil showed overall good agreement. It has been shown that with a proper representation of the static stall characteristics, this model can be used to predict dynamic stall for airfoil sections typical of those used for wind turbine applications. The unsteady airfoil model coupled with the blade model also adequately represented the three-dimensionality of the unsteady flow field for a parked blade, under both steady and unsteady flow conditions. The wake geometry solution integrated with the blade model was then used to predict the performance and airloads for a wind turbine tested under controlled conditions. It has been shown that it is important to accurately predict the transient wake aerodynamics to obtain accurate estimates of the unsteady airloads and power output. The skewed wake geometry behind an upwind wind turbine was successfully predicted in yawed flow conditions over a range of yaw angles and tip speed ratios. Measurements from the Phase VI of the NREL/NASA Ames wind tunnel test were used for validating the predictions of performance and airloads. The variation of the turbine thrust and the aerodynamic power output with wind speed was adequately predicted. Spanwise distributions of the aerodynamic coefficients were represented well, and encouraging agreement was obtained against the measured coefficients. The azimuthal variation of loads showed that the unsteady aerodynamic behavior of the wind turbine was adequately represented, with some exceptions.

Gupta, Sandeep

81

How to perform measurements in a hovering animal's wake: physical modelling of the vortex wake of the hawkmoth, Manduca sexta.  

PubMed Central

The vortex wake structure of the hawkmoth, Manduca sexta, was investigated using a vortex ring generator. Based on existing kinematic and morphological data, a piston and tube apparatus was constructed to produce circular vortex rings with the same size and disc loading as a hovering hawkmoth. Results show that the artificial rings were initially laminar, but developed turbulence owing to azimuthal wave instability. The initial impulse and circulation were accurately estimated for laminar rings using particle image velocimetry; after the transition to turbulence, initial circulation was generally underestimated. The underestimate for turbulent rings can be corrected if the transition time and velocity profile are accurately known, but this correction will not be feasible for experiments on real animals. It is therefore crucial that the circulation and impulse be estimated while the wake vortices are still laminar. The scaling of the ring Reynolds number suggests that flying animals of about the size of hawkmoths may be the largest animals whose wakes stay laminar for long enough to perform such measurements during hovering. Thus, at low advance ratios, they may be the largest animals for which wake circulation and impulse can be accurately measured.

Tytell, Eric D; Ellington, Charles P

2003-01-01

82

How to perform measurements in a hovering animal's wake: physical modelling of the vortex wake of the hawkmoth, Manduca sexta.  

PubMed

The vortex wake structure of the hawkmoth, Manduca sexta, was investigated using a vortex ring generator. Based on existing kinematic and morphological data, a piston and tube apparatus was constructed to produce circular vortex rings with the same size and disc loading as a hovering hawkmoth. Results show that the artificial rings were initially laminar, but developed turbulence owing to azimuthal wave instability. The initial impulse and circulation were accurately estimated for laminar rings using particle image velocimetry; after the transition to turbulence, initial circulation was generally underestimated. The underestimate for turbulent rings can be corrected if the transition time and velocity profile are accurately known, but this correction will not be feasible for experiments on real animals. It is therefore crucial that the circulation and impulse be estimated while the wake vortices are still laminar. The scaling of the ring Reynolds number suggests that flying animals of about the size of hawkmoths may be the largest animals whose wakes stay laminar for long enough to perform such measurements during hovering. Thus, at low advance ratios, they may be the largest animals for which wake circulation and impulse can be accurately measured. PMID:14561347

Tytell, Eric D; Ellington, Charles P

2003-09-29

83

Application of Wind Tunnel Free-Flight Technique for Wake Vortex Encounters  

NASA Technical Reports Server (NTRS)

A wind tunnel investigation was conducted in the Langley 30- by 60-Foot Tunnel to assess the free-flight test technique as a tool in research on wake vortex encounters. A typical 17.5-percent scale business-class jet airplane model was flown behind a stationary wing mounted in the forward portion of the wind tunnel test section. The span ratio (model span-generating wingspan) was 0.75. The wing angle of attack could be adjusted to produce a vortex of desired strength. The test airplane model was successfully flown in the vortex and through the vortex for a range of vortex strengths. Data obtained included the model airplane body axis accelerations, angular rates, attitudes, and control positions as a function of vortex strength and relative position. Pilot comments and video records were also recorded during the vortex encounters.

Brandon, Jay M.; Jordan, Frank L., Jr.; Stuever, Robert A.; Buttrill, Catherine W.

1997-01-01

84

Design of airport wake vortex monitoring system based on 1.5-?m pulsed coherent Doppler lidar  

NASA Astrophysics Data System (ADS)

To shun the vortex hazard, the airport wake vortex monitoring system based on 1.5-?m pulsed coherent Doppler lidar is designed successfully in this paper. Based on the realistic analytical model, the wake vortex generated by airbus A340 under typical flight condition is simulated. Then the principle of airport wake vortex monitoring is introduced, and the work flow of the monitoring system is also presented. Moreover, based on the mechanism of vortex coherent detection and typical system parameters, both detection SNR and detection precision are obtained through numerical simulations. When the system outputs 2 J energy, the coherent detection SNR at 10 km distance is up to 23.452, and detection precision can reach 0.328 m/s. With the wake vortex monitoring experiment of A340, some vortex parameters are estimated. Due to these results comparatively coinciding with the previous simulation conclusions, the ability of Doppler lidar for full-scale wake vortex characterization and real time measurement is demonstrated. The study shows that the wake vortex detection based on 1.5-?m pulsed coherent Doppler lidar has the advantages of high accuracy and far distance, and the designed airport wake vortex monitoring system has proved to be effective and feasible, which has significant development and application prospect in the aspect of assuring flight security and increasing airport capacity.

Wu, Yong-Hua; Hu, Yi-Hua; Xu, Shi-Long; Li, Jin-Ming; Dai, Ding-Chuan

2011-07-01

85

Evaluation of Fast-Time Wake Vortex Prediction Models  

NASA Technical Reports Server (NTRS)

Current fast-time wake models are reviewed and three basic types are defined. Predictions from several of the fast-time models are compared. Previous statistical evaluations of the APA-Sarpkaya and D2P fast-time models are discussed. Root Mean Square errors between fast-time model predictions and Lidar wake measurements are examined for a 24 hr period at Denver International Airport. Shortcomings in current methodology for evaluating wake errors are also discussed.

Proctor, Fred H.; Hamilton, David W.

2009-01-01

86

Effects of spoilers and gear on B-747 wake vortex velocities  

NASA Technical Reports Server (NTRS)

Vortex velocities were measured in the wakes of four configurations of a 0.61-m span model of a B-747 aircraft. The wakes were generated by towing the model underwater in a ship model basin. Tangential and axial velocity profiles were obtained with a scanning laser velocimeter as the wakes aged to 35 span lengths behind the model. A 45 deg deflection of two outboard flight spoilers with the model in the landing configuration resulted in a 36 percent reduction in wake maximum tangential velocity, altered velocity profiles, and erratic vortex trajectories. Deployment of the landing gear with the inboard flaps in the landing position and outboard flaps retracted had little effect on the flap vortices to 35 spans, but caused the wing tip vortices to have: (1) more diffuse velocity profiles; (2) a 27 percent reduction in maximum tangential velocity; and (3) a more rapid merger with the flap vortices.

Luebs, A. B.; Bradfute, J. G.; Ciffone, D. L.

1976-01-01

87

An experimental and numerical study of the vortex structure in the wake of a wind turbine  

Microsoft Academic Search

An experimental investigation into the properties of the vortex wake behind a wind turbine rotor has been carried out at model scale, using Particle Image Velocimetry (PIV). The two-blade model was operated at tip speed ratios in the range ?=3–8, and chord Reynolds numbers Re=6400–16?000. The blades were untwisted, with flat-plate aerofoil profile. Measurements of wake velocity and vorticity were

J Whale; C. G Anderson; R Bareiss; S Wagner

2000-01-01

88

A study of the interaction between a wake vortex and an encountering airplane  

NASA Technical Reports Server (NTRS)

An analytical study of the mutual interaction between a wake vortex and an encountering airplane has been conducted. The interaction was calculated on the basis of conservation of angular momentum of the wake vortex and the encountering airplane in the roll axis. For the nominal conditions assumed in the study, the interaction reduced the roll response in the lust second by 20 percent compared to the classical method of calculating the response. However, depending on the conditions assumed, the reduction varied from 0 to 30 percent which could be highly significant in determining separation standards for airplanes. Controlled experiments are needed to determine the exact amount of interaction.

Stewart, E. C.

1993-01-01

89

Periodic vortex shedding in the supersonic wake of a planar plate  

NASA Technical Reports Server (NTRS)

Vortex sheets in the wake have been mainly studied in incompressible flows and in the transonic region. Heinemann et al. (1976) have shown that for the subsonic region the Strouhal number is nearly independent of the Mach number. Motallebi and Norbury (1981) have observed an increase in the Strouhal number in transonic supersonic flow at Mach numbers up to 1.25. The present investigation is concerned with an extension of the studies of vortex shedding to higher supersonic Mach numbers, taking into account questions regarding the possibility of a generation of stable von Karman vortex paths in the considered Mach number range. It is found that the vortex sheet observed in a supersonic wake behind a rough plate is only stable and reproducible in cases involving a certain surface roughness and certain aspects of trailing edge geometry.

Xing, W. F.; Marenbach, G.

1985-01-01

90

Large Eddy Simulation of Aircraft Wake Vortices in a Homogeneous Atmospheric Turbulence: Vortex Decay and Descent  

NASA Technical Reports Server (NTRS)

The effects of ambient turbulence on decay and descent of aircraft wake vortices are studied using a validated, three-dimensional: large-eddy simulation model. Numerical simulations are performed in order to isolate the effect of ambient turbulence on the wake vortex decay rate within a neutrally-stratified atmosphere. Simulations are conducted for a range of turbulence intensities, by injecting wake vortex pairs into an approximately homogeneous and isotropic turbulence field. The decay rate of the vortex circulation increases clearly with increasing ambient turbulence level, which is consistent with field observations. Based on the results from the numerical simulations, simple decay models are proposed as functions of dimensionless ambient turbulence intensity (eta) and dimensionless time (T) for the circulation averaged over a range of radial distances. With good agreement with the numerical results, a Gaussian type of vortex decay model is proposed for weak turbulence: while an exponential type of Tortex decay model can be applied for strong turbulence. A relationship for the vortex descent based on above vortex decay model is also proposed. Although the proposed models are based on simulations assuming neutral stratification, the model predictions are compared to Lidar vortex measurements observed during stable, neutral, and unstable atmospheric conditions. In the neutral and unstable atmosphere, the model predictions appear to be in reasonable agreement with the observational data, while in the stably-stratified atmosphere, they largely underestimate the observed circulation decay with consistent overestimation of the observed vortex descent. The underestimation of vortex decay during stably-stratified conditions suggests that stratification has an important influence on vortex decay when ambient levels of turbulence are weak.

Han, Jongil; Lin, Yuh-Lang; Arya, S. Pal; Proctor, Fred H.

1999-01-01

91

Trailing Vortex Measurements in the Wake of a Hovering Rotor Blade with Various Tip Shapes  

NASA Technical Reports Server (NTRS)

This work examined the wake aerodynamics of a single helicopter rotor blade with several tip shapes operating on a hover test stand. Velocity field measurements were conducted using three-component laser Doppler velocimetry (LDV). The objective of these measurements was to document the vortex velocity profiles and then extract the core properties, such as the core radius, peak swirl velocity, and axial velocity. The measured test cases covered a wide range of wake-ages and several tip shapes, including rectangular, tapered, swept, and a subwing tip. One of the primary differences shown by the change in tip shape was the wake geometry. The effect of blade taper reduced the initial peak swirl velocity by a significant fraction. It appears that this is accomplished by decreasing the vortex strength for a given blade loading. The subwing measurements showed that the interaction and merging of the subwing and primary vortices created a less coherent vortical structure. A source of vortex core instability is shown to be the ratio of the peak swirl velocity to the axial velocity deficit. The results show that if there is a turbulence producing region of the vortex structure, it will be outside of the core boundary. The LDV measurements were supported by laser light-sheet flow visualization. The results provide several benchmark test cases for future validation of theoretical vortex models, numerical free-wake models, and computational fluid dynamics results.

Martin, Preston B.; Leishman, J. Gordon

2003-01-01

92

Analysis of the Radar Reflectivity of Aircraft Vortex Wakes.  

National Technical Information Service (NTIS)

Radar has been proposed as a way to track wake vortices to reduce aircraft spacing and tests have revealed radar echoes from aircraft wakes in clear air. The results are always interpreted qualitatively using Tatarski's theory of weak scattering by isotro...

K. Shariff A. Wray

2000-01-01

93

Estimates of the effectiveness of automatic control in alleviating wake vortex induced roll excursions  

NASA Technical Reports Server (NTRS)

Estimates of the effectiveness of a model following type control system in reducing the roll excursion due to a wake vortex encounter were obtained from single degree of freedom computations with inputs derived from the results of wind tunnel, flight, and simulation experiments. The analysis indicates that the control power commanded by the automatic system must be roughly equal to the vortex induced roll acceleration if effective limiting of the maximum bank angle is to be achieved.

Tinling, B. E.

1977-01-01

94

A Parametric Study of Accelerations of an Airplane Due to a Wake Vortex System  

NASA Technical Reports Server (NTRS)

A study was conducted using strip theory to systematically investigate the effects of progressively more complete descriptions of the interaction of an airplane with a wake vortex system. The emphasis was in roll-dominant, parallel, vortex encounters. That is, the simulated airplane's longitudinal axis was nearly parallel to the rotation axis of the vortex system for most of the results presented. The study began with a drag-less rectangular wing in the flow field of a single vortex and progressed to a complete airplane with aerodynamic surfaces possessing taper, sweep, dihedral, and stalling and immersed in the flow field of a vortex pair in ground effect. The effects of the pitch, roll, and yaw attitudes of the airplane on the calculated accelerations were also investigated. The airplane had the nominal characteristics of a Boeing 757, and the vortex flow field had the nominal characteristics of the wake of a Boeing 767. The Bumham-Hallock model of a vortex flow field was used throughout the study. The data are presented mainly in terms of contours of equal acceleration in a two-dimensional area centered on the vortex pair and having dimensions of 300 feet by 300 feet.

Stewart, Eric C.

1999-01-01

95

Experimental investigation of wing fin configurations for alleviation of vortex wakes of aircraft  

NASA Technical Reports Server (NTRS)

A variety of fin configurations were tested on a model of the Boeing B747 in 40 by 80 foot wind tunnels. The test results confirmed that a reduction in wake rolling moment was brought about by the vortex shed by the fins so that a wide range of designs can be used to achieve wake alleviation. It was also found that the reduction in wake-induced rolling moments was especially sensitive to the location of the smaller fins on the wing and that the penalties in lift and drag can probably be made negligible by proper fin design.

Rossow, V. J.

1978-01-01

96

Spectral Characteristics of Wake Vortex Sound During Roll-Up  

NASA Technical Reports Server (NTRS)

This report presents an analysis of the sound spectra generated by a trailing aircraft vortex during its rolling-up process. The study demonstrates that a rolling-up vortex could produce low frequency (less than 100 Hz) sound with very high intensity (60 dB above threshold of human hearing) at a distance of 200 ft from the vortex core. The spectrum then drops o rapidly thereafter. A rigorous analytical approach has been adopted in this report to derive the spectrum of vortex sound. First, the sound pressure was solved from an alternative treatment of the Lighthill s acoustic analogy approach [1]. After the application of Green s function for free space, a tensor analysis was applied to permit the removal of the source term singularity of the wave equation in the far field. Consequently, the sound pressure is expressed in terms of the retarded time that indicates the time history and spacial distribution of the sound source. The Fourier transformation is then applied to the sound pressure to compute its spectrum. As a result, the Fourier transformation greatly simplifies the expression of the vortex sound pressure involving the retarded time, so that the numerical computation is applicable with ease for axisymmetric line vortices during the rolling-up process. The vortex model assumes that the vortex circulation is proportional to the time and the core radius is a constant. In addition, the velocity profile is assumed to be self-similar along the aircraft flight path, so that a benchmark vortex velocity profile can be devised to obtain a closed form solution, which is then used to validate the numerical calculations for other more realistic vortex profiles for which no closed form solutions are available. The study suggests that acoustic sensors operating at low frequency band could be profitably deployed for detecting the vortex sound during the rolling-up process.

Booth, Earl R., Jr. (Technical Monitor); Zhang, Yan; Wang, Frank Y.; Hardin, Jay C.

2003-01-01

97

Dynamic Wind Loads and Vortex Structures in the Wake of a Wind Turbine  

Microsoft Academic Search

We report an experimental study to characterize the dynamic wind loads and evolution of wake vortex flow structures downstream of a horizontal axis wind turbine (HAWT). The experiments were conducted in a wind tunnel with a wind turbine model placed in a boundary layer flow developed over rough and smooth surfaces in order to study the effects of roughness and

Hui Hu; Zifeng Yang; Partha Sarkar

2010-01-01

98

Rotor blade-vortex interaction impulsive noise source identification and correlation with rotor wake predictions  

Microsoft Academic Search

An acoustic source localization scheme applicable to noncompact moving sources is developed and applied to the blade-vortex interaction (BVI) noise data of a 40-percent scale BO-105 model rotor. A generalized rotor wake code is employed to predict possible VBI locations on the rotor disk and is found quite useful in interpreting the acoustic localization results. The highly varying directivity patterns

W. R. Splettstoesser; K. J. Schultz; Ruth M. Martin

1987-01-01

99

FLUTTER CLEARANCE FLIGHT TESTS OF AN OV-10A AIRPLANE MODIFIED FOR WAKE VORTEX FLIGHT EXPERIMENTS  

Microsoft Academic Search

SUMMARY Envelope expansion, flight flutter tests of a modified OV-10A aircraft are described. For a wake vortex research program, the airplane was modified to incorporate three, forward-extending instrumentation booms, one extending forward from each wing tip and one from the right side of the fuselage. The booms are instrumented with sensors to measure the velocity and direction of local air

Robert V. Doggett; Eric C. Stewart

100

Vortex arrangement in the wake of rigid and flexible rapidly pitching airfoils at low Reynolds number  

NASA Astrophysics Data System (ADS)

An experimental investigation of the wake of an airfoil undergoing rapid pitch oscillation is conducted in a water tunnel at a chord Reynolds number of about 2000. Flow visualization is utilized to characterize the vortical patterns in the wake of the airfoil, which is constructed from a NACA 0036 profile fitted with an extended trailing edge with controllable flexibility. The spatial configuration of the vortices is extracted in terms of streamwise and cross-flow spacing over a range of pitching frequencies and amplitudes. We discuss how different levels of flexibility alter the vortex spacing parameters and the conditions under which the traditional Karman vortex pattern, corresponding to a wake profile, changes to the reverse Karman pattern associated with a jet profile.

Monnier, Bruno; Naguib, Ahmed; Koochesfahani, Manoochehr

2011-11-01

101

Measurement of Vortex Strength and Core Diameter in the Wake of a Hovering Rotor  

NASA Technical Reports Server (NTRS)

Detailed hot wire measurements have been acquired in the tip vortex of a three-bladed model tilt rotor in hover. Testing was conducted at a rotor tip speed of 752 ft/sec, a Reynolds number (based on blade tip chord) of 1.77 x 10(exp 6), for thrust coefficients up to 0.0160. A figure shows the hot wire mounted above the inverted rotor at the Outside Aerodynamic Rotor Facility (OARF) at NASA Ames Research Center. Strobed shadowgraph flow visualization was used to define the vortex trajectory as an aid in hot wire positioning. Considerable variations in tip vortex structure with time were observed, even from the same blade, under essentially uniform test conditions. The only velocity signatures analyzed were those corresponding to passage of the probe directly through the center of the vortex. These time histories were ensemble averaged after compensating for jitter in the vortex arrival time at the probe, thereby retaining the core structure with minimal smearing. An example of a mean velocity signature, after ensemble averaging, is shown. The mean velocity signature was analyzed under the assumption of constant (unknown) translation speed of the vortex filament past the fixed probe. The translation speed of the vortex is deduced and the vortex strength and core diameter inferred. The results were highly unexpected. The indicated vortex strength is seen to decrease rapidly after first blade passage. In addition, the core radius is seen to decrease with increasing wake age, not increase as might be expected from simple diffusion.

Wadcock, Alan J.

1997-01-01

102

Exploration of the Relationship Between Wake Vortex Parameters and Thrust Force on Oscillating Airfoils Using a Vortex Array Model  

NASA Astrophysics Data System (ADS)

Recently, we demonstrated the ability of a simple model, based on an array of finite-core Gaussian vortices, to accurately reproduce the unsteady velocity field in the wake of, and drag/thrust force acting on harmonically/non-harmonically pitching airfoils. In the present work, this model is employed to explore how the thrust force varies with wake vortex parameters; i.e. circulation, core radius and streamwise/cross-flow spacing of the vortices. Insight from this investigation will be helpful to draw links between trailing-edge flexibility and the detailed process of generation of wake vortices. Such links may have the potential for providing a path towards a rational, yet efficient, approach for tailoring trailing-edge flexibility to obtain desirable force characteristics for flapping-wings Micro Air Vehicles.

Naguib, Ahmed; Koochesfahani, Manoochehr

2011-11-01

103

Validation of Vortex-Lattice Method for loads on wings in lift-generated wakes  

NASA Technical Reports Server (NTRS)

A study is described that evaluates the accuracy of vortex-lattice methods when they are used to compute the loads induced on aircraft as they encounter lift-generated wakes. The evaluation is accomplished by use of measurements made in the 80- by 120-foot wind tunnel of the lift, rolling-moment, and downwash in the wake of three configurations of a model of a subsonic transport aircraft. The downwash measurements are used as input for a vortex-lattice code in order to compute the lift and rolling moment induced on wings that have a span of 0.186, 0.510, or 1.022 times the span of the wake-generating model. Comparison of the computed results with the measured lift and rolling moment distributions are used to determine the accuracy of the vortex-lattice code. It was found that the vortex-lattice method is very reliable as long as the span of the encountering of following wing is less than about 0.2 of the generator span. As the span of the following wing increases above 0.2, the vortex-lattice method continues to correctly predict the trends and nature of the induced loads, but it overpredicts the magnitude of the loads by increasing amounts. The increase in deviation of the computed from the measured loads with size of the following wing is attributed to the increase in distortion of the structure of the vortex wake as it approaches and passes the larger following wings.

Rossow, J.

1994-01-01

104

Point vortex model for asymmetric inviscid wakes past bluff bodies  

NASA Astrophysics Data System (ADS)

Wakes past bluff bodies are modeled by means of point vortices standing in equilibrium. The consistency of the adopted model is discussed with respect to the asymptotic model proposed by Batchelor. It is shown that, in general, when symmetry is broken, the wake configuration may be neither closed, as for the Batchelor model, nor open, as for the Kirchhoff model. The proposed model has three degrees of freedom, which reduce to one when the locations of separation are prescribed. A further condition has been established for the closure of the wake which reduces the degrees of freedom to zero as for the asymptotic Batchelor model. The existence of multiple solutions, suggestive for real world phenomena, is discussed.

Elcrat, A.; Ferlauto, M.; Zannetti, L.

2014-06-01

105

Interfacing comprehensive rotorcraft analysis with advanced aeromechanics and vortex wake models  

NASA Astrophysics Data System (ADS)

This dissertation describes three aspects of the comprehensive rotorcraft analysis. First, a physics-based methodology for the modeling of hydraulic devices within multibody-based comprehensive models of rotorcraft systems is developed. This newly proposed approach can predict the fully nonlinear behavior of hydraulic devices, and pressure levels in the hydraulic chambers are coupled with the dynamic response of the system. The proposed hydraulic device models are implemented in a multibody code and calibrated by comparing their predictions with test bench measurements for the UH-60 helicopter lead-lag damper. Predicted peak damping forces were found to be in good agreement with measurements, while the model did not predict the entire time history of damper force to the same level of accuracy. The proposed model evaluates relevant hydraulic quantities such as chamber pressures, orifice flow rates, and pressure relief valve displacements. This model could be used to design lead-lag dampers with desirable force and damping characteristics. The second part of this research is in the area of computational aeroelasticity, in which an interface between computational fluid dynamics (CFD) and computational structural dynamics (CSD) is established. This interface enables data exchange between CFD and CSD with the goal of achieving accurate airloads predictions. In this work, a loose coupling approach based on the delta-airloads method is developed in a finite-element method based multibody dynamics formulation, DYMORE. To validate this aerodynamic interface, a CFD code, OVERFLOW-2, is loosely coupled with a CSD program, DYMORE, to compute the airloads of different flight conditions for Sikorsky UH-60 aircraft. This loose coupling approach has good convergence characteristics. The predicted airloads are found to be in good agreement with the experimental data, although not for all flight conditions. In addition, the tight coupling interface between the CFD program, OVERFLOW-2, and the CSD program, DYMORE, is also established. The ability to accurately capture the wake structure around a helicopter rotor is crucial for rotorcraft performance analysis. In the third part of this thesis, a new representation of the wake vortex structure based on Non-Uniform Rational B-Spline (NURBS) curves and surfaces is proposed to develop an efficient model for prescribed and free wakes. NURBS curves and surfaces are able to represent complex shapes with remarkably little data. The proposed formulation has the potential to reduce the computational cost associated with the use of Helmholtz's law and the Biot-Savart law when calculating the induced flow field around the rotor. An efficient free-wake analysis will considerably decrease the computational cost of comprehensive rotorcraft analysis, making the approach more attractive to routine use in industrial settings.

Liu, Haiying

106

Introduction to Jet-Engine Exhaust and Trailing Vortex Wakes.  

National Technical Information Service (NTIS)

The report is a result of a survey of the immediately available literature on aircraft wakes. While it should not be considered the final word on the subject, it is a good general representation of techniques and problems involved. (Author) The simple, ap...

D. N. Jones

1970-01-01

107

A Method for Calculating Helicopter Vortex Paths and Wake Velocities.  

National Technical Information Service (NTIS)

A simple method is developed for calculating the time averaged velocity field induced at large distances from the rotor by a helicopter in steady horizontal motion. The influence of the ground plane and of horizontal winds on the rotor wake and velocity f...

E. S. Levinsky T. Strand

1970-01-01

108

Validation of Vortex-Lattice Method for Loads on Wings in Lift-Generated Wakes  

NASA Technical Reports Server (NTRS)

A study is described that evaluates the accuracy of vortex-lattice methods when they are used to compute the loads induced on aircraft as they encounter lift-generated wakes. The evaluation is accomplished by the use of measurements made in the 80 by 120 ft Wind Tunnel of the lift, rolling moment, and downwash in the wake of three configurations of a model of a subsonic transport aircraft. The downwash measurements are used as input for a vortex-lattice code in order to compute the lift and rolling moment induced on wings that have a span of 0.186, 0.510, or 1.022 times the span of the wake-generating model. Comparison of the computed results with the measured lift and rolling-moment distributions the vortex-lattice method is very reliable as long as the span of the encountering or following wing is less than about 0.2 of the generator span. As the span of the following wing increases above 0.2, the vortex-lattice method continues to correctly predict the trends and nature of the induced loads, but it overpredicts the magnitude of the loads by increasing amounts.

Rossow, Vernon J.

1995-01-01

109

Numerical Modeling Studies of Wake Vortex Transport and Evolution Within the Planetary Boundary Layer  

NASA Technical Reports Server (NTRS)

In support of the wake vortex effect of the Terminal Area Productivity program, we have put forward four tasks to be accomplished in our proposal. The first task is validation of two-dimensional wake vortex-turbulence interaction. The second task is investigation of three-dimensional interaction between wake vortices and atmospheric boundary layer (ABL) turbulence. The third task is ABL studies. The, fourth task is addition of a Klemp-Durran condition at the top boundary for TASS model. The accomplishment of these tasks will increase our understanding of the dynamics of wake vortex and improve forecasting systems responsible for air safety and efficiency. The first two tasks include following three parts: (a) Determine significant length scale for vortex decay and transport, especially the length scales associated with the onset of Crow instability (Crow, 1970); (b) Study the effects of atmospheric turbulence on the decay of the wake vortices; and (c) Determine the relationships between decay rate, transport properties and atmospheric parameters based on large eddy simulation (LES) results and the observational data. These parameters may include turbulence kinetic energy, dissipation rate, wind shear and atmospheric stratification. The ABL studies cover LES modeling of turbulence structure within planetary boundary layer under transition and stable stratification conditions. Evidences have shown that the turbulence in the stable boundary layer can be highly intermittent and the length scales of eddies are very small compared to those in convective case. We proposed to develop a nesting grid mesh scheme and a modified Klemp-Durran conditions (Klemp and Wilhelmson, 1978) at the top boundary for TASS model to simulate planetary boundary layer under stable stratification conditions. During the past year, our group has made great efforts to carry out the above mentioned four tasks simultaneously. The work accomplished in the last year will be described in the next section.

Lin, Yuh-Lang; Arya, S. Pal; Kaplan, Michael L.; Shen, Shaohua

1998-01-01

110

Wake Vortex Prediction Models for Decay and Transport Within Stratified Environments  

NASA Technical Reports Server (NTRS)

This paper proposes two simple models to predict vortex transport and decay. The models are determined empirically from results of three-dimensional large eddy simulations, and are applicable to wake vortices out of ground effect and not subjected to environmental winds. The results, from the large eddy simulations assume a range of ambient turbulence and stratification levels. The models and the results from the large eddy simulations support the hypothesis that the decay of the vortex hazard is decoupled from its change in descent rate.

Switzer, George F.; Proctor, Fred H.

2002-01-01

111

Prediction and control of vortex-dominated and vortex-wake flows  

NASA Technical Reports Server (NTRS)

This progress report documents the accomplishments achieved in the period from December 1, 1992 until November 30, 1993. These accomplishments include publications, national and international presentations, NASA presentations, and the research group supported under this grant. Topics covered by documents incorporated into this progress report include: active control of asymmetric conical flow using spinning and rotary oscillation; supersonic vortex breakdown over a delta wing in transonic flow; shock-vortex interaction over a 65-degree delta wing in transonic flow; three dimensional supersonic vortex breakdown; numerical simulation and physical aspects of supersonic vortex breakdown; and prediction of asymmetric vortical flows around slender bodies using Navier-Stokes equations.

Kandil, Osama

1993-01-01

112

Far-Field Turbulent Vortex-Wake/Exhaust Plume Interaction for Subsonic and HSCT Airplanes  

NASA Technical Reports Server (NTRS)

Computational study of the far-field turbulent vortex-wake/exhaust plume interaction for subsonic and high speed civil transport (HSCT) airplanes is carried out. The Reynolds-averaged Navier-Stokes (NS) equations are solved using the implicit, upwind, Roe-flux-differencing, finite-volume scheme. The two-equation shear stress transport model of Menter is implemented with the NS solver for turbulent-flow calculation. For the far-field study, the computations of vortex-wake interaction with the exhaust plume of a single engine of a Boeing 727 wing in a holding condition and two engines of an HSCT in a cruise condition are carried out using overlapping zonal method for several miles downstream. These results are obtained using the computer code FTNS3D. The results of the subsonic flow of this code are compared with those of a parabolized NS solver known as the UNIWAKE code.

Kandil, Osama A.; Adam, Ihab; Wong, Tin-Chee

1996-01-01

113

A family of vortex wakes for bird flight at different speeds.  

NASA Astrophysics Data System (ADS)

Theoretical models of bird flight based on presumed distributions of vorticity in the wake are popular and occasionally sophisticated. However, owing to the practical difficulties in making direct measurements on unsteady flows behind animals in free flight, they are based primarily on presumed, rather than observed, wake structures. Furthermore, the data that do exist in the literature are at one or two characteristic flight speeds only, and so the critical questions concerning possible changes in wake structure with flight speed for an individual have been answered by creative extrapolation from very few data points. Here, for the first time, the wake structre of a single bird (a thrush nightingale) is measured over its entire range of natural flight speeds (4-11 m/s). The bird was trained to fly steadily in a low-turbulence wind tunnel, and the wake flow was measured using the custom BPIV method described in a companion talk in the Experimental Methods session. At most flight speeds the wake structure is qualitatively and quantitatively different from the commonly-invoked vortex loop or constant circulation models. Some consequences of these differences will be briefly discussed.

Hedenstrom, A.; Rosen, M.; Spedding, G. R.

2002-11-01

114

Turbulent wake and vortex shedding for a stack partially immersed in a turbulent boundary layer  

Microsoft Academic Search

The effect of the jet-to-cross-flow velocity ratio, R, on the turbulent wake and Kármán vortex shedding for a cylindrical stack of aspect ratio AR=9 was investigated in a low-speed wind tunnel using thermal anemometry. The cross-flow Reynolds number was ReD=2.3×104, the jet Reynolds number ranged from Red=7.6×103 to 4.7×104, and R was varied from 0 to 3. The stack was

M. S. Adaramola; D. Sumner; D. J. Bergstrom

2007-01-01

115

Real-Time Visualization of Wake-Vortex Simulations Using Computational Steering and Beowulf Clusters  

Microsoft Academic Search

In this paper, we present the design and implementation of POSSE, a new, lightweight computational steering system based on a client\\/server pro- gramming model. We demonstrate the effectiveness of this software system by illustrating its use for a visualization client designed for a particularly demand- ing real-time application—wake-vortex simulations for multiple aircraft running on a parallel Beowulf cluster. We describe

Anirudh Modi; Lyle N. Long; Paul E. Plassmann

2002-01-01

116

A dynamic prescribed vortex wake model for the FAST/AeroDyn wind energy conversion simulation code  

NASA Astrophysics Data System (ADS)

A Dynamic Prescribed Vortex Wake model for analysis of horizontal axis wind turbines has been developed. This model extends the HAWTDAWG steady state prescribed wake code to dynamic flows. This extension assumes wake vortices follow prescription functions valid at the time each vortex is generated. This allows modeling of dynamic wake effects known to exist. This assumption is supported through analysis and comparison to UAE Phase VI test data. The new Dynamic Prescribed Vortex Wake model is built into AeroDyn as a third aerodynamic model and uses the FAST structural dynamic model. It implements the wind models and dynamic stall model in AeroDyn. FAST structural degrees of freedom are implemented. Comparisons are made to UAE Phase VI wind tunnel data, and to the other two AeroDyn models, Blade Element Momentum and Generalized Dynamic Wake. Both steady, to verify the base model, and dynamic, to validate the extension to dynamic flow, conditions are considered. Both axial and yawed flow are analyzed. Dynamic UAE test data analyzed include rapid pitch, Sequence Q, and yaw release, Sequence E. The Dynamic Prescribed Wake model compares favorably to test data and to other AeroDyn models. Small rapid dynamic response is noted in each model and the test data. The new Dynamic Prescribed Vortex Wake model shows promise. Release of the code for experimental use and further validation is recommended.

Currin, Hugh D.

117

On the development of a wake vortex in inviscid flow  

NASA Astrophysics Data System (ADS)

The evolution of an initial perturbation in an axisymmetric subsonic normal inviscid gas flow through a pipe is directly simulated. The basic (unperturbed) flow has a zero radial velocity component, while its axial velocity component (along the axis of symmetry) increases or decreases linearly with the radius. The perturbation is specified as a swirl (rotation about the axis) with a positive or negative velocity vanishing on the central axis and the lateral surface. Irrespective of its direction, the swirl gives rise to a steady-state vortex carried by the flow. It shape is spherical (contiguous to the rotation axis) or circular (sliding along the impermeable lateral surface).

Belotserkovskii, O. M.; Belotserkovskaya, M. S.; Denisenko, V. V.; Eriklintsev, I. V.; Kozlov, S. A.; Oparina, E. I.; Troshkin, O. V.; Fortova, S. V.

2014-01-01

118

A modified wake oscillator model for vortex-induced vibration of circular cylinders for a wide range of mass-damping ratio  

NASA Astrophysics Data System (ADS)

In this paper, the behavior of an elastically mounted cylinder, subjected to vortex-induced vibrations (VIV), is investigated by a low-dimensional model. The classical wake oscillator model, as a standard model, predicts the behavior of the system at high mass-damping ratios but fails in modeling the system at low mass-damping ratios. A modified wake oscillator model is introduced in order to describe the response of the system over a wide range of mass-damping ratios. The results of this new model are compared to experimental results from the literature and shown to be in good agreement. The new model can describe most of the features of vortex-induced vibration phenomenology, such as the Griffin plot and lock-in domains.

Farshidianfar, A.; Zanganeh, H.

2010-04-01

119

Experiments on the vortex wake of a swimming knifefish  

NASA Astrophysics Data System (ADS)

The knifefish species propels itself by generating a reverse Kármán street using an anal fin, and the propulsion of this species is known to be highly efficient (Blake in Can J Zool 61:1432-1441, 1983). Previous studies have suggested that there is an optimal swimming range for fish based on the amplitude and frequency of the reverse Kármán street. In the current study, experiments have been performed to measure the ratio between the amplitude and wavelength of vortices in the wake of a knifefish. It is suggested that the wave efficiency can be estimated by optimizing the thrust created by the reverse Kármán street for a given spacing ratio, and present observations have an average value of 0.89. The relationship established between spacing ratio and wave efficiency, in addition to the measured parameters, will be invaluable for bio-inspired designs based on the knifefish.

Taylor, Zachary J.; Liberzon, Alexander; Gurka, Roi; Holzman, Roi; Reesbeck, Thomas; Diez, F. Javier

2013-08-01

120

Numerical investigation of medium wavelength instabilities in four vortex wake systems  

NASA Astrophysics Data System (ADS)

Wake vortex systems generated by wings with modified span loading can generate, after rollup, multiple vortex systems. We here consider the four vortex system investigated experimentally by Ortega et al.: two pairs of counter-rotating vortices with circulation ratio ?_2/?_1=-0.37 and inner spacing ratio b_2/b1 = 0.48. Different numerical tools are used to investigate the dynamics of the most unstable medium wavelength (?/b_1?1) instability: a vortex filament method with random initial perturbations and of longitudinal extent equivalent to many times ?; a spectral code run on a single processor (about 9 millions grid points for a simulation with an extent of ?), and on multiple processors (for simulations with an extent of many ?) thus also allowing for random initial perturbations and eventually also capturing the Crow-type instability of the equivalent two vortex system. The spectral code is run in both DNS mode (for moderate Reynolds number) and ``quasi-inviscid" mode (using a hyper-viscosity of type nabla^8 u. Results are presented on stability (modes, growth rates, saturation), vortex topological changes by reconnection, and related energy decay. Comparisons are also made with the experimental results of Ortega et al.

Dufresne, Louis; Winckelmans, Gregoire; Capart, Raphael

2003-11-01

121

Comparisons of Crosswind Velocity Profile Estimates Used in Fast-Time Wake Vortex Prediction Models  

NASA Technical Reports Server (NTRS)

Five methods for estimating crosswind profiles used in fast-time wake vortex prediction models are compared in this study. Previous investigations have shown that temporal and spatial variations in the crosswind vertical profile have a large impact on the transport and time evolution of the trailing vortex pair. The most important crosswind parameters are the magnitude of the crosswind and the gradient in the crosswind shear. It is known that pulsed and continuous wave lidar measurements can provide good estimates of the wind profile in the vicinity of airports. In this study comparisons are made between estimates of the crosswind profiles from a priori information on the trajectory of the vortex pair as well as crosswind profiles derived from different sensors and a regional numerical weather prediction model.

Pruis, Mathew J.; Delisi, Donald P.; Ahmad, Nashat N.

2011-01-01

122

Modeling von Karman vortex shedding in cylinder wake to examine energetic coherent motions on hydrokinetic turbines  

NASA Astrophysics Data System (ADS)

Numerous investigators have examined vortex-shedding in the wake of cylinders. This is a classical flow problem that has many engineering applications, including pronounced flow disturbance, turbulence generation, and sediment scour in the wakes of in stream structures, e.g. bridge piers and towers for marine and hydrokinetic (MHK) turbines. It is also important to understand the contribution of large coherent motions on the unsteady loading and performance of hydrokinetic turbines. Unsteady vortex shedding is caused by flow separation and detachment within the near-wall region along the cylinder surface. Our aim is to examine the unsteady flow field and von Karman vortex shedding resulting from unsteady turbulent flow around an emergent cylinder mounted perpendicular to a fixed surface by conducting physical and numerical modeling experiments. The numerical simulation emulates an open-channel flow experiment at the St. Anthony Falls Laboratory at the University of Minnesota, where instantaneous velocity was measured using three synchronized acoustic Doppler velocimeters (ADVs). The open-channel flume is 80 m long, and 2.75 m wide. The flow depth is 1.15 m. The cylinder diameter is 0.116 m. The flow is turbulent, with a cylinder Reynolds number equal to 5.44E4. We use the commercial CFD software, STAR-CCM+, to generate the computational mesh that models the flow geometry around the cylinder, and to numerically solve the unsteady Reynolds-Averaged Navier-Stokes (URANS) equations. The generated mesh is fine enough (> 2 million elements) to resolve the coherent structures of vortex shedding. The Frost high-performance cluster (an ORNL supercomputer) is used to run the simulation. The results show how a validated CFD model can be used to design the layout and spacing of synchronized ADV point measurements to characterize essential features of the Karman shedding in the cylinder wake. A similar approach can be used to design field ADV arrays for measuring more complex vortex shedding, e.g. the tip vortices, occurring in the wakes of MHK turbine rotors.; Numerical simulation of Karman shedding in the wake of cylinder (diameter 0.116 m, Reynolds number, 5.44E4).

