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1

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

2

Vortex interactions and decay in aircraft wakes  

NASA Technical Reports Server (NTRS)

The dynamic interaction of aircraft wake vortices was investigated using both inviscid and viscous models. For the viscous model, a computer code was developed using a second-order closure model of turbulent transport. The phenomenon of vortex merging which results in the rapid aging of a vortex wake was examined in detail. It was shown that the redistribution of vorticity during merging results from both convective and diffusive mechanisms.

Bilanin, A. J.; Teske, M. E.; Dupdonaldson, C.; Williamson, G. G.

1977-01-01

3

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

4

Effects of fog droplets on wake vortex decay rate  

NASA Technical Reports Server (NTRS)

A simple model for the motion of particles in a laminar line vortex is discussed. The energy required to accelerate a set of these particles was determined and shown to be only a small fraction of the energy content of the vortex flow. It is shown that this energy transfer is unlikely to be sufficient to significantly modify the vortex decay rate. It is further argued that the effect of the particle on the viscous properties of the resulting two phase fluid leads to a slower decay rate than in single phase air flow. However, this conclusion may not necessarily follow for turbulence flows. Results show that the migration of particles to the outer flow results in a redistribution of the velocity profile in the vortex and in a non-uniform two phase viscosity across the core. It is suggested that these effects may accelerate vortex bursting.

Moulden, T. H.; Frost, W.

1976-01-01

5

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

6

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

7

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

8

Passive Wake Vortex Control  

SciTech Connect

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 operations, which occurred in deep-water environments, future submarine conflicts are anticipated to occur in shallow, littoral regions that are complex and noisy. Consequently, non-acoustic signatures will become increasingly important and the submarine stealth technology designed for deep-water operations may not be effective in these environments. One such non-acoustic signature is the surface detection of a submarine's trailing vortex wake. If a submarine runs in a slightly buoyant condition, its diving planes must be inclined at a negative angle of attack to generate sufficient downforce, which keeps the submarine from rising to the surface. As a result, the diving planes produce a pair of counter-rotating trailing vortices that propagate to the water surface. In previous deep-water operations, this was not an issue since the submarines could dive deep enough so that the vortex pair became incoherent before it reached the water surface. However, in shallow, littoral environments, submarines do not have the option of diving deep and, hence, the vortex pair can rise to the surface and leave a distinct signature that might be detectable by synthetic aperture radar. Such detection would jeopardize not only the mission of the submarine, but also the lives of military personnel on board. There has been another attempt to solve this problem and reduce the intensity of trailing vortices in the wakes of military submarines. The research of Quackenbush et al. over the past few years has been directed towards an idea called ''vortex leveraging.'' This active concept works by placing shape memory alloy (SMA) control surfaces on the submarine's diving planes and periodically oscillating them. The modulated control vortices generated by these surfaces interact with the tip vortices on the diving planes, causing an instability to rapidly occur. Though several numerical simulations have been presented, experimental verification does not appear to be available in the open literature. The authors address this problem through a concept called passive wake vortex control (PWVC), which has been demonstrated to rapidly break apart a trailing vortex wake and render it incoherent. PWVC functions by introducing unequal strength, counter-rotating control vortices next to the tip vortices. The presence of these control vortices destabilizes the vortex wake and produces a rapidly growing wake instability.

Ortega, J M

2001-10-18

9

Wake Vortex Sensors Requirements Overview  

NASA Technical Reports Server (NTRS)

The presentation includes discussions of primary wake vortex system requirements, evolution models, sensor evolution, site specific sensor tradeoffs, wake sensor functions, deployment considerations, the operational test bed system and additional sensor requirements.

Hinton, David A.

1997-01-01

10

NASA Wake Vortex Research for Aircraft Spacing  

NASA Technical Reports Server (NTRS)

The National Aeronautics and Space Administration (NASA) is addressing airport capacity enhancements during instrument meteorological conditions through the Terminal Area Productivity (TAP) program. Within TAP, the Reduced Spacing Operations (RSO) subelement at the NASA Langley Research Center is developing an Aircraft Vortex Spacing System (AVOSS). AVOSS will integrate the output of several inter-related areas to produce weather dependent, dynamic wake vortex spacing criteria. These areas include current and predicted weather conditions, models of wake vortex transport and decay in these weather conditions, real-time feedback of wake vortex behavior from sensors, and operationally acceptable aircraft/wake interaction criteria. In today's ATC system, the AVOSS could inform ATC controllers when a fixed reduced separation becomes safe to apply to large and heavy aircraft categories. With appropriate integration into the Center/TRACON Automation System (CTAS), AVOSS dynamic spacing could be tailored to actual generator/follower aircraft pairs rather than a few broad aircraft categories.

Perry, R. Brad; Hinton, David A.; Stuever, Robert A.

1996-01-01

11

Wake-Vortex Hazards During Cruise  

NASA Technical Reports Server (NTRS)

Even though the hazard posed by lift-generated wakes of subsonic transport aircraft has been studied extensively for approach and departure at airports, only a small amount of effort has gone into the potential hazard at cruise altitude. This paper reports on a studio of the wake-vortex hazard during cruise because encounters may become more prevalent when free-flight becomes available and each aircraft, is free to choose its own route between destinations. In order to address the problem, the various fluid-dynamic stages that vortex wakes usually go through as they age will be described along with estimates of the potential hazard that each stage poses. It appears that a rolling-moment hazard can be just as severe at cruise as for approach at airports, but it only persists for several minutes. However, the hazard posed by the downwash in the wake due to the lift on the generator aircraft persists for tens of minutes in a long narrow region behind the generating aircraft. The hazard consists of severe vertical loads when an encountering aircraft crosses the wake. A technique for avoiding vortex wakes at cruise altitude will be described. To date the hazard posed by lift-generated vortex wakes and their persistence at cruise altitudes has been identified and subdivided into several tasks. Analyses of the loads to be encounter and are underway and should be completed shortly. A review of published literature on the subject has been nearly completed (see text) and photographs of vortex wakes at cruise altitudes have been taken and the various stages of decay have been identified. It remains to study and sort the photographs for those that best illustrate the various stages of decay after they are shed by subsonic transport aircraft at cruise altitudes. The present status of the analysis and the paper are described.

Rossow, Vernon J.; James, Kevin D.; Nixon, David (Technical Monitor)

1998-01-01

12

Evaluation of Fast-Time Wake Vortex Models using Wake Encounter Flight Test Data  

NASA Technical Reports Server (NTRS)

This paper describes a methodology for the integration and evaluation of fast-time wake models with flight data. The National Aeronautics and Space Administration conducted detailed flight tests in 1995 and 1997 under the Aircraft Vortex Spacing System Program to characterize wake vortex decay and wake encounter dynamics. In this study, data collected during Flight 705 were used to evaluate NASA's fast-time wake transport and decay models. Deterministic and Monte-Carlo simulations were conducted to define wake hazard bounds behind the wake generator. The methodology described in this paper can be used for further validation of fast-time wake models using en-route flight data, and for determining wake turbulence constraints in the design of air traffic management concepts.

Ahmad, Nashat N.; VanValkenburg, Randal L.; Bowles, Roland L.; Limon Duparcmeur, Fanny M.; Gloudesman, Thijs; van Lochem, Sander; Ras, Eelco

2014-01-01

13

Wake Vortex Advisory System (WakeVAS) Concept of Operations  

NASA Technical Reports Server (NTRS)

NASA Langley Research Center has a long history of aircraft wake vortex research, with the most recent accomplishment of demonstrating the Aircraft VOrtex Spacing System (AVOSS) at Dallas/Forth Worth International Airport in July 2000. The AVOSS was a concept for an integration of technologies applied to providing dynamic wake-safe reduced spacing for single runway arrivals, as compared to current separation standards applied during instrument approaches. AVOSS included state-of-the-art weather sensors, wake sensors, and a wake behavior prediction algorithm. Using real-time data AVOSS averaged a 6% potential throughput increase over current standards. This report describes a Concept of Operations for applying the technologies demonstrated in the AVOSS to a variety of terminal operations to mitigate wake vortex capacity constraints. A discussion of the technological issues and open research questions that must be addressed to design a Wake Vortex Advisory System (WakeVAS) is included.

Rutishauser, David; Lohr, Gary; Hamilton, David; Powers, Robert; McKissick, Burnell; Adams, Catherine; Norris, Edward

2003-01-01

14

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

15

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

16

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

17

Investigation of aircraft vortex wake structure  

NASA Astrophysics Data System (ADS)

In this work we analyze the mechanisms of formation of the vortex wake structure of aircraft with different wing shape in the plan flying close to or away from the underlying surface cleaned or released mechanization wing.

Baranov, N. A.; Turchak, L. I.

2014-11-01

18

A flight evaluation of methods for predicting vortex wake effects on trailing aircraft  

NASA Technical Reports Server (NTRS)

The results of four current analytical methods for predicting wing vortex strength and decay rate are compared with the results of a flight investigation of the wake characteristics of several large jet transport aircraft. An empirical expression defining the strength and decay rate of wake vortices is developed that best represents most of the flight-test data. However, the expression is not applicable to small aircraft that would be immersed in the vortex wake of large aircraft.

Robinson, G. H.; Larson, R. R.

1972-01-01

19

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

20

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

21

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

22

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

23

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

24

Recent NASA Wake-Vortex Flight Tests, Flow-Physics Database and Wake-Development Analysis  

NASA Technical Reports Server (NTRS)

A series of flight tests over the ocean of a four engine turboprop airplane in the cruise configuration have provided a data set for improved understanding of wake vortex physics and atmospheric interaction. An integrated database has been compiled for wake characterization and validation of wake-vortex computational models. This paper describes the wake-vortex flight tests, the data processing, the database development and access, and results obtained from preliminary wake-characterization analysis using the data sets.

Vicroy, Dan D.; Vijgen, Paul M.; Reimer, Heidi M.; Gallegos, Joey L.; Spalart, Philippe R.

1998-01-01

25

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

26

Passive propulsion in vortex wakes  

NASA Astrophysics Data System (ADS)

A dead fish is propelled upstream when its flexible body resonates with oncoming vortices formed in the wake of a bluff cylinder, despite being well outside the suction region of the cylinder. Within this passive propulsion mode, the body of the fish extracts sufficient energy from the oncoming vortices to develop thrust to overcome its own drag. In a similar turbulent wake and at roughly the same distance behind a bluff cylinder, a passively mounted high-aspect-ratio foil is also shown to propel itself upstream employing a similar flow energy extraction mechanism. In this case, mechanical energy is extracted from the flow at the same time that thrust is produced. These results prove experimentally that, under proper conditions, a body can follow at a distance or even catch up to another upstream body without expending any energy of its own. This observation is also significant in the development of low-drag energy harvesting devices, and in the energetics of fish dwelling in flowing water and swimming behind wake-forming obstacles.

Beal, D. N.; Hover, F. S.; Triantafyllou, M. S.; Liao, J. C.; Lauder, G. V.

27

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

28

Proceedings of the NASA First Wake Vortex Dynamic Spacing Workshop  

NASA Technical Reports Server (NTRS)

A Government and Industry workshop on wake vortex dynamic spacing systems was conducted on May 13-15, 1997, at the NASA Langley Research Center. The purpose of the workshop was to disclose the status of ongoing NASA wake vortex R&D to the international community and to seek feedback on the direction of future work to assure an optimized research approach. Workshop sessions examined wake vortex characterization and physics, wake sensor technologies, aircraft/wake encounters, terminal area weather characterization and prediction, and wake vortex systems integration and implementation. A final workshop session surveyed the Government and Industry perspectives on the NASA research underway and related international wake vortex activities. This document contains the proceedings of the workshop including the presenters' slides, the discussion following each presentation, the wrap-up panel discussion, and the attendees' evaluation feedback.

Creduer, Leonard (Editor); Perry, R. Brad (Editor)

1997-01-01

29

Vortex shedding in high-speed compressor blade wakes  

NASA Technical Reports Server (NTRS)

The wakes of highly loaded compressor blades are generally considered to be turbulent flows. Recent work has suggested that the blade wakes are dominated by a vortex streetlike structure. The experimental evidence supporting the wake vortex structure is reviewed. This structure is shown to redistribute thermal energy within the flowfield. The effect of the wake structure on conventional aerodynamic measurements of compressor performance is noted. A two-dimensional, time-accurate, viscous numerical simulation of the flow exhibits both vortex shedding in the wake and a lower-frequency flow instability that modulates the shedding. The numerical results are shown to agree quite well with the measurement from transonic compressor rotors.

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

1988-01-01

30

An approximate model of vortex decay in the atmosphere  

NASA Technical Reports Server (NTRS)

An approximate analysis of atmospheric effects on wake vortex motion and decay is presented. The effects of density stratification, turbulence, and Reynolds number are combined in a single model so that the relative importance of different parameters can be determined. Predicted wake motion is shown to be in good agreement with limited data from both ground facility and flight test measurements taken under low turbulence conditions. Wake decay was found to depend strongly on both density stratification and turbulence. For typical levels of turbulence, wake decay was found to result from the 'Crow instability' except under strongly stratified conditions.

Greene, G. C.

1985-01-01

31

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

32

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

33

Dynamics of the vortex wakes of flying and swimming vertebrates.  

PubMed

The vortex wakes of flying and swimming animals provide evidence of the history of aero- and hydrodynamic force generation during the locomotor cycle. Vortex-induced momentum flux in the wake is the reaction of forces the animal imposes on its environment, which must be in equilibrium with inertial and external forces. In flying birds and bats, the flapping wings generate lift both to provide thrust and to support the weight. Distinct wingbeat and wake movement patterns can be identified as gaits. In flow visualization experiments, only two wake patterns have been identified: a vortex ring gait with inactive upstroke, and a continuous vortex gait with active upstroke. These gaits may be modelled theoretically by free vortex and lifting line theory to predict mechanical energy consumption, aerodynamic forces and muscle activity. Longer-winged birds undergo a distinct gait change with speed, but shorter-winged species use the vortex ring gait at all speeds. In swimming fish, the situation is more complex: the wake vortices form a reversed von Kármán vortex street, but little is known about the mechanism of generation of the wake, or about how it varies with speed and acceleration or with body form and swimming mode. An unresolved complicating factor is the interaction between the drag wake of the flapping fish body and the thrusting wake from the tail. PMID:8571221

Rayner, J M

1995-01-01

34

Wake Vortex Inverse Model User's Guide  

NASA Technical Reports Server (NTRS)

NorthWest Research Associates (NWRA) has developed an inverse model for inverting landing aircraft vortex data. The data used for the inversion are the time evolution of the lateral transport position and vertical position of both the port and starboard vortices. The inverse model performs iterative forward model runs using various estimates of vortex parameters, vertical crosswind profiles, and vortex circulation as a function of wake age. Forward model predictions of lateral transport and altitude are then compared with the observed data. Differences between the data and model predictions guide the choice of vortex parameter values, crosswind profile and circulation evolution in the next iteration. Iterations are performed until a user-defined criterion is satisfied. Currently, the inverse model is set to stop when the improvement in the rms deviation between the data and model predictions is less than 1 percent for two consecutive iterations. The forward model used in this inverse model is a modified version of the Shear-APA model. A detailed description of this forward model, the inverse model, and its validation are presented in a different report (Lai, Mellman, Robins, and Delisi, 2007). This document is a User's Guide for the Wake Vortex Inverse Model. Section 2 presents an overview of the inverse model program. Execution of the inverse model is described in Section 3. When executing the inverse model, a user is requested to provide the name of an input file which contains the inverse model parameters, the various datasets, and directories needed for the inversion. A detailed description of the list of parameters in the inversion input file is presented in Section 4. A user has an option to save the inversion results of each lidar track in a mat-file (a condensed data file in Matlab format). These saved mat-files can be used for post-inversion analysis. A description of the contents of the saved files is given in Section 5. An example of an inversion input file, with preferred parameters values, is given in Appendix A. An example of the plot generated at a normal completion of the inversion is shown in Appendix B.

Lai, David; Delisi, Donald

2008-01-01

35

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

36

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

37

An Experimental Study of Wake Vortex Instabilities Brian M. Babie  

E-print Network

poses a flight safety hazard to other aircraft that may encounter this wake. This flight safety threat of axial bending mode = azimuthal coordinate x = mean axial vorticity I. Introduction LL aircraft create a trailing vortex wake that may persist for several miles behind the generating aircraft, dependent

Nelson, Robert C.

38

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

39

Implementation of vortex wake control using SMA-actuated devices  

NASA Astrophysics Data System (ADS)

Mitigation of the undesirable effects of trailing vortex wakes has been a long-standing priority for both reduction of submarine wake signature and alleviation of aircraft vortex wake hazard. A recent study established the feasibility of using relatively weak, secondary vortices with carefully selected unsteady amplitude and phasing to accelerate the breakup of the primary vortex system of a lifting surface, a technique denoted `vortex leveraging'. This paper will summarize progress on the development of SMA-actuated devices for implementing vortex leveraging for hydrodynamic applications. The methods being applied to the hydrodynamic design of these deformable Smart Vortex Leveraging Tabs (SVLTs) will be described, and the results of a preliminary assessment of SVLT performance in achieving wake breakup will be presented. Also, previous work on the design and testing of deformable control surfaces actuated via embedded SMA agonist wires will be reviewed and the design process being employed in the present applications will be discussed. Finally, the plans for near-term computational and experimental work to validate the use of SMA-driven devices for the wake mitigation task will be briefly outlined.

Quackenbush, Todd R.; Bilanin, Alan J.; Batcho, P. F.; McKillip, Robert M., Jr.; Carpenter, Bernie F.

1997-05-01

40

Numerical Study of a Long-Lived, Isolated Wake Vortex in Ground Effect  

NASA Technical Reports Server (NTRS)

This paper examines a case observed during the 1990 Idaho Falls Test program, in which a wake vortex having an unusually long lifetime was observed while in ground effect. A numerical simulation is performed with a Large Eddy Simulation model to understand the response of the environment in affecting this event. In the simulation, it was found that one of the vortices decayed quickly, with the remaining vortex persisting beyond the time-bound of typical vortex lifetimes. This unusual behavior was found to be related to the first and second vertical derivatives of the ambient crosswind.

Proctor, Fred H.

2014-01-01

41

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

42

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

43

Civil aircraft vortex wake. TsAGI's research activities  

NASA Astrophysics Data System (ADS)

This paper provides a review of research conducted in TsAGI (Central AeroHydrodynamic Institute) concerning a vortex wake behind an airliner. The research into this area of theoretical and practical importance have been done both in Russia and in other countries, for which these studies became a vital necessity at the end of the 20th century. The paper describes the main methods and ratios on which software systems used to calculate the evolution of a vortex wake in a turbulent atmosphere are based. Verification of calculation results proved their acceptable consistency with the known experimental data. The mechanism of circulation loss in a vortex wake which is based on the analytical solution for the problem of two vortices diffusing in a viscous fluid is also described. The paper also describes the model of behavior of an aircraft which has deliberately or unintentionally entered a vortex wake behind another aircraft. Approximated results of calculations performed according to this model by means of artificial neural networks enabled the researchers to model the dynamics of an aircraft in a vortex wake on flight simulators on-line.

Chernyshev, S. L.; Gaifullin, A. M.; Sviridenko, Yu. N.

2014-11-01

44

Vortex wake alleviation studies with a variable twist wing  

NASA Technical Reports Server (NTRS)

Vortex wake alleviation studies were conducted in a wind tunnel and a water towing tank using a multisegmented wing model which provided controlled and measured variations in span load. Fourteen model configurations are tested at a Reynolds number of one million and a lift coefficient of 0.6 in the Langley 4- by 7-Meter Tunnel and the Hydronautics Ship Model Basin water tank at Hydronautics, Inc., Laurel, Md. Detailed measurements of span load and wake velocities at one semispan downstream correlate well with each other, with inviscid predictions of span load and wake roll up, and with peak trailing-wing rolling moments measured in the far wake. Average trailing-wing rolling moments are found to be an unreliable indicator of vortex wake intensity because vortex meander does not scale between test facilities and free-air conditions. A tapered-span-load configuration, which exhibits little or no drag penalty, is shown to offer significant downstream wake alleviation to a small trailing wing. The greater downstream wake alleviation achieved with the addition of spoilers to a flapped-wing configuration is shown to result directly from the high incremental drag and turbulence associated with the spoilers and not from the span load alteration they cause.

Holbrook, G. T.; Dunham, D. M.; Greene, G. C.

1985-01-01

45

Evaluation of a Wake Vortex Upset Model Based on Simultaneous Measurements of Wake Velocities and Probe-Aircraft Accelerations  

NASA Technical Reports Server (NTRS)

Simultaneous measurements were made of the upset responses experienced and the wake velocities encountered by an instrumented Learjet probe aircraft behind a Boeing 747 vortex-generating aircraft. The vortex-induced angular accelerations experienced could be predicted within 30% by a mathematical upset response model when the characteristics of the wake were well represented by the vortex model. The vortex model used in the present study adequately represented the wake flow field when the vortices dissipated symmetrically and only one vortex pair existed in the wake.

Short, B. J.; Jacobsen, R. A.

1979-01-01

46

The vortex street wakes of a rotating circular cylinder  

SciTech Connect

The evolution of the vortex stress behind a rotating circular cylinder in uniform free stream is investigated numerically at high Reynolds number. The ratio of the cylinder surface velocity to the free stream velocity, {alpha} is in the range 0 {le} {alpha} {le} 3. The method used to calculate the flow can be considered as a combination between the diffusion-vortex method and the vortex-in-cell method. The lift and drag forces exerted by the fluid on the cylinder surface as well as the Strouhal number of vortex shedding, are determined together with the flow patterns in the wake. It was found that the vortex shedding and wake development behind the cylinder vary significantly depending on the magnitude of the rotation parameter {alpha}. When {alpha} {le} 2, the vortex street behind the cylinder in the near-wake inclines as a whole towards the direction of rotation as {alpha} increases. The Karman vortex street structure begins to deteriorate as soon as {alpha} exceeds 2 and finally disappears for {alpha} = 3.

Cheng, M.; Chew, Y.T.; Luo, S.C. [National Univ. of Singapore (Singapore)

1994-12-31

47

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

48

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

49

Measurements of Aircraft Wake Vortex Separation at High Arrival Rates and a Proposed New Wake Vortex Separation Philosophy  

NASA Technical Reports Server (NTRS)

This paper presents data and a proposed new aircraft wake vortex separation standard that argues for a fundamental re-thinking of international practice. The current static standard, under certain atmospheric conditions, presents an unnecessary restriction on system capacity. A new approach, that decreases aircraft separation when atmospheric conditions dictate, is proposed based upon the availability of new instrumentation and a better understanding of wake physics.

Rutishauser, David; Donohue, George L.; Haynie, Rudolph C.

2003-01-01

50

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

51

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

52

ASSESSMENT OF WAKE VORTEX SEPARATION DISTANCES USING THE WAVIR TOOLSET  

E-print Network

methods, designed for evaluation of technologies/products, such as the Fault Tree Analysis (FTA of the system behavior, and the absence of historic data for rare-events [7, 8]. Qualitative safety methods a quantitative method for wake vortex safety analysis (WAVIR), 0-7803-8539-X/04/$20.00 © 2004 IEEE 2.E.2-1 #12

53

Far Wake Rotorcraft Vortex Tumbling James H. Stephenson  

E-print Network

-scale, 1 m diame- ter, four-bladed rotor during hover is studied using vortex methods combined during hover conditions. The nominal operating condition of the rotor is at a rotational speed of 1520RPM validation of CFD codes. I. Introduction The aerodynamic wake of a helicopter in hover is highly complex

Tinney, Charles E.

54

Development of a rotor wake-vortex model, volume 1  

NASA Technical Reports Server (NTRS)

Certain empirical rotor wake and turbulence relationships were developed using existing low speed rotor wave data. A tip vortex model was developed by replacing the annulus wall with a row of image vortices. An axisymmetric turbulence spectrum model, developed in the context of rotor inflow turbulence, was adapted to predicting the turbulence spectrum of the stator gust upwash.

Majjigi, R. K.; Gliebe, P. R.

1984-01-01

55

Transitions in the vortex wake behind the plunging profile  

NASA Astrophysics Data System (ADS)

In this study we investigate numerically the vortex wake formation behind the profile performing simple harmonic motion known in the literature as plunging. This research was inspired by the flapping motion which is appropriate for birds, insects and fishes. We assume the two dimensional model of flow. Depending on the parameters such as plunging amplitude, frequency and the Reynolds number, we demonstrate many different types of vortex street behind the profile. It is well known that the type of vortex wake determines the hydrodynamic forces acting on the profile. Dependences of the plunging amplitude, the Strouhal number and various topology vortices are established by constructing the phase transition diagram. The areas in the diagram related to the drag, thrust, and lift force generation are captured. We notice also the areas where the vorticity field is disordered. The disordered vorticity field does not allow maintenance of the periodic forces on the profile. An increase in the Reynolds number leads to the transition of the vortex wake behind the profile. The transition is caused by the phenomenon of boundary layer eruption. Further increase of the Reynolds number causes the vortex street related to the generation of the lift force to vanish.

Koz?owski, Tomasz; Kudela, Henryk

2014-12-01

56

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

57

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

58

Vortex dynamics in the wake of a mechanical fish  

NASA Astrophysics Data System (ADS)

This study focuses on the three-dimensional flow around a mechanical fish model, which reproduces the typical undulatory body and fin motion of a carangiform swimmer. The mechanical model consists of a flexible skeleton embedded in a soft transparent silicone body, which is connected with two cams to a flapping and bending hinge generating a traveling wave motion with increasing amplitude from anterior to posterior, extending to a combined heaving and pitching motion at the fin. The model is submerged in a water tank and towed at the characteristic swimming speed for the neutral swimming mode at U/V = 1. The method of Scanning Particle Image Velocimetry was used to analyze the three-dimensional time-dependent flow field in the axial and saggital planes. The results confirm the earlier observations that the wake develops into a chain of vortex rings which travel sidewards perpendicular to the swimming direction. However, instead of one single vortex shed at each tail beat half-cycle we observed a pair of two vortex rings being shed. Each pair consists of a larger main vortex ring corresponding to the tail beat start stop vortex, while the second vortex ring is due to the body bending motion. The existence of the second vortex reflects the role of the body in undulatory swimming. A simplified model of the fish body comparing it to a plate with a hinged flap demonstrates the link between the sequence of kinematics and vortex shedding.

Brücker, Christoph; Bleckmann, Horst

2007-11-01

59

Vortex dynamics in the wake of a mechanical fish  

NASA Astrophysics Data System (ADS)

This study focuses on the three-dimensional flow around a mechanical fish model, which reproduces the typical undulatory body and fin motion of a carangiform swimmer. The mechanical model consists of a flexible skeleton embedded in a soft transparent silicone body, which is connected with two cams to a flapping and bending hinge generating a traveling wave motion with increasing amplitude from anterior to posterior, extending to a combined heaving and pitching motion at the fin. The model is submerged in a water tank and towed at the characteristic swimming speed for the neutral swimming mode at U/V = 1. The method of Scanning Particle Image Velocimetry was used to analyze the three-dimensional time-dependent flow field in the axial and saggital planes. The results confirm the earlier observations that the wake develops into a chain of vortex rings which travel sidewards perpendicular to the swimming direction. However, instead of one single vortex shed at each tail beat half-cycle we observed a pair of two vortex rings being shed. Each pair consists of a larger main vortex ring corresponding to the tail beat start-stop vortex, while the second vortex ring is due to the body bending motion. The existence of the second vortex reflects the role of the body in undulatory swimming. A simplified model of the fish body comparing it to a plate with a hinged flap demonstrates the link between the sequence of kinematics and vortex shedding.

Brücker, Christoph; Bleckmann, Horst

60

Geometrical influence on vortex shedding in turbulent axisymmetric wakes  

NASA Astrophysics Data System (ADS)

We investigate the structures generated by the vortex shedding mechanism in turbulent axisymmetric wakes of non-axisymmetric plates, including a square plate and a series of fractal plates, and compare the results to a disk. For a given characteristic length ?, all plates have the same frontal area A, since ? = A0.5, but the length of the perimeter and the irregularity of the perimeter were varied in a fractal manner thus allowing us to investigate the effect of boundary conditions. Measurements were taken over a large range of downstream and radial distances in order to obtain a more robust measure for the vortex shedding energy. It was found that the fractal plates are able to reduce the vortex shedding energy by as much as 60% compared to the disk and square plates. It was also found that the frequency at which the vortex shedding structures are generated and the manner in which they organise themselves in the wake are independent of the boundary conditions of the wake generator. The results suggest that the main function of the multi-scale segments around the perimeter of the plates is to re-distribute the energy to a broader range of scales in the flow, which could explain the previously observed increase in drag coefficient for the fractal edged plates.

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

2015-03-01

61

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

62

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

63

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

64

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

65

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

66

A new methodology for free wake analysis using curved vortex elements  

NASA Technical Reports Server (NTRS)

A method using curved vortex elements was developed for helicopter rotor free wake calculations. The Basic Curve Vortex Element (BCVE) is derived from the approximate Biot-Savart integration for a parabolic arc filament. When used in conjunction with a scheme to fit the elements along a vortex filament contour, this method has a significant advantage in overall accuracy and efficiency when compared to the traditional straight-line element approach. A theoretical and numerical analysis shows that free wake flows involving close interactions between filaments should utilize curved vortex elements in order to guarantee a consistent level of accuracy. The curved element method was implemented into a forward flight free wake analysis, featuring an adaptive far wake model that utilizes free wake information to extend the vortex filaments beyond the free wake regions. The curved vortex element free wake, coupled with this far wake model, exhibited rapid convergence, even in regions where the free wake and far wake turns are interlaced. Sample calculations are presented for tip vortex motion at various advance ratios for single and multiple blade rotors. Cross-flow plots reveal that the overall downstream wake flow resembles a trailing vortex pair. A preliminary assessment shows that the rotor downwash field is insensitive to element size, even for relatively large curved elements.

Bliss, Donald B.; Teske, Milton E.; Quackenbush, Todd R.

1987-01-01

67

Radar monitoring of a wake vortex: Electromagnetic reflection of wake turbulence in clear air  

NASA Astrophysics Data System (ADS)

This article deals with X-band radar trial campaigns in 2006 and 2007 at Orly Airport, and in June 2008 at Paris-CDG Airport. An X-band Doppler radar has been deployed to assess short range (inferior to 2000 m) wake vortex monitoring capabilities in all weather conditions (dry and wet conditions). Recorded data have been correlated with electromagnetic and fluid mechanical models of wake turbulences for better and more accurate understanding of roll-up radar cross section (RCS) and Doppler signature.

Barbaresco, Frédéric; Meier, Uwe

2010-01-01

68

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

69

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

70

An Investigation of Candidate Sensor-Observable Wake Vortex Strength Parameters for the NASA Aircraft Vortex Spacing System (AVOSS)  

NASA Technical Reports Server (NTRS)

The counter-rotating pair of wake vortices shed by flying aircraft can pose a threat to ensuing aircraft, particularly on landing approach. To allow adequate time for the vortices to disperse/decay, landing aircraft are required to maintain certain fixed separation distances. The Aircraft Vortex Spacing System (AVOSS), under development at NASA, is designed to prescribe safe aircraft landing approach separation distances appropriate to the ambient weather conditions. A key component of the AVOSS is a ground sensor, to ensure, safety by making wake observations to verify predicted behavior. This task requires knowledge of a flowfield strength metric which gauges the severity of disturbance an encountering aircraft could potentially experience. Several proposed strength metric concepts are defined and evaluated for various combinations of metric parameters and sensor line-of-sight elevation angles. Representative populations of generating and following aircraft types are selected, and their associated wake flowfields are modeled using various wake geometry definitions. Strength metric candidates are then rated and compared based on the correspondence of their computed values to associated aircraft response values, using basic statistical analyses.

Tatnall, Chistopher R.

1998-01-01

71

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

72

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

73

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 hot-wire 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 velocity fluctuations accompany the periodic formation of vortices. 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. The transition first occurs in the range r=150 to 300. Spectrum and statistical measurements were made to study the velocity fluctuations.