Neary, V. S.; Gunawan, B.; Chamorro, L. P.; Stekovic, S.; Hill, C.

2012-12-01

123

Flight Test Analysis of the Forces and Moments Imparted on a B737-100 Airplane During Wake Vortex Encounters  

NASA Technical Reports Server (NTRS)

Aircraft travel has become a major form of transportation. Several of our major airports are operating near their capacity limit, increasing congestion and delays for travelers. As a result, the National Aeronautics and Space Administration (NASA) has been working in conjunction with the Federal Aviation Administration (FAA), airline operators, and the airline industry to increase airport capacity without sacrificing public safety. One solution to the problem is to increase the number of airports and build new. runways; yet, this solution is becoming increasingly difficult due to limited space. A better solution is to increase the production per runway. This solution increases the possibility that one aircraft will encounter the trailing wake of another aircraft. Hazardous wake vortex encounters occur when an aircraft encounters the wake produced by a heavier aircraft. This heavy-load aircraft produces high-intensity wake turbulence that redistributes the aerodynamic loads of trailing smaller aircraft. This situation is particularly hazardous for smaller aircraft during takeoffs and landings. In order to gain a better understanding of the wake-vortex/aircraft encounter phenomena, NASA Langley Research Center conducted a series of flight tests from 1995 through 1997. These tests were designed to gather data for the development a wake encounter and wake-measurement data set with the accompanying atmospheric state information. This data set is being compiled into a database that can be used by wake vortex researchers to compare with experimental and computational results. The purpose of this research is to derive and implement a procedure for calculating the wake-vortex/aircraft interaction portion of that database by using the data recorded during those flight tests. There were three objectives to this research. Initially, the wake-induced forces and moments from each flight were analyzed based on varying flap deflection angles. The flap setting alternated between 15 and 30 degrees while the separation distance remained constant. This examination was performed to determine if increases in flap deflection would increase or decrease the effects of the wake-induced forces and moments. Next, the wake-induced forces and moments from each flight were analyzed based on separation distances of 1-3 nautical miles. In this comparison, flap deflection was held constant at 30 degrees. The purpose of this study was to determine if increased separation distances reduced the effects of the wake vortex on the aircraft. The last objective compared the wake-induced forces and moments of each flight as it executed a series of maneuvers through the wake-vortex. This analysis was conducted to examine the impact of the wake on the B737 as it traversed the wake horizontally and vertically. Results from the first analysis indicated that there was no difference in wake effect at flap deflections of 15 and 30 degrees. This conclusion is evidenced in the cases of the wake-induced sideforce, rolling moment, and yawing moment. The wake-induced lift, drag, and pitching moment cases yielded less conclusive results. The second analysis compared the wake-induced forces and moments at separation distances of 1-3 nautical miles. Results indicated that there was no significant difference in the wake-induced lift, drag, sideforce, or yawing moment coefficients. The analysis compared the wake-induced forces and moments based on different flight maneuvers. It was found that the wake-induced forces and moments had the greatest impact on out-to-in and in-to-out maneuvers.

Roberts, Chistopher L.

2001-01-01

124

Vortex wake and flight kinematics of a swift in cruising flight in a wind tunnel.  

PubMed

In this paper we describe the flight characteristics of a swift (Apus apus) in cruising flight at three different flight speeds (8.0, 8.4 and 9.2 m s(-1)) in a low turbulence wind tunnel. The wingbeat kinematics were recorded by high-speed filming and the wake of the bird was visualized by digital particle image velocimetry (DPIV). Certain flight characteristics of the swift differ from those of previously studied species. As the flight speed increases, the angular velocity of the wingbeat remains constant, and so as the wingbeat amplitude increases, the frequency decreases accordingly, as though the flight muscles were contracting at a fixed rate. The wings are also comparatively inflexible and are flexed or retracted rather little during the upstroke. The upstroke is always aerodynamically active and this is reflected in the wake, where shedding of spanwise vorticity occurs throughout the wingbeat. Although the wake superficially resembles those of other birds in cruising flight, with a pair of trailing wingtip vortices connected by spanwise vortices, the continuous shedding of first positive vorticity during the downstroke and then negative vorticity during the upstroke suggests a wing whose circulation is gradually increasing and then decreasing during the wingbeat cycle. The wake (and implied wing aerodynamics) are not well described by discrete vortex loop models, but a new wake-based model, where incremental spanwise and streamwise variations of the wake impulse are integrated over the wingbeat, shows good agreement of the vertical momentum flux with the required weight support. The total drag was also estimated from the wake alone, and the calculated lift:drag ratio of approximately 13 for flapping flight is the highest measured yet for birds. PMID:18281334

Henningsson, P; Spedding, G R; Hedenström, A

2008-03-01

125

A Sensitivity Study of the Aircraft Vortex Spacing System (AVOSS) Wake Predictor Algorithm to the Resolution of Input Meteorological Profiles  

NASA Technical Reports Server (NTRS)

The AVOSS project demonstrated the feasibility of applying aircraft wake vortex sensing and prediction technologies to safe aircraft spacing for single runway arrivals. On average, AVOSS provided spacing recommendations that were less than the current FAA prescribed spacing rules, resulting in a potential airport efficiency gain. Subsequent efforts have included quantifying the operational specifications for future Wake Vortex Advisory Systems (WakeVAS). In support of these efforts, each of the candidate subsystems for a WakeVAS must be specified. The specifications represent a consensus between the high-level requirements and the capabilities of the candidate technologies. This report documents the beginnings of an effort to quantify the capabilities of the AVOSS Prediction Algorithm (APA). Specifically, the APA horizontal position and circulation strength output sensitivity to the resolution of its wind and turbulence inputs is examined. The results of this analysis have implications for the requirements of the meteorological sensing and prediction systems comprising a WakeVAS implementation.

Rutishauser, David K.; Butler, Patrick; Riggins, Jamie

2004-01-01

126

Dynamic Wind Loads and Vortex Structures in the Wake of a Wind Turbine  

NASA Astrophysics Data System (ADS)

We report an experimental study to characterize the dynamic wind loads and evolution of wake vortex flow structures downstream of a horizontal axis wind turbine (HAWT). The experiments were conducted in a wind tunnel with a wind turbine model placed in a boundary layer flow developed over rough and smooth surfaces in order to study the effects of roughness and the resulting velocity and turbulence fields on the wake characteristics and fatigue loads acting on the wind turbine. In addition to measuring dynamic wind loads (both aerodynamic forces and moments) acting on the wind turbine model using a six-component load cell, a high-resolution Particle Image Velocimetry (PIV) system was used to make phase-locked flow field measurements to quantify the time-evolution of the wake vortex and turbulence flow structures shedding from wind turbine blades. The detailed flow field measurements were correlated with the wind load measurements to elucidate the underlying physics associated with turbine power generation and fatigue loads acting on wind turbines.

Hu, Hui; Yang, Zifeng; Sarkar, Partha

2010-11-01

127

Computation of vortex wake flows and control of their effects on trailing wings  

NASA Technical Reports Server (NTRS)

The near-vortex-wake flow of a large aspect-ratio rectangular wing is accurately computed by using the thin-layer and full Navier-Stokes (NS) equations. The chordwise section of the wing is a NACA-0012 airfoil and its tip is round. The computations have been carried out on a fine C-O grid using an implicit, upwind, flux-difference splitting, finite-volume scheme. The thin-layer NS results have been obtained with and without flux limiters, and the full NS results have been obtained without flux limiters. Flow transition from laminar to turbulent is mimicked by turning-on the Baldwin-Lomax algebraic model at an experimentally prescribed chord-station location of 0.05. Comparison of computed results and experimental data shows that the full NS results give the best resolution of the near-vortex-wake flow. Next, the strength of the wing-tip vortex has been reduced substantially without reducing the lift coefficient by using flow-injection from a slot along a portion of the wing tip. The flow injection is directed in the wing plane at 45 deg with the wing-tip chord.

Wong, Tin-Chee; Kandil, Osama A.; Liu, C. H.

1992-01-01

128

Effect of wake structure on blade-vortex interaction phenomena: Acoustic prediction and validation  

NASA Technical Reports Server (NTRS)

During the Higher Harmonic Control Aeroacoustic Rotor Test, extensive measurements of the rotor aerodynamics, the far-field acoustics, the wake geometry, and the blade motion for powered, descent, flight conditions were made. These measurements have been used to validate and improve the prediction of blade-vortex interaction (BVI) noise. The improvements made to the BVI modeling after the evaluation of the test data are discussed. The effects of these improvements on the acoustic-pressure predictions are shown. These improvements include restructuring the wake, modifying the core size, incorporating the measured blade motion into the calculations, and attempting to improve the dynamic blade response. A comparison of four different implementations of the Ffowcs Williams and Hawkings equation is presented. A common set of aerodynamic input has been used for this comparison.

Gallman, Judith M.; Tung, Chee; Schultz, Klaus J.; Splettstoesser, Wolf; Buchholz, Heino

1995-01-01

129

Unsteady vortex lattice techniques applied to wake formation and performance of the statically thrusting propeller  

NASA Technical Reports Server (NTRS)

The application is considered of vortex lattice techniques to the problem of describing the aerodynamics and performance of statically thrusting propellers. A numerical lifting surface theory to predict the aerodynamic forces and power is performed. The chordwise and spanwise loading is modelled by bound vortices fixed to a twisted flat plate surface. In order to eliminate any apriori assumptions regarding the wake shape, it is assumed the propeller starts from rest. The wake is generated in time and allowed to deform under its own self-induced velocity field as the motion of the propeller progresses. The bound circulation distribution is then determined with time by applying the flow tangency boundary condition at certain selected control points on the blades. The aerodynamics of the infinite wing and finite wing are also considered. The details of wake formation and roll-up are investigated, particularly the localized induction effect. It is concluded that proper wake roll-up and roll-up rates can be established by considering the details of motion at the instant of start.

Hall, G. F.

1975-01-01

130

Flutter clearance flight tests of an OV-10A airplane modified for wake vortex flight experiments  

NASA Technical Reports Server (NTRS)

The envelope expansion, flight flutter tests of a modified OV-10A aircraft are described. For the wake vortex research program, the airplane was modified to incorporate three forward-extending instrumentation booms, one extending forward from each wing tip and one from the right side of the fuselage. The booms were instrumented with sensors to measure the velocity and direction of local air flow. The flutter test results show that the modified OV-10A aircraft is free from flutter at speeds up to 330 KEAS at 5000 feet altitude.

Doggett, Robert V., Jr.; Rivera, Jose A., Jr.; Stewart, Eric C.

1995-01-01

131

On the response and wake modes of a cylinder undergoing streamwise vortex-induced vibration  

NASA Astrophysics Data System (ADS)

This brief communication elaborates on aspects of the free response and wake modes of a flexibly-supported cylinder undergoing vortex-induced vibration in the streamwise direction only in view of some recent studies in the literature. It discusses the meaning of the term 'lock-in' employed in the context of forced excitation and free response studies, the influence of added mass on the response frequency, the competition between alternating and symmetrical modes and associated excitation mechanisms, and the drop in amplitude in the range separating the two response branches. A few similarities between single-degree-of-freedom streamwise and transverse vibrations are brought to light. Finally, attention is drawn to some discrepancies between published results which call for further understanding of the complicated nature of vortex-induced vibration.

Konstantinidis, Efstathios

2014-02-01

132

Numerical studies of three-dimensional breakdown in trailing vortex wakes  

NASA Technical Reports Server (NTRS)

Finite element, three dimensional relaxation methods are used to calculate the development of vortex wakes behind aircraft for a considerable downstream distance. The inclusion of a self-induction term in the solution, dependent upon local curvature and vortex core radius, permits calculation of finite lifetimes for systems for which infinite life would be predicted two dimensionally. The associated computer program is described together with single-pair, twin-pair, and multiple-pair studies carried out using it. It is found, in single-pair studies, that there is a lower limit to the wavelengths at which the Crow-type of instability can occur. Below this limit, self-induction effects cause the plane of the disturbance waves to rotate counter to the vortex direction. Self induction in two dimensionally generated twin spiral waves causes an increase in axial length which becomes more marked with decreasing initial wavelength. The time taken for vortex convergence toward the center plane is correspondingly increased. The limited parametric twin-pair study performed suggests that time-to-converge increases with increasing flap span. Limited studies of Boeing 747 configurations show correct qualitative response to removal of the outer flap and to gear deployment, as compared with wind tunnel and flight test experience.

Evans, P. F.; Hackett, J. E.

1976-01-01

133

Volumetric imaging of shark tail hydrodynamics reveals a three-dimensional dual-ring vortex wake structure  

PubMed Central

Understanding how moving organisms generate locomotor forces is fundamental to the analysis of aerodynamic and hydrodynamic flow patterns that are generated during body and appendage oscillation. In the past, this has been accomplished using two-dimensional planar techniques that require reconstruction of three-dimensional flow patterns. We have applied a new, fully three-dimensional, volumetric imaging technique that allows instantaneous capture of wake flow patterns, to a classic problem in functional vertebrate biology: the function of the asymmetrical (heterocercal) tail of swimming sharks to capture the vorticity field within the volume swept by the tail. These data were used to test a previous three-dimensional reconstruction of the shark vortex wake estimated from two-dimensional flow analyses, and show that the volumetric approach reveals a different vortex wake not previously reconstructed from two-dimensional slices. The hydrodynamic wake consists of one set of dual-linked vortex rings produced per half tail beat. In addition, we use a simple passive shark-tail model under robotic control to show that the three-dimensional wake flows of the robotic tail differ from the active tail motion of a live shark, suggesting that active control of kinematics and tail stiffness plays a substantial role in the production of wake vortical patterns.

Flammang, Brooke E.; Lauder, George V.; Troolin, Daniel R.; Strand, Tyson

2011-01-01

134

Volumetric imaging of shark tail hydrodynamics reveals a three-dimensional dual-ring vortex wake structure.  

PubMed

Understanding how moving organisms generate locomotor forces is fundamental to the analysis of aerodynamic and hydrodynamic flow patterns that are generated during body and appendage oscillation. In the past, this has been accomplished using two-dimensional planar techniques that require reconstruction of three-dimensional flow patterns. We have applied a new, fully three-dimensional, volumetric imaging technique that allows instantaneous capture of wake flow patterns, to a classic problem in functional vertebrate biology: the function of the asymmetrical (heterocercal) tail of swimming sharks to capture the vorticity field within the volume swept by the tail. These data were used to test a previous three-dimensional reconstruction of the shark vortex wake estimated from two-dimensional flow analyses, and show that the volumetric approach reveals a different vortex wake not previously reconstructed from two-dimensional slices. The hydrodynamic wake consists of one set of dual-linked vortex rings produced per half tail beat. In addition, we use a simple passive shark-tail model under robotic control to show that the three-dimensional wake flows of the robotic tail differ from the active tail motion of a live shark, suggesting that active control of kinematics and tail stiffness plays a substantial role in the production of wake vortical patterns. PMID:21543357

Flammang, Brooke E; Lauder, George V; Troolin, Daniel R; Strand, Tyson

2011-12-22

135

Turbulent wake and vortex shedding for a stack partially immersed in a turbulent boundary layer  

NASA Astrophysics Data System (ADS)

The effect of the jet-to-cross-flow velocity ratio, R, on the turbulent wake and Kármán vortex shedding for a cylindrical stack of aspect ratio AR=9 was investigated in a low-speed wind tunnel using thermal anemometry. The cross-flow Reynolds number was ReD=2.3×104, the jet Reynolds number ranged from Red=7.6×103 to 4.7×104, and R was varied from 0 to 3. The stack was partially immersed in a flat-plate turbulent boundary layer, with a boundary layer thickness-to-stack-height ratio of ?/H=0.5 at the location of the stack. From the behaviour of the turbulent wake and the vortex shedding, the flow around the stack could be classified into three regimes depending on the value of R, which were the downwash (R<0.7), cross-wind-dominated (0.7?R<1.5), and jet-dominated (R?1.5) flow regimes. Each flow regime had a distinct structure to the mean velocity (streamwise and wall-normal directions), turbulence intensity (streamwise and wall-normal directions), and Reynolds shear stress fields, as well as the variation of the Strouhal number and the power spectrum along the stack height.

Adaramola, M. S.; Sumner, D.; Bergstrom, D. J.

2007-11-01

136

Numerical study of particle distribution in wake of liquid-particle flows past a circular cylinder using discrete vortex method  

Microsoft Academic Search

Particle-laden water flows past a circular cylinder were numerically investigated. The discrete vortex method (DVM) was employed\\u000a to evaluate the unsteady water flow fields and a Lagrangian approach was applied for tracking individual solid particles.\\u000a A dispersion function was defined to represent the dispersion scale of the particle. The wake vortex patterns, the distributions\\u000a and the time series of dispersion

Yuan-dong Huang; Wen-quan Wu

2006-01-01

137

B-747 in Flight during Vortex Study with Learjet and T-37 Fly Through the Wake  

NASA Technical Reports Server (NTRS)

In this 1974 NASA Flight Research Center (FRC) photograph, the two chase aircraft, a Learjet and a Cessna T-37, are shown in formation off the right wing tip of the Boeing B-747 jetliner. The two chase aircraft were used to probe the trailing wake vortices generated by the airflow around the wings of the B-747 aircraft. The vortex trail behind the right wing tip was made visible by a smoke generator mounted under the wing of the B-747 aircraft. In 1974 the NASA Flight Research Center (later Dryden Flight Research Center, Edwards, California) used a Boeing 747 as part of the overall NASA study of trailing vortices. Trailing vortices are the invisible flow of spiraling air that trails from the wings of large aircraft and can 'upset' smaller aircraft flying behind them. The 747 that NASA used was on loan from the Johnson Space Center where it was part of the Space Shuttle Program. The data gathered in the 747 studies complemented data from the previous (1973-74) joint NASA Flight Research Center and Federal Aviation Administration (FAA) Boeing727 wake vortices study. Six smoke generators were installed under the wings of the 747 to provide a visual image of the trailing vortices. The object of the experiments was to test different configurations and mechanical devices on the747 that could be used to break up or lessen the strength of the vortices. The results of the tests could lead to shorter spacing between landings and takeoffs, which, in turn, could alleviate air-traffic congestion. For approximately 30 flights the 747 was flown using various combinations of wing air spoilers in an attempt to reduce wake vortices. To evaluate the effectiveness of the different configurations, chase aircraft were flown into the vortex sheets to probe their strengths and patterns at different times. Two of the chase planes used were the Flight Research Center's Cessna T-37 and the NASA Ames Research Center's Learjet. These aircraft represented the types of smaller business jets and other small aircraft that might encounter large passenger aircraft on approach or landings around major airports or in flight. Tests without the 747's wing spoilers deployed produced violent 'upset' problems for the T-37 aircraft at a distance of approximately 3 miles. From the magnitude of the problems found, distances of as much as ten miles might be required if spoilers were not used. With two spoilers on the outer wing panels, the T-37 could fly at a distance of three miles and not experience the 'upset' problem. The wake vortex study continued even after the 747 was returned to its primary mission of carrying the Space Shuttle.

1974-01-01

138

Optical vortex behavior in dynamic speckle fields.  

PubMed

The dynamic behavior of phase singularities, or optical vortices, in the pseudo-phase representation of dynamic speckle patterns is investigated. Sequences of band-limited, dynamic speckle patterns with predetermined Gaussian decorrelation behavior were generated, and the pseudo-phase realizations of the individual speckle patterns were calculated via a two-dimensional Hilbert transform algorithm. Singular points in the pseudo-phase representation are identified by calculating the local topological charge as determined by convolution of the pseudo-phase representations with a series of 2×2 nabla filters. The spatial locations of the phase singularities are tracked over all frames of the speckle sequences, and recorded in three-dimensional space (x,y,f), where f is frame number in the sequence. The behavior of the phase singularities traces 'vortex trails' which are representative of the speckle dynamics. Slowly decorrelating speckle patterns results in long, relatively straight vortex trails, while rapidly decorrelating speckle patterns results in tortuous, relatively short vortex trails. Optical vortex analysis such as described herein can be used as a descriptor of biological activity, flow, and motion. PMID:22612119

Kirkpatrick, Sean J; Khaksari, Kosar; Thomas, Dennis; Duncan, Donald D

2012-05-01

139

Three-dimensional vortex breakdown in swirling jets and wakes: direct numerical simulation  

NASA Astrophysics Data System (ADS)

Vortex breakdown of nominally axisymmetric, swirling incompressible flows with jet- and wake-like axial velocity distributions issuing into a semi-infinite domain is studied by means of direct numerical simulations. By selecting a two-parametric velocity profile for which the steady axisymmetric breakdown is well-studied (Grabowski & Berger 1976), issues are addressed regarding the role of three-dimensionality and unsteadiness with respect to the existence, mode selection, and internal structure of vortex breakdown, in terms of the two governing parameters and the Reynolds number. Low Reynolds numbers are found to yield flow fields lacking breakdown bubbles or helical breakdown modes even for high swirl. In contrast, highly swirling flows at large Reynolds numbers exhibit bubble, helical or double-helical breakdown modes, where the axisymmetric mode is promoted by a jet-like axial velocity profile, while a wake-like profile renders the flow helically unstable and ultimately yields non-axisymmetric breakdown modes. It is shown that a transition from super- to subcritical flow, as defined by Benjamin (1962), accurately predicts the parameter combination yielding breakdown, if applied locally to flows with supercritical inflow profiles. Thus the basic form of breakdown is axisymmetric, and a transition to helical breakdown modes is shown to be caused by a sufficiently large pocket of absolute instability in the wake of the bubble, giving rise to a self-excited global mode. Two distinct eigenfunctions corresponding to azimuthal wavenumbers m {=} {-}1 and m {=} {-}2 have been found to yield a helical or double-helical breakdown mode, respectively. Here the minus sign represents the fact that the winding sense of the spiral is opposite to that of the flow.

Ruith, M. R.; Chen, P.; Meiburg, E.; Maxworthy, T.

2003-07-01

140

Vortex wake, downwash distribution, aerodynamic performance and wingbeat kinematics in slow-flying pied flycatchers  

PubMed Central

Many small passerines regularly fly slowly when catching prey, flying in cluttered environments or landing on a perch or nest. While flying slowly, passerines generate most of the flight forces during the downstroke, and have a ‘feathered upstroke’ during which they make their wing inactive by retracting it close to the body and by spreading the primary wing feathers. How this flight mode relates aerodynamically to the cruising flight and so-called ‘normal hovering’ as used in hummingbirds is not yet known. Here, we present time-resolved fluid dynamics data in combination with wingbeat kinematics data for three pied flycatchers flying across a range of speeds from near hovering to their calculated minimum power speed. Flycatchers are adapted to low speed flight, which they habitually use when catching insects on the wing. From the wake dynamics data, we constructed average wingbeat wakes and determined the time-resolved flight forces, the time-resolved downwash distributions and the resulting lift-to-drag ratios, span efficiencies and flap efficiencies. During the downstroke, slow-flying flycatchers generate a single-vortex loop wake, which is much more similar to that generated by birds at cruising flight speeds than it is to the double loop vortex wake in hovering hummingbirds. This wake structure results in a relatively high downwash behind the body, which can be explained by the relatively active tail in flycatchers. As a result of this, slow-flying flycatchers have a span efficiency which is similar to that of the birds in cruising flight and which can be assumed to be higher than in hovering hummingbirds. During the upstroke, the wings of slowly flying flycatchers generated no significant forces, but the body–tail configuration added 23 per cent to weight support. This is strikingly similar to the 25 per cent weight support generated by the wing upstroke in hovering hummingbirds. Thus, for slow-flying passerines, the upstroke cannot be regarded as inactive, and the tail may be of importance for flight efficiency and possibly manoeuvrability.

Muijres, Florian T.; Bowlin, Melissa S.; Johansson, L. Christoffer; Hedenstrom, Anders

2012-01-01

141

Vortex wake, downwash distribution, aerodynamic performance and wingbeat kinematics in slow-flying pied flycatchers.  

PubMed

Many small passerines regularly fly slowly when catching prey, flying in cluttered environments or landing on a perch or nest. While flying slowly, passerines generate most of the flight forces during the downstroke, and have a 'feathered upstroke' during which they make their wing inactive by retracting it close to the body and by spreading the primary wing feathers. How this flight mode relates aerodynamically to the cruising flight and so-called 'normal hovering' as used in hummingbirds is not yet known. Here, we present time-resolved fluid dynamics data in combination with wingbeat kinematics data for three pied flycatchers flying across a range of speeds from near hovering to their calculated minimum power speed. Flycatchers are adapted to low speed flight, which they habitually use when catching insects on the wing. From the wake dynamics data, we constructed average wingbeat wakes and determined the time-resolved flight forces, the time-resolved downwash distributions and the resulting lift-to-drag ratios, span efficiencies and flap efficiencies. During the downstroke, slow-flying flycatchers generate a single-vortex loop wake, which is much more similar to that generated by birds at cruising flight speeds than it is to the double loop vortex wake in hovering hummingbirds. This wake structure results in a relatively high downwash behind the body, which can be explained by the relatively active tail in flycatchers. As a result of this, slow-flying flycatchers have a span efficiency which is similar to that of the birds in cruising flight and which can be assumed to be higher than in hovering hummingbirds. During the upstroke, the wings of slowly flying flycatchers generated no significant forces, but the body-tail configuration added 23 per cent to weight support. This is strikingly similar to the 25 per cent weight support generated by the wing upstroke in hovering hummingbirds. Thus, for slow-flying passerines, the upstroke cannot be regarded as inactive, and the tail may be of importance for flight efficiency and possibly manoeuvrability. PMID:21676971

Muijres, Florian T; Bowlin, Melissa S; Johansson, L Christoffer; Hedenström, Anders

2012-02-01

142

Atmospheric Boundary Layer Sensors for Application in a Wake Vortex Advisory System  

NASA Technical Reports Server (NTRS)

Remote sensing of the atmospheric boundary layer has advanced in recent years with the development of commercial off-the-shelf (COTS) radar, sodar, and lidar wind profiling technology. Radio acoustic sounding systems for vertical temperature profiles of high temporal scales (when compared to routine balloon soundings- (radiosondes) have also become increasingly available as COTS capabilities. Aircraft observations during landing and departures are another source of available boundary layer data. This report provides an updated assessment of available sensors, their performance specifications and rough order of magnitude costs for a potential future aircraft Wake Vortex Avoidance System (WakeVAS). Future capabilities are also discussed. Vertical profiles of wind, temperature, and turbulence are anticipated to be needed at airports in any dynamic wake avoidance system. Temporal and spatial resolution are dependent on the selection of approach and departure corridors to be protected. Recommendations are made for potential configurations of near-term sensor technologies and for testing some of the sensor systems in order to validate performance in field environments with adequate groundtruth.

Zak, J. Allen; Rutishauser, David (Technical Monitor)

2003-01-01

143

Overview of the preparation and use of an OV-10 aircraft for wake vortex hazards flight experiments  

NASA Technical Reports Server (NTRS)

An overview is presented of the development, use, and current flight-test status of a highly instrumented North American Rockwell OV-10A Bronco as a wake-vortex-hazards research aircraft. A description of the operational requirements and measurements criteria, the resulting instrumentation systems and aircraft modifications, system-calibration and research flights completed to date, and current flight status are included. These experiments are being conducted by the National Aeronautics and Space Administration as part of an effort to provide the technology to safely improve the capacity of the nation's air transportation system and specifically to provide key data in understanding and predicting wake vortex decay, transport characteristics, and the dynamics of encountering wake turbulence. The OV-10A performs several roles including meteorological measurements platform, wake-decay quantifier, and trajectory-quantifier for wake encounters. Extensive research instrumentation systems include multiple airdata sensors, video cameras with cockpit displays, aircraft state and control-position measurements, inertial aircraft-position measurements, meteorological measurements, and an on-board personal computer for real-time processing and cockpit display of research data. To date, several of the preliminary system check flights and two meteorological-measurements deployments have been completed. Several wake encounter and wake-decay-measurements flights are planned for the fall of 1995.

Stuever, Robert A.; Stewart, Eric C.; Rivers, Robert A.

1995-01-01

144

Flight Test Analysis of the Forces and Moments Imparted on a B737-100 Aircraft During Wake Vortex Encounters  

NASA Technical Reports Server (NTRS)

Several of our major airports are operating at or near their capacity limit, increasing congestion and delays for travelers. As a result, the National Aeronautics and Space Administration (NASA) has been working in conjunction with the Federal Aviation Administration (FAA), airline operators, and the airline industry to increase airport capacity and safety. As more and more airplanes are placed into the terminal area the probability of encountering wake turbulence is increased. The NASA Langley Research Center conducted a series of flight tests from 1995 through 1997 to develop a wake encounter and wake-measurement data set with the accompanying atmospheric state information. The purpose of this research is to use the data from those flights to compute the wake-induced forced and moments exerted on the aircraft The calculated forces and moments will then be compiled into a database that can be used by wake vortex researchers to compare with experimental and computational results.

Roberts, Christopher L.; Smith, Sonya T.; Vicroy, Dan D.

2000-01-01

145

Study of the feasibility of using a sailplane as an instrument platform for the study of wake vortex phenomena  

NASA Technical Reports Server (NTRS)

The feasibility of utilizing instrumentation mounted on a sailplane wing is investigated to determine vortex wakes from a large aircraft. The instrumentation consisted of static and total pressure tubes and a rotating vane vorticity meter mounted in a pod on the glider wing tip. It was concluded that the study was not feasible.

Ormsbee, A. I.

1974-01-01

146

Dynamics and flow structures in the turbulent wake of rigid and flexible cylinders subject to vortex-induced vibrations  

Microsoft Academic Search

We present simulation results of vortex-induced vibrations of an infinitely long flexible cylinder at Reynolds number Re = 1000, corresponding to a `young' turbulent wake (i.e. exhibiting a small inertial subrange). The simulations are based on a new class of spectral methods suitable for unstructured and hybrid grids. To obtain different responses of the coupled flow structure system we vary

Constantinos Evangelinos; George Em Karniadakis

1999-01-01

147

Ground-Based Measurements of the Wake Vortex Characteristics of a B-747 Aircraft in Various Configurations.  

National Technical Information Service (NTIS)

A Boeing 747 aircraft flew 54 passes at low altitude over ground-based sensors. Vortex velocities were measured by a laser Doppler velocimeter, an array of monostatic acoustic sounders, and an array of propeller anemometers. Flow visualization of the wake...

D. C. Burnham J. N. Hallock I. H. Tombach M. R. Brashears M. R. Barber

1978-01-01

148

Study of the far wake vortex field generated by a rectangular airfoil in a water tank  

NASA Technical Reports Server (NTRS)

Underwater towing experiments were carried out with a rectangular airfoil of aspect ratio 5.3 at 4 and 8 deg angles of attack and at chord-based Reynolds numbers between 2 x 100,000 and 7.5 x 100,000. Quantitative measurements by means of the hydrogen bubble technique indicated lower peak swirl velocities in the range of 100 to 1000 lenghts downstream than have been measured in wind tunnel of flight tests. The maximum circumferential velocity decayed whereas the turbulent eddy viscosity increased. This behavior and other known rates of vortex decay are explained in terms of an analytical solution for the vortex problem with time varying eddy viscosity. It is shown that this case corresponds to nonequilibrium turbulent vortex flow.

Lezius, D. K.

1973-01-01

149

Computation of potential flows with embedded vortex rings and applications to helicopter rotor wakes  

NASA Technical Reports Server (NTRS)

A finite difference scheme for solving the motion of a number of vortex rings is developed. The method is an adaptation of the 'cloud-in-cell' technique to axisymmetric flows, and is thus a combined Eulerian-Lagrangian technique. A straightforward adaptation of the cloud-in-cell scheme to an axisymmetric flow field is shown to introduce a grid dependent self-induced velocity to each vortex ring. To correct this behavior the potential is considered to consist of two parts, a local and a global field. An improved difference formula is derived, allowing the accurate calculation of the potential at points near vortex locations. The local potential is then subtracted before calculating the velocity, leaving only the influences of the remaining vortices. The correct self-induced velocity is then explicitly added to the vortex velocity. Calculations of the motion of one and two vortex rings are performed, demonstrating the ability of the new method to eliminate the grid dependence of the self-induced velocity. The application of the method to the calculation of helicopter rotor flows in hover is attempted.

Roberts, T. W.

1983-01-01

150

Rotor Wake Vortex Definition: Initial Evaluation of 3-C PIV Results of the Hart-II Study  

NASA Technical Reports Server (NTRS)

An initial evaluation is made of extensive three-component (3C) particle image velocimetry (PIV) measurements within the wake across a rotor disk plane. The model is a 40 percent scale BO-105 helicopter main rotor in forward flight simulation. This study is part of the HART II test program conducted in the German-Dutch Wind Tunnel (DNW). Included are wake vortex field measurements over the advancing and retreating sides of the rotor operating at a typical descent landing condition important for impulsive blade-vortex interaction (BVI) noise. Also included are advancing side results for rotor angle variations from climb to steep descent. Using detailed PIV vector maps of the vortex fields, methods of extracting key vortex parameters are examined and a new method was developed and evaluated. An objective processing method, involving a center-of-vorticity criterion and a vorticity 'disk' integration, was used to determine vortex core size, strength, core velocity distribution characteristics, and unsteadiness. These parameters are mapped over the rotor disk and offer unique physical insight for these parameters of importance for rotor noise and vibration prediction.

Burley, Casey L.; Brooks, Thomas F.; vanderWall, Berend; Richard, Hughes; Raffel, Markus; Beaumier, Philippe; Delrieux, Yves; Lim, Joon W.; Yu, Yung H.; Tung, Chee

2002-01-01

151

Effect of natural ventilation on the boundary layer separation and near-wake vortex shedding characteristics of a sphere  

Microsoft Academic Search

Experiments were conducted in water and wind tunnels on spheres in the Reynolds number range 6?×?103 to 6.5?×?105 to study the effect of natural ventilation on the boundary layer separation and near-wake vortex shedding characteristics.\\u000a In the subcritical range of Re (5), ventilation caused a marginal downstream shift in the location of laminar boundary layer separation; there was only a

G. K. Suryanarayana; A. Prabhu

2000-01-01

152

An experimental study of the unsteady vortex structures in the wake of a root-fixed flapping wing  

Microsoft Academic Search

An experimental study was conducted to characterize the evolution of the unsteady vortex structures in the wake of a root-fixed\\u000a flapping wing with the wing size, stroke amplitude, and flapping frequency within the range of insect characteristics for\\u000a the development of novel insect-sized nano-air-vehicles (NAVs). The experiments were conducted in a low-speed wing tunnel\\u000a with a miniaturized piezoelectric wing (i.e.,

Hui Hu; Lucas Clemons; Hirofumi Igarashi

2011-01-01

153

Numerical simulation of unsteady vortex structures in near wake of poorly streamlined bodies on multiprocessor computer system  

NASA Astrophysics Data System (ADS)

On the basis of the conservative difference method, spatially unsteady flows near complexly shaped objects are studied. The mathematical model is based on the inviscid gas model. For subsonic, transonic, and supersonic regimes, the nonstationary aerodynamics of various aerospace objects is examined. The three-dimensional structure of the unsteady vortex near wake and its influence on the basic aerodynamic characteristics of aerial vehicles are visualized. The numerical simulation is performed using parallel algorithms on supercomputers of cluster architecture.

Babakov, A. V.; Novikov, P. A.

2011-02-01

154

Serotonin control of sleep-wake behavior.  

PubMed

Based on electrophysiological, neurochemical, genetic and neuropharmacological approaches, it is currently accepted that serotonin (5-HT) functions predominantly to promote wakefulness (W) and to inhibit REM (rapid eye movement) sleep (REMS). Yet, under certain circumstances the neurotransmitter contributes to the increase in sleep propensity. Most of the serotonergic innervation of the cerebral cortex, amygdala, basal forebrain (BFB), thalamus, preoptic and hypothalamic areas, raphe nuclei, locus coeruleus and pontine reticular formation comes from the dorsal raphe nucleus (DRN). The 5-HT receptors can be classified into at least seven classes, designated 5-HT(1-7). The 5-HT(1A) and 5-HT(1B) receptor subtypes are linked to the inhibition of adenylate cyclase, and their activation evokes a membrane hyperpolarization. The actions of the 5-HT(2A), 5-HT(2B) and 5-HT(2C) receptor subtypes are mediated by the activation of phospholipase C, with a resulting depolarization of the host cell. The 5-HT(3) receptor directly activates a 5-HT-gated cation channel which leads to the depolarization of monoaminergic, aminoacidergic and cholinergic cells. The primary signal transduction pathway of 5-HT(6) and 5-HT(7) receptors is the stimulation of adenylate cyclase which results in the depolarization of the follower neurons. Mutant mice that do not express 5-HT(1A) or 5-HT(1B) receptor exhibit greater amounts of REMS than their wild-type counterparts, which could be related to the absence of a postsynaptic inhibitory effect on REM-on neurons of the laterodorsal and pedunculopontine tegmental nuclei (LDT/PPT). 5-HT(2A) and 5-HT(2C) receptor knock-out mice show a significant increase of W and a reduction of slow wave sleep (SWS) which has been ascribed to the increase of catecholaminergic neurotransmission involving mainly the noradrenergic and dopaminergic systems. Sleep variables have been characterized, in addition, in 5-HT(7) receptor knock-out mice; the mutants spend less time in REMS that their wild-type counterparts. Direct infusion of the 5-HT(1A) receptor agonists 8-OH-DPAT and flesinoxan into the DRN significantly enhances REMS in the rat. In contrast, microinjection of the 5-HT(1B) (CP-94253), 5-HT(2A/2C) (DOI), 5-HT(3) (m-chlorophenylbiguanide) and 5-HT(7) (LP-44) receptor agonists into the DRN induces a significant reduction of REMS. Systemic injection of full agonists at postsynaptic 5-HT(1A) (8-OH-DPAT, flesinoxan), 5-HT(1B) (CGS 12066B, CP-94235), 5-HT(2C) (RO 60-0175), 5-HT(2A/2C) (DOI, DOM), 5-HT(3) (m-chlorophenylbiguanide) and 5-HT(7) (LP-211) receptors increases W and reduces SWS and REMS. Of note, systemic administration of the 5-HT(2A/2C) receptor antagonists ritanserin, ketanserin, ICI-170,809 or sertindole at the beginning of the light period has been shown to induce a significant increase of SWS and a reduction of REMS in the rat. Wakefulness was also diminished in most of these studies. Similar effects have been described following the injection of the selective 5-HT(2A) receptor antagonists volinanserin and pruvanserin and of the 5-HT(2A) receptor inverse agonist nelotanserin in rodents. In addition, the effects of these compounds have been studied on the sleep electroencephalogram of subjects with normal sleep. Their administration was followed by an increase of SWS and, in most instances, a reduction of REMS. The administration of ritanserin to poor sleepers, patients with chronic primary insomnia and psychiatric patients with a generalized anxiety disorder or a mood disorder caused a significant increase in SWS. The 5-HT(2A) receptor inverse agonist APD-125 induced also an increase of SWS in patients with chronic primary insomnia. It is known that during the administration of benzodiazepine (BZD) hypnotics to patients with insomnia there is a further reduction of SWS and REMS, whereas both variables tend to remain decreased during the use of non-BZD derivatives (zolpidem, zopiclone, eszopiclone, zaleplon). Thus, the association of 5-HT(2A) antagonists or 5-HT(2A) inverse agonists with BZD and non-BZD hypnoti

Monti, Jaime M

2011-08-01

155

Vortex dynamics of a cylinder wake in proximity to a wall  

NASA Astrophysics Data System (ADS)

Large-eddy simulations (LES) are used to investigate the modifications of wake dynamics and turbulence characteristics behind a circular cylinder placed near a wall for varying gap-to-diameter (G/D) ratios (where G signifies the gap between the wall and the cylinder, and D the cylinder diameter). The three-dimensional (3-D), time-dependent, incompressible Navier-Stokes equations with a dynamic subgrid-scale model are solved using a symmetry-preserving finite-difference scheme of second-order spatial and temporal accuracy. The immersed boundary (IB) method is employed to impose the no-slip boundary condition on the cylinder surface. Flow visualizations along with turbulence statistics are presented to gain insight into the flow structures that are due to interaction between the shear layers and the approaching boundary layer. Apart from the vortex shedding mechanism, the paper illustrates the physics involving the shear layer transition, stretching, breakdown and turbulence generation, either qualitatively or quantitatively, in the presence of a wall for a Reynolds number of Re=1440 (based on D and the inlet free-stream velocity U?).

Sarkar, S.; Sarkar, Sudipto

2010-01-01

156

Vortex dynamics and associated fluid forcing in the near wake of a light and heavy tethered sphere in uniform flow  

NASA Astrophysics Data System (ADS)

Time-resolved particle image velocimetry measurements of vortex-induced vibrations of a negatively ("heavy") and positively ("light") buoyant tethered sphere in uniform flow, and its wake characteristics were performed in a closed-loop water channel. Experiments for both spheres were performed at similar bulk velocities, ranging between 0.048 < U < 0.32 m/s, corresponding to reduced velocities, 2.2 < U * < 13.5. Initially stationary, with increasing U, the amplitude response displayed periodic oscillations beyond the Hopf bifurcation as a result of "lock-in" between vortex shedding and the natural structural frequency. However, while the heavy sphere's amplitude decreased beyond U * = 7.0, the light sphere's amplitude continuously increased. In the periodic oscillation region, flow field characteristics in the wakes of both spheres (at comparable U * ) were similar, characterized by alternately shed hairpin vortices having a horizontal symmetry plane. Primary vortex trajectories in the frame of reference of the sphere collapsed for different U * (but not for different m * ) when scaled by f 2,s/ U, where f 2,s is the sphere's transverse oscillation frequency. This allows determination of vortex positions based on sphere dynamics and bulk flow conditions only. Associated vortex convection velocities as a function of downstream position from the sphere also nearly collapsed when normalized by U. In addition, fluid forcing and energy transfer from fluid to sphere were estimated based on an analogy between aircraft trailing vortices and hairpin vortices. Maximum forcing occurred at vortex pinch-off. For the highest comparable U * , despite different amplitudes, total transferred energy during one oscillation period was similar for both spheres. Changes in sphere dynamics must therefore be related to differences in inertia.

Krakovich, A.; Eshbal, L.; van Hout, R.

2013-11-01

157

On Use of Global Positioning Technology for Solution of Wake Vortex Problem  

NASA Technical Reports Server (NTRS)

Improved precision of the flight paths used by aircraft to approach and depart airports is becoming available when the Global Positioning System (GPS) is implemented at airports. An overview will be given of published information on how GPS precision guidance at airports can be used to avoid encounters with the lift-generated vortices shed by preceding aircraft during landing. It is pointed out that GPS provides two needed services to bring about improved avoidance capability. Firstly, GPS pseudolites are being built and installed at airports so that, when coupled with autopilot systems currently available on subsonic transport aircraft, the aircraft can make precision approaches for zero visibility landings. The same equipment can also be used for precision approaches for wake-vortex avoidance. Secondly, regular monitoring of atmospheric motions along the approach corridor can be obtained by use of GPS equipment on board aircraft that are in the flight corridors. That is, wind velocity is determined by use of GPS equipment to measure the ground speed of the aircraft and then combined with onboard instrumentation to measure the airspeed of the aircraft. The difference between the two measurements yields the wind velocity. When the measured wind velocities are transmitted to an airport ground station they are used to monitor whether air motions adverse for safety in the flight corridor are present. If any parts of the corridor are unsafe, the spacing of the aircraft, or the location of the flight corridor being used, is modified. It is estimated that the spacings between any combination of aircraft can then be safely reduced to a uniform 3 n. mi. Information to be presented is contained in an article published in the AIAA Journal of Aircraft, May-June 1996.

Rossow, Vernon J.; Olson, Lawerence E. (Technical Monitor)

1997-01-01

158

Vortex-Body Interactions: A Critical Assessment. Coupled Gap-Wake Instabilities/Turbulence: A Source of Noise  

NASA Technical Reports Server (NTRS)

This program has involved, first of all, a critical state-of-the-art assessment of vortex-body interactions. Then, efforts were focused on experimental investigation on coupled-wake instabilities and turbulence occurring in a two-cylinder system. An extensive review was undertaken on the effect of incident vortices on various types of bodies. These incident vortices have a length scale of the same order of magnitude as the scale of the body. The body can take on various forms, including, for example, a circular cylinder, a blade or a wing. The classes of vortex-body interaction that were critically assessed include: (1) Periodic distortion of the incident (primary) vortex and shedding of secondary vorticity from the surface of the body. (2) Modulated vortex distortion and shedding at a leading-edge or surface due to incidence of a complex system of vortices. (3) Vortex distortion and shedding in presence of body oscillation. (4) Three-dimensional vortex interaction and shedding. For all of these classes of vortex-body interaction, quantitative topologies of the vorticity distributions and streamline patterns were found to be central to a unified description of mechanisms of vortex distortion and shedding. In most cases, it was possible to define relationships between vortex interactions and unsteady loading at the body surface. This phase of the program was an experimental investigation of a two-cylinder system, which simulated a central aspect of a four-wheel bogie on a large-scale commercial aircraft. The overall aim of this experimental research program was to determine the crucial elements of the unsteadiness in the gap and near-wake regions as a function of time using cinema-based techniques. During the research program, various image evaluation techniques were employed. They involved assessment of instantaneous velocity fields, streamline topology and patterns of vorticity. Experiments were performed in a large-scale water channel using a high-resolution version of digital particle image velocimetry. The program has focused on acquisition of images of velocity and vorticity for varying gap widths between the two-cylinder system. As a result of analysis of a relatively large number of images, it is demonstrated that low frequency instabilities can occur in the gap region between the cylinder. These low frequency instabilities are hypothesized to influence the near-wake structure of the entire two-cylinder system. The nature of the unstable shear layers in the gap region involves generation of small-scale Kelvin-Helmholtz instabilities. These unsteady shear layers then impinge upon the upper and lower surfaces of the cylinders, thereby influencing both the unsteady structure and the time-averaged patterns of the near-wake. Initial efforts have focused on characterization of the patterns of instantaneous and averaged streamlines using topological concepts. The end result of this investigation is a series of documented instantaneous images. They will serve as a basis for various types of post-processing, which will lead to a fuller understanding of the instantaneous and time-averaged unstable-turbulent fields in the gap region and downstream of the two-cylinder system. This further assessment is the focus of a subsequent program.

Rockwell, Donald

1999-01-01

159

Evaluation of the discrete vortex wake cross flow model using vector computers. Part 1: Theory and application  

NASA Technical Reports Server (NTRS)

The current program had the objective to modify a discrete vortex wake method to efficiently compute the aerodynamic forces and moments on high fineness ratio bodies (f approximately 10.0). The approach is to increase computational efficiency by structuring the program to take advantage of new computer vector software and by developing new algorithms when vector software can not efficiently be used. An efficient program was written and substantial savings achieved. Several test cases were run for fineness ratios up to f = 16.0 and angles of attack up to 50 degrees.

1979-01-01

160

Characterizing the wake vortex signature for an active line of sight remote sensor. M.S. Thesis Technical Report No. 19  

NASA Technical Reports Server (NTRS)

A recurring phenomenon, described as a wake vortex, develops as an aircraft approaches the runway to land. As the aircraft moves along the runway, each of the wing tips generates a spiraling and expanding cone of air. During the lifetime of this turbulent event, conditions exist over the runway which can be hazardous to following aircraft, particularly when a small aircraft is following a large aircraft. Left to themselves, these twin vortex patterns will converge toward each other near the center of the runway, harmlessly dissipating through interaction with each other or by contact with the ground. Unfortunately, the time necessary to disperse the vortex is often not predictable, and at busy airports can severely impact terminal area productivity. Rudimentary methods of avoidance are in place. Generally, time delays between landing aircraft are based on what is required to protect a small aircraft. Existing ambient wind conditions can complicate the situation. Reliable detection and tracking of a wake vortex hazard is a major technical problem which can significantly impact runway productivity. Landing minimums could be determined on the basis of the actual hazard rather than imposed on the basis of a worst case scenario. This work focuses on using a windfield description of a wake vortex to generate line-of-sight Doppler velocity truth data appropriate to an arbitrarily located active sensor such as a high resolution radar or lidar. The goal is to isolate a range Doppler signature of the vortex phenomenon that can be used to improve detection. Results are presented based on use of a simplified model of a wake vortex pattern. However, it is important to note that the method of analysis can easily be applied to any vortex model used to generate a windfield snapshot. Results involving several scan strategies are shown for a point sensor with a range resolution of 1 to 4 meters. Vortex signatures presented appear to offer potential for detection and tracking.

Heil, Robert Milton

1994-01-01

161

A Scanning laser-velocimeter technique for measuring two-dimensional wake-vortex velocity distributions. [Langley Vortex Research Facility  

NASA Technical Reports Server (NTRS)

A rapid scanning two dimensional laser velocimeter (LV) has been used to measure simultaneously the vortex vertical and axial velocity distributions in the Langley Vortex Research Facility. This system utilized a two dimensional Bragg cell for removing flow direction ambiguity by translating the optical frequency for each velocity component, which was separated by band-pass filters. A rotational scan mechanism provided an incremental rapid scan to compensate for the large displacement of the vortex with time. The data were processed with a digital counter and an on-line minicomputer. Vaporized kerosene (0.5 micron to 5 micron particle sizes) was used for flow visualization and LV scattering centers. The overall measured mean-velocity uncertainity is less than 2 percent. These measurements were obtained from ensemble averaging of individual realizations.

Gartrell, L. R.; Rhodes, D. B.

1980-01-01

162

Vortex-induced vibrations of two cylinders in tandem arrangement in the proximity-wake interference region  

PubMed Central

We investigate numerically vortex-induced vibrations (VIV) of two identical two-dimensional elastically mounted cylinders in tandem in the proximity–wake interference regime at Reynolds number Re = 200 for systems having both one (transverse vibrations) and two (transverse and in-line) degrees of freedom (1-DOF and 2-DOF, respectively). For the 1-DOF system the computed results are in good qualitative agreement with available experiments at higher Reynolds numbers. Similar to these experiments our simulations reveal: (1) larger amplitudes of motion and a wider lock-in region for the tandem arrangement when compared with an isolated cylinder; (2) that at low reduced velocities the vibration amplitude of the front cylinder exceeds that of the rear cylinder; and (3) that above a threshold reduced velocity, large-amplitude VIV are excited for the rear cylinder with amplitudes significantly larger than those of the front cylinder. By analysing the simulated flow patterns we identify the VIV excitation mechanisms that lead to such complex responses and elucidate the near-wake vorticity dynamics and vortex-shedding modes excited in each case. We show that at low reduced velocities vortex shedding provides the initial excitation mechanism, which gives rise to a vertical separation between the two cylinders. When this vertical separation exceeds one cylinder diameter, however, a significant portion of the incoming flow is able to pass through the gap between the two cylinders and the gap-flow mechanism starts to dominate the VIV dynamics. The gap flow is able to periodically force either the top or the bottom shear layer of the front cylinder into the gap region, setting off a series of very complex vortex-to-vortex and vortex-to-cylinder interactions, which induces pressure gradients that result in a large oscillatory force in phase with the vortex shedding and lead to the experimentally observed larger vibration amplitudes. When the vortex shedding is the dominant mechanism the front cylinder vibration amplitude is larger than that of the rear cylinder. The reversing of this trend above a threshold reduced velocity is associated with the onset of the gap flow. The important role of the gap flow is further illustrated via a series of simulations for the 2-DOF system. We show that when the gap-flow mechanism is triggered, the 2-DOF system can develop and sustain large VIV amplitudes comparable to those observed in the corresponding (same reduced velocity) 1-DOF system. For sufficiently high reduced velocities, however, the two cylinders in the 2-DOF system approach each other, thus significantly reducing the size of the gap region. In such cases the gap flow is entirely eliminated, and the two cylinders vibrate together as a single body with vibration amplitudes up to 50% lower than the amplitudes of the corresponding 1-DOF in which the gap flow is active. Three-dimensional simulations are also carried out to examine the adequacy of two-dimensional simulations for describing the dynamic response of the tandem system at Re = 200. It is shown that even though the wake transitions to a weakly three-dimensional state when the gap flow is active, the three-dimensional modes are too weak to affect the dynamic response of the system, which is found to be identical to that obtained from the two-dimensional computations.

BORAZJANI, IMAN; SOTIROPOULOS, FOTIS

2009-01-01

163

Vortex wakes generated by robins Erithacus rubecula during free flight in a wind tunnel  

PubMed Central

The wakes of two individual robins were measured in digital particle image velocimetry (DPIV) experiments conducted in the Lund wind tunnel. Wake measurements were compared with each other, and with previous studies in the same facility. There was no significant individual variation in any of the measured quantities. Qualitatively, the wake structure and its gradual variation with flight speed were exactly as previously measured for the thrush nightingale. A procedure that accounts for the disparate sources of circulation spread over the complex wake structure nevertheless can account for the vertical momentum flux required to support the weight, and an example calculation is given for estimating drag from the components of horizontal momentum flux (whose net value is zero). The measured circulations of the largest structures in the wake can be predicted quite well by simple models, and expressions are given to predict these and other measurable quantities in future bird flight experiments.

Hedenstrom, A; Rosen, M; Spedding, G.R

2005-01-01

164

Vortex wakes generated by robins Erithacus rubecula during free flight in a wind tunnel.  

PubMed

The wakes of two individual robins were measured in digital particle image velocimetry (DPIV) experiments conducted in the Lund wind tunnel. Wake measurements were compared with each other, and with previous studies in the same facility. There was no significant individual variation in any of the measured quantities. Qualitatively, the wake structure and its gradual variation with flight speed were exactly as previously measured for the thrush nightingale. A procedure that accounts for the disparate sources of circulation spread over the complex wake structure nevertheless can account for the vertical momentum flux required to support the weight, and an example calculation is given for estimating drag from the components of horizontal momentum flux (whose net value is zero). The measured circulations of the largest structures in the wake can be predicted quite well by simple models, and expressions are given to predict these and other measurable quantities in future bird flight experiments. PMID:16849236

Hedenström, A; Rosén, M; Spedding, G R

2006-04-22

165

Characteristics of wake vortex generated by a Boeing 727 jet transport during two-segment and normal ILS approach flight paths  

NASA Technical Reports Server (NTRS)

A series of flight tests was conducted to evaluate the vortex wake characteristics of a Boeing 727 (B727-200) aircraft during conventional and two-segment ILS approaches. Twelve flights of the B727, which was equipped with smoke generators for vortex marking, were flown and its vortex wake was intentionally encountered by a Lear Jet model 23 (LR-23) and a Piper Twin Comanche (PA-30). Location of the B727 vortex during landing approach was measured using a system of photo-theodolites. The tests showed that at a given separation distance there were no readily apparent differences in the upsets resulting from deliberate vortex encounters during the two types of approaches. Timed mappings of the position of the landing configuration vortices showed that they tended to descend approximately 91 m(300 ft) below the flight path of the B727. The flaps of the B727 have a dominant effect on the character of the trailed wake vortex. The clean wing produces a strong, concentrated vortex but as the flaps are lowered, the vortex system becomes more diffuse. Pilot opinion and roll acceleration data indicate that 4.5 n.mi. would be a minimum separation distance at which roll control of light aircraft (less than 5,670 kg (12,500 lb) could be maintained during parallel encounters of the B727's landing configuration wake. This minimum separation distance is generally in scale with results determined from previous tests of other aircraft using the small roll control criteria.

Kurkowski, R. L.; Barber, M. R.; Garodz, L. J.

1976-01-01

166

Time-resolved vortex wake of a common swift flying over a range of flight speeds  

PubMed Central

The wake of a freely flying common swift (Apus apus L.) is examined in a wind tunnel at three different flight speeds, 5.7, 7.7 and 9.9 m s?1. The wake of the bird is visualized using high-speed stereo digital particle image velocimetry (DPIV). Wake images are recorded in the transverse plane, perpendicular to the airflow. The wake of a swift has been studied previously using DPIV and recording wake images in the longitudinal plane, parallel to the airflow. The high-speed DPIV system allows for time-resolved wake sampling and the result shows features that were not discovered in the previous study, but there was approximately a 40 per cent vertical force deficit. As the earlier study also revealed, a pair of wingtip vortices are trailing behind the wingtips, but in addition, a pair of tail vortices and a pair of ‘wing root vortices’ are found that appear to originate from the wing/body junction. The existence of wing root vortices suggests that the two wings are not acting as a single wing, but are to some extent aerodynamically detached from each other. It is proposed that this is due to the body disrupting the lift distribution over the wing by generating less lift than the wings.

Henningsson, P.; Muijres, F. T.; Hedenstrom, A.

2011-01-01

167

Measurements on a wind turbine wake: 3D effects and bluff body vortex shedding  

NASA Astrophysics Data System (ADS)

The velocity field in the wake of a two-bladed wind turbine model (diameter 180 mm) has been studied under different conditions using a two-component hot wire. All three velocity components were measured both for the turbine rotor normal to the oncoming flow as well as with the turbine inclined to the freestream direction (the yaw angle was varied from 0° to 20°). The measurements showed, as expected, a wake rotation in the opposite direction to that of the turbine. A yawed turbine is found to clearly deflect the wake flow to the side, showing the potential of controlling the wake by yawing the turbine. An unexpected feature of the flow was that spectra from the time signals showed the appearance of a low-frequency fluctuation both in the wake and in the flow outside the wake. This fluctuation was found both with and without freestream turbulence and also with a yawed turbine. The frequency expressed as a Strouhal number was shown to be independent of the freestream velocity or turbulence level, but the low frequency was only observed when the tip speed ratio (or equivalently the drag coefficient) was high. The shedding frequency changed also with the yaw angle. This is in agreement with the idea that the turbine sheds structures as a bluff body. The phenomenon, noticeable in all the velocity components, was further investigated using two-point cross-correlations of the velocity signals. Copyright

Medici, D.; Alfredsson, P. H.

2006-05-01

168

Simulating Virtual Terminal Area Weather Data Bases for Use in the Wake Vortex Avoidance System (Wake VAS) Prediction Algorithm  

NASA Technical Reports Server (NTRS)

During the research project, sounding datasets were generated for the region surrounding 9 major airports, including Dallas, TX, Boston, MA, New York, NY, Chicago, IL, St. Louis, MO, Atlanta, GA, Miami, FL, San Francico, CA, and Los Angeles, CA. The numerical simulation of winter and summer environments during which no instrument flight rule impact was occurring at these 9 terminals was performed using the most contemporary version of the Terminal Area PBL Prediction System (TAPPS) model nested from 36 km to 6 km to 1 km horizontal resolution and very detailed vertical resolution in the planetary boundary layer. The soundings from the 1 km model were archived at 30 minute time intervals for a 24 hour period and the vertical dependent variables as well as derived quantities, i.e., 3-dimensional wind components, temperatures, pressures, mixing ratios, turbulence kinetic energy and eddy dissipation rates were then interpolated to 5 m vertical resolution up to 1000 m elevation above ground level. After partial validation against field experiment datasets for Dallas as well as larger scale and much coarser resolution observations at the other 8 airports, these sounding datasets were sent to NASA for use in the Virtual Air Space and Modeling program. The application of these datasets being to determine representative airport weather environments to diagnose the response of simulated wake vortices to realistic atmospheric environments. These virtual datasets are based on large scale observed atmospheric initial conditions that are dynamically interpolated in space and time. The 1 km nested-grid simulated datasets providing a very coarse and highly smoothed representation of airport environment meteorological conditions. Details concerning the airport surface forcing are virtually absent from these simulated datasets although the observed background atmospheric processes have been compared to the simulated fields and the fields were found to accurately replicate the flows surrounding the airport where coarse verification data were available as well as where airport scale datasets were available.

Kaplan, Michael L.; Lin, Yuh-Lang

2004-01-01

169

Flight test to determine feasibility of a proposed airborne wake vortex detection concept  

NASA Technical Reports Server (NTRS)

This investigation was conducted to determine the radial extent at which aircraft mounted flow vanes or roll rate gyros can sense the circulatory flow field that exists around the lift induced vortex system generated by an aircraft in flight. The probe aircraft was equipped with wingtip sensors for measuring angle of attack and angle of sideslip, and with a fuselage mounted gyroscope for measuring roll rate. Analysis of flight test data indicated that the vortex was detectable at a lateral distance of about 105 feet (best results) using unsophisticated equipment. Measurements were made from the centerline of the probe aircraft to the center of the nearest vortex with the probe aircraft flying between one half and one and one half miles behind the vortex generating aircraft.

Branstetter, James R.; Hastings, E. C., Jr.; Patterson, James C., Jr.

1991-01-01

170

Behavioral and Polygraphic Study of 'Sleep' and 'Wakefulness' in Chronic Decerebrate Cats.  

National Technical Information Service (NTIS)

The behavioral and physiological manifestations of sleep and wakefulness in decerebrate animals showed no systematic relationship with the ECoG and behavioral manifestations of the rostral cerveau isole. On the basis of previous as well as the present fin...

J. Villablanca

1966-01-01

171

Measurements and modeling of flow structure in the wake of a low profile wishbone vortex generator  

NASA Technical Reports Server (NTRS)

The results of an experimental examination of the vortex structures shed from a low profile 'wishbone' generator are presented. The vortex generator height relative to the turbulent boundary layer was varied by testing two differently sized models. Measurements of the mean three-dimensional velocity field were conducted in cross-stream planes downstream of the vortex generators. In all cases, a counter-rotating vortex pair was observed. Individual vortices were characterized by three descriptors derived from the velocity data; circulation, peak vorticity, and cross-stream location of peak vorticity. Measurements in the cross plane at two axial locations behind the smaller wishbone characterize the downstream development of the vortex pairs. A single region of stream wise velocity deficit is shared by both vortex cores. This is in contrast to conventional generators, where each core coincides with a region of velocity deficit. The measured cross-stream velocities for each case are compared to an Oseen model with matching descriptors. The best comparison occurs with the data from the larger wishbone.

Wendt, B. J.; Hingst, W. R.

1994-01-01

172

Spatio-temporal behavior of spiral vortex flow  

NASA Astrophysics Data System (ADS)

Experimental realizations of Taylor-Couette flow often include rigid end plates at bottom and top of the system. As a consequence of such end plates the bifurcation behavior of the basic laminar flow as well as the spatio-temporal properties of the emerging pattern, such as e.g. spiral vortex flow, can change. The latter point is in the focus of our present experimental study. The spatio-temporal behavior of spiral vortex flow in a Taylor-Couette system with rigid end plates is analyzed by a measurement technique based on Doppler-shift. This enables us to determine the spatial amplitude profile of up- and downward propagating spiral vortices within oscillatory flow states. Our study confirms experimentally recent numerical results of Hoffmann et al. [1] on the spatio-temporal properties of the spiral vortex state in finite systems with rigid end plates.

Heise, M.; Külter, D.; Abshagen, J.; Pfister, G.

2008-11-01

173

TR PIV Experimental Investigation on Bypass Transition Induced by a Cylinder Wake  

NASA Astrophysics Data System (ADS)

The process of laminar to turbulent transition induced by a cylinder wake is studied by time-resolved (TR) particle image velocimetry (PIV) in a water channel. The combination of multi-scale local-averaged structure function analysis with criteria is used to identify the generation of secondary transverse vortex structure and to track its evolution along the streamwise. At the beginning of transition, with the decent of cylinder wake vortex, the secondary vortex structure is induced near the wall. As the secondary vortex moves downstream, it is induced to lift up by the wake vortex, meanwhile they are diffused and dissipated. According to the method of spatial conditional average, a low-speed hump is found in the near-wall region along the bypass transition zone, accompanied by a low-speed region in the free stream occupied by the wake vortex. With further downstream, the hump in the near-wall region becomes more and more obvious. At the later stage of transition zone, hairpin vortex can be seen by conditional-averaged low-pass filtered vorticity. The hairpin head is almost vertical to the wall with an inclination angle of about 90°, which is attributed to the additional lift-up behavior induced by wake vortex. It can be concluded that in the process of bypass transition, the wake vortex would not only induce the secondary vortex but also leaven its growth and evolution, resulting in the robust and rapidly growing hairpin vortex.

Tang, Zhan-Qi; Jiang, Nan

2011-05-01

174

Measurements on a wind turbine wake: 3D effects and bluff body vortex shedding  

Microsoft Academic Search

The velocity field in the wake of a two-bladed wind turbine model (diameter 180 mm) has been studied under different conditions using a two-component hot wire. All three velocity components were measured both for the turbine rotor normal to the oncoming flow as well as with the turbine inclined to the freestream direction (the yaw angle was varied from 0°

D. Medici; P. H. Alfredsson

2006-01-01

175

A family of vortex wakes generated by a thrush nightingale in free flight in a wind tunnel over its entire natural range of flight speeds.  

PubMed

In view of the complexity of the wing-beat kinematics and geometry, an important class of theoretical models for analysis and prediction of bird flight performance entirely, or almost entirely, ignores the action of the wing itself and considers only the resulting motions in the air behind the bird. These motions can also be complicated, but some success has previously been recorded in detecting and measuring relatively simple wake structures that can sometimes account for required quantities used to estimate aerodynamic power consumption. To date, all bird wakes, measured or presumed, seem to fall into one of two classes: the closed-loop, discrete vortex model at low flight speeds, and the constant-circulation, continuous vortex model at moderate to high speeds. Here, novel and accurate quantitative measurements of velocity fields in vertical planes aligned with the freestream are used to investigate the wake structure of a thrush nightingale over its entire range of natural flight speeds. At most flight speeds, the wake cannot be categorised as one of the two standard types, but has an intermediate structure, with approximations to the closed-loop and constant-circulation models at the extremes. A careful accounting for all vortical structures revealed with the high-resolution technique permits resolution of the previously unexplained wake momentum paradox. All the measured wake structures have sufficient momentum to provide weight support over the wingbeat. A simple model is formulated and explained that mimics the correct, measured balance of forces in the downstroke- and upstroke-generated wake over the entire range of flight speeds. Pending further work on different bird species, this might form the basis for a generalisable flight model. PMID:12796450

Spedding, G R; Rosén, M; Hedenström, A

2003-07-01

176

Knuckleball and Flying Disk: Boundary Layer Transitions, Separations and Vortex Wakes in Sports Aerodynamics  

NASA Astrophysics Data System (ADS)

In spite of their popularity, flow structures over common baseball and flying disks have not been studied in detail. A slowly rotating baseball is subject to erratic flight paths, and is known as a knuckleball. In the present experiment, the characteristic of force acting on a baseball was obtained and the velocity vector field near the surface of the ball and the wake were measured with the DPIV technique. The seam triggered the boundary layer transition or caused the boundary layer separation itself. The laminar/turbulent boundary layer separations were identified with specific ball orientations. Corresponding three-dimensional wake pattern and the side force result in unpredictable trajectories. In the second part of the talk, flow physics regarding a spin-stabilized flying disk is addressed. The roll-up of trailing vortices was visualized in detail and their vorticity field was measured with the DPIV. The vortical flow over the disk produced flow reattachment at a very high angle of attack. The boundary layer at low angles of attack was affected by the surface motion with asymmetric boundary layer transitions as evidenced by the flow visualization and the hot wire survey. The flow separation and attachment on the underside cavity were also affected by the rotation.

Higuchi, Hiroshi; Kiura, Toshiro; Goto, Yuichiro; Hiramoto, Riho

2001-11-01

177

Implementation and validation of a wake model for vortex-surface interactions in low speed forward flight  

NASA Technical Reports Server (NTRS)

The wake model was implemented using a VAX 750 and a Microvax II workstation. Online graphics capability using a DISSPLA graphics package. The rotor model used by Beddoes was significantly extended to include azimuthal variations due to forward flight and a simplified scheme for locating critical points where vortex elements are placed. A test case was obtained for validation of the predictions of induced velocity. Comparison of the results indicates that the code requires some more features before satisfactory predictions can be made over the whole rotor disk. Specifically, shed vorticity due to the azimuthal variation of blade loading must be incorporated into the model. Interactions between vortices shed from the four blades of the model rotor must be included. The Scully code for calculating the velocity field is being modified in parallel with these efforts to enable comparison with experimental data. To date, some comparisons with flow visualization data obtained at Georgia Tech were performed and show good agreement for the isolated rotor case. Comparison of time-resolved velocity data obtained at Georgia Tech also shows good agreement. Modifications are being implemented to enable generation of time-averaged results for comparison with NASA data.

Komerath, Narayanan M.; Schreiber, Olivier A.

1987-01-01

178

The effect of asymmetric vortex wake characteristics on a slender delta wing undergoing wing rock motion  

NASA Technical Reports Server (NTRS)

An experimental investigation into the fluid mechanisms responsible for wing rock on a slender delta wing with 80 deg leading edge sweep has been conducted. Time history and flow visualization data are presented for a wide angle-of-attack range. The use of an air bearing spindle has allowed the motion of the wing to be free from bearing friction or mechanical hysteresis. A bistable static condition has been found in vortex breakdown at an angle of attack of 40 deg which causes an overshoot of the steady state rocking amplitude. Flow visualization experiments also reveal a difference in static and dynamic breakdown locations on the wing. A hysteresis loop in dynamic breakdown location similar to that seen on pitching delta wings was observed as the wing was undergoing the limit cycle oscillation.

Arena, A. S., Jr.; Nelson, R. C.

1989-01-01

179

Presumed REM sleep behavior disorder arising from cataplexy and wakeful dreaming.  

PubMed

REM sleep behavior disorder (RBD) is an injurious parasomnia occasionally associated with narcolepsy. RBD usually occurs within REM sleep. We report a case of RBD in a narcoleptic patient arising from cataplexy and wakeful dreaming. The problem of state boundary control inherent in narcolepsy together with the pharmacological effects of tricyclic antidepressants used to treat cataplexy are usually the reason of RBD in narcoleptics. By similar mechanism our patient has presumed RBD out of REM intrusions into wakefulness. PMID:10767654

Attarian; Schenck; Mahowald

2000-04-01

180

Wind turbine design using a free-wake vortex method with winglet application  

NASA Astrophysics Data System (ADS)

Wind turbine blades are traditionally designed with blade element momentum theory (BEMT). This method is incapable of accurately analyzing non-conventional or non-planar blade planforms. Modern wind turbine blade design thus requires non-standard modeling that can effectively analyze the effects of a non-planar blade, such as a blade with a winglet. The free-wake, distributed vorticity element (FW-DVE) method meets these analysis goals. Previous work applied the FW-DVE method to wind turbines, but did not include the influence of profile forces and did not include any design applications. The present research focused on developing the FW-DVE method into a design tool for wind turbine design applications and on the validation of this tool. In the research presented in this thesis, the FW-DVE method was modified to include the effect of airfoil profile drag and to account for the effects of stall and a non-linear lift-curve. A design tool was created to aid in using the WindDVE analysis code for trade space exploration. The method was used to analyze and design a winglet for a small-scale wind turbine, which was tested in a wind tunnel at the University of Waterloo where it exhibited a 9% increase in the maximum coefficient of power of the rotor. The performance results from this test have been used to validate the FW-DVE method for wind turbine design, along with an analysis of the National Renewable Energy Laboratory's Unsteady Aerodynamics Experiment Phase VI wind turbine.

Maniaci, David

181

Transitions in effective scaling behavior of accelerometric time series across sleep and wake  

NASA Astrophysics Data System (ADS)

We study the effective scaling behavior of high-resolution accelerometric time series recorded at the wrists and hips of 100 subjects during sleep and wake. Using spectral analysis and detrended fluctuation analysis we find long-term correlated fluctuations with a spectral exponent \\beta \\approx 1.0 (1/f noise). On short time scales, ? is larger during wake (\\approx 1.4 ) and smaller during sleep (\\approx 0.6 ). In addition, characteristic peaks at 0.2-0.3 Hz (due to respiration) and 4-10 Hz (probably due to physiological tremor) are observed in periods of weak activity. Because of these peaks, spectral analysis is superior in characterizing effective scaling during sleep, while detrending analysis performs well during wake. Our findings can be exploited to detect sleep-wake transitions.

Wohlfahrt, Patrick; Kantelhardt, Jan W.; Zinkhan, Melanie; Schumann, Aicko Y.; Penzel, Thomas; Fietze, Ingo; Pillmann, Frank; Stang, Andreas

2013-09-01

182

Assessing the Influence of Wake Dynamics on the Performance and Aeroelastic Behavior of Wind Turbines  

NASA Astrophysics Data System (ADS)

While wind turbine farms are currently rapidly expanding, there are numerous technological challenges that must be overcome before wind energy represents a significant contributor to energy generation in the United States. One of the primary challenges is accurately accounting for the aerodynamic environment. This dissertation is focused on improving the aerodynamic modeling through the incorporation of wake effects. A comprehensive verification and validation of the NREL FAST code, which has been enhanced to include a Free Vortex Wake (FVW) model was conducted. The verification and validation is carried out through a comparison of wake geometry, blade lift distribution, wind turbine power and force and moment coefficients using a combination of Computational Fluid Dynamics (CFD) and experimental data. The results are also compared against Blade Element Momentum Theory (BEM), and results from an extensive experimental campaign by NREL on the prediction capabilities of wind turbine modeling tools. Results indicate that the enhanced aeroelastic code generally provides improved predictions. However, in several notable cases the predictions are only marginally improved, or even worse, than those generated using Blade Element Momentum Theory aerodynamics. After verification and validation of the model, the impact of including the free vortex wake model in the presence of turbulent flow was also examined. The inclusion of turbulence created large differences between BEM and FVW in predictions of rotor loading and power, however the amplitude of the turbulence did not have a large impact on the difference between the FVW and BEM. In addition to loading and power predictions, the structural response (tip deflections and root bending moments) of the wind turbine is investigated in the presence of turbulent inflow. The results indicate that the turbulence intensity and spectral model have a significant effect on the importance of the wake dynamics in modeling the tip deflections and root moments. From the dissertation results, it is concluded that modeling of the aerodynamic environment remains incomplete, even after inclusion of wake effects. One important aspect identified for future improvements is modeling of the unsteady aerodynamic lift characteristics of the rotor.

Kecskemety, Krista Marie

183

Interfacing comprehensive rotorcraft analysis with advanced aeromechanics and vortex wake models  

Microsoft Academic Search

This dissertation describes three aspects of the comprehensive rotorcraft analysis. First, a physics-based methodology for the modeling of hydraulic devices within multibody-based comprehensive models of rotorcraft systems is developed. This newly proposed approach can predict the fully nonlinear behavior of hydraulic devices, and pressure levels in the hydraulic chambers are coupled with the dynamic response of the system. The proposed

Haiying Liu

2008-01-01

184

Time-resolved particle image velocimetry measurements of vortex and shear layer dynamics in the near wake of a tethered sphere  

NASA Astrophysics Data System (ADS)

The coupling between shear layer, near wake dynamics, and structural oscillations downstream of a tethered spherical pendulum undergoing vortex induced vibrations (VIV) has been experimentally investigated using time-resolved particle image velocimetry in a wind tunnel. One quarter of the sphere was imaged in the field of view (spatial resolution 0.043D) that extended to 1.17D from the sphere's center (D is the sphere diameter). Reynolds numbers based on D, ranged between 493 <= Re <= 2218 and reduced velocities between 3.18 <= U* <= 14.1, covering a non-oscillating sphere, periodic oscillations, and the onset of non-stationary sphere oscillations. After the first Hopf bifurcation, the sphere exhibited large amplitude periodic oscillations and the near-wake vortices periodically interacted with the sphere and flapping shear layer. At U* = 5.97, a ``secondary'' counterclockwise rotating vortex seemed to facilitate shear layer pinch-off. In agreement with the onset of shear layer instabilities for a stationary sphere, only at Re = 2218 power spectra of velocity fluctuations inside the shear layer indicated a weak, broad frequency peak centered at 15 Hz similar as those measured for stationary cylinders and spheres. This peak was consistent with the results of linear instability theory indicating that despite the inherent three-dimensionality of the shear layer, its instability characteristics (at least for the Re investigated here) can be considered to be quasi-two-dimensional. Small-scale, near-wake structures were observed in the instantaneous swirling strength maps at all U* and it is conjectured here that their interaction with the sphere and separating shear layer is the feedback mechanism through which VIV occurs and is sustained.

van Hout, R.; Katz, A.; Greenblatt, D.

2013-07-01

185

Passive Wake Vortex Control  

Microsoft Academic Search

The collapse of the Soviet Union and ending of the Cold War brought about many significant changes in military submarine operations. The enemies that the US Navy faces today and in the future will not likely be superpowers armed with nuclear submarines, but rather smaller, rogue nations employing cheaper diesel\\/electric submarines with advanced air-independent propulsion systems. Unlike Cold War submarine

2001-01-01

186

Vortex  

NSDL National Science Digital Library

In this activity, learners create a tornado in a bottle to observe a spiraling, funnel-shaped vortex. A simple connector device allows water to drain from a 2-liter bottle into a second bottle. Learners can observe the whirling water and then repeat the process by inverting the bottle. Use this activity to talk about surface tension, pressure, gravity, friction, angular momentum, and centripetal force.

Exploratorium, The

2012-06-26

187

Genetic and Environmental Contributions to Sleep-Wake Behavior in 12-Year-Old Twins  

PubMed Central

Study Objectives: To examine the role of genetic and environmental factors on sleep behavior in 12-year-old twins matched for family environment. Design: Population-based twin cohort. Setting: Participants were assessed in their home environment. Patients or Participants: One hundred thirty-two adolescent twins comprising 25 monozygotic (MZ) and 41 dizygotic (DZ) twin pairs; aged 12.2 ± 0.1 y (mean ± standard deviation). Interventions: N/A. Measurements and Results: For 2 weeks in their home environment, participants wore a wrist activity monitor and completed a daily sleep diary. Sleep diaries included reports of bedtime, wake time, and estimated sleep onset time. Mean timing, duration, and quality of sleep during the 2 weeks were calculated for each individual and compared within twin pairs. MZ twin correlations were higher than the DZ correlations for total sleep time (MZr = 0.64; DZr = 0.38) and sleep onset latency (MZr = 0.83; DZr = 0.53) and significantly higher for wake after sleep onset (MZr = 0.66; DZr = 0.04) and sleep efficiency (MZr = 0.82; DZr = 0.10). Univariate modeling showed additive genetic factors accounted for 65% of the variance in total sleep time, 83% in sleep onset latency, and 52% and 57% of the variance in wake after sleep onset and sleep efficiency, respectively. A predominant influence of shared environment was found on the timing of sleep (67% for sleep start time, 86% for sleep end time). Conclusions: There is a strong genetic influence on the sleep-wake patterns of 12-year-old adolescents. Genes have a greater influence on sleep initiation and sleep maintenance and a smaller role in sleep timing, likely to be influenced by family environment. Citation: Sletten TL; Rajaratnam SMW; Wright MJ; Zhu G; Naismith S; Martin NG; Hickie I. Genetic and environmental contributions to sleep-wake behavior in 12-year-old twins. SLEEP 2013;36(11):1715-1722.

Sletten, Tracey L.; Rajaratnam, Shantha M.W.; Wright, Margaret J.; Zhu, Gu; Naismith, Sharon; Martin, Nicholas G.; Hickie, Ian

2013-01-01

188

Statistical Properties of Sleep-Wake Behavior in the Rat and Their Relation to Circadian and Ultradian Phases  

PubMed Central

Study Objectives: To examine the statistical characteristics of short-term sleep-wake architecture and to evaluate their dependence on ultradian and circadian phase. Design: Observational, time series. Setting: Laboratory. Participants: Ten male adult Sprague-Dawley rats. Interventions: N/A. Measurements and Results: States of wakefulness (WAKE), rapid eye movement sleep (REM) and nonrapid eye movement sleep (NREM) were recorded in 5-sec epochs over 7 consecutive days. State bout durations were analyzed using parametric regression of survival curves, comparing exponential, biexponential, and power law models. WAKE survival curves were best fit by biexponential models, suggesting that there are two statistically distinct stochastic mechanisms generating two types of WAKE—“brief” WAKE and “long” WAKE. Exponential time constants varied as a function of circadian and ultradian phase, with “long” WAKE showing the largest effect. NREM survival curves exhibited biexponential and monoexponential distributions in light and dark, respectively, with weak effects of ultradian phase. REM survival curves approximated a monoexponential distribution that varied with circadian but not ultradian phase. ?2 analysis was used in a three-state Markov model to evaluate whether conditional state transition probabilities exhibit the property of first-order dependence. This was partially confirmed, but only after accounting for heterogeneity associated with circadian and ultradian phase. However, there was evidence of residual second-order dependence indicating that additional sources of statistical heterogeneity may remain to be identified. Conclusions: Sleep-wake state is regulated over short timescales by stochastic mechanisms. When the major sources of heterogeneity are taken into account, including two-component WAKE and NREM states, the sleep-wake system of the rat behaves, to a reasonable approximation, as a Markovian system that is modulated over ultradian and circadian timescales. Citation: Stephenson R; Famina S; Caron AM; Lim J. Statistical properties of sleep-wake behavior in the rat and their relation to circadian and ultradian phases. SLEEP 2013;36(9):1377-1390.