Roshko, Anatol

1954-01-01

74

Development of a Wake Vortex Spacing System for Airport Capacity Enhancement and Delay Reduction  

NASA Technical Reports Server (NTRS)

The Terminal Area Productivity project has developed the technologies required (weather measurement, wake prediction, and wake measurement) to determine the aircraft spacing needed to prevent wake vortex encounters in various weather conditions. The system performs weather measurements, predicts bounds on wake vortex behavior in those conditions, derives safe wake spacing criteria, and validates the wake predictions with wake vortex measurements. System performance to date indicates that the potential runway arrival rate increase with Aircraft VOrtex Spacing System (AVOSS), considering common path effects and ATC delivery variance, is 5% to 12% depending on the ratio of large and heavy aircraft. The concept demonstration system, using early generation algorithms and minimal optimization, is performing the wake predictions with adequate robustness such that only 4 hard exceedances have been observed in 1235 wake validation cases. This performance demonstrates the feasibility of predicting wake behavior bounds with multiple uncertainties present, including the unknown aircraft weight and speed, weather persistence between the wake prediction and the observations, and the location of the weather sensors several kilometers from the approach location. A concept for the use of the AVOSS system for parallel runway operations has been suggested, and an initial study at the JFK International Airport suggests that a simplified AVOSS system can be successfully operated using only a single lidar as both the weather sensor and the wake validation instrument. Such a selfcontained AVOSS would be suitable for wake separation close to the airport, as is required for parallel approach concepts such as SOIA.

Hinton, David A.; OConnor, Cornelius J.

2000-01-01

75

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

E-print Network

Real-Time Visualization of Wake-Vortex Simulations using Computational Steering and Beowulf real-time application--wake-vortex simulations for multiple aircraft running on a parallel Beowulf computational steering system based on a client/server pro- gramming model. We demonstrate the effectiveness

76

Meteorology and Wake Vortex Influence on American Airlines FL-587 Accident  

NASA Technical Reports Server (NTRS)

The atmospheric environment surrounding the crash of American Airlines Flight 587 is investigated. Examined are evidence for any unusual atmospheric conditions and the potential for encounters with aircraft wake vortices. Computer simulations are carried out with two different vortex prediction models and a Large Eddy Simulation model. Wind models are proposed for studying aircraft and pilot response to the wake vortex encounter.

Proctor, Fred H.; Hamilton, David W.; Rutishauser, David K.; Switzer, George F.

2004-01-01

77

Shortle and Jeddi 1 Probabilistic Analysis of Wake Vortex Hazards for Landing Aircraft Using Multilateration  

E-print Network

Shortle and Jeddi 1 Probabilistic Analysis of Wake Vortex Hazards for Landing Aircraft Using revised from original submittal. #12;Shortle and Jeddi 2 ABSTRACT Wake vortices are a safety hazard to landing aircraft. A landing aircraft that encounters a wake may roll, resulting in a fatal crash

78

The decay of confined vortex rings  

NASA Astrophysics Data System (ADS)

Vortex rings are produced during the ejection of fluid through a nozzle or orifice, which occurs in a wide range of biological conditions such as blood flow through the valves of the heart or through arterial constrictions. Confined vortex ring dynamics, such as these, have not been previously studied despite their occurrence within the biological flow conditions mentioned. In this work, we investigate laminar vortex rings using particle image velocimetry and develop a new semi-empirical model for the evolution of vortex ring circulation subject to confinement. Here we introduce a decay parameter ? which exponentially grows with increasing vortex ring confinement ratio, the ratio of the vortex ring diameter ( D VR) to the confinement diameter ( D), with the relationship ?=4.38 exp(9.5D_VR/D), resulting in a corresponding increase in the rate of vortex ring circulation decay. This work enables the prediction of circulation decay rate based on confinement, which is important to understanding naturally occurring confined vortex ring dynamics.

Stewart, K. C.; Niebel, C. L.; Jung, S.; Vlachos, P. P.

2012-07-01

79

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

NASA Astrophysics Data System (ADS)

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.

2010-07-01

80

Two Dimensional Wake Vortex Simulations in the Atmosphere: Preliminary Sensitivity Studies  

NASA Technical Reports Server (NTRS)

A numerical large-eddy simulation model is currently being used to quantify aircraft wake vortex behavior with meteorological observables. The model, having a meteorological framework, permits the interaction of wake vortices with environments characterized by crosswind shear, stratification, and humidity. The addition of grid-scale turbulence as an initial condition appeared to have little consequence. Results show that conventional nondimensionalizations work very well for vortex pairs embedded in stably stratified flows. However, this result is based on simple environments with constant Brunt-Vaisala frequency. Results presented here also show that crosswind profiles exert important and complex interactions on the trajectories of wake vortices. Nonlinear crosswind profiles tended to arrest the descent of wake vortex pairs. The member of the vortex pair with vorticity of same sign as the vertical change in the ambient along-track vorticity may be deflected upwards.

Proctor, F. H.; Hinton, D. A.; Han, J.; Schowalter, D. G.; Lin, Y.-L.

1998-01-01

81

Free Wake Techniques for Rotor Aerodynamic Analylis. Volume 2: Vortex Sheet Models  

NASA Technical Reports Server (NTRS)

Results of computations are presented using vortex sheets to model the wake and test the sensitivity of the solutions to various assumptions used in the development of the models. The complete codings are included.

Tanuwidjaja, A.

1982-01-01

82

Wind turbine wake stability investigations using a vortex ring modelling approach  

NASA Astrophysics Data System (ADS)

In the present study, a simple inviscid vortex ring (VR) modelling approach is used to represent the developing rotor wake. This allows a straightforward investigation and comparison of the impact of uniform, yawed and sheared flow conditions on the development of the rotor wake, with the additional possibility of including ground effect. The effect of instabilities on the development of the wake is manually introduced in the form of perturbations of strength, ring position and size. The phenomenon of vortex filament interaction or leapfrogging, could play a role in the observation of unsteady phenomena and is therefore also addressed. Such a study is hence performed in light of recent conflicting views on the causes of wake meandering: is the observed dynamic wake behaviour a result of large scale turbulent forcing or do more subtle and intrinsic wake instabilities play a role? This study concludes that the presence of the ground and external perturbations, most notably changes in the wake pitch and the rotor thrust coefficient, can significantly affect the steady development of the wake. The mutual vortex pairing instability, whilst displaying interesting periodic behaviour, does not correlate with periodic wake behaviour reported by Medici et al. [1]. However, in the absence of unsteady inflow, it is shown that the wake of a Horizontal Axis Wind Turbine (HAWT) is certainly prone to displaying unstable, dynamic behaviour caused by these additional factors.

Baldacchino, Daniel; van Bussel, Gerard J. W.

2014-12-01

83

Development of a rotor wake/vortex model. Volume 2: User's manual for computer program  

NASA Technical Reports Server (NTRS)

The principal objective was to establish a verified rotor wake/vortex model for specific application to fan and compressor rotor-stator interaction and resulting noise generation. A description and flow chart of the Rotor Wake/Vortex Model computer program, a listing of the program, definitions of the input/output parameters, a sample input/output case, and input files for Rotor 55, the JT15D rotor, and Rotor 67, Stage 1 are provided.

Majjigi, R. K.; Gliebe, P. R.

1984-01-01

84

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

85

The role of vortex wake dynamics in the flow-induced vibration of tube arrays  

E-print Network

The role of vortex wake dynamics in the flow-induced vibration of tube arrays N.K.-R. Kevlahan Keywords: Fluid­structure interaction Vortex-induced vibration Tube arrays Potential flow a b s t r a c in the non-resonant flow-induced vibration of periodic tube arrays. This dual approach untangles the effects

Kevlahan, Nicholas

86

Three-dimensional vortex wake structure of flapping wings in hovering flight.  

PubMed

Flapping wings continuously create and send vortices into their wake, while imparting downward momentum into the surrounding fluid. However, experimental studies concerning the details of the three-dimensional vorticity distribution and evolution in the far wake are limited. In this study, the three-dimensional vortex wake structure in both the near and far field of a dynamically scaled flapping wing was investigated experimentally, using volumetric three-component velocimetry. A single wing, with shape and kinematics similar to those of a fruitfly, was examined. The overall result of the wing action is to create an integrated vortex structure consisting of a tip vortex (TV), trailing-edge shear layer (TESL) and leading-edge vortex. The TESL rolls up into a root vortex (RV) as it is shed from the wing, and together with the TV, contracts radially and stretches tangentially in the downstream wake. The downwash is distributed in an arc-shaped region enclosed by the stretched tangential vorticity of the TVs and the RVs. A closed vortex ring structure is not observed in the current study owing to the lack of well-established starting and stopping vortex structures that smoothly connect the TV and RV. An evaluation of the vorticity transport equation shows that both the TV and the RV undergo vortex stretching while convecting downwards: a three-dimensional phenomenon in rotating flows. It also confirms that convection and secondary tilting and stretching effects dominate the evolution of vorticity. PMID:24335561

Cheng, Bo; Roll, Jesse; Liu, Yun; Troolin, Daniel R; Deng, Xinyan

2014-02-01

87

Three-dimensional vortex wake structure of flapping wings in hovering flight  

PubMed Central

Flapping wings continuously create and send vortices into their wake, while imparting downward momentum into the surrounding fluid. However, experimental studies concerning the details of the three-dimensional vorticity distribution and evolution in the far wake are limited. In this study, the three-dimensional vortex wake structure in both the near and far field of a dynamically scaled flapping wing was investigated experimentally, using volumetric three-component velocimetry. A single wing, with shape and kinematics similar to those of a fruitfly, was examined. The overall result of the wing action is to create an integrated vortex structure consisting of a tip vortex (TV), trailing-edge shear layer (TESL) and leading-edge vortex. The TESL rolls up into a root vortex (RV) as it is shed from the wing, and together with the TV, contracts radially and stretches tangentially in the downstream wake. The downwash is distributed in an arc-shaped region enclosed by the stretched tangential vorticity of the TVs and the RVs. A closed vortex ring structure is not observed in the current study owing to the lack of well-established starting and stopping vortex structures that smoothly connect the TV and RV. An evaluation of the vorticity transport equation shows that both the TV and the RV undergo vortex stretching while convecting downwards: a three-dimensional phenomenon in rotating flows. It also confirms that convection and secondary tilting and stretching effects dominate the evolution of vorticity. PMID:24335561

Cheng, Bo; Roll, Jesse; Liu, Yun; Troolin, Daniel R.; Deng, Xinyan

2014-01-01

88

The role of vortex wake dynamics in the flow-induced vibration of tube arrays  

E-print Network

The role of vortex wake dynamics in the flow-induced vibration of tube arrays N.K.-R. Kevlahan­structure interaction Vortex-induced vibration Tube arrays Potential flow a b s t r a c t Potential flow and 2-D Navier­Stokes calculations are used to investigate the role of vortex shedding in the non-resonant flow-induced vibration

Kevlahan, Nicholas

89

Evolution and breakdown of helical vortex wakes behind a wind turbine  

NASA Astrophysics Data System (ADS)

The wake behind a three-bladed Glauert model rotor in a water channel was investigated. Planar particle image velocimetry was used to measure the velocity fields on the wake centre-line, with snapshots phase-locked to blade position of the rotor. Phase- locked averages of the velocity and vorticity fields are shown, with tip vortex interaction and entanglement of the helical filaments elucidated. Proper orthogonal decomposition and topology-based vortex identification are used to filter the PIV images for coherent structures and locate vortex cores. Application of these methods to the instantaneous data reveals unsteady behaviour of the helical filaments that is statistically quantifiable.

Nemes, A.; Sherry, M.; Lo Jacono, D.; Blackburn, H. M.; Sheridan, J.

2014-12-01

90

A free wake vortex lattice model for vertical axis wind turbines: Modeling, verification and validation  

NASA Astrophysics Data System (ADS)

Since the 1970s several research activities had been carried out on developing aerodynamic models for Vertical Axis Wind Turbines (VAWTs). In order to design large VAWTs of MW scale, more accurate aerodynamic calculation is required to predict their aero-elastic behaviours. In this paper, a 3D free wake vortex lattice model for VAWTs is developed, verified and validated. Comparisons to the experimental results show that the 3D free wake vortex lattice model developed is capable of making an accurate prediction of the general performance and the instantaneous aerodynamic forces on the blades. The comparison between momentum method and the vortex lattice model shows that free wake vortex models are needed for detailed loads calculation and for calculating highly loaded rotors.

Meng, Fanzhong; Schwarze, Holger; Vorpahl, Fabian; Strobel, Michael

2014-12-01

91

Initialization and Simulation of Three-Dimensional Aircraft Wake Vortices  

NASA Technical Reports Server (NTRS)

This paper studies the effects of axial velocity profiles on vortex decay, in order to properly initialize and simulate three-dimensional wake vortex flow. Analytical relationships are obtained based on a single vortex model and computational simulations are performed for a rather practical vortex wake, which show that the single vortex analytical relations can still be applicable at certain streamwise sections of three-dimensional wake vortices.

Ash, Robert L.; Zheng, Z. C.

1997-01-01

92

Design, fabrication, and test planning for an SMA-actuated vortex wake control system  

NASA Astrophysics Data System (ADS)

This paper describes ongoing design and fabrication work on a vortex wake control system for submarines that employs SMA-actuated devices. Previous work has described the theoretical basis and feasibility studies for this system, which is based on a novel wake control scheme known as vortex leveraging. The critical item in the realization of this system is a Smart Vortex Leveraging Tab (SVLT), whose design and fabrication is the principal focus of this work. This paper outlines the background of the effort and the design principles involved, but will chiefly deal with three closely interrelated topics; the hydrodynamic design requirements and control surface layout for the vortex leveraging system; the detail design and fabrication techniques being used in the construction of a prototype SVLT; and the test planning and experiment design process currently underway for test of both the overall vortex leveraging concept and SVLT device itself.

Quackenbush, Todd R.; Batcho, P. F.; Bilanin, Alan J.; Carpenter, Bernie F.

1998-06-01

93

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. PMID:14561347

Tytell, Eric D; Ellington, Charles P

2003-01-01

94

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

95

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

96

Correlation of turbulent trailing vortex decay data  

NASA Technical Reports Server (NTRS)

A correlation function, derived on the basis of self similar variable eddy viscosity decay, is introduced and utilized to correlate aircraft trailing vortex velocity data from ground and flight experiments. The correlation function collapses maximum tangential velocity data from scale model and flight tests to a single curve. The resulting curve clearly shows both the inviscid plateau and the downstream decay regions. A comparison between experimental data and numerical solution shows closer agreement with the variable eddy viscosity solution than the constant viscosity analytical solution.

Iversen, J. D.

1974-01-01

97

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

98

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

99

Near-wake vortex motions behind a circular cylinder at low Reynolds number  

Microsoft Academic Search

A topological point of view is taken to investigate vortex motions in the near-wake region of a circular cylinder, where the Taylor hypothesis does not hold. Three-dimensional flow fields in the wake-transition regime are constructed by synthesizing time-resolved PIV data obtained in several planes of view. The convection velocities of the Kármán and secondary vortices are evaluated from the trajectories

J. Sung; J. Y. Yoo

2003-01-01

100

The prediction of blade wake interaction noise based on a turbulent vortex model  

NASA Technical Reports Server (NTRS)

Blade wake interaction is defined as the broadband noise generated by the interaction of helicopter rotor blades with their own wake. Experimental observations have shown that this is a strong function of advance ratio and tip path plane angle. This paper describes how this noise source can be associated with the blade vortex interactions in the forward sector of the rotor. Measured levels of turbulence in the vortex core are used to predict the broadband noise levels with some success. However, more detailed information on the turbulence spectrum and the trajectory of the shed vortices is required before more accurate noise predictions can be made.

Glegg, Stewart A. L.

1989-01-01

101

Simulating Wake Vortex Detection with the Sensivu Doppler Wind Lidar Simulator  

NASA Technical Reports Server (NTRS)

In support of NASA's Atmospheric Environment Safety Technologies NRA research topic on Wake Vortex Hazard Investigation, Aerospace Innovations (AI) investigated a set of techniques for detecting wake vortex hazards from arbitrary viewing angles, including axial perspectives. This technical report describes an approach to this problem and presents results from its implementation in a virtual lidar simulator developed at AI. Threedimensional data volumes from NASA's Terminal Area Simulation System (TASS) containing strong turbulent vortices were used as the atmospheric domain for these studies, in addition to an analytical vortex model in 3-D space. By incorporating a third-party radiative transfer code (BACKSCAT 4), user-defined aerosol layers can be incorporated into atmospheric models, simulating attenuation and backscatter in different environmental conditions and altitudes. A hazard detection algorithm is described that uses a twocomponent spectral model to identify vortex signatures observable from arbitrary angles.

Ramsey, Dan; Nguyen, Chi

2014-01-01

102

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

103

Elliptic tip effects on the vortex wake of an axisymmetric body at incidence  

NASA Astrophysics Data System (ADS)

The effectiveness of a new version of an elliptic cross-section tip in controlling the asymmetry of the vortex wake of an axisymmetric body at angle of attack has been studied. The elliptic cross sections were generated using two sixth-degree polynomials such that the tip radius, slope, and curvature would match those of a right circular cone at the point where the polynomial became tangent to a cone generator. The tip was found to be effective in varying the vortex wake geometry of a right circular cone at large angle of attack. The measured side force coefficient varied smoothly with tip roll angle for the two lowest angles of attack studied and exhibited square-wave and more undesirable variations for the larger angles of attack studied. These square-wave and peak, reduction-in-magnitude, and change-in-sign variations were caused by vortex breakaway, which allowed vortex crossover to occur. Ahead of vortex breakaway, the elliptic cross-section tip yielded smooth variations of vortex wake asymmetry with tip roll angle, indicating that the tip would possibly be a feasible control device for high-performance fighter aircraft at high angle of attack.

Bridges, David H.; Hornung, Hans G.

1994-07-01

104

On the spanwise variation of vortex shedding in the wake of slender cones  

NASA Astrophysics Data System (ADS)

The present work aims to study the spanwise variation of vortex shedding in the wake of slender cones at low Reynolds numbers. The occurrence of modulated velocity fluctuations is associated with cellular vortex shedding, with the discontinuities responsible for these modulations occurring at discrete locations along the span. Hot-wire data showed that the increase in amount of taper resulted in the increase in the number of vortex shedding cells in the wake, with the velocity fluctuations being modulated at all locations along the span of highly tapered cones. Comparing flow visualization images, hot-wire signals and corresponding streamwise velocity contours, one can confirm that the activity seen in the junction of parallel and tilted vortex lines is indeed the discontinuity and the discontinuity appears to move along the span of the cone in time. The tilting of vortices is periodic and continues along the span of the cone as time progresses and is surmised that as the vortex lines reach a certain large angle the process gets disrupted, resulting in the so-called discontinuity. This process then occurs at the next spanwise location, until it reaches a local diameter where the Reynolds number is too low for periodic vortex shedding. Thus the movement of the discontinuity in the wake of the highly tapered cone could lead to a scenario where the so-called cells could be considered to be non-stationary, or in other words moving.

Jagadeesh, Chetan; Gaster, Michael

2010-11-01

105

Vortex-wake interactions of a flapping foil that models animal swimming and flight.  

PubMed

The fluid dynamics of many swimming and flying animals involves the generation and shedding of vortices into the wake. Here we studied the dynamics of similar vortices shed by a simple two-dimensional flapping foil in a soap-film tunnel. The flapping foil models an animal wing, fin or tail in forward locomotion. The vortical flow induced by the foil is correlated to (the resulting) thickness variations in the soap film. We visualized these thickness variations through light diffraction and recorded it with a digital high speed camera. This set-up enabled us to study the influence of foil kinematics on vortex-wake interactions. We varied the dimensionless wavelength of the foil (lambda*=4-24) at a constant dimensionless flapping amplitude (A*=1.5) and geometric angle of attack amplitude (A(alpha,geo)=15 degrees ). The corresponding Reynolds number was of the order of 1000. Such values are relevant for animal swimming and flight. We found that a significant leading edge vortex (LEV) was generated by the foil at low dimensionless wavelengths (lambda*<10). The LEV separated from the foil for all dimensionless wavelengths. The relative time (compared with the flapping period) that the unstable LEV stayed above the flapping foil increased for decreasing dimensionless wavelengths. As the dimensionless wavelength decreased, the wake dynamics evolved from a wavy von Kármán-like vortex wake shed along the sinusoidal path of the foil into a wake densely packed with large interacting vortices. We found that strongly interacting vortices could change the wake topology abruptly. This occurred when vortices were close enough to merge or tear each other apart. Our experiments show that relatively small changes in the kinematics of a flapping foil can alter the topology of the vortex wake drastically. PMID:18165254

Lentink, David; Muijres, Florian T; Donker-Duyvis, Frits J; van Leeuwen, Johan L

2008-01-01

106

A self-learning coupled map lattice for vortex shedding in cable and cylinder wakes.  

PubMed

A coupled map lattice (CML) with self-learning features is developed to model flow over freely vibrating cables and stationary cylinders at low Reynolds numbers. Coupled map lattices that combine a series of low-dimensional circle maps with a diffusion model have been used previously to predict qualitative features of these flows. However, the simple nature of these CML models implies that there will be unmodeled wake features if a detailed, quantitative comparison is made with laboratory or simulated wake flows. Motivated by a desire to develop an improved CML model, we incorporate self-learning features into a new CML that is first trained to precisely estimate wake patterns from a target numerical simulation. A new convective-diffusive map that includes additional wake dynamics is developed. The new self-learning CML uses an adaptive estimation scheme (multivariable least-squares algorithm). Studies of this approach are conducted using wake patterns from a Navier-Stokes solution (spectral element-based NEKTAR simulation) of freely vibrating cable wakes at Reynolds numbers Re=100. It is shown that the self-learning model accurately and efficiently estimates the simulated wake patterns. The self-learning scheme is then successfully applied to vortex shedding patterns obtained from experiments on stationary cylinders. This constitutes a first step toward the use of the self-learning CML as a wake model in flow control studies of laboratory wake flows. PMID:15189057

Balasubramanian, G; Olinger, D J; Demetriou, M A

2004-06-01

107

Tomographic particle image velocimetry of desert locust wakes: instantaneous volumes combine to reveal hidden vortex elements and rapid wake deformation  

PubMed Central

Aerodynamic structures generated by animals in flight are unstable and complex. Recent progress in quantitative flow visualization has advanced our understanding of animal aerodynamics, but measurements have hitherto been limited to flow velocities at a plane through the wake. We applied an emergent, high-speed, volumetric fluid imaging technique (tomographic particle image velocimetry) to examine segments of the wake of desert locusts, capturing fully three-dimensional instantaneous flow fields. We used those flow fields to characterize the aerodynamic footprint in unprecedented detail and revealed previously unseen wake elements that would have gone undetected by two-dimensional or stereo-imaging technology. Vortex iso-surface topographies show the spatio-temporal signature of aerodynamic force generation manifest in the wake of locusts, and expose the extent to which animal wakes can deform, potentially leading to unreliable calculations of lift and thrust when using conventional diagnostic methods. We discuss implications for experimental design and analysis as volumetric flow imaging becomes more widespread. PMID:22977102

Bomphrey, Richard J.; Henningsson, Per; Michaelis, Dirk; Hollis, David

2012-01-01

108

Generation of vortex rings by nonstationary laser wake field  

SciTech Connect

A new concept of generating quasistatic magnetic fields, vortex rings, and electron jets in an isotropic homogeneous plasma is presented. The propagation of plasma waves, generated by a relativistically intense short pulse laser, is investigated by using the kinetic model and a novel nonpotential, time-dependent ponderomotive force is derived by obtaining a hydrodynamic equation of motion. This force can in turn generate quasistatic magnetic fields, vortex rings, and electron jets. It is also shown that the vortex rings can become a means for accelerating electrons, which are initially in equilibrium. The conservation of canonical momentum circulation and the frozen-in condition for the vorticity is discussed. The excitation of the vortex waves by the modulation of the amplitude of the plasma waves is considered. These vortex waves, which generate a lower hybrid mode propagating across the generated magnetic field, are also investigated.

Tsintsadze, N.L.; Murtaza, G.; Shah, H.A. [Department of Physics, Tbilisi State University, Chavchavadze 3 (Georgia); National Centre for Mathematics, G.C. University, Lahore 54000 (Pakistan); Department of Physics, G.C. University, Lahore 54000 (Pakistan)

2006-01-15

109

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

110

QUANTITATIVE ESTIMATION OF WAKE VORTEX SAFETY USING THE P2P MODEL  

E-print Network

safety issue in aircraft final approach phase. The evaluation of the risk is crucial when the industryQUANTITATIVE ESTIMATION OF WAKE VORTEX SAFETY USING THE P2P MODEL Yue Xie, yxie@gmu.edu, John takes the effort to reduce aircraft spacing for the purpose of capacity gain. This paper proposes

111

Structure of the vortex wake in hovering Anna's hummingbirds (Calypte anna).  

PubMed

Hummingbirds are specialized hoverers for which the vortex wake has been described as a series of single vortex rings shed primarily during the downstroke. Recent findings in bats and birds, as well as in a recent study on Anna's hummingbirds, suggest that each wing may shed a discrete vortex ring, yielding a bilaterally paired wake. Here, we describe the presence of two discrete rings in the wake of hovering Anna's hummingbirds, and also infer force production through a wingbeat with contributions to weight support. Using flow visualization, we found separate vortices at the tip and root of each wing, with 15% stronger circulation at the wingtip than at the root during the downstroke. The upstroke wake is more complex, with near-continuous shedding of vorticity, and circulation of approximately equal magnitude at tip and root. Force estimates suggest that the downstroke contributes 66% of required weight support, whereas the upstroke generates 35%. We also identified a secondary vortex structure yielding 8-26% of weight support. Lift production in Anna's hummingbirds is more evenly distributed between the stroke phases than previously estimated for Rufous hummingbirds, in accordance with the generally symmetric down- and upstrokes that characterize hovering in these birds. PMID:24174113

Wolf, M; Ortega-Jimenez, V M; Dudley, R

2013-12-22

112

Structure of the vortex wake in hovering Anna's hummingbirds (Calypte anna)  

PubMed Central

Hummingbirds are specialized hoverers for which the vortex wake has been described as a series of single vortex rings shed primarily during the downstroke. Recent findings in bats and birds, as well as in a recent study on Anna's hummingbirds, suggest that each wing may shed a discrete vortex ring, yielding a bilaterally paired wake. Here, we describe the presence of two discrete rings in the wake of hovering Anna's hummingbirds, and also infer force production through a wingbeat with contributions to weight support. Using flow visualization, we found separate vortices at the tip and root of each wing, with 15% stronger circulation at the wingtip than at the root during the downstroke. The upstroke wake is more complex, with near-continuous shedding of vorticity, and circulation of approximately equal magnitude at tip and root. Force estimates suggest that the downstroke contributes 66% of required weight support, whereas the upstroke generates 35%. We also identified a secondary vortex structure yielding 8–26% of weight support. Lift production in Anna's hummingbirds is more evenly distributed between the stroke phases than previously estimated for Rufous hummingbirds, in accordance with the generally symmetric down- and upstrokes that characterize hovering in these birds. PMID:24174113

Wolf, M.; Ortega-Jimenez, V. M.; Dudley, R.

2013-01-01

113

TRAILING VORTEX MEASUREMENTS IN THE WAKE OF A HOVERING ROTOR BLADE WITH VARIOUS TIP SHAPES  

Microsoft Academic Search

This work examined the wake aerodynamics of a sin- gle helicopter rotor blade with several tip shapes operat- ing on a hover test stand. Velocity eld measurements were conducted using three-component laser Doppler velocimetry (LDV). The objective of these measure- ments was to document the vortex velocity proles and then extract the core properties, such as the core radius, peak

Preston B. Martin; J. Gordon Leishman

2002-01-01

114

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

115

Relationship between vortex ring in tail fin wake and propulsive force  

NASA Astrophysics Data System (ADS)

Our aim was to investigate the three-dimensional (3D) vortex ring in the wake of a tail fin and to clarify the propulsion mechanism of dolphins and fish. In this study, we replaced a tail fin in pitching motion with an oscillating wing having a drive unit. The flow fields around the wing were measured by stereoscopic particle image velocimetry. To visualize the 3D structure of the vortex in the wake, we determined the flow fields in equally spaced cross-sectional planes. We reconstructed the 3D velocity fields from the velocity data with three components in two dimensions. We visualized the 3D vortex structure from these velocity data and plotted an iso-vorticity surface. As a result, we found that the vortex ring was generated by the kick-down and kick-up motions of the wing and that the wake structure was comparable with that obtained numerically. Moreover, we calculated the propulsive forces from the temporal variations in circulation and in the area surrounded by the vortex ring.

Imamura, Naoto; Matsuuchi, Kazuo

2013-10-01

116

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

117

Models of Wake-Vortex Spreading Mechanisms and Their Estimated Uncertainties  

NASA Technical Reports Server (NTRS)

One of the primary constraints on the capacity of the nation's air transportation system is the landing capacity at its busiest airports. Many airports with nearly-simultaneous operations on closely-spaced parallel runways (i.e., as close as 750 ft (246m)) suffer a severe decrease in runway acceptance rate when weather conditions do not allow full utilization. The objective of a research program at NASA Ames Research Center is to develop the technologies needed for traffic management in the airport environment so that operations now allowed on closely-spaced parallel runways under Visual Meteorological Conditions can also be carried out under Instrument Meteorological Conditions. As part of this overall research objective, the study reported here has developed improved models for the various aerodynamic mechanisms that spread and transport wake vortices. The purpose of the study is to continue the development of relationships that increase the accuracy of estimates for the along-trail separation distances available before the vortex wake of a leading aircraft intrudes into the airspace of a following aircraft. Details of the models used and their uncertainties are presented in the appendices to the paper. Suggestions are made as to the theoretical and experimental research needed to increase the accuracy of and confidence level in the models presented and instrumentation required or more precise estimates of the motion and spread of vortex wakes. The improved wake models indicate that, if the following aircraft is upwind of the leading aircraft, the vortex wakes of the leading aircraft will not intrude into the airspace of the following aircraft for about 7s (based on pessimistic assumptions) for most atmospheric conditions. The wake-spreading models also indicate that longer time intervals before wake intrusion are available when atmospheric turbulence levels are mild or moderate. However, if the estimates for those time intervals are to be reliable, further study is necessary to develop the instrumentation and procedures needed to accurately define when the more benign atmospheric conditions exist.

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

2006-01-01

118

Large-Eddy Simulations and Lidar Measurements of Vortex-Pair Breakup in Aircraft Wakes  

NASA Technical Reports Server (NTRS)

Results of large-eddy simulations of an aircraft wake are compared with results from ground-based lidar measurements made at NASA Langley Research Center during the Subsonic Assessment Near-Field Interaction Flight Experiment field tests. Brief reviews of the design of the field test for obtaining the evolution of wake dispersion behind a Boeing 737 and of the model developed for simulating such wakes are given. Both the measurements and the simulations concentrate on the period from a few seconds to a few minutes after the wake is generated, during which the essentially two-dimensional vortex pair is broken up into a variety of three-dimensional eddies. The model and experiment show similar distinctive breakup eddies induced by the mutual interactions of the vortices, after perturbation by the atmospheric motions.

Lewellen, D. C.; Lewellen, W. S.; Poole, L. R.; DeCoursey, R. J.; Hansen, G. M.; Hostetler, C. A.; Kent, G. S.

1998-01-01

119

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

120

Turbulent vortex streets and the entrainment mechanism of the turbulent wake  

NASA Technical Reports Server (NTRS)

The results of an experimental investigation of a turbulent vortex street in the range from 1000 to 20,000 are presented. The vortex street was created by the motion of a circular cylinder in a motionless fluid (mercury). Photographs obtained showed that the turbulent street, created by the vortex shedding behind the cylinder, persisted at longer downstream distances and higher Reynolds numbers than previously reported in the literature. A theory was developed to account for the experimental measurements pertaining to the change of the geometrical characteristics, (the distance between the two rows of vortices and the longitudinal distance between two consecutive vortices on the same row), of the street in the downstream direction. The implications of the structure of the vortex street on the entrainment mechanism of the turbulent wake are discussed.

Papailiou, D. D.; Lykoudis, P. S.

1974-01-01

121

A wake oscillator with frequency dependent coupling for the modeling of vortex-induced vibration  

NASA Astrophysics Data System (ADS)

The aim of this paper is to improve the phenomenological modeling of vortex-induced vibration of an elastically mounted cylinder in fluid flow. To this end an attempt is made to introduce a wake oscillator model that conforms to both the free and forced vibration experiments. This approach is new as in the past wake oscillator models have been tuned to the free vibration experiments only. First, an existing wake oscillator model is improved by properly including the effect of stall and that of the relatively large attack angles in the course of vortex-induced vibration. Then, to comply with the forced vibration experiments, the model is enhanced by introducing frequency dependent coupling. Such a coupling allows reproduction of the measured frequency dependence of the fluid force on the cylinder. The time domain representation of this coupling is a convolution integral. It is found in this paper that if the wake oscillator is modeled with a Van der Pol equation, it is impossible to find one set of frequency dependent coefficients that conforms to the forced vibration experiments at all amplitudes of cylinder motion. Moreover, the frequency dependencies identified for each frequency separately do not seem to satisfy the Kramers-Kronig relations. Based on the above findings, it is concluded that the nonlinearities in the wake oscillator model used in this paper cannot accurately model the results of vortex-induced vibration measurements. The idea proposed in this paper, that a consistent wake oscillator model must conform to the forced vibration experiments as well, is expected to be a powerful tool in the search for the correct nonlinearity.