Stephenson, Richard; Famina, Svetlana; Caron, Aimee M.; Lim, Joonbum

2013-01-01

189

Euromech 160 on Periodic Flow and Wake Phenomena  

NASA Astrophysics Data System (ADS)

Several areas of periodic flow and wake phenomena are addressed including vortex shedding, oscillator model theory, unsteady pressure, velocity fields, vortex formation, bluff body wakes, and wind induced vibrations.

190

Rapid assessment of sleep-wake behavior in mice.  

PubMed

Sleep is a fundamental biological rhythm involving the interaction of numerous brain structures and diverse neurotransmitter systems. The primary measures used to define sleep are the electroencephalogram (EEG) and electromyogram (EMG). However, EEG-based methods are often unsuitable for use in high-throughput screens as they are time-intensive and involve invasive surgery. As such, the dissection of sleep mechanisms and the discovery of novel drugs that modulate sleep would benefit greatly from further development of rapid behavioral assays to assess sleep in animal models. Here is described an automated noninvasive approach to evaluate sleep duration, latency, and fragmentation using video tracking of mice in their home cage. This approach provides a high correlation with EEG/EMG measures under both baseline conditions and following administration of pharmacological agents. Moreover, the dose-dependent effects of sedatives, stimulants, and light can be readily detected. This approach is robust yet relatively inexpensive to implement and can be easily incorporated into ongoing screening programs to provide a powerful first-pass screen for assessing sleep and allied behaviors. PMID:22306973

Fisher, Simon P; Godinho, Sofia I H; Pothecary, Carina A; Hankins, Mark W; Foster, Russell G; Peirson, Stuart N

2012-02-01

191

Time-resolved volumetric particle tracking velocimetry of large-scale vortex structures from the reattachment region of a laminar separation bubble to the wake  

NASA Astrophysics Data System (ADS)

The present paper presents time-resolved volumetric Particle Tracking Velocimetry measurements in a water towing tank on a SD7003 airfoil, performed at a Reynolds number of 60,000 and a 4° angle of attack. The SD7003 airfoil was chosen because of its long mid-chord and stable laminar separation bubble (LSB), occurring on the suction side of the airfoil at low Reynolds numbers. The present study focuses on the temporal resolution of unsteady large-scale vortex structures emitted from the LSB. In contrast to other studies, where only the observation of the flow in the transition region was examined, the entire flow from the leading edge to the far wake of the airfoil was investigated here.

Wolf, E.; Kähler, C. J.; Troolin, D. R.; Kykal, C.; Lai, W.

2011-04-01

192

Developments and Validations of Fully Coupled CFD and Practical Vortex Transport Method for High-Fidelity Wake Modeling in Fixed and Rotary Wing Applications  

NASA Technical Reports Server (NTRS)

A novel Computational Fluid Dynamics (CFD) coupling framework using a conventional Reynolds-Averaged Navier-Stokes (BANS) solver to resolve the near-body flow field and a Particle-based Vorticity Transport Method (PVTM) to predict the evolution of the far field wake is developed, refined, and evaluated for fixed and rotary wing cases. For the rotary wing case, the RANS/PVTM modules are loosely coupled to a Computational Structural Dynamics (CSD) module that provides blade motion and vehicle trim information. The PVTM module is refined by the addition of vortex diffusion, stretching, and reorientation models as well as an efficient memory model. Results from the coupled framework are compared with several experimental data sets (a fixed-wing wind tunnel test and a rotary-wing hover test).

Anusonti-Inthra, Phuriwat

2010-01-01

193

Information Requirements for Supervisory Air Traffic Controllers in Support of a Mid-Term Wake Vortex Departure System  

NASA Technical Reports Server (NTRS)

A concept focusing on wind dependent departure operations has been developed the current version of this concept is called the Wake Turbulence Mitigation for Departures (WTMD). This concept takes advantage the fact that cross winds of sufficient velocity blow wakes generated by "heavy" and B757 category aircraft on the downwind runway away from the upwind runway. Supervisory Air Traffic Controllers would be responsible for authorization of the Procedure. An investigation of the information requirements necessary to for Supervisors to approve monitor and terminate the Procedure was conducted. Results clearly indicated that the requisite information is currently available in air traffic control towers and that additional information was not required.

Lohr, Gary W.; Williams, Daniel M.; Trujillo, Anna C.; Johnson, Edward J.; Domino, David A.

2008-01-01

194

Histamine from Brain Resident MAST Cells Promotes Wakefulness and Modulates Behavioral States  

PubMed Central

Mast cell activation and degranulation can result in the release of various chemical mediators, such as histamine and cytokines, which significantly affect sleep. Mast cells also exist in the central nervous system (CNS). Since up to 50% of histamine contents in the brain are from brain mast cells, mediators from brain mast cells may significantly influence sleep and other behaviors. In this study, we examined potential involvement of brain mast cells in sleep/wake regulations, focusing especially on the histaminergic system, using mast cell deficient (W/Wv) mice. No significant difference was found in the basal amount of sleep/wake between W/Wv mice and their wild-type littermates (WT), although W/Wv mice showed increased EEG delta power and attenuated rebound response after sleep deprivation. Intracerebroventricular injection of compound 48/80, a histamine releaser from mast cells, significantly increased histamine levels in the ventricular region and enhanced wakefulness in WT mice, while it had no effect in W/Wv mice. Injection of H1 antagonists (triprolidine and mepyramine) significantly increased the amounts of slow-wave sleep in WT mice, but not in W/Wv mice. Most strikingly, the food-seeking behavior observed in WT mice during food deprivation was completely abolished in W/Wv mice. W/Wv mice also exhibited higher anxiety and depression levels compared to WT mice. Our findings suggest that histamine released from brain mast cells is wake-promoting, and emphasizes the physiological and pharmacological importance of brain mast cells in the regulation of sleep and fundamental neurobehavior.

Chikahisa, Sachiko; Kodama, Tohru; Soya, Atsushi; Sagawa, Yohei; Ishimaru, Yuji; Sei, Hiroyoshi; Nishino, Seiji

2013-01-01

195

Development and testing of laser Doppler system components for wake vortex monitoring. Volume 1: Scanner development, laboratory and field testing and system modeling  

NASA Technical Reports Server (NTRS)

A servo-controlled range/elevation scanner for the laser Doppler velocimeter (LDV) was developed and tested in the field to assess its performance in detecting and monitoring aircraft trailing vortices in an airport environment. The elevation scanner provides a capability to manually point the LDV telescope at operator chosen angles from 3.2 deg. to 89.6 deg within 0.2 deg, or to automatically scan the units between operator chosen limits at operator chosen rates of 0.1 Hz to 0.5 Hz. The range scanner provides a capability to manually adjust the focal point of the system from a range of 32 meters to a range of 896 meters under operator control, or to scan between operator chosen limits and at rates from 0.1 Hz to 6.9 Hz. The scanner controls are designed to allow simulataneous range and elevation scanning so as to provide finger scan patterns, arc scan patterns, and vertical line scan patterns. The development and testing of the unit is discussed, along with a fluid dynamic model of the wake vortex developed in a laser Doppler vortex sensor simulation program.

Wilson, D. J.; Krause, M. C.; Coffey, E. W.; Huang, C. C.; Edwards, B. B.; Shrider, K. R.; Jetton, J. L.; Morrison, L. K.

1974-01-01

196

Evaluation of the discrete vortex wake cross flow model using vector computers. Part 2: User's manual for DIVORCE  

NASA Technical Reports Server (NTRS)

The users manual for the Discrete Vortex Cross flow Evaluator (DIVORCE) computer program is presented. DIVORCE was developed in FORTRAN 4 for the DCD 6600 and CDC 7600 machines. Optimal calls to a NASA vector subroutine package are provided for use with the CDC 7600.

Deffenbaugh, F. D.; Vitz, J. F.

1979-01-01

197

The Dynamics Of Multiple Vortex Breakdown Bubbles In Confined Swirling Flows: Quasi-Periodic Order In The Wake Of Chaos  

Microsoft Academic Search

We study numerically and experimentally the Lagrangian characteristics of the flow in the interior of vortex breakdown (VB) bubbles in a closed container with a rotating lid for governing parameters within the steady, two-bubble regime. The dynamics of the first bubble are chaotic and consistent with those previously uncovered for VB bubbles in the single-bubble regime. In striking contrast, however,

Tahirih C. Lackey; Fotis Sotiropoulos; Donald R. Webster

2000-01-01

198

Static and dynamic behavior of the vortex-electrohydrodynamic instability in freely suspended layers of nematic liquid crystals  

Microsoft Academic Search

The vortex–electrohydrodynamic instability occurring in thin freely suspended layers of some nematic liquid crystals is investigated. The vortex pattern is observed both in the anisotropic and in the isotropic phase of the nematic liquid crystal. In this paper, we study the behavior of the vortex pattern when dc and ac electric fields are applied parallel to the free surfaces. The

S. Faetti; L. Fronzoni; P. A. Rolla

1983-01-01

199

Vortex dynamics and scalar transport in the wake of a bluff body driven through a steady recirculating flow  

NASA Astrophysics Data System (ADS)

The air ventilation system in wide-body aircraft cabins provides passengers with a healthy breathing environment. In recent years, the increase in global air traffic has amplified contamination risks by airborne flu-like diseases and terrorist threats involving the onboard release of noxious materials. In particular, passengers moving through a ventilated cabin may transport infectious pathogens in their wake. This paper presents an experimental investigation of the wake produced by a bluff body driven through a steady recirculating flow. Data were obtained in a water facility using particle image velocimetry and planar laser induced fluorescence. Ventilation attenuated the downward convection of counter-rotating vortices produced near the free-end corners of the body and decoupled the downwash mechanism from forward entrainment, creating stagnant contaminant regions.

Poussou, Stephane B.; Plesniak, Michael W.

2012-09-01

200

Signals from the Brainstem Sleep/Wake Centers Regulate Behavioral Timing via the Circadian Clock  

PubMed Central

Sleep-wake cycling is controlled by the complex interplay between two brain systems, one which controls vigilance state, regulating the transition between sleep and wake, and the other circadian, which communicates time-of-day. Together, they align sleep appropriately with energetic need and the day-night cycle. Neural circuits connect brain stem sites that regulate vigilance state with the suprachiasmatic nucleus (SCN), the master circadian clock, but the function of these connections has been unknown. Coupling discrete stimulation of pontine nuclei controlling vigilance state with analytical chemical measurements of intra-SCN microdialysates in mouse, we found significant neurotransmitter release at the SCN and, concomitantly, resetting of behavioral circadian rhythms. Depending upon stimulus conditions and time-of-day, SCN acetylcholine and/or glutamate levels were augmented and generated shifts of behavioral rhythms. These results establish modes of neurochemical communication from brain regions controlling vigilance state to the central circadian clock, with behavioral consequences. They suggest a basis for dynamic integration across brain systems that regulate vigilance states, and a potential vulnerability to altered communication in sleep disorders.

Abbott, Sabra M.; Arnold, Jennifer M.; Chang, Qing; Miao, Hai; Ota, Nobutoshi; Cecala, Christine; Gold, Paul E.; Sweedler, Jonathan V.; Gillette, Martha U.

2013-01-01

201

Lidar and numerical studies on the different evolution of vortex pair and secondary wake in young contrails  

Microsoft Academic Search

Vortex-regime evolution of contrails is investigated by focusing on the role of ambient humidity. Lidar cross-section measurements and observational analysis are combined with numerical simulations of fluid dynamics and microphysics. Contrail evolution behind four-turbofan aircraft is classified into three different scenarios. In the case of ice-subsaturated air, a visible pair of wingtip vortices is formed that disappears at the end

Ralf Sussmann; Klaus M. Gierens

1999-01-01

202

The Efiect of Vortex Persistence on Boundary Layer Behavior  

Microsoft Academic Search

Flow visualization experiments in a water tunnel at moderate Reynolds number reveal that the addition of a stationary vortex to an otherwise turbulent boundary layer results in laminar ?ow. This result is consis- tent with earlier heat transfer experiments of Touel (1) and Balle (2), who found laminar wall heat ?uxes under stationary vortices. Cotel's theory of persistent vortices (3)

Olivia R. Dawson; Robert E. Breidenthal

203

Wake effects on the aerodynamic performance of horizontal axis wind turbines  

Microsoft Academic Search

Success of vortex theories in the performance analysis of horizontal axis wind turbines depends greatly upon accurate specification of the geometry of the vortex wake. Two analysis methods were developed, a new simplified free wake method (SFW) and a prescribed wake method. An earlier wake model of helicopter rotors is extended for wind turbine applications, the fast free wake method

A. A. Afjeh; A. A. A. K

1984-01-01

204

On vortex bursting  

NASA Technical Reports Server (NTRS)

Vortex bursting is studied by means of visualization. The physical behavior of the phenomenon is emphasized, and its similarity with boundary layer separation or wake bursting becomes apparent. The essential influence of an increasing pressure gradient on the initiation, the position and the type of bursting is clearly confirmed. The evolution of the phenomena as a function of several parameters is analyzed in the case of delta wings, alone or installed on aircraft models, and compared with the results of similar wind tunnel or flight tests.

Werle, H.

1984-01-01

205

Remote measurement utilizing NASA's scanning laser Doppler systems. Volume 1. Laser Doppler wake vortex tracking at Kennedy Airport  

NASA Technical Reports Server (NTRS)

Test operations of the Scanning Laser Doppler System (SLDS) at Kennedy International Airport (KIA) during August 1974 through June 1975 are reported. A total of 1,619 data runs was recorded with a totally operational system during normal landing operations at KIA. In addition, 53 data runs were made during cooperative flybys with the C880 for a grand total of 1672 recorded vortex tracks. Test crews were in attendance at KIA for 31 weeks, of which 25 weeks were considered operational and the other six were packing, unpacking, setup and check out. Although average activity equates to 67 recorded landing operations per week, two periods of complete runway inactivity spanned 20 days and 13 days, respectively. The operation frequency therefore averaged about 88 operations per week.

Krause, M. C.; Wilson, D. J.; Howle, R. E.; Edwards, B. B.; Craven, C. E.; Jetton, J. L.

1976-01-01

206

Point vortex dynamics: Recent results and open problems  

SciTech Connect

The concept of point vortex motion, a classical model in the theory of two-dimensional, incompressible fluid mechanics, was introduced by Helmholtz in 1858. Exploration of the solutions to these equations has made fitful progress since that time as the point vortex model has been brought to bear on various physical situations: atomic structure, large-scale weather patterns, ''vortex street'' wakes, vortex lattices in superfluids and superconductors, etc. The point vortex equations also provide an interesting example of transition to chaotic behavior. We give a brief historical introduction to these topics and develop two of them in particular to the point of current understanding: steadily moving configurations of point vortices; and collision dynamics of vortex pairs. 26 refs.

Aref, H.; Kadtke, J.B.; Zawadzki, I.; Campbell, L.J.; Eckhardt, B.

1987-01-01

207

Downstream Development of the Wakes behind Cylinders  

Microsoft Academic Search

Wake development behind circular cylinders and flat plates was investigated in the research water tanks. The aluminium dust method was used to observe the flow patterns. At the intermediate Reynolds number range the Kármán vortex streets are formed in the wakes behind cylindrical obstacles. But these primary Kármán vortex streets are not stable. They are more and more deformed as

Sadatoshi Taneda

1959-01-01

208

Design of an Aircraft Vortex Spacing System for Airport Capacity Improvement  

NASA Technical Reports Server (NTRS)

The National Aeronautics and Space Administration (NASA) is addressing airport capacity enhancements through the Terminal Area Productivity (TAP) program. Within TAP, the Reduced Spacing Operations element at the NASA Langley Research Center is developing an Aircraft VOrtex Spacing System (AVOSS). AVOSS will integrate the output of several systems to produce weather dependent, dynamic wake vortex spacing criteria. These systems provide current and predicted weather conditions, models of wake vortex transport and decay in these weather conditions, and real-time feedback of wake vortex behavior from sensors. The goal of the NASA program is to provide the research and development to demonstrate an engineering model AVOSS, in real-time operation, at a major airport. A wake vortex system test facility was established at the Dallas-Fort Worth International Airport (DFW) in 1997 and tested in 1998. Results from operation of the initial AVOSS system, plus advances in wake vortex prediction and near-term weather forecast models, "nowcast", have been integrated into a second-generation system. This AVOSS version is undergoing final checkout in preparation for a system demonstration in 2000. This paper describes the revised AVOSS system architecture, subsystem enhancements, and initial results with AVOSS version 2 from a deployment at DFW in the fall of 1999.

Hinton, David A.; Charnock, James K.; Bagwell, Donald R.

2000-01-01

209

Response of plane shear layers and wakes to strong three-dimensional disturbances  

Microsoft Academic Search

Shear layers and wakes formed downstream of a splitter plate whose trailing edge has spanwise variations were briefly explored using the reacting flow visualization technique. A fundamental difference in behavior was observed between the two types of flows. The shear layer (which has only one sign of mean vorticity) rapidly forms its characteristic two-dimensional vortex structures, whereas, in contrast, the

R. Breidenthal

1980-01-01

210

Wake effects on the aerodynamic performance of horizontal axis wind turbines  

SciTech Connect

Success of vortex theories in the performance analysis of horizontal axis wind turbines depends greatly upon accurate specification of the geometry of the vortex wake. In this study, two methods of analysis are developed: a new simplified free wake method (SFW) and a prescribed wake method. In addition, an earlier wake model of helicopter rotors is extended for wind turbine applications. This method is referred to as the fast free wake method (FFW). The FFW was accomplished by partitioning the flow field downstream of the rotor into three regions: the near wake, modeled as a series of straight vortex lines; the intermediate wake, modeled as a number of vortex rings; and the far wake, taken to be a semi-infinite cylindrical wake. In the SFW, a new wake model is proposed. The model assumes that the wake is composed of an intense tip vortex and a diffused inboard wake, consistent with the experimentally observed wake of hovering helicopters. However, due to the complexity of the tip vortex formation and due to the lack of such experimental data for wind turbines, it was assumed that the vortex formation was almost immediate as opposed to the actual gradual rolling-up of the tip vortex. For the prescribed wake analysis the expansion of the wake must be known. Unfortunately, detailed wake measurements for wind turbines are sparse in number; hence, the method was demonstrated by assuming the wake expansion could be represented by an analytical expression.

Afjeh, A.A.

1984-08-01

211

Rotor Wake Development During the First Revolution  

NASA Technical Reports Server (NTRS)

The wake behind a two-bladed model rotor in light climb was measured using particle image velocimetry, with particular emphasis on the development of the trailing vortex during the first revolution of the rotor. The distribution of vorticity was distinguished from the slightly elliptical swirl pattern. Peculiar dynamics within the void region may explain why the peak vorticity appeared to shift away from the center as the vortex aged, suggesting the onset of instability. The swirl and axial velocities (which reached 44 and 12 percent of the rotor-tip speed, respectively) were found to be asymmetric relative to the vortex center. In particular, the axial flow was composed of two concentrated zones moving in opposite directions. The radial distribution of the circulation rapidly increased in magnitude until reaching a point just beyond the core radius, after which the rate of growth decreased significantly. The core-radius circulation increased slightly with wake age, but the large-radius circulation appeared to remain relatively constant. The radial distributions of swirl velocity and vorticity exhibit self-similar behaviors, especially within the core. The diameter of the vortex core was initially about 10 percent of the rotor-blade chord, but more than doubled its size after one revolution of the rotor. According to vortex models that approximate the measured data, the core-radius circulation was about 79 percent of the large-radius circulation, and the large-radius circulation was about 67 percent of the maximum bound circulation on the rotor blade. On average, about 53 percent of the maximum bound circulation resides within the vortex core during the first revolution of the rotor.

McAlister, Kenneth W.

2003-01-01

212

Maternal Depressive Symptoms, Dysfunctional Cognitions, and Infant Night Waking: The Role of Maternal Nighttime Behavior  

ERIC Educational Resources Information Center

Mechanisms were examined to clarify relations between maternal depressive symptoms, dysfunctional cognitions, and infant night waking among 45 infants (1-24 months) and their mothers. A mother-driven mediational model was tested in which maternal depressive symptoms and dysfunctional cognitions about infant sleep predicted infant night waking via…

Teti, Douglas M.; Crosby, Brian

2012-01-01

213

Temporal behavior and vortex topology of topographic wave-breaking  

NASA Astrophysics Data System (ADS)

At low Froude numbers or strong stratification, the internal waves generated by flow over an obstacle or mountain will overturn and break. Surprisingly litte is known, however, of the dynamics of the wave breaking itself. Afanasyev and Peltier (1998) investigated the wave breaking region via LES and Eiff et al. (2005) via PIV measurements, but both presumed a statistically stationary wave-breaking process after the initial wave overturning. Here, we propose to take a closer look at this assumption by closely analyzing the spatio-temporal structure of internal wave breaking region and the surrounding flow. The analysis is based on Hovmöller diagrams and spatial correlations obtained from 2D-PIV measurements of flows generated in uniform stratified flow over 2D and quasi-2D obstacles in salt-stratified hydraulic channels at different Reynolds numbers ranging from laminar to turbulent. The results reveal low frequency variations throughout the flow field, in and outside the wave-breaking region. This characteristic frequency can be related to be due to a sequence of growth and decay of wave-breaking. Finally, new 3D-3C PIV measurements at high Reynolds numbers reveal a first glimpse of the 3D vortex topology.

Eiff, Olivier; Boulanger, Nicolas; Leroux, Karine; Paci, Alexandre

2010-11-01

214

Influence of Initial Vorticity Distribution on Axisymmetric Vortex Breakdown and Reconnection  

NASA Technical Reports Server (NTRS)

An analytical treatment has been developed to study some of the axisymmetric vortex breakdown and reconnection fluid dynamic processes underlying body-vortex interactions that are frequently manifested in rotorcraft and propeller-driven fixed-wing aircraft wakes. In particular, the presence of negative vorticity in the inner core of a vortex filament (one example of which is examined in this paper) subsequent to "cutting" by a solid body has a profound influence on the vortex reconnection, leading to analog flow behavior similar to vortex breakdown phenomena described in the literature. Initial vorticity distributions (three specific examples which are examined) without an inner core of negative vorticity do not exhibit vortex breakdown and instead manifest diffusion-like properties while undergoing vortex reconnection. Though this work focuses on laminar vortical flow, this work is anticipated to provide valuable insight into rotary-wing aerodynamics as well as other types of vortical flow phenomena.

Young, Larry A.

2007-01-01

215

Aircraft Vortex Spacing System (AVOSS) Initial 1997 System Deployment at Dallas/Ft. Worth (DFW) Airport  

NASA Technical Reports Server (NTRS)

The potential hazard of aircraft encounters with the wake turbulence of preceding aircraft requires the use of minimum separations on landing that are significant constraint on airport arrival capacity during instrument flight rules (IFR) conditions. The National Aeronautics and Space Administration (NASA) Langley research Center has been researching the development of the Aircraft Vortex Spacing System (AVOSS) which would dynamically change aircraft arrival separations based on the forecasted weather conditions and vortex behavior.

Dasey, T. J.; Cole, R. E.; Heinrichs, R. M.; Matthews, M. P.; Perras, G. H.

1998-01-01

216

Vortex safety in aviation  

NASA Astrophysics Data System (ADS)

The objective is the general review of impact of aircraft wake vortices on the follower aircraft encountering the wake. Currently, the presence of wake vortices past aircraft limits the airspace capacity and flight safety level for aircraft of different purposes. However, wake vortex nature and evolution have not been studied in full measure. A mathematical model simulating the process of near wake generation past bodies of different shapes, as well as the wake evolution after rolling-up into wake vortices (far wake) is developed. The processes are suggested to be modeled by means of the Method of Discrete Vortices. Far wake evolution is determined by its complex interaction with the atmosphere and ground boundary layer. The main factors that are supposed to take into account are: wind and ambient turbulence 3Ddistributions, temperature stratification of the atmosphere, wind shear, as well as some others which effects will be manifested as considerable during the investigation. The ground boundary layer effects on wake vortex evolution are substantial at low flight altitudes and are determined through the boundary layer separation.

Turchak, L. I.

2012-10-01

217

Relationship Between Vortex Meander and Ambient Turbulence  

NASA Technical Reports Server (NTRS)

Efforts are currently underway to increase the capacity of airports by use of closely-spaced parallel runways. If such an objective is to be achieved safely and efficiently during both visual and instrument flight conditions, it will be necessary to develop more precise methods for the prediction of the motion and spread of the hazard posed by the lift-generated vortex-wakes of aircraft, and their uncertainties. The purpose of the present study is to relate the motion induced in vortex filaments by turbulence in the ambient flow field to the measured turbulence in the flow field. The problem came about when observations made in the two largest NASA wind tunnels indicated that extended exposure of vortex wakes to the turbulence in the wind tunnel air stream causes the centers of the vortices to meander about with time at a given downstream station where wake measurements are being made. Although such a behavior was expected, the turbulence level based on the maximum amplitude of meander was much less than the root-mean-squared value measured in the free-stream of the wind tunnel by use of hot-film anemometers. An analysis of the time-dependent motion of segments of vortex filaments as they interact with an eddy, indicates that the inertia of the filaments retards their motion enough in the early part of their travel to account for a large part of the difference in the two determinations of turbulence level. Migration of vortex filaments from one turbulent eddy to another (probably with a different orientation), is believed to account for the remainder of the difference. Methods that may possibly be developed for use in the measurement of the magnitude of the more intense eddies in turbulent flow fields and how they should be adjusted to predict vortex meander are then discussed.

Rossow, Vernon J.; Hardy, Gordon H.; Meyn, Larry A.

2006-01-01

218

Large Eddy Simulation of Aircraft Wake Vortices: Atmospheric Turbulence Effects  

NASA Technical Reports Server (NTRS)

Crow instability can develop in most atmospheric turbulence levels, however, the ring vortices may not form in extremely strong turbulence cases due to strong dissipation of the vortices. It appears that strong turbulence tends to accelerate the occurrences of Crow instability. The wavelength of the most unstable mode is estimated to be about 5b(sub 0), which is less than the theoretical value of 8.6b(sub 0) (Crow, 1970) and may be due to limited domain size and highly nonlinear turbulent flow characteristics. Three-dimensional turbulence can decay wake vortices more rapidly. Axial velocity may be developed by vertical distortion of a vortex pair due to Crow instability or large turbulent eddy motion. More experiments with various non-dimensional turbulence levels are necessary to get useful statistics of wake vortex behavior due to turbulence. Need to investigate larger turbulence length scale effects by enlarging domain size or using grid nesting.

Han, Jongil; Lin, Yuh-Lang; Arya, S. Pal; Kao, C.-T.

1997-01-01

219

Critical behavior at depinning of driven disordered vortex matter in 2H-NbS2  

NASA Astrophysics Data System (ADS)

We report unusual jamming in driven ordered vortex flow in 2H-NbS2. Reinitiating movement in these jammed vortices with a higher driving force and halting it thereafter once again with a reduction in drive leads to a critical behavior centered around the depinning threshold via divergences in the lifetimes of transient states, validating the predictions of a recent simulation study [Reichhardt and Olson Reichhardt, Phys. Rev. Lett.0031-900710.1103/PhysRevLett.103.168301 103, 168301 (2009)] which also pointed out a correspondence between plastic depinning in vortex matter and the notion of random organization proposed [Corte , Nat. Phys.1745-247310.1038/nphys891 4, 420 (2008)] in the context of sheared colloids undergoing diffusive motion.

Shaw, Gorky; Mandal, Pabitra; Banerjee, S. S.; Niazi, A.; Rastogi, A. K.; Sood, A. K.; Ramakrishnan, S.; Grover, A. K.

2012-05-01

220

PIV study of near-field tip vortex behind perforated Gurney flaps  

NASA Astrophysics Data System (ADS)

The impact of Gurney flaps, of different heights and perforations, on the growth and development of a tip vortex, both along the tip and in the near field of a finite NACA 0012 wing, at Re = 1.05 × 105 was investigated by using particle image velocimetry (PIV). Wind-tunnel force balance measurements were also made to supplement the PIV results. This study is a continuation of the work of Lee and Ko (Exp Fluids 46(6):1005-1019, 2009) on the near-wake measurements behind perforated Gurney flaps. The present results show that along the tip, the overall behavior of the secondary vortices and their interaction with the primary, or tip, vortex remained basically unchanged, regardless of flap height and perforation. The peak vorticity of the tip vortex, however, increased with flap height and always exhibited a local maximum at x/ c = 0.8 (from the leading edge). In the near field, the strength and structure of the near-field tip vortex were found to vary greatly with the flap height and perforation. The small flaps produced a more concentrated tip vortex with an increased circulation, while the large Gurney flaps caused a disruption of the tip vortex. The disrupted vortex can, however, be re-established by the addition of flap perforation. The larger the flap perforation the more organized the tip vortex. The Gurney flaps have the potential to serve as an alternative off-design wake vortex control device.

Lee, T.

2011-02-01

221

Crosswind Shear Gradient Affect on Wake Vortices  

NASA Technical Reports Server (NTRS)

Parametric simulations with a Large Eddy Simulation (LES) model are used to explore the influence of crosswind shear on aircraft wake vortices. Previous studies based on field measurements, laboratory experiments, as well as LES, have shown that the vertical gradient of crosswind shear, i.e. the second vertical derivative of the environmental crosswind, can influence wake vortex transport. The presence of nonlinear vertical shear of the crosswind velocity can reduce the descent rate, causing a wake vortex pair to tilt and change in its lateral separation. The LES parametric studies confirm that the vertical gradient of crosswind shear does influence vortex trajectories. The parametric results also show that vortex decay from the effects of shear are complex since the crosswind shear, along with the vertical gradient of crosswind shear, can affect whether the lateral separation between wake vortices is increased or decreased. If the separation is decreased, the vortex linking time is decreased, and a more rapid decay of wake vortex circulation occurs. If the separation is increased, the time to link is increased, and at least one of the vortices of the vortex pair may have a longer life time than in the case without shear. In some cases, the wake vortices may never link.

Proctor, Fred H.; Ahmad, Nashat N.

2011-01-01

222

Initial Circulation and Peak Vorticity Behavior of Vortices Shed from Airfoil Vortex Generators  

NASA Technical Reports Server (NTRS)

An extensive parametric study of vortices shed from airfoil vortex generators has been conducted to determine the dependence of initial vortex circulation and peak vorticity on elements of the airfoil geometry and impinging flow conditions. These elements include the airfoil angle of attack, chord length, span, aspect ratio, local boundary layer thickness, and free stream Mach number. In addition, the influence of airfoil-to-airfoil spacing on the circulation and peak vorticity has been examined for pairs of co-rotating and counter-rotating vortices. The vortex generators were symmetric airfoils having a NACA-0012 cross-sectional profile. These airfoils were mounted either in isolation, or in pairs, on the surface of a straight pipe. The turbulent boundary layer thickness to pipe radius ratio was about 17 percent. The circulation and peak vorticity data were derived from cross-plane velocity measurements acquired with a seven-hole probe at one chord-length downstream of the airfoil trailing edge location. The circulation is observed to be proportional to the free-stream Mach number, the angle-of-attack, and the span-to-boundary layer thickness ratio. With these parameters held constant, the circulation is observed to fall off in monotonic fashion with increasing airfoil aspect ratio. The peak vorticity is also observed to be proportional to the free-stream Mach number, the airfoil angle-of-attack, and the span-to-boundary layer thickness ratio. Unlike circulation, however, the peak vorticity is observed to increase with increasing aspect ratio, reaching a peak value at an aspect ratio of about 2.0 before falling off again at higher values of aspect ratio. Co-rotating vortices shed from closely spaced pairs of airfoils have values of circulation and peak vorticity under those values found for vortices shed from isolated airfoils of the same geometry. Conversely, counter-rotating vortices show enhanced values of circulation and peak vorticity when compared to values obtained in isolation. The circulation may be accurately modeled with an expression based on Prandtl's relationship between finite airfoil circulation and airfoil geometry. A correlation for the peak vorticity has been derived from a conservation relationship equating the moment at the airfoil tip to the rate of angular momentum production of the shed vortex, modeled as a Lamb (ideal viscous) vortex. This technique provides excellent qualitative agreement to the observed behavior of peak vorticity for low aspect ratio airfoils typically used as vortex generators.

Wendt, Bruce J.; Biesiadny, Tom (Technical Monitor)

2001-01-01

223

High lift wake investigation  

NASA Technical Reports Server (NTRS)

The behavior of wakes in adverse pressure gradients is critical to the performance of high-lift systems for transport aircraft. Wake deceleration is known to lead to sudden thickening and the onset of reversed flow; this 'wake bursting' phenomenon can occur while surface flows remain attached. Although known to be important for high-lift systems, few studies of such decelerated wakes exist. In this study, the wake of a flat plate has been subjected to an adverse pressure gradient in a two-dimensional diffuser, whose panels were forced to remain attached by use of slot blowing. Pitot probe surveys, L.D.V. measurements, and flow visualization have been used to investigate the physics of this decelerated wake, through the onset of reversed flow.

Sullivan, J. P.; Schneider, S. P.; Hoffenberg, R.

1996-01-01

224

Large Eddy Simulation of Wake Vortices in the Convective Boundary Layer  

NASA Technical Reports Server (NTRS)

The behavior of wake vortices in a convective boundary layer is investigated using a validated large eddy simulation model. Our results show that the vortices are largely deformed due to strong turbulent eddy motion while a sinusoidal Crow instability develops. Vortex rising is found to be caused by the updrafts (thermals) during daytime convective conditions and increases with increasing nondimensional turbulence intensity eta. In the downdraft region of the convective boundary layer, vortex sinking is found to be accelerated proportional to increasing eta, with faster speed than that in an ideal line vortex pair in an inviscid fluid. Wake vortices are also shown to be laterally transported over a significant distance due to large turbulent eddy motion. On the other hand, the decay rate of the, vortices in the convective boundary layer that increases with increasing eta, is larger in the updraft region than in the downdraft region because of stronger turbulence in the updraft region.

Lin, Yuh-Lang; Han, Jongil; Zhang, Jing; Ding, Feng; Arya, S. Pal; Proctor, Fred H.

2000-01-01

225

Wake effects on the aerodynamic performance of horizontal axis wind turbines  

NASA Astrophysics Data System (ADS)

Success of vortex theories in the performance analysis of horizontal axis wind turbines depends greatly upon accurate specification of the geometry of the vortex wake. Two analysis methods were developed, a new simplified free wake method (SFW) and a prescribed wake method. An earlier wake model of helicopter rotors is extended for wind turbine applications, the fast free wake method (FFW). The FFW was accomplished by partitioning the flow field downstream of the rotor into three regions: (1) the near wake, modeled as a series of straight vortex lines; (2) the intermediate wake, modeled as a number of vortex rings; and (3) the far wake, taken to be a semi-infinite cylindrical wake. In the SFW, a new wake model is proposed. The model assumes that the wake is composed of an intense tip vortex and a diffused inboard wake, consistent with the experimentally observed wake of hovering helicopters. It is assumed that the vortex formation was almost immediate as opposed to the actual gradual rolling-up of the tip vortex. The method is demonstrated by assuming that the wake expansion can be represented by an analytical expression.

Afjeh, A. A.

1984-08-01

226

Glassy vortex behavior in superconducting SrPd2Ge2 single crystals  

NASA Astrophysics Data System (ADS)

In this study we report the vortex-glass behavior of superconducting ternary germanide SrPd2Ge2 single crystals with a ThCr2Si2-type structure. We observed flux trapping and its nonexponential decay with time after the magnetic field was turned off at T = 2 K. In addition, we found that the diamagnetism in the zero field cooling (ZFC) mode below Tc was irreversible, depending on the temperature and field history, whereas the diamagnetism in the field-cooled warming (FCW) mode was reversible if the applied magnetic field was parallel to the c-axis. An irreversibility line Tr(H) was determined by the ZFC and FCW measurements at various magnetic fields, and the temperature dependence of Tr(H) was found to agree with the de Almeida-Thouless relation, H = H0[1-Tr(H)/Tc(0)]?, where ? = 3/2. Including these vortex-glass behaviors, we discuss the critical current density, Jc(T), determined from isothermal magnetization measurements at various temperatures, and the pinning potential, determined from the slope of an Arrhenius plot, lnR(T,B) versus 1/T.

Sung, N. H.; Jo, Y. J.; Cho, B. K.

2012-07-01

227

Wake shape and its effects on aerodynamic characteristics  

NASA Technical Reports Server (NTRS)

The wake shape under symmetrical flight conditions and its effects on aerodynamic characteristics are examined. In addition, the effect of wake shape in sideslip and discrete vortices such as strake or forebody vortex on lateral characteristics is presented. The present numerical method for airplane configurations, which is based on discretization of the vortex sheet into vortex segments, verified the symmetrical and asymmetrical roll-up process of the trailing vortices. Also, the effect of wing wake on tail planes is calculated. It is concluded that at high lift the assumption of flat wake for longitudinal and lateral-directional characteristics should be reexamined.

Emdad, H.; Lan, C. E.

1986-01-01

228

Video images of smoke dispersion in the near wake of a model building. Part 1. Temporal and spatial scales of vortex shedding  

Microsoft Academic Search

In a wind-tunnel study, recorded video images of smoke dispersion in the wake of a rectangular-shaped building were analyzed. A continuous source of smoke was emitted at floor level, midway along the leeward side of the building. Smoke was observed to build up within a region adjacent to the building. Then the smoke was periodically swept away by vortices shed

1988-01-01

229

Wake effects of the aerodynamic performance of horizontal axis wind turbines  

SciTech Connect

Success of vortex theories in the performance prediction of horizontal axis wind turbines largely depends upon accurate specification of the geometry of the vortex wake. In this study, two methods of vortex wake analysis are developed: a new simplified free wake method (SFW) and a prescribed wake method. In addition, an earlier wake model of helicopter rotors, referred to as the fast free wake method (FFW), is extended for wind turbine applications. In the FFW model, the flow field downstream of the rotor was partitioned into three regions: the near wake, modeled as a series of straight vortex lines; the intermediate wake, modeled as a number of vortex rings; and the far wake, taken to be a semi-infinite cylindrical wake. The methods of this work were compared with an existing unconstrained free wake analysis, with an existing rigid wake analysis, with a popular blade element momentum method and with existing experimental data. Airload parameters obtained by using the present methods were found to be in good agreement with those of a full free wake analysis. However, the computational times were greatly reduced. Furthermore, the predicted performance agrees well with the experimental data. Both the FFW and SFW methods out performed the rigid wake and the blade element momentum methods. A parametric study using the prescribed wake analysis indicated that at some expansion rates, corresponding to low wind conditions, the predicted power exceeded that of the rigid wake prediction and could in fact exceeds the Betz limit.

Afjeh, A.A.A.K.

1984-01-01

230

Zebrafish Behavioral Profiling Links Drugs to Biological Targets and Rest\\/Wake Regulation  

Microsoft Academic Search

A major obstacle for the discovery of psychoactive drugs is the inability to predict how small molecules will alter complex behaviors. We report the development and application of a high-throughput, quantitative screen for drugs that alter the behavior of larval zebrafish. We found that the multidimensional nature of observed phenotypes enabled the hierarchical clustering of molecules according to shared behaviors.

Jason Rihel; David A. Prober; Anthony Arvanites; Kelvin Lam; Steven Zimmerman; Sumin Jang; Stephen J. Haggarty; David Kokel; Lee L. Rubin; Randall T. Peterson; Alexander F. Schier

2010-01-01

231

Flight Data Reduction of Wake Velocity Measurements Using an Instrumented OV-10 Airplane  

NASA Technical Reports Server (NTRS)

A series of flight tests to measure the wake of a Lockheed C- 130 airplane and the accompanying atmospheric state have been conducted. A specially instrumented North American Rockwell OV-10 airplane was used to measure the wake and atmospheric conditions. An integrated database has been compiled for wake characterization and validation of wake vortex computational models. This paper describes the wake- measurement flight-data reduction process.

Vicroy, Dan D.; Stuever, Robert A.; Stewart, Eric C.; Rivers, Robert A.

1999-01-01

232

Oscillating airfoils and their wake  

NASA Technical Reports Server (NTRS)

The unsteady phenomena in the wake of an oscillating wing or rotor blade are examined theoretically using the Prandtl approximation of the vortex-transport equation. A mathematical model is developed and applied to such problems as the effect of winglets on the performance of fixed wings and the possibly of employing similar designs in rotor blades. Model predictions for several profiles are compared with published and experimental measurements, and good agreement is found. Graphs and diagrams are provided.

Send, W.

1985-01-01

233

Cylinder wakes in flowing soap films  

SciTech Connect

We present an experimental characterization of cylinder wakes in flowing soap films. From instantaneous velocity and thickness fields, we find the vortex-shedding frequency, mean-flow velocity, and mean-film thickness. Using the empirical relationship between the Reynolds and Strouhal numbers obtained for cylinder wakes in three dimensions, we estimate the effective soap-film viscosity and its dependence on film thickness. We also compare the decay of vorticity with that in a simple Rankine vortex model with a dissipative term to account for air drag. [copyright] [ital 1999] [ital The American Physical Society

Vorobieff, P.; Ecke, R.E. (Center for Nonlinear Studies, Condensed Matter and Thermal Physics Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)); Vorobieff, P. (Dynamic Experimentation Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States))

1999-09-01

234

Turbulent vortex flows  

NASA Astrophysics Data System (ADS)

Mean and fluctuating velocity measurements in three types of longitudinal vortex imbedded in turbulent boundary layers in nominally zero pressure gradients are presented. Vortex generators were installed upstream of the wind tunnel contraction, so that the vortices entering the working section did not have large associated total pressure wakes. Measurements include all three components of mean velocity, all second and third order (and a few fourth order) mean products of fluctuating velocities, and surface shear stress, all for at least two streamwise positions for each configuration. Temperature conditioned sampling measurements, and some flow visualization results, were also acquired.

Bradshaw, P.; Shataka, I. M. M. A.; Mehta, R. D.

1982-03-01

235

Coherent structures in an airfoil boundary layer and wake at low Reynolds numbers  

NASA Astrophysics Data System (ADS)

Boundary layer and turbulent wake development for a NACA 0025 airfoil at low Reynolds numbers was studied experimentally. Wind tunnel experiments were carried out for a range of Reynolds numbers and three angles of attack. Laminar boundary layer separation occurs on the upper surface of the airfoil for all Reynolds numbers and angles of attack examined. Two flow regimes are investigated (i) boundary layer separation without reattachment and (ii) separation bubble formation. The results suggest that coherent structures form in the separated flow region and the wake of the airfoil for both flow regimes. The formation of the roll-up vortices in the separated shear layer is linked to inviscid spatial growth of disturbances and is attributed to the Kelvin-Helmholtz instability. Linear stability theory can be employed to adequately describe the salient characteristics of such vortices and the initial stage of the separated shear layer transition. The development of the roll-up vortices leads to boundary layer transition, and the vortices break down during the transition process. Vortex shedding also occurs in the airfoil wake and vortices form in the near-wake region. It is shown that the boundary layer behavior has a profound effect on the identified coherent structures, and each of the two flow regimes is associated with distinctly different vortex shedding characteristics.

Yarusevych, Serhiy; Sullivan, Pierre E.; Kawall, John G.

2006-04-01

236

An experimental investigation of the surface flow and wake dynamics associated with transverse flow over wavy cylinders  

SciTech Connect

Fluid flow over wavy and right circular cylinders was investigated experimentally in the TAMU 2' x 3' wind tunnel and 2' x 3' water tunnel. Surface-pressure and three-component laser-Doppler-velocimetry measurements were obtained at a Reynolds number of 20,000 based on mean diameter. Flow-visualization tests were conducted for right circular cylinders at Reynolds numbers from 330 to 21,000 and for wavy cylinders at Reynolds numbers of 5,000, 10,000 and 20,000. These tests revealed new information concerning the secondary streamwise vortical structures (ribs) in the immediate wake of a right circular cylinder. The formation of the ribs was observed to be linked to an interaction between the near-surface flow on the leeward side of the cylinder and each von Karman vortex as it advected from the vortex-formation region. The spanwise spacing of the ribs in the immediate wake was independent of Reynolds number over the range of Reynolds numbers tested. The ribs significantly affected the von Karman vortices at the upstream end of each braid and rapidly distorted stream surfaces as they were entrained into the wake. The wavy-cylinder flow field exhibited streamwise trailing vortices originating at the boundary-layer separation lines near the geometric nodes. The trailing vortices caused the width of the wake to shrink behind the geometric nodes and expand behind the geometric saddles. The behavior of these structures, in response to the von Karman vortex shedding, indicated that pairing of counter-rotating streamwise vortices can be suppressed by the application of an axial strain field. The wavy-cylinder geometry had no significant effect on the spanwise spacing or the spatial locations of the rib structures. Despite large spanwise variations in the vortex-formation region, the velocity and Reynolds shear-stress fields in the wake of a wavy cylinder rapidly approached a state of spanwise uniformity.

Bays-muchmore, B.

1991-01-01

237

Large eddy simulations of aircraft wake vortices in a homogeneous atmospheric turbulence  

NASA Astrophysics Data System (ADS)

This dissertation is composed of two separate papers where atmospheric turbulence effects on the behavior of aircraft wake vortices are studied using a validated large eddy simulation model. Information from this study may contribute to the development of a predictor algorithm for the National Aeronautic and Space Administration's (NASA) Aircraft Vortex Spacing System (AVOSS), which will determine safe operating spacing between arriving and departing aircraft and provide a safe reduction in separation of aircraft compared to the now-existing flight rules. In the first paper (Chapter 2), the development of Crow instability in a homogeneous field of turbulence is studied. The Crow instability becomes well developed in most atmospheric turbulence levels, but in strong turbulence the vortex pair deforms more irregularily due to turbulence advection. The maximum Crow instability wavelength decreases with increasing nondimensional turbulence intensity (/eta) while it increases with increasing turbulence integral length scale. The vortex lifespan is controlled primarily by ? and decreases with increasing ?, while the effect of integral scale of turbulence on vortex lifespan is of minor importance. The lifespan of vortex in our numerical simulations is estimated to be about 40% larger than the theoretically predicted value. This larger lifespan compared with an analytic theory agrees very well with the data from laboratory experiments and is caused by slower growth in the middle of the vortex lifespan. The maximum deviation in lifespan from the average value due to ambient turbulence alone is about 7% of the mean for small ? and about 20% of the mean for large ?, showing much less scatter in lifespan compared with atmospheric observations. In the second paper (Chapter 3), the vortex decay and descent in a homogeneous field of turbulence is investigated. The decay rate of the vortex circulation increases clearly with increasing ambient turbulence level but decreases with increasing radial distance, which is consistent with field observations. Simple vortex decay models are proposed as functions of ? and radial distance. A Gaussian type of vortex decay model can be applied at larger radial distances, while an exponential type of vortex decay model can be applied at smaller radial distances near the core, but the latter can be extended up to a radial distance of about half of the initial vortex separation distance (b0) for strong turbulence. The circulation averaged over radial distances from 2.5 rc (rc is the initial core radius) to 0.5 b0, which is related to the rolling moment of an encountering aircraft, shows a Gaussian decay for weak and moderate turbulence and an exponential decay for strong turbulence. A model for the vortex descent based on the vortex decay model is also proposed as a function of ? and dimensionless time. (Abstract shortened by UMI.)

Han, Jongil

238

Stationary Vortices in Karman Grooves. I.Vortex Growth Rate  

Microsoft Academic Search

The effect of a stationary vortex on wall fluxes in turbulent flow is predicted by the vortex persistence theory of turbulence. As a first step to test the theory, the feasibility of holding a vortex sufficiently stationary whilst embedded in a turbulent boundary layer is investi gated. Exploiting the stationarity of von Karman vortices in a wake, the dividing stream

Gregory J. Balle; Thiemo M. Kier; Robert E. Breidenthal

1999-01-01

239

Formal optimization of hovering performance using free wake lifting surface theory  

NASA Technical Reports Server (NTRS)

Free wake techniques for performance prediction and optimization of hovering rotor are discussed. The influence functions due to vortex ring, vortex cylinder, and source or vortex sheets are presented. The vortex core sizes of rotor wake vortices are calculated and their importance is discussed. Lifting body theory for finite thickness body is developed for pressure calculation, and hence performance prediction of hovering rotors. Numerical optimization technique based on free wake lifting line theory is presented and discussed. It is demonstrated that formal optimization can be used with the implicit and nonlinear objective or cost function such as the performance of hovering rotors as used in this report.

Chung, S. Y.

1986-01-01

240

Bubble and bubble wake characteristics in a gas-liquid-solid fluidized bed  

SciTech Connect

A light transmittance technique involving a dual optical fiber probe was used to measure the local solids holdup in a three-dimensional gas-liquid-solid fluidized bed. The bubble could be clearly identified in the same signal, thus permitting the simultaneous determination of the local solids holdup profile in the wake of a single bubble and the rise velocity and chord length of the bubble. The solids holdup behavior was studied in the wake of single bubbles rising in a liquid-solid fluidized bed for different liquid velocities, particle sizes, and bubble sizes. The wake solids holdup was found to decrease with decreasing mean solids holdup in the bed, to be essentially independent of the bubble Reynolds number over the range investigated in this study (4000 < Re{sub b} < 8000), and to decrease with increasing particle size. In freely bubbling bed at low gas holdups ({epsilon}{sub g} < 0.03), both the solids holdup in the wake and the particulate phase solids holdup were measured. The light transmittance probe developed for the single bubble system was also used for these measurements. Axial variations in the particulate solids holdup indicated that the relative wake solids holdup increased with axial distance from the distributor. The best agreement between the single bubble measurements and the measurements in the freely bubbling bed occurred in the bubble formation region. And finally, a simple model of the bubble wake was developed based on equations available in the literature for a spherical vortex to predict both the wake size and the relative wake solids holdup. The predictions showed good agreement with the experimentally observed values.

Kreischer, B.E.

1989-01-01

241

Sound Generation by Aircraft Wake Vortices  

NASA Technical Reports Server (NTRS)

This report provides an extensive analysis of potential wake vortex noise sources that might be utilized to aid in their tracking. Several possible mechanisms of aircraft vortex sound generation are examined on the basis of discrete vortex dynamic models and characteristic acoustic signatures calculated by application of vortex sound theory. It is shown that the most robust mechanisms result in very low frequency infrasound. An instability of the vortex core structure is discussed and shown to be a possible mechanism for generating higher frequency sound bordering the audible frequency range. However, the frequencies produced are still low and cannot explain the reasonably high-pitched sound that has occasionally been observed experimentally. Since the robust mechanisms appear to generate only very low frequency sound, infrasonic tracking of the vortices may be warranted.

Hardin, Jay C.; Wang, Frank Y.

2003-01-01

242

Enhancement of behavioral and electroencephalographic indices of waking following stimulation of noradrenergic beta-receptors within the medial septal region of the basal forebrain.  

PubMed

Previous studies in halothane-anesthetized rat documented potent electroencephalographic (EEG) modulatory actions of the locus coeruleus (LC) noradrenergic system, with LC neuronal activity causally related to the maintenance of EEG activity patterns associated with enhanced arousal/alertness. Recent studies, also in halothane-anesthetized rat, demonstrated that the region of the basal forebrain encompassing the medial septum/vertical limb of the diagonal band of Broca (MS) is a site at which noradrenergic efferents act to influence EEG state via actions at beta-receptors. These and other observations are consistent with the hypothesis that the LC noradrenergic system participates in the modulation of behavioral state. However, the degree to which this system modulates EEG state in the absence of anesthesia and to what extent such actions are accompanied by behavioral modulatory actions remain to be determined. The current studies examined whether small infusions of isoproterenol (ISO), a beta-adrenergic agonist, into MS alter behavioral, EEG, and electromyographic (EMG) measures of sleep and waking in the resting, undisturbed rat. These infusions resulted in a significant increase in time spent awake, defined by both behavioral and EEG/EMG measures, and in the nearly complete suppression of REM sleep. EEG/EMG responses either coincided with or preceded behavioral responses by 10-320 sec. The pattern of behavioral responses observed following MS-ISO infusions was qualitatively similar to that associated with normal waking. Infusions of vehicle into MS or ISO into sites adjacent to MS did not elicit consistent alterations in behavioral state. These results suggest that the LC noradrenergic system exerts potent behavioral and EEG-activating effects via actions of norepinephrine at beta-receptors located within MS. PMID:8824336

Berridge, C W; Foote, S L

1996-11-01

243

NASA AVOSS Fast-Time Wake Prediction Models: User's Guide  

NASA Technical Reports Server (NTRS)

The National Aeronautics and Space Administration (NASA) is developing and testing fast-time wake transport and decay models to safely enhance the capacity of the National Airspace System (NAS). The fast-time wake models are empirical algorithms used for real-time predictions of wake transport and decay based on aircraft parameters and ambient weather conditions. The aircraft dependent parameters include the initial vortex descent velocity and the vortex pair separation distance. The atmospheric initial conditions include vertical profiles of temperature or potential temperature, eddy dissipation rate, and crosswind. The current distribution includes the latest versions of the APA (3.4) and the TDP (2.1) models. This User's Guide provides detailed information on the model inputs, file formats, and the model output. An example of a model run and a brief description of the Memphis 1995 Wake Vortex Dataset is also provided.

Ahmad, Nash'at N.; VanValkenburg, Randal L.; Pruis, Matthew

2014-01-01

244

Perpendicular blade vortex interaction and its implications for helicopter noise prediction: Wave-number frequency spectra in a trailing vortex for BWI noise prediction  

NASA Technical Reports Server (NTRS)

Perpendicular blade vortex interactions are a common occurrence in helicopter rotor flows. Under certain conditions they produce a substantial proportion of the acoustic noise. However, the mechanism of noise generation is not well understood. Specifically, turbulence associated with the trailing vortices shed from the blade tips appears insufficient to account for the noise generated. The hypothesis that the first perpendicular interaction experienced by a trailing vortex alters its turbulence structure in such a way as to increase the acoustic noise generated by subsequent interactions is examined. To investigate this hypothesis a two-part investigation was carried out. In the first part, experiments were performed to examine the behavior of a streamwise vortex as it passed over and downstream of a spanwise blade in incompressible flow. Blade vortex separations between +/- one eighth chord were studied for at a chord Reynolds number of 200,000. Three-component velocity and turbulence measurements were made in the flow from 4 chord lengths upstream to 15 chordlengths downstream of the blade using miniature 4-sensor hot wire probes. These measurements show that the interaction of the vortex with the blade and its wake causes the vortex core to loose circulation and diffuse much more rapidly than it otherwise would. Core radius increases and peak tangential velocity decreases with distance downstream of the blade. True turbulence levels within the core are much larger downstream than upstream of the blade. The net result is a much larger and more intense region of turbulent flow than that presented by the original vortex and thus, by implication, a greater potential for generating acoustic noise. In the second part, the turbulence measurements described above were used to derive the necessary inputs to a Blade Wake Interaction (BWI) noise prediction scheme. This resulted in significantly improved agreement between measurements and calculations of the BWI noise spectrum especially for the spectral peak at low frequencies, which previously was poorly predicted.

Devenport, William J.; Glegg, Stewart A. L.

1993-01-01

245

Neurobiological Mechanisms for the Regulation of Mammalian Sleep-Wake Behavior: Reinterpretation of Historical Evidence and Inclusion of Contemporary Cellular and Molecular Evidence  

PubMed Central

At its most basic level, the function of mammalian sleep can be described as a restorative process of the brain and body; recently, however, progressive research has revealed a host of vital functions to which sleep is essential. Although many excellent reviews on sleep behavior have been published, none have incorporated contemporary studies examining the molecular mechanisms that govern the various stages of sleep. Utilizing a holistic approach, this review is focused on the basic mechanisms involved in the transition from wakefulness, initiation of sleep and the subsequent generation of slow-wave sleep and rapid eye movement (REM) sleep. Additionally, using recent molecular studies and experimental evidence that provides a direct link to sleep as a behavior, we have developed a new model, the Cellular-Molecular-Network model, explaining the mechanisms responsible for regulating REM sleep. By analyzing the fundamental neurobiological mechanisms responsible for the generation and maintenance of sleep-wake behavior in mammals, we intend to provide a broader understanding of our present knowledge in the field of sleep research.

Datta, Subimal; MacLean, Robert Ross

2007-01-01

246

MFM study of magnetic vortex cores in circular permalloy dots: behavior in external field  

Microsoft Academic Search

In a circular dot of permalloy with an appropriate size, a vortex structure with perpendicular (turned-up) magnetization at the core is realized. The existence of the perpendicular magnetization spot has been confirmed and the direction of the magnetization, up or down, has been determined by magnetic force microscopy (MFM) for permalloy dots with the diameter of 0.1–1?m. The switching field

T. Okuno; K. Shigeto; T. Ono; K. Mibu; T. Shinjo

2002-01-01

247

Large-scale vortical structure of turbulent separation bubble affected by unsteady wake  

Microsoft Academic Search

The large-scale vortical structure of a turbulent separation bubble under the influence of an unsteady wake was investigated. The unsteady wake was generated by a spoked-wheel type wake generator installed in front of the separation bubble. This wake generator was rotated either clockwise or counter-clockwise at Re H=5.600. The mechanism of vortex shedding from the separation bubble was analyzed in

S. Chun; H. J. Sung

2003-01-01

248

Cross-wire measurements in the wake of an airfoil at low Reynolds numbers with and without acoustic excitation  

Microsoft Academic Search

The wake structure and vortex shedding characteristics of a NACA 0025 airfoil were studied experimentally. Wind tunnel experiments were carried out for three Reynolds numbers and three angles of attack by means of cross-wire measurements, spectral analysis and complementary surface flow visualization. Evidence of wake vortex shedding and flow separation was obtained for most of the cases examined, and dependence

Serhiy Yarusevych; Pierre E Sullivan; John G Kawall

2002-01-01

249

Delayed Orexin Signaling Consolidates Wakefulness and Sleep: Physiology and Modeling  

PubMed Central

Orexin-producing neurons are clearly essential for the regulation of wakefulness and sleep because loss of these cells produces narcolepsy. However, little is understood about how these neurons dynamically interact with other wake- and sleep-regulatory nuclei to control behavioral states. Using survival analysis of wake bouts in wild-type and orexin knockout mice, we found that orexins are necessary for the maintenance of long bouts of wakefulness, but orexin deficiency has little impact on wake bouts <1 min. Since orexin neurons often begin firing several seconds before the onset of waking, this suggests a surprisingly delayed onset (>1 min) of functional effects. This delay has important implications for understanding the control of wakefulness and sleep because increasing evidence suggests that different mechanisms are involved in the production of brief and sustained wake bouts. We incorporated these findings into a mathematical model of the mouse sleep/wake network. Orexins excite monoaminergic neurons and we hypothesize that orexins increase the monoaminergic inhibition of sleep-promoting neurons in the ventrolateral preoptic nucleus. We modeled orexin effects as a time-dependent increase in the strength of inhibition from wake- to sleep-promoting populations and the resulting simulated behavior accurately reflects the fragmented sleep/wake behavior of narcolepsy and leads to several predictions. By integrating neurophysiology of the sleep/wake network with emergent properties of behavioral data, this model provides a novel framework for investigating network dynamics and mechanisms associated with normal and pathologic sleep/wake behavior.

Diniz Behn, C. G.; Kopell, N.; Brown, E. N.; Mochizuki, T.; Scammell, T. E.

2011-01-01

250

Near-wake flow structure downwind of a wind turbine in a turbulent boundary layer  

NASA Astrophysics Data System (ADS)

Wind turbines operate in the surface layer of the atmospheric boundary layer, where they are subjected to strong wind shear and relatively high turbulence levels. These incoming boundary layer flow characteristics are expected to affect the structure of wind turbine wakes. The near-wake region is characterized by a complex coupled vortex system (including helicoidal tip vortices), unsteadiness and strong turbulence heterogeneity. Limited information about the spatial distribution of turbulence in the near wake, the vortex behavior and their influence on the downwind development of the far wake hinders our capability to predict wind turbine power production and fatigue loads in wind farms. This calls for a better understanding of the spatial distribution of the 3D flow and coherent turbulence structures in the near wake. Systematic wind-tunnel experiments were designed and carried out to characterize the structure of the near-wake flow downwind of a model wind turbine placed in a neutral boundary layer flow. A horizontal-axis, three-blade wind turbine model, with a rotor diameter of 13 cm and the hub height at 10.5 cm, occupied the lowest one-third of the boundary layer. High-resolution particle image velocimetry (PIV) was used to measure velocities in multiple vertical stream-wise planes ( x- z) and vertical span-wise planes ( y- z). In particular, we identified localized regions of strong vorticity and swirling strength, which are the signature of helicoidal tip vortices. These vortices are most pronounced at the top-tip level and persist up to a distance of two to three rotor diameters downwind. The measurements also reveal strong flow rotation and a highly non-axisymmetric distribution of the mean flow and turbulence structure in the near wake. The results provide new insight into the physical mechanisms that govern the development of the near wake of a wind turbine immersed in a neutral boundary layer. They also serve as important data for the development and validation of numerical models.

Zhang, Wei; Markfort, Corey D.; Porté-Agel, Fernando

2012-05-01

251

On the characteristics of the wake meandering of a marine hydrokinetic turbine  

NASA Astrophysics Data System (ADS)

Recently Kang et al. (Journal of Fluid Mechanics, submitted) showed that the hub vortex breakdown occurring downstream of a hydrokinetic turbine plays an important role in enhancing wake meandering. In this study the hub vortex breakdown and wake meandering phenomena are further examined using large-eddy simulation (Kang et al., Advances in Water Resources, 2012). Specifically, the effect of the incoming turbulence, the presence of hub and nacelle geometries, and the tip speed ratio of the rotor on the wake meandering and the hub vortex breakdown are examined.

Kang, S.

2013-12-01

252

Measurements in 80- by 120-foot wind tunnel of hazard posed by lift-generated wakes  

NASA Technical Reports Server (NTRS)

The large, low speed wind tunnel at NASA-Ames has been used to study the characteristics of lift-generated vortices involved in the definition of aircraft-separation criteria, in order to enhance airport capacity without compromising safety. Attention is given to the potential hazard caused by the vortex wake of several configurations of a subsonic transport. Measured downwash distributions in the wake of three different wake-generator configurations are obtained by means of a vortex-lattice method, in order to predict the lift and rolling moment on several models of wake-following aircraft.

Rossow, V. J.; Sacco, J. N.; Askins, P. A.; Bisbee, L. S.; Smith, S. M.

1993-01-01

253

Sleep-wake differences in scaling behavior of the human heartbeat: Analysis of terrestrial and long-term space flight data  

Microsoft Academic Search

We compare scaling properties of the cardiac dynamics during sleep and wake periods for healthy individuals, cosmonauts during orbital flight, and subjects with severe heart disease. For all three groups, we find a greater degree of anticorrelation in the heartbeat fluctuations during sleep compared to wake periods. The sleep-wake difference in the scaling exponents for the three groups is comparable

P. Ch. Ivanov; Armin Bunde; L. A. N. Amaral; Shlomo Havlin; Janice Fritsch-Yelle; R. M. Baevsky; H. E. Stanley; A. L. Goldberger

1999-01-01

254

Dynamics of wake structure in clapping propulsion  

NASA Astrophysics Data System (ADS)

Some animals such as insects and frogs use a pair of symmetric flaps for locomotion. In some cases, these flappers operate in close proximity or even touch each other. In order to understand the underlying physics of these kinds of motion, we have studied the wake structures induced by clapping and their associated thrust performance. A simple mechanical model with two acrylic plates was used to simulate the power stroke of the clapping motion and three-dimensional flow fields were obtained using defocusing digital particle image velocimetry. Our studies show that the process of vortex connection plays a critical role in forming a downstream closed vortex loop. Under some kinematic conditions, this vortex loop changes its shape dynamically, which is analogous to the process of an elliptical vortex ring switching its minor and major axis. As the length of the plate along the rotating shaft decreases to change an aspect ratio, the downstream motion of the vortex is retarded due to the outward motion of side edge vortices and less propulsive force is generated per the surface area of the plate. The impact of compliance and stroke angle of the plate on wake structures and thrust magnitudes are also presented.

Kim, Daegyoum; Gharib, Morteza

2009-11-01

255

A Critical Review of the Transport and Decay of Wake Vortices in Ground Effect  

NASA Technical Reports Server (NTRS)

This slide presentation reviews the transport and decay of wake vortices in ground effect and cites a need for a physics-based parametric model. The encounter of a vortex with a solid body is always a complex event involving turbulence enhancement, unsteadiness, and very large gradients of velocity and pressure. Wake counter in ground effect is the most dangerous of them all. The interaction of diverging, area-varying, and decaying aircraft wake vortices with the ground is very complex because both the vortices and the flow field generated by them are altered to accommodate the presence of the ground (where there is very little room to maneuver) and the background turbulent flow. Previous research regarding vortex models, wake vortex decay mechanisms, time evolution within in ground effect of a wake vortex pair, laminar flow in ground effect, and the interaction of the existing boundary layer with a convected vortex are reviewed. Additionally, numerical simulations, 3-dimensional large-eddy simulations, a probabilistic 2-phase wake vortex decay and transport model and a vortex element method are discussed. The devising of physics-based, parametric models for the prediction of (operational) real-time response, mindful of the highly three-dimensional and unsteady structure of vortices, boundary layers, atmospheric thermodynamics, and weather convective phenomena is required. In creating a model, LES and field data will be the most powerful tools.

Sarpkaya, T.

2004-01-01

256

Wake fields and wake field acceleration  

SciTech Connect

In this lecture we introduce the concepts of wake fields and wake potentials, examine some basic properties of these functions, show how they can be calculated, and look briefly at a few important applications. One such application is wake field acceleration. The wake field accelerator is capable of producing the high gradients required for future very high energy e/sup +/e/sup -/ linear colliders. The principles of wake field acceleration, and a brief description of experiments in progress in this area, are presented in the concluding section. 40 references, 27 figures.

Bane, K.L.F.; Wilson, P.B.; Weiland, T.

1984-12-01

257

The entrainment eddies in a wake flow  

NASA Astrophysics Data System (ADS)

The behavior of a plane turbulent wake generated by a porous body, such as a strip of square mesh, were investigated to find additional evidence in support for the finding of Louchez (1985) that blips are characteristic features of plane turbulent wakes and that the statistical properties of these blips are the same for all wakes at the same normalized streamwise location. The velocity signals from the intermittent region of the flow were analyzed using the triple-choice conditional sampling technique of Tabatabai et al. (1987). The present results compare well with those of Tabatabai et al. obtained for the wake behind a circular cylinder and support the hypothesis concerning the existence of entrainment-eddy-type structures (blips) in the outer region of the plane wake. The statistical properties of the blips pertaining to different wake generators were found to be the same, consistent with the findings of Louchez.

Tabatabai, M.; Kawall, J. G.; Keffer, J. F.

258

Hybrid vortex method for lifting surfaces with free-vortex flow  

NASA Technical Reports Server (NTRS)

A Nonlinear Hybrid Vortex method (NHV-method) has been developed for predicting the aerodynamic characteristics of wings exhibiting leading- and side-edge separations. This method alleviates the drawbacks of the Nonlinear Discrete Vortex method (NDV-method, also known as the multiple line vortex method.) The NHV-method combines continuous-vorticity and vortex-line representations of the wing and its separated free shear layers. Continuous vorticity is used in the near-field calculations, while discrete vortex-lines are used in the far-field calculations. The wing and its free shear layers are divided into quadrilateral vortex panels having second-order vorticity distributions. The aerodynamic boundary conditions and continuity of the vorticity distributions are satisfied at certain nodal points on the vortex panels. An iterative technique is used to satisfy these conditions in order to obtain the vorticity distribution and the wake shape. Distributed and total aerodynamic loads are then calculated.

Kandil, O. A.; Chu, L.-C.; Yates, E. C., Jr.

1980-01-01

259

Parallel 3D computation of unsteady wake flows with complex geometries and fluid-structure interactions  

NASA Astrophysics Data System (ADS)

New powerful parallel computational tools are developed for 3D simulation of unsteady wake flows with complex geometries and fluid-structure interactions. The base method for flow simulation is a finite element formulation for the Navier-Stokes equations. The finite element formulation is based on the streamline-upwind/Petrov-Galerkin (SUPG) and pressure-stabilizing/Petrov-Galerkin (PSPG) techniques. These stabilization techniques facilitate simulation of flows with high Reynolds numbers, and allow us to use equal-order interpolation functions for velocity and pressure without generating numerical oscillations. A multi-domain computational method is developed to simulate wake flow both in the near and far downstream. The formulations lead to coupled nonlinear equation systems which are solved, at every time step, with the Newton-Raphson method. The overall formulation and solution techniques are implemented on parallel platforms such as the CRAY T3E and SGI PowerChallenge. Two phases of vortex shedding for flow past a cylinder is simulated to verify the accuracy of this method. The Enhanced-Discretization Interface Capturing Technique (EDICT) is utilized to simulate wake flow accurately. Fluid-structure coupling solution method based on the Deforming-Spatial-Domain/Stabilized Space-Time (DSD/SST) formulation is applied to simulate a parachute behavior in the unsteady wake.

Osawa, Yasuo

260

Airloads and Wake Geometry Calculations for an Isolated Tiltrotor Model in a Wind Tunnel  

NASA Technical Reports Server (NTRS)

Comparisons of measured and calculated aerodynamic behavior of a tiltrotor model are presented. The test of the Tilt Rotor Aeroacoustic Model (TRAM) with a single, 0.25-scale V-22 rotor in the German-Dutch Wind Tunnel (DNW) provides an extensive set of aeroacoustic, performance, and structural loads data. The calculations were performed using the rotorcraft comprehensive analysis CAMRAD II. Presented are comparisons of measured and calculated performance for hover and helicopter mode operation, and airloads for helicopter mode. Calculated induced power, profile power, and wake geometry provide additional information about the aerodynamic behavior. An aerodynamic and wake model and calculation procedure that reflects the unique geometry and phenomena of tiltrotors has been developed. There are major differences between this model and the corresponding aerodynamic and wake model that has been established for helicopter rotors. In general, good correlation between measured and calculated performance and airloads behavior has been shown. Two aspects of the analysis that clearly need improvement are the stall delay model and the trailed vortex formation model.

Johnson, Wayne

2001-01-01

261

Dependence of the wake on inclination of a stationary cylinder  

Microsoft Academic Search

Three-dimensional vorticity in the wake of an inclined stationary circular cylinder was measured simultaneously using a multi-hot\\u000a wire vorticity probe over a streamwise range of x\\/d = 10–40. The study aimed to examine the dependence of the wake characteristics on cylinder inclination angle ? (=0°–45°).\\u000a The validity of the independence principle (IP) for vortex shedding was also examined. It was found that

T. Zhou; S. F. Mohd. Razali; Y. Zhou; L. P. Chua; L. Cheng

2009-01-01

262

The Effects of Aircraft Wake Dynamics on Contrail Development  

Microsoft Academic Search

Results of large-eddy simulations of the development of young persistent ice contrails are presented, con- centrating on the interactions between the aircraft wake dynamics and the ice cloud evolution over ages from a few seconds to ;30 min. The 3D unsteady evolution of the dispersing engine exhausts, trailing vortex pair interaction and breakup, and subsequent Brunt-Vaisalaoscillations of the older wake

D. C. Lewellen; W. S. Lewellen

2001-01-01

263

Contrail Formation in Aircraft Wakes Using Large-Eddy Simulations  

NASA Technical Reports Server (NTRS)

In this work we analyze the issue of the formation of condensation trails ("contrails") in the near-field of an aircraft wake. The basic configuration consists in an exhaust engine jet interacting with a wing-tip training vortex. The procedure adopted relies on a mixed Eulerian/Lagrangian two-phase flow approach; a simple micro-physics model for ice growth has been used to couple ice and vapor phases. Large eddy simulations have carried out at a realistic flight Reynolds number to evaluate the effects of turbulent mixing and wake vortex dynamics on ice-growth characteristics and vapor thermodynamic properties.

Paoli, R.; Helie, J.; Poinsot, T. J.; Ghosal, S.

2002-01-01

264

ROTORCRAFT AERODYNAMIC AND AEROACOUSTIC MODELLING USING VORTEX PARTICLE METHODS  

Microsoft Academic Search

The initial development of a combined active aeroelastic aeroacoustic rotorcraft code is discussed. The GENeral Unsteady Vortex Particle code (GENUVP) is used as the aerodynamic component of the combined code. A brief discussion of the theory behind GENUVP's use of an unsteady panel method with a vortex particle wake is presented. Modifications to GENUVP to increase its computational efficiency are

Daniel G. Opoku; Dimitris G. Triantos; Fred Nitzsche; Spyros G. Voutsinas

2002-01-01

265

Vortex dynamics around pitching plates  

NASA Astrophysics Data System (ADS)

Vortex dynamics of wakes generated by rectangular aspect-ratio 2 and 4 and two-dimensional pitching flat plates in free stream are examined with direct numerical simulation and water tunnel experiments. Evolution of wake vortices comprised of tip, leading-edge, and trailing-edge vortices is compared with force history for a range of pitch rates. The plate pivots about its leading edge with reduced frequency from ?/8 to ?/48, which corresponds to pitching over 1 to 6 chord lengths of travel. Computations have reasonable agreement with experiments, despite large differences in Reynolds number. Computations show that the tip effects are confined initially near the wing tips, but begin to strongly affect the leading-edge vortex as the motion of the plate proceeds, with concomitant effects on lift and drag history. Scaling relations based on reduced frequency are shown to collapse aerodynamic force history for the various pitch rates.

Jantzen, Ryan T.; Taira, Kunihiko; Granlund, Kenneth O.; Ol, Michael V.

2014-05-01

266

Laser-optical investigation of turbine wake flow  

Microsoft Academic Search

The vortex street of a turbine blade profile (VKI-1, 58 mm chord length) was investigated by particle image velocimetry (PIV) and laser vibrometry in a Mach number range between 0.26 and 0.78. While the laser vibrometer (LV) measured the frequency of the density fluctuations in the wake and around the profile, PIV helped to clarify the vortex shedding process. The

J. Woisetschläger; N. Mayrhofer; B. Hampel; H. Lang; W. Sanz

2003-01-01

267

Wake of forced flow around elliptical leading edge plates  

Microsoft Academic Search

Previous investigations have shown that flows around rectangular plates with transverse forcing involve interactions between vortices shed from the leading and trailing edges and vortex merging in the wakes. The Strouhal number of vortex shedding at which peak base drag occurs varies with chord-to-thickness ratio in a stepwise fashion, similar to the self-sustained oscillations at low Reynolds number for unforced

R. Mills; J. Sheridan; K. Hourigan

2005-01-01

268

Thrust Production and Wake Structure of an Actuated Lamprey Model  

NASA Astrophysics Data System (ADS)

Thrust generation is studied for a flexible lamprey model which is actuated periodically to produce a streamwise traveling wave. Shape memory alloy actuators are used to achieve this deformation. The flow field is investigated using DPIV and flow visualization for a range of Strouhal numbers based on peak-to-peak amplitude of the trailing edge. The vortex kinematics in the spanwise and streamwise planes are examined, and a three-dimensional unsteady vortex model of the wake will be discussed.

Buchholz, James; Smits, Alexander

2004-11-01

269

Studies on Unsteady Vortex Motions Including Thermo - Interactions.  

NASA Astrophysics Data System (ADS)

An inviscid unsteady vortex dynamics study is presented here to study the flow details in several free and bounded shear flows. The specific flows considered here are the starting vortex behind sharp edges, the vortex street behind bluff body wakes, the plane mixing layers between two streams and the wall bounded vortex street. Using the unsteady vortex dynamics models developed for each of these flows and through the Lagrangian dynamics computational approach, the streakline and pathline pattern have been computed for these flows. From the combined study of the computed streakline and pathline pattern, various flow visualization details and the illusions created by the streaklines have been identified and explained. With the accuracy of the unsteady vortex model for the vortex street wake having been verified by the good agreement obtained between the computed and flow visualization results, the model has been used to investigate the different flow interactions present in this complex flow. The first case chosen for analysis is the thermo-fluid interaction that exists in this flow. It has been shown, through the computed instantaneous total temperature, that the bluff body wake exhibits regions of separated energy. Further, the well known phenomenon of Eckert-Weise effect, which predicts a cold near wake, has been obtained by temporally averaging the instantaneous total temperature, thus proving that the cause of this phenomenon is the unsteady vortex motion in the wake. In a similar manner, the familiar result of a loss in total pressure in the near wake can be interpreted to result from the time averaged total pressure separation phenomenon. The second case considered here for the vortex interaction study is the interference caused by the channel walls in a wall bounded vortex street wake. From the computed streakline pattern and also through the flow visualization studies, an interesting vortex dynamics result that forces the flow from both sides of the wake to interchange their sides has been obtained. This result explains the mechanism for the static temperature reversal between the two sides of a wall bounded wake that was observed by Minchin. Further, the present computational study highlights the important inviscid entrainment effects of the convecting vortices by showing that the eruption-like appearance of the wall layer streaklines observed in the flow visualization pictures is predominantly due to the inviscid entrainment effects and not merely caused by the viscous effects.

Sundaram, P.

270

Unsteady wing surface pressures in the wake of a propeller  

NASA Technical Reports Server (NTRS)

The unsteady nature of the propeller slipstream interacting with a wing has been studied by flow visualization and unsteady wing surface pressure measurements. Flow visualization was performed by marking the propeller tip vortex with smoke. Unsteady wing surface pressures were measured by traversing a wing instrumented with a chordwise array of 16 microphones in a spanwise direction through the propeller wake. This work yielded information on the motion of the propeller wake as it passes over the wing. As the propeller wake passed over the wing: the propeller tip vortex experienced an inviscid interaction at the leading edge; viscous action at the leading edge severed the propeller tip vortex; the propeller tip vortex experienced significant spanwise and chordwise displacements and then deformed in order to reconnect at the trailing edge; axial velocity in the vortex core caused the helical vortex to thicken or stretch near the wing surface; and, the magnitude of the pressure fluctuations decreased in magnitude with distance traveled along the chord.

Johnston, R. T.; Sullivan, J. P.

1992-01-01

271

Neuropeptidergic control of sleep and wakefulness.  

PubMed

Sleep and wake are fundamental behavioral states whose molecular regulation remains mysterious. Brain states and body functions change dramatically between sleep and wake, are regulated by circadian and homeostatic processes, and depend on the nutritional and emotional condition of the animal. Sleep-wake transitions require the coordination of several brain regions and engage multiple neurochemical systems, including neuropeptides. Neuropeptides serve two main functions in sleep-wake regulation. First, they represent physiological states such as energy level or stress in response to environmental and internal stimuli. Second, neuropeptides excite or inhibit their target neurons to induce, stabilize, or switch between sleep-wake states. Thus, neuropeptides integrate physiological subsystems such as circadian time, previous neuron usage, energy homeostasis, and stress and growth status to generate appropriate sleep-wake behaviors. We review the roles of more than 20 neuropeptides in sleep and wake to lay the foundation for future studies uncovering the mechanisms that underlie the initiation, maintenance, and exit of sleep and wake states. PMID:25032501

Richter, Constance; Woods, Ian G; Schier, Alexander F

2014-07-01

272

Characterization of cavity wakes  

NASA Astrophysics Data System (ADS)

Scope and Method of Study. This research focused on flow over deep cavities at subsonic speeds with emphasis on the wake downstream of the cavity. Cavity wake behaviors have not been studied in detail and are a major concern for air vehicles with cavities and in particular for optical sensor systems installed in cavities. Other key behaviors for sensor survival and performance are cavity resonance and turbulence scales in the shear layer. A wind tunnel test apparatus was developed to explore cavity and wake characteristics. It consisted of a test section insert for the OSU Indraft Wind Tunnel with an additional contraction cone for significantly increased speed. The test section included a variable depth cavity in a boundary layer splitter plate/fairing assembly, a Y-Z traverse and pitot rake with in-situ pressure transducers for high frequency response. Flows were measured over clean cavities with length to depth (L/D) ratios of 4 to 1/2 and on cavities with a porous fence for resonance suppression. Measurements were taken in streamwise and cross-stream sections to three cavity lengths downstream of the cavity trailing edge. Flow visualization using laser sheet and smoke injection was also used. Findings and Conclusions. The high speed insert demonstrated a significant new capability for the OSU wind tunnel, reaching speeds of 0.35 Mach (390 feet/second) in a 14"x14" test section. Inlet room flow was found to be quite unsteady and recommendations are made for improved flow and quantitative visualization. Key findings for cavity wake flow include its highly three dimensional nature with asymmetric peaks in cross section with boundary layer thicknesses and integral length scales several times that of a normal flat plate turbulent boundary layer (TBL). Turbulent intensities (TI) of 35% to 55% of freestream speeds were measured for the clean configuration. Fence configuration TI's were 20% to 35% of free stream and, in both configurations, TI's decayed to approximately that of a flat plate TBL by 3 cavity lengths downstream from the cavity trailing edge. Fence flow visualization showed edge vortices and jets through the perforations that suggest the potential for minimizing turbulence intensity and scales while still suppressing cavity resonance.