Ogink, R. H. M.; Metrikine, A. V.

2010-12-01

122

Application of panel method to wake vortex/wing interaction and comparison with experiment  

NASA Technical Reports Server (NTRS)

The ability of a low-order panel method to calculate the aerodynamic loads on wings caused by interaction with wake vortices was studied. The loads were calculated for various positions of a downstream following wing relative to an upstream vortex-generating wing. Calculated vortex-induced span loads and rolling-moment coefficients on the following wing were compared with experimental data. A good agreement with experiment was obtained when the following wing was located more than one following-wing chord length from the tip vortex. The predictions deteriorated as the following wing was placed closer to the vortex. At large downstream distances (approximately 10 generating-wing chord lengths), induced rolling-moment coefficients on the following wing were consistently overestimated. Despite the strong interaction between the wake-vortex filaments and surface doublet panels, the accuracy of the calculations was in most cases independent of the panel distribution and density. A good agreement between theoretical and experimental loads was obtained with a minimum of experimentation with panel arrangements.

Smith, B. E.; Ross, J. C.

1984-01-01

123

Aircraft Wake Vortex Spacing System (AVOSS) Performance Update and Validation Study  

NASA Technical Reports Server (NTRS)

An analysis has been performed on data generated from the two most recent field deployments of the Aircraft Wake VOrtex Spacing System (AVOSS). The AVOSS provides reduced aircraft spacing criteria for wake vortex avoidance as compared to the FAA spacing applied under Instrument Flight Rules (IFR). Several field deployments culminating in a system demonstration at Dallas Fort Worth (DFW) International Airport in the summer of 2000 were successful in showing a sound operational concept and the system's potential to provide a significant benefit to airport operations. For DFW, a predicted average throughput increase of 6% was observed. This increase implies 6 or 7 more aircraft on the ground in a one-hour period for DFW operations. Several studies of performance correlations to system configuration options, design options, and system inputs are also reported. The studies focus on the validation performance of the system.

Rutishauser, David K.; OConnor, Cornelius J.

2001-01-01

124

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

125

An Evaluation of the Measurement Requirements for an In-Situ Wake Vortex Detection System  

NASA Technical Reports Server (NTRS)

Results of a numerical simulation are presented to determine the feasibility of estimating the location and strength of a wake vortex from imperfect in-situ measurements. These estimates could be used to provide information to a pilot on how to avoid a hazardous wake vortex encounter. An iterative algorithm based on the method of secants was used to solve the four simultaneous equations describing the two-dimensional flow field around a pair of parallel counter-rotating vortices of equal and constant strength. The flow field information used by the algorithm could be derived from measurements from flow angle sensors mounted on the wing-tip of the detecting aircraft and an inertial navigation system. The study determined the propagated errors in the estimated location and strength of the vortex which resulted from random errors added to theoretically perfect measurements. The results are summarized in a series of charts and a table which make it possible to estimate these propagated errors for many practical situations. The situations include several generator-detector airplane combinations, different distances between the vortex and the detector airplane, as well as different levels of total measurement error.

Fuhrmann, Henri D.; Stewart, Eric C.

1996-01-01

126

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

127

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

128

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

129

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

130

Energy contents and vortex dynamics in Mode-C transition of wired-cylinder wake  

NASA Astrophysics Data System (ADS)

The 3D transition of the flow behind a circular cylinder with a near-wake wire disturbance has been investigated experimentally. The flow is oriented horizontally and the wire is positioned in the upper half of the wake. We performed flow visualization and particle image velocimetry experiments to investigate the influence of the wire on various properties of the flow, such as the dynamics of the spanwise structures. Experiments were performed in the Reynolds number range of Re = 165-300. It is shown that in Mode-C transition of the wired cylinder wake, some part of the streamwise vorticity content of the upper von Kármán vortices located at the perturbed side, is transferred to the secondary vortices. This vorticity transfer results in upper von Kármán vortices which are weaker than the lower ones. The analysis of the discrete energy content of the wake supports this analysis by showing that the energy intensity at von Kármán vortex shedding frequency f0 at the perturbed side of the wake is less than the energy intensity in the lower half. This leads to conclusion that the excess energy is transferred to the subharmonic frequency f1 ? f0/2.

Yildirim, I.; Rindt, C. C. M.; van Steenhoven, A. A.

2013-05-01

131

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

132

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

133

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

134

Validation of a vortex ring wake model suited for aeroelastic simulations of floating wind turbines  

NASA Astrophysics Data System (ADS)

In order to evaluate aerodynamic loads on floating offshore wind turbines, advanced dynamic analysis tools are required. As a unified model that can represent both dynamic inflow and skewed inflow effects in it basic formulation, a wake model based on a vortex ring formulation is discussed. Such a model presents a good intermediate solution between computationally efficient but simple momentum balance methods and computationally expensive but complete computational fluid dynamics models. The model introduced is shown to be capable of modelling typical steady and unsteady test cases with reasonable accuracy.

de Vaal, J. B.; Hansen, M. O. L.; Moan, T.

2014-12-01

135

On the spread and decay of wind turbine wakes in ambient turbulence  

NASA Astrophysics Data System (ADS)

The decay of the downstream wake of a wind turbine plays an important role in the performance of wind farms. The spread and decay of a wake depend both on wake meandering (advection of the wake as a whole) and wake diffusion (widening of the wake within its meandering frame of reference). Both of these effects depend strongly on the intensity of the ambient turbulence relative to the velocity deficit in the wake, and on the integral length scale of the turbulence relative to the wake width. Recent theory, which we review here, shows how intense large-scale turbulence can lead to a rapid x?2 decay in the time-averaged centreline velocity deficit, as compared to a x?1 decay for smaller scale turbulence, where x is distance downstream. We emphasise in this paper that common wind farm models do not predict this rapid decay. We present new experimental measurements of the velocity deficit downstream of a porous disc in relatively large-scale ambient turbulence which corroborate predictions of a x?2 decay, and we show theoretically that the commonly used k-epsilon model does not capture this effect. We further show that a commercial CFD package, configured to match our experiments and employing the k-epsilon model, fails to predict such rapid decay. We conclude that steady simulations of wind turbine wake dynamics are insufficient for informing wind farm layout optimisation.

Johnson, P. B.; Jonsson, C.; Achilleos, S.; Eames, I.

2014-12-01

136

Secondary vortex street in the wake of two tandem circular cylinders at low Reynolds number.  

PubMed

The experiments on two tandem circular cylinders were conducted in a horizontal soap film tunnel for the Reynolds number Re=60 , 80, and 100 and the nondimensional center-to-center spacing Gamma ranging in 1 approximately 12. The flow patterns were recorded by a high-speed camera and the vortex shedding frequency was obtained by a spatiotemporal evolution method. The secondary vortex formation (SVF) mode characterized by the formation of a secondary vortex street in the wake of the downstream cylinder was found at large gamma. Moreover, some typical modes predicted by previous investigations, including the single bluff-body, shear layer reattachment, and synchronization of vortex shedding modes, were also revisited in our experiments. Further, numerical simulations were carried out using a space-time finite-element method and the results confirmed the existence of the SVF mode. The mechanism of SVF mode was analyzed in terms of the numerical results. The dependence of the Strouhal number Sr on Gamma was given and the flow characteristics relevant to the critical spacing values and the hysteretic mode transitions were investigated. PMID:20365852

Wang, Si-ying; Tian, Fang-bao; Jia, Lai-bing; Lu, Xi-yun; Yin, Xie-zhen

2010-03-01

137

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

138

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

NASA Technical Reports Server (NTRS)

Closely Space Parallel Runway (CSPR) configurations are capacity limited for departures due to the requirement to apply wake vortex separation standards from traffic departing on the adjacent parallel runway. To mitigate the effects of this constraint, a concept focusing on wind dependent departure operations has been developed, known as the Wake Turbulence Mitigation for Departures (WTMD). This concept takes advantage of the fact that crosswinds of sufficient velocity blow wakes generated by aircraft departing from the downwind runway away from the upwind runway. Consequently, under certain conditions, wake separations on the upwind runway would not be required based on wakes generated by aircraft on the downwind runway, as is currently the case. It follows that information requirements, and sources for this information, would need to be determined for airport traffic control tower (ATCT) supervisory personnel who would be charged with decisions regarding use of the procedure. To determine the information requirements, data were collected from ATCT supervisors and controller-in-charge qualified individuals at Lambert-St. Louis International Airport (STL) and George Bush Houston Intercontinental Airport (IAH). STL and IAH were chosen as data collection sites based on the implementation of a WTMD prototype system, operating in shadow mode, at these locations. The 17 total subjects (STL: 5, IAH: 12) represented a broad-base of air traffic experience. Results indicated that the following information was required to support the conduct of WTMD operations: current and forecast weather information, current and forecast traffic demand and traffic flow restrictions, and WTMD System status information and alerting. Subjects further indicated that the requisite information is currently available in the tower cab with the exception of the WTMD status and alerting. Subjects were given a demonstration of a display supporting the prototype systems and unanimously stated that the WTMD status information they felt important was represented. Overwhelmingly, subjects felt that approving, monitoring and terminating the WTMD procedure could be integrated into their supervisory workload.

Lohr, Gary W.; Williams, Daniel M.; Trujillo, Anna C.

2008-01-01

139

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

140

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

141

The nonlinear decay of vortex flows in a rotating fluid  

NASA Astrophysics Data System (ADS)

The manifestations of the cyclone-anticyclone asymmetry effect, which is observed in nature [1-3] and in laboratory experiments with a rotating fluid [4-6], have been studied in the scope of the vorticity evolution generalized equation. Under laboratory conditions, asymmetry is observed as a faster decay of cyclonic vortices as compared to anticyclonic ones. This effect has been analytically described simply based on the problem of the vortex spot decay and spatially periodic vortex lattice. It has been indicated that anticyclonic vortices compress and cyclonic ones expand when a lattice damps. Examples of exact solutions of the vorticity equation [7] (describing, specifically, collapsing of an axisymmetric vortex core with anticyclonic vorticity) have been constructed. The effect of the Ekman friction on the interaction between singular vortices is discussed.

Kalashnik, M. V.; Chkhetiani, O. G.

2014-06-01

142

The decay of longitudinal vortices shed from airfoil vortex generators  

NASA Technical Reports Server (NTRS)

An experimental study is conducted to examine the crossplane structure and streamwise decay of vortices shed from airfoil-type vortex generators. The vortex generators are set in a counter-rotating array spanning the full circumference of a straight pipe. The span of the vortex generators above the duct surface, h, is approximately equal to the local turbulent boundary layer thickness, delta. Measurement of three-component mean flow velocity in downstream crossplanes are used to characterize the structure of the shed vortices. Measurements in adjacent crossplanes (closely spaced along the streamwise coordinate) characterize the interaction and decay of the embedded vortices. A model constructed by the superposition of Oseen vortices is compared to the data for one test case.

Wendt, Bruce J.; Reichert, Bruce A.; Foster, Jeffry D.

1995-01-01

143

The vortex wake is the fluid dynamic footprint of swimming and flying animals. When an animal moves through fluid,  

E-print Network

3519 The vortex wake is the fluid dynamic footprint of swimming and flying animals. When an animal moves through fluid, Newton's second and third laws together dictate that the locomotive force exerted by the fluid on the animal has a magnitude equal to the rate at which the animal imparts momentum to the fluid

Dabiri, John O.

144

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

145

Spanwise vortex dislocation in the wake of segmented blunt trailing edge  

NASA Astrophysics Data System (ADS)

Dislocation of the Karman vortex is forced in the near wake behind a two dimensional blunt trailing edge aerofoil to induce strong three dimensionality to weaken the Karman vortex and inhibit its periodic shedding by segmenting the trailing edge in a novel way, different from a rectangular segmented trailing edge tried in the past. Symmetrical trapezoidal prismatic blocks, with the major and the minor sides being equal to 4 and 2 base heights, respectively, are attached to the base at regular intervals along the span which could be varied in order to render multiple wavelengths of spanwise discontinuity to ensure that at least one of the modes of dislocation is triggered independent of the Reynolds number. Hot-wire measurements confirm effectiveness of the trailing edge configurations with trapezoidal prismatic blocks in creating controlled dislocation along the span which annihilates the Karman vortices and suppresses their periodic shedding completely. Flow visualization in a water tunnel reinforces the hot-wire results and clearly shows that the classical Karman vortex street, seen behind the plain base model, disappears when segmented trailing edge is used. In comparison, the rectangular segmented trailing edge is found to attenuate the Karman vortex strength only partially. However, the base pressure measurements have shown improvement in reducing the associated base drag only by 3-4%. The segmented trailing edges designed for the present study are found to generate strong streamwise vortices that effectively transfer energy from the Karman vortices resulting in suppression of the unsteadiness but, perhaps, lower pressures in their core may be restricting the further rise in the base pressure due to the induced effect.

Deshpande, P. J.; Sharma, S. D.

2012-10-01

146

Motion, decay and merging of vortex filaments  

NASA Technical Reports Server (NTRS)

The asymptotic solutions of Navier-Stokes equations for vortex filaments of finite strength with small effective vortical cores are summarized. Emphases are placed on the physical meaning and the practical limit to the applicability of the asymptotic solution. Finite-difference solutions of Navier-Stokes equations for the merging of the filament(s) are described. It is focused on the development of the approximate boundary conditions for the computational domain.

Liu, C. H.; Ting, L.

1988-01-01

147

Dynamic decay of a single vortex into vortex-antivortex pairs  

SciTech Connect

A variety of metastable states, including vortices, antivortices, and their combinations, is typical for magnetically soft, thin films and patterned structures. The physics of individual spin vortices in patterned structures has been rather extensively explored. In contrast, there are few studies of the vortex–antivortex–vortex (v-av-v) system, in part because the configuration is rather challenging to obtain experimentally. We demonstrate herein how a recently proposed resonant-spin-ordering technique can be used to induce the dynamic decay of a single vortex into v-av states in elongated elements. The approach is based on first driving the system from the linear regime of constant vortex gyrations to the non-linear regime of vortex-core reversals at a fixed excitation frequency, and then subsequently reducing the excitation field back to the linear regime. This procedure stabilizes the system into a v-av-v state that is completely decoupled from the initialization excitation frequency. The newly acquired state is stable in remanence. The dynamic response of this system is expected to demonstrate a number of collective modes, depending on the combination of the vortex core polarities, and/or the excitation field direction, and, hence, is of interest for future studies.

Lendínez, Sergi [Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 (United States); Universidad Barcelona, Departamento Fisica Fonamental, E-08028 Barcelona (Spain); Jain, Shikha; Novosad, Valentyn, E-mail: novosad@anl.gov; Fradin, Frank Y.; Pearson, John E. [Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 (United States); Tejada, Javier [Universidad Barcelona, Departamento Fisica Fonamental, E-08028 Barcelona (Spain); Bader, Samuel D. [Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 (United States); Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439 (United States)

2014-05-07

148

Adaptive proportional control of a low-order model for vortex shedding in cylinder wakes  

NASA Astrophysics Data System (ADS)

An adaptive scheme is proposed for the control of wake structures behind circular cylinders predicted by a previously developed coupled map lattice. The map consists of circle map oscillators along the cylinder span coupled by a simple diffusion model. Highly efficient low-order models with control features are desirable for flow control studies in cylinder wakes. Chaotic states associated with disordered vortex shedding patterns are observed when forcing the cylinder outside the classical lock-on region. Earlier methods concentrated on controlling these states via an application of a small-amplitude periodic perturbation of a system parameter as was proposed by Ott, Grebogi and York. In the present work, the control gain is varied adaptively by employing Lyapunov redesign methods. In essence the adaptive scheme forces the system energy, associated with the error system derived from the difference of the chaotic states from the target states, to be decreasing. The effectiveness of the proposed adaptive scheme is compared to the previously developed chaos control and which overwhelmingly demonstrates an improved performance with a considerable control energy reduction.

Demetriou, Michael A.; Raman Balasubramanian, Ganapathi; Olinger, David J.

2001-11-01

149

Investigation and Optimization of Blade Tip Winglets Using an Implicit Free Wake Vortex Method  

NASA Astrophysics Data System (ADS)

Novel outer-blade geometries such as tip winglets can increase the aerodynamic power that can be extracted from the wind by tailoring the relative position and strengths of trailed vorticity. This design space is explored using both parameter studies and gradient-based optimization, with the aerodynamic analysis carried out using LibAero, a free wake vortex-based code introduced in previous work. The starting design is the NREL 5MW reference turbine, which allows comparison of the aerodynamic simulation for the unmodified blade with other codes. The code uses a Prandtl-Weissinger lifting line model to represent the blade, and vortex filaments as the flow elements. A fast multipole method is implemented to accelerate the influence calculations and reduce the computational cost. This results in higher fidelity aerodynamic simulations that can capture the effects of novel geometries while maintaining sufficiently fast run-times (on the order of an hour) to allow the use of optimization. Gradients of the objective function with respect to design variables are calculated using the complex step method which is accurate and efficient. Since the vortex structure behind the rotor is being resolved in detail, insight is also gained into the mechanisms by which these new blade designs affect performance. It is found that adding winglets can increase the power extracted from the wind by around 2%, with a similar increase in thrust. It is also possible to create a winglet that slightly lowers the thrust while maintaining very similar power compared to the standard straight blade.

Lawton, Stephen; Crawford, Curran

2014-06-01

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

Decay of helical Kelvin waves on a quantum vortex filament  

SciTech Connect

We study the dynamics of helical Kelvin waves moving along a quantum vortex filament driven by a normal fluid flow. We employ the vector form of the quantum local induction approximation (LIA) due to Schwarz. For an isolated filament, this is an adequate approximation to the full Hall-Vinen-Bekarevich-Khalatnikov dynamics. The motion of such Kelvin waves is both translational (along the quantum vortex filament) and rotational (in the plane orthogonal to the reference axis). We first present an exact closed form solution for the motion of these Kelvin waves in the case of a constant amplitude helix. Such solutions exist for a critical wave number and correspond exactly to the Donnelly-Glaberson instability, so perturbations of such solutions either decay to line filaments or blow-up. This leads us to consider helical Kelvin waves which decay to line filaments. Unlike in the case of constant amplitude helical solutions, the dynamics are much more complicated for the decaying helical waves, owing to the fact that the rate of decay of the helical perturbations along the vortex filament is not constant in time. We give an analytical and numerical description of the motion of decaying helical Kelvin waves, from which we are able to ascertain the influence of the physical parameters on the decay, translational motion along the filament, and rotational motion, of these waves (all of which depend nonlinearly on time). One interesting finding is that the helical Kelvin waves do not decay uniformly. Rather, such waves decay slowly for small time scales, and more rapidly for large time scales. The rotational and translational velocity of the Kelvin waves depend strongly on this rate of decay, and we find that the speed of propagation of a helical Kelvin wave along a quantum filament is large for small time while the wave asymptotically slows as it decays. The rotational velocity of such Kelvin waves along the filament will increase over time, asymptotically reaching a finite value. These decaying Kelvin waves correspond to wave number below the critical value for the Donnelly-Glaberson instability, and hence our results on the Schwarz quantum LIA correspond exactly to what one would expect from prior work on the Donnelly-Glaberson instability.

Van Gorder, Robert A., E-mail: rav@knights.ucf.edu [Department of Mathematics, University of Central Florida, Orlando, Florida 32816-1364 (United States)

2014-07-15

152

Influence of different flap settings on the wake-vortex structure of a rectangular wing with flaps and means of alleviation with wing fins  

Microsoft Academic Search

This paper presents the results of an experimental investigation of the wake-vortex structure behind a flapped rectangular wing. The wind-tunnel test is part of a project funded by the German Research Association (DFG) to investigate the flow structure in the wake of wings with different flap extensions. The purpose of this investigation is to study the main features of lift

Ingmar Schell; Erol Özger; Dieter Jacob

2000-01-01

153

Flight test investigation of the vortex wake characteristics behind a Boeing 727 during two-segment and normal ILS approaches (A joint NASA/FAA report)  

NASA Technical Reports Server (NTRS)

Flight tests were performed to evaluate the vortex wake characteristics of a Boeing 727 aircraft during conventional and two-segment instrument landing approaches. Smoke generators were used for vortex marking. The vortex was intentionally intercepted by a Lear Jet and a Piper Comanche aircraft. The vortex location during landing approach was measured using a system of phototheodolites. The tests showed that at a given separation distance there are no readily apparent differences in the upsets resulting from deliberate vortex encounters during the two types of approaches. The effect of the aircraft configuration on the extent and severity of the vortices is discussed.

Barber, M. R.; Kurkowski, R. L.; Garodz, L. J.; Robinson, G. H.; Smith, H. J.; Jacobsen, R. A.; Stinnett, G. W., Jr.; Mcmurtry, T. C.; Tymczyszyn, J. J.; Devereaux, R. L.

1975-01-01

154

Wake Geometry Effects on Rotor Blade-Vortex Interaction Noise Directivity  

NASA Technical Reports Server (NTRS)

Acoustic measurements from a model rotor wind tunnel test are presented which show that the directionality of rotor blade vortex interaction (BVI) noise is strongly dependent on the rotor advance ratio and disk attitude. A rotor free wake analysis is used to show that the general locus of interactions on the rotor disk is also strongly dependent on advance ratio and disk attitude. A comparison of the changing directionality of the BVI noise with changes in the interaction locations shows that the strongest noise radiation occurs in the direction of motion normal to the blade span at the time of interaction, for both advancing and retreating side BVI. For advancing side interactions, the BVI radiation angle down from the tip-path plane appears relatively insensitive to rotor operating condition and is typically between 40 and 55 deg below the disk. However, the azimuthal radiation direction shows a clear trend with descent speed, moving towards the right of the flight path with increasing descent speed. The movement of the strongest radiation direction is attributed to the movement of the interaction locations on the rotor disk with increasing descent speed.

Martin, R. M.; Marcolini, Michael A.; Splettstoesser, W. R.; Schultz, K.-J.

1990-01-01

155

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

156

Three-dimensional vortex wake structure of a flapping-wing micro aerial vehicle in forward flight configuration  

NASA Astrophysics Data System (ADS)

This paper investigates the formation and evolution of the unsteady three-dimensional wake structures generated by the flapping wings of the DelFly II micro aerial vehicle in forward flight configuration. Time-resolved stereoscopic particle image velocimetry (Stereo-PIV) measurements were carried out at several spanwise-aligned planes in the wake, so as to allow a reconstruction of the temporal development of the wake of the flapping wings throughout the complete flapping cycle. Simultaneous thrust-force measurements were performed to explore the relation between the wake formation and the aerodynamic force generation mechanisms. The three-dimensional wake configuration was subsequently reconstructed from the planar PIV measurements by two different approaches: (1) a spatiotemporal wake reconstruction obtained by convecting the time-resolved, three-component velocity field data of a single measurement plane with the free-stream velocity; (2) for selected phases in the flapping cycle a direct three-dimensional spatial wake reconstruction is interpolated from the data of the different measurement planes, using a Kriging regression technique. Comparing the results derived from both methods in terms of the behavior of the wake formations, their phase and orientation indicate that the spatiotemporal reconstruction method allows to characterize the general three-dimensional structure of the wake, but that the spatial reconstruction method can reveal more details due to higher streamwise resolution. Comparison of the wake reconstructions for different values of the reduced frequency allows assessing the impact of the flapping frequency on the formation and interaction characteristics of the vortical structures. For low values of the reduced frequency, it is observed that the vortex structure formation of instroke and outstroke is relatively independent of each other, but that increasing interaction occurs at higher reduced frequencies. It is further shown that there is a phase lag in the appearance of the structures for increasing flapping frequency, which is in correlation with the generation of the forces. Comparison of thrust generated during the instroke and the outstroke phases of the flapping motion in conjunction with the development of the wake structures indicates that wing-wing interaction at the start of outstroke (peel motion) becomes a dominant feature for reduced frequencies greater than 0.62.

Percin, M.; van Oudheusden, B. W.; Eisma, H. E.; Remes, B. D. W.

2014-09-01

157

An unsteady vortex lattice method model of a horizontal axis wind turbine operating in an upstream rotor wake  

NASA Astrophysics Data System (ADS)

An unsteady formulation of the vortex lattice method, VLM, is presented that uses a force- free representation of the wake behind a horizontal axis wind turbine, HAWT, to calculate the aerodynamic loading on a turbine operating in the wake of an upstream rotor. A Cartesian velocity grid is superimposed over the computational domain to facilitate the representation of the atmospheric turbulence surrounding the turbine and wind shear. The wake of an upstream rotor is modelled using two methods: a mean velocity deficit with superimposed turbulence, based on experimental observations, and a purely numeric periodic boundary condition. Both methods are treated as frozen and propagated with the velocity grid. Measurements of the mean thrust and blade root bending moment on a three bladed horizontal axis rotor modelling a 5 MW HAWT at 1:250 scale were carried out in a wind tunnel. Comparisons are made between operation in uniform flow and in the wake of a similarly loaded rotor approximately 6.5 diameters upstream. The measurements were used to validate the output from the VLM simulations, assuming a completely rigid rotor. The trends in the simulation thrust predictions are found to compare well with the uniform flow case, except at low tip speed ratios where there are losses due to stall which are yet to be included in the model. The simple wake model predicts the mean deficit, whilst the periodic boundary condition captures more of the frequency content of the loading in an upstream wake. However, all the thrust loads are over-predicted. The simulation results severely overestimate the bending moment, which needs addressing. However, the reduction in bending due to the simple wake model is found to reflect the experimental data reasonably well.

Hankin, D.; Graham, J. M. R.

2014-12-01

158

Application of digital particle image velocimetry to insect aerodynamics: measurement of the leading-edge vortex and near wake of a Hawkmoth  

NASA Astrophysics Data System (ADS)

Some insects use leading-edge vortices to generate high lift forces, as has been inferred from qualitative smoke visualisations of the flow around their wings. Here we present the first Digital Particle Image Velocimetry (DPIV) data and quantitative analysis of an insect’s leading-edge vortex and near wake at two flight speeds. This allows us to describe objectively 2D slices through the flow field of a tethered Tobacco Hawkmoth ( Manduca sexta). The near-field vortex wake appears to braodly resemble elliptical vortex loops. The presence of a leading-edge vortex towards the end of the downstroke is found to coincide with peak upward force production measured by a six-component force-moment balance. The topology of Manduca’s leading-edge vortex differs from that previously described because late in the downstroke, the structure extends continuously from wingtip across the thorax to the other wingtip.

Bomphrey, Richard J.; Lawson, Nicholas J.; Taylor, Graham K.; Thomas, Adrian L. R.

2006-04-01

159

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

160

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

161

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

162

Self-Consistent Mean Flow Description of the Nonlinear Saturation of the Vortex Shedding in the Cylinder Wake  

NASA Astrophysics Data System (ADS)

The Bénard-von Kármán vortex shedding instability in the wake of a cylinder is perhaps the best known example of a supercritical Hopf bifurcation in fluid dynamics. However, a simplified physical description that accurately accounts for the saturation amplitude of the instability is still missing. Here, we present a simple self-consistent model that provides a clear description of the saturation mechanism and quantitatively predicts the saturated amplitude and flow fields. The model is formally constructed by a set of coupled equations governing the mean flow together with its most unstable eigenmode with finite size. The saturation amplitude is determined by requiring the mean flow to be neutrally stable. Without requiring any input from numerical or experimental data, the resolution of the model provides a good prediction of the amplitude and frequency of the vortex shedding as well as the spatial structure of the mean flow and the Reynolds stress.

Manti?-Lugo, Vladislav; Arratia, Cristóbal; Gallaire, François

2014-08-01

163

Self-consistent mean flow description of the nonlinear saturation of the vortex shedding in the cylinder wake.  

PubMed

The Bénard-von Kármán vortex shedding instability in the wake of a cylinder is perhaps the best known example of a supercritical Hopf bifurcation in fluid dynamics. However, a simplified physical description that accurately accounts for the saturation amplitude of the instability is still missing. Here, we present a simple self-consistent model that provides a clear description of the saturation mechanism and quantitatively predicts the saturated amplitude and flow fields. The model is formally constructed by a set of coupled equations governing the mean flow together with its most unstable eigenmode with finite size. The saturation amplitude is determined by requiring the mean flow to be neutrally stable. Without requiring any input from numerical or experimental data, the resolution of the model provides a good prediction of the amplitude and frequency of the vortex shedding as well as the spatial structure of the mean flow and the Reynolds stress. PMID:25192100

Manti?-Lugo, Vladislav; Arratia, Cristóbal; Gallaire, François

2014-08-22

164

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

165

Numerical studies of motion and decay of vortex filaments  

NASA Technical Reports Server (NTRS)

A computational code is developed for the integro-differential equations governing the motion of the centerlines of vortex filaments submerged in a background potential flow. These equations, which are derived from the method of matched asymptotic analysis, include the effect of decaying large-magnitude circumferential and axial velocity components in the vortical cores. Numerical examples are presented to assess the effect of large axial velocity and of nonsimilar initial profiles in vortical cores. The initial configurations of the filaments are chosen so as to fulfill the basic assumption of asymptotic analysis, which is the effective vortical core size is much smaller than all other length scales in the flowfield, e.g., the radius of curvature and interfilament distance. The computations are continued until the basic assumption is no longer valid, that is, when the merging or intersection of filaments have begun. Various types of local or global merging or intersection of filaments are classified and demonstrated by numerical examples.

Liu, C. H.; Tavantzis, J.; Ting, L.

1986-01-01

166

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

PubMed

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. PMID:19693281

Borazjani, Iman; Sotiropoulos, Fotis

2009-01-01

167

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. PMID:19693281

BORAZJANI, IMAN; SOTIROPOULOS, FOTIS

2009-01-01

168

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. PMID:16849236

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

2005-01-01

169

Wind-tunnel measurements in the wakes of structures  

NASA Technical Reports Server (NTRS)

Detailed measurements of longitudinal mean velocity, turbulence intensity, space correlations, and spectra made in the wake of two rectangular scaled models in simulated atmospheric boundary-layer winds are presented. The model buildings were 1:50 scale models of two trailers. Results of a flow visualization study of the wake geometry are analyzed with some singular point theorems. Two hypothetical flow patterns of the detailed wake geometry are proposed. Some preliminary studies of the vortex wake, effects of the model size, model aspect ratios, and boundary layer characteristics on the decay rate and extent of the wake are also presented and discussed.

Woo, H. G. C.; Peterka, J. A.; Cermak, J. E.

1977-01-01

170

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

NASA Astrophysics Data System (ADS)

An experimental study was conducted to characterize the evolution of the unsteady vortex structures in the wake of a root-fixed flapping wing with the wing size, stroke amplitude, and flapping frequency within the range of insect characteristics for the development of novel insect-sized nano-air-vehicles (NAVs). The experiments were conducted in a low-speed wing tunnel with a miniaturized piezoelectric wing (i.e., chord length, C = 12.7 mm) flapping at a frequency of 60 Hz (i.e., f = 60 Hz). The non-dimensional parameters of the flapping wing are chord Reynolds number of Re = 1,200, reduced frequency of k = 3.5, and non-dimensional flapping amplitude at wingtip h = A/C = 1.35. The corresponding Strouhal number (Str) is 0.33 , which is well within the optimal range of 0.2 < Str < 0.4 used by flying insects and birds and swimming fishes for locomotion. A digital particle image velocimetry (PIV) system was used to achieve phased-locked and time-averaged flow field measurements to quantify the transient behavior of the wake vortices in relation to the positions of the flapping wing during the upstroke and down stroke flapping cycles. The characteristics of the wake vortex structures in the chordwise cross planes at different wingspan locations were compared quantitatively to elucidate underlying physics for a better understanding of the unsteady aerodynamics of flapping flight and to explore/optimize design paradigms for the development of novel insect-sized, flapping-wing-based NAVs.