Kidd, James A.

273

Dynamics of Tab-Wake Vortices  

NASA Astrophysics Data System (ADS)

The dynamics of vortex structures in the wake of surface-mounted trapezoidal tab at Re=600 based on tab height was studied in detail using time-series, 2D particle image velocimetry. From a total of over 20,000 PIV realizations acquired in x-y, x-z, and y-z planes, we successfully identified vortex structures using the methods proposed by Jeong and Hussain (JFM, vol 285, 1995) and proposed by Chong, Perry, and Cantwell (Phys. Fluids A2, 1990), and cross-checked them with conventional velocity subtraction. Similar to prior measurement at Re=2080, secondary vortices, reverse vortices, and tertiary vortices were observed frequently in the present study. Higher PIV spatial resolution and higher temporal resolution (relative to the flow periodicity) allow us to investigate these dynamical phenomena in much greater detail and confidence. Furthermore, y-z measurements demonstrate that hairpin vortex legs, taking the shape of streamwise vortices, pair with their neighbor counterparts while traveling downstream, and possibly merge with each other. Circulation distribution of the hairpin vortex heads along the x direction shows that it increases at the very near-tab region with the help of pressure induced counter-rotating vortex pairs, but gradually decreases very slowly with the increasing downstream distance, indicating that hairpin vortices are long-lived vortex structures.

Yang, W.; Meng, H.

1999-11-01

274

A Theoretical Study of the Aerodynamics of Slender Cruciform-Wing Arrangements and Their Wakes  

NASA Technical Reports Server (NTRS)

A theoretical study is made of the pressures, loadings, forces, and vortex wake associated with certain cruciform wing arrangements. For 45 degree bank, the wake of a cruciform wing is treated numerically with 40 vortices and analytically with 4 vortices. Comparisons are made with water-tank measurements, and the calculation of loads on cruciform tails by reverse flow considered.

Spreiter, John R; Sacks, Alvin H

1957-01-01

275

A near wake model for trailing vorticity compared with the blade element momentum theory  

Microsoft Academic Search

A near wake model for trailing vorticity originally proposed by Beddoes for high-resolution helicopter blade vortex interaction computations has been implemented and compared with the usual blade element momentum models used for wind turbine calculations. The model is in principle a lifting line model for the rotating blade, where only a quarter revolution of the wake system behind the blade

Helge Aagaard Madsen; Flemming Rasmussen

2004-01-01

276

Vortex equations: Singularities, numerical solution, and axisymmetric vortex breakdown  

NASA Technical Reports Server (NTRS)

A method of weighted residuals for the computation of rotationally symmetric quasi-cylindrical viscous incompressible vortex flow is presented and used to compute a wide variety of vortex flows. The method approximates the axial velocity and circulation profiles by series of exponentials having (N + 1) and N free parameters, respectively. Formal integration results in a set of (2N + 1) ordinary differential equations for the free parameters. The governing equations are shown to have an infinite number of discrete singularities corresponding to critical values of the swirl parameters. The computations point to the controlling influence of the inner core flow on vortex behavior. They also confirm the existence of two particular critical swirl parameter values: one separates vortex flow which decays smoothly from vortex flow which eventually breaks down, and the second is the first singularity of the quasi-cylindrical system, at which point physical vortex breakdown is thought to occur.

Bossel, H. H.

1972-01-01

277

Waking self-hypnosis efficacy in cognitive-behavioral treatment for pathological gambling: an effectiveness clinical assay.  

PubMed

Cognitive-behavioral therapy for pathological gambling has a long-term success rate of more than 50%. This study evaluated the effect of self-hypnosis in cognitive-behavioral treatment of pathological gamblers. Forty-nine participants were assigned to 2 groups. Both groups received a cognitive-behavioral protocol, and Group 1, the no-hypnosis group, received an 11-session intervention and Group 2, the hypnosis group, received 7 sessions that included self-hypnosis. Both groups were equal in gambling chronicity, frequency, intensity, change motivation, and problems derived from gambling. All participants reported significant improvement in gambling behavior and consequences at both treatment end and 6-month follow-up. Data show no differences between the interventions in abstinence, therapeutic compliance, fulfillment, and satisfaction. Results suggest that self-hypnosis reinforces treatment and can be a supportive technique for future brief interventions. PMID:24256479

Lloret, Daniel; Montesinos, Rosa; Capafons, Antonio

2014-01-01

278

Wind turbine wake properties from Doppler lidar measurements  

NASA Astrophysics Data System (ADS)

Wake properties were estimated from the High-Resolution Doppler Lidar (HRDL) measurements during the Turbine Wake and Inflow Characterization Study (TWICS) in the spring of 2011. Velocity deficit, wake downwind extent, and wake meandering were obtained by detailed analysis of both lidar vertical-slice scans, performed straddling along the lidar-turbine centerline, and lidar conical scans, performed in narrow, nearly horizontal sectors that include the wind turbine inflow, and its wake at four levels. Simultaneous measurements of inflow and turbine outflow were corrected by terrain and wind direction to obtain mean wake properties. It has been found out that an operating wind turbine generates a wake with the maximum velocity deficit varying from 20% to 70% extending up to 10 rotor diameters downstream of the turbine, depending on the wind strength and atmospheric turbulence. Details including images and animations of the wake behavior will be presented.

Pichugina, Y.; Banta, R. M.; Brewer, A.; Lundquist, J. K.

2012-12-01

279

The effects of vortex modeling on blade-vortex interaction noise prediction  

NASA Astrophysics Data System (ADS)

The use of a blade vortex interaction noise prediction scheme, based on CAMRAD/JA, FPR and RAPP, quantifies the effects of errors and assumptions in the modeling of the helicopter's shed vortex on the acoustic predictions. CAMRAD/JA computes the wake geometry and inflow angles that are used in FPR to solve for the aerodynamic surface pressures. RAPP uses these surface pressures to predict the acoustic pressure. Both CAMRAD/JA and FPR utilize the Biot-Savart Law to determine the influence of the vortical velocities on the blade loading and both codes use an algebraic vortex model for the solid body rotation of the vortex core. Large changes in the specification of the vortex core size do not change the inplane wake geometry calculated by CAMRAD/JA and only slightly affect the out-of-plane wake geometry. However, the aerodynamic surface pressure calculated by FPR changes in both magnitude and character with small changes to the core size used by the FPR calculations. This in turn affects the acoustic predictions. Shifting the CAMRAD/JA wake geometry away from the rotor plane by 1/4 chord produces drastic changes in the acoustic predictions indicating that the prediction of acoustic pressure is extremely sensitive to the miss distance between the vortex and the blade and that this distance must be calculated as accurately as possible for acceptable noise predictions. The inclusion or exclusion of a vortex in the FPR-RAPP calculation allows for the determination of the relative importance of that vortex as a BVI noise source.

Gallman, Judith M.; Tung, Chee; Low, Scott L.

280

Wind turbine wake aerodynamics  

Microsoft Academic Search

The aerodynamics of horizontal axis wind turbine wakes is studied. The contents is directed towards the physics of power extraction by wind turbines and reviews both the near and the far wake region. For the near wake, the survey is restricted to uniform, steady and parallel flow conditions, thereby excluding wind shear, wind speed and rotor setting changes and yawed

L. J. Vermeer; J. N. Sørensen; A. Crespo

2003-01-01

281

Numerical Study of Tip Vortex Flows  

NASA Technical Reports Server (NTRS)

This paper presents an overview and summary of the many different research work related to tip vortex flows and wake/trailing vortices as applied to practical engineering problems. As a literature survey paper, it outlines relevant analytical, theoretical, experimental and computational study found in literature. It also discusses in brief some of the fundamental aspects of the physics and its complexities. An appendix is also included. The topics included in this paper are: 1) Analytical Vortices; 2) Experimental Studies; 3) Computational Studies; 4) Wake Vortex Control and Management; 5) Wake Modeling; 6) High-Lift Systems; 7) Issues in Numerical Studies; 8) Instabilities; 9) Related Topics; 10) Visualization Tools for Vertical Flows; 11) Further Work Needed; 12) Acknowledgements; 13) References; and 14) Appendix.

Dacles-Mariani, Jennifer; Hafez, Mohamed

1998-01-01

282

The effect of vortex shedding on the unsteady pressure distribution around the trailing edge of a turbine blade  

Microsoft Academic Search

The wakes behind turbine blade trailing edges are characterized by large-scale periodic vortex patterns known as the von Karman vortex street. The failure of steady-state Navier-Stokes calculations in modeling wake flows appears to be mainly due to ignoring this type of flow instabilities. In an effort to contribute to a better understanding of the time-varying wake flow characteristics behind turbine

G. Cicatelli; C. H. Sieverding

1997-01-01

283

Numerical simulation and control of horseshoe vortex around an appendage-body junction  

NASA Astrophysics Data System (ADS)

The horseshoe vortex generated around the appendage-body junction of submarines strongly influences the non-uniformity of submarine wakes at the propeller discs. The flow characteristics around the appended submarine body are numerically simulated and analyzed, and a new method on the vortex control baffle is presented. Then, the influence of the vortex control baffle on the horseshoe vortex generated at the sail-body junction is numerically studied, and the flow phenomena caused by the vortex control baffle with different transverse positions is investigated further. Results show that the vortex control baffle can induce a kind of attached vortex in a rotational direction opposite to the horseshoe vortex; these two kinds of vortices undermine each other. Furthermore, when the transverse position of the vortex control baffle is close to the horseshoe vortex, the state of the horseshoe vortex is directly affected, and the flow structure becomes even more complex. We adapt the vortex control baffle for the horseshoe vortex generated at the stern foil-body junction. Results from the numerical simulation of the flow around the fully appended submarine model indicate that the effect of the vortex control baffle greatly improves the performance of the submarine wake. The circumferential non-uniformity of the axial, tangential, and radial velocity components are decreased markedly. The engineering applicability of the vortex control baffle has been well presented.

Zhihua, Liu; Ying, Xiong; Chengxu, Tu

2011-01-01

284

Rotorcraft acoustic radiation prediction based on a refined blade-vortex interaction model  

NASA Astrophysics Data System (ADS)

The analysis of rotorcraft aerodynamics and acoustics is a challenging problem, primarily due to the fact that a rotorcraft continually flies through its own wake. The generation mechanism for a rotorcraft wake, which is dominated by strong, concentrated blade-tip trailing vortices, is similar to that in fixed wing aerodynamics. However, following blades encounter shed vortices from previous blades before they are swept downstream, resulting in sharp, impulsive loading on the blades. The blade/wake encounter, known as Blade-Vortex Interaction, or BVI, is responsible for a significant amount of vibratory loading and the characteristic rotorcraft acoustic signature in certain flight regimes. The present work addressed three different aspects of this interaction at a fundamental level. First, an analytical model for the prediction of trailing vortex structure is discussed. The model as presented is the culmination of a lengthy research effort to isolate the key physical mechanisms which govern vortex sheet rollup. Based on the Betz model, properties of the flow such as mass flux, axial momentum flux, and axial flux of angular momentum are conserved on either a differential or integral basis during the rollup process. The formation of a viscous central core was facilitated by the assumption of a turbulent mixing process with final vortex velocity profiles chosen to be consistent with a rotational flow mixing model and experimental observation. A general derivation of the method is outlined, followed by a comparison of model predictions with experimental vortex measurements, and finally a viscous blade drag model to account for additional effects of aerodynamic drag on vortex structure. The second phase of this program involved the development of a new formulation of lifting surface theory with the ultimate goal of an accurate, reduced order hybrid analytical/numerical model for fast rotorcraft load calculations. Currently, accurate rotorcraft airload analyses are limited by the massive computational power required to capture the small time scale events associated with BVI. This problem has two primary facets: accurate knowledge of the wake geometry, and accurate resolution of the impulsive loading imposed by a tip vortex on a blade. The present work addressed the second facet, providing a mathematical framework for solving the impulsive loading problem analytically, then asymptotically matching this solution to a low-resolution numerical calculation. A method was developed which uses continuous sheets of integrated boundary elements to model the lifting surface and wake. Special elements were developed to capture local behavior in high-gradient regions of the flow, thereby reducing the burden placed on the surrounding numerical method. Unsteady calculations for several classical cases were made in both frequency and time domain to demonstrate the performance of the method. Finally, a new unsteady, compressible boundary element method was applied to the problem of BVI acoustic radiation prediction. This numerical method, combined with the viscous core trailing vortex model, was used to duplicate the geometry and flight configuration of a detailed experimental BVI study carried out at NASA Ames Research Center. Blade surface pressure and near- and far-field acoustic radiation calculations were made. All calculations were shown to compare favorably with experimentally measured values. The linear boundary element method with non-linear corrections proved sufficient over most of the rotor azimuth, and particular in the region of the blade vortex interaction, suggesting that full non-linear CFD schemes are not necessary for rotorcraft noise prediction.

Rule, John Allen

1997-08-01

285

Flow visualizations of perpendicular blade vortex interactions  

NASA Technical Reports Server (NTRS)

Helium bubble flow visualizations have been performed to study perpendicular interaction of a turbulent trailing vortex and a rectangular wing in the Virginia Tech Stability Tunnel. Many combinations of vortex strength, vortex-blade separation (Z(sub s)) and blade angle of attack were studied. Photographs of representative cases are presented. A range of phenomena were observed. For Z(sub s) greater than a few percent chord the vortex is deflected as it passes the blade under the influence of the local streamline curvature and its image in the blade. Initially the interaction appears to have no influence on the core. Downstream, however, the vortex core begins to diffuse and grow, presumably as a consequence of its interaction with the blade wake. The magnitude of these effects increases with reduction in Z(sub s). For Z(sub s) near zero the form of the interaction changes and becomes dependent on the vortex strength. For lower strengths the vortex appears to split into two filaments on the leading edge of the blade, one passing on the pressure and one passing on the suction side. At higher strengths the vortex bursts in the vicinity of the leading edge. In either case the core of its remnants then rapidly diffuse with distance downstream. Increase in Reynolds number did not qualitatively affect the flow apart from decreasing the amplitude of the small low-frequency wandering motions of the vortex. Changes in wing tip geometry and boundary layer trip had very little effect.

Rife, Michael C.; Davenport, William J.

1992-01-01

286

Simulation of unsteady motion of a propeller in a fluid including free wake modeling  

Microsoft Academic Search

The problem of flow around a marine propeller performing a general 3D unsteady motion in an infinitely extended fluid is formulated and solved using a boundary element method. Hydrodynamic modeling of the freely moving—unsteady—trailing vortex sheet emanating from each blade is achieved, using vortex filaments and a time stepping method. Thus vortex wake–blade interaction can be taken correctly into consideration.

Gerasimos K. Politis

2004-01-01

287

Wake structure of a finite circular cylinder of small aspect ratio  

NASA Astrophysics Data System (ADS)

The wake of a finite circular cylinder of small aspect ratio was studied with a seven-hole probe and thermal anemometry. The cylinder was mounted normal to a ground plane and was partially immersed in a turbulent boundary layer. The time-averaged velocity and streamwise vorticity fields showed the development of the tip vortex structures, the extent of the near-wake recirculation zone, the downwash phenomenon and base vortex structures within the boundary layer. The wake structure and power spectra were similar for cylinder aspect ratios of 5 to 9, but a distinctly different behaviour was observed for an aspect ratio of 3.

Sumner, D.; Heseltine, J. L.; Dansereau, O. J. P.

2004-11-01

288

A vortex-lattice method for calculating lifting-surface interference  

NASA Technical Reports Server (NTRS)

The interference effect of closely coupled lifting surfaces was investigated. A nonsteady vortex-lattice method was applied to calculate the steady-state and nonsteady lift characteristics of configurations having interference effects. The configurations consist of low-aspect-ratio delta wings, at low and high angles of attack. Leading-edge separation and wake roll-up are simulated by a time-dependent wake-shedding procedure. For steady-state flow conditions, this numerical procedure saves a considerable amount of computer time, compared with iterative methods, and yields the same results. A better understanding of the interference effect can be gained by using the method to study the transient behavior. The nonsteady approach also offers the capability of calculating various nonsteady motions, as is demonstrated in the calculation of the longitudinal damping.

Levin, D.

1981-01-01

289

Axisymmetric Turbulent Wakes with New Nonequilibrium Similarity Scalings  

NASA Astrophysics Data System (ADS)

The recently discovered nonequilibrium turbulence dissipation law implies the existence of axisymmetric turbulent wake regions where the mean flow velocity deficit decays as the inverse of the distance from the wake-generating body and the wake width grows as the square root of that distance. This behavior is different from any documented boundary-free turbulent shear flow to date. Its existence is confirmed in wind tunnel experiments of wakes generated by plates with irregular edges placed normal to an incoming free stream. The wake characteristics of irregular bodies such as buildings, bridges, mountains, trees, coral reefs, and wind turbines are critical in many areas of environmental engineering and fluid mechanics.

Nedi?, J.; Vassilicos, J. C.; Ganapathisubramani, B.

2013-10-01

290

Sleep-wake differences in scaling behavior of the human heartbeat: analysis of terrestrial and long-term space flight data  

NASA Technical Reports Server (NTRS)

We compare scaling properties of the cardiac dynamics during sleep and wake periods for healthy individuals, cosmonauts during orbital flight, and subjects with severe heart disease. For all three groups, we find a greater degree of anticorrelation in the heartbeat fluctuations during sleep compared to wake periods. The sleep-wake difference in the scaling exponents for the three groups is comparable to the difference between healthy and diseased individuals. The observed scaling differences are not accounted for simply by different levels of activity, but appear related to intrinsic changes in the neuroautonomic control of the heartbeat.

Bunde, A.; Amaral, L. A.; Havlin, S.; Fritsch-Yelle, J.; Baevsky, R. M.; Stanley, H. E.; Goldberger, A. L.

1999-01-01

291

Flow visualization of the wake of a transport aircraft model with lateral-control oscillations  

NASA Technical Reports Server (NTRS)

An exploratory flow visualization study conducted in the Langley Vortex Research Facility to investigate the effectiveness of lateral control surface oscillations as a potential method for wake vortex attenuation on a 0.03 scale model of a wide body jet transport aircraft is described. Effects of both asymmetric surface oscillation (control surfaces move as with normal lateral control inputs) and symmetric surface oscillation (control surfaces move in phase) are presented. The asymmetric case simulated a flight maneuver which was previously investigated on the transport aircraft during NASA/FAA flight tests and which resulted in substantial wake vortex attenuation. Effects on the model wake vortex systems were observed by propelling the model through a two dimensional smoke screen perpendicular to the model flight path. Results are presented as photographic time histories of the wake characteristics recorded with high speed still cameras. Effects of oscillation on the wake roll up are described in some detail, and the amount of vortex attenuation observed is discussed in comparative terms. Findings were consistent with flight test results in that only a small amount of rotation was observed in the wake for the asymmetric case. A possible aerodynamic mechanism contributing to this attenuation is suggested.

Jordan, F. L., Jr.

1983-01-01

292

Near wake structure for a generic ASTV configuration  

NASA Technical Reports Server (NTRS)

Results of a numerical study are presented for hypersonic low-density flow about a 70-deg blunt cone using the direct simulation Monte Carlo method. Particular emphasis is given to the near wake flow and its sensitivity to rarefaction and other parametric variations. The flow conditions simulated are attainable in existing low-density hypersonic wind tunnels; that is, Mach 20 nitrogen flow encompassing freestream Knudsen numbers of 0.03 to 0.001. A stable vortex forms in the near wake at and below a freestream Knudsen number of 0.01 and the size of the vortex increases with decreasing freestream Knudsen number. The base region of the flow remains in thermal nonequilibrium for all cases. There is no formation of a lip separation shock or a distinct wake shock at these rarefied conditions.

Dogra, Virendra K.; Moss, James N.; Price, Joseph M.

1993-01-01

293

The Unsteady Wake Generated by a Rotor in Ground Effect  

NASA Astrophysics Data System (ADS)

Helicopters flying close to the ground encounter many handling qualities problems. Large unsteady forces near the main and tail rotors are believed to be a major cause of these problems. We investigate the structure of the unsteady wake generated by multibladed rotors in ground effect. A vortex lattice method with a free wake model is used to simulate the tip-vortex and the method of images is used to model the ground and the blades are modeled using the lifting surface method. The rotor wake is advanced in time until periodicity is attained. Unsteadiness in the form of instantaneous velocities is studied at different planes on the advancing and retreating sides of the tail rotor to identify the plane with the maximum unsteadiness. The root-mean-square of the deviation of the instantaneous velocity from the time averaged velocity over a given period is used to quantify the unsteadiness

Pulla, Devi

2005-11-01

294

Identifying optimal vortex spacing for swimming and flying animals  

NASA Astrophysics Data System (ADS)

Swimming and flying animals generate thrust by creating an unsteady vortex wake through the oscillation of their appendages. To determine the vortex spacing that maximizes propulsive efficiency, a finite core vortex array model was developed to compute the unsteady velocity field generated by vortex streets representative of bio- inspired propulsion. The model systematically varies the streamwise and transverse spacing between vortex cores to determine the time averaged velocity field induced by a reverse von Karman vortex street and a uniform freestream velocity. Experimental particle image velocimetry was conducted in the wake of a rigid pitching panel to determine the size and strength of the vortex cores to input to the model. Viscosity is accounted for by assuming a Gaussian vorticity distribution around the vortex core. A linear spatial stability analysis was performed on the computed velocity profiles to determine which vortex configuration leads to efficient propulsion. Here it is assumed that efficient propulsion proceeds when the driving frequency of the vortex street matches the resonant frequency of velocity jet.

Dewey, Peter A.; Moored, Keith W.; Quinn, Daniel B.; Smits, Alexander J.

2011-11-01

295

Wake Oscillation of Column Wall Jet in Uniform Flow  

NASA Astrophysics Data System (ADS)

Both experiment and calculation demonstrated to clarify the effect of a column wall jet in uniform fluid flow on the characteristic of wake oscillation. The vortex intensity decreased and the oscillations attenuated when the jet direction matched the uniform flow. When the jet flow was reversed, the vortex intensity grew and the oscillations increased in magnitude. It has been found that the Strouhal number based on the half width of the flow velocity distribution was nearly constant. Also, the frequency depended on the vortex structure of the wake, which was further dependent on the jet flow velocity. In addition, the situations that gave twin peaks in the oscillation spectrum were found both in experiment and in calculation.

Yoshida, Yohei; Sato, Kotaro; Ono, Yoichi

296

A fluctuating state and the critical behavior of the depinning transition in driven vortex matter in superconductors  

NASA Astrophysics Data System (ADS)

A status report is presented on explorations in 2H-NbS2 which have revealed a new protocol that generates vortex states corresponding to two limiting ends of critical current density (Jlc(H) and Jhc (H)) for a given vortex density deep in the mixed state. An ordered vortex array set in motion with transport current I (Ilc < I < Ihc) explores its other option of non-moving jammed state of higher end Ihc while steadily enhancing the driving force. Attempts to further unjam the re-pinned state unmask (i) a moving fluctuating state and (ii) the diverging (life) times along with an underlying critical behaviour at the threshold limit of de-pinning the disordered-jammed state, from both below as well as above Ihc.

Shaw, Gorky; Mandal, Pabitra; Banerjee, S. S.; Niazi, A.; Rastogi, A. K.; Sood, A. K.; Tulapurkar, A.; Ramakrishnan, S.; Grover, A. K.

2012-06-01

297

Numerical investigations on the wake structures of micro-ramp and micro-vanes  

NASA Astrophysics Data System (ADS)

Based on large eddy simulation, combined with the high-order WENO (weighted essentially non-oscillatory schemes) scheme, immersed boundary method and adaptive mesh refinement technique, the supersonic flow past a wall-mounted micro-ramp and two micro-vanes have been simulated. The different wake structures are presented and discussed. Our numerical results showed that wake structures behind the micro-ramp are more complicated, including ring-like vortex train, and streamwise vortex tubes etc. However, the wake structures of the micro-vanes are quite simple; they are mainly the two counter-rotating streamwise vortex tubes. The control of boundary flow of both is achieved through the energy exchange between the main stream and the boundary layer and is presented mainly by the upwash and downwash motion of gases under the entrainment of vortex tubes.

DaWen, Xue; ZhiHua, Chen; XiaoHai, Jiang; BaoChun, Fan

2014-02-01

298

Interaction of Aircraft Wakes From Laterally Spaced Aircraft  

NASA Technical Reports Server (NTRS)

Large Eddy Simulations are used to examine wake interactions from aircraft on closely spaced parallel paths. Two sets of experiments are conducted, with the first set examining wake interactions out of ground effect (OGE) and the second set for in ground effect (IGE). The initial wake field for each aircraft represents a rolled-up wake vortex pair generated by a B-747. Parametric sets include wake interactions from aircraft pairs with lateral separations of 400, 500, 600, and 750 ft. The simulation of a wake from a single aircraft is used as baseline. The study shows that wake vortices from either a pair or a formation of B-747 s that fly with very close lateral spacing, last longer than those from an isolated B-747. For OGE, the inner vortices between the pair of aircraft, ascend, link and quickly dissipate, leaving the outer vortices to decay and descend slowly. For the IGE scenario, the inner vortices ascend and last longer, while the outer vortices decay from ground interaction at a rate similar to that expected from an isolated aircraft. Both OGE and IGE scenarios produce longer-lasting wakes for aircraft with separations less than 600 ft. The results are significant because concepts to increase airport capacity have been proposed that assume either aircraft formations and/or aircraft pairs landing on very closely spaced runways.

Proctor, Fred H.

2009-01-01

299

Panel method for the wake effects on the aerodynamics of vertical-axis wind turbines  

NASA Astrophysics Data System (ADS)

A formulation based on the panel method is implemented for studying the unsteady aerodynamics of straight-bladed vertical-axis wind turbines. A combination of source and vortex distributions is used to represent an airfoil in Darrieus type motion. Our approach represents a low-cost computational technique that takes into account the dynamic changes in angle of attack of the blade during a cycle. A time-stepping mechanism is introduced for the wake convection, and its effects on the aerodynamic forces on the blade are discussed. The focus of the study is to describe the effect of the trailing wakes on the upstream flow conditions and coefficient of performance of the turbines. Results show a decrease in Cp until the wake structure develops and assumes a quasi-steady behavior. A comparison with other models such as single and multiple streamtubes is discussed, and optimization of the blade pitch angle is performed to increase the instantaneous torque and hence the power output from the turbine.

Goyal, Udit; Rempfer, Dietmar

2011-11-01

300

The structure of vortex breakdown  

NASA Technical Reports Server (NTRS)

The term 'vortex breakdown', as used in the reported investigation, refers to a disturbance characterized by the formation of an internal stagnation point on the vortex axis, followed by reversed flow in a region of limited axial extent. Two forms of vortex breakdown, which predominate, are shown in photographs. One form is called 'near-axisymmetric' (sometimes 'axisymmetric'), and the other is called 'spiral'. A survey is presented of work published since the 1972 review by Hall. Most experimental data taken since Hall's review have been in tubes, and the survey deals primarily with such cases. It is found that the assumption of axial-symmetry has produced useful results. The classification of flows as supercritical or subcritical, a step that assumes symmetry, has proved universally useful. Experiments show that vortex breakdown is always preceded by an upstream supercritical flow and followed by a subcritical wake. However, a comparison between experiments and attempts at prediction is less than encouraging. For a satisfactory understanding of the structure of vortex breakdown it is apparently necessary to take into account also aspects of asymmetry.

Leibovich, S.

1978-01-01

301

Toward Understanding Wake Vortices and Atmospheric Turbulence Interactions Using Large-Eddy Simulation  

NASA Technical Reports Server (NTRS)

The vortices produced by an aircraft in flight are a complex phenomena created from a 'sheet of vorticity' leaving the trailing edge of the aircraft surfaces. This sheet tends to roll-up into two counter-rotating vortices. After a few spans downstream of the aircraft, the roll-up process is complete and the vortex pair may be characterized in a simple manner for modeling purposes. Our research will focus on what happens to these post roll-up vortices in the vicinity of an airport terminal. As the aircraft wake vortices descend, they are transported by the air mass which they are embedded and are decayed by both internal and external processes. In the vicinity of the airport, these external influences are usually due to planetary boundary layer (PBL) turbulence. Using large-eddy simulation (LES), one may simulate a variety of PBL conditions. In the LES method, turbulence is generated in the PBL as a response to surface heat flux, horizontal pressure gradient, wind shear, and/or stratification, and may produce convective or unstably stratified, neutral, or stably stratified PBL's. Each of these PBL types can occur during a typical diurnal cycle of the PBL. Thus it is important to be able to characterize these conditions with the LES method. Once this turbulent environment has been generated, a vortex pair will be introduced and the interactions are observed. The objective is to be able to quantify the PBL turbulence vortex interaction and be able to draw some conclusions of vortex behavior from the various scale interactions. This research is ongoing, and we will focus on what has been accomplished to date and the future direction of this research. We will discuss the model being used, show results that validate its use in the PBL, and present a nested-grid method proposed to analyze the entire PBL and vortex pair simultaneously.

DeCroix, David; Lin, Yuh-Lang; Arya, S. Pal; Kao, C.-T.; Shen, S.

1997-01-01

302

Application of Three-Component PIV to a Hovering Rotor Wake  

NASA Technical Reports Server (NTRS)

The key to accurate predictions of rotorcraft aerodynamics, acoustics, and dynamics lies in the accurate representation of the rotor wake. The vortical wake computed by rotorcraft CFD analyses typically suffer from numerical dissipation before the first blade passage. With some a priori knowledge of the wake trajectory, grid points can be concentrated along the trajectory to minimize the dissipation. Comprehensive rotorcraft analyses based on lifting-line theory rely on classical vortex models and/or semi-empirical information about the tip vortex structure. Until the location, size, and strength of the trailed tip vortex can be measured over a range of wake ages, the analyses will continue to be adjusted on a trial and error basis in order to correctly predict blade airloads, acoustics, dynamics, and performance. Using the laser light sheet technique, tip vortex location can be acquired in a straightforward manner. Measuring wake velocities and vortex core size, however, has been difficult and tedious using point-measurement techniques such as laser velocimetry. Recently, the Particle Image Velocimetry (PIV) technique has proven to be an efficient method for acquiring velocity measurements over relatively large areas and volumes of a rotor wake. The work reported to date, however, has been restricted to 2-component velocity measurements of the rotor wake. Three-component velocity measurements of a hovering rotor wake were acquired at NASA Ames Research Center in May 1999. This experiment represents a major step toward understanding the detailed structure of a rotor wake. This paper will focus primarily on the experimental technique used in acquiring this data. The accuracy and limitations of the current technique will also be discussed. Representative velocity field measurements will be included.

Yamauchi, Gloria K.; Lourenco, Luiz; Heineck, James T.; Wadcock, Alan J.; Abrego, Anita I.; Aiken, Edwin W. (Technical Monitor)

2000-01-01

303

Aerodynamic flow vectoring of a wake using asymmetric synthetic jet actuation  

NASA Astrophysics Data System (ADS)

This paper reports an experimental investigation on the wake of a blunt-based, flat plate subjected to aerodynamic flow vectoring using asymmetric synthetic jet actuation. Wake vectoring was achieved using a synthetic jet placed at the model base 2.5 mm from the upper corner. The wake Reynolds number based on the plate thickness was 7,200. The synthetic jet actuation frequency was selected to be about 75 % the vortex shedding frequency of the natural wake. At this actuation frequency, the synthetic jet delivered a periodic flow with a momentum coefficient, C ?, of up to 62 %. Simultaneous measurements of the streamwise and transverse components of the velocity were performed using particle image velocimetry (PIV) in the near wake. The results suggested that for significant wake vectoring, vortex shedding must be suppressed first. Under the flow conditions cited above, C ? values in the range of 10-20 % were required. The wake vectoring angle seemed to asymptote to a constant value of about 30° at downstream distances, x/ h, larger than 4 for C ? values ranging between 24 and 64 %. The phase-averaged vorticity contours and the phase-averaged normal lift force showed that most of the wake vectoring is produced during the suction phase of the actuation, while the blowing phase was mostly responsible for vortex shedding suppression.

Ben Chiekh, Maher; Ferchichi, Mohsen; Béra, Jean-Christophe

2012-12-01

304

Building wake diffusion  

SciTech Connect

A recent review of building-wake diffusion models for the Nuclear Regulatory Commission (NRC) compares model predictions of centerline concentrations in building wakes with concentrations observed in experiments at seven nuclear reactors. In general, the model predictions are conservative in that they tend to predict concentrations that are greater than those actually observed. However, the models show little skill in accounting for variations in the observed concentrations. Analysis of experimental data indicates that the general form of the standard wake diffusion models is inconsistent with observed variation of concentrations in the wakes. The inconsistency is especially marked for ground-level releases. As an interim measure, multiple linear regression techniques have been used to develop a statistical building wake model alternative to the current models. This paper describes the statistical wake model and compares it with other models. 11 refs., 4 figs.

Ramsdell, J.V.

1989-01-01

305

The Reykjavik wake  

NASA Astrophysics Data System (ADS)

We present preliminary results from the wake flights in the MOSO 2 campaign that took place in Iceland, autumn 2011. The results include RPAS measurements (soundings) of an orographic wake in southwestern Iceland during a northerly wind storm. The results reveal a wake structure dominated by the local as well as the larger scale topography. The RPAS dataset is augmented with measurements from a network of automatic weather stations and simulations from a numerical weather model.

Jonassen, Marius O.; Ólafsson, Haraldur; Rögnvaldsson, Ólafur; Ágústsson, Hálfdán

2013-04-01

306

Mesoscale Simulation Data for Initializing Fast-Time Wake Transport and Decay Models  

NASA Technical Reports Server (NTRS)

The fast-time wake transport and decay models require vertical profiles of crosswinds, potential temperature and the eddy dissipation rate as initial conditions. These inputs are normally obtained from various field sensors. In case of data-denied scenarios or operational use, these initial conditions can be provided by mesoscale model simulations. In this study, the vertical profiles of potential temperature from a mesoscale model were used as initial conditions for the fast-time wake models. The mesoscale model simulations were compared against available observations and the wake model predictions were compared with the Lidar measurements from three wake vortex field experiments.

Ahmad, Nashat N.; Proctor, Fred H.; Vanvalkenburg, Randal L.; Pruis, Mathew J.; LimonDuparcmeur, Fanny M.

2012-01-01

307

Standard Test Case for a Low Speed, Turbulent Junction Vortex Flow.  

National Technical Information Service (NTIS)

The mean flow structure upstream, around, within and in the near wake of a turbulent junction or horseshoe vortex is reported for an incompressible, subsonic flow. Measurements of the primitive variables of velocity and pressure are reported on all surfac...

F. J. Pierce M. D. Harsh C. M. Kim S. K. Nath J. Shin

1988-01-01

308

Arctic Vortex  

... about 650 kilometers northeast of Iceland in the north Atlantic Ocean. Jan Mayen's Beerenberg volcano rises about 2.2 kilometers above ... Arctic Vortex location:  Arctic Ocean Atlantic Ocean thumbnail:  ...

2013-06-26

309

Vortex-Based Aero- and Hydrodynamic Estimation  

NASA Astrophysics Data System (ADS)

Flow control strategies often require knowledge of unmeasurable quantities, thus presenting a need to reconstruct flow states from measurable ones. In this thesis, the modeling, simulation, and estimator design aspects of flow reconstruction are considered. First, a vortex-based aero- and hydrodynamic estimation paradigm is developed to design a wake sensing algorithm for aircraft formation flight missions. The method assimilates wing distributed pressure measurements with a vortex-based wake model to better predict the state of the flow. The study compares Kalman-type algorithms with particle filtering algorithms, demonstrating that the vortex nonlinearities require particle filters to yield adequate performance. Furthermore, the observability structure of the wake is shown to have a negative impact on filter performance regardless of the algorithm applied. It is demonstrated that relative motions can alleviate the filter divergence issues associated with this observability structure. In addition to estimator development, the dissertation addresses the need for an efficient unsteady multi-body aerodynamics testbed for estimator and controller validation studies. A pure vortex particle implementation of a vortex panel-particle method is developed to satisfy this need. The numerical method is demonstrated on the impulsive startup of a flat plate as well as the impulsive startup of a multi-wing formation. It is clear, from these validation studies, that the method is able to accommodate the unsteady wake effects that arise in formation flight missions. Lastly, successful vortex-based estimation is highly dependent on the reliability of the low-order vortex model used in representing the flow of interest. The present treatise establishes a systematic framework for vortex model improvement, grounded in optimal control theory and the calculus of variations. By minimizing model predicted errors with respect to empirical data, the shortcomings of the baseline vortex model can be revealed and reconciled. Here, the method is demonstrated on an impulse matching model for canonical unsteady wing maneuvers and reveals the shortcomings of the Kutta condition in such flows. The resulting analysis sheds light on the governing physical processes and provides guidance for model improvement for the unsteady aerodynamics associated with these canonical wing maneuvers.

Hemati, Maziar Sam

310

Doppler radar detection of vortex hazard indicators  

NASA Technical Reports Server (NTRS)

Wake vortex experiments were conducted at White Sands Missile Range, NM using the AN/MPS-39 Multiple Object Tracking Radar (MOTR). The purpose of these experiments was twofold. The first objective was to verify that radar returns from wake vortex are observed for some time after the passage of an aircraft. The second objective was to verify that other vortex hazard indicators such as ambient wind speed and direction could also be detected. The present study addresses the Doppler characteristics of wake vortex and clear air returns based upon measurements employing MOTR, a very sensitive C-Band phased array radar. In this regard, the experiment was conducted so that the spectral characteristics could be determined on a dwell to-dwell basis. Results are presented from measurements of the backscattered power (equivalent structure constant), radial velocity and spectral width when the aircraft flies transverse and axial to the radar beam. The statistics of the backscattered power and spectral width for each case are given. In addition, the scan strategy, experimental test procedure and radar parameters are presented.

Nespor, Jerald D.; Hudson, B.; Stegall, R. L.; Freedman, Jerome E.

1994-01-01

311

New scanning technique for the optical vortex microscope.  

PubMed

In the optical vortex microscopy the focused Gaussian beam with optical vortex scans a sample. An optical vortex can be introduced into a laser beam with the use of a special optical element--a vortex lens. When moving the vortex lens, the optical vortex changes its position inside the spot formed by a focused laser beam. This effect can be used as a new precise scanning technique. In this paper, we study the optical vortex behavior at the sample plane. We also estimate if the new scanning technique results in observable effects that could be used for a phase object detection. PMID:22505087

Augustyniak, Ireneusz; Popio?ek-Masajada, Agnieszka; Masajada, Jan; Drobczy?ski, S?awomir

2012-04-01

312

Preliminary study of a wing-tip vortex using laser velocimetry  

NASA Technical Reports Server (NTRS)

Measurements have been made in the wake of a semi-span NACA 0015 airfoil with emphasis on the region of the wing tip vortex. The spanwise and streamwise velocity components were measured using a two-component laser Doppler velocimeter. The purpose of the study was to initiate the operation of a laser velocimeter system and to perform preliminary wake measurements in preparation for a more extensive study of the structure and near field development of a tip vortex.

Takahashi, R. K.; Mcalister, K. W.

1987-01-01

313

The effect of flow perturbations on the near wake characteristics of a circular cylinder  

NASA Astrophysics Data System (ADS)

The mean and fluctuating velocity fields in the near wake of a circular cylinder subjected to an incident mean flow with periodic velocity perturbations superimposed upon it were examined using laser Doppler anemometry. From these measurements the wake was characterized in terms of the recirculation bubble length, vortex formation length, maximum intensity of the velocity fluctuations and the wavelength of the vortex street. The well-known `lock-on' phenomenon was observed for perturbation frequencies around two times the natural vortex shedding frequency. It is shown that the wake structure is modified in a systematic manner within the lock-on range. The forced wake shares many basic characteristics as that of a cylinder oscillating either transversely or in line, relative to the flow direction. These include the shortening of the recirculation bubble and the vortex formation region as well as the variation of the longitudinal vortex spacing with perturbation frequency. Differences but also similarities between forced wakes at low (less than 350) and relatively higher Reynolds numbers (greater than 350) are discussed.

Konstantinidis, E.; Balabani, S.; Yianneskis, M.

2003-09-01

314

Intelligent and mass vortex flowmeters  

SciTech Connect

In nature, Karman vortices are quite common. For instance, they happen when an airstream flows past a mountain, house, pole, tower, or skyscraper, or, more simply, when it blows among branches of a tree. The typical spiral shape of these swirls is invisible because there is no tracing element, such as the clouds in the satellite photo. Also, the observation point is rarely above or below the plane of these classic spiral shapes. Or you can watch the alternating whirlpool train that a river or stream makes behind bridge piers. Regular Karman vortices form downstream of a bluff body along two distinct wakes: the vortices of one wake rotate clockwise, those of the other rotate counterclockwise. Close to the bluff body, the wake distance is always constant and depends on bluff body shape and dimensions. The distance between two adjacent vortices is also constant and independent of fluid parameters such as velocity, pressure, density, and temperature. Vortices interact with their surrounding space by stimulating or choking every other nearby swirl on the verge of birth and development. Two Karman vortices cannot be generated simultaneously, but only one at a time, alternately on the left and right side of the bluff body. The process works just like a fluidic flip-flop. This natural phenomenon can be created artificially by placing a trapezoidal, or similarly symmetrical, bar across the diameter of a pipe section. Parallelism of the internal walls of the pipe and the corners of the trapezoidal bar ensure stability of the separation point of the boundary layer. Consequently, the separation point of each vortex with respect to the bar remains stable and the vortex train is regular. If the fluid speed doubles, creation of swirls doubles while the small volume encompassed by each vortex remains constant. So, by counting the number of swirls passing a fixed point during a defined time interval, one can compute the total passed fluid volume. 3 figs.

Ribolini, E. [Yokagawa Italia, Milan (Italy)

1996-02-01

315

Cosmic string wakes  

NASA Technical Reports Server (NTRS)

Accretion of matter onto wakes left behind by horizon-sized pieces of cosmic string is investigated, and the effects of wakes on the large-scale structure of the universe are determined. Accretion of cold matter onto wakes, the effects of a long string on fluids with finite velocity dispersion or sound speeds, the interactions between loops and wakes, and the conditions for wakes to survive disruption by loops are discussed. It is concluded that the most important wakes are those which were formed at the time of equal matter and radiation density. This leads to sheetlike overdense regions of galaxies with a mean separation in agreement with the scale of the bubbles of de Lapparent, Geller, and Huchra (1986). However, for the value of G(mu) favored from galaxy formation considerations in a universe with cold dark matter, a wake accretes matter from a distance of only about 1.5 Mpc, which is much less than the distance between the wakes.

Stebbins, Albert; Veeraraghavan, Shoba; Silk, Joseph; Brandenberger, Robert; Turok, Neil

1987-01-01

316

Stabilization of vortices in the wake of a circular cylinder using harmonic forcing.  

PubMed

We explore whether vortex flows in the wake of a fixed circular cylinder can be stabilized using harmonic forcing. We use Föppl's point vortex model augmented with a harmonic point source-sink mechanism which preserves conservation of mass and leaves the system Hamiltonian. We discover a region of Lyapunov-stable vortex motion for an appropriate selection of parameters. We identify four unique parameters that affect the stability of the vortices: the uniform flow velocity, vortex equilibrium positions, forcing amplitude, and forcing frequency. We assess the robustness of the controller using a Poincaré section. PMID:21797478

Chamoun, Georges C; Schilder, Frank; Brøns, Morten

2011-06-01

317

Vortical structures in the wake of an undulating fin  

NASA Astrophysics Data System (ADS)

Batoid fish such as the manta ray propel themselves through the water by producing a traveling wave motion along the chord of their pectoral fin. Such a motion produces thrust through the development of an unsteady vortex street that results in a jet-like average flow. Digital particle image velocimetry (DPIV) is used to characterize the vortical patterns and structures developed in the wake of a manta ray-like fin. A DC servo motor powers a gear train to produce the traveling wave motion; whose frequency and wave length can be varied. The amplitude of the traveling wave motion linearly increases along the span of the fin. Wake morphologies for a wide spectrum of oscillation frequencies and traveling wave wavelengths are identified. A bifurcation from a 2S wake structure to a 2P wake structure is observed as the traveling wave wavelength is decreased, which corresponds to a decrease in efficiency as reported by Clark and Smits (2006). Alteration of the oscillation frequency, and thus Strouhal number, affects vortex interaction and is found to significantly modify the resulting velocity profiles in the wake of the fin. Notably, increasing the Strouhal number beyond optimal conditions, reported by Clark and Smits, corresponds to a reduction in the extent that the jet-like average flow is observed downstream of the fin.

Dewey, Peter A.; Carriou, Antoine; Smits, Alexander J.

2010-11-01

318

The Interaction Vortex Flow Around Two Bluff Cylinders  

NASA Astrophysics Data System (ADS)

In this study, the interaction vortex flow features around a pair of parallel arranged bluff cylinders were observed by visualizing water flow experiment at the range of the gap ratio G/d=0~3. It was obtained that the result of established wind tunnel test and the result of this water tank test agreed about the characteristics of vortex shedding when varying the distance of circular cylinder gap. The flow pattern and vortex shedding frequency of another type bluff cylinder (triangular and square cylinder) were also investigated. As a result of the experiment, it was shown that the flow pattern of wake flow was divided into three kinds (coupled vortex streets, biased gap flow and single vortex street) regardless of the cylinder section shape and cylinder size. Then, the region of the appearance of flow pattern was shown about each case. In the case where two each other independent vortex streets were formed, three typical flow patterns of vortex formation (in-phase coupled vortex streets, out-of-phase coupled vortex streets and complication coupled vortex streets) were observed. It was known that three configuration of vortex formation appear intermittently and alternatively.

Yokoi, Y.; Hirao, K.

2013-04-01

319

Quantum analogues of classical wakes in Bose–Einstein condensates  

NASA Astrophysics Data System (ADS)

We show that an elliptical obstacle moving through a Bose–Einstein condensate generates wakes of quantum vortices which resemble those of classical viscous flow past a cylinder or sphere. The role of ellipticity is to facilitate the interaction of the vortices nucleated by the obstacle. Initial steady symmetric wakes lose their symmetry and form clusters of like-signed vortices, in analogy to the classical Bénard–von Kármán vortex street. Our findings, demonstrated numerically in both two and three dimensions, confirm the intuition that a sufficiently large number of quanta of circulation reproduce classical physics.

Stagg, G. W.; Parker, N. G.; Barenghi, C. F.

2014-05-01

320

Manipulating thrust wakes: A parallel with biomimetic propulsion  

NASA Astrophysics Data System (ADS)

We present an experiment to investigate the role of shed vortices in thrust production. Using two different mechanisms, harmonic acoustic forcing and vortex trapping, a dry-air jet was manipulated to give the wake a typical propulsive-like pattern as observed behind swimmers or flyers. Our results show that even in a thrust production configuration, wakes with asymmetric roll-up of vortices are always associated to performance loss. By contrast, cases involving symmetric modes show thrust enhancement, as reported in very recent studies on pulsed jet propulsion.

Raspa, V.; Gaubert, C.; Thiria, B.

2012-02-01

321

Wake structure and wing motion in bat flight  

NASA Astrophysics Data System (ADS)

We report on experiments concerning the wake structure and kinematics of bat flight, conducted in a low-speed wind tunnel using time-resolved PIV (200Hz) and 4 high-speed cameras to capture wake and wing motion simultaneously. 16 Lesser dog-faced fruit bats (C. brachyotis) were trained to fly in the wind tunnel at 3-6.5m/s. The PIV recordings perpendicular to the flow stream allowed observing the development of the tip vortex and circulation over the wing beat cycle. Each PIV acquisition sequence is correlated with the respective kinematic history. Circulation within wing beat cycles were often quite repeatable, however variations due to maneuvering of the bat are clearly visible. While no distinct vortex structure was observed at the upper reversal point (defined according the vertical motion of the wrist) a tip vortex was observed to develop in the first third of the downstroke, growing in strength, and persisting during much of the upstroke. Correlated to the presence of a strong tip vortex the circulation has almost constant strength over the middle half of the wing beat. At relatively low flight speeds (3.4 m/s), a closed vortex structure behind the bat is postulated.

Hubel, Tatjana; Breuer, Kenneth; Swartz, Sharon

2008-11-01

322

Functional anatomy of the sleep-wakefulness cycle: wakefulness.  

PubMed

Sleep is a necessary, diverse, periodic, and an active condition circadian and homeostatically regulated and precisely meshed with waking time into the sleep-wakefulness cycle (SWC). Photic retinal stimulation modulates the suprachiasmatic nucleus, which acts as the pacemaker for SWC rhythmicity. Both the light period and social cues adjust the internal clock, making the SWC a circadian, 24-h period in the adult human. Bioelectrical and behavioral parameters characterize the different phases of the SWC. For a long time, lesions and electrical stimulation of brain structures, as well as connection studies, were the main methods used to decipher the foundations of the functional anatomy of the SWC. That is why the first section of this review presents these early historical studies to then discuss the current state of our knowledge based on our understanding of the functional anatomy of the structures underlying the SWC. Supported by this description, we then present a detailed review and update of the structures involved in the phase of wakefulness (W), including their morphological, functional, and chemical characteristics, as well as their anatomical connections. The structures for W generation are known as the "ascending reticular activating system", and they keep and maintain the "thalamo-cerebral cortex unit" awake. This system originates from the neuronal groups located within the brainstem, hypothalamus, and basal forebrain, which use known neurotransmitters and whose neurons are more active during W than during the other SWC states. Thus, synergies among several of these neurotransmitters are necessary to generate the cortical and thalamic activation that is characteristic of the W state, with all the plastic qualities and nuances present in its different behavioral circumstances. Each one of the neurotransmitters exerts powerful influences on the information and cognitive processes as well as attentional, emotional, motivational, behavioral, and arousal states. The awake "thalamo-cerebral cortex unit" controls and adjusts the activation pattern through a top-down action on the subcortical cellular groups that are the origin of the "ascending reticular activating system". PMID:21166301

Reinoso-Suárez, Fernando; de Andrés, Isabel; Garzón, Miguel

2011-01-01

323

Wakes in viscous quark-gluon plasma  

NASA Astrophysics Data System (ADS)

With the dielectric function derived from the chromohydrodynamic approach, we investigate wakes in induced charge density and wake potential induced by a fast parton traveling through the viscous quark-gluon plasma (QGP). When the fast parton moves with a speed v = 0.55c which is less than the phase velocity of plasmon vp in QGP, the equicharge lines show a sign flip in the backward-forward spaces. While for v = 0.99c which is larger than vp, the equicharge lines show an oscillatory behavior. A Lennard-Jones potential and an oscillatory wake potential are found in the backward direction for v = 0.55c and v = 0.99c respectively. In addition, the viscous effect on wakes is also speed-dependent. When v = 0.55c, shear viscosity has a trivial impact on the wakes. While for v = 0.99c, shear viscosity modifies the strength and structure of the wakes significantly.

Jiang, Bing-Feng; Hou, De-Fu; Li, Jia-Rong

2014-04-01

324

Approaches to Measuring the Effects of Wake-Promoting Drugs: A Focus on Cognitive Function  

PubMed Central

Objectives In clinical drug development, wakefulness and wake-promotion maybe assessed by a large number of scales and questionnaires. Objective assessment of wakefulness is most commonly made using sleep latency/maintenance of wakefulness tests, polysomnography and/or behavioral measures. The purpose of the present review is to highlight the degree of overlap in the assessment of wakefulness and cognition, with consideration of assessment techniques and the underlying neurobiology of both concepts. Design Reviews of four key areas were conducted: commonly used techniques in the assessment of wakefulness; neurobiology of sleep/wake and cognition; targets of wake promoting and/or cognition enhancing drugs; and ongoing clinical trials investigating wake promoting effects. Results There is clear overlap between the assessment of wakefulness and cognition. There are common techniques which may be used to assess both concepts; aspects of the neurobiology of both concepts may be closely related; and wake promoting drugs may have nootropic properties (and vice-versa). Clinical trials of wake promoting drugs often, though not routinely, assess aspects of cognition. Conclusions Routine and broad assessment of cognition in the development of wake promoting drugs may reveal important nootropic effects, which are not secondary to alertness/wakefulness, whilst existing cognitive enhancers may have under explored or unknown wake promoting properties.

Edgar, Christopher J.; Pace-Schott, Edward F.; Wesnes, Keith A.

2009-01-01

325

Effect of the secondary flow on Karman vortex shedding from a yawed cylinder  

Microsoft Academic Search

Karman vortex shedding from a circular cylinder with a yaw angle in a uniform flow was investigated using a wind tunnel. The yaw angle made the vortex shedding irregular and its frequency low. In addition, an intense secondary flow along the cylinder generatrix in the wake just behind the cylinder was induced by the yawing. When a pair of parallel

M. Shirakashi; S. Wakiya; A. Hasegawa

1986-01-01

326

Vortex shedding from a circular cylinder near a moving ground  

NASA Astrophysics Data System (ADS)

The flow and force characteristics have been experimentally investigated of a circular cylinder with an aspect ratio of 8.33, with and without end-plates, placed near and parallel to a moving ground, on which substantially no boundary layer developed to interfere with the cylinder. Mean drag and lift measurements, surface oil flow visualization, and particle image velocimetry (PIV) measurements were carried out at two upper-subcritical Reynolds numbers of 0.4 and 1.0×105 (based on the cylinder diameter d) to investigate the mechanisms of the ground effect, i.e., the effect of the gap-to-diameter ratio h/d, where h is the gap between the cylinder and the ground. For the cylinder with end-plates, on which the oil flow patterns were observed to be essentially two-dimensional, the drag rapidly decreased as h/d decreased to less than 0.5 but became constant for h/d of less than 0.35, unlike that usually observed near a fixed ground. This critical drag behavior was found to be directly related to a global change in the near wake structure of the cylinder; as h/d decreased the Kármán-type vortex shedding became intermittent at h/d=0.4, and then totally ceased and instead two nearly parallel shear layers were formed behind the cylinder at h/d=0.3 and below. For the cylinder without end-plates, however, no such critical change in drag was observed as the Kármán-type vortices were not generated in the near wake region at all h/d investigated. Based on the experimental results obtained, further discussions are also given to the essential cause of the cessation of the Kármán vortex shedding in the ground effect.

Nishino, Takafumi; Roberts, Graham T.; Zhang, Xin

2007-02-01

327

Numerical Simulation of Wake Vortices Measured During the Idaho Falls and Memphis Field Programs  

NASA Technical Reports Server (NTRS)

A numerical large-eddy simulation model is under modification and testing for application to aircraft wake vortices. The model, having a meteorological framework, permits the interaction of wake vortices with environments characterized by crosswind shear, stratification, and humidity. As part of the validation process, model results are compared with measured field data from the 1990 Idaho Falls and the 1994-1995 Memphis field experiments. Cases are selected that represent different aircraft and a cross section of meteorological environments. Also included is one case with wake vortex generation in ground effect. The model simulations are initialized with the appropriate meteorological conditions and a post roll-up vortex system. No ambient turbulence is assumed in our initial set of experiments, although turbulence can be self generated by the interaction of the model wakes with the ground and environment.

Proctor, Fred H.

1996-01-01

328

Flame-vortex interaction and mixing behaviors of turbulent non-premixed jet flames under acoustic forcing  

SciTech Connect

This study examines the effect of acoustic excitation using forced coaxial air on the flame characteristics of turbulent hydrogen non-premixed flames. A resonance frequency was selected to acoustically excite the coaxial air jet due to its ability to effectively amplify the acoustic amplitude and reduce flame length and NO{sub x} emissions. Acoustic excitation causes the flame length to decrease by 15% and consequently, a 25% reduction in EINO{sub x} is achieved, compared to coaxial air flames without acoustic excitation at the same coaxial air to fuel velocity ratio. Moreover, acoustic excitation induces periodical fluctuation of the coaxial air velocity, thus resulting in slight fluctuation of the fuel velocity. From phase-lock PIV and OH PLIF measurement, the local flow properties at the flame surface were investigated under acoustic forcing. During flame-vortex interaction in the near field region, the entrainment velocity and the flame surface area increased locally near the vortex. This increase in flame surface area and entrainment velocity is believed to be a crucial factor in reducing flame length and NO{sub x} emission in coaxial jet flames with acoustic excitation. Local flame extinction occurred frequently when subjected to an excessive strain rate, indicating that intense mass transfer of fuel and air occurs radially inward at the flame surface. (author)

Kim, Munki; Choi, Youngil; Oh, Jeongseog; Yoon, Youngbin [School of Mechanical and Aerospace Engineering, Seoul National University, Seoul (Korea)

2009-12-15

329

Supersonic shock wave/vortex interaction  

NASA Technical Reports Server (NTRS)

Although shock wave/vortex interaction is a basic and important fluid dynamics problem, very little research has been conducted on this topic. Therefore, a detailed experimental study of the interaction between a supersonic streamwise turbulent vortex and a shock wave was carried out at the Penn State Gas Dynamics Laboratory. A vortex is produced by replaceable swirl vanes located upstream of the throat of various converging-diverging nozzles. The supersonic vortex is then injected into either a coflowing supersonic stream or ambient air. The structure of the isolated vortex is investigated in a supersonic wind tunnel using miniature, fast-response, five-hole and total temperature probes and in a free jet using laser Doppler velocimetry. The cases tested have unit Reynolds numbers in excess of 25 million per meter, axial Mach numbers ranging from 2.5 to 4.0, and peak tangential Mach numbers from 0 (i.e., a pure jet) to about 0.7. The results show that the typical supersonic wake-like vortex consists of a non-isentropic, rotational core, where the reduced circulation distribution is self similar, and an outer isentropic, irrotational region. The vortex core is also a region of significant turbulent fluctuations. Radial profiles of turbulent kinetic energy and axial-tangential Reynolds stress are presented. The interactions between the vortex and both oblique and normal shock waves are investigated using nonintrusive optical diagnostics (i.e. schlieren, planar laser scattering, and laser Doppler velocimetry). Of the various types, two Mach 2.5 overexpanded-nozzle Mach disc interactions are examined in detail. Below a certain vortex strength, a 'weak' interaction exists in which the normal shock is perturbed locally into an unsteady 'bubble' shock near the vortex axis, but vortex breakdown (i.e., a stagnation point) does not occur. For stronger vortices, a random unsteady 'strong' interaction results that causes vortex breakdown. The vortex core reforms downstream of the rear stagnation point, and the reduced circulation distribution once again becomes self-similar in this region. A-new model of this interaction is proposed. Finally, a curve defining the approximate limits of supersonic vortex breakdown is presented.

Settles, G. S.; Cattafesta, L.

1993-01-01

330

Vortex and structural dynamics of a flexible cylinder in cross-flow  

NASA Astrophysics Data System (ADS)

A low-density, flexible cantilevered cylinder was permitted to vibrate freely under the influence of vortex shedding in the laminar flow regime. We find that the vortex-induced vibrations (VIV) of a flexible cantilever depart from those of a flexible cylinder that is fixed at both ends. In particular, we find discontinuous regions of VIV behavior - here called states - as a function of the reduced velocity U*. These states are demarcated by discrete changes in the dominant eigenmodes of the structural response as the cylinder vibrates in progressively higher structural modes with increasing U*. The contribution of structural modes can be identified readily by a modal projection of the cylinder oscillation onto known cantilever beam modes. Oscillation frequencies do not monotonically increase with U*. The wake response between different states is also found to have distinct characteristics; of particular note is the occurrence of a P+S wake over one of these regions, which is associated with a high-amplitude vibration of the cylinder that is due to the constructive interference of contributing eigenmodes.

Shang, Jessica K.; Stone, Howard A.; Smits, Alexander J.

2014-05-01

331

A Piloted Simulation Study of Wake Turbulence on Final Approach  

NASA Technical Reports Server (NTRS)

A piloted simulation study has been conducted in a research simulator to provide a means to estimate the effects of different levels of wake turbulence on final approach. A worst-case methodology was used to ensure conservative estimates. Fourteen airline pilots voluntarily participated in the study and flew almost 1000 approaches. The pilots rated the subjective severity of the disturbances using a special rating scale developed for this study. Several objective measures of the airplane/pilot response to the simulated wake turbulence were also made. All the data showed a large amount of variation between pilots and to a lesser extent for a given pilot. Therefore, the data were presented at 50, 70, 90 percentile levels as a function of vortex strength. The data allow estimates of the vortex strength for a given subjective or objective response and vice versa. The results of this study appear to be more conservative than the results of previous studies.

Stewart, Eric C.

1998-01-01

332

Wake Turbulence: An Obstacle to Increased Air Traffic Capacity  

NASA Technical Reports Server (NTRS)

Wingtip vortices were first described by British aerodynamicist F.W. Lanchester in 1907. A product of lift on a finite-span wing, these counterrotating masses of air trail behind an aircraft, gradually diffusing while convecting downward and moving about under mutual induction and the influence of wind and stratification. Should a smaller aircraft happen to be following the first aircraft, it could be buffeted and even flipped if it flew into the vortex, with dangerous consequences. Given the amount of air traffic in 1907, the wake vortex hazard was not initially much of a concern. The demand for air transportation continues to increase, and it is estimated that demand could double or even triple by 2025. One factor in the capacity of the air transportation system is wake turbulence and the consequent separation distances that must be maintained between aircraft to ensure safety.

2008-01-01

333

Effect of leading-edge vortex flaps on aerodynamic performance of delta wings  

NASA Technical Reports Server (NTRS)

The effect of leading-edge vortex flaps on the aerodynamic characteristics of highly swept-back wings is analytically investigated, using the free vortex sheet method. The method, based on a three-dimensional inviscid flow model, is an advanced panel type employing quadratic doublet distributions to represent the wing surface, rolled-up vortex sheet and wake and is capable of computing forces, moments and surface pressures.

Reddy, C. S.

1981-01-01

334

Bénard-von Kármán vortex street in an exciton-polariton superfluid  

NASA Astrophysics Data System (ADS)

The dynamics of an exciton-polariton superfluid resonantly injected into a semiconductor microcavity are investigated numerically. The results reveal that a Bénard-von Kármán vortex street is generated in the wake behind an obstacle potential, in addition to the generation of quantized vortex dipoles and dark solitons. The vortex street can be observed in the presence of a disorder potential in a realistic sample, and it can be observed even in time-integrated measurements.

Saito, Hiroki; Aioi, Tomohiko; Kadokura, Tsuyoshi

2012-07-01

335

Aircraft Wake RCS Measurement  

NASA Technical Reports Server (NTRS)

A series of multi-frequency radar measurements of aircraft wakes at altitudes of 5,000 to 25,00 ft. were performed at Kwajalein, R.M.I., in May and June of 1990. Two aircraft were tested, a Learjet 35 and a Lockheed C-5A. The cross-section of the wake of the Learjet was too small for detection at Kwajalein. The wake of the C-5A, although also very small, was detected and measured at VHF, UHF, L-, S-, and C-bands, at distances behind the aircraft ranging from about one hundred meters to tens of kilometers. The data suggest that the mechanism by which aircraft wakes have detectable radar signatures is, contrary to previous expectations, unrelated to engine exhaust but instead due to turbulent mixing by the wake vortices of pre-existing index of refraction gradients in the ambient atmosphere. These measurements were of necessity performed with extremely powerful and sensitive instrumentation radars, and the wake cross-section is too small for most practical applications.

Gilson, William H.

1994-01-01

336

Three-dimensional structure of straight and curved plane wakes  

NASA Technical Reports Server (NTRS)

Although the plane wake is marked by the formation of strong spanwise vortices, the initially two-dimensional Karman-like vortices soon develop a three-dimensional structure in the form of secondary streamwise vortices. So far, this streamwise vortex structure has been studied mostly through flow visualization and at relatively low Reynolds numbers. The primary objective of the present program was to investigate the origin and evolution of the three-dimensional structure of straight and curved plane wakes at relatively high Reynolds numbers (Re(sub b) = 28,000) through detailed measurements of the mean and turbulent properties at several streamwise locations. The experiments were conducted in three phases. In the first phase, the development of a straight plane wake was investigated. In the second phase, the effects of imposed streamwise curvature on the wake development were examined. The streamwise curvature was of constant radius and very mild in terms of the curvature ratio (b/(square root of R) is less than 2 percent). In both the first and second phases, the role of initial conditions was examined in wakes generated from both untripped (laminar) and tripped (turbulent) initial boundany layers. In the third phase, the effects of injecting streamwise vorticity and the effects of increased Reynolds number on the tripped wake structure and development were investigated.

Weygandt, James H.; Mehta, Rabindra D.

1993-01-01

337

Recent Developments on Airborne Forward Looking Interferometer for the Detection of Wake Vortices  

NASA Technical Reports Server (NTRS)

A goal of these studies was development of the measurement methods and algorithms necessary to detect wake vortex hazards in real time from either an aircraft or ground-based hyperspectral Fourier Transform Spectrometer (FTS). This paper provides an update on research to model FTS detection of wake vortices. The Terminal Area Simulation System (TASS) was used to generate wake vortex fields of 3-D winds, temperature, and absolute humidity. These fields were input to the Line by Line Radiative Transfer Model (LBLRTM), a hyperspectral radiance model in the infrared, employed for the FTS numerical modeling. An initial set of cases has been analyzed to identify a wake vortex IR signature and signature sensitivities to various state variables. Results from the numerical modeling case studies will be presented. Preliminary results indicated that an imaging IR instrument sensitive to six narrow bands within the 670 to 3150 per centimeter spectral region would be sufficient for wake vortex detection. Noise floor estimates for a recommended instrument are a current research topic.

Daniels, Taumi S.; Smith, William L.; Kirev, Stanislav

2012-01-01

338

Dynamic analysis of marine risers with vortex excitation  

SciTech Connect

The basic equations for nonplanar transverse vibrations of marine risers are derived from the theory of elastic rods. A numerical method is developed for solution of the equations by time integration. Spatial discretization is accomplished by a hybrid finite element method. Vortex excitation is modeled by the coupled wake oscillator proposed by Iwan and Blevins. The vortex oscillator equations are integrated numerically in time along with the riser equations. By way of example, several typical riser problems are analyzed including forced vibration and vortex-induced vibration.

Nordgren, R.P.

1982-03-01

339

Experimental study on effect of a new vortex control baffler and its influencing factor  

NASA Astrophysics Data System (ADS)

The horseshoe vortex generated around the sail-body junction of submarine has an important influence on the non-uniformity of submarine wake at propeller disc. The flow characteristics in the horseshoe vortex generated area are analyzed, and a new method of vortex control baffler is presented. The influence of vortex control baffler on the flow field around submarine main body with sail is numerically simulated. The wind tunnel experiment on submarine model is carried out, and it is proved that the vortex control baffler can weaken the horseshoe vortex and decrease the non-uniformity of the wake at propeller disc. It is shown from the experiment results that the effect of vortex control baffler depends on its installation position; with a proper installation position, the non-uniform coefficient of submarine wake would be declined by about 50%; the Reynolds number of submarine model has an influence on the effect of vortex control baffler too, and the higher the Reynolds number is, the better the effect of the vortex control baffler is.

Liu, Zhi-Hua; Xiong, Ying; Wang, Zhan-Zhi; Wang, Song; Tu, Cheng-Xu

2011-03-01

340

The interaction of helical tip and root vortices in a wind turbine wake  

NASA Astrophysics Data System (ADS)

Analysis of the helical vortices measured behind a model wind turbine in a water channel are reported. Phase-locked measurements using planar particle image velocimetry are taken behind a Glauert rotor to investigate the evolution and breakdown of the helical vortex structures. Existing linear stability theory predicts helical vortex filaments to be susceptible to three unstable modes. The current work presents tip and root vortex evolution in the wake for varying tip speed ratio and shows a breaking of the helical symmetry and merging of the vortices due to mutual inductance between the vortical filaments. The merging of the vortices is shown to be steady with rotor phase, however, small-scale non-periodic meander of the vortex positions is also observed. The generation of the helical wake is demonstrated to be closely coupled with the blade aerodynamics, strongly influencing the vortex properties which are shown to agree with theoretical predictions of the circulation shed into the wake by the blades. The mutual inductance of the helices is shown to occur at the same non-dimensional wake distance.

Sherry, Michael; Nemes, András; Lo Jacono, David; Blackburn, Hugh M.; Sheridan, John

2013-11-01

341

Downwash and wake behind plain and flapped airfoils  

NASA Technical Reports Server (NTRS)

Extensive experimental measurements have been made of the downwash angles and the wake characteristics behind airfoils with and without flaps and the data have been analyzed and correlated with the theory. A detailed study was made of the errors involved in applying lifting-line theory, such as the effects of a finite wing chord, the rolling-up of the trailing vortex sheet, and the wake. The downwash angles, as computed from the theoretical span load distribution by means of the Biot-Savart equation, were found to be in satisfactory agreement with the experimental results. The rolling-up of the trailing vortex sheet may be neglected, but the vertical displacement of the vortex sheet requires consideration. By the use of a theoretical treatment indicated by Prandtl, it has been possible to generalize the available experimental results so the predictions can be made of the important wake parameters in terms of the distance behind the airfoil trailing edge and the profile-drag coefficient. The method of application of the theory to design and the satisfactory agreement between predicted and experimental results when applied to an airplane are demonstrated.

Silverstein, Abe; Katzoff, S; Bullivant, W Kenneth

1939-01-01

342

Experimental modeling of vortex tube refrigerator  

Microsoft Academic Search

An experimental investigation has been performed to realize thorough behavior of a vortex tube system. In this work attention has been focussed on the classification of the parameters affecting vortex tube operation. The effective parameters are divided into two different types, namely geometrical and thermo-physical ones. A reliable test rig has been designed and constructed to investigate the effect of

M. H. Saidi; M. S. Valipour

2003-01-01

343

Experimental Investigation of the Wake behind a Sphere at Low Reynolds Numbers  

Microsoft Academic Search

Wakes produced by a sphere moving in a tank of water were photo-graphically investigated at Reynolds numbers from 5 to 300. Main results obtained are as follows. 1. The critical Reynolds number at which the parmanent vortex-ring begins to form in the rear of a sphere is about 24. 2.The size of vortex-ring is nearly proportional to the logarithm of

Sadatoshi Taneda

1956-01-01

344

An experimental investigation of vortex stability, tip shapes, compressibility, and noise for hovering model rotors  

NASA Technical Reports Server (NTRS)

Schlieren methods of flow visualization and hot-wire anemometry for velocity measurements were used to investigate the wakes generated by hovering model propellers and rotors. The research program was directed toward investigating (1) the stability of the tip vortex, (2) the effects produced by various tip shapes on performance and tip vortex characteristics, and (3) the shock formation and noise characteristics associated with various tip shapes. A free-wake analysis was also conducted for comparison with the vortex stability experimental results. Schlieren photographs showing wake asymmetry, interaction, and instability are presented along with a discussion of the effects produced by the number of blades, collective pitch, and tip speed. Two hot-wire anemometer techniques, used to measure the maximum circumferential velocity in the tip vortex, are discussed.

Tangler, J. L.; Wohlfeld, R. M.; Miley, S. J.

1973-01-01

345

Vortex perturbation dynamics  

NASA Technical Reports Server (NTRS)

An initial value approach is used to examine the dynamics of perturbations introduced into a vortex under strain. Both the basic vortex considered and the perturbations are taken as fully three-dimensional. An explicit solution for the time evolution of the vorticity perturbations is given for arbitrary initial vorticity. Analytical solutions for the resulting velocity components are found when the initial vorticity is assumed to be localized. For more general initial vorticity distributions, the velocity components are determined numerically. It is found that the variation in the radial direction of the initial vorticity disturbance is the most important factor influencing the qualitative behavior of the solutions. Transient growth in the magnitude of the velocity components is found to be directly attributable to the compactness of the initial vorticity.

Criminale, W. O.; Lasseigne, D. G.; Jackson, T. L.

1995-01-01

346

Brownian vortexes  

NASA Astrophysics Data System (ADS)

Mechanical equilibrium at zero temperature does not necessarily imply thermodynamic equilibrium at finite temperature for a particle confined by a static but nonconservative force field. Instead, the diffusing particle can enter into a steady state characterized by toroidal circulation in the probability flux, which we call a Brownian vortex. The circulatory bias in the particle’s thermally driven trajectory is not simply a deterministic response to the solenoidal component of the force but rather reflects interplay between advection and diffusion in which thermal fluctuations extract work from the nonconservative force field. As an example of this previously unrecognized class of stochastic heat engines, we consider a colloidal sphere diffusing in a conventional optical tweezer. We demonstrate both theoretically and experimentally that nonconservative optical forces bias the particle’s fluctuations into toroidal vortexes whose circulation can reverse direction with temperature or laser power.

Sun, Bo; Lin, Jiayi; Darby, Ellis; Grosberg, Alexander Y.; Grier, David G.

2009-07-01

347

Firing Variability in Cat Association Cortex During Sleep And Wakefulness.  

National Technical Information Service (NTIS)

Spontaneous discharges of single neurons in the association (middle suprasylvian) cortex of the cat were studied during natural sleep and wakefulness. Changes in the temporal pattern of neuronal discharges associated with the behavioral shifts were evalua...

H. Noda W. R. Adey

1969-01-01

348

Dissipation of turbulence in the wake of a wind turbine  

NASA Astrophysics Data System (ADS)

The wake of a wind turbine is characterized by increased turbulence and decreased wind speed. Turbines are generally deployed in large groups in wind farms, and so the behavior of an individual wake as it merges with other wakes and propagates downwind is of great importance in assessing wind farm power production as well as impacts of wind energy deployment on local and regional environments. The rate of turbulence dissipation in the wake quantifies the wake behavior as it propagates. In situ field measurements of turbulence dissipation rate in the wake of wind turbines have not been previously collected although correct modeling of dissipation rate is required for accurate simulations of wake evolution. In Fall 2012, we collected in situ measurements of winds and turbulence dissipation from the wake region of a multi-MW turbine, using the University of Colorado at Boulder's Tethered Lifting System (TLS). The TLS is a unique state-of-the-art tethersonde, proven in numerous boundary-layer field experiments to be able to measure turbulence kinetic energy dissipation rates. Ambient flow measurements were provided from sonic anemometers on a meteorological tower located upwind of the turbine, from a profiling lidar upwind, and from a scanning lidar measuring both inflow to and wake from the turbine. Measurements collected within the wake indicate that dissipation rates are higher in the turbine wake than in the ambient flow. Profiles of dissipation and turbulence throughout the rotor disk suggest that dissipation peaks near the hub height of the turbine. Suggestions for incorporating this information into wind turbine modeling approaches will be provided.

Lundquist, J. K.; Bariteau, L.

2013-12-01

349

The stability of laminar symmetric separated wakes  

NASA Astrophysics Data System (ADS)

Time-dependent computations of the two-dimensional incompressible uniform-velocity laminar flow past a normal flat plate (of unit half-width) in a channel are presented. Attention is restricted to cases in which the well-known anti-symmetric (von Kármán-type) vortex shedding is suppressed by the imposition of a symmetry plane on the downstream plate centreline. With a further symmetry plane at the channel's upper boundary, the only two governing parameters in the problem are the channel half-width, H, and the Reynolds number, Re (based on the body half-width and the upstream velocity, U). The former is restricted to the range 3 {?} H {?} 30 and the interest lies in determining the nature of the initial instability which occurs in the separated wake as Re is gradually increased. It is found that for sufficiently large H and at a critical Re, a long-time-scale global (supercritical) instability is initiated, which in its saturated (limit) state takes the form of ‘lumps’ of vorticity being periodically shed from the tail end of the separated bubble. Stability calculations of corresponding mean flow profiles (typical of those found in the separated wake) are undertaken by examining the impulse response of particular profiles via appropriate solution of the Orr Sommerfeld equation. The results of this analysis extend those available from related published work and are consistent with the behaviour found from the numerical computations. Taken together, all the results suggest that this type of global instability may be generic to many kinds of separated wakes and, indeed, may provide the fundamental explanation for the very low-frequency oscillations often noticed in fully turbulent wake bubbles.

Castro, Ian P.

2005-06-01

350

3D visualization of unsteady 2D airplane wake vortices  

NASA Technical Reports Server (NTRS)

Air flowing around the wing tips of an airplane forms horizontal tornado-like vortices that can be dangerous to following aircraft. The dynamics of such vortices, including ground and atmospheric effects, can be predicted by numerical simulation, allowing the safety and capacity of airports to be improved. In this paper, we introduce three-dimensional techniques for visualizing time-dependent, two-dimensional wake vortex computations, and the hazard strength of such vortices near the ground. We describe a vortex core tracing algorithm and a local tiling method to visualize the vortex evolution. The tiling method converts time-dependent, two-dimensional vortex cores into three-dimensional vortex tubes. Finally, a novel approach calculates the induced rolling moment on the following airplane at each grid point within a region near the vortex tubes and thus allows three-dimensional visualization of the hazard strength of the vortices. We also suggest ways of combining multiple visualization methods to present more information simultaneously.

Ma, Kwan-Liu; Zheng, Z. C.

1994-01-01

351

Enhanced Airport Capacity Through Safe, Dynamic Reductions in Aircraft Separation: NASA's Aircraft VOrtex Spacing System (AVOSS)  

NASA Technical Reports Server (NTRS)

An aspect of airport terminal operations that holds potential for efficiency improvements is the separation criteria applied to aircraft for wake vortex avoidance. These criteria evolved to represent safe spacing under weather conditions conducive to the longest wake hazards, and are consequently overly conservative during a significant portion of operations. Under many ambient conditions, such as moderate crosswinds or turbulence, wake hazard durations are substantially reduced. To realize this reduction NASA has developed a proof-of-concept Aircraft Vortex Spacing System (AVOSS). Successfully operated in a real-time field demonstration during July 2000 at the Dallas Ft. Worth International Airport, AVOSS is a novel integration of weather sensors, wake sensors, and analytical wake prediction algorithms. Gains in airport throughput using AVOSS spacing as compared to the current criteria averaged 6%, with peak values approaching the theoretical maximum of 16%. The average throughput gain translates to 15-40% reductions in delay when applied to realistic capacity ratios at major airports.

OConnor, Cornelius J.; Rutishauser, David K.

2001-01-01

352

DIELECTRIC WAKE FIELD RESONATOR ACCELERATOR MODULE  

SciTech Connect

Results are presented from experiments, and numerical analysis of wake fields set up by electron bunches passing through a cylindrical or rectangular dielectric-lined structure. These bunches excite many TM-modes, with Ez components of the wake fields sharply localized on the axis of the structure periodically behind the bunches. The experiment with the cylindrical structure, carried out at ATF Brookhaven National Laboratory, used up to three 50 MeV bunches spaced by one wake field period (21 cm) to study the superposition of wake fields by measuring the energy loss of each bunch after it passed through the 53-cm long dielectric element. The millimeter-wave spectrum of radiation excited by the passage of bunches is also studied. Numerical analysis was aimed not only to simulate the behavior of our device, but in general to predict dielectric wake field accelerator performance. It is shown that one needs to match the radius of the cylindrical dielectric channel with the bunch longitudinal rms-length to achieve optimal performance.

Hirshfield, Jay L.

2013-11-06

353

Laser Doppler velocimeter system simulation for sensing aircraft wake vortices  

NASA Technical Reports Server (NTRS)

A hydrodynamic model of aircraft vortex wakes in an irregular wind shear field near the ground is developed and used as a basis for modeling the characteristics of a laser Doppler detection and vortex location system. The trailing vortex sheet and the wind shear are represented by discrete free vortices distributed over a two-dimensional grid. The time dependent hydrodynamic equations are solved by direct numerical integration in the Boussinesq approximation. The ground boundary is simulated by images, and fast Fourier Transform techniques are used to evaluate the vorticity stream function. The atmospheric turbulence was simulated by constructing specific realizations at time equal to zero, assuming that Kolmogoroff's law applies, and that the dissipation rate is constant throughout the flow field. The response of a simulated laser Doppler velocimeter is analyzed by simulating the signal return from the flow field as sensed by a simulation of the optical/electronic system.

Thomson, J. A. L.; Meng, J. C. S.

1974-01-01

354

Waking with the hypothalamus.  