Hu, Hui; Clemons, Lucas; Igarashi, Hirofumi

2011-08-01

171

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. PMID:21131333

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

2011-01-01

172

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

173

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

174

The Development of a Plan for the Assessment, Improvement and Deployment of a Radar Acoustic Sounding System (RASS) for Wake Vortex Detection  

NASA Technical Reports Server (NTRS)

This report describes the activities completed under a grant from the NASA Langley Research Center to develop a plan for the assessment, improvement, and deployment of a Radar Acoustic Sounding System (RASS) for the detection of wake vortices. A brief review is provided of existing alternative instruments for wake vortex detection. This is followed by a review of previous implementations and assessment of a RASS. As a result of this review, it is concluded that the basic features of a RASS have several advantages over other commonly used wake vortex detection and measurement systems. Most important of these features are the good fidelity of the measurements and the potential for all weather operation. To realize the full potential of this remote sensing instrument, a plan for the development of a RASS designed specifically for wake vortex detection and measurement has been prepared. To keep costs to a minimum, this program would start with the development an inexpensive laboratory-scale version of a RASS system. The new instrument would be developed in several stages, each allowing for a critical assessment of the instrument s potential and limitations. The instrument, in its initial stages of development, would be tested in a controlled laboratory environment. A jet vortex simulator, a prototype version of which has already been fabricated, would be interrogated by the RASS system. The details of the laboratory vortex would be measured using a Particle Image Velocimetry (PIV) system. In the early development stages, the scattered radar signal would be digitized and the signal post-processed to determine how extensively and accurately the RASS could measure properties of the wake vortex. If the initial tests prove to be successful, a real-time, digital signal processing system would be developed as a component of the RASS system. At each stage of the instrument development and testing, the implications of the scaling required for a full-scale instrument would be considered. It is concluded that a RASS system, developed for the specific application of wake vortex detection, could become part of a robust Aircraft Vortex Spacing System (AVOSS). This system, in turn, could contribute to Reduced Spacing Operations (RSO) in US airports and improvements in Terminal Area productivity (TAP).

Morris, Philip J.; McLaughlin, Dennis K.; Gabrielson, Thomas B.; Boluriaan, Said

2004-01-01

175

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

176

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

177

Motion of a curved vortex filament with decaying vortical core and axial velocity  

NASA Technical Reports Server (NTRS)

The motion and decay of a curved vortex filament having large axial and circumferential velocity components in a three-dimensional stream are analyzed by using the method of matched asymptotic expansions of the incompressible Navier-Stokes equations. The small parameter is the square root of the ratio of the kinematic viscosity to the circulation. The outer region is analyzed by the classical Biot-Savart law, and its solution is matched to that of the inner region, where viscous effects are important. Equations describing the coupling between the inner vortex structure and the motion of the vortex filament as well as the time evolution of the inner vortex structure are obtained. Equations are derived for the motion of the vortex filament and for the change and decay in time and space of the leading-order circumferential and axial velocity and vorticity components. Solutions are constructed for these components in terms of initial data.

Callegari, A. J.; Ting, L.

1978-01-01

178

Cylinder wakes in flowing soap films.  

PubMed

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. PMID:11970100

Vorobieff, P; Ecke, R E

1999-09-01

179

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

180

Mean velocity and decay characteristics of the near- and far-wake of a compressor rotor blade of moderate loading  

NASA Technical Reports Server (NTRS)

This paper reports the experimental study of the three-dimensional characteristics of the mean velocity in the wake of a moderately loaded compressor rotor blade. The measurements were taken with a three-sensor hot-wire probe rotating with the rotor. The wake was surveyed at several radial and axial stations. The loading was found to have substantial effect and this was reflected not only in the axial and tangential components, but also in the radial component. The radial velocities were found to be high very near the trailing-edge and this exhibits the characteristics prevalent in a trailing vortex system. The static pressures across the wake were measured using a direction insensitive spherical head static-stagnation pressure probe. The static pressure was found to be higher inside the wake. These and other measurements are reported and correlated in this paper.

Ravindranath, A.; Lakshminarayana, B.

1979-01-01

181

TURBULENT DIFFUSION BEHIND VEHICLES: EXPERI-MENTALLY DETERMINED INFLUENCE OF VORTEX PAIR IN VEHICLE WAKE  

EPA Science Inventory

The wake of a moving vehicle was simulated using a wind tunnel with a moving floor. he vehicle models, both scale and block-shaped, were held in a fixed position while the floor moved at the upstream air speed. his simulates an automobile traveling on a straight highway in still ...

182

Vortex-wake interactions of a flapping foil that models animal swimming and flight  

Microsoft Academic Search

The fluid dynamics of many swimming and flying animals involves the generation and shedding of vortices into the wake. Here we studied the dynamics of similar vortices shed by a simple two-dimensional flapping foil in a soap-film tunnel. The flapping foil models an animal wing, fin or tail in forward locomotion. The vortical flow induced by the foil is correlated

D. Lentink; F. T. Muijres; F. J. Donker-Duyvis; J. L. van Leeuwen

2008-01-01

183

Vortex Ring Formation in the Wake of Biologically Inspired Flapping Foils  

NASA Astrophysics Data System (ADS)

The design of biologically inspired propulsion mechanisms for underwater vehicles continues to generate significant interest in the hydrodynamics of fish swimming. Flapping foils, mimicking fish fins, have been shown to produce significant thrust and have been implemented on prototype underwater vehicles. Here, the three-dimensional vortical structures in the wake of a finite aspect ratio flapping foil are investigated in order to model the three dimensional propulsive signature of swimming fish and flapping foils. The vortical patterns in the wake of a flapping foil are visualized using qualitative fluorescent dye methods, imaged in three views: planform, wing-tip and isometric. Reynolds number based on foil chord length is 165. The foil is forced to heave and pitch with a prescribed motion mimicking that of a swimming fish tail. The visualizations reveal the formation of a pair of coherent, curved, and interconnected ring-like vortices for each full flapping cycle. The wake evolution shows a dependence on Strouhal number and foil motion kinematics; Strouhal number was varied between 0.1 and 0.4. Experimental visualization results compare well with recent numerical simulations using the same parameters. An analogy the model of the wake of a swimming fish is also explored.

Read, M. B.

2005-11-01

184

Oscillating pendulum decay by emission of vortex rings Diogo Bolster,1  

E-print Network

Oscillating pendulum decay by emission of vortex rings Diogo Bolster,1 Robert E. Hershberger,2 studied oscillation of a pendulum in water using spherical bobs. By measuring the loss in potential energy also studied the decay of amplitude of the pendulum over time. As observed previously, at small

Bolster, Diogo

185

Motion and decay of vortex rings submerged in a rotational flow  

NASA Technical Reports Server (NTRS)

The interaction between vortex rings of finite strength and an axisymmetric rotational background flow is studied by a singular perturbation method, because it is difficult to use a finite-difference method to analyze the viscous decay in the small core of a vortex ring. The analysis is carried out by combining a composite solution of a vortex ring and an unsteady Euler solution for the background rotational flow. Using the method of averaging, a numerical scheme is developed to obtain an Euler solution in which the grid and time-step sizes depend solely on the length and velocity scales of the background flow. Numerical results are presented to illustrate the interaction between the trajectories and decay rates of the vortex rings and the background rotational flow.

Ishii, K.; Liu, C. H.

1987-01-01

186

Flow mapping of the reversing vortex wake of a cylinder in planar harmonic flow  

SciTech Connect

The reversing near flow of a cylinder in planar harmonic flow at Keulegan Carpenter numbers in the drag-inertia dominated flow regime and at subcritical Reynolds numbers is investigated. LDV measurements of streamwise and cross-stream velocities near the cylinder and streamwise velocities at the far flow field are taken and recorded simultaneously. Visualization of the flow around the cylinder is performed by seeding the flow with small wax particles which scatter light when moving through a laser light sheet. Velocity data are reduced via mode phase averaging to average velocity cycles representing the positive and negative flow modes--dominant vortex shedding and convecting from the top and the bottom of the cylinder, respectively. Flow mapping, based on the mode phase averages of measured velocity data, captures the basic features of the vortex patterns that have been identified by flow visualization.

Sibetheros, I.A.; Miksad, R.W.; Ventre, A.V. [Univ. of Texas, Austin, TX (United States); Lambrakos, K.F. [Exxon Production Research Co., Houston, TX (United States)

1994-12-31

187

A Probabilistic Wake Vortex Lateral Transport Model Using Data from SFO and DEN  

NASA Technical Reports Server (NTRS)

In a previous report, we considered the behavior of the lateral position of vortices as a function of time after vortex formation for Out of Ground Effects (OGE) data for aircraft landing at San Francisco International Airport (SFO). We quantified the spread in lateral position as a function of time and examined how predictable lateral position is under a variety of assumptions. The combination of spread and predictability allowed us to derive probability distribution functions (PDFs) for lateral position given observed crosswind (CW) velocities. In this study, we examine the portability of these PDFs with respect to other landing sites. To this end, we consider OGE data obtained by the Federal Aviation Administration for landings at Denver International Airport (DEN) between 04/05/2006 and 06/03/2006. We consider vortices from both B733 (Boeing 737 models 200-500) and B757 (Boeing 757) aircraft. The data set contains 635 B733 landings and 506 B757 landings. The glide slope altitude for these measurements was 280 m, determined by the average initial vortex observation adjusted for a 3-second delay in the initial observation. The comparable SFO altitude was 158 m. We note that the principal mechanism for lateral transport in the OGE regime is advection by the ambient wind. This implies that a simple crosswind correction may be effective in explaining much of the variation in the lateral transport data. In this study, we again consider the use of ASOS data and average Lidar crosswind data over the vortex altitude range to predict vortex location as a function of time.

Mellman, George R.; Delisi, Donald P.

2008-01-01

188

Airloads, wakes, and aeroelasticity  

NASA Technical Reports Server (NTRS)

Fundamental considerations regarding the theory of modeling of rotary wing airloads, wakes, and aeroelasticity are presented. The topics covered are: airloads and wakes, including lifting-line theory, wake models and nonuniform inflow, free wake geometry, and blade-vortex interaction; aerodynamic and wake models for aeroelasticity, including two-dimensional unsteady aerodynamics and dynamic inflow; and airloads and structural dynamics, including comprehensive airload prediction programs. Results of calculations and correlations are presented.

Johnson, Wayne

1990-01-01

189

Vortex-dominated flow with viscous core structure  

NASA Technical Reports Server (NTRS)

Recent theoretical studies of vortex-dominated flows are reviewed with special emphasis on those for which the viscous core structures play an important role. The problems to be described are: The interaction and merging of two-dimensional vortices and of curved vortex filaments, the roll-up and decay of trailing far wakes, and the initiation of vortex breakdown. The analysis utilizes finite-difference solutions of the Navier-Stokes equations complemented by asymptotic expansion techniques.

Liu, C. H.; Krause, E.; Ting, L.

1985-01-01

190

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

191

Secondary frequencies in the wake of a circular cylinder with vortex shedding  

NASA Technical Reports Server (NTRS)

A detailed numerical study of two-dimensional flow past a circular cylinder at moderately low Reynolds numbers was conducted using three different numerical algorithms for solving the time-dependent compressible Navier-Stokes equations. It was found that if the algorithm and associated boundary conditions were consistent and stable, then the major features of the unsteady wake were well-predicted. However, it was also found that even stable and consistent boundary conditions could introduce additional periodic phenomena reminiscent of the type seen in previous wind-tunnel experiments. However, these additional frequencies were eliminated by formulating the boundary conditions in terms of the characteristic variables. An analysis based on a simplified model provides an explanation for this behavior.

Abarbanel, Saul S.; Don, Wai Sun; Gottlieb, David; Rudy, David H.; Townsend, James C.

1991-01-01

192

Secondary frequencies in the wake of a circular cylinder with vortex shedding  

NASA Technical Reports Server (NTRS)

A detailed numerical study of two-dimensional flow past a circular cylinder at moderately low Reynolds numbers was conducted using three different numerical algorithms for solving the time-dependent compressible Navier-Stokes equations. It was found that if the algorithm and associated boundary conditions were consistent and stable, then the major features of the unsteady wake were well-predicted. However, it was also found that even stable and consistent boundary conditions could introduce additional periodic phenomena reminiscent of the type seen in previous wind-tunnel experiments. However, these additional frequencies were eliminated by formulating the boundary conditions in terms of the characteristic variables. An analysis based on a simplified model provides an explanation for this behavior.

Abarbanel, Saul S.; Don, Wai Sun; Gottlieb, David; Rudy, David H.; Townsend, James C.

1990-01-01

193

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

194

Visualization on fish's wake  

NASA Astrophysics Data System (ADS)

In this paper an experiment on wake of Goldfish swimming unrestricted was conducted in a water tunnel. Method of color liquid was used to visualize the wake. Results show that there is reverse Karman vortex street in symmetrical plane of the wake and the Strouhal frequency of the fish is in the range 0.25-0.35. A 3D vortex ring chain model was presented.

Li, Xuemin; Lu, Xiyun; Yin, Xiezhen

2002-05-01

195

Commercial aircraft wake vortices  

Microsoft Academic Search

This paper discusses the problem of wake vortices shed by commercial aircraft. It presents a consolidated European view on the current status of knowledge of the nature and characteristics of aircraft wakes and of technical and operational procedures of minimizing and predicting the vortex strength and avoiding wake encounters. Methodological aspects of data evaluation and interpretation, like the description of

Thomas Gerza; Frank Holz

196

Decay of the vortex tangle at zero temperature and quasiclassical turbulence  

NASA Astrophysics Data System (ADS)

We review and analyze a series of works, both experimental and numerical and theoretical, dealing with the decay of quantum turbulence at zero temperature. Free decay of the vortex tangle is a key argument in favor of the idea that a chaotic set of quantum vortices can mimic classical turbulence, or at least reproduce many of the basic features. The corresponding topic is referred as the quasiclassical turbulence. Appreciating significance of the challenging problem of classical turbulence it can be expressed that the idea to study it in terms of quantized line is indeed very important and may be regarded as a breakthrough. For this reason, the whole theory, together with the supporting experimental results and numerical simulations should be carefully scrutinized. One of the main arguments, supporting the idea of quasiclassical turbulence is the fact that vortex tangle decays at zero temperature, when the mutual friction is absent. Since all other possible mechanisms of dissipation of the vortex energy, discussed in literature, are related to the small scales, it is natural to suggest that the Kolmogorov cascade takes place with the flow of the energy in space of scales, just like as in the classical turbulence. In the present work we discuss an alternative mechanism of decay of the vortex tangle, which is not associated with dissipation at small scales. This mechanism is a diffusive-like spreading of the vortex tangle due to evaporation of small vortex loops. We discuss a number of experiments and numerical simulations, considering them from the point of view of alternative mechanism.

Nemirovskii, Sergey K.

2013-10-01

197

Decay of a potential vortex in a generalized Oldroyd-B fluid  

Microsoft Academic Search

The velocity field and the adequate shear stress corresponding to the decay of a potential vortex in a generalized Oldroyd-B fluid are determined by means of Hankel and Laplace transforms. The exact solutions, written in terms of the generalized G and R functions, are presented as a sum of the Newtonian solutions and the adequate non-Newtonian contributions. These solutions can

Corina Fetecau; M. Khan; D. Vieru

2008-01-01

198

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

199

Comparison of the lifting-line free vortex wake method and the blade-element-momentum theory regarding the simulated loads of multi-MW wind turbines  

NASA Astrophysics Data System (ADS)

Design load simulations for wind turbines are traditionally based on the blade- element-momentum theory (BEM). The BEM approach is derived from a simplified representation of the rotor aerodynamics and several semi-empirical correction models. A more sophisticated approach to account for the complex flow phenomena on wind turbine rotors can be found in the lifting-line free vortex wake method. This approach is based on a more physics based representation, especially for global flow effects. This theory relies on empirical correction models only for the local flow effects, which are associated with the boundary layer of the rotor blades. In this paper the lifting-line free vortex wake method is compared to a state- of-the-art BEM formulation with regard to aerodynamic and aeroelastic load simulations of the 5MW UpWind reference wind turbine. Different aerodynamic load situations as well as standardised design load cases that are sensitive to the aeroelastic modelling are evaluated in detail. This benchmark makes use of the AeroModule developed by ECN, which has been coupled to the multibody simulation code SIMPACK.

Hauptmann, S.; Bülk, M.; Schön, L.; Erbslöh, S.; Boorsma, K.; Grasso, F.; Kühn, M.; Cheng, P. W.

2014-12-01

200

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

201

NASA Aircraft Vortex Spacing System Development Status  

NASA Technical Reports Server (NTRS)

The National Aeronautics and Space Administration (NASA) is addressing airport capacity enhancements during instrument meteorological conditions through the Terminal Area Productivity (TAP) program. Within TAP, the Reduced Spacing Operations (RSO) subelement 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. The demonstration is only of concept feasibility, and additional effort is required to deploy an operational system for actual aircraft spacing reduction. This paper describes the AVOSS system architecture, a wake vortex facility established at the Dallas-Fort Worth International Airport (DFW), initial operational experience with the AVOSS system, and emerging considerations for subsystem requirements. Results of the initial system operation suggest a significant potential for reduced spacing.

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

1999-01-01

202

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.

2012-06-26

203

Rotor Vortex Filaments: Living on the Slipstream's Edge  

NASA Technical Reports Server (NTRS)

The purpose of this paper is to gain a better understanding of rotor wake evolution in hover and axial flow by deriving an analytical solution for the time dependent behavior of vortex filament circulation and core size. This solution is applicable only for vortex filaments in the rotor far-wake. A primarily inviscid vortex/shear layer interaction (where the slipstream boundary is modeled as a shear layer) has been identified in this analytical treatment. This vortex/shear layer interaction results in decreasing, vortex filament circulation and core size with time. The inviscid vortex/shear layer interaction is shown, in a first-order treatment, to be of greater magnitude than viscous diffusion effects. The rate of contraction, and ultimate collapse, of the vortex filament core is found to be directly proportional to the rotor inflow velocity. This new insight into vortex filament decay promises to help reconcile several disparate observations made in the literature and will, hopefully, promote new advances in theoretical modeling of rotor wakes.

Young, Larry A.

1997-01-01

204

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

205

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

206

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

207

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

208

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

209

The effect of single-horn glaze ice on the vortex structures in the wake of a horizontal axis wind turbine  

NASA Astrophysics Data System (ADS)

The present study experimentally investigated the effect of a simulated single-horn glaze ice accreted on rotor blades on the vortex structures in the wake of a horizontal axis wind turbine by using the stereoscopic particle image velocimetry (Stereo-PIV) technique. During the experiments, four horizontal axis wind turbine models were tested, and both "free-run" and "phase-locked" Stereo-PIV measurements were carried out. Based on the "free-run" measurements, it was found that because of the simulated single-horn glaze ice, the shape, vorticity, and trajectory of tip vortices were changed significantly, and less kinetic energy of the airflow could be harvested by the wind turbine. In addition, the "phase-locked" results indicated that the presence of simulated single-horn glaze ice resulted in a dramatic reduction of the vorticity peak of the tip vortices. Moreover, as the length of the glaze ice increased, both root and tip vortex gaps were found to increase accordingly.

Jin, Zhe-Yan; Dong, Qiao-Tian; Yang, Zhi-Gang

2015-02-01

210

Investigation of Some Wake Vortex Characteristics of an Inclined Ogive-Cylinder Body at Mach Number 2  

NASA Technical Reports Server (NTRS)

For a body consisting of a fineness-ratio-3 ogival nose tangent to a cylindrical afterbody 7.3 diameters long, pitot-pressure distributions in the flow field, pressure distributions over the body, and downwash distributions along a line through the vortex centers have been measured for angles of attack to 20 degrees. The Reynolds numbers, based on body diameter, were 0.15 x 10 to the 6th power and 0.44 x 10 to the 6th power. Comparisons of computed and measured vortex paths and downwash distributions are made. (author)

Jorgensen, Leland H; Perkins, Edward W

1958-01-01

211

Numerical simulations of swirling pipe flows- decay of swirl and occurrence of vortex structures  

NASA Astrophysics Data System (ADS)

The present work aims at better understanding of the physics underlying swirling flows in pipes by means of numerical simulations. Direct numerical simulations have been carried out by using two different inlet swirl conditions. In one case, rotating honeycomb is used as the means to generate swirl whereas in the other case a solid body rotation is provided at the inlet. The inlet swirl intensity is varied in order to scan the underlying physics. Reynolds number 1730 is selected so that the flow remains in laminar regime. The results are compared with those obtained from the experiments using a similar experimental set-up. It is shown that the increase in the inlet swirl intensity leads to a faster decay of swirl downstream of the pipe. Similarly certain specific vortex structures are observed in the radial velocity contours. These structures are thought to be analogous with those found in the Taylor-Couette flow between a stationary outer cylinder and inner rotating cone. The reported investigations reveal dependence of swirl decay on the inlet swirl intensity and occurrence of vortex structures.

Vaidya, H. A.; Ertunç, Ö.; Genç, B.; Beyer, F.; Köksoy, Ç.; Delgado, A.

2011-12-01

212

Wake Turbulence Mitigation for Arrivals (WTMA)  

NASA Technical Reports Server (NTRS)

The preliminary Wake Turbulence Mitigation for Arrivals (WTMA) concept of operations is described in this paper. The WTMA concept provides further detail to work initiated by the Wake Vortex Avoidance System Concept Evaluation Team and is an evolution of the Wake Turbulence Mitigation for Departure concept. Anticipated benefits about reducing wake turbulence separation standards in crosswind conditions, and candidate WTMA system considerations are discussed.

Williams, Daniel M.; Lohr, Gary W.; Trujillo, Anna C.

2008-01-01

213

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

214

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

215

Kinematics of flight and the relationship to the vortex wake of a Pallas' long tongued bat (Glossophaga soricina).  

PubMed

To obtain a full understanding of the aerodynamics of animal flight, the movement of the wings, the kinematics, needs to be connected to the wake left behind the animal. Here the detailed 3D wingbeat kinematics of bats, Glossophaga soricina, flying in a wind tunnel over a range of flight speeds (1-7 m s(-1)) was determined from high-speed video. The results were compared with the wake geometry and quantitative wake measurements obtained simultaneously to the kinematics. The wingbeat kinematics varied gradually with flight speed and reflected the changes observed in the wake of the bats. In particular, several of the kinematic parameters reflected the differences in the function of the upstroke at low and high flight speeds. At lower flight speeds the bats use a pitch-up rotation to produce a backward flick which creates thrust and some weight support. At higher speeds this mechanism disappears and the upstroke generates weight support but no thrust. This is reflected by the changes in e.g. angle of attack, span ratio, camber and downstroke ratio. We also determined how different parameters vary throughout a wingbeat over the flight speeds studied. Both the camber and the angle of attack varied over the wingbeat differently at different speeds, suggesting active control of these parameters to adjust to the changing aerodynamic conditions. This study of the kinematics strongly indicates that the flight of bats is governed by an unsteady high-lift mechanism at low flight speeds and points to differences between birds and bats. PMID:20511529

Wolf, Marta; Johansson, L Christoffer; von Busse, Rhea; Winter, York; Hedenström, Anders

2010-06-15

216

A note on decay of potential vortex in an Oldroyd-B fluid through a porous space  

Microsoft Academic Search

The problem of decay of a potential vortex in an Oldroyd-B fluid filling the porous space is studied. The flow problem is first modeled and then solved by employing the Hankel transform. Analytical expressions of the velocity field and the associated tangential tension are developed. The well known solutions for a Newtonian fluid as well as those corresponding to a

M. Husain; T. Hayat; C. Fetecau

2009-01-01

217

Measurements of fish's wake by PIV  

NASA Astrophysics Data System (ADS)

In this paper an experiment on measurements of the wake of Goldfish carassius auratus swimming unrestricted was conducted in a water tunnel. Color liquid was used to visualize the wake of the fish and PIV was used to measure velocity field of the wake. Results show that there is reverse Karman vortex street in symmetrical plane of the fish's wake and the Strouhal frequency of the fish is about 0.35 udner the different experimental conditions. The distribution of velocity and vorticity in the wake of Goldfish was measured by PIV and formation of reverse Karman vortex street in the wake was studied in a model experiment.

Li, Xuemin; Wu, Yanfeng; Lu, Xiyun; Yin, Xiezhen

2003-04-01

218

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

219

Wingtip Vortex-Augmented Turbopusher Propeller Thrust  

NASA Technical Reports Server (NTRS)

Thrust of propeller enhanced by tip vortex. Wingtip-Mounted Nacelle provides turboprop vortex velocity recovery. Thrust of turbopusher propeller increased by flow of lift-induced vortex. As result of weaker vortex, reduction in induced drag of wing afforded by propeller-wake mass injection into core of vortex, causing it to break down.

Patterson, J. C., Jr.

1985-01-01

220

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

221

Numerical Modeling Studies of Wake Vortices: Real Case Simulations  

NASA Technical Reports Server (NTRS)

A three-dimensional large-eddy simulation model, TASS, is used to simulate the behavior of aircraft wake vortices in a real atmosphere. The purpose for this study is to validate the use of TASS for simulating the decay and transport of wake vortices. Three simulations are performed and the results are compared with the observed data from the 1994-1995 Memphis field experiments. The selected cases have an atmospheric environment of weak turbulence and stable stratification. The model simulations are initialized with appropriate meteorological conditions and a post roll-up vortex system. The behavior of wake vortices as they descend within the atmospheric boundary layer and interact with the ground is discussed.

Shen, Shao-Hua; Ding, Feng; Han, Jongil; Lin, Yuh-Lang; Arya, S. Pal; Proctor, Fred H.

1999-01-01

222

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

223

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

224

Vortex cloud model for body vortex shedding and tracking  

NASA Technical Reports Server (NTRS)

The discrete vortex cloud approach models a missile airframe's vortex wake by means that are capable of treating a variety of configurations over a range of flow conditions. Attention is given to the sheets of vorticity formed on the lee side of a missile at moderate angles-of-attack. While three-dimensional attached flow models are used to represent the missile body, two-dimensional, incompressible, separated flow models are used to represent the separated vortex wake. The predicted pressure distribution of the body under the influence of the freestream and the separation vortex wake are used to calculate aerodynamic loads on the body. The separation vortex wake is represented by clouds of discrete vortices in cross flow planes normal to the body axis.

Mendenhall, Michael R.; Perkins, Stanley C., Jr.

1986-01-01

225

Vortex cloud model for body vortex shedding and tracking  

NASA Technical Reports Server (NTRS)

The present engineering prediction method addresses the aerodynamic characteristics and associated flowfields of circular and noncircular bodies at large incidence angles and arbitrary roll angles. 3D attached-flow models are used to represent the body, and 2D, incompressible, separated flow models are used to represent the separated vortex wake. The predicted pressure distribution of the body, under the influence of both the freestream and the separation vortex wake, is used to calculate aerodynamic loads. The separation vortex wake is represented by clouds of discrete vortices in crossflow planes normal to the body axis.

Mendenhall, Michael R.; Perkins, Stanley C., Jr; Lesieutre, Daniel J.

1992-01-01

226

Wake Studies of Ornithopters  

NASA Astrophysics Data System (ADS)

This paper details experiments using a mechanical ornithopter flying in a low speed wind tunnel. Experiments were conducted for a Strouhal number of 0.3 and Reynolds number of 2300, Particle Image Velocimetry (PIV) and flow visualization was used to develop quantitative and qualitative information about the nature of the wake. The data shows that the wake is made of a series of discrete vortex rings. The impulse of these rings has been estimated with PIV data and the results correlate well with the lift required to sustain the ornithopter in flight.

Juarez, Alfredo; Harlow, Jacob; Allen, James; Ferreira de Sousa, Paulo

2006-11-01

227

Decay of a potential vortex and propagation of a heat wave in a second grade fluid  

Microsoft Academic Search

The propagation of a heat wave in an incompressible second grade fluid within the context of a potential vortex is studied. The solutions for the Newtonian fluid can be obtained from those for fluids of second grade as a limiting case.

C. Fetecau; J. Zierep

2002-01-01

228

High Resolution Wake Capturing Methodology for Hovering Rotors Karthikeyan Duraisamy  

E-print Network

edge is lacking in the literature, reasonable validation of the vortex velocity profiles-bladed rotor case, the tip vortex could be tracked up to two rotor revolutions with minimal diffusion ) wake age (degrees) Introduction The wake generated by a helicopter blade is characterized by concen

Alonso, Juan J.

229

Three dimensional mean flow and turbulence characteristics of the near wake of a compressor rotor blade  

NASA Technical Reports Server (NTRS)

The investigation was carried out using the rotating hot wire technique. Measurements were taken inside the end wall boundary layer to discern the effect of annulus and hub wall boundary layer, secondary flow, and tip leakage on the wake structure. Static pressure gradients across the wake were measured using a static stagnation pressure probe insensitive to flow direction changes. The axial and the tangential velocity defects, the radial component of velocity, and turbulence intensities were found to be very large as compared to the near and far wake regions. The radial velocities in the trailing edge region exhibited characteristics prevalent in a trailing vortex system. Flow near the blade tips found to be highly complex due to interaction of the end wall boundary layers, secondary flows, and tip leakage flow with the wake. The streamwise curvature was found to be appreciable near the blade trailing edge. Flow properties in the trailing edge region are quite different compared to that in the near and far wake regions with respect to their decay characteristics, similarity, etc. Fourier decomposition of the rotor wake revealed that for a normalized wake only the first three coefficients are dominant.

Ravindranath, A.; Lakshminarayana, B.

1980-01-01

230

Wake characteristics of a model ornithopter  

NASA Astrophysics Data System (ADS)

This paper details unsteady wake measurements from a model Ornithopther flying in a wind tunnel at representative flight conditions. Testing over a range of Strouhal number, 0.1-0.3, shows that the unsteady wake is composed of coherent vortical structures that resemble vortex rings. A single ring is formed in the wake of each wing during one wing beat. Momentum balance from velocity field measurements are reconciled with unsteady lift and drag measurements from a drag balance.

Juarez, Alfredo; Harlow, Jacob; Allen, James; Ferreira de Sousa, Paulo

2006-03-01

231

Decay of a potential vortex in an Oldroyd-B fluid  

Microsoft Academic Search

Analytical expressions for the velocity field and the associated tangential tension corresponding to a potential vortex in an Oldroyd-B fluid are determined by means of the Hankel transform. The well-known solutions for a Navier–Stokes fluid as well as those corresponding to a Maxwell fluid and a second grade one, appear as limiting cases of our solutions. Finally, some comparative diagrams

C. Fetecau

2005-01-01

232

Propeller tip vortex interactions  

NASA Technical Reports Server (NTRS)

Propeller wakes interacting with aircraft aerodynamic surfaces are a source of noise and vibration. For this reason, flow visualization work on the motion of the helical tip vortex over a wing and through the second stage of a counterrotation propeller (CRP) has been pursued. Initially, work was done on the motion of a propeller helix as it passes over the center of a 9.0 aspect ratio wing. The propeller tip vortex experiences significant spanwise displacements when passing across a lifting wing. A stationary propeller blade or stator was installed behind the rotating propeller to model the blade vortex interaction in a CRP. The resulting vortex interaction was found to depend on the relative vortex strengths and vortex sign.

Johnston, Robert T.; Sullivan, John P.

1990-01-01

233

Holographic flow visualization. [of aircraft wakes  

NASA Technical Reports Server (NTRS)

Holographic visualization techniques are presented of the vortex wake of a lifting wing. The motions of tracer particles in vortical flows are described along with the development of a liquid-drop tracer generator. An analysis is presented of the motion of particles of arbitrary density and size in solid body and potential vortex flows.

Charwat, A. F.; Fourney, M. E.

1976-01-01

234

VIDEO IMAGES OF SMOKE DISPERSION IN THE NEAR WAKE OF A BUILDING PART I. TEMPORAL AND SPATIAL SCALES OF VORTEX SHEDDING  

EPA Science Inventory

In a wind-tunnel study, recorded video images of smoke dispersion in the wake of a rectangular-shaped building web analyzed. ontinuous source of smoke was emitted at floor level, midway along the leeward side of the building. moke was observed to build up within a region adjacent...

235

Turbulence Measurements in the Near Field of a Wingtip Vortex  

NASA Technical Reports Server (NTRS)

The roll-up of a wingtip vortex, at Reynolds number based on chord of 4.6 million was studied with an emphasis on suction side and near wake measurements. The research was conducted in a 32 in. x 48 in. low-speed wind tunnel. The half-wing model had a semi-span of 36 in. a chord of 48 in. and a rounded tip. Seven-hole pressure probe measurements of the velocity field surrounding the wingtip showed that a large axial velocity of up to 1.77 U(sub infinity) developed in the vortex core. This level of axial velocity has not been previously measured. Triple-wire probes have been used to measure all components of the Reynolds stress tensor. It was determined from correlation measurements that meandering of the vortex was small and did not appreciably contribute to the turbulence measurements. The flow was found to be turbulent in the near-field (as high as 24 percent RMS w - velocity on the edge of the core) and the turbulence decayed quickly with streamwise distance because of the nearly solid body rotation of the vortex core mean flow. A streamwise variation of the location of peak levels of turbulence, relative to the core centerline, was also found. Close to the trailing edge of the wing, the peak shear stress levels were found at the edge of the vortex core, whereas in the most downstream wake planes they occurred at a radius roughly equal to one-third of the vortex core radius. The Reynolds shear stresses were not aligned with the mean strain rate, indicating that an isotropic-eddy-viscosity based prediction method cannot accurately model the turbulence in the cortex. In cylindrical coordinates, with the origin at the vortex centerline, the radial normal stress was found to be larger than the circumferential.

Chow, Jim; Zilliac, Greg; Bradshaw, Peter

1997-01-01

236

Helicopter rotor wake geometry and its influence in forward flight. Volume 1: Generalized wake geometry and wake effect on rotor airloads and performance  

NASA Technical Reports Server (NTRS)

An analytic investigation to generalize wake geometry of a helicopter rotor in steady level forward flight and to demonstrate the influence of wake deformation in the prediction of rotor airloads and performance is described. Volume 1 presents a first level generalized wake model based on theoretically predicted tip vortex geometries for a selected representative blade design. The tip vortex distortions are generalized in equation form as displacements from the classical undistorted tip vortex geometry in terms of vortex age, blade azimuth, rotor advance ratio, thrust coefficient, and number of blades. These equations were programmed to provide distorted wake coordinates at very low cost for use in rotor airflow and airloads prediction analyses. The sensitivity of predicted rotor airloads, performance, and blade bending moments to the modeling of the tip vortex distortion are demonstrated for low to moderately high advance ratios for a representative rotor and the H-34 rotor. Comparisons with H-34 rotor test data demonstrate the effects of the classical, predicted distorted, and the newly developed generalized wake models on airloads and blade bending moments. Use of distorted wake models results in the occurrence of numerous blade-vortex interactions on the forward and lateral sides of the rotor disk. The significance of these interactions is related to the number and degree of proximity to the blades of the tip vortices. The correlation obtained with the distorted wake models (generalized and predicted) is encouraging.

Egolf, T. A.; Landgrebe, A. J.

1983-01-01

237

Dynamics and control of hydrofoil wakes  

NASA Astrophysics Data System (ADS)

The problem of rotor-stator interaction (RSI) is an issue within the field of turbomachinery. The flow field entering the rotor cascade will depend on the stator blade to blade velocity distributions, and the viscous wake trailing cascade blades. This flow field is also dependent on the mode of operation, e.g by changing the angle of each blade in hydroturbines. Manipulating the stator viscous wakes is one method to minimize the problems associated RSI; i.e. noise and vibration. In order to explore this concept, a comprehensive experimental program was carried out in a high-speed water tunnel utilizing a series of NACA 0015 hydrofoils. Baseline wake data were collected with a hydraulically smooth foil and compared with two foils modified with two sizes of vortex generators (VG) positioned close to the leading edge of the foil. Not only was the effect of the modifications on wake spreading investigated but also the effect on wake dynamics such as vortex shedding was studied. A high frame-rate PIV system was used at recording rates of 1 and 10 kHz to map the near wake region, extending roughly 1 chord-length downstream the trailing edge, over a range of angles of attack and velocities. The results show that wake dynamics and wake characteristics, i.e. velocity deficit and width, scale with average drag. It was demonstrated that the use of VGs can improve both the dynamics and spreading characteristics of the wake.

Kjeldsen, Morten; Wosnik, Martin; Arndt, Roger

2008-11-01

238

Micro-vortex generators for shock wave/boundary layer interactions  

NASA Astrophysics Data System (ADS)

The effect of micro-vortex generators (MVGs) on shock wave/boundary layer interactions (SBLIs) has been reviewed. Experimental and computational evidence has been presented about the dominant role it has in the suppression of shock-induced separation the pair of counter-rotating streamwise vortices, which appears downstream of the types of micro-vortex generators used in SBLIs; these streamwise vortices entrain high momentum fluid, increasing the boundary layer velocity near the wall. The structure of the wake is examined in detail, with emphasis on the strength and decay of the streamwise vortices and on the ring-type or hairpin vortices which have been detected in the instantaneous flow around the wake. Evaluation of the ability of various types of MVGs to suppress shock-induced separation is done. Topics which need to be further studied, like the structure of the flow around devices of small size and the effect of the Reynolds number, are suggested.

Panaras, Argyris G.; Lu, Frank K.

2015-04-01

239

Vortex dynamics in 4 Banavara N. Shashikanth  

E-print Network

of oblique vortex shedding behind a heated circular cylinder in laminar wake regime Phys. Fluids 24, 011701 using a local induction approximation. The regularized self-induced velocity field is then shown

Shashikanth, Banavara N.

240

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

241

Mesoscale wake clouds in Skylab pictures.  

NASA Technical Reports Server (NTRS)

The recognition of cloud patterns formed in the wake of orographic obstacles was investigated using pictures from Skylab, for the purpose of estimating atmospheric motions. The existence of ship-wake-type wave clouds in contrast to vortex sheets were revealed during examination of the pictures, and an attempt was made to characterize the pattern of waves as well as the transition between waves and vortices. Examples of mesoscale cloud patterns which were analyzed photogrammetrically and meteorologically are presented.

Fujita, T. T.; Tecson, J. J.

1974-01-01

242

Thrust-augmented vortex attenuation  

NASA Technical Reports Server (NTRS)

An experimental investigation was conducted to determine the vortex attenuating effect of engine thrust. Tests were made using a 0.03-scale model of the Boeing 747 transport aircraft as a vortex generating model. A Learjet-class probe model was used to measure the vortex induced rolling moment at a scale separation distance of 1.63 km. These tests were conducted at a lift coefficient of 1.4 at a model velocity of 30.48 m/s. The data presented indicate that engine thrust is effective as a vortex attenuating device when the engines are operated at high thrust levels and are positioned to direct the high energy engine wake into the core of the vortex. The greatest thrust vortex attenuation was obtained by operating the inboard engine thrust reversers at one-quarter thrust and the outboard engines at maximum forward thrust.

Patterson, J. C., Jr.; Jordan, F. L., Jr.

1977-01-01

243

Control of submersible vortex flows  

NASA Technical Reports Server (NTRS)

Vortex flows produced by submersibles typically unfavorably influence key figures of merit such as acoustic and nonacoustic stealth, control effectiveness/maneuverability, and propulsor efficiency/body drag. Sources of such organized, primarily longitudinal, vorticity include the basic body (nose and sides) and appendages (both base/intersection and tip regions) such as the fairwater, dive planes, rear control surfaces, and propulsor stators/tips. Two fundamentally different vortex control approaches are available: (1) deintensification of the amplitude and/or organization of the vortex during its initiation process; and (2) downstream vortex disablement. Vortex control techniques applicable to the initiation region (deintensification approach) include transverse pressure gradient minimization via altered body cross section, appendage dillets, fillets, and sweep, and various appendage tip and spanload treatment along with the use of active controls to minimize control surface size and motions. Vortex disablement can be accomplished either via use of control vortices (which can also be used to steer the vortices off-board), direct unwinding, inducement of vortex bursting, or segmentation/tailoring for enhanced dissipation. Submersible-applicable vortex control technology is also included derived from various aeronautical applications such as mitigation of the wing wake vortex hazard and flight aircraft maneuverability at high angle of attack as well as the status of vortex effects upon, and mitigation of, nonlinear control forces on submersibles. Specific suggestions for submersible-applicable vortex control techniques are presented.

Bushnell, D. M.; Donaldson, C. D.

1990-01-01

244

Vortex Jitter in Hover Swathi M. Mula  

E-print Network

Vortex Jitter in Hover Swathi M. Mula , James Stephenson , Charles E. Tinney , Jayant Sirohi during hover is studied using vortex methods combined with a center of mass analysis approach of the unsteady wake produced by rotorcraft during hover, descent, ascent and forward flight conditions

Tinney, Charles E.

245

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

246

Strouhal-Reynolds number relationship for vortex streets.  

PubMed

A rationale for the empirically observed Strouhal-Reynolds number relation for vortex shedding in the wake of a cylinder is provided. This rationale derives from a mechanism of vortex formation observed in numerical simulations of two-dimensional vortex shedding coupled with an order of magnitude estimate of the terms in the vorticity transport equation based on this mechanism. PMID:15447193

Ponta, Fernando L; Aref, Hassan

2004-08-20

247

Effects of Magnetic Field on the Turbulent Wake of a Cylinder in MHD Channel Flow  

SciTech Connect

Results from a free-surface MHD flow experiment are presented detailing the modi cation of vortices in the wake of a circular cylinder with its axis parallel to the applied magnetic fi eld. Experiments were performed with a Reynolds number near Re ~ 104 as the interaction parameter, N = |j x#2; B| / |? (? ? ?), was increased through unity. By concurrently sampling the downstream fluid velocity at sixteen cross-stream locations in the wake, it was possible to extract an ensemble of azimuthal velocity profi les as a function of radius for vortices shed by the cylinder at varying strengths of magnetic field. Results indicate a signi cant change in vortex radius and rotation as N is increased. The lack of deviations from the vortex velocity pro file at high magnetic fi elds suggests the absence of small-scale turbulent features. By sampling the wake at three locations downstream in subsequent experiments, the decay of the vortices was examined and the effective viscosity was found to decrease as N-049±0.4. This reduction in effective viscosity is due to the modi cation of the small-scale eddies by the magnetic fi eld. The slope of the energy spectrum was observed to change from a k-1.8 power-law at low N to a k-3.5 power-law for N > 1. Together, these results suggest the flow smoothly transitioned to a quasi-two-dimensional state in the range 0 < N < 1.

John Rhoads, Eric Edlund and Hantao Ji

2013-04-17

248

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

249

Computation and analysis of a cylinder wake flow  

NASA Technical Reports Server (NTRS)

The Karman vortex wake of a circular cylinder at low Reynolds number was computed by a time-accurate, two-dimensional compressible Navier-Stokes equation solver which uses the MacCormack predictor-corrector finite-difference scheme and a nonreflecting boundary condition on the outer flow boundary. The results from a large number of time steps were analyzed using Fast Fourier Transform techniques to identify the important frequency components for comparison with published experimental data. A strong low-frequency component was found below the vortex shedding frequency and not harmonically related to it. The experimentally discovered low-frequency fluctuations in the cylinder wake are considered possibly to be precursors to transition from laminar to turbulent flow conditions. The present finding of similar frequencies in a computed wake tends to confirm their existence as a real wake phenomenon. This computational work provides a complementary means to experimental investigations of wake phenomena.

Townsend, J. C.; Rudy, D. H.; Sirovich, L.

1987-01-01

250

Transitions in the wake of a flapping foil  

NASA Astrophysics Data System (ADS)

We study experimentally the vortex streets produced by a flapping foil in a hydrodynamic tunnel, using two-dimensional particle image velocimetry. An analysis in terms of a flapping frequency-amplitude phase space allows the identification of (i) the transition from the well-known Bénard-von Kármán (BvK) wake to the reverse BvK vortex street that characterizes propulsive wakes, and (ii) the symmetry breaking of this reverse BvK pattern giving rise to an asymmetric wake. We also show that the transition from a BvK wake to a reverse BvK wake precedes the actual drag-thrust transition and we discuss the significance of the present results in the analysis of flapping systems in nature.

Godoy-Diana, Ramiro; Aider, Jean-Luc; Wesfreid, José Eduardo

2008-01-01

251

Some aspects of streamwise vortex behavior during oblique shock wave\\/vortex interaction  

Microsoft Academic Search

.   An experimental study of the flowfield generated by the interaction of a streamwise vortex having a strong wake-type axial\\u000a Mach number profile and a two-dimensional oblique shock wave was conducted in a Mach 2.49 flow. The experiments were aimed\\u000a at investigating the dynamics of supersonic vortex distortion and to study downstream behavior of a streamwise vortex during\\u000a a strong

M. K. Smart; I. M. Kalkhoran; S. Popovic

1998-01-01

252

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

253

Wake Geometry Measurements and Analytical Calculations on a Small-Scale Rotor Model  

NASA Technical Reports Server (NTRS)

An experimental investigation was conducted in the Langley 14- by 22-Foot Subsonic Tunnel to quantify the rotor wake behind a scale model helicopter rotor in forward level flight at one thrust level. The rotor system in this test consisted of a four-bladed fully articulated hub with blades of rectangular planform and an NACA 0012 airfoil section. A laser light sheet, seeded with propylene glycol smoke, was used to visualize the vortex geometry in the flow in planes parallel and perpendicular to the free-stream flow. Quantitative measurements of wake geometric proper- ties, such as vortex location, vertical skew angle, and vortex particle void radius, were obtained as well as convective velocities for blade tip vortices. Comparisons were made between experimental data and four computational method predictions of experimental tip vortex locations, vortex vertical skew angles, and wake geometries. The results of these comparisons highlight difficulties of accurate wake geometry predictions.

Ghee, Terence A.; Berry, John D.; Zori, Laith A. J.; Elliott, Joe W.

1996-01-01

254

Theoretical study of lift generated vortex sheets designed to avoid roll up  

NASA Technical Reports Server (NTRS)

The random motions of the vortex elements behind a wing that sheds a disturbed, translating array of vortices are analyzed. The analysis indicates that the wake would diffuse and decay rapidly when viscosity is present and would produce small rolling moments on encountering aircraft. It was found that comparable results could also be achieved with an array consisting of vortices that are equal in magnitude but which alternate in sign. This observation indicates that random motion can probably be achieved with a variety of stepped loadings.

Rossow, V. J.

1973-01-01

255

Devices that Alter the Tip Vortex of a Rotor  

NASA Technical Reports Server (NTRS)

Small devices were attached near the tip of a hovering rotor blade 'in order to alter the structure and trajectory of the trailing vortex. Stereo particle image velocimetry (PIV) images were used to quantify the wake behind the rotor blade during the first revolution. A procedure for analyzing the 3D-velocity field is presented that includes a method for accounting for vortex wander. The results show that a vortex generator can alter the trajectory of the trailing vortex and that a major change in the size and intensity of the trailing vortex can be achieved by introducing a high level of turbulence into the core of the vortex.

McAlister, Kenneth W.; Tung, Chee; Heineck, James T.

2001-01-01

256

Wake Vortex Systems Cost/Benefits Analysis  

NASA Technical Reports Server (NTRS)

The goals of cost/benefit assessments are to provide quantitative and qualitative data to aid in the decision-making process. Benefits derived from increased throughput (or decreased delays) used to balance life-cycle costs. Packaging technologies together may provide greater gains (demonstrate higher return on investment).

Crisp, Vicki K.

1997-01-01

257

Decay  

NSDL National Science Digital Library

Decay is the process of organic rotting. Decay can take many different forms, such as tooth decay. Many times, you can see the visible effects of decay. Some types of decay can be prevented or can be fought against; others are inevitable. Microbes are responsible for decay in fruits, vegetables, and other organisms and products.

Olivia Worland (Purdue University; Biological Sciences)

2008-06-16

258

Development of new tip-loss corrections based on vortex theory and vortex methods  

NASA Astrophysics Data System (ADS)

A new analytical formulation of the tip-loss factor is established based on helical vortex filament solutions. The derived tip-loss factor can be applied to wind-turbines, propellers or other rotary wings. Similar numerical formulations are used to assess the influence of wake expansion on tip-losses. Theodorsen's theory is successfully applied for the first time to assess the wake expansion behind a wind turbine. The tip-loss corrections obtained are compared with the ones from Prandtl and Glauert and implemented within a new Blade Element Momentum(BEM) code. Wake expansion is seen to reduce tip-losses and have a greater influence than wake distortion.

Branlard, Emmanuel; Gaunaa, Mac

2014-12-01

259

Three-dimensional Navier-Stokes calculations of multiple interacting vortex rings  

NASA Technical Reports Server (NTRS)

Results from a finite-difference Navier-Stokes code for three-dimensional, unsteady, vortical flows in unbounded domains are presented and analyzed in this paper. The vortical flows presented are representative of vortex rings and other closed vortical tubes or structures in fluid mechanics. Such structures are important elements in fluid flows such as jets, atmospheric turbulence, and the far-field wakes of aircraft, and studies of their interaction may aid in an understanding of complex fluid flows. The paper demonstrates that computational methods can be used as a viable alternative or supplement to experimental techniques for studying the physics of vortex flows. The separate visualization of vortex stretching, convection, and diffusion is presented in this paper for a single elliptical vortex ring.The calculations employ a truncated series expansion technique to simulate the unbounded nature of the fluid flow with a finite computational domain, which is a more accurate technique than the conventional freestream boundary specification. The numerical divergence of the three-dimensional vorticity field is considered as a useful estimate of truncation error, and the use of a kinetic energy decay law as a calculation check is demonstrated. Results from the Navier-Stokes code are presented for the unsteady motion of two and four vortex rings along parallel axes, and the results agree qualitatively with experimental flow visualization.

Chamberlain, J. P.; Weston, R. P.

1984-01-01

260

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

261

On the structure of the turbulent vortex  

NASA Technical Reports Server (NTRS)

The trailing vortex generated by a lifting surface, the structure of its turbulent core and the influence of axial flow within the vortex on its initial persistence and on its subsequent decay are described. Similarity solutions of the turbulent diffusion equation are given in closed form and results are expressed in sufficiently simple terms that the influence of the lifting surface parameters on the length of persistence and the rate of decay of the vortex can be evaluated.

Roberts, L.

1985-01-01

262

On the wake of a Darrieus turbine  

NASA Technical Reports Server (NTRS)

The theory and experimental measurements on the aerodynamic decay of a wake from high performance vertical axis wind turbine are discussed. In the initial experimental study, the wake downstream of a model Darrieus rotor, 28 cm diameter and a height of 45.5 cm, was measured in a Boundary Layer Wind Tunnel. The wind turbine was run at the design tip speed ratio of 5.5. It was found that the wake decayed at a slower rate with distance downstream of the turbine, than a wake from a screen with similar troposkein shape and drag force characteristics as the Darrieus rotor. The initial wind tunnel results indicated that the vertical axis wind turbines should be spaced at least forty diameters apart to avoid mutual power depreciation greater than ten per cent.

Base, T. E.; Phillips, P.; Robertson, G.; Nowak, E. S.

1981-01-01

263

The 3-D wake measurements near a hovering rotor for determining profile and induced drag  

NASA Technical Reports Server (NTRS)

Primarily an experimental effort, this study focuses on the velocity and vorticity fields in the near wake of a hovering rotor. Drag terminology is reviewed, and the theory for separately determining the profile-and-induced-drag components from wake quantities is introduced. Instantaneous visualizations of the flow field are used to center the laser velocimeter (LV) measurements on the vortex core and to assess the extent of the positional mandering of the trailing vortex. Velocity profiles obtained at different rotor speeds and distances behind the rotor blade clearly indicate the position, size, and rate of movement of the wake sheet and the core of the trailing vortex. The results also show the distribution of vorticity along the wake sheet and within the trailing vortex.

Mcalister, K. W.; Schuler, C. A.; Branum, L.; Wu, J. C.

1995-01-01

264

Measurement of velocity and vorticity fields in the wake of an airfoil in periodic pitching motion  

NASA Technical Reports Server (NTRS)

The velocity field created by the wake of an airfoil undergoing a prescribed pitching motion was sampled using hot wire anemometry. Data analysis methods concerning resolution of velocity components from cross wire data, computation of vorticity from velocity time history data, and calculation of vortex circulation from vorticity field data are discussed. These data analysis methods are applied to a flow field relevant to a two dimensional blade-vortex interaction study. Velocity time history data were differentiated to yield vorticity field data which are used to characterize the wake of the pitching airfoil. Measurement of vortex strength in sinusoidal and nonsinusoidal wakes show vortices in the sinusoidal wake have stronger circulation and more concentrated vorticity distributions than the tailored nonsinusoidal wake.

Booth, Earl R., Jr.

1987-01-01

265

Evaluation of Aircraft Wing-Tip Vortex Using PIV  

NASA Astrophysics Data System (ADS)

The formation and development of a wing-tip vortex in a near and extended near filed were studied experimentally. Particle image velocimetry was used in a wind tunnel to measure the tip vortex velocity field and hence investigate the flow structure in a wake of aircraft half-wing model. The purpose of this investigation is to evaluate the main features of the lift generated vortices in order to find ways to alleviate hazardous wake vortex encounters for follower airplanes during start and approach such that the increase in airport capacity can be achieved. First the wake structure at successive downstream planes crosswise to the axis of the wake vortices was investigated by measuring parameters such as core radius, maximum tangential velocities, vorticities and circulation distributions. The effect of different angles of attack setting on vortex parameters was examined at one downstream location. In very early stages the vortex sheet evolution makes the tip vortex to move inward and to the suction side of the wing. While the core radius and circulation distributions hardly vary with the downstream distance, noticeable differences for the same vortex parameters at different angles of attack settings were observed. The center of the wing tip vortices scatter in a circle of radius nearly equal to 1% of the mean wing chord and wandering amplitudes shows no direct dependence on the vortex strength but linearly increase with the downstream distance.

Alsayed, Omer A.; Asrar, Waqar; Omar, Ashraf A.

2010-06-01

266

Vortex rings  

Microsoft Academic Search

The vortex-ring problem in fluid mechanics is examined generally in terms of formation, the steady state, the duration of the rings, and vortex interactions. The formation is studied by examining the generation of laminar and turbulent vortex rings and their resulting structures with attention given to the three stages of laminar ring development. Inviscid dynamics is addressed to show how

Karim Shariff; Anthony Leonard

1992-01-01

267

On vortex shedding from low aspect ratio dual step cylinders  

NASA Astrophysics Data System (ADS)

A dual-step cylinder is comprised of two cylinders of different diameters. A large diameter cylinder (D) with low aspect ratio (L/D) is attached to the mid-span of a small diameter cylinder (d). The present study investigates the effect of Reynolds number (ReD) and L/D on dual step cylinder wake development for D/d=2, 0.2?L/D?3, and two Reynolds numbers, ReD=1050 and 2100. Experiments have been performed in a water flume facility utilizing flow visualization, Laser Doppler Velocimetry (LDV), and Particle Image Velocimetry (PIV). The results show that vortex shedding occurs from both the large and small diameter cylinders for 1?L/D?3 at ReD=2100 and 2?L/D?3 at ReD=1050. At these conditions, large cylinder vortices predominantly form vortex loops in the wake and small cylinder vortices form half-loop vortex connections. At lower aspect ratios, vortex shedding from the large cylinder ceases, with the dominant frequency in the large cylinder wake attributed to the passage of vortex filaments connecting small cylinder vortices. At these lower aspect ratios, the presence of the large cylinder induces periodic vortex dislocations. Increasing L/D increases the frequency of occurrence of vortex dislocations and decreases the dominant frequency in the large cylinder wake. The identified changes in wake topology are related to substantial variations in the location of boundary layer separation on the large cylinder, and, consequently, changes in the size of the vortex formation region. The results also show that the Reynolds number has a substantial effect on wake vortex shedding frequency, which is more profound than that expected for a uniform cylinder.

Morton, C.; Yarusevych, S.

2014-01-01

268

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.

269

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

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

A prescribed wake rotor inflow and flow field prediction analysis, user's manual and technical approach  

NASA Technical Reports Server (NTRS)

A user's manual is provided which includes the technical approach for the Prescribed Wake Rotor Inflow and Flow Field Prediction Analysis. The analysis is used to provide the rotor wake induced velocities at the rotor blades for use in blade airloads and response analyses and to provide induced velocities at arbitrary field points such as at a tail surface. This analysis calculates the distribution of rotor wake induced velocities based on a prescribed wake model. Section operating conditions are prescribed from blade motion and controls determined by a separate blade response analysis. The analysis represents each blade by a segmented lifting line, and the rotor wake by discrete segmented trailing vortex filaments. Blade loading and circulation distributions are calculated based on blade element strip theory including the local induced velocity predicted by the numerical integration of the Biot-Savart Law applied to the vortex wake model.

Egolf, T. A.; Landgrebe, A. J.

1982-01-01

272

AIAA-982909 Characterization of the Near Wake of a Helicopter Rotor*  

E-print Network

AIAA-982909 Characterization of the Near Wake of a Helicopter Rotor* Raghav Mahalingam of the tip-vortex in the near-wake of a rotor are studied in this paper, in a test case of low-speed forward be carefully examined. In these experiments, it was found that essentially perfect periodicity could

273

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

274

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

275

Structure of leading-edge vortex flows including vortex breakdown  

SciTech Connect

An experimental investigation of the structure of leading-edge vortex flows on thin sharp-edged delta wings was carried out at low Reynolds numbers. Flow-visualization techniques were used to study the topology of the vortex and the phenomenon of vortex breakdown. Seven-hole probe-wake surveys and laser-doppler-anemometer measurements were obtained and compared. Delta wings with sweep angles of 70, 75, 80, and 85/sup 0/ were tested at angles of attack of 10, 20, 30, and 40/sup 0/. The test were conducted in a Reynolds number range of 8.5 x 10/sup 4/ to 6.4 x 10/sup 5/. Smoke-flow visualization revealed the presence of small Kelvin-Helmholtz type vortical structures in the shear layer of a leading-edge vortex. These shear-layer vortices follow a helical path and grow in the streamwise direction as they wind into the vortex core where the individual shear layers merge. The phenomenon of vortex breakdown was studied using high-speed cinema photography. The bubble and spiral types of breakdown were observed and appear to represent the extremes in a continuum of breakdown forms.

Payne, F.M.

1987-01-01

276

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

277

Analytical investigation of rotor wake formation and geometry  

NASA Technical Reports Server (NTRS)

A number of refinements in the computer code were worked out and tested. Three codes have been written to date. One program is for an isolated wing and is being used to compare with data for the vortex wake (Weston). The second code is for an isolated wing with a streamwise vortex passing above it. This program is being used to validate the computational procedure for incorporating the vortex into the Euler equation calculations. The third program is the hovering rotor code which is the overall objective of the research. The optimization calculations for a hovering helicopter rotor have been completed.

Miller, R.; Murman, E. M.

1986-01-01

278

Vortex rings  

NASA Technical Reports Server (NTRS)

The vortex-ring problem in fluid mechanics is examined generally in terms of formation, the steady state, the duration of the rings, and vortex interactions. The formation is studied by examining the generation of laminar and turbulent vortex rings and their resulting structures with attention given to the three stages of laminar ring development. Inviscid dynamics is addressed to show how core dynamics affects overall ring motion, and laminar vortex structures are described in two dimensions. Viscous and inviscid structures are related in terms of 'leapfrogging', head-on collisions, and collisions with a no-slip wall. Linear instability theory is shown to successfully describe observational data, although late stages in the breakdown are not completely understood. This study of vortex rings has important implications for key aerodynamic issues including sound generation, transport and mixing, and vortex interactions.

Shariff, Karim; Leonard, Anthony

1992-01-01

279

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

280

The psychology of waking  

Microsoft Academic Search

More than 200 dreams were secured and all the results were in harmony. The dream is recalled as the cause of and as occurring at the time of waking. The dream presents a situation that is strange, frightful, astonishing, puzzling, surprising, unreal or otherwise shocking––a crisis. In regular waking it is least shocking and very elusive. In voluntary waking at

N. B. Bond

1929-01-01

281

The Vortex Lattice Method for the Rotor-Vortex Interaction Problem  

NASA Technical Reports Server (NTRS)

The rotor blade-vortex interaction problem and the resulting impulsive airloads which generate undesirable noise levels are discussed. A numerical lifting surface method to predict unsteady aerodynamic forces induced on a finite aspect ratio rectangular wing by a straight, free vortex placed at an arbitrary angle in a subsonic incompressible free stream is developed first. Using a rigid wake assumption, the wake vortices are assumed to move downsteam with the free steam velocity. Unsteady load distributions are obtained which compare favorably with the results of planar lifting surface theory. The vortex lattice method has been extended to a single bladed rotor operating at high advance ratios and encountering a free vortex from a fixed wing upstream of the rotor. The predicted unsteady load distributions on the model rotor blade are generally in agreement with the experimental results. This method has also been extended to full scale rotor flight cases in which vortex induced loads near the tip of a rotor blade were indicated. In both the model and the full scale rotor blade airload calculations a flat planar wake was assumed which is a good approximation at large advance ratios because the downwash is small in comparison to the free stream at large advance ratios. The large fluctuations in the measured airloads near the tip of the rotor blade on the advance side is predicted closely by the vortex lattice method.

Padakannaya, R.

1974-01-01

282

Benard-von Karman Vortex Street in a Bose-Einstein Condensate  

SciTech Connect

Vortex shedding from an obstacle potential moving in a Bose-Einstein condensate is investigated. Long-lived alternately aligned vortex pairs are found to form in the wake, which is similar to the Benard-von Karman vortex street in classical viscous fluids. Various patterns of vortex shedding are systematically studied and the drag force on the obstacle is calculated. It is shown that the phenomenon can be observed in a trapped system.

Sasaki, Kazuki; Suzuki, Naoya; Saito, Hiroki [Department of Engineering Science, University of Electro-Communications, Tokyo 182-8585 (Japan)

2010-04-16

283

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

284

Computation of rotor aerodynamic loads with a constant vorticity contour free wake model  

NASA Technical Reports Server (NTRS)

An analytical method is presented which facilitates the study of isolated rotors with an improved approach to wake simulation. Vortex filaments are simulated along contours of constant sheet strength for the sheet of vorticity resulting from each rotor blade. Curved vortex elements comprise the filaments which can be distorted by the local velocity field. Called the Constant Vorticity Contour wake model, the approach permits the simulation of the blades' wakes corresponding to the full span of the rotor blade. The discretization of the wake of the rotor blade produces spacing and structure that are consistent with the spatial and temporal variations in the loading. A vortex-lattice aerodynamic model of the blade is also included which introduces a finite-element structural model of the blade and consideration of the force and moment trim analysis. Results of the present version of the simulation, called RotorCRAFT, are found to correlate well with H-34 flight-test data.

Quackenbush, Todd R.; Wachspress, Daniel A.; Boschitsch, Alexander H.

1991-01-01

285

Inviscid Interactions Between Wake Vortices and Shear Layers  

NASA Technical Reports Server (NTRS)

Aircraft trailing vortices can be influenced significantly by atmospheric conditions such as crosswind, turbulence, and stratification. According to the NASA 1994 and 1995 field measurement program in Memphis, Tennessee, the descending aircraft wake vortices could stall or be deflected at the top of low-level temperature inversions that usually produce pronounced shear zones. Numerical simulations of vortex/shear interactions with ground effects have been performed by several groups. Burnham used a series of evenly spaced line vortices at a particular altitude to model the ground shear layer of the cross- wind. He found that the wind shear was swept up around the downwind vortex and caused the downwind vortex to move upward, and claimed that the effect was actually produced by the vertical gradient in the wind shear rather than by the wind shear directly, because uniformly distributed wind-shear vortices would have no effect on the trailing vortex vertical motion. Recently, Proctor et al. numerically tested the effects of narrow shear zones on the behavior of the vortex pair, motivated by the observation of the Memphis field data. The shear-layer sensitivity tests indicated that the downwind vortex was more sensitive and deflected to a higher altitude than its upwind counterpart. The downstream vortex contained vorticity of opposite sign to that of the shear. There was no detectable preference for the downwind vortex (or upwind vortex) to weaken (or strengthen) at a greater rate.

Zheng, Z. C.; Baek, K.

1998-01-01

286

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

287

Experimental study of the coupled wakes of two spheres  

NASA Astrophysics Data System (ADS)

We have studied the coupled wakes of two spheres (diameter d), separated by a transverse distance h between their centers and aligned normal to a uniform flow (velocity U). The dynamics of this system is controlled by the Reynolds number Re = Ud/nu, and the transverse spacing h/d. The vortex structures have been visualized by injection of dye in a small hole drilled downstream of the center of the spheres. Each sphere is hold by a thin pipe, slighty inclined on the flow direction in order to control the orientation of the wake[1]. The wake of a single sphere is periodic in the Reynolds number range [280, 400]. When the spacing h/d is much larger than 1, the vortices shed behind the spheres exhibit three-dimensional characteristic horeshoe shape. A small asymmetry, for instance due to the dye injection, might lead to different frequencies of vortex shedding. For intermediate values of h/d (e.g. between 1.3 and 2.5 for Re = 360) locked regimes of simultaneous or alternate vortex shedding have been observed. Finally, when h/d is lower than 1.3 (also for Re =360), the system behaves like a single wake and gives rise to a double alternate vortex street, similar to the Benard-von Karman street. In this case the two-sphere wake behave like a small aspect ratio cylinder. The coupling of these two oscillators along the spanwise direction is coherent with the Ginzburg-Landau model used to describe the vortex street behind a cylinder[2]. [1] Leweke T., Ormieres D., Provansal M., Schouveiler L. 1999, Proc. Fluvisu'99, Toulouse, France, 103 [2] Albarede P., Provansal M. 1995, J. Fluid Mech. 291, 191

Provansal, Michel; Schouveiler, Lionel

1999-11-01

288

Active control of a cylinder wake flow by using a streamwise oscillating foil  

NASA Astrophysics Data System (ADS)

In this study, numerical experiments are carried out to control the vortex shedding of a circular cylinder by utilizing an oscillating foil. The thin foil of elliptic shape undergoes prescribed harmonic oscillations in the streamwise direction in the near wake region. This simplified model is intended to study how wake dynamics are modified via localized wake disturbance, and then to stabilize the global wake instability. The results show that, at proper gap spacing, the oscillating foil can completely suppress the wake unsteadiness and recover the recirculating bubble type flow. The global instability suppression is then established on the imposition of local symmetry into the reversed flow behind the cylinder. It is revealed that the dynamic interaction between the main shears layer and oscillatory boundary layers is responsible for the wake stabilization mechanism. In addition, the kinematic/dynamic parameters related to foil motions and flow properties are widely discussed to reveal their effects on the performance of wake stabilization and drag reduction.