PubMed

An essential component of the whole-body homoeostasis provided by the hypothalamus is the management of available energy. This includes the regulation of sleeping and waking, feeding and drinking, body temperature and activity, as well as the endocrinium. The waking brain, in particular the cerebral cortex, needs to be activated through neuronal pathways ascending from the brainstem reticular formation (ascending reticular activating system, ARAS) and reaching the cortical structures by a dorsal route through the thalamus and a ventral route, including the hypothalamus and the basal forebrain. This review concentrates on the more recently explored ventral route and the hypothalamus with its different regions involved in the control of the waking state. PMID:21796339

Haas, Helmut L; Lin, Jian-Sheng

2012-01-01

355

Collinear wake field acceleration  

NASA Astrophysics Data System (ADS)

In the Voss-Weiland scheme of wake field acceleration a high current, ring shaped driving bunch is used to accelerate a low current beam following along an axis. In such a structure, the transformer ratio, i.e., the ratio of maximum voltage that can be gained by the on-axis beam and the voltage lost by the driving beam, can be large. In contrast, it was observed that for an arrangement in which driving and driven bunches follow the same path, and where the current distribution of both bunches is gaussian, the transformer ratio is not normally greater than two. Some of the possibilities and limitations of a collinear acceleration scheme are explored. In addition to its application to wake field acceleration in structures, this study is also of interest for the understanding of the plasma wake field accelerator.

Bane, K. I. F.; Chen, P.; Wilson, P. B.

1985-04-01

356

Collinear wake field acceleration  

SciTech Connect

In the Voss-Weiland scheme of wake field acceleration a high current, ring-shaped driving bunch is used to accelerate a low current beam following along on axis. In such a structure, the transformer ratio, i.e., the ratio of maximum voltage that can be gained by the on-axis beam and the voltage lost by the driving beam, can be large. In contrast, it has been observed that for an arrangement in which driving and driven bunches follow the same path, and where the current distribution of both bunches is gaussian, the transformer ratio is not normally greater than two. This paper explores some of the possibilities and limitations of a collinear acceleration scheme. In addition to its application to wake field acceleration in structures, this study is also of interest for the understanding of the plasma wake field accelerator. 11 refs., 4 figs.

Bane, K.L.F.; Chen, P.; Wilson, P.B.

1985-04-01

357

Stereo-Video Data Reduction of Wake Vortices and Trailing Aircrafts  

NASA Technical Reports Server (NTRS)

This report presents stereo image theory and the corresponding image processing software developed to analyze stereo imaging data acquired for the wake-vortex hazard flight experiment conducted at NASA Langley Research Center. In this experiment, a leading Lockheed C-130 was equipped with wing-tip smokers to visualize its wing vortices, while a trailing Boeing 737 flew into the wake vortices of the leading airplane. A Rockwell OV-10A airplane, fitted with video cameras under its wings, flew at 400 to 1000 feet above and parallel to the wakes, and photographed the wake interception process for the purpose of determining the three-dimensional location of the trailing aircraft relative to the wake. The report establishes the image-processing tools developed to analyze the video flight-test data, identifies sources of potential inaccuracies, and assesses the quality of the resultant set of stereo data reduction.

Alter-Gartenberg, Rachel

1998-01-01

358

Flows over wings with leading-edge vortex separation  

NASA Technical Reports Server (NTRS)

The unsteady cross flow analogy reduces the steady three dimensional separation flow problem into an unsteady two dimensional flow problem in which the section shape changes with time. The two dimensional VORSEP code is extended to the case of arbitrary body growth rates in order to generate the initial vortex structures for the three dimensional free vortex sheet (FVS) code. Automatic procedures to reduce the wing geometry definition to a set of cross flow plane sections corresponding to the locations of the time step solutions and to generate the effective source distribution on each cross flow section to represent the section normal growth across the following steps are incorporated in the VORSEP code. Also, the wake shedding model is improved by adopting a redistribution scheme which improves the stability of the free sheet development with time. The improved wake shedding model combined with the redistribution scheme alleviated the numerical instabilities associated with the vortex rollup.

Rao, B. M.; Maskew, B.

1982-01-01

359

VORTEX FORMATION DURING TETHERED FLIGHT OF FUNCTIONALLY AND MORPHOLOGICALLY TWO-WINGED INSECTS, INCLUDING EVOLUTIONARY CONSIDERATIONS ON INSECT FLIGHT  

Microsoft Academic Search

Summary Tethered flight of six insect species (two pentatomid bugs, a moth, a butterfly, a muscid fly and a crane fly) was studied using several modifications of a dust flow visualization procedure. The spatial structure of the near vortex wake of f lying specimens was reconstructed on the basis of two-dimensional flow pictures. The dynamics of the wake was followed

DMITRY L. GRODNITSKY; PAHVEL P. MOROZOV

1993-01-01

360

Hypersonic rarefied wake characterization  

NASA Technical Reports Server (NTRS)

Results of a numerical study using the direct simulation Monte Carlo (DSMC) method are presented for hypersonic rarefied flow over an aeroassisted space transfer vehicle (ASTV). The emphasis of the study is the characterization of the near wake region which includes the ASTV payload. The study covered the transitional flow regime from near continuum to free molecular. Calculations show that the character of the near wake is significantly affected by the presence of the payload. Flow separation occurs when an afterbody is present throughout the transitional flow regime. In contrast, when no afterbody is present, no separation is observed until the flow approaches continuum.

Brewer, E. B.

1993-01-01

361

Half-loop and full-loop shedding in the wake of wall-mounted square cylinders due to boundary layer-wake interaction  

NASA Astrophysics Data System (ADS)

The vortical flow in the finite wall-mounted obstacle wakes can be important in heat transfer devices, turbomachinery components, and flame stabilizer devices, and is of fundamental importance since it displays fully three-dimensional states of K'arm'an vortex streets that are found in 2D bluff body wakes. The turbulent state of the wake of a finite square cylinder (height-to-width ratio h/w=8) has been found to be sensitive to the conditions of the approach boundary layer. The energetic quasi-periodic vortical structure topology is found to change between two topological states. Boundary layer thicknesses ?/d=0.7 and 1.6 yield half-loop and full-loop structures, respectively. This modification of the structure topology has significant consequences for modifying the mixing, momentum transfer, turbulence production, and Reynolds stresses in the wake. Using synchronized particle image velocimetry (PIV) and surface pressure measurements for these two boundary layers, the coherent structures in the wake of the cylinder are reconstructed and analyzed. Vortical connector strands which tie together subsequently shed structures lead to high incoherent Reynolds stresses, streamwise vorticity, vortex stretching, and turbulence production in their neighbourhood, but do not appear in the lower regions of the wake for the half-loop topology.

Bourgeois, Jason; Hosseini, Zahra; Martinuzzi, Robert

2011-11-01

362

Meander of a fin trailing vortex and the origin of its turbulence  

NASA Astrophysics Data System (ADS)

The low-frequency meander of a trailing vortex shed from a tapered fin installed on a wind tunnel wall has been studied using stereoscopic particle image velocimetry in the near-wake at Mach 0.8. Distributions of the instantaneous vortex position reveal that the meander amplitude increases with downstream distance and decreases with vortex strength, indicating meander is induced external to the vortex. Trends with downstream distance suggest meander begins on the fin surface, prior to vortex shedding. Mean vortex properties are unaltered when considered in the meandering reference frame, apparently because turbulent fluctuations in the vortex shape and strength dominate positional variations. Conversely, a large peak of artificial turbulent kinetic energy is found centered in the vortex core, which almost entirely disappears when corrected for meander, though some turbulence remains near the core radius. Turbulence originating at the wind tunnel wall was shown to contribute to vortex meander by energizing the incoming boundary layer using low-profile vortex generators and observing a substantial increase in the meander amplitude, while greater turbulent kinetic energy penetrates the vortex core. An explanatory mechanism has been hypothesized, in which the vortex initially forms at the apex of the swept leading edge of the fin where it is exposed to turbulent fluctuations within the wind tunnel wall boundary layer, introducing an instability into the incipient vortex core.

Beresh, Steven J.; Henfling, John F.; Spillers, Russell W.

2010-09-01

363

Axisymmetric turbulent wakes with new nonequilibrium similarity scalings.  

PubMed

The recently discovered nonequilibrium turbulence dissipation law implies the existence of axisymmetric turbulent wake regions where the mean flow velocity deficit decays as the inverse of the distance from the wake-generating body and the wake width grows as the square root of that distance. This behavior is different from any documented boundary-free turbulent shear flow to date. Its existence is confirmed in wind tunnel experiments of wakes generated by plates with irregular edges placed normal to an incoming free stream. The wake characteristics of irregular bodies such as buildings, bridges, mountains, trees, coral reefs, and wind turbines are critical in many areas of environmental engineering and fluid mechanics. PMID:24138244

Nedi?, J; Vassilicos, J C; Ganapathisubramani, B

2013-10-01

364

Probabilistic Analysis of Impact of Wake Vortices on Closely-Spaced Parallel Approaches  

NASA Technical Reports Server (NTRS)

One of the primary constraints on the capacity of the nation's air transportation system is the landing capacity of its largest airports. Many airports with closely spaced parallel runways suffer a severe runway acceptance rate when the weather conditions do not allow full utilization of these parallel runways. The present requirement for simultaneous independent landings in Instrument Meteorological Conditions, IMC, is at least 4300 feet of lateral runway spacing (as close as 3000 feet for runways with a Precision Runway Monitor). Operations in Visual Meteorological Conditions, VMC, to Closely Spaced Parallel Approaches only require a lateral runway spacing greater than 750 feet. A study by Hardy and Lewis integrated and extended earlier studies and concepts in lateral traffic separation, longitudinal station keeping, wake prediction, wake display, and the concepts of R N P into a preliminary system concept for Closely Spaced Parallel Approaches in IMC. This system allows IMC airport acceptance rates to approach those for VMC. The system concept that was developed, presented traffic and wake information on the NAVigation Display, NAV, and developed operational procedures for a mix of conventional and Runway Independent Aircraft with different approach speeds to Closely Spaced Parallel Runways. This paper first describes some improvements made on the technology needed to better predict and formulate a probabilistic representation for the time-dependent motion and spreading of the hazardous region associated with the lift-generated vortex wakes of preceding aircraft. In this way, the time at which the vortex wakes of leading aircraft intrude into the airspace of adjacent flight-corridor/runway combinations can be more reliably predicted. Such a prediction is needed because it determines restraints to be placed on in-trail separation distances; or, the allowable time intervals between aircraft executing nearly simultaneous landings or takeoffs on very closely-spaced runways. Improved estimates of wake spreading are achieved by inclusion of representations in the equations for wake spreading due to ambient turbulence and due to the long-wave instability of a vortex pair. Wake motion and spreading due to the time-averaged wind and its variations with time, are retained. The more detailed representation of wake spreading presented here permits the development of probabilistically-based uncertainty estimates for wake spreading. Measurements needed within actual aircraft wake vortices to validate and support this analysis are also described. The second part of the paper uses the improvements in the accuracy of the location of wake vortices to extend the preliminary system concept for Closely Spaced Parallel Approaches described earlier with more robust operational procedures. Additionally, improvements in longitudinal station keeping, wake display, and risk assessment methodologies are incorporated and described.

Hardy, Gordon H.; Rossow, Vernon J.; Meyn, Larry A.

2005-01-01

365

Radar Reflectivity in Wingtip-Generated Wake Vortices  

NASA Technical Reports Server (NTRS)

This report documents new predictive models of radar reflectivity, with meter-scale resolution, for aircraft wakes in clear air and fog. The models result from a radar design program to locate and quantify wake vortices from commercial aircraft in support of the NASA Aircraft Vortex Spacing System (AVOSS). The radar reflectivity model for clear air assumes: 1) turbulent eddies in the wake produce small discontinuities in radar refractive index; and 2) these turbulent eddies are in the 'inertial subrange' of turbulence. From these assumptions, the maximum radar frequency for detecting a particular aircraft wake, as well as the refractive index structure constant and radar volume reflectivity in the wake can be obtained from the NASA Terminal Area Simulation System (TASS) output. For fog conditions, an empirical relationship is used to calculate radar reflectivity factor from TASS output of bulk liquid water. Currently, two models exist: 1) Atlas-based on observations of liquid water and radar reflectivity factor in clouds; and 2) de Wolf- specifically tailored to a specific measured dataset (1992 Vandenberg Air Force Base).

Marshall, Robert E.; Mudukutore, Ashok; Wissel, Vicki

1997-01-01

366

Exploration of Terminal Procedures Enabled by NASA Wake VAS Technologies  

NASA Technical Reports Server (NTRS)

The National Aeronautics and Space Administration (NASA) tasked The MITRE Corporation's Center for Advanced Aviation System Development (CAASD) to investigate potential air traffic control (ATC) procedures that could benefit from technology used or developed in NASA's Wake Vortex Advisory System (WakeVAS). The task also required developing an estimate of the potential benefits of the candidate procedures. The main thrust of the investigation was to evaluate opportunities for improved capacity and efficiency in airport arrival and departure operations. Other procedures that would provide safety enhancements were also considered. The purpose of this investigation was to provide input to the WakeVAS program office regarding the most promising areas of development for the program. A two-fold perspective was desired: First, identification of benefits from possible procedures enabled by both incremental components and the mature state of WakeVAS technology; second identification of procedures that could be expected to evolve from the current Federal Aviation Administration (FAA) procedures. The evolution of procedures should provide meaningful increments of benefit and a low risk implementation of the WakeVAS technologies.

Lunsford, Clark R.; Smith, Arthur P., III; Cooper, Wayne W., Jr.; Mundra, Anand D.; Gross, Amy E.; Audenaerd, Laurence F.; Killian, Bruce E.

2004-01-01

367

Waking Up to Waste  

ERIC Educational Resources Information Center

All homes and schools produce waste. Children may have been astonished at how much people throw away, and this could be the "wake-up call" that arouses their interest. At Carymoor Environmental Centre (an Eco-Centre in South Somerset) getting children involved in active waste reduction and recycling is a priority. Carymoor tries to model waste…

Vrdlovcova, Jill

2005-01-01

368

Diffusion in building wakes  

SciTech Connect

Straight-line Gaussian models adequately describe atmospheric diffusion for many applications. They have been modified for use in estimating diffusion in building wakes by adding terms that include projected building area and by redefining the diffusion coefficients so that the coefficients have minimum values that are related to building dimensions. In a recent study, Ramsdell reviewed the building-wake dispersion models used by the Nuclear Regulatory Commission (NRC) in its control room habitability assessments. The review included comparison of model estimates of centerline concentrations with concentrations observed in experiments at seven nuclear reactors. In general, the models are conservative in that they tend to predict concentrations that are greater than those actually observed. However, the models show little skill in accounting for variations in the observed concentrations. Subsequently, the experimental data and multiples linear regression techniques have been used to develop a new building wake diffusion model. This paper describes the new building wake model and compares it with other models. 8 refs., 2 figs.

Ramsdell, J.V.

1988-03-01

369

Singular vortex  

SciTech Connect

During implementation of the PODMODELI-program for the equation of gas dynamics it became clear that the equations have many partial invariant solutions, most of which have not been studied previously. Such solutions require special analysis, which sometimes is not trivial. Our attention was attracted by solutions generated by the rotation group O(3), which is allowed by the equations of gas dynamics. What is specific here is that solutions invariant under O(3), which are known as spherically symmetric solutions, are singular invariant solutions from the standpoint of group analysis. For the group O(3), however, the necessary conditions for the existence of nonsingular partially invariant solutions are rank two and defect one. These solutions are characterized by the fact that their invariant components are spherically symmetric, but their velocity vector component tangential to the sphere is nonzero. It turned out that a fairly broad class of new solutions is opened up here. The aim of this study is to demonstrate that such solutions do exist and to make a general analysis of them. The kinematics and dynamics of the respective motions of the gas are very involved and the details are not yet very clear. A singular vortex is distinguished as an exact solution with a special initial distribution of the tangential component. Particular examples of such exact solutions are given here. In addition, we consider the case of steady-state flow of an incompressible liquid, where solutions of the singular vortex type exist and are fairly foreseeable.

Ovsyannikov, L.V. [M.A. Lavrent`ev Hydrodynamics Institute, Novosibirsk (Russian Federation)

1995-11-01

370

Demonstration of rapid-scan two-dimensional laser velocimetry in the Langley Vortex Research Facility for research in aerial applications  

NASA Technical Reports Server (NTRS)

Tests were conducted to demonstrate a rapid scan two dimensional laser velocimeter (LV) measurement technique for aerial applications research. The LV system is capable of simultaneously measuring both vertical and axial flow velocity components in a near or far field vortex system. Velocity profiles were successfully measured in the wake vortex of a representative agricultural aircraft model, with the vortex system rapidly transporting in ground effect. Results indicate that the laser velocimetry technique can provide quantitative information of wake vortex characteristics in ground effect.

Gartrell, L. R.; Jordan, F. L., Jr.

1977-01-01

371

TR PIV Experimental Investigation on Bypass Transition Induced by a Cylinder Wake  

Microsoft Academic Search

The process of laminar to turbulent transition induced by a cylinder wake was studied by TR PIV in a water channel. The generation of secondary transverse vortex structure and its evolution along the streamwise are tracked, also, a low-speed hump is found in the near-wall region along the bypass transition zone, which becomes more obvious further downstream. At the later

Z. Q. Tang; N. Jiang

2011-01-01

372

A Hybrid Vortex Method for Two-Dimensional Flow Over Tube Bundles  

SciTech Connect

A hybrid vortex method is presented for computing flows about objects that accurately resolves the boundary layer details while keeping the number of free vortices at a reasonable level. The method uses a wall layer model close to the body surface and discrete vortex blobs in the free wake. Details of the wall layer implementation are presented, and results of sample calculations are compared with known analytical solutions and with calculations from other vortex codes. These results show that the computed boundary layer details are accurate to approximately 0.3 percent of analytical solutions while using three orders of magnitude fewer vortices than other vortex simulations.

Strickland, J.H.; Wolfe, W.P.

1998-11-13

373

Automatic Tip Vortex Core Profiling for Numerical Flow Simulations of Rotorcraft in Hover  

NASA Technical Reports Server (NTRS)

An automated approach is presented that extracts visual and quantitative data from vortex cores produced by Navier-Stokes simulations of rotorcraft in hover mode. This approach extracts contiguous rotor tip vortex-core trajectories, cross-flow velocity profiles, and vortex-core diameter variation with wake age (azimuth angle). This automated approach is faster and more accurate than a conventional manual approach. Moreover, this new approach allows for an efficient way to quantitatively compare vortex-core profiles from different flow simulations, e.g., grid resolution studies, and validate computed results with experimental data

Kao, David L.; Chaderjian, Neal M.

2010-01-01

374

Wind tunnel investigation on wind turbine wakes and wind farms  

NASA Astrophysics Data System (ADS)

The interaction between atmospheric boundary layer and wind farms leads to flow modifications, which need to be deeply characterized in order to relate them to wind farm performance. The wake flow produced from a wind farm is the result of a strong interaction between multiple turbine wakes, so that the wind farm configuration turns out to be one of the dominant features to enhance power production. For the present work a wind tunnel investigation was carried out with hot-wire anemometry and velocity measurements performed with multi-hole pressure probes. The tested wind farms consist of miniature three-bladed wind turbine models. Preliminarily, the wake flow generated from a single wind turbine is surveyed, which is characterized by a strong velocity defect lying in proximity of the wind turbine hub height. The wake gradually recovers by moving downstream; the characteristics of the incoming boundary layer and wind turbulence intensity can strongly affect the wake recovery, and thus performance of following wind turbines. An increased turbulence level is typically detected downstream of each wind turbine for heights comparable to the wind turbine blade top-tip. These wake flow fluctuations produce increased fatigue loads on the following wind turbines within a wind farm, which could represent a significant hazard for real wind turbines. Dynamics of vorticity structures present in wind turbine wakes are also investigated; particular attention is paid to the downstream evolution of the tip helicoidal vortices and to oscillations of the hub vortex. The effect of wind farm layout on power production is deeply investigated. Particular emphasis is placed on studying how the flow adjusts as it moves inside the wind farm and can affect the power production. Aligned and staggered wind farm configurations are analysed, also with varying separation distances in the streamwise and spanwise directions. The present experimental results are being used to test and guide the development of improved parameterizations of wind turbines in high-resolution numerical models, such as large-eddy simulations (LES).

Iungo, G. V.; Coëffé, J.; Porté-Agel, F.

2012-04-01

375

Study of the Mutual Interaction Between a Wing Wake and an Encountering Airplane  

NASA Technical Reports Server (NTRS)

In an effort to increase airport productivity, several wind-tunnel and flight-test programs are currently underway to determine safe reductions in separation standards between aircraft. These programs are designed to study numerous concepts from the characteristics and detection of wake vortices to the wake-vortex encounter phenomenon. As part of this latter effort, computational tools are being developed and utilized as a means of modeling and verifying wake-vortex hazard encounters. The objective of this study is to assess the ability of PMARC, a low-order potential-flow panel method, to predict the forces and moments imposed on a following business-jet configuration by a vortex interaction. Other issues addressed include the investigation of several wake models and their ability to predict wake shape and trajectory, the validity of the velocity field imposed on the following configuration, modeling techniques and the effect of the high-lift system and the empennage. Comparisons with wind-tunnel data reveal that PMARC predicts the characteristics for the clean wing-body following configuration fairly well. Non-linear effects produced by the addition of the high-lift system and empennage, however, are not so well predicted.

Walden, A. B.; vanDam, C. P.

1996-01-01

376

Three-Centimeter Doppler Radar Observations of Wingtip-Generated Wake Vortices in Clear Air  

NASA Technical Reports Server (NTRS)

This report documents a high risk, high pay-off experiment with the objective of detecting, for the first time, the presence of aircraft wake vortices in clear air using X-band Doppler radar. Field experiments were conducted in January 1995 at the Wallops Flight Facility (WFF) to demonstrate the capability of the 9.33 GHz (I=3 cm) radar, which was assembled using an existing nine-meter parabolic antenna reflector at VVTT and the receiver/transmitter from the NASA Airborne Windshear Radar-Program. A C-130-aircraft, equipped with wingtip smoke generators, created visually marked wake vortices, which were recorded by video cameras. A C-band radar also observed the wake vortices during detection attempts with the X-band radar. Rawinsonde data was used to calculate vertical soundings of wake vortex decay time, cross aircraft bearing wind speed, and water vapor mixing ratio for aircraft passes over the radar measurement range. This experiment was a pathfinder in predicting, in real time, the location and persistence of C-130 vortices, and in setting the flight path of the aircraft to optimize X-band radar measurement of the wake vortex core in real time. This experiment was conducted in support of the NASA Aircraft Vortex Spacing System (AVOSS).

Marshall, Robert E.; Mudukutore, Ashok; Wissel, Vicki L. H.; Myers, Theodore

1997-01-01

377

Measured Changes in C-Band Radar Reflectivity of Clear Air Caused by Aircraft Wake Vortices  

NASA Technical Reports Server (NTRS)

Wake vortices from a C-130 airplane were observed at the NASA Wallops Flight Facility with a ground-based, monostatic C-band radar and an antenna-mounted boresight video camera. The airplane wake was viewed from a distance of approximately 1 km, and radar scanning was adjusted to cross a pair of marker smoke trails generated by the C-130. For each airplane pass, changes in radar reflectivity were calculated by subtracting the signal magnitudes during an initial clutter scan from the signal magnitudes during vortex-plus-clutter scans. The results showed both increases and decreases in reflectivity on and near the smoke trails in a characteristic sinusoidal pattern of heightened reflectivity in the center and lessened reflectivity at the sides. Reflectivity changes in either direction varied from -131 to -102 dBm(exp -1); the vortex-plus-clutter to noise ratio varied from 20 to 41 dB. The radar recordings lasted 2.5 min each; evidence of wake vortices was found for up to 2 min after the passage of the airplane. Ground and aircraft clutter were eliminated as possible sources of the disturbance by noting the occurrence of vortex signatures at different positions relative to the ground and the airplane. This work supports the feasibility of vortex detection by radar, and it is recommended that future radar vortex detection be done with Doppler systems.

Mackenzie, Anne I.

1997-01-01

378

Wake interference behind two flat plates normal to the flow - A finite-element study  

NASA Technical Reports Server (NTRS)

A finite-element model of the Navier-Stokes equations is used for numerical simulation of flow past two normal flat plates arranged side by side at Reynolds number 80 and 160. The results from this simulation indicate that when the gap between the plates is twice the width of a single plate, the individual wakes of the plates behave independently, with the antiphase vortex shedding being dominant. At smaller gap sizes, the in-phase vortex shedding, with strong wake interaction, is favored. The gap flow in those cases becomes biased, with one of the wakes engulfing the other. The direction of the biased flow was found to be switching at irregular intervals, with the time histories of the indicative flow parameters and their power spectra resembling those of a chaotic system.

Behr, M.; Tezduyar, T. E.; Higuchi, H.

1991-01-01

379

The Wake of St. Vincent.  

NASA Astrophysics Data System (ADS)

The island of St. Vincent and the other Windward Islands in the southeastern Caribbean were chosen as a field site for the study of weak mountain wakes. By the authors' definition, a `weak wake' forms when the potential vorticity generated by a mountain is not strong enough to advect itself into eddies; rather, it is simply advected downstream by the ambient flow. GOES-8 and Landsat sunglint images unambiguously revealed that the mountainous Windward Islands have remarkably long straight wakes. The length of St. Vincent's wake exceeds 300 km although its width is only 20 km. Near the islands, the wake structures reflect the details of the island topography. These wakes do not exhibit any obvious diurnal effect.Boat surveys in the lee of St. Vincent confirmed the existence of features seen in the images: the sharp wake boundary, the small valley-induced jet embedded in the near wake, and the absence of any reverse flow. Aircraft surveys gave evidence of descent over the island and showed that the wake air is relatively warm and dry. The length of the wake (L) agrees with the formula L = H/2CD (where H is the wake depth and CD is the surface drag coefficient), implying that the reacceleration of the wake air is caused by the ambient streamwise pressure gradient rather than by lateral entrainment of momentum or geostrophic adjustment.Two numerical models were used to simulate St. Vincent's wake, a single-layer hydrostatic model and a 3D nonhydrostatic model. Both models indicated that air descent, acceleration, wave breaking, and weak potential vorticity generation occur over the island, causing a long straight wake.

Smith, Ronald B.; Gleason, Arthur C.; Gluhosky, Paul A.; Grubii, Vanda

1997-03-01

380

NASA aircraft trailing vortex research  

NASA Technical Reports Server (NTRS)

A brief description is given of NASA's comprehensive program to study the aircraft trailing vortex problem. Wind tunnel experiments are used to develop the detailed processes of wing tip vortex formation and explore different means to either prevent trailing vortices from forming or induce early break-up. Flight tests provide information on trailing vortex system behavior behind large transport aircraft, both near the ground, as in the vicinity of the airport, and at cruise/holding pattern altitudes. Results from some flight tests are used to show how pilots might avoid the dangerous areas when flying in the vicinity of large transport aircraft. Other flight tests will be made to verify and evaluate trailing vortex elimination schemes developed in the model tests. Laser Doppler velocimeters being developed for use in the research program and to locate and measure vortex winds in the airport area are discussed. Field tests have shown that the laser Doppler velocimeter measurements compare well with those from cup anemometers.

Mcgowan, W. A.

1971-01-01

381

Exploration of the vortex wake behind of wind turbine rotor  

Microsoft Academic Search

The present paper describes a wind tunnel study of flow downstream a small horizontal axis wind turbine (HAWT). The experimental investigations were carried out with the use of particle image velocimetry (PIV). To obtain the flow field in the rotating frame of reference, the phase-locked technique was applied. Explorations were carried out in azimuth planes with different angles. The 3D

F Massouh; I Dobrev

2007-01-01

382

Instabilities of a cylinder wake in a stratified fluid  

NASA Astrophysics Data System (ADS)

The goal of this study is to analyse experimentally, numerically and theoretically how a linear density stratification modifies a cylinder wake, which is well known to exhibit a rich dynamics in a homogeneous fluid. In a first part, we focus on the 2D dynamics of the wake. We show that the von Karman vortex street is stabilised by a moderate stratification for tilted and horizontal cylinders, in agreement with the stabilisation of shear flows at large Richardson numbers. However, it is curious to see that the von Karman vortices reappear for a strong stratification in the case of a tilted cylinder. This new unstable mode can be explained by the presence of tilted vortices with no vertical velocity, i.e. with horizontal streamlines. In a second part, we focus on the 3D instabilities of the cylinder wake. For a vertical cylinder, the well known mode A can be nicely visualised by shadowgraph and seems to be enhanced by a moderate stratification. For a tilted cylinder, the structure of the instability is strongly modified, with the presence of thin undulated dark lines in the shadowgraph images. These structures are similar to the Kelvin-Helmholtz billows which have been observed recently in the critical layer of a tilted stratified vortex.

Meunier, Patrice

2011-11-01

383

Brain Wake-Ups  

NSDL National Science Digital Library

Wake-Up_Brain - Fire up those synapses each Monday morning. It's Monday morning and caffeine is slowly percolating into your system but your brain is still covered with weekend sludge. You need something to get those synapses firing, a brain booster to stimulate those billions of gray matter cells. You need Good Morning Thinkers! ... an absolutely free brain wake-up service offered to you by the Innovative Thinking Network, a professional membership association of leaders forging the revitalization of organizations through the powerful use of Innovation, Creativity and Group Thinking Skills. Every Monday morning subscribers receive a short, light-hearted message designed to help wipe away the fog and open the door to more powerful, creative thinking.

1997-01-01

384

Tip Vortex Development of an Adaptive Arc Airfoil  

NASA Astrophysics Data System (ADS)

The roll-up and development of the wing-tip vortex from an adaptive circular arc airfoil is observed experimentally. A piezoceramic actuator with a steel substrate serves as the adaptive airfoil. The baseline (zero potential) radius of curvature Rc of the airfoil is 16 cm. Rc is varied by changing the voltage applied across the arc airfoil. A maximum input voltage of ± 600 V varies Rc ± 2% resulting in a maximum change in lift coefficient Cl of approximately 7%. Rc can be varied at a frequency f up to 20 kHz. Maximum ? Rc occurs in the range f<25 Hz. This allows a simple method to examine the effect of unsteady Cl on tip vortex development. The arc airfoil is mounted as a wing semi-span in a subsonic wind tunnel and observed at a chord-based Reynolds number from 1.0 \\cdot 10^4 <= Rec <= 1.0 \\cdot 10^5. A helium filled soap bubble generator is used to create tracer particles for DPIV to examine the unsteady wake and a 7-hole Pitot probe is used to measure the steady-state wake. The wing-tip vortex is observed up to 20 chord-lengths downstream for varying f. The phenomenon of aero-elestic coupling between unsteady pressure forces and piezoelectric forces is explored. Potential application to wake vortex alleviation is also discussed.

Jou Pern, Nan; Jacob, J. D.

1998-11-01

385

Aircraft wake turbulence avoidance  

NASA Technical Reports Server (NTRS)

Analytical studies and flight tests are used to describe the formation and severity of trailing vortices and the spatial extent of their influence. This information is then used to outline procedures for ready application by pilots, tower operators, and others concerned with the flow of traffic. The procedures provide the necessary appreciation of the physical attributes of trailing vortices, the potential hazards involved when encountering them, and how best to avoid the dangerous portions of the wake during flight operations.

Mcgowan, W. A.

1971-01-01

386

Field Dependence of Magnetic Vortex Dynamics  

NASA Astrophysics Data System (ADS)

We have used time-resolved Kerr microscopy (TRKM) to investigate the dynamical behavior of micron diameter disks patterned from sputtered Permalloy (Py) films. Different growth conditions yielded grain diameters of ˜35 nm and ˜80 nm while average roughness (Ra) remained less than 1 nm for 50 nm thick films. The magnetization of the disks relaxes into a vortex ground state, in which broadband spin dynamics include a low frequency vortex translational mode (vortex mode) that is expected to be nearly independent of field, based on simulations and analytical theory. We measured the field dependence of the vortex dynamics of individual disks using 5 Oe field steps from 0 Oe through the vortex annihilation field (Ha). For a 1 ?m diameter disk the vortex mode has a mean frequency of ˜300 MHz, but the frequency fluctuates throughout the entire field range (Ha˜ 350 Oe) with a magnitude ?f˜200 MHz and a characteristic period ˜30 Oe. The fluctuations are not symmetric about zero field and look different in detail for different disks, but are highly repeatable for the same disk. We have also observed non-linear effects including the presence of up to 3 higher harmonics of the vortex mode, with a higher harmonic occasionally dominating the spectrum. A consistent interpretation is that the vortex core samples a distribution of pinning potentials, some of which are anharmonic, as it traverses the disk under the influence of the static applied field. Supported by NSF DMR 04-06029.

Compton, R. L.; Park, J. P.; Crowell, P. A.

2006-03-01

387

Vortex loops and Majoranas  

SciTech Connect

We investigate the role that vortex loops play in characterizing eigenstates of interacting Majoranas. We give some general results and then focus on ladder Hamiltonian examples as a test of further ideas. Two methods yield exact results: (i) A mapping of certain spin Hamiltonians to quartic interactions of Majoranas shows that the spectra of these two examples coincide. (ii) In cases with reflection-symmetric Hamiltonians, we use reflection positivity for Majoranas to characterize vortices in the ground states. Two additional methods suggest wider applicability of these results: (iii) Numerical evidence suggests similar behavior for certain systems without reflection symmetry. (iv) A perturbative analysis also suggests similar behavior without the assumption of reflection symmetry.

Chesi, Stefano [Department of Physics, McGill University, Montreal, Quebec H3A 2T8 (Canada) [Department of Physics, McGill University, Montreal, Quebec H3A 2T8 (Canada); CEMS, RIKEN, Wako, Saitama 351-0198 (Japan); Jaffe, Arthur [Harvard University, Cambridge, Massachusetts 02138 (United States) [Harvard University, Cambridge, Massachusetts 02138 (United States); Department of Physics, University of Basel, Basel (Switzerland); Institute for Theoretical Physics, ETH Zürich, Zürich (Switzerland); Loss, Daniel [CEMS, RIKEN, Wako, Saitama 351-0198 (Japan) [CEMS, RIKEN, Wako, Saitama 351-0198 (Japan); Department of Physics, University of Basel, Basel (Switzerland); Pedrocchi, Fabio L. [Department of Physics, University of Basel, Basel (Switzerland)] [Department of Physics, University of Basel, Basel (Switzerland)

2013-11-15

388

Organized structures and their interactions in the turbulent wake of circular cylinders  

NASA Astrophysics Data System (ADS)

This thesis presents an experimental study of organized structures in the turbulent near wakes of single, two and three side-by-side circular cylinders, covering four topics. Firstly, the relationships between logitudinal (ribs) and spanwise vortical structures in the turbulent near wake of a circular cylinder were investigated. Three components of vorticity in the wake of a circular cylinder were measured simultaneously with a combination of four X-wire probes. Secondly, interactions between spanwise vortical structures, momentum and heat transport in the turbulent wake of two side-by-side circular cylinders were studied. The results point to a significant effect of T/ d on the topology and transport characteristics. The streamwise evolution of the vortex street is also discussed in detail. Thirdly, interaction between turbulent simple wakes is determined. Using the same three-wire probe mentioned above, velocity and temperature fluctuations were measured in the turbulent wakes generated by single, two and three side-by-side circular cylinders (one of them was heated slightly). The superposition hypothesis, as proposed by Bradshaw and his co-workers, is also examined for the present complex wakes. Finally, the effects of unequal cylinder spacing on the vortex streets behind the three side-by-side cylinders were studied. Various techniques were used in this experimental study, including hot-wire technique, laser Doppler anemometry (LDA) and laser-illuminated flow visualization the equally and unequally spaced cylinders in terms of the pressure around the cylinders, drag, lift, dominant frequencies, vortex formation and downstream evolution. The flow topology (patterns) is also discussed in detail. (Abstract shortened by UMI.)

Zhang, Hong-Jun

389

A Numerical Model of Unsteady, Subsonic Aeroelastic Behavior. Ph.D Thesis  

NASA Technical Reports Server (NTRS)

A method for predicting unsteady, subsonic aeroelastic responses was developed. The technique accounts for aerodynamic nonlinearities associated with angles of attack, vortex-dominated flow, static deformations, and unsteady behavior. The fluid and the wing together are treated as a single dynamical system, and the equations of motion for the structure and flow field are integrated simultaneously and interactively in the time domain. The method employs an iterative scheme based on a predictor-corrector technique. The aerodynamic loads are computed by the general unsteady vortex-lattice method and are determined simultaneously with the motion of the wing. Because the unsteady vortex-lattice method predicts the wake as part of the solution, the history of the motion is taken into account; hysteresis is predicted. Two models are used to demonstrate the technique: a rigid wing on an elastic support experiencing plunge and pitch about the elastic axis, and an elastic wing rigidly supported at the root chord experiencing spanwise bending and twisting. The method can be readily extended to account for structural nonlinearities and/or substitute aerodynamic load models. The time domain solution coupled with the unsteady vortex-lattice method provides the capability of graphically depicting wing and wake motion.

Strganac, Thomas W.

1987-01-01

390

Wake flowfields for Jovian probe  

NASA Technical Reports Server (NTRS)

The wake flow field developed by the Galileo probe as it enters the Jovian atmosphere was modeled. The wake produced by the probe is highly energetic, yielding both convective and radiative heat inputs to the base of the probe. A component mathematical model for the inviscid near and far wake, the viscous near and far wake, and near wake recirculation zone was developed. Equilibrium thermodynamics were used for both the ablation and atmospheric species. Flow fields for three entry conditions were calculated. The near viscous wave was found to exhibit a variable axial pressure distribution with the neck pressure approximately three times the base pressure. Peak wake flow field temperatures were found to be in proportion to forebody post shock temperatures.

Engel, C. D.; Hair, L. M.

1980-01-01

391

Vortex Buckyball  

NASA Astrophysics Data System (ADS)

Buckminsterfullerene, C60, is a molecule with perfect symmetry made up of 60 carbon atoms arranged in the shape of a soccer ball and resembling a geodesic dome. The hexagonal and pentagonal patches making up the soccer ball are sewn together such that there are exactly 60 vertices with 3 edges intersecting at each vertex. Geometric structure of the C60 molecule is that of a truncated icosahedron with a single carbon atom occupying each vertex. Such a structure is obtained from an icosahedron by truncating each of the 12 vertices, resulting in a 5-membered ring at the location of each vertex and a 6-membered ring corresponding to each icosahedral face. In this paper, it will be shown that 60 point vortex atoms corresponding to the buckyball configuration undergo spontaneous clustering, each cluster undergoing periodic motion with characteristic frequency proportional to number of co-orbiting vortices. Generalizing the frequency relation for an isolated ring, the interacting clusters are shown to modify the ring frequencies such that the latter can be scale-fitted by an r^-a relation where r is the radius of the periodic orbit and 'a' is a constant.

Khushalani, Bharat

2004-05-01

392

B-747 in Flight during Vortex Study  

NASA Technical Reports Server (NTRS)

In this 1974 NASA Flight Research Center photograph, a Boeing B-747 jetliner is shown taking part in the trailing wake vortex study. In the photograph, the two wing tip vortex trails, being the strongest, stay in tight cylindrical rolls. The 'strength' of the vortices decreases toward the midspan of each wing, and the trails become less defined. In 1974 the NASA Flight Research Center (later Dryden Flight Research Center, Edwards, California) used a Boeing 747 as part of the overall NASA study of trailing vortices. Trailing vortices are the invisible flow of spiraling air that trails from the wings of large aircraft and can 'upset' smaller aircraft flying behind them. The 747 that NASA used was on loan from the Johnson Space Center where it was part of the Space Shuttle Program. The data gathered in the 747 studies complemented data from the previous (1973-74) joint NASA Flight Research Center and Federal Aviation Administration (FAA) Boeing727 wake vortices study. Six smoke generators were installed under the wings of the 747 to provide a visual image of the trailing vortices. The object of the experiments was to test different configurations and mechanical devices on the747 that could be used to break up or lessen the strength of the vortices. The results of the tests could lead to shorter spacing between landings and takeoffs, which, in turn, could alleviate air-traffic congestion. For approximately 30 flights the 747 was flown using various combinations of wing air spoilers in an attempt to reduce wake vortices. To evaluate the effectiveness of the different configurations, chase aircraft were flown into the vortex sheets to probe their strengths and patterns at different times. Two of the chase planes used were the Flight Research Center's Cessna T-37 and the NASA Ames Research Center's Learjet. These aircraft represented the types of smaller business jets and other s