Bao, Y.; Tao, J.

2013-05-01

289

Analysis of Wake VAS Benefits Using ACES Build 3.2.1: VAMS Type 1 Assessment  

NASA Technical Reports Server (NTRS)

The FAA and NASA are currently engaged in a Wake Turbulence Research Program to revise wake turbulence separation standards, procedures, and criteria to increase airport capacity while maintaining or increasing safety. The research program is divided into three phases: Phase I near term procedural enhancements; Phase II wind dependent Wake Vortex Advisory System (WakeVAS) Concepts of Operations (ConOps); and Phase III farther term ConOps based on wake prediction and sensing. The Phase III Wake VAS ConOps is one element of the Virtual Airspace Modelling and Simulation (VAMS) program blended concepts for enhancing the total system wide capacity of the National Airspace System (NAS). This report contains a VAMS Program Type 1 (stand-alone) assessment of the expected capacity benefits of Wake VAS at the 35 FAA Benchmark Airports and determines the consequent reduction in delay using the Airspace Concepts Evaluation System (ACES) Build 3.2.1 simulator.

Smith, Jeremy C.

2005-01-01

290

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

291

Influence of atmospheric boundary layer on turbulence in wind turbine wake  

NASA Astrophysics Data System (ADS)

Full-scale wind turbines (WT) operate in the atmospheric boundary layer. The atmospheric boundary layer structure significantly influences the turbulence generated in the wake of the WT. As Atmospheric boundary layer structure is dictated by the stratification of the atmosphere, hence stratifications effects are critical in accurate representation of the turbine wake physics. Due to the dependency of several factors, such as turbulence scales, buoyancy flux, momentum flux, the atmospheric boundary layer turbulence capturing is really challenging. Large Eddy Simulation (LES) has been used as a tool to understand the effects of atmospheric stability on turbine wake turbulence. The differences between the stable and unstable atmosphere on wake of 5-MW turbine has been explored. Differences in tip and root vortex interactions, wake expansion and recovery have been analyzed. The study has revealed for stable ABL low level jets play an important role in wake dynamics and increasing stability delays the wake recovery. Tip vortex is unconditionally unstable in all stability conditions due to mutual inductance mode of stability leading to vortex merging. The study is one of the first studies that accounts for realistic atmospheric boundary turbulence on wake development.

Debnath, Mithu Chandra

292

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

293

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

294

Hawkmoth flight stability in turbulent vortex streets.  

PubMed

Shedding of vortices is a common phenomenon in the atmosphere over a wide range of spatial and temporal scales. However, it is unclear how these vortices of varying scales affect the flight performance of flying animals. In order to examine these interactions, we trained seven hawkmoths (Manduca sexta) (wingspan ~9 cm) to fly and feed in a wind tunnel under steady flow (controls) and in the von Kármán vortex street of vertically oriented cylinders (two different cylinders with diameters of 10 and 5 cm) at speeds of 0.5, 1 and 2 m s(-1). Cylinders were placed at distances of 5, 25 and 100 cm upstream of the moths. Moths exhibited large amplitude yaw oscillations coupled with modest oscillations in roll and pitch, and slight increases in wingbeat frequency when flying in both the near (recirculating) and middle (vortex dominated) wake regions. Wingbeat amplitude did not vary among treatments, except at 1 m s(-1) for the large cylinder. Yaw and roll oscillations were synchronized with the vortex shedding frequencies in moths flying in the wake of the large cylinder at all speeds. In contrast, yaw and pitch were synchronized with the shedding frequency of small vortices at speeds ?1 m s(-1). Oscillations in body orientation were also substantially smaller in the small cylinder treatment when compared with the large cylinder, regardless of temporal or non-dimensional spatial scale. Moths flying in steady conditions reached a higher air speed than those flying into cylinder wakes. In general, flight effects produced by the cylinder wakes were qualitatively similar among the recirculating and vortex-dominated wake regions; the magnitude of those effects, however, declined gradually with downstream distance. PMID:24072794

Ortega-Jimenez, Victor Manuel; Greeter, Jeremy S M; Mittal, Rajat; Hedrick, Tyson L

2013-12-15

295

Dissipative dynamics of a vortex state in a trapped Bose-condensed gas  

Microsoft Academic Search

We discuss dissipative dynamics of a vortex state in a trapped Bose-condensed gas at finite temperature and draw a scenario of decay of this state in a static trap. The interaction of the vortex with the thermal cloud transfers energy from the vortex to the cloud and induces the motion of the vortex core to the border of the condensate.

P. O. Fedichev; G. V. Shlyapnikov

1999-01-01

296

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

297

Turbulent Plane Wakes Subjected to Successive Strains  

NASA Technical Reports Server (NTRS)

Six direct numerical simulations of turbulent time-evolving strained plane wakes have been examined to investigate the response of a wake to successive irrotational plane strains of opposite sign. The orientation of the applied strain field has been selected so that the flow is the time-developing analogue of a spatially developing wake evolving in the presence of either a favourable or an adverse streamwise pressure gradient. The magnitude of the applied strain rate a is constant in time t until the total strain e(sup at) reaches about four. At this point, a new simulation is begun with the sign of the applied strain being reversed (the original simulation is continued as well). When the total strain is reduced back to its original value of one, yet another simulation is begun with the sign of the strain being reversed again back to its original sign. This process is done for both initially "favourable" and initially "adverse" strains, providing simulations for each of these strain types from three different initial conditions. The evolution of the wake mean velocity deficit and width is found to be very similar for all the adversely strained cases, with both measures rapidly achieving exponential growth at the rate associated with the cross-stream expansive strain e(sup at). In the "favourably" strained cases, the wake widths approach a constant and the velocity deficits ultimately decay rapidly as e(sup -2at). Although all three of these cases do exhibit the same asymptotic exponential behaviour, the time required to achieve this is longer for the cases that have been previously adversely strained (by at approx. equals 1). These simulations confirm the generality of the conclusions drawn in Rogers (2002) regarding the response of plane wakes to strain. The evolution of strained wakes is not consistent with the predictions of classical self-similar analysis; a more general equilibrium similarity solution is required to describe the results. At least for the cases considered here, the wake Reynolds number and the ratio of the turbulent kinetic energy to the square of the wake mean velocity deficit are determined nearly entirely by the total strain. For these measures the order in which the strains are applied does not matter and the changes brought about by the strain are nearly reversible. The wake mean velocity deficit and width, on the other hand, differ by about a factor of three when the total strain returns to one, depending on whether the wake was first "favourably" or "adversely" strained. The strain history is important for predicting the evolution of these quantities.

Rogers, Michael M.

2003-01-01

298

Experimental investigation of the vortical activity in the close wake of a simplified military transport aircraft  

NASA Astrophysics Data System (ADS)

This paper focuses on the experimental characterization of the vortex structures that develop in the aft fuselage region and in the wake of a simplified geometry of a military transport aircraft. It comes within the framework of the military applications of airflow influence on airdrop operations. This work relies on particle image velocimetry measurements combined with a vortex-tracking approach. Complex vortex dynamics is revealed, in terms of vortex positions, intensities, sizes, shapes and fluctuation levels, for both closed and opened cargo-door and ramp airdrop configurations.

Bury, Yannick; Jardin, Thierry; Klöckner, Andreas

2013-05-01

299

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

300

Vortex methods and vortex statistics  

SciTech Connect

Vortex methods originated from the observation that in incompressible, inviscid, isentropic flow vorticity (or, more accurately, circulation) is a conserved quantity, as can be readily deduced from the absence of tangential stresses. Thus if the vorticity is known at time t = 0, one can deduce the flow at a later time by simply following it around. In this narrow context, a vortex method is a numerical method that makes use of this observation. Even more generally, the analysis of vortex methods leads, to problems that are closely related to problems in quantum physics and field theory, as well as in harmonic analysis. A broad enough definition of vortex methods ends up by encompassing much of science. Even the purely computational aspects of vortex methods encompass a range of ideas for which vorticity may not be the best unifying theme. The author restricts himself in these lectures to a special class of numerical vortex methods, those that are based on a Lagrangian transport of vorticity in hydrodynamics by smoothed particles (``blobs``) and those whose understanding contributes to the understanding of blob methods. Vortex methods for inviscid flow lead to systems of ordinary differential equations that can be readily clothed in Hamiltonian form, both in three and two space dimensions, and they can preserve exactly a number of invariants of the Euler equations, including topological invariants. Their viscous versions resemble Langevin equations. As a result, they provide a very useful cartoon of statistical hydrodynamics, i.e., of turbulence, one that can to some extent be analyzed analytically and more importantly, explored numerically, with important implications also for superfluids, superconductors, and even polymers. In the authors view, vortex ``blob`` methods provide the most promising path to the understanding of these phenomena.

Chorin, A.J.

1993-05-01

301

Analytical model of rotor wake aerodynamics in ground effect  

NASA Technical Reports Server (NTRS)

The model and the computer program developed provides the velocity, location, and circulation of the tip vortices of a two-blade helicopter in and out of the ground effect. Comparison of the theoretical results with some experimental measurements for the location of the wake indicate that there is excellent accuracy in the vicinity of the rotor and fair amount of accuracy far from it. Having the location of the wake at all times enables us to compute the history of the velocity and the location of any point in the flow. The main goal of out study, induced velocity at the rotor, can also be calculated in addition to stream lines and streak lines. Since the wake location close to the rotor is known more accurately than at other places, the calculated induced velocity over the disc should be a good estimate of the real induced velocity, with the exception of the blade location, because each blade was replaced only by a vortex line. Because no experimental measurements of the wake close to the ground were available to us, quantitative evaluation of the theoretical wake was not possible. But qualitatively we have been able to show excellent agreement. Comparison of flow visualization with out results has indicated the location of the ground vortex is estimated excellently. Also the flow field in hover is well represented.

Saberi, H. A.

1983-01-01

302

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

303

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

304

Titan's Winter Polar Vortex  

NASA Technical Reports Server (NTRS)

Titan's atmosphere has provided an interesting study in contrasts and similarities with Earth's. While both have N$_2$ as the dominant constituent and comparable surface pressures $\\sim1$ bar, Titan's next most abundant molecule is CH$_4$, not O$_2$, and the dissociative breakup of CH$_4$ and N$_2$ by sunlight and electron impact leads to a suite of hydrocarbons and nitriles, and ultimately the photochemical smog that enshrouds the moon. In addition, with a 15.95-day period, Titan is a slow rotator compared to Earth. While the mean zonal terrestrial winds are geostrophic, Titan's are mostly cyclostrophic, whipping around the moon in as little as 1 day. Despite the different dynamical regime, Titan's winter stratosphere exhibits several characteristics that should be familiar to terrestrial meteorologists. The cold winter pole near the 1 -mbar level is circumscribed by strong winds (up to 190 m/s) that act as a barrier to mixing with airmasses at lower latitudes. There is evidence of enhancement of several organic species over the winter pole, indicating subsidence. The adiabatic heating associated with this subsidence gives rise to a warm anomaly at the 0.01-mbar level, raising the stratopause two scale heights above its location at equatorial latitudes. Condensate ices have been detected in Titan's lower stratosphere within the winter polar vortex from infrared spectra. Although not always unambiguously identified, their spatial distribution exhibits a sharp gradient, decreasing precipitously across the vortex away from the winter pole. The interesting question of whether there is important heterogeneous chemistry occurring within the polar vortex, analogous to that occurring in the terrestrial polar stratospheric clouds in the ozone holes, has not been addressed. The breakup of Titan's winter polar vortex has not yet been observed. On Earth, the polar vortex is nonlinearly disrupted by interaction with large-amplitude planetary waves. Large-scale waves have not been identified in Titan's atmosphere, so the decay of its polar vortex may be more gradual than on Earth. Observations from an extended Cassini mission into late northern spring should provide critical data indicating whether the vortex goes away with a bang or just fades away.

Flasar, F.M.; Achterberg, R.K.; Schinder, P.J.

2008-01-01

305

Volumetric visualization of the near- and far-field wake in flapping wings.  

PubMed

The flapping wings of flying animals create complex vortex wake structure; understanding its spatial and temporal distribution is fundamental to animal flight theory. In this study, we applied the volumetric 3-component velocimetry to capture both the near- and far-field flow generated by a pair of mechanical flapping wings. For the first time, the complete three-dimensional wake structure and its evolution throughout a wing stroke were quantified and presented experimentally. The general vortex wake structure maintains a quite consistent form: vortex rings in the near field and two shear layers in the far field. Vortex rings shed periodically from the wings and are linked to each other in successive strokes. In the far field, the shed vortex rings evolve into two parallel shear layers with dominant vorticity convected from tip and root vortices. The shear layers are nearly stationary in space compared to the periodic vortex rings shed in the near field. In addition, downwash passes through the centers of the vortex rings and extends downward between the two shear layers. PMID:23924871

Liu, Yun; Cheng, Bo; Barbera, Giovanni; Troolin, Daniel R; Deng, Xinyan

2013-09-01

306

Vortex methods  

SciTech Connect

Vortex methods originated from the observation that in incompressible inviscid flow vorticity (or, more accurately, circulation) is a conserved quantity, as can be readily deduced from the absence of tangential stresses. Thus, if the vorticity is known at time t=0, one can find the flow at a later time by simply following the vorticity. In this narrow context, a vortex method is a numerical method that follows vorticity. The author restricts himself in these lectures to a special class of numerical vortex methods, those that are based on a Lagrangian transport of vorticity in hydrodynamics by smoothed particles (blobs) and those whose analysis contributes to the understanding of blob methods. Blob methods started in the 1930`s.

Chorin, A.J. [California Univ., Berkeley, CA (United States). Dept. of Mathematics]|[Lawrence Berkeley Lab., CA (United States)

1993-06-01

307

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

308

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

309

Experimental investigation of the wake behind a model of wind turbine in a water flume  

NASA Astrophysics Data System (ADS)

The flow behind the model of wind turbine rotor is investigated experimentally in a water flume using Particle Image Velocimetry. The study carried out involves rotors of three bladed wind turbine designed using Glauert's optimization. The transitional regime, generally characterized as in between the regime governed by stable organized vortical structures and the turbulent wake, develops from disturbances of the tip and root vorticies through vortex paring and further complex behaviour towards the fully turbulent wake. Our PIV measurements pay special attention to the onset of the instabilities. The near wake characteristics (development of expansion, tip vortex position, deficit velocity and rotation in the wake) have been measured for different tip speed ratio to compare with main assumptions and conclusions of various rotor theories.

Okulov, V. L.; Naumov, I. N.; Kabardin, I.; Mikkelsen, R.; Sřrensen, J. N.

2014-12-01

310

On the evolution of the wake structure produced by a low-aspect-ratio pitching panel  

PubMed Central

Flow visualization is used to interrogate the wake structure produced by a rigid flat panel of aspect ratio (span/chord) 0.54 pitching in a free stream at a Strouhal number of 0.23. At such a low aspect ratio, the streamwise vorticity generated by the plate tends to dominate the formation of the wake. Nevertheless, the wake has the appearance of a three-dimensional von Kármán vortex street, as observed in a wide range of other experiments, and consists of horseshoe vortices of alternating sign shed twice per flapping cycle. The legs of each horseshoe interact with the two subsequent horseshoes in an opposite-sign, then like-sign interaction in which they become entrained. A detailed vortex skeleton model is proposed for the wake formation. PMID:19746198

BUCHHOLZ, JAMES H. J.; SMITS, ALEXANDER J.

2009-01-01

311

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

312

Arctic Vortex  

Atmospheric Science Data Center

... of the obstacle. These turbulence patterns are known as von Karman vortex streets. In these images from NASA's Multi-angle Imaging ... placed over your left eye. Fluid dynamicist Theodore von Karman was the first to derive the conditions under which these turbulence ...

2013-06-26

313

Fluid-Structure Interaction Analysis of Hingeless Rotor Blades in Hover Considering Wake Effects  

Microsoft Academic Search

Aeroelastic analysis of hingeless rotor blades in hover was performed. Large deflection beam theory was applied to analyze blade motions with effects of geometric structural nonlinearity. Aerodynamic loads for aeroelastic analysis were calculated through a three-dimensional aerodynamic model which is based on the unsteady vortex lattice method. Wake geometry was described using a time-marching free-wake method. Lead-lag damping ratio and

Seung-Jae Yoo; Min-Soo Jeong; In Lee

2010-01-01

314

Wake Signature Detection  

NASA Astrophysics Data System (ADS)

An accumulated body of quantitative evidence shows that bluff-body wakes in stably stratified environments have an unusual degree of coherence and organization, so characteristic geometries such as arrays of alternating-signed vortices have very long lifetimes, as measured in units of buoyancy timescales, or in the downstream distance scaled by a body length. The combination of pattern geometry and persistence renders the detection of these wakes possible in principle. It now appears that identifiable signatures can be found from many disparate sources: Islands, fish, and plankton all have been noted to generate features that can be detected by climate modelers, hopeful navigators in open oceans, or hungry predators. The various types of wakes are reviewed with notes on why their signatures are important and to whom. A general theory of wake pattern formation is lacking and would have to span many orders of magnitude in Reynolds number.

Spedding, Geoffrey R.

2014-01-01

315

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

316

Dissipative dynamics of a vortex state in a trapped Bose-condensed gas  

E-print Network

We discuss dissipative dynamics of a vortex state in a trapped Bose-condensed gas at finite temperature and draw a scenario of decay of this state in a static trap. The interaction of the vortex with the thermal cloud transfers energy from the vortex to the cloud and induces the motion of the vortex core to the border of the condensate. Once the vortex reaches the border, it immediately decays through the creation of excitations. We calculate the characteristic life-time of a vortex state and address the question of how the dissipative dynamics of vortices can be studied experimentally.

P. O. Fedichev; G. V. Shlyapnikov

1999-06-15

317

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

318

On the absence of asymmetric wakes for periodically plunging finite wings  

NASA Astrophysics Data System (ADS)

It has previously been shown that, at high Strouhal numbers, oscillating airfoils can produce deflected jets that can create very high lift-coefficients for otherwise symmetric scenarios. These deflected jets form through pairing of the trailing-edge vortices to create asymmetric vortex couples that self-propel at an angle to the freestream, resulting in an asymmetric flow field and non-zero lift. In this paper results are presented that indicate these high-lift deflected jets cannot form for finite wings. Instead of the straight vortex tubes that pair and convect at an angle to the freestream observed for effectively infinite wings, finite wings exhibit vortex tubes that break into two branches near the tip forming double helix structures. One branch connects with the last vortex; one branch connects with the next vortex. This creates a long "daisy chain" of interconnected trailing edge vortices forming a long series of vortex loops. These symmetric flow fields are shown to persist for finite wings even to Strouhal numbers more than twice those required to produce asymmetric wakes on plunging airfoils. Two contributing reasons are discussed for why deflected jets are not observed. First the tip vortex creates three-dimensionality that discourages vortex coupling. Second, the symmetry of the circulation of the interconnected vortex loops, which has been confirmed by the experiments, is a natural consequence of the vortex topology. Therefore, the asymmetry in trailing edge vortex strength previously observed as characteristic of deflected jets cannot be supported for finite wings.

Calderon, D. E.; Cleaver, D. J.; Gursul, I.; Wang, Z.

2014-07-01

319

Wake flow behaviour behind a smaller cylinder oscillating in the wake of an upstream stationary cylinder  

NASA Astrophysics Data System (ADS)

The flow patterns around a cylinder oscillating freely in the wake of a larger cylinder upstream were investigated using the particle image velocimetry technique. The upstream cylinder was fixed at both ends while the downstream smaller cylinder was held by springs such that it was free to oscillate in the transverse direction. The flow patterns, amplitudes of oscillation and vortex shedding frequencies were compared with those of a single cylinder. In the presence of the upstream cylinder, the three parameters characterizing the oscillation response of the smaller cylinder—amplitude of oscillation, vortex shedding frequency and Reynolds stresses—were greatly reduced. While their magnitude increased with gap ratio, these three parameters were still smaller than the corresponding magnitudes for a single oscillating cylinder. The peak values of turbulence statistics such as Reynolds shear stress and normal stress behind the oscillating downstream cylinder were similarly reduced, and increased with gap ratios.

Gao, Yangyang; Sun, Zhilin; Tan, Danielle S.; Yu, Dingyong; Keat Tan, Soon

2014-04-01

320

Computation of rotor aerodynamic loads in forward flight using a full-span free wake analysis  

NASA Technical Reports Server (NTRS)

The development of an advanced computational analysis of unsteady aerodynamic loads on isolated helicopter rotors in forward flight is described. The primary technical focus of the development was the implementation of a freely distorting filamentary wake model composed of curved vortex elements laid out along contours of constant vortex sheet strength in the wake. This model captures the wake generated by the full span of each rotor blade and makes possible a unified treatment of the shed and trailed vorticity in the wake. This wake model was coupled to a modal analysis of the rotor blade dynamics and a vortex lattice treatment of the aerodynamic loads to produce a comprehensive model for rotor performance and air loads in forward flight dubbed RotorCRAFT (Computation of Rotor Aerodynamics in Forward Flight). The technical background on the major components of this analysis are discussed and the correlation of predictions of performance, trim, and unsteady air loads with experimental data from several representative rotor configurations is examined. The primary conclusions of this study are that the RotorCRAFT analysis correlates well with measured loads on a variety of configurations and that application of the full span free wake model is required to capture several important features of the vibratory loading on rotor blades in forward flight.

Quackenbush, Todd R.; Bliss, Donald B.; Wachspress, Daniel A.; Boschitsch, Alexander H.; Chua, Kiat

1990-01-01

321

Generalization of the JTZ model to open plane wakes  

E-print Network

The JTZ model [C. Jung, T. T\\'el and E. Ziemniak, Chaos {\\bf 3}, (1993) 555], as a theoretical model of a plane wake behind a circular cylinder in a narrow channel at a moderate Reynolds number, has previously been employed to analyze phenomena of chaotic scattering. It is extended here to describe an open plane wake without the confined narrow channel by incorporating a double row of shedding vortices into the intermediate and far wake. The extended JTZ model is found in qualitative agreement with both direct numerical simulations and experimental results in describing streamlines and vorticity contours. To further validate its applications to particle transport processes, the interaction between small spherical particles and vortices in an extended JTZ model flow is studied. It is shown that the particle size has significant influences on the features of particle trajectories, which have two characteristic patterns: one is rotating around the vortex centers and the other accumulating in the exterior of vort...

Wu, Zuo-Bing

2011-01-01

322

Bifurcations and symmetry breaking in the wake of axisymmetric bodies  

NASA Astrophysics Data System (ADS)

We consider the generic problem of wake instabilities past fixed axisymmetric bodies, and focus on the extreme cases of a sphere and a flat disk. Numerical results reveal that the wakes of these two bodies evolve differently as the Reynolds number is increased. Especially, two new vortex shedding modes are identified behind a disk. To interpret these results, we introduce a model based on the theory of mode interactions in presence of O(2) symmetry. This model, which was initially developed for the Taylor-Couette system, allows us to explain the structural differences observed in the evolution of the two types of wakes and to accurately predict the evolution of the lift force.

Fabre, David; Auguste, Franck; Magnaudet, Jacques

2008-05-01

323

Visualization of the space-time impulse response of the subcritical wake of a cylinder  

PubMed

The well-known Benard-von Karman cylinder wake is one of the most challenging phenomena of fluid mechanics. As the Reynolds number of the flow around a cylinder passes through a critical value, alternating vortex shedding appears via a Hopf bifurcation. Theoretical studies of the wake have described the appearance of this self-sustained oscillation as the result of a convective to absolute transition resulting in the formation of a global mode. We illustrate here the convective global regime of the subcritical wake by analyzing visualizations of its impulse response. PMID:11088979

Le Gal P; Croquette

2000-09-01

324

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

325

Chaos Control in the Wake of an Oscillating Cylinder  

NASA Astrophysics Data System (ADS)

The nonlinear dynamics of vortex shedding behind circular cylinders are investigated using a previously developed spatial-temporal map lattice. The map studied consists of a series of circle map oscillators placed along the cylinder span coupled with a simple diffusion model. Chaotic states associated with disordered vortex shedding patterns are observed when forcing the cylinder outside the classical lock-on region. These are controlled through application of a small-amplitude periodic perturbation of a system parameter, as proposed by Ott, Grebogi, and Yorke. Periodic lace-like structures and parallel shedding patterns are realized by driving the chaotic system to the desired target state. A wide range of forcing frequency-amplitude combinations are studied along with manipulation of vortex lock-on region extents. Preliminary extensions of these chaos control techniques to a two-dimensional wake flow using finite element techniques are also discussed.

Balasubramanian, Ganapathi R.; Olinger, David J.

1997-11-01

326

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

327

An integrated Navier-Stokes - full potential - free wake method for rotor flows  

NASA Astrophysics Data System (ADS)

The strong wake shed from rotary wings interacts with almost all components of the aircraft, and alters the flow field thus causing performance and noise problems. Understanding and modeling the behavior of this wake, and its effect on the aerodynamics and acoustics of helicopters have remained as challenges. This vortex wake and its effect should be accurately accounted for in any technique that aims to predict rotor flow field and performance. In this study, an advanced and efficient computational technique for predicting three-dimensional unsteady viscous flows over isolated helicopter rotors in hover and in forward flight is developed. In this hybrid technique, the advantages of various existing methods have been combined to accurately and efficiently study rotor flows with a single numerical method. The flow field is viewed in three parts: (i) an inner zone surrounding each blade where the wake and viscous effects are numerically captured, (ii) an outer zone away from the blades where wake is modeled, and (iii) a Lagrangean wake which induces wake effects in the outer zone. This technique was coded in a flow solver and compared with experimental data for hovering and advancing rotors including a two-bladed rotor, the UH-60A rotor and a tapered tip rotor. Detailed surface pressure, integrated thrust and torque, sectional thrust, and tip vortex position predictions compared favorably against experimental data. Results indicated that the hybrid solver provided accurate flow details and performance information typically in one-half to one-eighth cost of complete Navier-Stokes methods.

Berkman, Mert Enis

1998-12-01

328

Comparison of calculated and measured model rotor loading and wake geometry  

NASA Technical Reports Server (NTRS)

The calculated blade bound circulation and wake geometry are compared with measured results for a model helicopter rotor in hover and forward flight. Hover results are presented for rectangular tip and ogee tip planform blades. The correlation is quite good when the measured wake geometry characteristics are used in the analysis. Available prescribed wake geometry models are found to give fair predictions of the loading, but they do not produce a reasonable prediction of the induced power. Forward flight results are presented for twisted and untwisted blades. Fair correlation between measurements and calculations is found for the bound circulation distribution on the advancing side. The tip vortex geometry in the vicinity of the advancing blade in forward flight was predicted well by the free wake calculation used, although the wake geometry did not have a significant influence on the calculated loading and performance for the cases considered.

Johnson, W.

1980-01-01

329

Interaction of a Boundary Layer with a Turbulent Wake  

NASA Technical Reports Server (NTRS)

The objective of this grant was to study the transition mechanisms on a flat-plate boundary layer interacting with the wake of a bluff body. This is a simplified configuration presented and designed to exemplify the phenomena that occur in multi-element airfoils, in which the wake of an upstream element impinges on a downstream one. Some experimental data is available for this configuration at various Reynolds numbers. The first task carried out was the implementation and validation of the immersed-boundary method. This was achieved by performing calculations of the flow over a cylinder at low and moderate Reynolds numbers. The low-Reynolds number results are discussed, which is enclosed as Appendix A. The high-Reynolds number results are presented in a paper in preparation for the Journal of Fluid Mechanics. We performed calculations of the wake-boundary-layer interaction at two Reynolds numbers, Re approximately equal to 385 and 1155. The first case is discussed and a comparison of the two calculations is reported. The simulations indicate that at the lower Reynolds number the boundary layer is buffeted by the unsteady Karman vortex street shed by the cylinder. This is shown: long streaky structures appear in the boundary layer in correspondence of the three-dimensionalities in the rollers. The fluctuations, however, cannot be self-sustained due to the low Reynolds-number, and the flow does not reach a turbulent state within the computational domain. In contrast, in the higher Reynolds-number case, boundary-layer fluctuations persist after the wake has decayed (due, in part, to the higher values of the local Reynolds number Re achieved in this case); some evidence could be observed that a self-sustaining turbulence generation cycle was beginning to be established. A third simulation was subsequently carried out at a higher Reynolds number, Re=3900. This calculation gave results similar to those of the Re=l155 case. Turbulence was established at fairly low Reynolds number, as a consequence of the high level of the free-stream perturbation. An instantaneous flow visualization for that case is shown. A detailed examination of flow statistics in the transitional and turbulent regions, including the evolution of the turbulent kinetic energy (TKE) budget and frequency spectra showed the formation and evolution of turbulent spots characteristic of the bypass transition mechanism. It was also observed that the turbulent eddies achieved an equilibrium, fully developed turbulent states first, as evidenced by the early agreement achieved by the terms in the TKE budget with those observed in turbulent flows. Once a turbulent Reynolds stress profile had been established, the velocity profile began to resemble a turbulent one, first in the inner region and later in the outer region of the wall layer. An extensive comparison of the three cases, including budgets, mean velocity and Reynolds stress profiles and flow visualization, is included. The results obtained are also presented.

Piomelli, Ugo

2004-01-01

330

Properties of a half-delta wing vortex  

NASA Technical Reports Server (NTRS)

The mean flow and turbulence structure of a single longitudinal vortex generated by a half-delta wing placed at a small angle of attack were investigated. Particular consideration was given to the near-field properties of the generator in order to establish the role of the generator wake in the initial rolling-up of the vortex, as well as to the far-field properties so that the approach to equilibrium could be studied. Measurements were made on fine cross-plane grids at seven streamwise locations using hot cross wires. The results show that the point of maximum vorticity and the generator wake do not merge until a streamwise distance equivalent to three generator heights is reached. Comparison with previous data on vortices produced by double-branched generators confirmed that the present vortex had achieved a fully developed state, and at a relatively short streamwise distance.

Mehta, Rabindra D.; Cantwell, Elizabeth R.

1987-01-01

331

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

332

Study of a Wake Recovery Mechanism in a High-Speed Axial Compressor Stage  

NASA Technical Reports Server (NTRS)

This work addresses the significant differences in compressor rotor wake mixing loss which exist in a stage environment relative to a rotor in isolation. The wake decay for a rotor in isolation is due solely to viscous dissipation which is an irreversible process and thus leads to a loss in both total pressure and efficiency. Rotor wake decay in the stage environment is due to both viscous mixing and the inviscid strain imposed on the wake fluid particles by the stator velocity field. This straining process, referred to by Smith (1993) as recovery, is reversible and for a 2D rotor wake leads to an inviscid reduction of the velocity deficit of the wake. A model for the rotor wake decay process is developed and used to quantify the viscous dissipation effects relative to those of inviscid wake stretching. The model is verified using laser anemometer measurements acquired in the wake of a transonic rotor operated in isolation and in a stage configuration at near peak efficiency and near stall operating conditions. Additional insight is provided by a time-accurate 3D Navier-Stokes simulation of the compressor stator flow field at the corresponding stage loading levels. Results from the wake decay model exhibit good agreement with the experimental data. Data from the model, laser anemometer measurements, and numerical simulations indicate that for the rotor/stator spacing used in this work, which is typical of core compressors, rotor wake straining (stretching) is the primary decay process in the stator passage with viscous mixing playing only a minor role. The implications of these results on compressor stage design are discussed.

VanZante, Dale E.

1998-01-01

333

Wake survey techniques for objects with highly turbulent wakes  

Microsoft Academic Search

The primary objective of this study is to develop practical and accurate wake survey techniques for determining the drag of bluff bodies that have highly turbulent wakes. The commonly used wake survey method, the simplified Jones' equation with pneumatic probe measurements, was found to be inadequate in such cases. This study consisted of an experimental investigation of several wind-tunnel models,

Biao Lu

2003-01-01

334

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

335

Wing-vortex interaction: unraveling the flowfield of a hovering rotor  

NASA Astrophysics Data System (ADS)

This paper focuses on one of the most prominent flow features of the hovering rotor wake, the close interaction of the tip vortex with a following blade. Such vortex interactions are fundamental determinants of rotor performance, loads, and noise. Yet, they are not completely understood, largely due to the lack of sufficiently comprehensive experimental data. The present study aims to perform such comprehensive measurements, not on hovering helicopter rotors (which hugely magnifies test complexity) but using fixed-wing models in controlled wind tunnel tests. The experiments were designed to measure, in considerable detail, the aerodynamic loading resulting from a vortex interacting with a semi-span wing, as well as the wake resulting from that interaction. The goal of the present study is to answer fundamental questions such as (a) the influence of a vortex passing below a wing on the lift, drag, tip vortex, and the wake of that wing and (b) the strength of the forming tip vortex and its relation to the wing loading and/or the tip loading. This paper presents detailed wing surface pressure measurements that result from the interaction of the wing with an interacting vortex trailing from an upstream wing. The data show large lift distribution changes for a range of wing-vortex interactions including the effects of close encounter with the vortex core. Significant asymmetry in the vortex-induced lift loading was observed, with the increase in wing sectional lift outboard of the interacting vortex (closer to the tip) being much smaller than the corresponding decrease inboard of the vortex.

Bhagwat, Mahendra J.; Caradonna, Francis X.; Ramasamy, Manikandan

2015-01-01

336

The Spectral and Statistical Properties of Turbulence Generated by a Vortex/Blade-Tip Interaction  

NASA Technical Reports Server (NTRS)

The perpendicular interaction of a streamwise vortex with the tip of a lifting blade was studied in incompressible flow to provide information useful to the accurate prediction of helicopter rotor noise and the understanding of vortex dominated turbulent flows. The vortex passed 0.3 chord lengths to the suction side of the blade tip, providing a weak interaction. Single and two-point turbulence measurements were made using sub-miniature four sensor hot-wire probes 15 chord lengths downstream of the blade trailing edge; revealing the mean velocity and Reynolds stress tensor distributions of the turbulence, as well as its spanwise length scales as a function of frequency. The single point measurements show the flow downstream of the blade to be dominated by the interaction of the original tip vortex and the vortex shed by the blade. These vortices rotate about each other under their mutual induction, winding up the turbulent wakes of the blades. This interaction between the vortices appears to be the source of new turbulence in their cores and in the region between them. This turbulence appears to be responsible for some decay in the core of the original vortex, not seen when the blade is removed. The region between the vortices is not only a region of comparatively large stresses, but also one of intense turbulence production. Velocity autospectra measured near its center suggests the presence quasi-periodic large eddies with axes roughly parallel to a line joining the vortex cores. Detailed two-point measurements were made on a series of spanwise cuts through the flow so as to reveal the turbulence scales as they would be seen along the span of an intersecting airfoil. The measurements were made over a range of probe separations that enabled them to be analyzed not only in terms of coherence and phase spectra but also in terms of wave-number frequency (kappa-omega) spectra, computed by transforming the measured cross-spectra with respect to the spanwise separation of the probes. These data clearly show the influence of the coherent eddies in the spiral wake and the turbulent region between the cores. These eddies produce distinct peaks in the upwash velocity kappa-omega spectra, and strong anisotropy manifested both in the decay of the kappa-omega spectrum at larger wave-numbers and in differences between the kappa-omega spectra of different components. None of these features are represented in the von Karman spectrum for isotropic turbulence that is often used in broadband noise computations. Wave-number frequency spectra measured in the cores appear to show some evidence that the turbulence outside sets tip core waves, as has previously been hypothesized. These spectra also provide for the first time a truly objective method for distinguishing velocity fluctuations produced by core wandering from other motions.

Devenport, William J.; Wittmer, Kenneth S.; Wenger, Christian W.

1997-01-01

337

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

338

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

339

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

340

Bifurcation theory for three-dimensional flow in the wake of a circular cylinder  

PubMed

A bifurcation scenario is presented for three-dimensional vortex shedding in the wake of a circular cylinder for Reynolds numbers up to 300. Amplitude equations are proposed to describe the nonlinear interaction between two three-dimensional modes of shedding with different spanwise wave numbers and different spatiotemporal symmetries. The amplitude equations explain many features of the transition scenario observed experimentally. PMID:11031572

Barkley; Tuckerman; Golubitsky

2000-05-01

341

American Institute of Aeronautics and Astronautics Effects of Wake Vortices on Commercial Aircraft  

E-print Network

Aircraft T. Economon+ University of Notre Dame, Notre Dame, Indiana, 46556 Commercial aircraft are becoming a trailing vortex which can linger behind the aircraft for miles. If a following aircraft penetrates the wake, it could potentially be a flight safety issue due to the induced roll on the airplane created

Alonso, Juan J.

342

Exhaust jet mixing and condensation effects in the near field of aircraft wakes  

Microsoft Academic Search

In order to investigate the process of contrail formation, an integral model and a two-dimensional direct numerical simulation have been used to analyse the mixing and entrainment processes of the engine exhaust through their interaction with the vortex wake of various aircraft. The objective of this study is to evaluate the partial vapour pressure of water and temperature in the

F Garnier; A Laverdant

1999-01-01

343

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

344

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

345

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

346

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

347

Measurement of parallel blade-vortex interaction at low Reynolds numbers  

NASA Astrophysics Data System (ADS)

In this study parallel blade-vortex interaction for a Schmidt-propeller configuration has been examined using particle image velocimetry (PIV). This tandem configuration consists of a leading airfoil (forefoil), used to generate a vortical wake of leading-edge vortices (LEVs) and trailing-edge vortices (TEVs) through a pitching or plunging motion, and a trailing airfoil (hindfoil), held fixed with a specified angle of attack and vertical spacing in its wake. The hindfoil incidence (loading) and not the vertical spacing to the incoming vortical wake has been found to dictate the nature of the interaction (inviscid vs. viscous). For cases where the vortex-blade offset is small and the hindfoil is loaded, vortex distortion and vortex-induced separations are observed. By tracking the circulation of the LEV and TEV, it has been found that the vortices are strengthened for the tandem arrangement and in certain cases dissipate quicker in the wake when interacting with the hindfoil. Time-averaged forces obtained using a standard control-volume analysis are then obtained and used to evaluate these vortex-interaction cases. A subsequent analysis of the varying pressure distribution over the suction side of the hindfoil is performed by integrating the Navier-Stokes equations through the velocity field. This allows for a direct comparison of the vortex-induced loading for the various configurations.

Rival, David; Manejev, Roland; Tropea, Cam

2010-07-01

348

Quadrupole Oscillation of a Single-Vortex Bose-Einstein Condensate: Evidence for Kelvin Modes  

Microsoft Academic Search

We study the two transverse quadrupole modes of a cigar-shaped Bose-Einstein condensate with a single centered vortex. We show that the counterrotating mode is more strongly damped than in the absence of a vortex, whereas the corotating mode is not affected appreciably by the vortex. We interpret this result as a decay of the counterrotating quadrupole mode into two excitations

V. Bretin; P. Rosenbusch; F. Chevy; G. V. Shlyapnikov; J. Dalibard

2003-01-01

349

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

350

POD analysis of a finite-length cylinder near wake  

NASA Astrophysics Data System (ADS)

The near wake of a wall-mounted finite-length square cylinder with an aspect ratio of 7 is investigated based on the proper orthogonal decomposition (POD) of the PIV data measured in three spanwise planes, i.e., z/ d = 6, 3.5 and 1.0, near the cylinder free end, mid-span and fixed end (wall), respectively. The Reynolds number based on free-stream velocity ( U ?) and cylinder width ( d) is 9,300. A two-dimensional (2D) square cylinder wake is also measured and analyzed at the same Reynolds number for the purpose of comparison. The structures of various POD modes show marked differences between the two flows. While the coefficients, a 1 and a 2, of the POD modes 1 and 2 occur within an annular area centered at a 1 = a 2 = 0 in the 2D wake, their counterparts are scattered all over the entire circular plane at z/ d = 1.0 and 3.5 of the finite-length cylinder wake. Flow at z/ d = 6 is dominated by POD mode 1, which corresponds to symmetrical vortex shedding and accounts for 54.0 % of the total turbulent kinetic energy (TKE). On the other hand, the POD modes 1 and 2, corresponding to anti-symmetrical vortex shedding, are predominant, accounting for about 45.0 % of the total TKE, at z/ d = 3.5 and 1. It has been found that the flow structure may be qualitatively and quantitatively characterized by the POD coefficients. For example, at z/ d = 6, a larger a 1 corresponds to a smaller length of flow reversal zone and a stronger downwash flow. At z/ d = 3.5 and 1, two typical flow modes can be identified from a 1 and a 2. While large a 1 and/or a 2 correspond to anti-symmetrical vortex shedding, as in a 2D cylinder wake, small a 1 and a 2 lead to symmetrical vortex shedding. Any values between the large and small a 1 and/or a 2 correspond to the flow structure between these two typical flow modes. As such, the probability of occurrence of a flow structure may be determined from the distribution of the POD coefficients.

Wang, H. F.; Cao, H. L.; Zhou, Y.

2014-08-01

351

A full potential flow analysis with realistic wake influence for helicopter rotor airload prediction  

NASA Technical Reports Server (NTRS)

A 3-D, quasi-steady, full potential flow solver was adapted to include realistic wake influence for the aerodynamic analysis of helicopter rotors. The method is based on a finite difference solution of the full potential equation, using an inner and outer domain procedure for the blade flowfield to accommodate wake effects. The nonlinear flow is computed in the inner domain region using a finite difference solution method. The wake is modeled by a vortex lattice using prescribed geometry techniques to allow for the inclusion of realistic rotor wakes. The key feature of the analysis is that vortices contained within the finite difference mesh (inner domain) were treated with a vortex embedding technique while the influence of the remaining portion of the wake (in the outer domain) is impressed as a boundary condition on the outer surface of the finite difference mesh. The solution procedure couples the wake influence with the inner domain solution in a consistent and efficient solution process. The method has been applied to both hover and forward flight conditions. Correlation with subsonic and transonic hover airload data is shown which demonstrates the merits of the approach.

Egolf, T. Alan; Sparks, S. Patrick

1987-01-01

352

Mapping optical ray trajectories through island wake vortices  

NASA Astrophysics Data System (ADS)

Optical wave propagation through the atmosphere is complicated by organized atmospheric structures, spanning a wide range of length and time scales, which induce spatio-temporal variability in refraction. Therefore, when considering long-range optical ray trajectories, the influence of such structures on the propagation path becomes significantly more complex compared to a hypothetically homogeneous atmosphere. In this paper, we use a coupled mesoscale model and ray tracing framework to analyze the refractive anomalies associated with the wake vortices induced by three geographically diverse islands under various meteorological conditions. We identify organized mesoscale wake vortices (e.g., von Kármán vortices) which are sometimes capable of distorting optical ray trajectories, through ray bending, tens of meters at a range of approximately 50 km. In addition, we find in some cases that vertical oscillations, or perturbations, to the simulated ray trajectories share a frequency with the vortex shedding frequency on the order of hours. At the same time, it is also observed that the intensity and predictability of the wake vortex-induced ray bending varies from case to case. Collectively, these results highlight the value of using mesoscale models in optical wave propagation studies above conventional approaches which do not explicitly consider horizontally heterogeneous atmospheres.

Nunalee, Christopher G.; He, Ping; Basu, Sukanta; Minet, Jean; Vorontsov, Mikhail A.

2015-01-01

353

Large-scale structures in dipole and quadrupole wakes of a wall-mounted finite rectangular cylinder  

NASA Astrophysics Data System (ADS)

Large-scale quasi-periodic vortex structures shed behind a wall-mounted rectangular cylinder were reconstructed from conditional averaging of several planar particle image velocimetry measurements based on the phase of the pressure at the cylinder surface. The measurements were taken for a square cross-section cylinder with height-to-width ratio of h/d = 8 partially immersed in two nominally thin turbulent boundary layers of thickness-to-height ratios of ?/h = 0.09 and 0.32. The Reynolds number based on the diameter was 12,000. For the thinner boundary layer in the time-averaged wake, one stream wise vortex pair was present at the free end (dipole wake) while for the thicker boundary layer, another pair was also observed at the wall junction (quadrupole wake). The detailed description of the shed structures giving rise to these time-averaged vortex pairs indicates more complex connections than previously proposed arch-type structures, which implies different vortex dynamic processes in the wake. The structures obtained for the dipole and quadrupole wakes were similar at the free end but significantly different at the junction resulting in distinct imprint on the mean and turbulent fields.

Hosseini, Z.; Bourgeois, J. A.; Martinuzzi, R. J.

2013-09-01

354

Flicker vortex structures in multiferroic materials  

NASA Astrophysics Data System (ADS)

Computer simulation of ferroelastic materials reveals dynamic polar vortex structures related to flexo-electricity between cation and anion lattices. At finite temperatures, the vortices are found to flicker in time and space. Widely spaced ferroelastic twin boundaries nucleate vortices while dense twin boundaries suppress them. The time averaged number of vortices at any site decays exponentially, indicating the highly mobile dynamics of the vortex lattice. Applied electric fields break the rotational symmetry of vortices and finally destroy them. The total number density of vortices follows a field and temperature dependence as N(E)=N0/[1+A exp(E/k(T-TVF))] with TVF < 0. The observed vortex structures are akin to those observed in magnetic and superconducting disordered vortex lattices.

Zhao, Z.; Ding, X.; Salje, E. K. H.

2014-09-01

355

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.

356

Spectral energy transport in two-dimensional quantum vortex dynamics  

NASA Astrophysics Data System (ADS)

We explore the possible regimes of decaying two-dimensional quantum turbulence, and elucidate the nature of spectral energy transport by introducing a dissipative point-vortex model with phenomenological vortex-sound interactions. The model is valid for a large system with weak dissipation, and also for systems with strong dissipation, and allows us to extract a meaningful and unambiguous spectral energy flux associated with quantum vortex motion. For weak dissipation and large system size we find a regime of hydrodynamic vortex turbulence in which energy is transported to large spatial scales, resembling the phenomenology of the transient inverse cascade observed in decaying turbulence in classical incompressible fluids. For strong dissipation the vortex dynamics are dominated by dipole recombination and exhibit no appreciable spectral transport of energy.

Billam, T. P.; Reeves, M. T.; Bradley, A. S.

2015-02-01

357

Dynamics of A Vortex Pair In Shear Flows  

NASA Astrophysics Data System (ADS)

The dynamics (more specifically, stability) of infinite sequences of such vortex pairs that initially have the form of a Karman street in a jet-type shear flow is investigated Such a geometry is most directly related to the wake behind a streamlined cylinder We assume that the characteristic scale of velocity variation in a shear flow is much larger than the sizes of vortex spots. This means that each vortex spot during its evolution is always in a flow with an almost linear velocity distribution and first of all undergoes general drift and weak (primarily elliptic due to its linear profile of the flow veloc- ity) deformation of its shape. In this formulation the problem can be solved using a previously developed perturbation theory for hydrodynamic vortices. It is shown that variations of the parameters of the chess structure and the value of the velocity of the jet are interdependent. For instance, at short times when the jet velocity is relative large , all vortices remain almost on the same line while in the late wake, when the jet slows down, the vortices form the chess structure.. Such vortex sheets were indeed observed in [[ Spedding G. R., Browand F. K., Fincham A. M. Turbulence, similar- ity scaling and vortex geometry in the wake of a towed sphere in a stably stratified fluid. J. Fluid Mech., 1996, v.31410]. Note that possible initial, short-lived large-scale perturbations were not realised under the experimental conditions of. , presumably because of the limited size of the tank. This in itself implies a concentration of flow in horizontal plane. Still, to make a positive conclusion, a more thorough analysis is needed including study of the effect of the vortex sheet onto the jet flow.

Gorshkov, K. A.; Soustova, I. A.

358

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

359

Control of vortex induced vibrations by suction and blowing  

NASA Astrophysics Data System (ADS)

K'arm'an vortex shedding behind bluff bodies is of interest in a wide range of technological applications. Vortex shedding past a stationary D-cylinder is controlled in our earlier investigations [1]. However, a flexibly mounted circular cylinder gives rise to vortex induced vibrations. The control of these vibrations is of both fundamental and practical interest as fluid submerged structures need suppression of vortex induced oscillations. Flow past a circular cylinder is numerically simulated by coupling mass, momentum conservation equations along with dynamical equations for the structure. An active flow control strategy based on suction and blowing is designed and implemented to assess the efficacy of this control methodology. This is achieved by suitably located suction and blowing slots on the cylinder surface. These actuators are designed such that, the suction and blowing together results in zero mass injection. This system is found to effectively annihilate the vortex induced vibrations, when the quantum of actuations is about thrice the free stream velocity. The blowing slot is located on the leeward side of the cylinder, while the suction slots are positioned at an angle of 100^o to the flow direction. The convective instability region is reduced, while the length of the wake formation region behind the body is controlled, with an attendant annihilation of the wake vortices. [4pt] [1] Patnaik BSV, Wei GW, Phy. Rev. Lett., 88, 054502, (2002).

Muralidharan, K.; Patnaik, B. S. V.

2010-11-01

360

The propeller tip vortex. A possible contributor to aircraft cabin noise  

NASA Technical Reports Server (NTRS)

Although the assumption is generally made that cabin noise levels are governed by the transmission of propeller generated noise through the fuselage sidewall, it was postulated that the propeller wake striking the wing, in particular pressure disturbances generated downstream of the propeller by the action of the propeller tip vortex, could be strong enough to excite the aircraft structure and contribute to the cabin noise level. Tests conducted to measure the strength of the propeller tip vortex support this hypothesis. It was found that the propeller tip vortex can produce a fluctuation pressure on a simulated wing surface in the wake of a propeller that exceeds by more than 15 dB the maximum direct noise that would strike the fuselage. Wing surface response to propeller tip vortex induced excitations, and the effectiveness of this response in radiating noise to the cabin interior, must be established to assess the full significance of these results.

Miller, B. A.; Dittmar, J. H.; Jeracki, R. J.

1981-01-01

361

Vortex/wall Interaction  

NASA Astrophysics Data System (ADS)

The evolution of a 2:1 aspect ratio elliptic vortex rings moving parallel to a wall has been studied by an inviscid model using the vortex filament method, a viscous direct numerical simulation, and flow visualization experiments. In the inviscid simulation a free slip wall was simulated by placement of an image vortex ring underneath the wall located perpendicular to the initial minor axis of the vortex ring. As the vortex ring undergoes an axis switching motion, a part of the vortex ring approaches the wall leading to the vortex/wall interaction. The simulation of a ring in the vicinity of a wall becomes invalid when the cores of the vortex ring and the image vortex ring overlap. The vortex ring with a core radius of 10% of the semi-major axis length fails to continue the second axis switching motion due to the interaction, if the initial distance from the wall to the closest point on the vortex ring is less than 70% of the semi-major axis length. This critical wall distance for the interaction is a function of core radius. Viscous interactions become important as the vortex ring approaches the wall. A viscous direct numerical simulation is used to study the details of the final stage of the interaction. The vortex ring propagating near the wall induces a boundary layer on the wall and then interacts with this boundary layer. After the vortex ring impacts the wall, strong upward flows exist near the wall which causes fluid on the wall surface to be entrained into the vortex ring. Flow visualization experiments in air using titanium tetrachloride smoke were conducted. The trajectory of the vortex core at the symmetry plane is quantified and the distortion of the vortex ring due to a wall is measured using an image processing system. The critical wall distance for the wall interaction suggested from the inviscid simulation is confirmed.

Chang, Tae Young

362

Modifying Ship Air-Wake Vortices for Aircraft Operations  

NASA Technical Reports Server (NTRS)

Columnar-vortex generators (CVG) have been proposed as means to increase the safety of takeoffs and landings of aircraft on aircraft or helicopter carriers and other ships at sea. According to the proposal, CVGs would be installed at critical edge locations on ships to modify the vortices in the air wakes of the ships. The desired effects of modifications are to smooth airflows over takeoff and landing deck areas and divert vortices from takeoff and landing flight paths. With respect to aircraft operations, the wake flows of primary interest are those associated with the bow and side edges of aircraft-carrier decks and with superstructures of ships in general (see Figure 1). The bow and deck-edge vortices can adversely affect airplane and helicopter operations on carriers, while the superstructure wakes can primarily affect operations of helicopters. The concept of the CVG is not new; what is new is the proposed addition of CVGs to ship structures to effect favorable modifications of air wakes. Figure 2 depicts a basic CVG, vertical and horizontal CVGs installed on a simple superstructure, and horizontal CVGs installed on the bow and deck edges. The vertical CVGs would be closed at the deck but open at the top. Each horizontal CVG would be open at both ends. The dimensions of the CVGs installed on the aft edges of the superstructure would be chosen so that the portion of the flow modified by the vertical CVGs would interact synergistically with the portion of the flow modified by the horizontal CVG to move the air wake away from the takeoff-and-landing zone behind the superstructure. The deck-edge CVGs would be mounted flush with, and would extend slightly ahead of the bow of, the flight deck. The overall length of each tube would exceed that of the flight deck. Each deck-edge CVG would capture that portion of the airflow that generates a deck-edge vortex and would generate a columnar vortex of opposite sense to that of the unmodified vortex. The vortex generated by the CVG could be dispersed at its base, thereby removing unwanted turbulence in the path of an approaching airplane. The deck-edge CVGs would promote smooth flow over the entire flight deck. In the case of a Nimitz-class aircraft carrier like that of Figure 1, there would be a CVG on each of the outer edges of the two left portions of the flight deck and a single CVG on the right side of the flight deck. The forward-most CVG on the left side would take the generated vortex underneath the angled flight deck. A CVG could also be installed on the bow of the flight deck to smooth the flow of air onto the flight deck. In the case of wind incident on the deck from an azimuth other than straight ahead, the vortex generated by the bow CVG could, perhaps, be used to feed the CVG(s) of the leeward side edge of the flight deck.

Lamar, John E.

2004-01-01

363

Wake in faint television meteors  

NASA Technical Reports Server (NTRS)

The two component dustball model was used in numerical lag computation. Detached grain lag is typically less than 2 km, with expected wakes of a few hundred meters. True wake in television meteors is masked by apparent wake due to the combined effects of image persistence and blooming. To partially circumvent this problem, we modified a dual MCP intensified CID video system by addition of a rotating shutter to reduce the effective exposure time to about 2.0 ms. Preliminary observations showed that only 2 of 27 analyzed meteors displayed statistically significant wake.

Robertson, M. C.; Hawkes, Robert L.

1992-01-01

364

PIV and LDA measurements of the wake behind a wind turbine model  

NASA Astrophysics Data System (ADS)

In the present work we review the results of a series of measurements of the flow behind a model scale of a horizontal axis wind turbine rotor carried out at the water flume at Technical University of Denmark (DTU). The rotor is three-bladed and designed using Glauert theory for tip speed ratio ? =5 with a constant design lift coefficient along the span, CLdesign= 0.8. The measurements include dye visualization, Particle Image Velocimetry and Laser Doppler Anemometry. The wake instability has been studied in the range ? =3 - 9 at different cross-sections from the very near wake up to 10 rotor diameters downstream from the rotor. The initial flume flow was subject to a very low turbulence level with a uniform velocity profile, limiting the influence of external disturbances on the development of the inherent vortex instability. Using PIV measurements and visualizations, special attention was paid to detect and categorize different types of wake instabilities and the development of the flow in the near and the far wake. In parallel to PIV, LDA measurements provided data for various rotor regimes, revealing the existence of three main regular frequencies governing the development of different processes and instabilities in the rotor wake. In the far wake a constant frequency corresponding to the Strouhal number was found for the long-scale instabilities. This Strouhal number is in good agreement with the well-known constant that usually characterizes the oscillation in wakes behind bluff bodies. From associated visualizations and reconstructions of the flow field, it was found that the dynamics of the far wake is associated with the precession (rotation) of a helical vortex core. The data indicate that Strouhal number of this precession is independent of the rotor angular speed.

Naumov, I. V.; Mikkelsen, R. F.; Okulov, V. L.; Sřrensen, J. N.

2014-06-01

365

Closed-loop bluff-body wake stabilization via fluidic excitation  

NASA Astrophysics Data System (ADS)

This article describes an experimental study aimed at stabilizing the wake of a shedding bluff-body by means of closed-loop active flow control at low Reynolds numbers. A D-shaped (6.5 mm thick) cylinder was used to allow a direct wake interaction rather than mixed wake-boundary-layer separation control. The fluidic actuators, installed inside the thin body, were ideally located at the separation locations, i.e., the trailing edges' upper and lower corners. The wake unsteadiness was monitored by a pair of hot wires (HWs), while a single surface-mounted hot-film (HF) sensor was used as a frequency and phase reference for closed-loop control. The HF signal was contaminated by noise. Hence, a technique for real-time tracking of a low signal-to-noise ratio (SNR) signal was necessary. This was achieved by means of a Phase-Locked Loop (PLL), common in communications systems. The closed-loop scheme was based on real-time measurement of the wake-state, using the surface-mounted HF sensor, and control authority imposed by the fluidic actuators. By using opposition control at frequencies close to the natural vortex shedding frequency (VSF), it was possible to significantly reduce the wake unsteadiness. Applying the same approach, but sensing the wake HW signal, rather than the surface-mounted HF signal, as the controller input did not result in wake stabilization. On the contrary, the unsteadiness increased at all the tested conditions. It is expected that a similar approach would work at much higher Reynolds numbers as well, as long as a clearly identifiable and nominally 2D vortex shedding occurs, even when the background flow is fully turbulent.

Stalnov, O.; Fono, I.; Seifert, A.

2011-06-01

366

About the effects of an oscillating miniflap upon the wake on an airfoil, all immersed in turbulent flow  

NASA Astrophysics Data System (ADS)

The present research analyzes the asymmetry in the rolling up shear layers behind the blunt trailing edge of an airfoil 4412 with a miniflap acting as active flow control device and its wake organization. Experimental investigations relating the asymmetry of the vortex flow in the near wake region, able to distort the flow increasing the downwash of an airfoil, have been performed. All of these in a free upstream turbulent flow (1.8% intensity). We examine the near wake region characteristics of a wing model with a 4412 airfoil without and with a rotating miniflap located on the lower surface, near the trailing edge. The flow in the near wake, for 3 x-positions (along chord line) and 20 vertical points in each x-position, was explored, for three different rotating frequencies, in order to identify signs of asymmetry of the initial counter rotating vortex structures. Experimental evidence is presented showing that for typical lifting conditions the shear layer rollup process within the near wake is different for the upper and lower vortices: the shear layer separating from the pressure side of the airfoil begins its rollup immediately behind the trailing edge, creating a stronger vortex while the shear layer from the suction side begins its rollup more downstream creating a weaker vortex. The experimental data were processed by classical statistics methods. Aspects of a mechanism connecting the different evolution and pattern of these initial vortex structures with lift changes and wake alleviating processes, due to these miniflaps, will be studied in future works.

S, Delnero J.; J, Marańón Di Leo; Colman; J; M, Camocardi; Sainz M, García; F, Muńoz

2011-12-01

367

Wake-Up Call  

NSDL National Science Digital Library

The main character of this case is Denise, who we first meet in the early morning hours as she wakes up in a cold sweat, gasping for breath. But it is her husband, Jeremy, who has been diagnosed with heart disease, not her. What’s going on? In this interrupted case study, in which the other main character is Denise’s heart (who we get to know through a series of “interior” monologues), students learn about the risk factors, symptoms, and consequences of a heart attack. The case is suitable for a course in pathophysiology, first year nursing, enzymology, advanced biology or anatomy, or nutrition.

Lisa Marie Rubin

2002-01-01

368

Kinetic energy entrainment in wind turbine and actuator disc wakes: an experimental analysis  

NASA Astrophysics Data System (ADS)

The present experimental study focuses on the comparison between the wake of a two-bladed wind turbine and the one of an actuator disk. The flow field at the middle plane of the wake is measured with a stereoscopic particle image velocimetry setup, in the low-speed Open Jet Facility wind tunnel of the Delft University of Technology. The wind turbine wake is characterized by the complex dynamics of the tip vortex development and breakdown. Analysis of the flow statistics show anisotropic turbulent fluctuations in the turbine wake, with stronger components in the radial direction. The wake of the actuator disc is instead characterized by isotropic random fluctuations. The mixing process in the shear layer is further analysed in terms of flux of mean flow kinetic energy, to show the main differences between the kinetic energy entrainment in the actuator and the turbine wake. This project is intended to provide the basis for understanding the origin of the limitations of the current wake models based on the actuator disc assumption.

Lignarolo, L. E. M.; Ragni, D.; Simăo Ferreira, C. J.; van Bussel, G. J. W.

2014-06-01

369

Achromatic optical vortex lens.  

PubMed

A vortex lens is a useful optical device having applications ranging from astronomy to microscopy. Current vortex masks operate across a narrow bandwidth. Two design schemes are proposed for creating a vortex across a bandwidth exceeding 100 nm in the visible region of the spectrum. PMID:16770426

Swartzlander, Grover A

2006-07-01

370

Wake Measurements in ECN's Scaled Wind Farm  

NASA Astrophysics Data System (ADS)

In ECN's scaled wind farm the wake evolution is studied in two different situations. A single wake is studied at two different locations downstream of a turbine and a single wake is studied in conjunction with a triple wake. Here, the wake is characterized by the relative wind speed, the turbulence intensity, the vertical wind speed and the turbulence (an)isotropy. Per situation all wake measurements are taken simultaneously together with the inflow conditions.

Wagenaar, J. W.; Schepers, J. G.

2014-12-01

371

The application of experimental data to blade wake interaction noise prediction  

NASA Technical Reports Server (NTRS)

Blade wake interaction noise (BWI) has been defined as the broadband noise generated by the ingestion of turbulent trailing tip vortices by helicopter rotors. This has been shown to be the dominant contributor to the subjectively important part of the acoustic spectrum for the approach stage of a helicopter flyover. A prediction method for BWI noise based on the calculated trailing vortex trajectories has been developed and estimates of the vortex turbulence have been made. These measurements were made on a trailing vortex from a split wing arrangement and did not give the spectrum of the velocity fluctuations. A recent experiment carried out to measure the turbulence associated with a trailing vortex and the application of the results to BWI noise prediction is described.

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

1991-01-01

372

. . . are going to wake up!  

E-print Network

, Number one, March 1906. Quote: Sooner or later the American people are going to wake up! Emma Goldman. . . are going to wake up! Sooner or later the American people . . . #12;With high hopes: A Documentary History of the American Years 1890-1919, our four-volume series. Volume 1: Made for America 1890

California at Berkeley, University of

373

A pilot rating scale for vortex hazard evaluation  

NASA Technical Reports Server (NTRS)

A pilot rating scale is developed for subjective assessment of hazard resulting from wake vortex encounter upsets. The development of the rating scale is based on a survey of 48 pilots regarding the semantic properties of various phrases and a choice of formats for the rating scale. The rating scale can be used to define a hazard/nonhazard boundary as well as to determine a measure of the hazard.

Hoh, R. H.

1975-01-01

374

Measurement and prediction of mean velocity and turbulence structure in the near wake of an airfoil  

NASA Technical Reports Server (NTRS)

An experimental investigation of the near wake of a thin airfoil at various incidence angles is reported in this paper. The airfoil (NACA 0012 basic thickness form) was located in a wind tunnel, and the wake structure was measured using hot-wire sensors. The measurements of mean-velocity, turbulence intensity and Reynolds-stress components across the wake at several distances downstream show the complex nature of the near wake and its asymmetrical behavior. The asymmetry in the wake property, which is maintained up to a length of 1.5 chords downstream of the trailing edge of the blade, is dependent on the incidence angle of the inlet flow. The streamwise velocity defect in an asymmetric wake decays more slowly compared to that of a symmetric wake. The streamline curvature due to the blade loading has a substantial effect on the mean velocity profile as well as the turbulence structure. The numerical study of the same wake indicates that the existing turbulence closure models need some modification to account for the asymmetric characteristics of the wake.

Hah, C.; Lakshminarayana, B.

1982-01-01

375

Spanwise loading distribution and wake velocity surveys of a semi-span wing  

NASA Technical Reports Server (NTRS)

The spanwise distribution of bound circulation on a semi-span wing and the flow velocities in its wake were measured in a wind tunnel. Particular attention was given to documenting the flow velocities in and around the development tip vortex. A two-component laser velocimeter was used to make the velocity measurements. The spanwise distribution of bound circulation, three components of the time-averaged velocities throughout the near wake their standard deviations, and the integrated forces and moments on a metric tip as measured by an internal strain gage balance are presented without discussion.

Felker, F. F., III; Piziali, R. A.; Gall, J. K.

1982-01-01

376

Crack street: the cycloidal wake of a cylinder tearing through a thin sheet.  

PubMed

When a cylindrical tool cuts through a thin sheet of a relatively brittle material, it leaves behind a visually arresting crack street in its wake, reminiscent of a vortex street in the wake of a cylinder moving through a fluid. We show that simple geometrical arguments based on the interplay of in-plane stretching and out-of-plane bending suffice to explain the cycloidal morphology of the curved crack. The coupling between geometry and dynamics also allows us to explain the "stick-slip"-like behavior of tearing and suggests that these oscillations should occur generically in the brittle fracture of thin solid films. PMID:14683318

Ghatak, A; Mahadevan, L

2003-11-21

377

Evidence of plasma vortices in the Venus plasma wake  

NASA Astrophysics Data System (ADS)

Measurements conducted with plasma instruments in the Pioneer Venus Orbiter (PVO) and in the Venus Express (VEX) spacecraft have led to the observation of plasma velocity distributions that resemble the presence of vortices in the Venus wake. The velocity vectors of plasma fluxes measured along several orbits of the PVO trajectory reveal cases in which they are sunward oriented, and also instances where the velocity vectors are anti-sunward oriented but they point into the inner plasma wake. At the same time measurements conducted with the ASPERA-4 instrument of the VEX spacecraft show velocity vectors in the plasma wake that are arranged in patterns that are consistent with vorticity vector structures in the downstream vicinity of the polar regions and also in the Venus central wake. A fluid dynamic interpretation of such measurements in terms of vortex structures lead to calculations of the viscosity coefficient with estimates of the damping that they experience through local viscosity forces and are in agreement with those derived from the transport of solar wind momentum to the Venus upper ionosphere. From the analysis of the vorticity equation applied to the plasma conditions in the wake it is found that their evolution requires values of the viscosity coefficient that are comparable to those inferred from calculations of the momentum equation of the solar wind as it streams by the flanks of the Venus ionosheath, and thus the results obtained stress the value of viscosity as an important property of the plasma in the interaction of the solar wind with the Venus ionosphere.

Perez de Tejada, H. A.; Intriligator, D. S.; Lundin, R.; Reyes-Ruiz, M.

2010-12-01

378

Downwash in Vortex Region Behind Rectangular Half-wing at Mach Number 1.91  

NASA Technical Reports Server (NTRS)

Results of an experimental investigation to determine downwash and wake characteristics in region of trailing vortex system behind a rectangular half-wing at Mach number 1.91 are presented. The wing had a 5-percent thick symmetric diamond cross section beveled to a knife edge at the tip. At small angles of attack, downwash angles were in close agreement with predictions of linearized theory based on the assumption of an undistorted vortex sheet. At higher angles of attack, the flow was greatly influenced by the rolling up of the vortex sheet.

Cummings, John L; Haefeli, Rudolph C

1950-01-01

379

The wake structure and thrust performance of a rigid low-aspect-ratio pitching panel  

PubMed Central

Thrust performance and wake structure were investigated for a rigid rectangular panel pitching about its leading edge in a free stream. For ReC = O(104), thrust coefficient was found to depend primarily on Strouhal number St and the aspect ratio of the panel AR. Propulsive efficiency was sensitive to aspect ratio only for AR less than 0.83; however, the magnitude of the peak efficiency of a given panel with variation in Strouhal number varied inversely with the amplitude to span ratio A/S, while the Strouhal number of optimum efficiency increased with increasing A/S. Peak efficiencies between 9 % and 21 % were measured. Wake structures corresponding to a subset of the thrust measurements were investigated using dye visualization and digital particle image velocimetry. In general, the wakes divided into two oblique jets; however, when operating at or near peak efficiency, the near wake in many cases represented a Kármán vortex street with the signs of the vortices reversed. The three-dimensional structure of the wakes was investigated in detail for AR = 0.54, A/S = 0.31 and ReC = 640. Three distinct wake structures were observed with variation in Strouhal number. For approximately 0.20 < St < 0.25, the main constituent of the wake was a horseshoe vortex shed by the tips and trailing edge of the panel. Streamwise variation in the circulation of the streamwise horseshoe legs was consistent with a spanwise shear layer bridging them. For St > 0.25, a reorganization of some of the spanwise vorticity yielded a bifurcating wake formed by trains of vortex rings connected to the tips of the horseshoes. For St > 0.5, an additional structure formed from a perturbation of the streamwise leg which caused a spanwise expansion. The wake model paradigm established here is robust with variation in Reynolds number and is consistent with structures observed for a wide variety of unsteady flows. Movies are available with the online version of the paper. PMID:19746195

BUCHHOLZ, JAMES H. J.; SMITS, ALEXANDER J.

2009-01-01

380

Glory, Vortex Street off Baja California  

NASA Technical Reports Server (NTRS)

On June 19, 2007, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite captured both a vortex street and a glory visible amid the lattice of clouds over the Pacific Ocean off Baja California. In this image, the swirling clouds known as vortex streets appear along the left edge of the image, stretching southward from Isla Guadalupe. Another NASA satellite captured an earlier example of vortex streets in June 2000. These atmospheric vortices, known as Von Karman vortex streets, often occur in the wake of an obstacle to air flow, such as an island. Stratocumulus clouds--low-lying, sheets of puffy clouds-- over the ocean show the impact of the island on air flow visible though their alternating pattern of clockwise and counter-clockwise swirls. Southeast of the vortex street, a glory, which resembles a rainbow, hovers above the cloud cover. The glory is faint but large, 200 to 300 kilometers long, along a north-south orientation. This phenomenon can occur when the satellite passes directly between the Sun and a bank of clouds below. (People also observe them while looking down on clouds from airplanes.) Not just any kind of cloud can produce a glory; only clouds composed entirely of water droplets (as opposed to ice crystals) can make them. The droplets that form glories generally have diameters of less than 50 micrometers (a micrometers is a millionth of a meter). The water droplets bend the light, showing its different wavelengths, or colors. In this glory, reds and oranges are most visible. NASA image by Jeff Schmaltz, MODIS Rapid Response Team, Goddard Space Flight Center.

2007-01-01

381

Vortex Formation and Vortex Shedding in Continuously Stratified Flows past Isolated Topography.  

NASA Astrophysics Data System (ADS)

The flow of a nonrotating atmosphere with uniform stratification and wind speed past an isolated three-dimensional topographic obstacle is investigated with a nonhydrostatic numerical model having a free-slip lower boundary. When the mountain is sufficiently high, the transient development of a quasi-steady flow occurs in two phases. During the first phase, which occurs over a dimensionless time of O(1), the flow is essentially inviscid and adiabatic, and potential vorticity (PV) is conserved. The transient evolution of the flow during the second phase, which occurs over a dimensionless time of O(10) to O(100), is controlled by dissipation and is accompanied by the generation of PV anomalies.Two cases are examined in which the flow is forced to remain left-right symmetric with respect to the axis of the incident flow. In the first, the dimensionless mountain height NH/U is 1.5, and gravity waves break over the mountain. In the second, NH/U = 3, and a quasi-steady recirculating wake containing a doublet of positive and negative vortices develops in the lee. In both cases potential vorticity anomalies are generated by dissipation, although the sources of dissipation are different in each case. The net effect of the dissipation on the PV budget is, nevertheless, similar as may be understood from the generalized Bernoulli theorem that equates the generation of potential vorticity fluxes to the development of a Bernoulli function gradient on quasi-steady isentropic surfaces. In these experiments a Bernoulli deficit develops either from strong localized dissipation in the wave-breaking region (dominant for NH/U = 1.5), or as the result of weak dissipation throughout the elongated wake (dominant for NH/U = 3).Oscillating von Kármán vortex streets appear if the flows are allowed to develop asymmetries with respect to the axis of the incident flow. It is shown that the transition into the vortex shedding regime is associated with an absolute instability of the symmetrical wake, which feeds upon the shear present at the edges of the wake. The most unstable global normal mode is diagnosed numerically and shows strong similarities with the corresponding mode in shallow-water theory. The doubling time of the instability is a few hours, which is consistent with the rapid formation of observed atmospheric vortex streets. The individual vortices in the fully developed vortex street are quasi-balanced warm-core vortices that are associated with both PV and surface potential temperature anomalies.

Schär, Christoph; Durran, Dale R.

1997-02-01

382

Quadrupole Oscillation of a Single-Vortex Condensate: Evidence for Kelvin Modes  

E-print Network

We study the two transverse quadrupole modes of a cigar-shaped Bose-Einstein condensate with a single centered vortex. We show that the counter-rotating mode is more strongly damped than in the absence of a vortex, whereas the co-rotating mode is not affected appreciably by the vortex. We interpret this result as a decay of the counter-rotating quadrupole mode into two excitations of the vortex line, the so-called Kelvin modes. This is supported by direct observation of the wiggling vortex line.

V. Bretin; P. Rosenbusch; F. Chevy; G. V. Shlyapnikov; J. Dalibard

2002-11-06

383

Quadrupole oscillation of a single-vortex Bose-Einstein condensate: evidence for Kelvin modes.  

PubMed

We study the two transverse quadrupole modes of a cigar-shaped Bose-Einstein condensate with a single centered vortex. We show that the counterrotating mode is more strongly damped than in the absence of a vortex, whereas the corotating mode is not affected appreciably by the vortex. We interpret this result as a decay of the counterrotating quadrupole mode into two excitations of the vortex line, the so-called Kelvin modes. This is supported by direct observation of the vortex line. PMID:12688984

Bretin, V; Rosenbusch, P; Chevy, F; Shlyapnikov, G V; Dalibard, J

2003-03-14

384

Description of Selected Algorithms and Implementation Details of a Concept-Demonstration Aircraft VOrtex Spacing System (AVOSS)  

NASA Technical Reports Server (NTRS)

A ground-based system has been developed to demonstrate the feasibility of automating the process of collecting relevant weather data, predicting wake vortex behavior from a data base of aircraft, prescribing safe wake vortex spacing criteria, estimating system benefit, and comparing predicted and observed wake vortex behavior. This report describes many of the system algorithms, features, limitations, and lessons learned, as well as suggested system improvements. The system has demonstrated concept feasibility and the potential for airport benefit. Significant opportunities exist however for improved system robustness and optimization. A condensed version of the development lab book is provided along with samples of key input and output file types. This report is intended to document the technical development process and system architecture, and to augment archived internal documents that provide detailed descriptions of software and file formats.

Hinton, David A.

2001-01-01

385

Bubbly wake of surface vessels  

NASA Astrophysics Data System (ADS)

We study the length of the bubbly wake of surface vessels. This wake is important for the boat security since it can extend to several ship length and thus increases the detectability of the ship by torpedoes. The image analysis of the wake of real scale ships reveals the sensitivity of the length to propellers. We have thus conducted a systematic study in the laboratory of the interaction bubble/propeller, trying to address several questions:- what is the role of cavitation?- is the propeller able to attract the bubbles present along the ship at the sea surface?- if attracted, can these bubble be broken by the propeller?

Caillé, François; Magnaudet, Jacques; Clanet, Christophe

2006-11-01

386

The role of turbulent mixing in wind turbine wake recovery and wind array performance  

NASA Astrophysics Data System (ADS)

The effect of wind turbine wakes in large offshore wind energy arrays can be a substantial factor in affecting the performance of turbines inside the array. Turbulent mixing plays a key role in the wake recovery, having a significant effect on the length over which the wake is strong enough to affect the performance other turbines significantly. We aim to highlight how turbulence affects wind turbine wakes, first by examining a high resolution CFD model of a single turbine wake validated by LIDAR measurements [1], and secondly with a much larger CFD simulation of Lillgrund offshore wind farm, validated with SCADA data [2]. By comparing the decay rates behind single turbines in environments of different surrounding surface features, ranging from ideal free-slip wind tunnels to mixed-vegetation hills, we suggest that the decay rate of turbine wakes are enhanced by free-stream turbulence, created by topography and ground features. In the context of Lillgrund wind farm, observations and computational results suggest that the wakes created by the turbines in the leading row facing the wind decay much slower than those in second row, or further into the turbine array. This observation can be explained by the diffusive action of upwind turbulence breaking up the wake generated by a turbine rotor. Angus CW Creech, Wolf-Gerrit Früh, Peter Clive (2012). Actuator volumes and hradaptive methods for threedimensional simulation of wind turbine wakes and performance. Wind Energy Vol.15, 847 - 863. Angus C.W. Creech, Wolf-Gerrit Früh, A. Eoghan Maguire (2013). High-resolution CFD modelling of Lillgrund Wind farm. Renewable Energies and Power Quality Journal, Vol. 11

Fruh, Wolf-Gerrit; Creech, Angus; Maguire, Eoghan

2014-05-01

387

Transient dynamics of the flow around a NACA 0015 airfoil using fluidic vortex generators  

Microsoft Academic Search

The unsteady activation or deactivation of fluidic vortex generators on a NACA 0015 airfoil is studied to understand the transient dynamics of flow separation control. The Reynolds number is high enough and the boundary layer is tripped, so the boundary layer is fully turbulent prior to separation. Conditional PIV of the airfoil wake is obtained phase-locked to the actuator trigger

W. L. Siauw; J.-P. Bonnet; J. Tensi; L. Cordier; B. R. Noack; L. Cattafesta

2010-01-01

388

NUMERICAL MODELING OF VORTEX-INDUCED VIBRATIONS OF TWO FLEXIBLE RISERS  

Microsoft Academic Search

Due to the complex interactions of vortex-induced vibrations (VIV) and downstream wake dynamics, the design of riser arrays has been an area of great uncertainty in the offshore industry. Numerical methodology can play an important role in predicting the hydrodynamic forces and motion of riser arrays from both design and operational standpoints. The focus of this study is to investigate

Marlow Springer; Rajeev K. Jaiman; Steve Cosgrove; Yiannis Constantinides

2009-01-01

389

A 3D Numerical Method for Studying Vortex For mation Behind a Moving Plate  

Microsoft Academic Search

In this paper, we introduce a three-dimensional numerical method for computing the wake behind a flat plate advancing perpendicular to the flow. Our numerical method is inspired by the panel method of J. Katz and A. Plotkin (J. Katz and A. Plotkin, Low- speed Aerodynamics, 2001) and the 2D vortex blob method of Krasny (R. Krasny, Lectures in Appl. Math.,

T. Y. Hou; V. G. Stredie; T. Y. Wu

2006-01-01

390

A Theoretical Investigation of Vortex-Sheet Deformation Behind a Highly Loaded Wing and Its Effect on Lift  

NASA Technical Reports Server (NTRS)

The induced drag polar is developed for wt-ngs capable of attaining extremely high loadings while possessing an elliptical distribution of circulation. This development is accomplished through a theoretical investigation of the vortex-wake deformation process and the deduction of the airfoil forces from the impulse and kinetic energy contents of the ultimate wake form. The investigation shows that the induced velocities of the wake limit the maximum lift coefficient to a value of 1.94 times the wing aspect ratio, for aspect ratios equal to or less than 6.5, and that the section properties of the airfoil limit the lift coefficient to 12.6 for aspect ratios greater than 6.5. Relations are developed for the rate of deformation of the vortex wake. It is also shown that linear wing theory is app1icable up to lift coefficients equal to 1.1 times the aspect ratio.

Cone, Clarence D., Jr.

1961-01-01

391

Interaction of a streamwise vortex with a turbulent mixing layer  

NASA Technical Reports Server (NTRS)

The interaction of a single streamwise vortex with a plane turbulent mixing layer is experimentally studied. Initially, near the mixing layer origin, the vortex rides just below the layer and only affects the mixing layer properties near the bottom side of the mixing layer. Once the vortex starts to become embedded within the mixing layer and active interaction has begun, the effect of the vortex is to grossly distort the mean velocity and turbulence contours over the whole width of the mixing layer. The induced cross-flow velocities in the mixing layer result in the production of additional normal Reynolds stresses which in turn act with mean flow gradients to generate extra shear stresses. In particular, the secondary shear stress (SSS) achieves an absolute peak value equivalent to about 40 percent of the maximum primary shear stress. The position and decay of the SSS peak are then found to be strongly correlated with those of the streamwise vortex.

Bell, James H.; Mehta, Rabindra D.

1990-01-01

392

Wake attenuation in large Reynolds number dispersed two-phase flows.  

PubMed

The dynamics of high Reynolds number-dispersed two-phase flow strongly depends on the wakes generated behind the moving bodies that constitute the dispersed phase. The length of these wakes is considerably reduced compared with those developing behind isolated bodies. In this paper, this wake attenuation is studied from several complementary experimental investigations with the aim of determining how it depends on the body Reynolds number and the volume fraction alpha. It is first shown that the wakes inside a homogeneous swarm of rising bubbles decay exponentially with a characteristic length that scales as the ratio of the bubble diameter d to the drag coefficient Cd, and surprisingly does not depend on alpha for 10(-2)wakes in a fixed array of spheres randomly distributed in space (alpha=2 x 10(-2)) is observed to be stronger than that of the wake of an isolated sphere in a turbulent incident flow, but similar to that of bubbles within a homogeneous swarm. It thus appears that the wakes in dispersed two-phase flows are controlled by multi-body interactions, which cause a much faster decay than turbulent fluctuations having the same energy and integral length scale. Decomposition of velocity fluctuations into a contribution related to temporal variations and that associated to the random character of the body positions is proposed as a perspective for studying the mechanisms responsible for multi-body interactions. PMID:18348974

Risso, Frédéric; Roig, Véronique; Amoura, Zouhir; Riboux, Guillaume; Billet, Anne-Marie

2008-06-28

393

Suppression of the Karman vortex street and reduction in the frontal drag of a circular cylinder with two vortex cells  

NASA Astrophysics Data System (ADS)

Numerical analysis of an unsteady-state two-dimensional incompressible flow at a Reynolds number of 40000 around a circular cylinder with two vortex cells is carried out on the basis of the finite-volume solution of the Reynolds equations closed by the Menter's shear-stress transport model. The vortex cells are fitted with slots that ensure suction into the central channel via a fan located and through outflow of a low-pressure jet. It is shown that the suction in small-size cells intensifies the circulatory flow inside it and leads to the rearrangement of a large-scale structure of the flow around the cylinder accompanied by suppression of the Karman vortex street and a slightly symmetrical stabilization of the wake. The frontal drag of the cylinder decreases almost by three times with an optimal coefficient of the sucked air rate.

Isaev, S. A.; Baranov, P. A.; Vatin, N. I.; Zhukova, Yu. V.; Sudakov, A. G.

2014-08-01

394

Irregular sleep-wake syndrome  

MedlinePLUS

... normal, but the body clock loses its normal circadian cycle. People with changing work shifts and travelers ... Kanuther N, Harrington J, Lee-Chiong T. Circadian rhythm sleep ... rhythm sleep disorder: irregular sleep wake rhythm. Sleep ...

395

Comparison between PIV measurements and computations of the near-wake of an actuator disc  

NASA Astrophysics Data System (ADS)

Experimental stereoscopic PIV measurements in the wake of a two-bladed rotor and a porous actuator disc are compared to numerical simulation of an actuator disc. Compared to previous literature, the focus of the present analysis is on the near wake, where the actuator discs fail to represent the complex flow structures correctly, which affects the downstream representation of the full wake behind a real rotor. The near wake region is characterised by the instability and breakdown of the tip-vortex helical system, which constitutes the onset of a stronger mixing process. The comparison focuses on the turbulent structures in the shear layer at the borders of the wake through the analysis of the Reynolds stresses and by employing POD on two separate regions. The analysis shows that the actuator discs fail to capture the details of the complex flow behind a rotor, but that the experimental and numerical actuator discs are generally comparable at a certain distance behind the actuator disc. This project is intended to provide the basis for understanding the origin of the limitations of the current wake models based on the actuator disc assumption.

Andersen, S. J.; Lignarolo, L. E. M.; Ragni, D.; Simăo Ferreira, C. J.; Sřrensen, J. N.; Mikkelsen, R. F.; van Bussel, G. J. W.

2014-06-01

396

Analysis of WakeVAS Benefits Using ACES Build 3.2.1  

NASA Technical Reports Server (NTRS)

The FAA and NASA are currently engaged in a Wake Turbulence Research Program to revise wake turbulence separation standards, procedures, and criteria to increase airport capacity while maintaining or increasing safety. The research program is divided into three phases: Phase I near term procedural enhancements; Phase II wind dependent Wake Vortex Advisory System (WakeVAS) Concepts of Operations (ConOps); and Phase III farther term ConOps based on wake prediction and sensing. This report contains an analysis that evaluates the benefits of a closely spaced parallel runway (CSPR) Phase I ConOps, a single runway and CSPR Phase II ConOps and a single runway Phase III ConOps. A series of simulation runs were performed using the Airspace Concepts Evaluation System (ACES) Build 3.21 air traffic simulator to provide an initial assessment of the reduction in delay and cost savings obtained by the use of a WakeVAS at selected U.S. airports. The ACES simulator is being developed by NASA Ames Research Center as part of the Virtual Airspace Modelling and Simulation (VAMS) program.

Smith, Jeremy C.

2005-01-01

397

GPU Based Fast Free-Wake Calculations For Multiple Horizontal Axis Wind Turbine Rotors  

NASA Astrophysics Data System (ADS)

Unsteady free-wake solutions of wind turbine flow fields involve computationally intensive interaction calculations, which generally limit the total amount of simulation time or the number of turbines that can be simulated by the method. This problem, however, can be addressed easily using high-level of parallelization. Especially when exploited with a GPU, a Graphics Processing Unit, this property can provide a significant computational speed-up, rendering the most intensive engineering problems realizable in hours of computation time. This paper presents the results of the simulation of the flow field for the NREL Phase VI turbine using a GPU-based in-house free-wake panel method code. Computational parallelism involved in the free-wake methodology is exploited using a GPU, allowing thousands of similar operations to be performed simultaneously. The results are compared to experimental data as well as to those obtained by running a corresponding CPU-based code. Results show that the GPU based code is capable of producing wake and load predictions similar to the CPU- based code and in a substantially reduced amount of time. This capability could allow free- wake based analysis to be used in the possible design and optimization studies of wind farms as well as prediction of multiple turbine flow fields and the investigation of the effects of using different vortex core models, core expansion and stretching models on the turbine rotor interaction problems in multiple turbine wake flow fields.

Türkal, M.; Novikov, Y.; Ü?enmez, S.; Sezer-Uzol, N.; Uzol, O.

2014-06-01

398

Effect of wing-wake interaction on aerodynamic force generation on a 2D flapping wing  

NASA Astrophysics Data System (ADS)

This paper is motivated by the works of Dickinson et al. (Science 284:1954-1960, 1999) and Sun and Tang (J Exp Biol 205:55-70, 2002) which provided two different perspectives on the influence of wing-wake interaction (or wake capture) on lift generation during flapping motion. Dickinson et al. (Science 284:1954-1960, 1999) hypothesize that wake capture is responsible for the additional lift generated at the early phase of each stroke, while Sun and Tang (J Exp Biol 205:55-70, 2002) believe otherwise. Here, we take a more fundamental approach to study the effect of wing-wake interaction on the aerodynamic force generation by carrying out simultaneous force and flow field measurements on a two-dimensional wing subjected to two different types of motion. In one of the motions, the wing at a fixed angle of attack was made to follow a motion profile described by "acceleration-constant velocity-deceleration". Here, the wing was first linearly accelerated from rest to a predetermined maximum velocity and remains at that speed for set duration before linearly decelerating to a stop. The acceleration and deceleration phase each accounted for only 10% of the stroke, and the stroke covered a total distance of three chord lengths. In another motion, the wing was subjected to the same above-mentioned movement, but in a back and forth manner over twenty strokes. Results show that there are two possible outcomes of wing-wake interaction. The first outcome occurs when the wing encounters a pair of counter-rotating wake vortices on the reverse stroke, and the induced velocity of these vortices impinges directly on the windward side of the wing, resulting in a higher oncoming flow to the wing, which translates into a higher lift. Another outcome is when the wing encounters one vortex on the reverse stroke, and the close proximity of this vortex to the windward surface of the wing, coupled with the vortex suction effect (caused by low pressure region at the center of the vortex), causes the net force on the wing to decrease momentarily. These results suggest that wing-wake interaction does not always lead to lift enhancement, and it can also cause lift reduction. As to which outcome prevails depend very much on the flapping motion and the timing of the reverse stroke.

Lua, K. B.; Lim, T. T.; Yeo, K. S.

2011-07-01

399

Twisted vortex state  

E-print Network

We study a twisted vortex bundle where quantized vortices form helices circling around the axis of the bundle in a "force-free" configuration. Such a state is created by injecting vortices into rotating vortex-free superfluid. Using continuum theory we determine the structure and the relaxation of the twisted state. This is confirmed by numerical calculations. We also present experimental evidence of the twisted vortex state in superfluid 3He-B.

V. B. Eltsov; A. P. Finne; R. Hanninen; J. Kopu; M. Krusius; M. Tsubota; E. V. Thuneberg

2006-08-15

400

Generalization of the JTZ model to open plane wakes  

NASA Astrophysics Data System (ADS)

The JTZ model [C. Jung, T. Tél, and E. Ziemniak, Chaos 3, 555 (1993)], as a theoretical model of a plane wake behind a circular cylinder in a narrow channel at a moderate Reynolds number, has previously been employed to analyze phenomena of chaotic scattering. It is extended here to describe an open plane wake without the confined narrow channel by incorporating a double row of shedding vortices into the intermediate and far wake. The extended JTZ model is found in qualitative agreement with both direct numerical simulations and experimental results in describing streamlines and vorticity contours. To further validate its applications to particle transport processes, the interaction between small spherical particles and vortices in an extended JTZ model flow is studied. It is shown that the particle size has significant influences on the features of particle trajectories, which have two characteristic patterns: one is rotating around the vortex centers and the other accumulating in the exterior of vortices. Numerical results based on the extended JTZ model are found in qualitative agreement with experimental ones in the normal range of particle sizes.

Wu, Zuo-Bing

2010-03-01

401

Generalization of the JTZ model to open plane wakes.  

PubMed

The JTZ model [C. Jung, T. Tel, and E. Ziemniak, Chaos 3, 555 (1993)], as a theoretical model of a plane wake behind a circular cylinder in a narrow channel at a moderate Reynolds number, has previously been employed to analyze phenomena of chaotic scattering. It is extended here to describe an open plane wake without the confined narrow channel by incorporating a double row of shedding vortices into the intermediate and far wake. The extended JTZ model is found in qualitative agreement with both direct numerical simulations and experimental results in describing streamlines and vorticity contours. To further validate its applications to particle transport processes, the interaction between small spherical particles and vortices in an extended JTZ model flow is studied. It is shown that the particle size has significant influences on the features of particle trajectories, which have two characteristic patterns: one is rotating around the vortex centers and the other accumulating in the exterior of vortices. Numerical results based on the extended JTZ model are found in qualitative agreement with experimental ones in the normal range of particle sizes. PMID:20370277

Wu, Zuo-Bing

2010-03-01

402

Gravitational wakes in Saturn's rings  

Microsoft Academic Search

Numerical simulations, including both gravitational interactions and dissipative impacts between particles, are used here to study realistic models for Saturn's rings. For the C-ring there is no instability, but for the B- and A-rings gravitational wakes form. In the A-ring these wakes are so strong that particles trapped in them from meter-sized aggregate particles, which themselves lead to further instability.

H. Salo

1992-01-01

403

Vortex generation and wave-vortex interaction over a concave plate with roughness and suction  

NASA Technical Reports Server (NTRS)

The generation and amplification of vortices by surface homogeneities, both in the form of surface waviness and of wall-normal velocity, is investigated using the nonlinear parabolic stability equations. Transients and issues of algebraic growth are avoided through the use of a similarity solution as initial condition for the vortex. In the absence of curvature, the vortex decays as the square root of 1/x when flowing over streamwise aligned riblets of constant height, and grows as the square root of x when flowing over a corresponding streamwise aligned variation of blowing/suction transpiration velocity. However, in the presence of wall inhomogeneities having both streamwise and spanwise periodicity, the growth of the vortex can be much larger. In the presence of curvature, the vortex develops into a Gortler vortex. The 'direct' and 'indirect' interaction mechanisms possible in wave-vortex interaction are presented. The 'direct' interaction does not lead to strong resonance with the flow conditions investigated. The 'indirect' interaction leads to K-type transition.

Bertolotti, Fabio

1993-01-01

404

Lift and wakes of flying snakes  

NASA Astrophysics Data System (ADS)

Flying snakes use a unique method of aerial locomotion: they jump from tree branches, flatten their bodies, and undulate through the air to produce a glide. The shape of their body cross-section during the glide plays an important role in generating lift. This paper presents a computational investigation of the aerodynamics of the cross-sectional shape. Two-dimensional simulations of incompressible flow past the anatomically correct cross-section of the species Chrysopelea paradisi show that a significant enhancement in lift appears at a 35° angle of attack, above Reynolds numbers 2000. Previous experiments on physical models also obtained an increased lift, at the same angle of attack. The flow is inherently three-dimensional in physical experiments, due to fluid instabilities, and it is thus intriguing that the enhanced lift also appears in the two-dimensional simulations. The simulations point to the lift enhancement arising from the early separation of the boundary layer on the dorsal surface of the snake profile, without stall. The separated shear layer rolls up and interacts with secondary vorticity in the near-wake, inducing the primary vortex to remain closer to the body and thus cause enhanced suction, resulting in higher lift.

Krishnan, Anush; Socha, John J.; Vlachos, Pavlos P.; Barba, L. A.

2014-03-01

405

A Family of Vortices to Study Axisymmetric Vortex Breakdown and Reconnection  

NASA Technical Reports Server (NTRS)

A new analytic model describing a family of vortices 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. The family of vortices incorporates a wide range of prescribed initial vorticity distributions -- including single or dual-core vorticity distributions. The result is analytical solutions for the vorticity and velocities for each member of the family of vortices. This model is of sufficient generality to further illustrate the dependence of vortex reconnection and breakdown on initial vorticity distribution as was suggested by earlier analytical work. This family of vortices, though laminar in nature, is anticipated to provide valuable insight into the vortical evolution of large-scale rotor and propeller wakes.

Young, Larry A.

2007-01-01

406

Investigation on 3D t wake flow structures of swimming bionic fish  

NASA Astrophysics Data System (ADS)

A bionic experimental platform was designed for the purpose of investigating time accurate three-dimensional flow field, using digital particle image velocimetry (DSPIV). The wake behind the flapping trail of a robotic fish model was studied at high spatial resolution. The study was performed in a water channel. A robot fish model was designed and built. The model was fixed onto a rigid support framework using a cable-supporting method, with twelve stretched wires. The entire tail of the model can perform prescribed motions in two degrees of freedom, mainly in carangiform mode, by driving its afterbody and lunate caudal fin respectively. The DSPIV system was set up to operate in a translational manner, measuring velocity field in a series of parallel slices. Phase locked measurements were repeated for a number of runs, allowing reconstruction of phase average flow field. Vortex structures with phase history of the wake were obtained. The study reveals some new and complex three-dimensional flow structures in the wake of the fish, including "reverse hairpin vortex" and "reverse Karman S-H vortex rings", allowing insight into physics of this complex flow.

Shen, G.-X.; Tan, G.-K.; Lai, G.-J.

2012-10-01

407

Properties of aircraft wake vortices measured by ground-based continuous-wave Doppler lidar  

NASA Astrophysics Data System (ADS)

The DLR ground-based cw Doppler lidar has been developed for wind and turbulence measurements in the Atmospheric Boundary Layer. Moreover, it is used for experimental investigations of aircraft wake vortices. By the present paper measurements of wake vortices generated by military-type fixed-wing as well as rotor-wing aircraft will be presented. The experimental data has been analyzed regarding the structure and circulation of wake vortices, partly also regarding their temporal development. The influence of flight parameters has been measured during several consecutive fly-bys. Small variations in the velocity profiles could be observed which were overlayed by the signatures of the variable wind and turbulence field, a problem which can only be solved by statistical treatment. The influence of the aircraft design could be demonstrated by comparison of the